1/cours/MIB 4228 Solid W… · UNIVERSITE DE YAOUNDE I FACULTE DES SCIENCES THE UNIVERSITY OF YAOUNDE I FACULTY OF SCIENCE DEPARTEMENT DE MICROBIOLOGIE DEPARTMENT OF …

UNIVERSITE DE YAOUNDE I FACULTE DES SCIENCES

THE UNIVERSITY OF YAOUNDE I FACULTY OF SCIENCE

DEPARTEMENT DE MICROBIOLOGIE

DEPARTMENT OF MICROBIOLOGY

MIB 4228: Solid waste management

Instructor

Blaise Bougnom

Department of Microbiology/University of Yaounde 1, Cameroon

Emails: [email protected]

Course Description –This course will focus on the characteristics of waste and the general

concept of waste management. It will lay emphasis on the use of microorganisms or their

components in organic waste management, namely composting, biogas production and

bioremediation. Knowledge of these mechanisms is being used to efficiently manage waste

for sustainable development.

Keywords: Municipal solid waste; waste management, compost; biogas

Solid Waste Management

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SummaryWith progressing urbanisation, solid waste management is becoming a ma-jor public health and environmental con-cern in urban areas of many developing countries. The overall goal of urban sol-id waste management is to collect, treat and dispose of solid waste generated by all urban population groups in an environ-mentally and socially satisfactory man-ner using the most economical means available.

However, a typical solid waste man-agement system in a developing country displays an array of problems, including low collection coverage, irregular collec-tion services, indiscriminate open dump-ing and burning without air and water pollution control, breeding of flies and

vermin, as well as handling and lack of control of informal waste picking or scav-enging. These public health, as well as environmental and management prob-lems are caused by various factors con-straining the development of effective solid waste management systems. (The World Bank, 2008)

This document provides an overview of the present state-of-the-art of solid waste production and management. It contains the characteristics of municipal solid waste and describes current waste treatment systems and technologies, as well as non-technical aspects like private sector involvement and financial arrange-ments.

Not included in Module 6Industrial hazardous wasteTechnical details for recycling and disposalTransboundary waste movements

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Summary

Figure 1: Solid waste management in the context of environmental sanitation.

�Sandec Training Tool: Module 6

Content

Content

Content

1 – Definitions & Objectives 41.1 How is municipal solid waste (MSW) characterised? 41.2 What are the objectives and main elements of an integrated solid waste

management? 5

2 – Introduction 72.1 What are the problems related to insufficient solid waste management? 72.2 What are the municipal solid waste categories and how much waste is

produced? 82.3 How is solid waste management related to rapid urbanisation? 102.4 What is the present state in SWM? 102.5 What role does organic waste play in the context of SWM? 12

3 – Technical Aspects 133.1 What are the main storage, collection and transport options? 13

Primary storage and transport 13Secondary storage (transfer point) and collection 15

3.2 What are the major organic waste treatment options? 17Composting 17Vermicomposting 20Biogasification 21

3.3 What are the recycling options for non-organic waste? 243.4 What are the final disposal options and how are they characterised? 25

Incineration 25Landfills 26Non-landfill disposal 27

4 – Hazardous Household Waste 284.1 What is hazardous household waste? 284.2 What are the dangers of hazardous waste? 284.3 What is e-waste? 29

5 – Non-technical Aspects 305.1 Who are the stakeholders to consider within SWM? 305.2 What are the characteristics of private sector involvement? 325.3 How do legal frameworks and international treaties influence MSW

management? 345.4 What are the financial arrangement options for SWM? 35

5 – References and Links 36References 36Weblinks <www> 37

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1 – Definitions & Objectives

1.1 How is municipal solid waste (MSW) characterised?

The characteristics of MSW are dependent on local culture, standard of living and natural resources.Ñ

Additional infoUNEP (2005). Solid Waste Management,

CalRecovery Inc. www.unep.or.jp/Ietc/Pub-lications/spc/Solid_Waste_Management/Vol_I/Binder1.pdf (last accessed 28.04.08)

Download available on the CD of Sandec’s Training Tool and from the Internet.

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Further questionsWhat is the purpose of segregating

waste at its point of generation, i. e. house-hold level?

Are there any major differences in household waste composition in different countries or regions?

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The main goal of municipal solid waste management (MSWM) is to reduce the waste volumes and to protect the health of the urban population, particularly that of low-income groups who suffer most from poor waste management. In ur-ban areas, solid waste is generated by households, commercial and industrial enterprises, as well as healthcare facil-ities and institutions. With the excep-tion of industrial, construction and debris

What is waste?The term ‘waste’ has a different mean-ing for different people. In general, waste is ‘unwanted’ for the person who dis-cards it; a product or material which is no longer valued by the first user and there-fore thrown away. However, ‘unwanted’ is subjective, as it could be of value for an-other person under different circumstanc-es or even in a different culture. There are many large industries using primarily or ex-clusively waste materials as their indus-trial feedstock – paper and metals are the most common. (Van de Klundert et al., 2001, p. 9)

waste, the aforementioned waste gener-ated in cities is referred to as municipal solid waste. (Zurbrügg, 2003a)

Semisolid waste, such as sludge or nightsoil, is dealt with by liquid waste management systems; whereas hazard-ous industrial or medical waste is, by definition, not a component of munici-pal solid waste. It is normally quite dif-ficult to separate from municipal solid waste, particularly when its sources are small and scattered. (Schüberler et al., 1996, p. 17)

Regional solid waste characteristics and quantity are dependent on the stand-ard of living and lifestyle of the inhab-itants of a region, but also on volume and type of available natural resourc-es. Urban waste can be subdivided into two major components – organic and inorganic.

Waste generated in countries locat-ed in humid, tropical and semitropical areas is usually characterised by a high concentration of plant debris; whereas the waste, generated in areas subject

to significant seasonal temperature var-iations or where coal or wood are used for cooking and heating, may contain an abundance of ash. The amount of ash may be substantially higher in win-ter. Waste is usually more or less con-taminated by nightsoil, irrespective of climatic conditions. (UNEP, 2005, p. 2)



Figure 2: National and local factors (circular boxes) influencing the core concepts of the waste management hierarchy (triangle), in which solid waste elements diminish in prior-ity from top to bottom. (Eawag/Sandec).

Figure 3: Elements of an integrated solid waste management. (Eawag/Sandec)

1.2 What are the objectives and main elements of an integrated solid waste management?

Integrated solid waste management includes all activities, which seek to minimise the health, environmental and aesthetic impacts of solid waste.

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The main goal of municipal solid waste management (MSWM) is to protect the health of the urban population, particu-larly that of low-income groups who suf-fer most from poor waste management. Secondly, MSWM aims to promote en-vironmental conditions by controlling pollution (including water, air, soil, and cross-media pollution) and to ensure the sustainability of ecosystems in the urban region. Thirdly, MSWM supports urban economic development by pro-viding the required waste management services and guaranteeing the efficient use and conservation of valuable mate-rials and resources. Fourthly, MSWM aims to generate employment and in-

come in the sector itself. The goals of MSWM are therefore:

To protect environmental healthTo promote the quality of the urban environmentTo support the efficiency and productivity of the economyTo generate employment and income

To achieve these goals, it is necessary to establish integrated and sustainable sys-tems of solid waste management that meet the needs of the entire urban pop-ulation, including the poor. The essential condition of sustainability implies that waste management systems must be ab-

1.2.

3.

4.

Sy st em Pr ocesses

Ac to rs and Stak eholder s

Planning , Mana ge ment , Ope ra tion & Main te nance and Moni to ri ng Pr ocesse s

Social, Instit utional/Leg al , Econimic and En vir onmental F ra me wo rk

collection/picking/recovering/scavenging of recy clables

treatment/processing of recy clables

use of rec yclables

Avoid-ance/reduc-

tion

Disposal s ys tems

designateddisposal sites

indiscriminat edisposal sites

incineratio n

seconda rycollectio n

Collection and storagesy stem s

seconda rycollectio n

bringsy stem

primarycollectio n

communi tycontainer

trans ferst ation

W ast egenerator s

Households

Commercia l

Health car efacility

Instit utions

Indust ry

Storagesy stem s

Heaps

Bin s

Sharedbins

Bags

Sharedbins


Further questionsIs waste avoidance and reduction gener-

ally given first priority by municipalities?Ñ


CalRecovery Inc. www.unep.or.jp/Ietc/Pub-lications/spc/Solid_Waste_Management/Vol_I/Binder1.pdf (last accessed 28.04.08)


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analysed together, since they are in fact interrelated, i. e. developments in one area frequently affect practices or ac-tivities in another area. (UNEP, 2005, p. 7 – 8)

An integrated approach is an impor-tant element of sound practice as:

Certain problems are more easily resolved in conjunction with other aspects of the waste system than on their own. Also, development of new or improved waste handling in one area can disrupt existing activities in another area, unless changes are han-dled in a coordinated manner.Integration allows for capacity or re-sources to be optimised and, thus, fully utilised. Economies of scale for equipment or management infrastruc-ture can be frequently reached only if all the waste in a region is managed as part of a single system.An integrated approach allows for pub-lic, private and informal sector partici-pation with roles adapted to each.Since some waste management prac-tices are more costly than others, in-tegrated approaches facilitate iden-tification and selection of low-cost

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Non-technical aspects Technical aspects

Setting policies

Developing and enforcing regulations

Planning and evaluating municipal SWM activities by system designers, users and other stakeholders

Marketing recovered materials to brokers or to end-users for industrial, commercial or small-scale manufacturing purposes

Establishing training programmes for MSWM workers

Carrying out public information and education programmes

Identifying financial mechanisms and cost recovery systems

Establishing prices for services and creating incentives

Managing public sector administrative and operation units

Incorporating private sector businesses, including informal sector collectors, processors and entrepreneurs

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Generation and storage of household waste (household is regarded here as the “source” of waste generation. It also includes commercial entities or institutional bodies generating waste)

Reuse and recycling at household level (including animal feed and composting)

Primary waste collection and transport to transfer station or community bin

Management of the transfer station or community bin

Secondary collection and transport to the waste disposal site

Waste disposal in landfills

Using waste characterisation studies to adapt the system to the types of waste generated

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Table 1: Technical and non-technical aspects associated with integrated solid waste management. (UNEP, 2005; Zurbrügg, 2003a)

sorbed and carried by the society and its local communities. In other words, these systems must be adapted to the particu-lar urban and area-specific circumstanc-es and problems. They must employ and develop the capacities of all stakehold-ers, including the households and com-munities requiring the service, private sector enterprises and workers (both for-mal and informal), as well as government agencies at the local, regional and na-tional level. (Schüberler et al., 1996)

Key to integrated solid waste man-agement is the development of a waste management hierarchy, integrating wide-spread elements of national and region-al policy – often considered as the most fundamental basis of modern MSWM practice. The hierarchy classifies waste management operations according to their environmental or energy benefits (UNEP, 2005, p. 8 – 9):

Prevent the production of waste or reduce the amount generated.Reduce toxicity or negative impacts of the waste generated.Reuse the materials recovered from the waste stream in their current forms.Recycle, compost or recover materi-als for use as direct or indirect inputs for new products.Recover energy by incineration, anaerobic digestion or similar proc-esses.Reduce the volume of waste prior to disposal.Dispose of residual solid waste in an environmentally sound manner, gen-erally in landfills.

In practically all countries, the hierarchy is similar to that described above, with higher priority given to the first points. (UNEP, 2005, p. 9)Figure 2 illustrates this hierarchy embed-ded in a set of factors related to SWM.The main elements of an integrated solid waste management system are depict-ed in Figure 3. These elements can be di-vided into technical and non-technical as-pects, as shown in Table 1.

Integrated waste management is based on the concept that all aspects (technical and non-technical) should be

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solutions. Some waste management activities cannot bear any charges; some will always be net expenses, while others may produce an income. An integrated system can result in a range of practices that complement each other in this regard.Failure to have an integrated system may mean that the revenue-produc-ing activities are “skimmed off” and treated as profitable, while activities related to maintaining public health and safety fail to secure adequate funding and are operated at low or insufficient levels.

(UNEP, 2005, p. 8)

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2 – Introduction

Photo 1: Polluted open sewer in Togo. (Source: Eawag/Sandec)

2.1 What are the problems related to insufficient solid waste management?

Uncontrolled dumping of solid waste can lead to severe health hazards for local inhabitants, and pollute natural resources like water, soil or air.

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According to a survey by the United Na-tions Development Programme (UNDP) of 151 mayors of cities around the world, the second most serious problem (after unemployment) faced by city dwellers is insufficient solid waste disposal. (UNDP, 1997, Draft Interim Report, Part 3)

The risks posed to the environment and public health arise from the way this waste, generated by human activities, is handled, stored, collected, and dis-posed of. Where intense human activi-ties concentrate, such as in urban cen-tres, appropriate and safe solid waste management (SWM) is of utmost impor-tance to allow healthy living conditions for the population. Although most gov-ernments have acknowledged this fact, many municipalities are struggling to pro-vide even the most basic services. (Zur-brügg, 2003b, p. 1)

The main problems and issues related to unsatisfactory SWM in most develop-ing countries are:

inadequate coverage of the popula-tion to be served;operational inefficiencies of municipal SW services and management;limited use of the recycling activities by the formal and informal sectors;problems related to the disposal of solid waste; andproblems concerning the manage-ment of non-industrial hazardous waste.

All these problems have common social, institutional, financial, and tech-nical denominators. (Schertenleib et al., 1992, p. 3)

Typically, one to two thirds of the solid waste generated are not collected (World Resources Institute et al., 1996, p. 1). As a result, the uncollected waste, which is often also mixed with human and animal excreta, is dumped indiscrim-inately on streets and in drains, thus con-tributing to flooding, breeding of insect and rodent vectors and to the resulting spread of diseases. (UNEP-IETC et al., 1996)

In low-income countries, most of the collected municipal solid waste is

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dumped on land in a more or less uncon-trolled manner. Such inadequate waste disposal practices create serious envi-ronmental problems affecting not only the health of humans and animals, but also giving rise to serious economic and other welfare losses. The environmen-tal degradation caused by inadequate waste disposal can be measured by the contamination of surface and groundwa-ter through leachate, soil contamination through direct waste contact or leachate, air pollution by waste burning, spread-ing of diseases by different vectors like birds, insects and rodents, or the uncon-trolled release of methane by anaerobic waste decomposition.

In cities of developing countries, the urban poor suffer most from the life-threatening conditions of deficient SWM (Kungskulniti, 1990; Lohani, 1984), as municipal authorities tend to allo-cate their limited financial resources to the richer areas with higher tax yields from citizens with more political power. Usually, wealthy residents use part of their income to avoid direct exposure to the environmental problems close to home by shifting them away from their neighbourhood. Thus, though environ-mental problems at the household or neighbourhood level may decrease in

higher-income areas, citywide and re-gional environmental degradation re-mains the same or increases due to a de-ficient SWM. (Zurbrügg, 2003b, p. 1)

In an attempt to accelerate its indus-trial development, an economically de-veloping nation may fail to pay adequate attention to solid waste management. Such a failure results in severe penal-

Photo 2: Open street site dump in India. (Source: Eawag/Sandec)


Figure 4: Generation of MSW (kg/capita/yr) in 11 cities and their GDP in 2005 (in US $, using pur-chasing power parity exchange rates) per capita according to the World Bank’s income classifica-tion of 2006 (low income: US $ 905 or less; middle income: US $ 906 to 11,115; and high income: US $ 11,116 or more). (Eawag/Sandec, 2008)

2.2 What are the municipal solid waste categories and how much waste is produced?

MSW generally consists of organic waste, paper, plastics, glass, metal, textiles, and other inert materials.Ñ

ties at a later time in the form of resourc-es needlessly lost and a staggering ad-verse impact on the environment, public health and safety. The penalty is neither avoided nor lessened by a resolve to do something about the waste in the future, when the country may be in a better po-sition to take appropriate measures. This is true because the rate of waste genera-tion generally increases in direct propor-tion to that of a nation’s advance in de-velopment. The greater the degradation of the environment, the greater is the effort required to restore its good quali-ty. In summary, the effort to preserve or enhance environmental quality should at least be commensurate with that afford-ed to the attainment of advance in devel-opment. (UNEP, 2005)

Environmental and health impactsThe organic fraction of MSW is an impor-tant component, as it constitutes a siza-ble fraction of the solid waste stream in a developing country, but also because of its potentially adverse impact upon public health and environmental quality.

Further questionsAre social, political, technical or rather

financial constraints the reasons for the general lack of solid waste management and services?

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Additional infoZurbrügg, C. (2003). Urban Solid Waste

Management in Low-Income Countries of Asia – How to Cope with the Garbage Cri-sis. SCOPE. Durban, South Africa. www.eawag.ch/organisation/abteilungen/sand-ec/publikationen/publications_swm/down-loads_swm/USWM-Asia.pdf (last accessed 28.04.08)


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According to Key Note, 2.02 billion tons of MSW were generated worldwide in 2006. The average quantity of municipal solid waste generated throughout Latin America, Asia, and some countries in Af-rica is in the order of 400 g/cap/day. This equals approximately 30 – 40 % of the daily per capita waste generated in the United States and in Western European countries. (UNEP, 2005, p. 53)

Growth in wealth and increase in waste are interlinked – the more afflu-ent a society, the more waste it gener-ates (compare Figure 4). As less wealthy nations develop, they too create more wealth, thus adding to the world’s waste output.

Key Note forecasts that total global MSW will increase by 37.3 % between 2007 and 2011. (Key Note, 2007, p. 16)

2 – Introduction

A major adverse effect is the attraction of rodents and vector insects for which it provides food and breeding grounds. Reduction of environmental quality takes the form of foul odours and un-sightliness. These impacts are not con-fined merely to the disposal site. On the contrary, they pervade the areas sur-rounding the sites wherever the waste is generated, spread or accumulated. Un-less organic waste is appropriately man-aged, its adverse impact will continue until it has fully decomposed or other-wise stabilised. Uncontrolled or poor-ly managed intermediate decomposition products can contaminate air, water and soil resources.

Studies have shown that a high per-centage of workers handling refuse, and individuals residing near or on dispos-al sites, are infected by gastrointestinal parasites, worms and related organisms. Contamination of this kind is likely at all points where waste is handled. Although it is certain that vector insects and ro-dents can transmit various pathogenic diseases (amoebic and bacillary dysen-

tery, typhoid fever, salmonellosis, vari-ous parasitoses, cholera, yellow fever, plague, and others), it is often difficult to trace the effects of such transmission to a specific population. Both public health and environmental quality benefit direct-ly and substantially from the implemen-tation of modern solid waste manage-ment practices. (UNEP, 2005, p. 3)


Table 2: Waste generation rates in Durban, South Africa as a function of average income level of neighbourhoods. (Zurbrügg, 2003a, p. 4)

Table 3: Waste generations rates (kg/cap day) of different sized cities

2 – Introduction

The influence of income level on waste generation is also revealed by studies looking at different urban income levels of neighbourhoods. Table 2 shows the example of Durban, South Africa.

Although economic standing is a key determinant of how much solid waste a city produces (World Resources Institute et al., 1996), this generalisation may not always be valid, but may vary according to the socio-cultural consumption habits. The example of high variability in relation to the income level of neighbourhoods in Durban also depicts the problem of data reliability when operating with city aver-ages or worse even, when using coun-try average waste generation rates. Not only income but also urbanisation level of a city indicated by its population size is interrelated with the waste genera-tion rates. Rural villages and small towns have significantly lower values of gen-erated waste per capita. Table 3 shows waste generation in relation to relative city size for Nepal, Egypt and Sri Lan-ka (Akolkar, 2001; Saber, 1998; UNEP, 1999 in Zurbrügg, 2003a). Thus, aver-age countrywide solid waste data can be very misleading if used in a specific city context.

Solid waste generation not only dif-fers in relation to the local context, but also to its waste composition. In indus-trialised countries, domestic waste con-sists mainly of packaging materials, such as paper and plastics, whereas waste from low and middle-income countries contains high biodegradable organic waste fractions (cf. Figure 5).

Income levelWaste generation (kg/cap/day)

High income 1.48

Middle income 0.41

Low income (formal) 0.13

Low income (informal) 0.11

Country Large cities Middle cities Small cities

Nepal 0.5 0.35 0.25

Egypt 1.0 – 1.3 0.5 – 0.8 0.25

Sri Lanka 0.65 – 0.85 0.45 – 0.65 0.2 – 0.45

Figure 5: Top: MSW composition (kg/capita/yr) in 12 countries grouped according to their gross national income (GNI). Bottom: MSW composition (kg/capita/yr) in 23 cities. (Eawag/Sandec, 2008)

Country-specific studies reveal that the physical characterisation of solid waste differs in categories. Most waste characterisation studies have established the following categories:

BiodegradablePaperPlasticGlassMetalTextiles and leatherInerts (ash, earth and others)

(Zurbrügg, 2003b, p. 4)

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Further questionsHow much waste do I produce every

year?

How much waste do people in other countries produce?

How much waste is recycled, burned and dumped?

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Additional infoEawag/Sandec (2008). Global Waste

Challenge - Situation in Developing Coun-tries. www.eawag.ch/organisation/abteilun-gen/sandec/publikationen/publications_swm/index_EN (last accessed 30.07.08)


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10Sandec Training Tool: Module 6

Figure 6: Global urban population categorised by the different economies. The economies are divided according to 1996 GNP per capita: low income US $ < 785; low middle income: US $ 786 – 3115; upper middle income: US$ 3116 – 9635; and high income: US $ > 9636. (www.worldbank.org/data/databytopic/class.htm). (Zurbrügg, 2003b)

Figure 7: Total MSW generated (kg/capita/yr) and collection coverage in % in 17 countries. (Eawag/Sandec, 2008)

2.3 How is solid waste management related to rapid urbanisation?

Development of solid waste systems generally lags behind urbanisation rates.Ñ

2 – Introduction

Rapid urbanisation is taking place espe-cially in low-income countries. In 1985, globally 41 % of the world’s population lived in urban areas: by 2015 the urban proportion is projected to rise to 60 % (Schertenleib et al., 1992). Of this popu-lation, 68 % will be living in cities of low-income and lower middle-income coun-tries (cf. Figure 6). (Zurbrügg, 2003b, p. 2)

The situation is acute, as slums are growing at an alarming rate and the mu-nicipal solid waste management servic-es are lagging behind the needs of the in-habitants, especially in urban poor areas. (UNEP, 1999)

UN News, 26 February 2008“By the end of this year, half of the world’s 6.7 billion people will live in urban areas, according to a report unveiled by the United Nations today, which also predicts that future growth of the world’s urban population will be concentrated in Asia and Africa.” (UN, 2008)

Further questionsHow should authorities deal with the

problem of equity of service access in areas where the population is too poor to pay the full cost of waste management?

How should municipal waste manage-ment systems be adapted to specific demands and requirements of residential populations, including, in particular those of women and low-income households?

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Additional infoUNEP (1999): Global Environment Out-

look, Geo-2000. UNEP. www.grid.unep.ch/geo2000/english/0070.htm. (last accessed 28.04.08)

Available from the Internet.

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2.4 What is the present state in SWM?

Most cities in the developing world collect only part of the overall waste, and only a tiny fraction of the collected waste is treated or properly disposed of.

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Even though municipal solid waste man-agement is a major responsibility of local governments, typically consuming be-tween 20 % and 50 % of municipal budg-ets, it has been estimated that about 30 – 50 % of the waste generated in developing countries is never collected (Schüberler et al., 1996; UNEP, 2005). Uncollected waste accumulates in vacant lots or is simply discharged into bodies of water. As a result of inadequate dis-posal and excessive littering, the burden of waste collection is, in many instances, transferred from the collection system to the street cleaning system.

Typical productivity of a refuse collec-tion worker in developing countries (de-fined as total weight of waste collect-ed by the entire system, divided by the number of collection workers) is approx-imately 250 kg/day. Average expendi-

26 %

13 %

24 %

37 %

20 %

12 %

23 %

45 %

1995 2015year

high-income countriesupper-middle-in-come countries

lower-middle-in-come countrieslow-income countries

11Sandec Training Tool: Module 6

Figure 8: Percentage of the commonly used MSW treatment and disposal technologies in 21 countries. (Eawag/Sandec, 2008)

ture (at 2002 price level) on solid waste management, including street cleaning and final disposal, ranges from about US $ 1 /cap/yr to nearly US $ 5 /cap/yr. (UNEP, 2005, p. 53 – 54)

With the mounting urgency of urban environmental problems identified (for example in Agenda 21, Chapters 7 and 21) and growing concern for capacity building at municipal management lev-el in recent years, MSWM has attract-ed increasing attention from bilateral and multilateral development agencies. With its broad organisational implications and close links to other sectors, MSWM constitutes an important entry point for integrated urban management support. (Schüberler et al., 1996, p. 15)

A word on the significance of collec-tion, transfer and street sweeping.Collection is by far the largest cost ele-ment in most MSWM systems, account-ing for 60 – 70 % of costs in industrial-ised countries, and 70 – 90 % of costs in developing and transition countries. Col-lection and street sweeping comprise the single largest category of expend-iture in many municipal budgets. Fail-ure or inadequacy of collection, especial-ly in developing countries, where there is frequently considerable human fae-cal waste in the municipal solid waste, can lead to public health hazards. Giv-en its high visibility and importance, one would expect collection to receive a high degree of scrutiny and analysis and, as a consequence, to be a highly efficient municipal or private operation. In fact, particularly in developing countries, the opposite is quite often true. Waste col-lection and street sweeping are often highly inefficient. Workers are frequent-ly poorly motivated, untrained, under- compensated, and disregarded. Further obstacles to efficiency are obsolete or non-functional equipment and inacces-sible routes, which have not kept pace with urban growth. Up to half of the poorer sections of developing cities are underserved or completely unserved. In some industrialised countries, waste col-

2 – Introduction

lection has recently received more at-tention, due partly to the introduction of source-separated collection of recy-clables and organics. Testing and anal-ysis, which has accompanied the de-velopment, introduction and monitoring of separate collection, has often had positive spin-off effects on collection of the rest of the waste. In developing countries, an influx of loans and grants for infrastructure development is just beginning to affect collection systems, sometimes for the better, sometimes for the worse. A further problem is that waste collection is often in jurisdictional no man’s land, where fiscal, operational and administrative responsibility is frag-mented between public health, public works and public cleansing departments, with budgetary and operational respon-sibility in conflict. The association with waste often means that waste collection

Further questionsWhat priority should be given to waste

minimisation and resource recovery in relation to waste treatment and disposal?

How should authorities deal with the service needs of irregularly illegal settlements?

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Additional infoEawag/Sandec (2008). Global Waste

Challenge - Situation in Developing Coun-tries. www.eawag.ch/organisation/abteilun-gen/sandec/publikationen/publications_swm/index_EN (last accessed 30.07.08)


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functions have low status, and managers and supervisors do not receive training, support or recognition. (UNEP-IETC et al., 1996, Chapter 1.3.1)

1�Sandec Training Tool: Module 6

Table 4: Waste characteristics * Countries with GDP < US $ 360 per year per capita; + countries with GDP > US $ 360, < US $ 3500 per year and capita. (Cointreau, 1982 and SAEFL, 1994 in Zurbrügg, 2003a)

2.5 What role does organic waste play in the context of SWM?

The high moisture content of organic waste (which makes up the main component in MSW, especially in low-income countries), influences the feasibility of collection and treatment options.

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2 – Introduction

A high biodegradable matter and inert material content in solid waste leads to a high density (weight to volume ratio) and high moisture content (compare Table 4). These physical characteristics signifi-cantly influence the feasibility of certain treatment options. Vehicles and systems operating well with low-density waste, prevalent in industrialised countries, will not be suitable or reliable under condi-tions where high organic content and density are common. The extra weight, abrasiveness of the inert material, such as sand and stones, and corrosiveness due to the high water content, may cause rapid deterioration of equipment. Waste with a high biodegradable, water or inert content will also have low calorif-ic value and, thus, be unsuitable for incin-eration. (Zurbrügg, 2003b, p. 5)

In general, the organic components of urban solid waste can be classified into three broad categories: putresci-ble, fermentable and non-fermentable. Putrescible waste tends to decompose rapidly and, unless carefully controlled, its decomposition produces malodours and visual unpleasantness. Fermentable waste tends to decompose rapidly, but without the unpleasant accompaniments of putrefaction. Non-fermentable waste tends to resist decomposition and to break down very slowly. A major source of putrescible waste is food preparation and consumption. Its nature varies with lifestyle, standard of living and seasonal-ity of foods. Fermentable waste is typi-fied by crop and market debris. (UNEP, 2005, p. 1)

Moisture contentMoisture content is determined as follows: The sample is weighed as received (“wet weight”) and then allowed to stand until it is air-dried, i. e. until its moisture content is equal to that of the ambient air. The mois-ture content is then obtained by the following formula:

Mc = Ww – Wd × 100

Ww

where:

Mc = moisture content (in %)

Ww = wet weight of sample

Wd = dry weight of sample

(UNEP, 2005, p. 39)

Low-Income Countries *

Middle-Income Countries+

High-Income Countries

Waste generated

(kg / cap. and day)0.� – 0.� 0.5 – 0.9

0.7 – 1.8

(CH: 1.1)

Waste density

(kg / m3)��0 – �00 170 – 330 100 – 170

Water content

(%)�0 – �0 40 – 60 20 – 30

Composition

Organic �0 – �� 20 – 65 20 – 50 (CH: 22)

Paper, cardboard 1 – 10 15 – 40 15 – 40 (21)

Glass and ceramics 1 – 10 1 – 10 4 – 10 (3)

Metal 1 – � 1 – 5 3 – 13 (6)

Plastics 1 – � 2 – 6 2 – 13 (13)

Dust and ash 1 – �0 1 – 30 1 – 20 (5)

Further questionsWhat are the main processing methods

of segregated organic wastes?Ñ


CalRecovery Inc. www.unep.or.jp/Ietc/Pub-lications/spc/Solid_Waste_Management/Vol_I/Binder1.pdf (last accessed 28.04.08):


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3 – Technical Aspects

Figure 9: Typical main elements of a solid waste management system in low or middle-income countries, where recovery and recycling elements and processes are indicated by the dotted arrows. (Eawag/Sandec)

3.1 What are the main storage, collection and transport options?

In developing countries, muscle-powered carts are feasible in most cases for primary transport and non-compactor trucks for secondary transport.

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Solid waste management facilities and equipment should be evaluated and ap-propriate technical solutions designed and selected, with careful attention to their operating characteristics, perform-ance, maintenance requirements and ex-pected life cycle costs. Technical evalua-tion requires data on waste composition and volumes, indications of important area-specific variations of waste genera-tion and its expected changes over time, an understanding of the disposal hab-its and requirements of different user groups, as well as an assessment of the technical capability of public and/or pri-vate sector organisations responsible for operating and maintaining the systems.

The technical systems established for primary collection, storage, transport, treatment, and final disposal are often poorly suited to the operational require-ments of the city. In many instances, the provision of imported equipment by international donors leads to the use of inappropriate technology and/or a diversity of equipment types, thus undermining the efficiency of operation and maintenance functions. (Schüberler et al., 1996, p. 47)

Some of the typical elements of a solid waste management system in de-veloping countries (depicted in Figure 9) will be described in detail in the following subchapters.

Primary storage and transport

Every MSWM system (except backyard composting or burying of the waste in one’s own back yard) includes collection in some form or another.

Primary storageMost collection systems depend on some kind of set-out container. In indus-trialised countries, this is usually a pa-per or plastic bag, or a metal or plastic garbage can. In developing countries or rural areas, set-out containers include bags, pots, plastic or paper bags, cane

or reed baskets, concrete or brick vats, urns, boxes, clay jars, or any other kind of container available.

In some places, waste is stored in a pit in front of the house while awaiting collection. In other places, any type of container can be used to store or organ-ise waste. Storage containers are often insufficient and waste is simply piled up or heaped in the street or on the ground to await collection. In places with com-munity transfer, residents use bags or baskets for carrying waste to the con-

tainers. The increasingly available non-biodegradable plastic bags are becoming a problem for composting. Industrialised countries have developed special con-tainers for certain recyclable materials.

The choice of set-out container has an important effect on collection effective-ness. Containers like baskets or paper bags allow waste to have contact with air, thus promoting decomposition while discouraging the formation of anaerobic odours. (UNEP-IETC et al., 1996, Chap-ter 1.3.2)

Animal f eed Middle me n Industry

Househol d

Primar y Collectio n

Marc hants

Compostin g Transposrt point

AgricultureHorticul ture

Seconda rycollectio n

Disposal


Photo 3: Kerbside containers in Thailand. (Source: Eawag/Sandec)


Communal collectionIn communal collection – very common in developing countries – individuals bring their waste directly to the collec-tion point, usually a container that can be accessed on foot. In a somewhat similar vein, some European cities require resi-dents to take their wheeled containers to a specific location on the day of col-lection and retrieve them when emptied. Industrialised countries also use commu-nal collection in rural areas, where most waste is brought by car, or separate collection of recyclables, household hazardous waste or specific materials such as leaves.

Communal collection is a particularly appropriate means of organising collec-tion, where household collection is im-possible or marginally feasible, where inadequate resources are allocated to poor areas, or where local customs promote it. The solid waste authority may choose to set up containers on street corners, at various spots along a densely populated road or at the edge of a neighbourhood accessible to both generators and collection vehicles.

An advantage of communal collection points for household waste drop-off is their more or less continuous access to disposal or materials recovery facilities. The disadvantages of communal collec-tion are that such facilities may receive little attention from municipal authori-

ties, and residents may deposit danger-ous materials in or near the container.

Sound practice in communal collec-tion design presupposes awareness of the inherent conflict between the phys-ical demands imposed by public con-venience in disposal and the strategies required to maintaining cleanliness and control waste pickers, odours, vectors, animals, flies, and other insects.

Sound practice also presupposes the availability of an adequate number of easy to use containers by the entire pop-ulation, including the children. The re-sponsible authority must carry out very frequent collection (often daily) and must be committed to cleaning up overflows. (UNEP-IETC et al., 1996, Chapter 1.3.2)

Primary transportMuscle-powered carts, wagons and rel-atively small rickshaws pulled, pushed or foot-pedalled, bicycles or animals are important sound practice for MSW col-lection in many developing countries and rural hilly areas of transition countries. Compared to other means of transport, such vehicles are inexpensive and easy to build and maintain. In many cases, muscle-powered vehicles represent the soundest mix of capital, labour and avail-able resources for waste or materials col-lection.

Small-scale collection can also be con-ducted by electric or propane-powered

Muscle-powered or micro-mechanical vehicles work well:

in densely populated areas of limited street access or unpaved roads;

in squatter settlements;

on hilly, wet or rough terrain; and

with relatively small waste volumes from a relatively large number of dense-ly populated housing units.

Muscle-powered vehicles exhibit the following disadvantages:

use of animals or human power is per-ceived by some as old-fashioned or shameful;

the vehicles have a limited travelling range and are generally slower than fuel-powered vehicles;

animals pulling such vehicles leave waste, which must be cleaned up;

weather exposure has a greater effect on humans and animals when they are not in motorised vehicles; and

problems associated with animal charac-teristics, health etc.

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Photo 4: Primary collection by wheelbarrow, Pakistan. (Source: Eawag/Sandec)

vehicles servicing a small or inaccessible area in combination with a larger “host” vehicle. Muscle-powered primary collec-tion (or micro-collection) may be coupled with transfer into a larger “host” vehicle at the edge of the neighbourhood. This is sometimes done with street sweep-ing or materials recovery in industrial-ised countries. (UNEP-IETC et al., 1996, Chapter 1.3.2)


Secondary storage (transfer point) and collection


Transfer refers to the movement of waste or materials from the primary col-lection vehicle to a secondary, generally larger and more efficient, transport vehi-cle. While virtually all waste systems in-clude collection, not all offer transfer.

The point of transfer is often re-ferred to as “transfer station” or “trans-fer point”. Primary collection vehicles bring their waste to a transfer station and dump it. It is then transferred, with or without compaction, to other vehi-cles for a longer haul to a disposal site. Transfer, which may include a short storage period, also provides a point of access to the waste or material stream and an opportunity to remove certain materials or perform processing, such as shredding, compacting, screening, wet-ting or drying.

Transfer stations are sound practice where i) vehicles servicing a collection route are required to travel a shorter dis-

tance, unload and quickly return to their primary task of collecting the waste; ii) in industrialised countries, and where waste from large urban areas in develop-ing countries is disposed of in large, new landfills, incinerators and composting facilities, increasingly designed to serve a number of communities or an entire re-gion and thus sited a considerable dis-tance from the collection service areas. In theses circumstance, transfer stations can be very attractive, since transporting waste from the route to the facility takes longer and uses more fuel.

Transfer tractor-trailers or compact-ing trucks can carry larger MSW volumes than regular collection vehicles, thus al-lowing them to travel longer distances and carry more waste. This lowers fuel costs, increases labour productivity and saves on vehicle wear. Drawbacks of transfer stations include the additional capital costs of purchasing vehicles and

Photo 5: Transfer point in Indonesia. (Source: Eawag/Sandec)

building transfer stations, and the extra time, labour and energy needed to trans-fer waste from collection vehicles to transfer trailers.

Some developing countries have transfer stations similar to the type described above, including non-mecha-nised, local transfer points serving the special needs of particular collection service areas. A micro-collection vehicle, designed to service a hilly or a densely populated area with narrow or congested streets, can transfer its load to a larger vehicle or a stationary container at such a transfer point. This even allows to serv-ice collection areas that are inaccessible to a truck. Such transfer points may also degenerate into unregulated dumps in the absence of institutional commitment and managerial capacity to ensure their efficient operation. (UNEP-IETC et al., 1996, Chapter 1.3.3)



Compactor trucks work well where:paved roads are wide enough to allow passage and turning;

waste is set out in containers or bags for the crews to pick them up quickly; and

density and moisture content of the waste is low.

Compactors work poorly where:the waste stream is either very dense or very wet, such as mixed waste in developing countries or newspapers in developed countries;

the materials collected are source- separated organics or materials with a septic content; compaction tends to squeeze out the moisture and discharge it as leachate;

collected materials are gritty or abrasive; and

roads are very dusty.

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Non-compactor trucksNon-compactor trucks are more effi-cient and cost-effective than compactor vehicles in small cities and areas where waste tends to be very dense and has a limited potential for compaction. Use of lighter, more energy-efficient box-trucks, vans and dump trucks can be appropri-ate for sparsely populated areas, where distance is the main constraint to collec-tion efficiency.

Non-compactor trucks used for waste collection usually require a dumping system to easily discharge the waste. Nevertheless, dump trucks with a high loading capacity may not offer the best choice for a non-compactor truck. Non- compactor trucks generally need to be covered to prevent residues from flying off the truck and/or rain from soaking the waste. (UNEP-IETC et al., 1996, Chap-ter 1.3.2)

Further questionsHow can operational integration and reli-

ability of technical systems be achieved despite diverse local collection needs, large number of different actors and decision-makers, as well as incremental develop-ment of facilities and equipment?

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Additional infoUNEP-IETC and HIID (1996): Internation-

al Source Book on Environmentally Sound Technologies for Municipal Solid Waste Management. UNEP, International Envi-ronmental Technology Centre. www.unep.or.jp/ietc/ESTdir/pub/MSW/index.asp (last accessed 28.04.08)


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Photo 6: Compactor truck in Vietnam. (Source: Eawag/Sandec)

Compactor trucksIn industrialised countries and cities, use of some type of compacting vehicle has become the standard sound practice of waste collection. A compactor truck:

allows waste containers to be emptied into the vehicle from the rear, the front or the side;densely compacts the waste by hydraulic or mechanical pressure;quickly removes the waste from pub-lic view; andinhibits vectors and insects from ac-cessing the waste during collection and transport.

The characteristics of compactor trucks include:

high capital costs;sensitive hydraulic mechanisms, which must be well maintained in or-der to function; i. e. they can break down if an attempt is made to com-pact already dense waste;high fuel and operating costs;moderate operating skills; andat least two persons to operate the truck under most conditions.

(UNEP-IETC et al., 1996, Chapter 1.3.2)

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Advantages of non-compactor trucksNon-compactor trucks are a sound technical practice for solid waste collection where:

the waste is generally very wet or dense;

labour is relatively inexpensive com-pared to capital;

highly skilled maintenance is scarce;

collection routes are long and relatively sparsely populated;

capital and operating costs are limit-ed; and

downtime for maintenance must be min-imised.

Disadvantages of non-compactor trucksThe main problem associated with the use of non-compactor trucks is of political rath-er than technical nature:

government officials, who attach a low status to non-compactor trucks, tend to see compactors as a means to modern-ise their waste collection system;

salesmen recommend compactor trucks as the only means of appropriate waste transport. This may be true for industrial-ised countries, but certainly not for most developing countries; and

donor agencies from industrialised coun-tries tend to recommend collection equipment considered efficient in their own countries, and thus assume that compactor trucks make adequate use of the funds provided.

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3.2 What are the major organic waste treatment options?

Composting is the main treatment option for organic waste.Biogasification and vermicomposting are gaining increasing recognition.

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Composting

Solid waste composting for use as a soil amendment, fertiliser or growth medi-um is of prime importance in many coun-tries. Asian countries in particular have a long-standing tradition of making and us-ing compost. In Western Europe, a range of modern technologies is applied to pro-duce compost.

Nonetheless, composting is the waste management system with the highest failure rate worldwide. In cit-ies of developing countries, most large mixed-waste composting plants, often designed and funded by foreign consult-ants, have failed or operate at less than 30 % capacity.

The most frequent problems cited for composting failures are: high operation and management costs, high transport costs, poor quality product as a result of poor pre-sorting (especially plastic and glass fragments), poor understanding of the composting process, and competi-tion from chemical fertilisers (which are often subsidised). In many urban areas, collection systems are too unreliable for urban authorities to ever consider run-ning composting facilities efficiently.

While many bio-waste compost-ing facilities have failed, the majority of source-separated composting sys-tems have succeeded. Yard, garden, res-taurant, and market waste composting projects quietly thrive in every corner of the globe. The biological composting process is so basic that it is very likely to succeed if there is an appropriate input stream and proper handling. (UNEP-IETC et al., 1996, Chapter 1.4.1)

Although the composting process is similar in all areas of the world, industr-ialised, transition and developing coun-tries reveal some practical differences. The main differences relate to the waste stream to be composted, the agricul-tural traditions associated with produc-tion and use of compost and the physi-cal infrastructure of the built and natural environment. As regards composting, transition countries exhibit similar infra-structures as industrialised countries,

however, their waste streams are com-parable to those of developing countries. (UNEP-IETC et al., 1996, Chapter 1.4.2)

Composting TechnologiesThere are two fundamental types of com-posting techniques: open or windrow composting, a slower process conduct-ed outdoors with simple equipment, and the enclosed system composting, where composting is performed in a building, tank, box, container or vessel. (Chapter adapted from: Dulac, 2001, p. 13 – 14)

In-vessel or enclosed systems. In- vessel systems, such as drum and agi-tated bed technologies or any technical system enclosed in a building, require complex equipment. These systems are highly engineered, capital-intensive and have to be managed on a daily ba-sis, since their automated systems and design have to prevent potential health risks to workers and the environment. Their energy consumption is also sub-stantial. Ongoing operation and mainte-nance is critical and less forgiving than more passive approaches, as they re-quire access to specialised pieces of equipment, which generally have to be manufactured and delivered at a high price. The equipment may have been designed for specific climatic conditions and may not be applicable universal-ly. They require less land and produce compost in a shorter time than open systems. Automated in-vessel systems cannot always meet the socio-economic conditions prevailing in different areas of the developing world (e. g. limited educa-tion and available institutional infrastruc-ture support, labour rich/capital-poor economies). Their operating costs usu-ally start at US $ 40 per ton, for the least expensive variant; more expensive sys-tems can cost up to US $ 100 per ton.

Open or windrow systems. Open composting processes are simpler and require less capital and energy. They generally rely more on land and labour

and less on machinery. They require far more land and longer periods to pro-duce compost than enclosed systems. In the labour-rich and capital-poor cities of Eastern Europe and the South (where enclosed systems have a long history of failure), they are usually more reliable and adapted to local needs, and the local authorities are capable of sustaining op-erations over a longer time period. Oper-ating costs range from US $ 5 to US $ 20 per ton, depending primarily on accessi-bility of the site and turning frequency.

Duration of compostingComposting is completed when the com-postable materials have entirely turned into humus. Compost stability can be tested by re-wetting the material to see whether it heats up again, thus revealing still uncom-posted materials in the pile. Most aerobic composting systems include a period of active composting, generally from 21 to 60 days, and a period of curing, generally from 6 to 24 months.

Composting can be accelerated by inten-sive aeration and inoculation of the piles with suitable bacteria. More land is re-quired when the period of composting is longer, as the throughput of waste is slower. In places where land for siting is scarce, sound practice may entail selection of more intensive management practices instead of more extensive land use.


Kitchen waste composting versus animal feeding in a waste management systemThere are many viable systems to feed kitchen waste to animals or to collect it for livestock feeding. In terms of the waste management hierarchy, this rep-resents a higher use of kitchen waste than composting, as more nutrient val-ue is productively used. (There are, how-ever, considerable health risks in feeding waste to animals).Whenever a compost system is being planned, it is important to evaluate the extent to which compostables are al-ready being diverted to animal feed. Mu-nicipal authorities are sometimes una-


Figure 10: The “safety zone diagram”. (Feachem et al., 1983)

3– Technical Aspects

ware of these processes. If people need their kitchen waste for animals they are unlikely to cooperate with central-ised composting systems. In developing countries, disruption or replacement of animal feeding systems with composting is generally not sound practice. (UNEP-IETC et al., 1996, Chapter 1.4.2)

Composting of mixed solid wasteComposting of mixed solid waste is a controversial issue. In industrialised and transition countries, the waste stream is generally too diverse and contains too many metals and plastics to al-low mixed-waste composting to be considered sound practice. Technical approaches to mixed-waste compost-ing have relied heavily on mechanical pre-processing and separation systems. These have generally failed to operate or to produce either a clean stream of com-postables or marketable recyclables. In developing countries, the waste stream contains high levels of organic waste, as the main non-compostables are not thrown away but picked out prior to final disposal, thus resulting in a highly com-postable waste stream. Composting it by low-cost technology can be consid-

ered sound practice, especially where ur-ban and peri-urban agriculture provides a strong demand for the resulting com-post. (UNEP-IETC et al., 1996, Chap-ter 1.4.2) However, waste separation at source (household level) is still prefer-able.

Siting and composting scaleMost compost systems require open land for establishing and handling com-post piles. In many ways, the type of land and sites available dictate the choice of composting system. Sound practice in siting for facilities other than backyard bins includes:

selection of a site with access adapt-ed to the type of transportation;availability of a buffer area between the site and nearby land users to minimise waste nuisance and com-post odours;appropriate soil for absorption or collection of leachate; andthe possibility to place the compost indoors to protect it from unfavour-able weather conditions or to buffer the surrounding environment.


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Heat treatmentHeat is one of the most effective ways of killing pathogens and the parameter used to achieve inactivation in some of the most widely applied processes, such as sewage sludge treatment. In Figure 10, inactivation of pathogens is plotted as a function of temperature and time. This creates a defined “safety zone” margin. If the corresponding temperature-time relationship is achieved throughout the exposed material, it can be considered microbiological-ly safe for handling and use. For example, efficient microbial inactivation occurs if a tempera-ture of > 55 °C is maintained for one to a few days. The time and temperature relationship for various pathogens have been widely accepted, though “new” pathogens have been identified and slight variations in the results have been observed.

To treat excreta, thermophilic digestion (50 °C for 14 days) or composting in aerated piles for one month at 55 – 60 °C (+ 2 – 4 months further maturation) are recommended and a gener-ally accepted procedure. Recommendations for treatment of e. g. sewage sludge and organic household waste (food waste) also rely on such temperatures (Danish EPA, 1996; EC, 2000; Swedish EPA, 2002).

Haug (1993) states that composting at 55 – 60 °C for a day or two should be suffi-cient to kill essentially all pathogens. The cited regulations above rely on longer peri-ods in order to provide a handling margin. It is common that cold zones are formed with-in the digested or compost material, result-ing in local areas with less inactivation and possible regrowth of pathogenic bacteria. Digestion and composting also aim at de-grading and stabilising organic material. For faeces, inactivation of pathogens is of key importance. A composting process will also decompose toilet paper, making the material more aesthetical and suitable for agricultural use.

(Schönning et al., 2004, p. 21 – 22)

Backyard compostingBackyard composting can be both an individual strategy for managing house-hold kitchen and garden waste and a formal strategy for managing the organic waste stream in a region. Backyard com-posting is the smallest composting scale and offers a sound approach if:

a significant number of households have individual or collective yards or gardens, and enough available space for a compost pile;composting is culturally accepted by most people; andthe waste stream to be composted contains primarily vegetable matter, as rodents and insects are easier to control if animal matter is scarce.


Decentralised neighbourhood, block or business-scale composingThe next larger composting scale is the neighbourhood, block or business-scale composting site. Such facilities can pro-vide a waste management opportunity to a small group of people at relatively low cost. Small-scale composting uses the waste of a number of households, shops or institutions. Sound practice for siting neighbourhood composting sites requires that they:

be accessible to all who want to use them;be clearly designated with signs that all users and non-users can read or interpret;be sited with the agreement of the surrounding land users;have adequate fencing or control to prevent their becoming an open dump; andhave appropriate soil to absorb lea-chate.

A compost monitor or supervisor should be elected from within the user commu-nity to maintain order and cleanliness. Sound practice generally requires the municipal authority to provide techni-cal and logistical support for removal of undesired items or turning of the piles. (UNEP-IETC et al., 1996, Chapter 1.4.2)

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Enteric viruses

Shigella spp.

Taenia spp.

Safety zone

Ascaris spp.

Vibrio cholerae

Entamoeba histolytica

Tem

pera

ture

(°C

)

Time (hours)

100001 day 1 week 1 month 1 year

Salmonella spp.



Example of sound practice: The Jakarta composting experimentsDevelopment of community-based com-posting in Jakarta is a good example of a sound composting practice in a develop-ing country. Aid from Australia, Germany, the Netherlands, and New Zealand helped to initiate pilot projects in Jakarta in the 1980s. Later, the Harvard Institute for In-ternational Development (HIID) and the Centre for Policy Implementation Stud-ies, supported by the Government of In-donesia and the Jakarta City government, worked on a model for operating small-scale, neighbourhood composting in Ja-karta. Starting around 1992, several small composting enterprises were set up in Ja-karta. The Jakarta experiments incorpo-rated sound practice in small-scale com-posting in similar cities, while enhancing the role of the informal sector. The project trained individuals already involved in ma-terials processing and taught them the ba-sics of composting. A second element was compost market stimulation through train-ing the intermediate buyers of recyclables to understand the physical and commer-cial properties of the compost. In the pilot project, measures were taken to protect the workers’ health, however, it is uncer-tain if these precautions will be observed when and if private entrepreneurs take over the model and operate it as a business. Sound practice would be a follow-up of the pilot projects by creating the necessary urban infrastructure to facilitate more enterprises and monitor the labour conditions.

The Jakarta research project provides a good example of how cities can begin to examine possible sound practices in mu-nicipal solid waste composting. An assess-ment of small-scale, multi-source com-posting projects in Jakarta and Bandung in 1994, suggested that such composting can achieve important waste reductions and contribute to improving the neighbourhood environment. Good management and mar-ket research as well as a consistent institu-tional support system are essential for the lasting success of such projects.


Centralised composting at the municipal scaleIn centralised composting of waste from multiple sources, the waste is transport-ed from several points to a facility capa-ble of receiving 10 to 200 tons per day.Municipal-scale composting plants re-ceive waste from a single jurisdiction, usually a city, including occasionally as-sociated suburbs or squatter settle-ments. Differences in scale, manage-ment, financing, and siting distinguish municipal-scale, centralised plants from regional facilities. At this scale, sound practice for siting the compost facility in industrialised countries must usually be a formal process that includes:

a technical assessment of the area, soil and geographic attributes of potential sites;the involvement of engineering and design professionals in site selection and design;an environmental assessment of potential sites, a formal evaluation and selection process to involve all stakeholders;a formal remediation or compensa-tion programme to minimise and/or compensate for nuisance from traffic, odour, leachate, and noise at the com-posting site;a separate collection and/or pre-processing system to ensure that only desired materials actually enter the composting system, and appropriate attention to the role of waste pickers or the informal sector in pre-process-ing and recovery of non-composta-bles; anda formal system for using and/or marketing the finished compost.

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Decentralised composting at village and community-scaleComposting is clearly sound practice for management of compostable waste streams at village or community-scale. Centralised composting of this type, whether privately or publicly developed, must fall under the jurisdiction of the mu-nicipal or community authorities, which accept responsibility for its operation. These facilities will generally be in the range of 2 to 50 tons per day, depend-

In addition to the siting and design re-quirements cited above, sound practice for regional-scale composting includes:

a siting process that takes into ac-count the equity effects of siting a compost plant for several jurisdictions within the boundaries of one of them. A frequent strategy here is to distrib-ute the sites for landfill, compost plant and incinerator (if part of the system) among the different municipalities.agreements between the participat-ing municipalities or jurisdictions for siting, design, financing, operations, maintenance, environmental compli-ance, and billing for services;enforceable protocols for the quali-ty and composition of the composta-ble materials delivered to the facility, since failure of separation from any one source can contaminate the com-post for all participating jurisdictions;agreements between the jurisdictions for use, take-back and marketing of the finished compost;waste delivery agreements and com-mitments from the various participat-ing jurisdictions; anddesignated routes for delivery of com-postables.


Composting at landfill and incineration sitesComposting facilities may be located at landfill sites, particularly in developing countries, but increasingly also in indus-trialised countries. This allows separate collection of organics or yard waste to be processed at the landfill. Siting is simpli-fied or rolled into the landfill siting proc-ess. Here, sound practice differs in in-dustrialised and developing countries. In industrialised countries, sound practice will usually require the composting op-erations to be separate from the land-fill, have their own scale or separate entrance, and resulting compost to be split between a low-quality product used in landfill operations as daily and final cover, and a high-quality used for other purposes.

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ing on the size of the community and volume of compostable materials in the waste stream.Siting is important, and sound practice requires neighbourhood composting op-erations to follow the siting guidelines listed above. At this scale, the site may have to accommodate more compost turning, processing, screening, and stor-age than at smaller scales. (UNEP-IETC et al., 1996, Chapter 1.4.2)

�0Sandec Training Tool: Module 6


Major composting factors to be consideredSiting: Compost facilities must be reasonably close to the input stream and potential users should meet the needs of the nearby community.

Input stream: Source-separated organics are best. However, in most developing countries, this is not always possible. Mixed waste can be processed to yield acceptable compost.

Selection of appropriate technology: The technology chosen must be adequate for the input stream and level of economic development of the country.

Scale: A smaller-scale facility often facilitates careful composting and formation of a good product.

Market development: Governments generally need to stimulate the compost market. Qual-ity standards are an important marketing element.

Existing compost practices using compost from dumps and garbage dump farming: These traditional activities, while often dangerous could, in some instances, be safe if they include an adequate testing programme.

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In developing countries, where the waste stream has a sufficiently high or-ganic fraction, waste may be left to de-compose at the landfill or dump. In cas-es of natural composting, sound practice requires clear decisions about the role of decomposition processes in landfill man-agement, whether or not remove the top layers of material once partially decom-posed, for further composting or use in agriculture, and whether farmers should be allowed to remove compost from the landfill or dump. (UNEP-IETC et al., 1996, Chapter 1.4.2)

Vermicomposting

Vermicomposting, also called vermicul-ture or worm composting, is a relatively cool but aerobic composting process in which certain varieties of redworms and earthworms can be used to break down organic materials. Worms mechanically break down compostables and partially decomposed materials by eating them, and biochemical decomposition occurs via bacteria and chemicals in the worms’ digestive system.

Vermiculture requires considerable labour and carefully controlled compost-ing conditions, including temperature, moisture and the mix of ingredients. Its success to date is limited to relative-ly small-scale or pilot programmes. The use of vermicomposting in centralised or village-scale composting systems is currently being explored in pilot projects. Considerable work was conducted in Manila in the 1970s; however, the mar-kets for the resulting worm castings did not develop.

Vermiculture can be carried out by small-scale enterprises in a cottage-- industry manner. Since worms are easily affected by impurities, the organic waste should be source-separated domestic or market waste.

Vermiculture produces a superior fertiliser-type product. However, the available information is not enough to indicate whether sufficient markets ex-ist to absorb worm castings on a scale

that would significantly contribute to mu-nicipal waste reduction. Since vermicul-ture does not necessarily kill all patho-gens, some viruses and parasites may survive the process. Therefore, if the in-put materials present a high pathogen

In this prepupae life stage, their bodies are rich in protein and fat, thus making them an ex-cellent component of animal feed for aquaculture or poultry production. Feeding experiments in aquaculture that replace fishmeal by larvae meal revealed highly promising results (Bond-ari and Sheppard, 1981; St-Hilaire et al., 2007). Such feedstuff of animal origin also becomes a very attractive and urgently needed alternative to the rapid and increasing global role of aquaculture and its ecologically and economically questionable demand for fishmeal, current-ly reflected by a steady increase in market prices. Given this situation, waste management processes using the Black Soldier Fly larvae may not only become a self-sustained waste treatment option, but also a profitable and flourishing business.

(Diener et al., 2008)

Adul t4d

Eggs4d

Pupa e

~ 14d

Prepupa e

Larvae~ 14d

Black Soldier FliesSandec is evaluating a new technology in a simple facility promising to combine waste treatment and the generation of a valuable (by-)product, i. e. an organism feeding on waste itself. The life cycle (cf. Fig. 11) of the non-pest Black Soldier Fly, Hermetia illucens fits this purpose extremely well.

The larvae voraciously feed on organic material and reduce its dry mass by 40 – 50 %. This figure is similar to the reduction achieved by composting or biogas diges-tion units. However, it is not only the abil-ity to reduce waste that makes the Black Soldier Fly a promising waste manager. After the larvae have fed extensively on waste, the last larval stage or so-called pre-pupae crawl out of the waste in search of a dry pupation site. This migration stage may be used to harvest the prepupae by simply channelling their migratory paths into a collection vessel.

Figure 11: Life cycle of the Black Soldier Fly, Hermetia illucens at 25 °C

risk, the finished product could still con-tain pathogens. This may be of particular concern in developing countries, where waste used in vermicomposting may not be source-separated. (UNEP-IETC et al., 1996, Chapter 1.4.2)


Biogasification


Biogas is produced by bacteria biode-grading organic matter under anaero-bic conditions. The natural generation of biogas is a key component of the bio- geochemical carbon cycle. Methanogens (methane producing bacteria) are the last link in a chain of microorganisms degrad-ing organic material and returning the decomposition products to the environ-ment. Biogas – a source of renewable energy – is generated in this process.

Biogas is a mixture of gases mainly composed of:

Methane (CH4): 40 – 70 vol.%Carbon dioxide (CO2): 30 – 60 vol.%Other gases: 1 – 5 vol.%

The calorific value of biogas is approx-imately 6 kWh/m3 and equivalent to about half a litre of diesel oil. The net cal-orific value depends on the efficiency of the burners or appliances. Methane is the valuable component of biogas when used as a fuel. (ISAT et al., p. 4 – 5)

“Methane fermentation”, “meth-ane production” and “anaerobic diges-tion” are among the terms frequently used to designate biogasification. Here, biogasification is defined as the decom-position of organic matter of biological origin under anaerobic conditions with an accompanying production primarily of methane (CH4) and secondarily of oth-er gases, mainly carbon dioxide (CO2). (UNEP, 2005, p. 259)

Every year, some 590 – 880 million tons of methane are released world-wide into the atmosphere by micro-bial activity. About 90 % of the emit-ted methane are derived from biogenic sources, i. e. from the decomposition of biomass. The remainder is of fos-sil origin, such as petrochemical proc-esses. (ISAT et al., p. 4). As indicated in Table 5, methane acts as a strong green-house gas.

Anaerobic digestion (AD) or biometh-anation of organic solid waste is con-sidered a promising treatment option

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Global Warming Potential (GWP) for Given Time Horizons

20-yr 100-yr 500-yr

Carbon dioxide 1 1 1

Methane 72 25 7.6

Table 5: Comparison of global warming potentials of CH4 and CO2 (GWP of CH4 includes indirect effects caused by enhanced ozone and stratospheric water vapour). (IPCC, 2007)

to digest waste. This process is already widespread in industrialised countries and is gaining increased importance giv-en the growing demand for renewable energy and high market prices for fuel. In low and middle-income countries, AD is currently common mainly in rural are-as, with livestock manure as major feed-stock. However, accessible knowledge and information on technical and opera-tional feasibility, challenges and oppor-tunities are limited as regards urban or peri-urban settings where predominantly organic solid waste is available as feed-stock. Nevertheless, in South India, nu-merous biogas plants treating kitchen and market waste have already been in operation for a few years. (Vögeli et al., 2008)

ARTIThe ARTI compact biogas plant, developed in India, is a small, household system designed to treat 1 – 2 kg of food waste per day. This already widespread system in South India is now also being promoted in Tanzania and Uganda.

Although it is considered a successful approach, data on its performance in Africa is rather scarce yet. More information will be needed to acquire a better assessment of this treatment option. Monitoring of an ARTI biogas plant at household level and experiments at the Ardhi University of Dar es Salaam shall provide reliable data on daily gas production, gas composi-tion, effluent quality, suitability of this technology for different feedstock, and operating con-venience. This project was launched in July 2008 in collaboration with the Ardhi University of Dar es Salaam and the University of Applied Sciences in Zurich. (Vögeli et al., 2008)

Photo 7: ARTI biogas plant in Dar es Salaam. (Source: Eawag/Sandec)

Advantages of the biogas technologyWell-functioning biogas systems can yield a wide range of benefits for their users, society and the environment in general:

production of energy (heat, light, elec-tricity);transformation of organic waste into a high quality fertiliser;improvement of hygienic conditions through reduction of pathogens, worm eggs and flies;reduction of workload, mainly for women, in firewood collection and cooking;environmental advantages through protection of soil, water, air, and woody vegetation;micro-economical benefits through energy and fertiliser substitution, additional sources of income and increasing yields from animal hus-bandry and agriculture;macro-economical benefits through decentralised energy generation, im-port substitution and environmental protection.

(ISAT et al., p. 5)

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��Sandec Training Tool: Module 6


Biogasification processThe process of biogasification can be divided into three steps. The overall process rests on the maintenance of a relatively critical balance between the re-spective activities of the three stages. An imbalance reduces the efficiency of the overall process and may lead to the complete standstill of all microbial activ-ity when no methane production occurs. Immediately after its initiation, the se-quence of readily observable reactions in a continuous culture is a gradual decline in pH level (the acid stage), followed by a similarly gradual rise in pH level, and eventually by the production of a methane-rich gas (the methane produc-tion stage). (UNEP, 2005, p. 260 – 261)

Parameters and process optimisationThe metabolic activity involved in micro-biological methanation is dependent on the following factors:

Substrate temperatureAvailable nutrientsRetention time (flow-through time)pH levelNitrogen inhibition and C:N ratioSubstrate solid content and agitationInhibitory factors

Each of the various types of bacteria re-sponsible for the three stages of metha-nogenesis is affected differently by the above parameters. Due to the interactive effects between the various determin-ing factors, accurate quantitative data on gas production as a function of the above factors is not available. (ISAT et al., p. 11)

Substrate temperature. Anaerobic fermentation is in principle possible between 3 °C and approximately 70 °C. Differentiation is generally made be-tween three temperature ranges:

the psychrophilic temperature lies be-low 20 °C,the mesophilic temperature ranges between 20 °C and 40 °C andthe thermophilic temperature lies above 40 °C.

The rate of bacteriological methane pro-duction increases with temperature. In general, unheated biogas plants perform satisfactorily only where mean annual

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temperatures are around 20 °C or above, or where the average daily tempera-ture is at least 18 °C. Within the range of 20 – 28 °C mean temperatures, gas pro-duction increases over-proportionally. If the temperature of the biomass is be-low 15 °C, gas production will be so low that biogas production is no longer eco-nomical.

The process of biomethanation is very sensitive to temperature fluctuations. The degree of sensitivity is, in turn, de-pendent on the temperature range. Brief fluctuations not exceeding the following limits may be regarded as still un-inhibi-tory with respect to the process of fer-mentation:

psychrophilic range: ± 2 °C/hmesophilic range: ± 1 °C/hthermophilic range: ± 0.5 °C/h

Temperature fluctuations between day and night are no great problem for plants built underground, since the temperature of the earth below a depth of one meter is practically constant. (ISAT et al., p. 11)

Retention time. For continuous sys-tems, the mean retention time is ap-proximated by dividing the digester vol-ume with the daily influent rate. Effective retention time may vary widely for the in-dividual substrate constituents, depend-ing on vessel geometry, type of mixing procedure etc. Selection of a suitable re-tention time thus depends not only on the process temperature, but also on the type of substrate used.

The following approximate values apply to liquid manure undergoing fer-

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mentation in the mesophilic tempera-ture range:

liquid cow manure: 20 – 30 daysliquid pig manure: 15 – 25 daysliquid chicken manure: 20 – 40 daysanimal manure mixed with plant material: 50 – 80 days

If retention time is too short, the bacteria in the digester are “washed out” faster than they can reproduce, and fermenta-tion practically comes to a standstill. This problem rarely occurs in agricultural bi-ogas systems. (ISAT et al., p. 12)

pH value. The methane-producing bac-teria live best under neutral to slight-ly alkaline conditions. Once the proc-ess of fermentation has stabilised under anaerobic conditions, the pH will normal-ly range between 7 and 8.5. Due to the buffer effect of carbon dioxide-bicarbo-nate (CO2 - HCO3

-) and ammonia-ammo-nium (NH3 - NH4

+), the pH level is rare-ly taken as a measure of substrate acids and/or potential biogas yield. A digester containing a high volatile-acid concentra-tion requires a somewhat higher-than-normal pH value. If the pH value drops below 6.2, the medium will have a tox-ic effect on the methanogenic bacteria. (ISAT et al., p. 12)

Nitrogen inhibition and C:N ratio. All substrates contain nitrogen. For higher pH values, even a relatively low nitrogen concentration may inhibit the process of fermentation. Noticeable inhibition oc-curs at a nitrogen concentration of rough-ly 1700 mg ammonium-nitrogen (NH4-N) per litre substrate. Nonetheless, given

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Figure 12: The three-stage anaerobic fermentation of biomass. (ISAT et al.)

Stage I Stage II Stage III

Methanogenic bacteria

H2, CO2, acelic acid

Protonic acid, butyric acid, various alcohols and other com-pounds

H2, CO2, acelic acid

Methane, CO2

Bacterial mass

Bacterial mass

Bacterial mass

Organic waste, carbohydrates, fats, proteins



enough time, the methanogens are ca-pable of adapting to NH4-N concentra-tions in the range of 5000 – 7000 mg/l substrate, the main prerequisite being that the ammonia level (NH3) does not exceed 200 – 300 mg NH3-N per litre substrate. The rate of ammonia dissoci-ation in water depends on the process temperature and pH value of the sub-strate slurry.

Microorganisms need both nitrogen and carbon for assimilation into their cell structures. Various experiments have shown that the metabolic activity of methanogenic bacteria can be optimised at an approximately 8 – 20 C:N ratio, whereby the optimum point varies from case to case, depending on the nature of the substrate. (ISAT et al., p. 12 – 14)

Substrate solids agitation. Many sub-strates and various modes of fermenta-tion require some sort of substrate ag-itation or mixing in order to maintain process stability within the digester. The

Further questionsWhat are the consequences of land-

spreading untreated organic waste?Ñ




Rothenberger, S., Zurbrügg, C., Sin-ha, M., Enayetullah, I.(2006): Decentral-ised Composting for Cities of Low and Middle-Income Countries. A Users’ Manu-al. Eawag/Sandec. www.eawag.ch/organ-isation/abteilungen/sandec/publikationen/publications_swm/downloads_swm/de-comp_Handbook_loRes.pdf (last accessed 28.04.08)

ISAT and GTZ: Biogas Digest. www.gtz.de/de/dokumente/en-biogas-volume1.pdf (last accessed 28.04.08)


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most important objectives of agitation are:

removal of the metabolites produced by the methanogens (gas)mixing of fresh substrate and bacterial population (inoculation)preclusion of scum formation and sed-imentationavoidance of pronounced temperature gradients within the digesterprovision of a uniform bacterial popu-lation densityprevention of the formation of dead spaces that would reduce the effec-tive digester volume.

(ISAT et al., p. 14)

Inhibitory factors. The presence of heavy metals, antibiotics (Bacitracin, Fla-vomycin, Lasalocid, Monensin, Spiramy-cin etc.) and detergents used in livestock husbandry can have an inhibitory effect on the process of biomethanation. (ISAT et al., p. 14)

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Using recycled plastics in road constructionReusing plastic waste to pave roads is an experiment that has been successfully conducted in many places, such as Kalamassery in Kerala, in Kolkata and Bangalore. The first technology approach, developed by Bangalore-based K K Plastic Waste Management Limited, entails the use of plastic waste along with bitumen - the conventional ingredient to pave roads. Not only does the road become a receptacle for plastic waste, but it also has a better grip.

The process

The plastic waste products (bags, cups and so forth) made out of polyethylene, polypropylene and polystyrene are separated, cleaned if necessary, and shredded into small pieces to allow their passage through a 4.35-millimetre sieve. The aggregate (granite) is heated to 170 °C in the mini hot-mix plant; the shredded plastic waste subsequently added softens and coats the aggregate. The hot bitumen (160 °C) is directly added and mixed well. As the polymer and bitumen are molten, they mix and the blend coats the surface of the aggregate. The mixture is transferred to the road for paving.

(Zhu et al., 2007)

refuse is mixed with water and ground into slurry in the wet pulper resembling a large kitchen disposal unit. Large piec-es of metal and other non-pulpable mate-rials are pulled out by a magnetic device before loading the slurry from the pulper into a centrifuge, the so-called liquid cy-clone. Here, the heavier non-combusti-bles, such as glass, metals and ceramics are separated out and sent on to a glass and metal-recovery system; other, light-er materials go to a paper fibre recovery system. The final residue is either incin-erated or used as landfill. (Abubakar et al., 2006, p. 6)

Further questionsIs full cost-recovery of solid waste

management possible through recycling?Ñ

Additional infoAbubakar, E. and Bello, M. (2006): Mu-

nicipal Solid Waste Management: Options for Developing Countries, IPAC Technical Meeting. EEMS Limited, Kaduna, Nigeria. www.eemslimited.com/issues/msw_op-tions.pdf (last accessed 28.04.08)


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3.3 What are the recycling options for non-organic waste?

While sophisticated waste processing units reclaim large amounts of MSW in industrialised countries, it is the informal sector in the DCs that sorts out waste and sells the recovered materials to vendors and specialised recycling units.

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Public interest in recycling has increased dramatically over the last 15 years throughout the industrialised world, and is presently gaining ground in develop-ing countries. This interest has been driv-en in the developed economies by a va-riety of factors, including concerns about increasing waste generation, dwindling landfill capacity, air pollution from incin-eration, and a general appreciation of the need for environmental protection. In re-sponse, a wide array of policies, regu-lations and programmes have been im-plemented. These include changing the requirements for recycling in households and businesses, banning recyclables from being landfilled, creating deposit-refund programmes and financial incen-tives for source separation and waste reduction. Other policies have been de-signed to stimulate the demand for recy-cled materials. These include guidelines for buying recycled products, require-ments for a minimum recycled content and tax incentives for products with re-cycled content.

In some countries, comprehensive extended producer responsibility (EPR) frameworks have been introduced to tar-get both supply and demand. EPR poli-cies shift the responsibility for meeting government-specified recycling targets to the industries that produce the recycla-bles. Governments are also increasingly encouraging industries to adopt envi-ronmental management systems (EMS). These holistically address waste genera-tion through source reduction, reuse and recycling. (Abubakar et al., 2006, p. 6)

In developing countries, recycling in-organic materials from municipal solid waste is often a well-developed activi-ty performed by the informal sector, al-though such activities are seldom rec-ognised, supported or promoted by the municipal authorities. Some of the

key factors affecting the potential for resource recovery comprise the cost of the separated materials, their purity, quantity, and location. Storage and trans-port costs are major factors governing the economic potential of resource re-covery.

In many low-income countries, the fraction of materials extracted for re-source recovery is extremely high, the work very labour-intensive and the in-come very low. In such situations, cre-ation of employment is the main eco-nomic benefit of resource recovery. The conditions in industrialised countries are totally different, as resource recovery is conducted by the formal sector within a legal framework and with a general pub-lic concern for the environment and gen-erally high costs. (Zurbrügg, 2002, p. 23)

Solid waste recycling is an ancient practice. In prehistoric times, the met-al fraction was melted and recast. Re-cyclable materials are currently recov-ered from municipal refuse by various methods, including shredding, magnet-ic separation of metals, air classification that separates light from heavy fractions, screening, and washing. Another meth-od of recovery in industrialised countries is the wet pulping process: Incoming


3.4 What are the final disposal options and how are they characterised?


Landfilling is the most common option for final disposal worldwide. In ICs, a significant fraction of MSW is incinerated and part of its energetic value thus reclaimed in the form of heat and electricity.

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Incineration

The primary benefit of MSW incinera-tion is a significant reduction in weight (up to 75 %) and volume (up to 90 %), which can be valuable if landfill space is limited. Generation of revenues from energy production, known as waste-to- energy incineration, can also partially off-set the cost of incineration; although typically less expensive forms of en-ergy production are available. Incinera-tion breaks down some hazardous, non-- metallic organic waste and destroys bacteria and viruses, which is the main benefit of medical waste incineration. If the MSW incineration option is consid-ered, decision-makers must weigh the benefits of incineration against the sig-nificant capital and operating costs, po-tential environmental impacts and techni-cal difficulties of operating an incinerator. (UNEP-IETC et al., 1996, Chapter 1.5.1)

MSW incineration is typically only cost-effective in regions where suita-ble landfill space is scarce. Such land-fill scarcity can arise due to geographic constraints, as with a highly urbanised region or island, or due to environmen-tal conditions, as in regions with a high water table. Jurisdictional and political boundaries can also constrain the size and number of sites available for land-

filling, thereby increasing the attractive-ness of incineration.

Some factors currently make incinera-tion difficult or not advisable in many de-veloping countries, i. e. high capital and operating costs in relation to national in-come levels and the comparatively low cost of sanitary landfilling. Due to its high moisture and low energy content, it is difficult to incinerate waste in many de-veloping countries. Moreover, the techni-cal infrastructure required to maintain-ing incineration facilities, including their pollution control equipment, is generally not yet available in developing countries. The frequently lacking infrastructural el-ements include highly trained person-nel, regular availability of technological-ly advanced testing and repair facilities and a well-functioning system to ensure the readily available spare parts. (UNEP-IETC et al., 1996, Chapter 1.5.1)

For environmentally sound incinera-tion, air pollution control equipment must be serviced regularly by highly special-ised personnel. Monitoring equipment is costly and requires thorough mainte-nance and servicing by trained techni-cians. In summary, incineration is expen-sive if conducted in a sustainable manner with low adverse health and environmen-tal risks. If poorly conducted (with low fi-nancial costs), it can become expensive in terms of human health and environ-mental impacts.

Some countries, which have emerged from developing country status, are defi-nitely able to incinerate their waste. Sin-gapore operates three MSW incinerators handling about 90 % of the MSW gener-ated. South Korea also operates numer-ous incinerators. MSW incineration is also being considered in Bangkok, where three incineration plants located at land-fills are already in operation, primarily to incinerate hazardous waste.

Pilot projects, supported by bilateral or international aid or joint ventures with foreign companies, may make such ini-

Factors influencing technology choiceMSW incineration may offer a sound practice only in situations where most or all of the following conditions apply:

suitable landfill space is scarce, making incineration a cost-effective alternative;

the necessary environmental protec-tion measures are properly installed and maintained;

the facility is adequately sized and sited to fit the other components of the MSWM system;

the materials to be burned are combusti-ble and have sufficient energy content; and

energy markets are available nearby.


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tiatives more feasible as they can make foreign capital and technology training available in developing countries. (UNEP-IETC et al., 1996, Chapter 1.5.1)

Use of energyIn waste-to-energy plants, heat from the burning waste is absorbed by water in the wall of the furnace chamber or in separate boilers. Water is heated to the boiling point and is converted to steam. At that point, either the steam is used for heating or to turn turbines to gener-ate electricity. The amount of energy re-covered from waste is calculated as a function of the amount of waste com-busted, of the energy value of the waste stream and efficiency of the combustion process. About one-fifth of the electric-ity produced in incineration facilities is used at the facilities for general opera-tions. The remaining electricity is sold to public and private utilities or nearby industries. In many countries, utilities provide a stable market for electricity generated from incinerators. The avail-ability of purchasing electricity and its sales rates will, however, vary accord-ing to region. (UNEP-IETC et al., 1996, Chapter 1.5.3)

Environmental impactsPotential pollution emissions into the air through exhaust stacks and into water through ash leachate are the main en-vironmental risks of MSW incinerators. Proper planning to minimise environmen-tal damage, as well as public educa-tion and involvement directly addressing these issues, are essential to successful incineration programmes. The combus-tion of any substance will generate by-product emissions likely to be released into the air. The following air emissions are usually associated with incinerators: metals, especially mercury, lead and cad-mium; organics, such as dioxins and furans; acid gases, such as sulphur di-oxide and hydrogen chloride; particulate



matter, such as dust and grit; nitrogen oxides (which are ozone precursors); and other substances, such as carbon mon-oxide.

People can be exposed to emissions directly by inhaling contaminated air or through skin contact with contaminated soil and dust. Exposure can also occur indirectly by eating foods contaminated by these substances. Aside from human health risks, plants and animals may also be adversely affected by emissions from incinerators. The ultimate effects are de-pendent on contaminant concentrations of the emissions, type of environmen-tal control measures adopted, height of the emission stack, location of the facil-ity, and prevailing weather and geograph-ic conditions.

A related contamination concern is as-sociated with the area close to the incin-erator, i. e. below its emission plume. All pollutants likely to escape will reach the ground closest to the incinerator. It is therefore particularly important to site an incinerator in an area as isolated as pos-sible. In general, industrial areas make more sense than other areas as their contamination levels may already have induced the taking of precautions.

However, adequate control of air emissions requires further pollution con-trol measures. MSW incinerators must be well-operated and well-maintained to ensure the lowest possible emissions. Good combustion practice, such as en-suring optimal levels of temperature in the combustion chamber and residence time of the MSW remaining in the com-bustion chamber, can lower emission levels. Major variations in these or oth-er incineration operations could lead to a limited but significant output of contami-nated air emissions.

Major technical requirements are among the obstacles to incineration in most developing countries.

Incinerator ash may contain concen-trations of heavy metals, such as lead, cadmium, mercury, arsenic, copper, and zinc released by plastics; coloured print-ing inks, batteries, certain rubber prod-ucts, and hazardous waste from house-holds and small industrial generators. Organic compounds such as dioxins and furans have also been detected in incin-erator ash.

Since incinerator ash is generally dis-posed of in an MSW landfill, the environ-mental pollution control measures typ-ically adapted for sustainable sanitary

landfill operations (e. g. liners and leach-ate collection/treatment) become all the more important.

Ash can be stabilised and solidified by encasing it in concrete prior to disposal, thereby reducing significantly the migrat-ing potential of the contaminant. Some also advocate managing fly ash and bot-tom ash separately, with additional sta-bilisation of the fly ash through vitrifica-tion or pyrolysis, as fly ash can contain higher metal concentrations. In addition to landfilling, incinerator ash has been used in the production of road bedding, concrete, brick, cinder block, and curb-ing. These uses are controversial as lea-chates may contain toxic constituents of these materials.

Most heavy metals (e. g. mercu-ry, cadmium and lead) originate from items commonly found in MSW, such as household batteries, thermostats, fluo-rescent lamps, plastics, and solder-bear-ing items (e. g. consumer electronics, light bulb sockets and plated metals). Removing these items from the waste stream, at household, commercial and industrial levels, may therefore lead to a significant reduction in the metals found in incinerator ash. (UNEP-IETC et al., 1996, Chapter 1.5.4)

All definitions of “sanitary landfill” call for the isolation of the landfilled waste from the environment until it is rendered innocuous through natural biological, chemical and physical processes. The major differences between the various definitions reside in the degree of isola-tion, means of accomplishing it, moni-toring prerequisites, and closing or main-taining the landfill after its active life. In industrialised nations, a far greater de-gree of isolation is usually required than actually needed in developing nations. This is not surprising as the means to at-tain a high degree of isolation in develop-ing nations are complex and expensive.A disposal site must meet the follow-ing three general but basic conditions to qualify as a sanitary landfill:

waste compaction;daily covering of the waste (with soil or other material) to protect it from environmental influences; and

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control and prevention of negative impacts on public health and the environment (e. g. odours, contami-nated water supplies etc.).

However, the meeting of all the specif-ic conditions may be technologically and economically impractical in many devel-oping countries. Therefore, the short-term goal should be to comply as far as possible with the more important condi-tions under the existing set of technical and financial circumstances. The long-term goal is to eventually meet all the specific requirements related to design and operating conditions. Only then can all the benefits associated with a sanitary landfill be yielded. Prevention of negative impacts on public health and the environ-ment is the most important prerequisite. (UNEP, 2005, p. 323 – 324)

A landfill is a vital component of any well-designed MSWM system. It is the

3. ultimate repository of a city’s MSW af-ter all other MSWM options have been exhausted. In many cases, the landfill is the only option available after MSW is collected. Safe and effective landfill op-eration depends on the sound planning, administration and management of the entire MSWM system. This begins with an institutional and environmental policy that views MSWM as an important com-ponent in the sustainable development plans of a city and country. It contin-ues with the implementation of MSWM regulations designed to protect human health and the environment, and with the funding driven by the needs of the sys-tem rather than by political expediency. It ends with the coordination of MSWM programmes to consolidate waste reduc-tion and resource recovery through col-lection, transfer and ultimate disposal into an integrated system. This system must provide a vital public service with-

Landfills



out compromising human health or the environment.

Landfill types range from uncontrolled open dumps to sound sanitary landfills. (UNEP-IETC et al., 1996, Chapter 6.1)

Left unmanaged and uncontrolled, solid waste openly dumped on land:

generates liquid and gaseous emis-sions (leachate and landfill gas) likely to pollute the environment; andpresents a breeding ground for disease-carrying animals and micro-organisms.

Uncontrolled land disposal of solid waste also leads to other public health, safety and environmental risks. (UNEP, 2005, p. 323)

Open dumps and the need to upgrade themOpen dumps are common in developing countries as their initial costs are low and they do not require expertise or equip-ment. However, remediation costs of these sites can easily exceed their total lifetime capital and operating costs. Con-taminated groundwater may never be re-turned to its usable condition, and other environmental impacts may take sever-al decades to be restored. The numer-ous birds that feed on the waste in open dumps could represent more serious dis-ease vectors than flies or rodents.

The practice of open dumping is a di-lemma for the poorer and smaller cities and towns of developing countries, and is certainly not sound practice. (Note however, in very poor countries, where cities are located near deserts (e. g.

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Landfills: Land and volume requirements

(UNEP, 2005, p. 332)

Figure 13: Land requirements for a landfill as a function of waste compaction.

Figure 14: Relationship between bulk waste density and required landfill volume.

North Africa and the Middle East), un-improved open dumps may conceivably be considered sound if the savings from not upgrading dumps are used to im-prove the collection service.) Managers are often told to close open dumps and construct controlled landfills. As a conse-quence of inadequate technical and man-agerial resources, solid waste managers attempt in many places to improve open dumping practices and gradually upgrade the sites.

A number of countries have acquired considerable experience with such low-cost upgrading methods. Solid waste de-partments can rent the heavy equipment necessary to improve the infrastructure and grading of the dump or can subcon-tract this work to a private engineering firm. The initial upgrading step consists in constructing perimeter drains to col-lect the run-off and leachates, the site should then be graded to minimise leach-

ing through the waste. Machines can be rented about every two months to regu-larly adjust the grading, construct trench-es for the deposit of waste (if necessary) and dig up cover material. Maintaining the grading and applying cover material can subsequently be conducted manu-ally by municipal workers. In some cas-es, a provincial ministry acquires the nec-essary earthmoving equipment, which is then rotated among the dumps of the jurisdiction. In places where equip-ment is acquired by the authority oper-ating the dump, such equipment should be kept as simple as possible to make operation and maintenance feasible. It is important to prove to municipal engi-neers that improvements can be made to open dumps with little capital outlay and additional costs. (UNEP-IETC et al., 1996, Chapter 6.3)

Non-landfill disposal

Some countries, including China, have a long-standing tradition of disposing garbage directly onto farmland. Farm-ers seek the nutrient value of the organ-ic portion of the waste as long as it con-tains little plastic, glass or metal. This is a hazardous practice, since uncompost-ed organic waste contains pathogens. Regulations in China require farmers to compost the waste first, however, these regulations are often not complied with.

Finally, some municipalities dispose their MSW at sea, on land near the ocean or on riverbanks, though many industr-ialised and developing countries have banned these practices. In general, these practices cannot be considered environ-mentally sustainable. (UNEP-IETC et al., 1996, Chapter 6.5)

Further questionsTo what extent should public subsidies

be used to promote environmentally safe waste disposal in landfills?

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4.1 What is hazardous household waste?

4 – Hazardous Household Waste

Households generate small quantities of hazardous waste, such as oil-based paints, paint thinners, wood preserva-tives, pesticides, household cleaners, used motor oil, antifreeze, and batteries. Hazardous household waste (HHW) in industrialised countries, such as the US, accounts for totally 0.5 % of all the waste generated at home. In developing coun-tries, the percentage is even lower.

No specific cost-effective sound prac-tices can be recommended for hazard-ous household waste management in developing countries. Since concentrat-ed waste tends to create more of a risk, hazardous household waste is best jointly landfilled with the MSW stream, where biological reactions tend to have a fixing effect on small amounts of toxic metals, while other toxic substances are diluted within the MSW.

Where financial resources are availa-ble (typically in industrialised countries), specific sound practices are in place for separating hazardous household waste from the regular MSW stream. (UNEP-IETC et al., 1996, Chapter 1.7.3)

What makes a waste hazardous?Hazardous waste come in many shapes and forms. It can be liquid, solid, contain gas or sludge. It can be the by-product of manufacturing processes or simply dis-carded commercial products, like clean-ing fluid or pesticide. Four defining char-acteristics of hazardous waste are:

Ignitability. Ignitable waste can create fires under certain conditions or is spontaneously combustible. Examples include waste oils and used solvents.

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Corrosivity. Corrosive waste in-cludes acids or bases capable of corroding metal, like storage tanks, containers, drums, and barrels. Bat-tery acid is a good example.Reactivity. Reactive waste is un-stable under “normal” conditions. It can cause explosions, toxic fumes, gases or vapours when mixed with water. Examples include lithium-sul-phur batteries and explosives.Toxicity. Toxic waste is harmful or fatal when ingested or absorbed. When toxic waste is disposed on land, contaminated liquid may drain (leach) from the waste and pol-lute groundwater. Certain chemical waste and heavy metals are exam-ples of potential toxic waste.

(UNEP, 2004, p. 34)

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4.2 What are the dangers of hazardous waste?

Inappropriate storage, collection and treatment of hazardous waste pose a high risk to natural resources and public health.

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Surface Water ContaminationChanges in the water chemistry due to surface water contamination can affect all levels of an ecosystem. It can impact the health of lower food chain organisms and, consequently, the availability of food up through the food chain. It can damage the health of wetlands and impair their ability to support healthy ecosystems, control flooding and filter pollutants from storm water runoff. The health of animals and humans are affected when they drink or bathe in contaminated water. Moreover, aquatic organisms, like fish and shellfish, can accumulate and concentrate contaminants in their bodies. When other animals or humans ingest these organisms, the dose of contaminants is much higher than when directly exposed to the original contami-nation.

Groundwater ContaminationContaminated groundwater can adverse-ly affect animals, plants and humans if it is removed from the ground by man-made or natural processes. Depending on the geology of the area, groundwa-ter may rise to the surface via springs

or seeps, flow laterally into nearby riv-ers, streams or ponds, or sink deep-er into the earth. In many parts of the world, groundwater is pumped out of the ground to be used for drinking, bathing, other household uses, agriculture, and industry.

Air ContaminationAir pollution can cause respiratory prob-lems and other adverse health effects, since contaminants are absorbed by the lungs and reach other parts of the body. Certain air contaminants can also harm animals and humans when they contact the skin. Plants rely on respiration for their growth and can also be affected by exposure to contaminants transport-ed in the air.

LeachateLeachate is the liquid that forms as water trickles through contaminated areas leaching out the chemicals. For example, the leaching of landfill can re-sult in a leachate containing a cocktail of chemicals. In agricultural areas, leach-ing may concentrate pesticides or ferti-lisers, and in feedlots, bacteria may be

leached from the soil. The movement of contaminated leachate may result in hazardous substances entering surface water, groundwater or soil.

Soil ContaminationContaminants in the soil can harm plants when they take up the contamination through their roots. Ingesting, inhaling or touching contaminated soil, as well as eating plants or animals that have accumulated soil contaminants can ad-versely impact the health of humans and animals.(Chapter adapted from: UNEP, 2004)

Further questionsWhat technical equipment and

procedures are required for optimal source-separation of hazardous waste?

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Additional infoUNEP (2004): Vital Waste Graphics, The

Seventh Meeting of the Conference of the Parties to the Basel Convention, Geneva. www.grida.no/publications/vg/ (last accessed 28.04.08)


<www> www.ewasteguide.info

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4.3 What is e-waste?

E-waste is any refuse created by discarded electronic devices and components as well as substances involved in their manufacture or use.

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4 – Hazardous Household Waste

The high tech boom has brought with it a new type of waste – electronic waste, a category that barely existed 20 years ago. Now e-waste represents the larg-est and fastest growing manufacturing waste. The black and white TV turned to colour, the basic mobile phone needed a camera, a personal organiser and mu-sic, and who wants last year’s compu-ter when it can’t handle the latest soft-ware? As we continually update and invent new products, the life of the old ones becomes shorter and shorter. Like ship breaking, e-waste recycling involves the major producers and users, shipping the obsolete products to Asia, Eastern Europe and Africa. But instead of being “green”, we are exporting a load of prob-lems to people who have to choose be-tween poverty or poison.

E-waste from computersOn average, a computer is made up of 23 % plastic, 32 % ferrous metals, 18 % non-ferrous metals (lead, cadmium, an-timony, beryllium, chromium, and mer-cury), 12 % electronic boards (gold, pal-ladium, silver, and platinum), and 15 %

Figure 15: Number of computers worldwide and their components. (UNEP, 2004)

What’s in a computer?

Let me give you a computerCommunities in West Africa receive used computers from donors in developed coun-tries. However, what was intended as a useful gift quickly becomes a waste prod-uct. When things go wrong, as they often do with computers (especially old ones), the lack of technical support means they end up on the scrap heap. It is estimated that the current number of personal com-puters worldwide amounts to over one bil-lion. In developed countries, these have an average service life of only two years. In the United States alone there are over 300 million obsolete computers. After its amendment, the Basel Convention banned the export of hazardous waste disposal to developing countries. Some countries (for example those in the European Union) have already implemented this proposed amendment. Moreover, countries like China have banned the import of e-waste, although significant volumes are still entering the country illegally.

Further questionsHow much e-waste is in the waste

stream?

How much e-waste is recycled?

How do I recycle my cell phone?

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Additional infoUNEP (2004): Vital Waste Graphics, The

Seventh Meeting of the Conference of the Parties to the Basel Convention, Gene-va. www.grida.no/publications/vg/ (last ac-cessed 28.04.08)


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glass. Only about 50 % of the computer is recycled, the rest is dumped. The tox-icity of the waste is mostly due to the lead, mercury and cadmium – non-recy-clable components of a single computer may contain almost 2 kg of lead. Much of the plastic used contains flame-retard-ants, which makes it difficult to recycle.

In many countries, entire communi-ties, including children, earn their liveli-hood by scavenging metals, glass and plastic from old computers. To extract the small quantity of gold, capacitors are melted down over a charcoal fire. The plastic on the electrical cords is burned in barrels to expose the copper wires. All in all, each computer yields about US $ 6 worth of material. Not very much when you consider that burning the plastic sends dioxin and other toxic gases into the air. And the large volume of worth-less parts is dumped nearby, allowing the remaining heavy metals to contami-nate the area. (UNEP, 2004)


5.1 Who are the stakeholders to consider within SWM?

5 – Non-technical Aspects

Stakeholders include households and communities requiring service, private sector enterprises and workers (formal and informal), and government agencies at the local, regional and national level.

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SWM cannot be solved with innova-tive technology or engineering alone. It is an urban problem that is closely re-lated to a number of issues, such as ur-ban lifestyle, resource consumption pat-tern, employment and income, and other socio-economic and cultural aspects. All these factors have to be consolidated in a common platform to ensure long-term solutions to urban waste disposal.

A large number of stakeholders are essential for the success of a solid waste management system. They influence ac-tivities on different spatial levels, such as household, neighbourhood, city, region, and nation.

Activities related to solid waste man-agement at the household level are pre-dominantly driven by socio-economic factors. Social responsibility and environ-mental awareness are driving forces for:

Central/provincial governmentCity councilNGOs and CBOsPrivate informal sectorPrivate formal sectorInternal and external support agencies

(Zurbrügg, 2003a, p. 7)

Similarly, actions to be taken at the state and national level are predominantly economic, political and administrative. Measures at the neighbourhood and city level cut across all themes.

Decisions and actions are embedded in a technological, environmental, social, financial and economic, organisational, administrative, institutional and political framework.

Experience in several countries has shown that cooperation and coordina-tion between the different stakeholder groups will ultimately lead to increased sustainability of a waste management system. (Klundert et al., 2000 in; Zur-brügg, 2003a)

As an alternative to the large (of-ten international) companies providing most or all of the solid waste servic-es to a city, the involvement of micro--

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enterprises or small enterprises (MSEs) should be considered. Since they often use simple equipment and labour-inten-sive methods, they can collect waste in places where the conventional trucks of large companies have no access. These MSEs may be started as a business to create income and employment, or they may be initiated by community members wishing to improve the immediate envi-ronment of their homes.

Appropriate practice in waste man-agement systems requires a clear defi-nition of jurisdiction and accountability, with all stakeholders participating in sys-tem design, including those affected at every level being made aware of their areas of responsibility.

Governments will generally have final jurisdiction and responsibility for over-all MSWM policy and management, ir-respective of whether or not they are di-rectly involved in waste management. The following participants have some im-portant relation to waste management and, in some cases, significant levels of responsibility for policies or operation.

Residential waste generators. Pref-erences of local residents for partic-ular types of waste services, their willingness to source separate recy-clable materials, their willingness to pay for the service, and their capac-ity to move waste to communal col-lection points, all have an impact on the overall waste system. Incentives can affect residents’ preferences and behaviour.Business waste generators. Since businesses also produce waste, the business sector can become a signifi-cant player in the waste management system, particularly since business-es are increasingly charged directly for the waste services. As with resi-dents, incentives can play an impor-tant role in shaping behaviour.Public health and sanitation departments. Maintenance of pub-lic health and sanitation is an impor-tant public responsibility, especially in

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developing and transition countries, where it generally falls under the juris-diction of the municipal public health department. In an integrated system, this department often has inspection and enforcement responsibilities, but is not directly involved in collection or disposal operations.Public works departments. These local government units often have op-erational responsibility for waste col-lection, transfer, treatment, and final disposal. Frequently, however, differ-ent department are responsible for collection of recoverable materials or management of private contractors, thus often creating conflicting goals and activities.Natural resource management agencies. Since these agencies are often responsible for activities relat-ing to materials recovery or compost-ing at the local or regional level, they are therefore separated from waste management functions. This results in poor integration as sound practice often places all the functions under the same agency or department.National or state/provincial en-vironmental ministries. Overall, waste management policy is gener-ally established at these levels. With respect to materials recovery policies, there is less policy-making at this level in developing countries. Sound practice includes not only the estab-lishment of policies, but putting pro-grammes in place, implement them and establish integration consistent with the policies.Municipal governments. In most countries, city or town governments assume the overall responsibility for waste management operations. They ensure regular collection services and delivery of the collected materials to processors, markets or disposal fa-cilities. The municipal government, which is ultimately responsible for the entire process, usually finances vehi-cles, crews and other equipment.

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Importance of scavengers/waste pickersIn developing countries, informal waste pickers or scavengers play an important role in sol-id waste management systems, acting in parallel with formal waste collection and disposal agents. Scavengers collect reusable and recyclable materials from streets, dumpsites or land-fills that can be reintegrated into the economy’s production process. Despite the benefits generated for society, waste pickers are ignored when waste management policies are formulated. (Moreno-Sánchez et al., 2006, p. 371)

In many developing countries, the socio-economic status of scavengers is usually very low. The general population and the authorities often view and treat them as ‘part of the rubbish they work with’ (ASMARE Street Scavenger’s Association, 1998). Low education levels and unhealthy working conditions combined with their popular status lead to a negative self- perception and lack of self-confidence. (UNESCO, 2001)

Medina states that even though scavengers are not always the poorest of the poor, their occupation is generally assigned the lowest status in society. Historically, outcasts and marginal groups, such as slaves, gypsies and migrants have performed waste collection and recycling activities in developing countries. In Muslim countries, non-Muslims usually perform refuse collection and recycling activities, since contact with waste materials is considered impure. (Medina, 2000).

In India, scavengers are mostly Dalits, or ‘untouchables’, not simply the lowest in the caste system but essentially outside it. The daily contact with garbage and sometimes even human excreta reinforces their ‘untouchable’ status. In other countries, such as Egypt, scavenger communities are groups of rural migrants who adopt scavenging as a way to survive in the city and end up specialising in this sector. In many countries, gypsies were the ethnic group involved in scavenging activities (cf. Fonseca, 1996). Aside from the day-to-day bad treatment that waste pickers experience, their low status can deter them from climbing the social ladder. NGOs and sometimes even governments strive for recognition of scavengers’ humanity and value. One way of tackling the ascribed and self-replicated low status of scavengers, whether or not related to ethnicity, is through the creation of co-operatives.

Besides raising income, this form of grassroots development can potentially provide scaven-gers with a certain status; they are recognised and accepted as part of the waste manage-ment system, which is beneficial to the entire population and increases both their self-esteem and self-reliance. (Nas et al., 2004, p. 345 – 346)

Photo 8: Waste pickers at a transfer station, Nepal. (Source: Eawag/Sandec)

Regional governments. Regional bodies or large city governments are often responsible for landfills, inciner-ators, composting facilities or the like, particularly in countries where there is a shortage of disposal space at the local level. Regional governments in charge of these facilities generally

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have access to sources of revenue from fees paid by waste collection companies for disposal.Private sector companies. Private sector companies tend to act as concessionaires or contractors of the responsible government authority in waste collection, street sweeping,

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materials recovery, and, increasing-ly, in construction and operation of landfills, incinerators and composting plants. Unlike governments, private companies do not have any direct re-sponsibility for maintaining public san-itation or health, so their involvement is limited to profit making functions. If there is no source of revenue, it is not reasonable to expect private sec-tor involvement. The necessary reve-nue can, however, come from direct charges or government allocations.Informal sector workers and enter-prises. In developing countries, but also increasingly in industrialised and transition countries, individual work-ers and unregistered, small enterpris-es recover materials from the waste stream, either by segregated or spe-cialised collection, by buying recycla-ble materials, or by picking through waste. These workers and enterpris-es clean and/or upgrade and sell the recovered materials, either to an in-termediate processor, a broker or a manufacturer. Informal sector work-ers sometimes manufacture new items from recovered materials, such as gaskets and shoe soles from dis-carded tires. These workers are often referred to as waste pickers or scav-engers.Non-governmental organisations. Non-governmental organisations (NGOs) are yet another set of partic-ipants in waste management opera-tions. NGOs are often commissioned to improve the environment or the quality of life of poor or marginalised populations, and may stimulate small-scale enterprises and other projects. Since waste materials often represent the only growing resource stream, these organisations frequently base their efforts in extracting certain ma-terials, currently not recovered, and in processing them to increase their value and produce revenue. This is how a number of composting projects were launched in Latin America.Community-based organisations. In some locations with insufficient collection or where neighbourhoods are underserved, community-based organisations play an active role in waste management operations. These smaller-scale organisations or

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panacea – a cure for all problems – even though their involvement has often re-sulted in very significant improvements in many situations. Experience has led some experts to believe that if a local government body has not been able to provide a satisfactory solid waste man-agement service using its own resourc-es, it will not be able to engage a private enterprise to provide a satisfactory serv-ice. Some assert that involving the pri-vate sector always results in increased corruption and misappropriation of pub-lic funds. However, most voices are in favour of private sector participation – some because of positive experience, some because of their political stand-point, and some out of a desperation nur-tured by the failure of the public sector.

Further questionsWhich MSWM functions, responsibilities

and powers should be assumed by which level of government?

What institutional arrangements and approaches would foster more demand- oriented solid waste services?

On what basis should authorities decide which waste management func-tions should be contracted out to private sector enterprises?

What is the potential role of community in local waste management, and what inputs are required to promote community-based waste management?

What instruments of awareness building and incentives should be employed to mobilise peoples’ contribution to waste minimisation and recovery?

What forms of collaboration between informal sector waste workers and munic-ipal authorities may be established to im-prove the productivity and working condi-tions of informal sector workers?

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Additional infoKlundert, A.v.d. and Anschütz, J. (2000):

The Sustainability of Alliances between Stakeholders in Waste Management. UWEP/CWG. www.gdrc.org/uem/waste/ISWM.pdf (last accessed 28.04.08):


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local NGOs are formed primarily as self-help or self-reliance units, which may, over time, evolve into service organisations that collect fees from their collection clients and from the sale of recovered materials. NGOs working with informal workers and community-based entrepreneurs of-ten seek recognition for these organi-sations as part of the waste manage-ment system.Poor and marginal populations in squatter areas. The waste service, much like other public services, fre-quently follows political power and clout, leaving the residents of poor and marginalised areas with inade-quate service (or no service at all), dirty streets and regular accumula-tion of refuse and faecal matter on

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streets and in other public areas. Very often, these people have the greatest need for improved or expanded waste service.Women. Waste handling dispropor-tionately touches the lives of women, particularly in some developing and transition countries. Women often collect the waste, set it out or move it to community transfer areas. Wom-en are far more likely to be involved in materials recovery than in other com-parable types of physical work. This is possible due to their daily contact with the waste in their homes, and probably because women tend to be among the most marginalised groups in some societies.

(UNEP, 2005, p. 9 – 11)

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5.2 What are the characteristics of private sector involvement?

Three key factors are decisive for the success of private sector involvement: Competition, accountability and transparency. Private sector participation can increase service quality and reduce costs through introduction of commercial principles.

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Provision of municipal solid waste serv-ices is a costly and vexing problem for local authorities everywhere. In cities of developing countries, service cover-age is low, resources are insufficient and uncontrolled dumping is widespread with resulting environmental problems. Moreover, substantial inefficiencies are typically observed. One solution com-monly proposed is to contract service provision out to the private sector in the belief that service efficiency and cover-age can be improved and environmental protection enhanced. The private sector assumes three important roles in solid waste management. First, where exist-ing public service delivery is either too costly or inadequate, private sector par-ticipation offers a means of enhancing

efficiency and lowering costs through the introduction of commercial principles and greater attention to customer satis-faction. Second, in situations where local public funds for investment are in chroni-cally short supply, the private sector may be able to mobilise needed investment funds. Third, the private sector is well situated to draw on local and internation-al experience in the waste management field and introduce proven and cost-ef-fective technologies along with manage-ment expertise. (Cointreau-Levine et al., 2000, p. 3)

However, opinion leaders, familiar with private sector participation, have urged that private enterprises involved in the provision of solid waste manage-ment services should not be seen as a



But opposition to the involvement of the private sector in the provision of pub-lic services can be expected in most situ-ations. This may result from:

political views;general resistance to change;opposition from labour unions;fears about corruption;fears of officials that they will lose power, influence or income;previous experience of private sector participation; anda belief that private companies take huge profits or other factors.

In general, a wide variety of arrange-ments can be implemented to take ad-vantage of the benefits of private sector participation. To be successful, however, cooperation with the public sector is cru-cial. Both sides should have rights that are upheld by the courts and duties that are backed up by the threat of sanctions. Such an equal partnership is much more likely to result in effective and economi-cal services that continue for a long pe-riod. Unfortunately, the public sector of-ten dominates, with little concern for the rights of the private sector, and the result can be the bankruptcy of the company or the reluctance of companies to bid for fu-ture work. (Coad, 2005, p. 3, 8, 25)

Key factors for successful private sector participationCompetition. There should be compe-tition between different private sector companies and, if possible, also between the private and public sectors. Compe-tition provides motivation to maintain effort. It sets a standard against which performance is compared or assessed. Furthermore, it provides a continual re-minder that there are others engaged

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in the same activity who could take the place of a competitor who is perform-ing poorly.

Accountability. Private sector service providers should sense that they are ac-countable to the people whose waste they collect and to the local govern-ment agency that has engaged or li-censed them. The companies know that if they fail to provide the required serv-ice in the required way, there will be consequences. They are not free to do as they please. Such accountability re-sults from a well-prepared contractual agreement, from effective enforcement of the terms of the agreement, and from the understanding that there will be fi-nancial penalties if expectations are not met. Microenterprises, which draw their workforces from the communities that are served, benefit from the accounta-bility that the laborers feel towards their neighbours who expect a fair and satis-factory service. The public sector agency (whether municipal or regional govern-ment) responsible for the service should also feel accountable to both the public and the elected representatives for the way it oversees the service. Often ca-pacity development will be required if government is to effectively discharge its responsibilities.

Transparency. There is growing con-cern about the crippling effects of cor-ruption and favouritism or “cronyism”. More and more emphasis is being placed on “good governance” at city, region-al and national levels. Financial dealings and decision-making should be trans-parent. The reasons for decisions – es-pecially the selection of private sector service providers – and the management

Further questionsWhat are the reasons leading to the

involvement of private enterprises?

Why do some oppose the participation of the private sector?

What are the steps to be taken and the questions to be answered when develop-ing a strategy for involving the private sector?

Why is it so important to work closely with the general public as recipients of the service?

What are the common shortcomings in the tendering process and in contractual documents?

What the most successful ways of im-plementing private sector participation?

Why is the monitoring of private sector service providers often ineffective and the cause of conflict?

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Additional infoCointreau-Levine, S. and Coad, A.

(2000): Private sector participation in municipal solid waste management - Guidance Pack. SKAT, St. Gallen, Swit-zerland. http://rru.worldbank.org/Docu-ments/Toolkits/waste_fulltoolkit.pdf (last accessed 28.04.08)


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of public funds should be open before the public. In this way the service can enjoy the support of the public and com-petition is encouraged, since the com-petitors are reassured that they will have the opportunity of competing on fair and equitable terms. Public support can be expected to result in more widespread payment of charges or taxes, and fair competition to result in lower costs and better services.(Cointreau-Levine et al., 2000, p. 8 – 9)



5.3 How do legal frameworks and international treaties influence MSW management?

Government enactments vary greatly in different countries. Action plans and guidelines, like the Agenda 21 or the Basel Convention, have been established at an international level.

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Currently, no convention or other meas-ure exists for the comprehensive man-agement of waste. Efforts to deal with waste on an international scale have been largely confined to managing the prob-lems associated with the trans-bound-ary transport of waste. Although some technical guidelines for the management of certain specific types of waste have

been drafted under the Basel Convention (1989), the effectiveness and impact of these guidelines on waste minimisation are yet to be determined. The environ-mental, economic and social implications of a rapidly increasing ‘waste problem’ have gained recognition over recent dec-ades, and the need for a response to the waste management problem on an in-ternational scale has long been accept-ed. At the 1992 UN Conference on Envi-ronment and Development (UNCED) in Rio de Janeiro, Brazil, the international community adopted the Rio Declaration on Environment and Development and Agenda 21, an action plan designed to guide the Earth’s development in a sus-tainable manner (United Nations Confer-ence on Environment and Development 1992. The same goals were reiterated ten years later at the World Summit on Sustainability Development. (Meyers et al., 2006, p. 505 – 506)

The prime driver behind improved waste management is legislation, but this does not fulfil its aims unless it is supported by effective enforcement. In-deed, a lack of enforcement gives rise to unscrupulous operators that appear to comply with the law, but in practice deal with waste incorrectly or even dump it illegally.

There is a legal international trade for reused or recycled materials, howev-

MSW Rules 2000 (India)In 1996, a public interest lawsuit was filed with the Supreme Court against the gov-ernment of India, state governments and municipal authorities for their failure to per-form their duty of managing MSW ade-quately. As a consequence, the Supreme Court appointed an expert committee to look into all aspects of MSW and make recommendations to improve the situation. On the basis of their report, the ministry issued in September 2000, the Municipal Solid Waste (Management and Handling) Rules 2000 under the Environment Protec-tion Act 1986.

These rules lay down the steps to be taken by all municipal authorities to ensure man-agement of solid waste according to best practice. The municipalities were mandat-ed to implement the rules by December 2003, with punishment if authorities fail to meet the standards prescribed. Neverthe-less, most municipalities did not meet the deadline. Some cities and towns have not even started to implement measures that could lead to compliance with these rules.

(Zhu et al., 2007)

er, some of the world’s wealthy nations are exporting mixed or even hazardous waste to poorer countries, where it is not properly treated.

Traditionally, municipalities have been responsible for municipal waste collec-tion and disposal, but commercial com-panies are increasingly being used for waste management tasks. In some coun-tries, commercial companies work with municipal organisations, while in oth-ers, the municipalities themselves have formed companies for waste-manage-ment work. The use of private waste management contractors is increasing. (Key Note, 2007)

Further questionsHow much importance should be at-

tached to alternative instruments of waste management (regulations and controls, economic incentives, non-economic motivations and solidarity)?

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Additional infoSchüberler, P., Wehrle, K. and Chris-

ten, J. (1996): Conceptual framework for municipal solid waste management in low-income countries. In: Working Pa-per no. 9. World Bank. Report Nr. 40096. http://go.worldbank.org/3I0WRR9IF0 (last accessed 28.04.08)


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5.4 What are the financial arrangement options for SWM?

Options include government support, private sector financing, fees, and charges.Ñ

Structuring financing for waste management systemsSound practice in financing waste man-agement systems usually entails differ-ing treatment of fixed and variable costs. Fixed costs, which establish waste or materials collection, processing or dis-posal capacity, may be paid from gener-al tax revenues. The rationale for this is that all members of society benefit from having the overall solid waste manage-ment system in place. Once societies reach a certain level of sophistication, they may be able to recover a certain portion of fixed costs from commercial-ised collection, processing and disposal operations, and not rely solely on general tax revenues to fund these activities. Di-rect or indirect fees can be allocated for payment of variable costs directly pro-portional to the same. One key to de-veloping sound cost recovery systems is to accurately track down all costs. A surprising number of municipal govern-ments do not actually know the total costs of collection or disposal, so they have no basis on which to set or defend fees. Establishing well-functioning and transparent full-cost accounting systems should be a high priority where they do not yet exist. (UNEP, 2005, p. 13)

Fees and chargesUntil recently, waste management, fi-nanced by general revenues, was con-sidered the responsibility of the govern-ment in most developing, transition and European socio-democratic countries. Partly as a result of austerity and struc-tural adjustment policies and pressures from multilateral financial institutions, and partly as a result of pressures to lim-it taxes, governments have, in recent years, increasingly focused on identify-ing specific revenue sources for waste management. This has led to a series of innovations relating to fees and charges for waste collection and disposal:

Charging directly for waste services.One approach to the financing of waste systems is to obtain payment from those who benefit from the services. On the simplest level, waste generators ben-efit from collection service, and there

have been some attempts, particular-ly in North America, to get households to pay directly for their own waste re-moval on the basis of how much waste they generate. The system of unit fees for waste removal works well and rep-resents sound practice when individuals want to get rid of their waste and can af-ford the fees. It works poorly when peo-ple are too poor to pay fees, when the fees are simply too high or when there are ready alternatives and no controls for waste disposal, such as unregulated disposal in the countryside. Fees can be used to finance waste collection or other aspects of the waste system, and also as incentives to create less waste.

Indirect charges. In some locations, the waste charges are linked to other pub-lic services that people are willing to pay, such as water or electricity. In ad-dition to the waste charges, water and (if present) sewer charges allow some cost recovery. Studies have revealed that consumption of water and electricity are rough indicators of waste generation.

Incentives and penalties. Charges and fees can also be used as incentives to en-courage “good behaviour” or discourage “bad behaviour”. For example, the price of disposal can be increased and the costs for materials recovery subsidised to provide incentives to source separate. In some instances, fines can be imposed to discourage illegal dumping.(Adapted from UNEP, 2005, p. 12 – 13)

Polluter pays principleA study by SAEFL (Swiss Agency for the Environment, Forests and Landscape), Switzerland, conducted between 2000 – 2003, examined the ecological and financial advantages and disadvantages of waste disposal charges based on the pol-luter pays principle. Case studies were conducted in 13 municipalities, where charges of this kind had mainly been intro-duced. Figure 16 shows changes in com-bustible refuse volumes between 1997 and 2001 for municipalities without waste bag charges (positive values) and for municipal-ities, which had adopted the new system (negative values), using data from one year prior to the system’s inception until 2001.

Conclusions of the study: Introduction of the polluter pays principle led to a 30 % reduction in combustible waste collected by the municipalities. The amount of col-lected recyclables increased by 30 %.

(Bischof et al., 2003)

Figure 16: Development of collected com-bustible waste volumes. (Basis: kg/inhabitant index)

Further questionsWhat steps should be taken to include

financial and economic analysis into strategic planning functions?

How can the use of appropriate cost accounting systems be promoted despite possible reluctance from municipal officials?

How may local governments ensure that MWSM revenues are used for the intended purpose?

How should incentives for cost reduc-tion and increased operational efficiency be incorporated into municipal cost reduction and service effectiveness?

In which task areas and under what conditions will private enterprises contrib-ute most effectively to cost reduction and service effectiveness?

What MSWM revenue collection system will attain adequate cost recovery while, at the same time, create real incentives for cost reduction and effectiveness?

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Additional infoUNEP (2005). Solid Waste Manage-

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5 – References and Links

References

Abubakar, E. and Bello, M. (�00�): Mu-nicipal Solid Waste Management: Options for Developing Countries, IPAC Technical Meeting. EEMS Limited, Kaduna, Nigeria.

Akolkar, A.B. (2001): Management of munici-pal solid waste in India - Status and Op-tions: An Overview, Asia Pacific Regional Workshop on Sustainable Waste Manage-ment. German Singapore Environmental Technology Agency (GSETA), Singapore.

ASMARE Street Scavenger’s Association (1998): Street Scavengers: Partners of the Selective Waste Collection of Belo Hori-zonte. Belo Horizonte (pamphlet), Brazil.

Bischof, R. et al. (2003): Die Sackgebühr aus Sicht der Bevölkerung und der Gemeinden. In: Schriftenreihe Umwelt Nr. 257. Bunde-samt für Umwelt, Wald und Landschaft. BUWAL, Berne, Switzerland.

Coad, A. (2005): Private Sector Involve-ment in Solid Waste Management - Avoid-ing Problems and Building on Successes. CWG, St. Gallen, Switzerland.

Cointreau-Levine, S. and Coad, A. (�000): Private sector participation in municipal solid waste management - Guidance Pack. SKAT, St. Gallen, Switzerland.

Danish EPA (1996): Statutory order from the ministry of environment and energy no. 823 of September 16, 1996, on application of waste products for agricultural purpos-es, Denmark.

Diener, S. and Zurbrügg, C. (2008): CORS-technologies - the Conversion of Organic Refuse by Saprophages. In: Sandec News (2008).

Dulac, N. (2001): The Organic Waste Flow in Integrated Sustainable Waste Manage-ment. Tools for Decision-makers. WASTE, Gouda, the Netherlands.

Eawag (Swiss Federal Institute of Aquat-ic Science and Technology) / Sandec (Department of Water and Sanitation in Developing Countries) (�00�): Global Waste Challenge – Situation in Developing Countries. Dübendorf, Switzerland.

EC (2000): Working document on sludge, 3rd draft. European Communities, Brussels.

Feachem, R.G., Bradley, D.J., Garelick, H. and Mara, D.D. (1983): Excreta and Waste-water Management. World Bank Studies in Water Supply and Sanitation #3. John Wi-ley & Sons, Chichester, England, 534 pp.

Fonseca, I. (1996): Bury Me Standing: The Gypsies and Their Journey. Vintage Books, New York.

Haug, R.T. (1993): The practical handbook of compost engineering. Lewis Publishers, Boca Raton, FL, USA.

IPCC (2007): The Physical Basis - Technical Summary. In: IPCC Fourth Assessment Re-port. Working Group I.

ISAT and GTZ: Biogas Digest, Volume I.

Keynote (2007): Global Waste Management. In: Market Research Brochure. Keynote.

Klundert, A.v.d. and Anschütz, J. (�000): The Sustainability of Alliances between Stakeholders in Waste Management. UWEP/CWG.

Kungskulniti, N. (1990): Report: Public Health Aspects of a Solid Waste Scavenger Com-munity in Thailand. Waste Management & Research, 8(2): 167-170.

Lohani, B.N. (1984): Recycling Potentials of Solid Waste in Asia through Organised Scavenging. Conservation & Recycling, 7(2-4): 181-190.

Medina, M. (2000): Scavenger Cooperatives in Asia and Latin America. Yale University.

Meyers, G.D., McLeod, G. and Anbarci, M.A. (2006): An international waste convention: measures for achieving sustainable devel-opment. Waste Management & Reserach, 24: 505-513.

Moreno-Sánchez, R.d.P. and Maldonado, J.H. (2006): Surviving from garbage: the role of informal waste pickers in a dynamic model of solid-waste management in developing countries. Environment and Development Economics, 11: 371-391.

Nas, P.J.M. and Jaffe, R. (2004): Informal Waste Management. Environment, Devel-opment and Sustainability, 6(3): 337-353.

Rothenberger, S., Zurbrügg, C., Sinha, M., Enayetullah, I.(�00�): Decentralised Composting for Cities of Low and Middle-Income Countries. A Users’ Manual. Ea-wag/Sandec.

Saber, M. (1998): Solid Waste Management in Egypt, Current Status and Future Out-look, 3rd Swedish Landfill Research Sym-posium, Lulae.

Sandec Eawag (2008): Global Waste Chal-lenge - Situation in Developing Countries.

Schertenleib, R. and Meyer, W. (1992): Mu-nicipal Solid Waste Management in DC’s: Problems and Issues; Need for Future Re-search. IRCWD News, No. 26.

Schönning, C. and Stenström, A. (2004): Guidelines for the Safe Use of Urine and Faeces in Ecological Sanitation Systems. In: EcoSanRes. Stockholm Environment Institute. Swedish Institute for Infectious Disease Control (SMI).

Schüberler, P., Wehrle, K. and Christen, J. (1��): Conceptual framework for mu-nicipal solid waste management in low-in-come countries. In: Working Paper No. 9. World Bank. Report No. 40096.

Swedish EPA (2002): Action plan for recy-cling of phosphorous and sludge (Aktions-plan för bra slam och fosfor i kretslopp). In: NV Report 5214, Stockholm, Sweden.

UN (2008): Half of the population will live in cities by the end of this year, predicts UN. UN News service, 26.02.2008.

UNDP (1997): Interim Report, International Conference on Governance for Sustainable Growth and Equity. UNDP, New York, 28-30 July 1997.

UNEP-IETC and HIID (1��): International Source Book on Environmentally Sound Technologies for Municipal Solid Waste Management. UNEP, International Environ-mental Technology Centre, 427 pp.

UNEP (�00�): Vital Waste Graphics, The Sev-enth Meeting of the Conference of the Par-ties to the Basel Convention, Geneva, 25.-28. October 2004.

UNEP (�00�): Solid Waste Management, I. CalRecovery Inc, 524 pp.

van de Klundert, A. and Anschütz, J. (2001): Integrated Sustainable Waste Manage-ment - the Concept. In: A. Scheinberg (Edi-tor), Tools for Decision-makers – Experi-ences from the Urban Waste Expertise Programme (1995-2001).

WASTE, Nieuwehaven.

Vögeli, Y. and Zurbrügg, C. (2008): Biogas in Cities – A New Trend? In: Sandec News (2008).

World Resources Institute, United Nations Environment Programme, United Nations Development Programme and The World Bank (1996): World Resources 1996-97 - The Urban Environment. Oxford University Press, Oxford, 365 pp.

Zhu, D., Asnani, P.U., Zurbrügg, C., Ana-polsky, S. and Mani, S. (2007): Improving Municipal Solid Waste Management in In-dia. The World Bank.

Zurbrügg, C. (2002): Solid Waste Manage-ment in Developing Countries. Eawag/San-dec.

Zurbrügg, C. (2003a): Environmental Sanita-tion in the Urban Environment of Low and Middle Income Countries - A Global Over-view -. Unpublished.

Zurbrügg, C. (�00�b): Urban Solid Waste Management in Low-Income Countries of Asia - How to Cope with the Garbage Cri-sis, SCOPE, Durban, South Africa, 2002.

Bold: The key readings (additional info) are available on the CD of Sandec’s Training Tool. They are open source products. The user must always give credit in citations to the original author, source and copyright holder.”

��

Compostage (biologie) 1

Compostage (biologie)

Compostage de d�chets de jardin en

r�cipient a�r�.

Poubelle � compost

Compost

Le compostage est un processus biologique de conversion et de

valorisation des mati�res organiques (sous-produits de la biomasse,

d�chets organiques d'origine biologique...) en un produit stabilis�,

hygi�nique, semblable � un terreau, riche en compos�s humiques, le

compost[1].

Composition du compost

Les organismes responsables du compostage ont besoin de trois

�l�ments pour vivre :

� de nourriture �quilibr�e, compos�e d'un m�lange de mati�res

carbon�es (brunes-dures-s�ches) et de mati�res azot�es

(vertes-molles-humides) ;

�� d'humidit�, contenue particuli�rement dans les mati�res azot�es ;

�� d'air, dont la circulation est favoris�e par les mati�res carbon�es

structurantes (dures).


Poubelle � compost commerciale

Pour ne pas polluer une partie du compost, il

faudrait �viter d'utiliser comme ici des bois

cr�osot�s (traverses de chemin de fer r�cup�r�es)

Poubelle � compost commerciale

avec une porte


R�sidus organiques compostables

D�chets dits carbon�s D�chets dits azot�s

� branches broy�es, feuilles mortes, paille (ces mati�res sont �

stocker de fa�on � toujours en avoir � sa disposition pour les

m�langer avec les mati�res azot�es) ;

� coquilles d'�uf, coquilles de noix ;

� liti�res biod�gradables des animaux herbivores ;

� papiers en �vitant ceux qui sont imprim�s, carton (il sert de refuge

aux vers de terre) ;

� morceaux de tissus en mati�res naturelles (laine, coton), etc.

� d�chets de maison (mouchoirs en papier, essuie-tout, cendres de

bois, sciures, copeaux, plantes d'int�rieur non malades).

� d�chets v�g�taux, de jardinage (tailles de haies, tontes de pelouse...),

feuilles vertes,

� d�chets m�nagers p�rissables (d�chets des l�gumes et de fruit

(alimentation humaine)s).

Il est ainsi possible de diminuer de 30-50 % sa quantit� d'ordures

m�nag�res et de diminuer d'autant la taille des d�charges et les volumes de

d�chets transport�s vers les incin�rateurs

Attention : certaines mati�res comme le marc de caf� se d�composent tr�s lentement. Les mati�res telles que la

viande, le poisson et les os ne sont pas recommand�es dans la plupart des m�thodes de compostage domestique.

Description du processusLe compostage est une op�ration qui consiste � d�grader, dans des conditions contr�l�es, des d�chets organiques en

pr�sence de l'oxyg�ne de l'air.

Deux ph�nom�nes se succ�dent dans un processus de compostage. Le premier, amenant les r�sidus � l'�tat de

compost frais, est une d�gradation a�robie intense. Il s'agit essentiellement de la d�composition de la mati�re

organique fra�che � haute temp�rature (50��70��C) sous l'action de bact�ries ; le deuxi�me, par une d�gradation

moins soutenue, va transformer le compost frais en un compost m�r, riche en humus. Ce ph�nom�ne de maturation,

qui se passe � temp�rature plus basse (35��45��C), conduit � la biosynth�se de compos�s humiques par des

champignons.

D�gradation

L'�volution de la temp�rature durant le processus de d�gradation s'effectue en trois phases :

� la temp�rature monte rapidement � 40��C - 45��C suite � la respiration des micro-organismes m�sophiles a�robies.

Les compos�s les plus d�gradables tels les sucres et l'amidon sont d'abord consomm�s.

Une phase pr�liminaire � cette premi�re phase est parfois d�crite. Au cours de cette phase on note, apr�s une courte

latence, une l�g�re augmentation de la temp�rature. Elle r�sulte de l'activit� respiratoire endog�ne de cellules

vivantes pr�sentes dans la masse � composter. Cette phase est donc tr�s courte et ne s'observe qu'en laboratoire

lorsque le m�lange � composter contient une forte proportion de tissus frais.

� la respiration �l�ve ensuite la temp�rature progressivement jusqu'� 60��C - 70��C, conduisant au remplacement

des micro-organismes m�sophiles par des thermophiles et des thermo-tol�rants.

� par leur respiration, les micro-organismes �puisent l'oxyg�ne de la masse en compostage et rendent le milieu

ana�robie. Des germes ana�robies se d�veloppent alors, conduisant � un abaissement de la temp�rature car leur

m�tabolisme est moins thermog�ne. Ils sont de plus responsables de la lib�ration de compos�s volatils

naus�abonds (m�thane, ammoniac, hydrog�ne sulfur�...).

Pour �viter cette putr�faction, il est n�cessaire de restaurer les conditions a�robies du milieu (voir a�ration

ci-dessous). Ainsi il sera possible de prolonger la fermentation � haute temp�rature. Les pathog�nes, parasites et

semences de mauvaises herbes seront d�truits par la temp�rature �lev�e, les mauvaises odeurs seront �vit�es, la

d�composition sera plus rapide. D�s que la temp�rature n'augmente plus apr�s a�ration, on peut consid�rer que la

d�gradation est termin�e.


Maturation

� ce moment, la quantit� de mati�re facilement utilisable par la microflore se rar�fie et la biosynth�se de compos�s

humiques devient pr�dominante. On assiste � la disparition des micro-organismes thermophiles au profit d'esp�ces

plus communes et de nouvelles esp�ces m�sophiles au fur et � mesure que la temp�rature d�cro�t au cours d'une

longue p�riode de m�rissement pour se stabiliser au niveau de la temp�rature ambiante.

Il faut encore signaler que la transition entre chacune des phases cit�es pr�c�demment r�sulte d'une �volution

continue. Il n'y a pas de fronti�re marqu�e entre les esp�ces m�sophiles et thermophiles. Chaque esp�ce poss�de une

gamme de temp�ratures vitales avec un optimum au milieu.

Influence de l'environnementLa progression du mat�riel de d�part vers le stade final, l'humus, d�pend d'un grand nombre de facteurs externes

comme la dimension des particules, la nature des nutriments, leur structure, le taux d'humidit�, l'a�ration, le pH�

D'autre part, en se multipliant, les micro-organismes changent constamment leur environnement et le rendent

souvent impropre � leur d�veloppement.

Conditions physiques

A�ration

Ce facteur est essentiel puisque le compostage est un processus a�robie. On estime que l'air devrait occuper au moins

50 % du volume du tas. L'ana�robiose commence lorsque le taux d'oxyg�ne du tas est inf�rieur � 10 % ; elle

pr�domine au-dessous de 5 % d'O2

(air = 21 % O2). Diverses techniques permettent de r�tablir l'a�robiose, elles

seront d�crites ci-dessous.

Humidit�

Comme pour un substrat de culture, l'a�ration et l'humidit� du compost sont li�es : un exc�s d'eau diminue la quantit�

d'air disponible dans le volume de compost. Un syst�me d'a�ration plus efficace sera alors n�cessaire.

La chaleur lib�r�e par la fermentation provoque l'�vaporation d'une grande quantit� d'eau. L'arrosage de la masse en

fermentation permet le cas �ch�ant de mani�re � maintenir un taux d'humidit� de 50 � 70 % de la masse fra�che

(c�est-�-dire l'�quivalent de la capacit� au champ pour un sol). D'autre part les pluies battantes comme l'�vaporation

excessive par le soleil peuvent aussi ralentir le processus. Une couverture, toiture ou b�che peut r�pondre � ce

probl�me.


Dimension des particules

Outre son r�le sur la porosit� � l'air et la r�tention en eau du milieu, l'un des effets de la dilac�ration pr�alable

(broyage) est d'augmenter la surface de contact entre les d�chets et la microflore. Une r�duction de la taille des

particules entra�ne donc un accroissement du taux de d�composition mais aussi une circulation d'air plus faible

(risque d'ana�robiose).

Temp�rature

Un tas de compost d�gageant de la vapeur un

matin froid.

Par leur respiration les micro-organismes d�gagent une chaleur telle

que les temp�ratures atteintes (80��C et m�me plus de 90��C dans un

tas bien isol�) peuvent devenir l�tales pour les cellules. L'optimisation

du processus consiste donc � veiller � ne pas d�passer une temp�rature

de 70��C.

Conditions chimiques

pH

G�n�ralement, les mati�res � composter pr�sentent un pH compris

entre 5 et 7, c�est-�-dire dans des limites acceptables. Le pH s'abaisse

pendant les premiers jours et remonte ensuite pour devenir neutre ou

l�g�rement alcalin. Certains auteurs recommandent cependant

l'adjonction d'un tampon ou d'une base faible (calcaires ou dolomie

broy�s, marne, craie phosphat�e...), d'autres s'y refusent car cela peut

provoquer un ralentissement du processus. Sans adjonction de tampon,

le pH final du compost est aux alentours de 8.

Forme du carbone

Elle influence beaucoup la vitesse de d�composition du compost. Certaines mol�cules, tels les glucides simples,

l'amidon, les h�micelluloses, les pectines et les acides amin�s, sont ais�ment d�gradables. La cellulose, polym�re

plus volumineux, est plus r�sistante. La lignine et les autres polym�res aromatiques, extr�mement solides, seront

d�grad�s plus tardivement, plus lentement et incompl�tement (conduisant � la formation d'humus).

Rapport C/N

La consommation du carbone organique par la microflore lib�re une grande quantit� de CO2. La diminution

progressive de la teneur en carbone du milieu a pour cons�quence une diminution sensible de la valeur du rapport

C/N. En effet l'azote, fix� dans les prot�ines microbiennes, reste dans la masse du compost (sauf pertes �ventuelles

par d�gagement d'ammoniac).

Un rapport C/N trop faible (inf�rieur � 15) conduit � des pertes d'azote ; un C/N trop �lev� ralentit la d�composition.

La quantit� d'azote � ajouter est difficile � estimer car il faut tenir compte du taux de fermentescibilit� du carbone.

Selon le degr� de fermentescibilit� du carbone composant les r�sidus, on consid�rera comme favorable un rapport

C/N de 20 � 40 en fin de maturation.

De nombreux auteurs citent un rapport C/N de 15 � 30 comme id�al. L'exp�rience pratique montre que, pour des

substrats riches en lignines ou autres formes de carbone peu fermentescibles, un rapport de 40 voire 50 ne provoque

pas de carence par immobilisation de l'azote. La d�gradation de ces compos�s carbon�s par les micro-organismes est

en effet tellement lente que la faible consommation d'azote qui en r�sulte ne concurrence pas la culture.


Le C/N est d�termin� chimiquement (m�thode d'analyse du C/N). Or, les r�actifs chimiques ne correspondent pas au

bagage enzymatique de la microflore pr�sente dans le compost. D'autre part, l'analyse chimique d�grade

compl�tement les particules de l'�chantillon, c�est-�-dire bien plus que la surface d'attaque imm�diatement accessible

aux enzymes microbiennes. Le C/N id�al sera donc � d�terminer dans chaque cas.

Rapport C/P

Le phosphore est essentiel aux r�actions �nerg�tiques des micro-organismes (ad�nosine triphosphate). Il entre aussi

dans la composition de nombreuses autres macro-mol�cules. Un rapport C/P de la mati�re � composter voisin de

celui de la microflore (75 � 150) conduit � une d�gradation plus rapide de la mati�re organique et � une plus grande

production d'humus.

Autres �l�ments min�raux

Les mati�res � composter doivent �tre consid�r�es comme un milieu de culture pour microbes, o� le facteur limitant

ne peut �tre que le carbone assimilable et non un autre constituant du milieu. Ces �l�ments sont en g�n�ral pr�sents

en quantit� suffisante dans la mati�re organique � composter.

Conditions biologiques

La vitesse et l'efficacit� du compostage sont li�es � la pr�sence d'une population microbienne ad�quate. Si la

pr�sence de ces milliards de bact�ries et champignons est indispensable, leur ensemencement ('activateurs' ou

'stimulateurs' de compostage) semble peu, voire pas utile. Les spores de ces micro-organismes existent en effet en

quantit�s suffisantes dans la nature et il est beaucoup plus important de veiller � cr�er un milieu (pH, humidit�,

a�ration, C/N...) favorable � leur d�veloppement. Le syst�me Bokashi permet de recycler les d�chets fermentescibles

en un substrat ferment� constituant un pr�-compostage acc�l�rant la production de compost[2].

L'inoculation des composts par des micro-organismes fixateurs d'azote atmosph�rique, tels que Azotobacter ne

semble pas non plus int�ressante pour le compostage, la d�pense d'�nergie de ces organismes pour fixer l'azote �tant

trop importante. Le seul int�r�t de ce type d'inoculation pourrait provenir d'une �ventuelle fixation d'azote,

post�rieure au compostage, pendant la culture des plantes sur les composts ainsi inocul�s. Des exp�riences devraient

�tre men�es afin de d�montrer la cr�dibilit� d'une telle hypoth�se.

Aptitude au compostage (CNFP)

L'aptitude au compostage est un param�tre form� d'un code de quatre lettres, majuscules ou minuscules, il repr�sente

les quatre aspects fondamentaux � r�unir pour r�aliser un bon compost : 'C' ou 'c' pour carbone, 'N' ou 'n' pour azote,

'F' ou 'f' pour le degr� de fermentescibilit� (c.-�-d. l'aptitude � fermenter du produit), et 'P' ou 'p' pour la porosit�

totale. Une lettre minuscule indique un apport correct pour cet aspect, une lettre majuscule indiquant des propri�t�s

am�liorantes. L'absence d'une lettre ('�') signifie un manque, � compl�menter par un produit ayant des propri�t�s

am�liorantes pour le m�me facteur. La r�alisation du compost se fera donc en combinant deux sous-produits (trois �

la rigueur) ayant des propri�t�s compl�mentaires de telle mani�re que les quatre lettres du code soient pr�sentes dans

le m�lange r�alis�.


'c' ou 'C'

'c' indique un produit poss�dant un rapport C/N correct (15 � 30). 'C' indique un produit � forte teneur en carbone,

c�est-�-dire ayant un rapport C/N sup�rieur � 75. Un tel produit devra �tre m�lang� � un produit de type 'N' ou

recevra un suppl�ment d'azote sous forme d'engrais min�ral (ur�e par exemple).

'n' ou 'N'

Compl�mentaire du facteur pr�c�dant, 'n' indique un C/N correct ; 'N' indique un C/N faible (inf�rieur � 10)

n�cessitant un m�lange avec un produit de type 'C' ; un mat�riau � C/N �lev� sera de type '�' pour ce facteur.

'f' ou 'F'

Donne une indication sur la forme du carbone pr�sent : 'f' repr�sente un �quilibre convenable entre les mol�cules �

fermentation rapide (sucres) et les mol�cules � d�gradation lente (lignines). Les mol�cules � d�gradation rapide sont

n�cessaires au d�marrage de la fermentation et � l'obtention d'une temp�rature �lev�e dans la masse de compost

('pasteurisation' du compost). Un mat�riau riche en ces mol�cules sera de type 'F', un mat�riau pauvre de type '�'.

Les mol�cules � d�gradation lente quant � elles serviront de base � la biosynth�se des compos�s humiques.

'p' ou 'P'

La porosit� � l'air du mat�riau est importante pour son r�le sur l'a�ration du compost et sur la r�tention en eau (la

porosit� � l'eau, exprim�e en pourcentage de la porosit� totale, est le compl�ment � 100 de la porosit� � l'air). Elle est

influenc�e principalement par la dimension des particules. Un mat�riau dont la porosit� � l'air est �lev�e ('P',

mat�riau de structuration) permettra par exemple de r�aliser des tas de composts de volume plus important sans

risquer un tassement qui emp�cherait la circulation de l'air. Il pourra aussi servir de mat�riau de base � m�langer

avec des mat�riaux sans structure ('�': boues de stations d'�puration ou eaux de process industriel par exemple). 'p'

repr�sente un mat�riau pr�sentant un bon �quilibre entre la porosit� � l'air et la porosit� � l'eau.

Il faut remarquer que le compostage, en soi, ne n�cessite pas un structurant d'origine organique. Des copeaux de

caoutchouc (issus de vieux pneus) peuvent �tre utilis�s, par exemple pour le compostage de boues de stations

d'�puration.

Flux massiques du compostage urbain

Le tableau ci-dessous pr�sente des flux massiques typiques, en tonnes par an (t/an), pour diff�rentes pratiques de

compostage urbain des r�sidus v�g�taux compostables (d�chets de cuisine, bio-d�chets m�nagers). Il est bas� sur les

hypoth�ses suivantes : un foyer urbain comporte deux personnes dont chacune produit environ 50 kg/an de restes

v�g�taux compostables (valeur typique, comprise notamment entre celle de Nantes, plus �lev�e, et celle de Paris).

Compostage Type de gestion Nombre de

foyers

R�sidus v�g�taux

(t/an)Structurant ajout� 1

(t/an)

Compost m�r 2

(t/an)

Dur�e 3

(mois)

domestique 4 individuel priv� 1 0,1 0 0,03 2 � 3

d'immeuble ou de parc et

jardin

collectif priv� ou

semi-public

10 � 50 1 � 5 0,33 � 1,67 0,33 � 1,67 6 � 12

de quartier collectif public ou

semi-public

100 � 500 10 � 50 3,3 � 16,7 3,33 � 16,7 6 � 12

1 Bois ram�al fragment� (BRF), broyat de taille, copeaux et sciures de bois de feuillus2 Masse volumique typique du compost m�r : 0,5 t/m3

3 Selon temp�rature et saison4 Lombricompostage avec vers Eisenia fetida (Lumbricidae).


Les diff�rentes m�thodes de compostageLes m�thodes d�crites ci-dessous ne concernent que la phase de fermentation active. La phase de maturation quant �

elle se d�roule habituellement � l'air libre en tas de grande dimension.

� l'air libre

On construira cependant un auvent au-dessus des composts en fermentation afin de les prot�ger des pluies excessives

ou de la dessiccation par le vent et le soleil.

En fosse

La m�thode de compostage en fosse est la pratique la plus anciennement employ�e mais conduit rapidement � des

conditions ana�robies. La fosse est creus�e dans un endroit abrit� et bien isol�. Les d�chets organiques y sont

dispos�s en couches d'une vingtaine de centim�tres d'�paisseur, alternant les produits riches en azote (type 'N') et

ceux riche en carbone (type 'C'). Ils sont ensuite recouverts d'une �paisse couche de paille (isolation) puis d'une

couche de terre d'environ 10�cm d'�paisseur. Cette m�thode est tr�s lente et partiellement ana�robie car aucun apport

ult�rieur d'eau ou d'air n'est effectu�. Elle est r�serv�e � l'amateurisme et aux climats frais (meilleure isolation) ou

secs (r�duction des pertes en eau). Sous un climat temp�r�, cette m�thode provoque l'apparition de mauvaises odeurs

(d�composition ana�robique).

En bac

Fabriqu� en bois, en b�ton, en treillis... le bac a un volume d'environ 400 � 1 000�litres (1�m�). Cette capacit� est

g�n�ralement suffisante pour un jardin d'une superficie de 3 � 10�ares. Pour faciliter le travail, 1, 2 ou 3 bacs (ou plus

si n�cessaire...) peuvent �tre construits. Le compost en formation �tant alors transf�r� d'un bac � l'autre pour

permettre une meilleure a�ration. En effet, les mati�res organiques ont tendance � se tasser avec le temps et l'a�ration

� l'int�rieur � diminuer (les processus ana�robiques apportant de mauvaises odeurs). La technique du compostage en

bac/silo(s) est sensiblement la m�me que le tas mais adapt�e � la quantit� de mati�re � traiter. Elle est plus simple �

g�rer et un peu plus propre.

En pavillon

La m�thode de compostage en pavillon (ou chalet) est analogue � la m�thode en bac mais pour des quantit�s plus

grandes, le pavillon ayant g�n�ralement un capacit� sup�rieure � un m�tre cube. Cette m�thode est pertinente pour le

compostage collectif et public, notamment, en milieu urbain, pour le compostage de quartier, � partir des restes

v�g�taux d'au moins une cinquantaine de foyers participants. Il est recommand� que le pavillon soit construit en bois,

sans fond pour que la mati�re � composter soit au contact de la terre, et avec plusieurs compartiments, soit pour

effectuer du transfert d'a�ration, soit pour continuer les apports lorsqu'un compartiment est plein.

En tas

C'est la m�thode de compostage la plus commune. Les d�chets sont rassembl�s en andains de longueur ind�finie et

dont la hauteur d�pend � la fois de la porosit� � l'air du compost (plus elle est �lev�e, type 'P', plus le tas peut �tre

haut) ainsi que de la fr�quence et de la m�thode d'a�ration choisie (une fr�quence �lev�e et/ou une a�ration par

ventilation forc�e autorisent des tas plus importants). � d�faut d'une bonne a�ration, des andins lin�aires de faible

hauteur (20 � 30�cm) sont � privil�gier de fa�on limiter la surchauffe et favoriser l'a�robiose.


En couloir

Cette m�thode est fort semblable � la pr�c�dente, mais les andains sont ici compris entre deux murets lat�raux. Elle

permet parfois une installation plus ais�e des dispositifs d'a�ration mais n�cessite un investissement plus important.

On dispose �galement de moins de flexibilit� pour l'organisation ou la modification du chantier de compostage.

En enceinte close ou digesteur

Le principe commun des proc�d�s de fermentation dite acc�l�r�e est bas�� sur le s�jour plus ou moins rapide des

d�chets dans des dispositifs appel�s digesteurs. Un digesteur est une enceinte ferm�e � l'int�rieur de laquelle il est

possible de contr�ler le d�roulement de la fermentation en agissant essentiellement sur l'a�ration. Les d�chets entrent

en g�n�ral par une extr�mit� du dispositif et ressortent, en fin de fermentation, � l'autre extr�mit�. Le brassage et

l'a�ration des mat�riaux sont le plus souvent r�alis�s en continu.

Silo vertical (tour)

De nombreux dispositifs existent, plus ou moins complexes, mais leur principe reste le m�me. Les d�chets sont

achemin�s, via une bande transporteuse, au sommet de la tour de digestion. Ils descendront soit au moyen de vis sans

fin ou de racleurs en suivant une succession de plateaux, soit par gravit�. � chaque niveau, ou dans la masse du

compost, sont install�s des tuyaux d'a�ration permettant d'oxyg�ner le milieu. En fin de fermentation, le compost est

r�cup�r� � la base de la tour.

Biostabilisateur

Le digesteur est dispos� ici, non plus verticalement, mais horizontalement. Il s'agit en fait d'un cylindre rotatif d'une

longueur de 25 � 35�m�tres et d'un diam�tre de 3 � 4�m�tres. La rotation continue du cylindre, � l'int�rieur duquel

sont fix�es des plaques d�flectrices h�lico�dales, permet d'assurer � la fois le brassage et l'a�ration du produit ainsi

que sa progression vers l'extr�mit� du dispositif. La dur�e de s�jour des d�chets � l'int�rieur du biostabilisateur est de

l'ordre de 4 � 6 jours, apr�s quoi ils sont transf�r�s sur l'aire de maturation.

Mode d'a�ration

Comme nous l'avons d�j� indiqu�, l'a�ration du m�lange en compostage est essentielle durant la phase de

fermentation active. Plusieurs m�thodes existent, mieux adapt�es � l'une ou l'autre m�thode de compostage ou � une

�chelle de travail plus ou moins grande.

A�ration passive et m�thode chinoise

Dans les syst�mes traditionnels de compostage en tas, seule la porosit� de celui-ci assure l'a�ration de la masse. On

est donc limit� � des tas de faibles dimensions et � des composts � porosit� tr�s �lev�e (type 'P', grosses particules).

Les Chinois ont am�lior� ce syst�me en installant des faisceaux de bambous lors de la constitution du tas. Ces

bambous sont ensuite retir�s apr�s 1 ou 2 jours, laissant libres des orifices plongeant jusqu'au milieu du tas et par

lesquels l'a�ration peut se faire plus activement.

Brassage des mat�riaux

L'oxyg�nation la plus efficace d'une masse en fermentation chaude est obtenue par son retournement. Le brassage

complet permet �galement d'assurer une fermentation plus homog�ne de toute la masse, chaque particule �voluant

suffisamment de temps au centre du compost, o� la temp�rature est la plus �lev�e. Entre les retournements, la partie

ext�rieure du tas �volue en a�robiose par a�ration passive (voir paragraphe pr�c�dent) pendant que le taux d'oxyg�ne

au centre du tas diminue rapidement. La fr�quence et la qualit� des retournements sont donc les param�tres

fondamentaux de cette technique.


Selon la dimension du chantier de compostage, le brassage se fera � la fourche (main d'�uvre manuelle), au moyen

d'un engin de travaux publics (pelle chargeuse sur pneus), ou au moyen de machines sp�cialis�es.

A�ration active par soufflerie

Contrairement aux techniques pr�c�dentes, l'apport d'oxyg�ne pendant la fermentation est ici continu. Les andains �

a�rer recouvrent un r�seau de tuyauteries perfor�es sur toute leur longueur et reli�es � un surpresseur. La puissance

du surpresseur est fonction du volume et du tassement de la masse � a�rer.

A�ration active par aspiration (m�thode Beltsville)

L'apport d'air frais est r�alis� ici par aspiration au travers des andains suivant un sch�ma identique � celui de la

m�thode pr�c�dente (si ce n'est que le surpresseur est remplac� par un aspirateur). Le dispositif par aspiration est �

pr�f�rer � celui par soufflerie car l'air aspir� a moins tendance � emprunter des chemins pr�f�rentiels, ce qui serait

pr�judiciable � l'efficacit� de l'a�ration. On pr�voira cependant un filtre, qui peut �tre simplement un tas de compost

mature, � la sortie de l'aspirateur afin d'�liminer les odeurs. La ventilation peut �tre combin�e, si on le d�sire, avec le

brassage des mat�riaux d�crit plus haut.

Compost de broussailles

Description et avantages

Branche broy�es pouvant entrer dans la

composition d'un compost de broussailles.

Le compost de broussailles est un proc�d� de compostage mis au point

dans les ann�es 1960 par Jean Pain, utilisant du bois ram�al fragment�.

Ce proc�d� permet :

� d'entretenir et de valoriser le patrimoine forestier ;

� d'�liminer les broussailles (r�duction du risque d'incendie) ;

� de valoriser celles-ci comme ressource naturelle en produisant :

� un excellent compost ;

� du biogaz utilisable aussi bien pour la cuisine que comme

carburant pour v�hicules, � partir d'une biomasse rapidement

renouvelable ;

� de chauffer de l'eau jusqu'� une temp�rature plus que suffisante pour les besoins d'une maisonn�e, sans pollution.

Jean Pain calcule que 1 000�hectares de for�t peuvent fournir chaque ann�e 6 000�tonnes d'engrais, 960 000�m�tres

cubes de biogaz et des millions de litres d'eau chaude. Exploiter la for�t dans ce sens, ne demande que 12 % de

l'�nergie que l'on recueille. Enfin, la broussaille se renouvelle tous les sept ans. L'op�ration s'inscrit donc

parfaitement dans le cadre des �nergies renouvelables et du d�veloppement durable. Le syst�me complet mis au

point par Jean Pain est sch�matis� ici [3].

Critiques

Une critique de la m�thode Jean Pain est que la pr�paration de la mati�re premi�re n�cessite le retournement

m�canique des empilements pour homog�n�iser correctement le processus d'humification. Les canadiens Guay,

Lachance et Lapointe ont subs�quemment mis au point la m�thode dite "sylvagraire" (voir bois ram�al fragment�),

qui en combine deux plus anciennes : celle de Jean Pain, et celle du compost de surface. Le but est d'obtenir les

r�sultats de Jean Pain en �vitant les pertes et les frais dus aux travaux de retournements m�caniques[4].


D�veloppements

Le Comit� Jean Pain �uvre � faire connaitre cette technique, avec un chantier-pilote de traitement des broussailles et

du bois d'�lagage � Londerzeel (province du Brabant flamand) et deux autres antennes en Belgique. Ce centre

propose des formations rapides de Maitre Composteur.

D�termination de la fin du compostageUn bon compost est un produit dont les constituants organiques ont subi une conversion biologique en des

substances moins agressives et plus stables. Les processus de d�gradation persistent cependant � un taux plus r�duit

au-del� m�me de la phase de fermentation. Il faut donc savoir quand et pour quel usage on pourra utiliser un compost

sans risque de phytotoxicit�.

Un compost frais, c�est-�-dire ayant subi un d�but de fermentation (de l'ordre de deux semaines), pourra �tre utilis�

en paillage (mulching) ou en champignonni�res. En fin de fermentation, le compost est stabilis� et pourra servir

comme amendement organique. Une utilisation comme substrat de culture requiert quant � elle un compost ayant

subi une longue p�riode de maturation (d'autant plus longue que les plantes sont sensibles : jeunes semis, laitue...).

�volution de la temp�rature

Un moyen d�roulement du processus de compostage consiste, comme mentionn� pr�c�demment, � utiliser des

sondes thermom�triques plongeant dans la masse en fermentation. Cette m�thode donne des informations sur le stade

de fermentation mais peu sur le niveau de maturit� du compost. Elle devra donc �tre compl�t�e par une ou plusieurs

autres m�thodes. La m�thode standardis�e pour mesurer le degr� de stabilit� d'un compost en fonction de la

temp�rature est le Rottegrade.

Chromatographie circulaire sur papier

Elle fut mise au point par Pfeifer (Biochemical Research Laboratory, Spring Valley - �tats-Unis) et test�e par

Hertelendy. Son principe est bas� sur le fait que des substances diff�rentes, dissoutes dans un m�me solvant (solution

de soude � 5 %), pr�sentent des affinit�s variables de migration capillaire sur une surface absorbante (feuille ronde

de papier filtre). Cette surface absorbante est pr�trait�e au nitrate d'argent, qui sert de r�v�lateur. La solution alcaline

de compost est apport�e goutte-�-goutte au centre du papier filtre. Apr�s migration de la solution, on laisse le

chromatogramme se d�velopper en lumi�re att�nu�e. Un compost peu �volu� est caract�ris� par une image sombre �

la p�riph�rie et une tache centrale claire ; un compost m�r montre au contraire une tache centrale sombre avec

tra�n�es claires vers la p�riph�rie.

Germination de plantes test

Il s'agit incontestablement de la m�thode la plus simple et la plus fiable. Les plantes test les plus couramment

utilis�es sont le cresson al�nois (Lepidium sativum) et la laitue (Lactuca sativa). Des graines de la plante test sont

sem�es sur le compost humidifi� en bocal herm�tiquement ferm�. Apr�s trois jours, la maturit� est �valu�e d'apr�s le

pourcentage de germination et, �ventuellement, la quantit� de mati�re verte obtenue.

Autres m�thodes

De nombreuses autres m�thodes existent, plus ou moins rapides, plus ou moins fiables, n�cessitant un appareillage

plus ou moins co�teux et un personnel plus ou moins qualifi�. Parmi celles-ci :

�� des crit�res empiriques - couleur plus ou moins sombre, aspect du compost au toucher, odeur...

� des m�thodes respirom�triques - O2

consomm�, CO2

�mis...

� des m�thodes physico-chimiques - dosage de la Demande Chimique en Oxyg�ne, dosage des formes de l'azote

min�ral (NH4

+ et NO3

-), dosage de la S.O.D et de la S.O.R. (Substance Organique D�composable et Substance


Organique R�sistante), mesure du pH...

� des m�thodes biologiques - dosage de l'ATP (Ad�nosine Tri-Phosphate), croissance de plantes ou de

micro-organismes tests...

Usages du compostLe compost peut �tre utilis� comme engrais sur prairie ou avant labour. Son usage am�liore la structure des sols

(apport de mati�re organique et amendement), ainsi que la biodisponibilit� en �l�ments nutritifs (azote, phosphore,

potasse). Il augmente �galement la biodiversit� de la p�dofaune.

Au jardin, il sert � fertiliser les plates-bandes, les arbres fruitiers et le potager. Il peut �galement �tre utilis� comme

terreau pour les plantes en pot et pour faire du nitrate de potassium (salp�tre). Il peut �tre extrait pour y multiplier les

micro-organismes et les transporter ainsi dans un liquide. Le but �tant alors de pulv�riser sur les parties foliaires des

cultures et cr�er une concurrence et une pr�dation contre les maladies (cryptogamiques ou bact�riennes) par action

pr�ventive ou curative. Le jus de compost peut aussi �tre arros� sur les cultures ; il participe alors � la diminution de

la fr�quence et des quantit�s astronomiques de compost sur les cultures de plusieurs hectares de SAU. Les

micro-organismes transport�s dans le sol vont entre autres aider � d�grader la mati�re organique pr�sente dans le sol

et dig�rer les pollutions.

Normes de qualit�Selon les pays et les �poques, des normes plus ou moins dures existent pour garantir que le compost commercialis�

ne pose pas de probl�mes sanitaires, toxicologiques ou �cotoxicologiques, c'est-�-dire qu'il n'ait pas �t� produit avec

des substances contenant des polluants non-biod�gradables et non biod�grad�s en quantit� excessive.

Pour FNE, la norme NF U 44-051 en vigueur seulement depuis mars 2009 pour le compost fran�ais n'offre pas de

garanties d'innocuit� et prot�ge mal les sols et l'environnement, car trop laxiste et en raison de l'absence de tri des

biod�chets � la source. Les d�chets mal tri�s et compost�s peuvent contenir des m�taux, des m�dicaments, des

r�sidus de pesticides, des cendres riches en m�taux lourds, pcb, dioxines, etc.

�� Par exemple, en France, le cadmium (puissant toxique r�nal) est autoris� jusqu'� 3 mg/kg, alors que la plupart des

autres pays le limitent � 0,7 � 1,5 mg/kg de mati�re s�che, et alors qu'en Europe, on en trouve 0,5 mg/kg en

moyenne dans le compost.

�� Une �tude de la Commission europ�enne publi�e en 2010 alerte sur le fait qu'utiliser un tel compost durant 25 ans

conduirait � polluer au del� des seuils tol�rables les sols en 50 ans. Pour le cuivre et le mercure, cette p�riode ne

serait que de 25 ans.

�� De m�me la France autorise-t-elle 2 % de verre et m�taux, 1,1 % de plastiques, soit jusqu'� 5 kg de verre/m�taux

et 2,7 kg de plastiques par m�tre cube. Ces produits peuvent notamment affecter les vers de terre et contaminer les

plantes cultiv�es.

Notes et r�f�rences[1] Michel Mustin, Le compost : Gestion de la mati�re organique.,Eds: Fran�ois Dubusc, Paris, 1987

[2] http:/ / www. actu-environnement. com/ ae/ fournisseur/ fiche/ seau_cuisine_compostage_bokashi_consomacteurs_associes_1080. php4

[3] http:/ / bonne-eau-bonne-terre. over-blog. com/ article-17340761. html

[4] Le bois ram�al et la p�dog�n�se: une influence agricole et foresti�re directe (http:/ / www. sbf. ulaval. ca/ brf/ bois_rameal. html), par Gilles

Lemieux, Professeur au D�partement de Sciences du Bois et de la For�t, Facult� de foresterie et de g�omatique, Groupe de Coordination sur

les Bois Ram�aux; et Alban Lapointe, Minist�re de l'�nergie et des Ressources (For�ts), Qu�bec.

3.1. Introduction

The biological cycling of nutrients is indispensable for life and is mediated throughmicroorganisms. Biotransformation is biological modification that alters the chemicalstructure. The prefix bio indicates biotic (living organism) activities (not, for example,the action of UV light or the action of isolated enzymes). Biotransformation may syn-thesize atoms or simple molecules into more complex molecules (biosynthesis), or viceversa (biodemolition, biodegradation, mineralization). Biodegradation is the breakdownof a molecular structure into its elemental components. Biodegradation may be a seg-mentation of complex into simpler compounds, or even atoms; this segmentation usuallyis not a simple fragmentation, but often other atoms also are incorporated to form newcompounds.

3.2. Definition of Composting (Process) and of Compost (Product)

The presence of mixed organic substrates is a prerogative of composting. More specif-ically, according to its etymological meaning, composting (from the Latin compositum,meaning mixture) refers to a biodegradation process of a mixture of substrates carriedout by a microbial community composed of various populations in aerobic conditions andin the solid state. Microbial transformation of pure substrates goes under the name offermentation or biooxidation, but not composting.

The exothermic process produces energy in the form of heat, which results in anincrease of the temperature in the mass. A spontaneous process, therefore, passes througha thermophilic phase, which is preceded and followed by two mesophilic phases. Duringcomposting there is a temporary release of phytotoxins (intermediary metabolites, ammo-nia, etc.). At the end of the process, this phytotoxicity is completely overcome and thefinal product is beneficial to plant growth. The composting process leads to the final pro-duction of carbon dioxide, water, minerals, and stabilized organic matter (compost). Theprocess starts with the oxidation of easily degradable organic matter; this first phase iscalled decomposition. The second phase, stabilization, includes not only the mineraliza-tion of slowly degradable molecules, but also includes more complex processes such asthe humification of ligno-cellulosic compounds.

From a technical point of view, the composting process is stopped at a phase in whichthe organic matter still is present in a relatively large quantity (more than 50% of the start-ing amount); otherwise the process would continue, environmental conditions permitting,until all of the organic components are completely mineralized.

The main product is called compost, which may be defined as the stabilized and san-itized product of composting, compatible and beneficial to plant growth. Compost hasundergone: (1) an initial, rapid stage of decomposition; (2) a stage of stabilization; and(3) an incomplete process of humification.

The transformation of fresh organic matter into compost is carried out mainly for threereasons: (1) to overcome the phytotoxicity of fresh non-stabilized organic matter; (2) toreduce the presence of agents (viruses, bacteria, fungi, parasites) that are pathogenic to

MICROBIOLOGY OF THE COMPOSTING PROCESS

1

Microbiology of the composting process

man, animals, and plants to a level that does not further constitute a health risk; and (3) toproduce an organic fertilizer or a soil conditioner, recycling organic wastes and biomass.

Many adjectives have been used for compost; some of them are correct, such as: aero-bic, solid state, hygienized, and quality. Some others are in contradiction with the definitionof compost and include: anaerobic, fresh, liquid state, etc., and thus should be avoided.

3.3. The Substrates

The decay of materials during composting follows the common biochemical pathways ofany other degradation process. Usually, the substrates are biogenic, i.e., they originate frombiological activity (e.g., photosynthesis, or consumer biomass). That means, essentially,that all available substrates are either of plant, animal, or microbial origin. Generally, plantmaterials make up the highest amounts, while animal tissues or microbial components areonly minor fractions of any mixture, but usually are the most nutrient-rich fractions. Themajor natural compounds found in compost substrates are listed in Table 3.1.

3.3.1. Lignin

Lignin is a major structural component of plants, and is the one that is degraded the slowest.The lignin content of wood varies from about 18 to 30%. The number of monomer unitsis not large, but due to the extraordinary variety of bonding among its basic monomercompounds (derivatives of phenylpropane, mainly coniferyl alcohol), its degradation isvery complex. Very often, lignin decomposition is of the co-metabolic type, since energyyield from lignin degradation is negligible.

The degradation of lignin is primarily accomplished by fungi, which are very oftenpathogens that thrive on the living plant. Lignin-degrading fungi are also known as white-rot fungi, like Trametes versicolor (Turkey Tail) or Stereum hirsutum (False Turkey Tail).They degrade the lignin and leave the pale-colored cellulose parts. Some fungi, such asPleurotus ostreatus, degrade cellulose and lignin at the same time.

3.3.2. Cellulose

Cellulose is the most abundant component of plants. Cellulose is found in almost everytype of organic waste. It is most prominent in wastes that are dominated by plant remainswith a high percentage of structural elements (wastes from the wood industry, agriculturalwaste, and domestic wastes). Cellulose molecules are chains of β-d-glucose with a poly-merization degree of 40,000. The glucose molecules are combined by β-1,4-glycosidicbonding. Enzymatic cleavage results from the activity of three enzymes:

1. Endo-β-1,4-Glucanases cleave the β-1,4 bonds within the molecule, resulting in longchains with free ends.

2. Exo-β-1,4-Glucanases separate the disaccharide cellobiose from the free ends.3. Beta-glucosidases hydrolyze the cellobiose, and the resulting glucose is taken up by

the microorganisms.

2

Tabl

e3.

1.M

ajor

natu

ral

com

poun

dsth

atar

eth

esu

bstr

ates

for

deco

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Under aerobic conditions, many fungi, bacteria, and also myxomycetes are involved inthe degradation of cellulose. The catalytic action (mechanical destruction of large structuralelements) of the microfauna is considered important. Fungi are, in general, more impor-tant for cellulose degradation than bacteria, which is especially the case if the cellulose isencrusted with lignin (e.g., in wood or straw). Since cellulose is rich in C but does not con-tain N or other essential elements, the mycelial structure of fungi is a competitive advan-tage. A few fungi to mention are Chaetomium, Fusarium, and Aspergillus. Among thebacteria, it is mainly the group of myxomycetes or related taxonomic groups (Cytophaga,Polyangium, Sorangium). Also, Pseudomonas and related genera are known to degradecellulose, but only few Actinobacteria (formerly called Actinomycetes) are involved.

Under anaerobic conditions, cellulose is mainly degraded by mesophilic and ther-mophilic Clostridia.

3.3.3. Hemicelluloses

Xylan, among the hemicelluloses, is the most important and is found in straw, bagasse(up to 30%), and wood (2–25%). Xylan is made up of pentoses (xylose, arabinose) orhexoses (glucose, mannose, galactose). The degree of polymerization is 30–100. The maindegrading enzymes are xylanases, produced by many bacteria and fungi (in some cases,constitutively).Pectin (polygalacturonides) is made up from unbranched chains of polygalacturonic acid.It is degraded by pectinase, which is very common among fungi and bacteria. Many plantpathogens produce pectinases.Starch is composed of amylose (20%) and amylopectine. Amyloses are unbranched chainsof d-glucose (due to 1,4-position β-glycosidic bonding amylose is, in contrast to cellu-lose, helical). Amylopectin is, in addition, branched at the 1,6 position and containsphosphate residues and Ca and Mg ions. Two types of enzymatic starch degradation areimportant:

• Phosphorolysis by phosphorylases, starting at the free, non-reducing end of the amylosechain, releasing single glucose-1-phosphate molecules. At the 1,6 branches, the enzymecomes to a halt, and only continues after action of amylo-1,6-glucosidase.

• Hydrolysis: α-amylase cleaves the α-1,4 bonds within the molecule.

3.3.4. Murein

Murein consists of unbranched chains of N -acetylglucosamine and N -acetylmuramic acid.Muramic acid is bound through lactyl groups to variable amino acids. Murein is the maincomponent of the cell wall of most bacteria.

3.3.5. Chitin

Chitin is, considering the masses, less important than cellulose. Chemically, celluloseand chitin are very similar. While the monomer of cellulose is glucose, the monomer of

5


chitin is N -acetylglucosamine. The main difference for degraders is the high concentrationof nitrogen in chitin (approximately 7% N, the C/N of chitin is approximately 5).

Many fungi (e.g., Aspergillus) and bacteria (e.g., Flavobacterium, Cytophaga,Pseudomonas) are able to use chitin as a nitrogen and carbon source. Chitin isdegraded through exoenzymes to N -acetylglucosamine, which is resorbed, transformedto fructose-6-P, and thus incorporated into the carbohydrate metabolism.

Chitin is the most important structural compound in the cell walls of fungi, and it is thesubstance that makes up the exoskeleton of insects and crustaceans. In areas with shellfishindustry, chitin is an important waste product.

3.4. Composting as a Discontinuous Process

Degradation of organic compounds under natural conditions usually occurs in soils andsediments, on the soil surface, or in water bodies. In most cases, the decomposing sub-strates have physical contact with the degraded material, or with an external matrix. Thus,an exchange of nutrients of the degrading material and the matrix (e.g., soil, sediment, orwater) is usually possible, and due to the dispersal of the degrading material, the decom-position takes place at ambient temperature. There are exceptions to this, for example, theaccumulation of leaves during the fall when exothermal processes during decompositionare strong enough to elevate the temperature of the material. Notably, it is mostly humanactivities that accumulate substrates at a certain place to an extent that allows self-heating,which is a typical feature of any composting process. Since any chemical or biochemicalreaction is temperature dependent, many chemical, physical, and biological propertiesalso change during the process (Fig. 3.1).

The present level of understanding of the microbiota involved in composting dependslargely on studies made with traditional methods, e.g., isolation and identification of bacte-ria, including actinobacteria, and fungi (Miller, 1996). Composting induces high metabolicactivities of microorganisms at high densities (up to 1012 cells g−1). The constant change inconditions (temperature, pH, aeration, moisture, availability of substrates) results in stagesof exponential growth and stationary phases of various organisms. The microbial consortiapresent at any point of time are replaced by others in short intervals. The problem, however,is that despite their viability, only a minor fraction of the microbes can be cultivated.

Biodegradation processes in nature are commonly comparable to a continuous culture(from a microbiological view), and the most important determinant factors are external(substrate quality, temperature, moisture, etc.). Composting, in contrast, resembles a batchculture with steady changes in substrate composition and biochemical conditions. Contin-uous composting processes may be regarded as a sequence of continuous cultures, each ofthem with their own physical (e.g., temperature), chemical (the available substrate), andbiological (e.g., the microbial community composition) properties and feedback effects.These changes make it difficult to study the process, which virtually is impossible to sim-ulate in the laboratory since temperature, aeration, moisture, etc. are directly related tothe surface/volume ratio. However, it is generally accepted that composting is essentiallya four-phase process that may be summarized as follows.

6

0

10

20

30

40

50

60

70

80

90 easily degradable -------------------------- recalcitrant

mainly liquid --------------------------------mainly solid

Fungal flora

Bacterial flora

Total microbial community

Temperature

Tem

pera

ture

(°C

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Fig. 3.1. Microbial communities during the composting process: temperature feedback.

3.4.1. Mesophilic Phase (25–40◦C)

In this first phase (also called starting phase), energy-rich, easily degradable compoundslike sugars and proteins are abundant and are degraded by fungi, actinobacteria, and bac-teria, generally referred to as primary decomposers. Provided that mechanical influences(like turning) are small, also compost worms, mites, millipedes, and other mesofaunadevelop, which mainly act as catalysts. Depending on the composting method, the contri-bution of these animals is either negligible or, as in the special case of vermicomposting,considerable. It has been demonstrated that the number of mesophilic organisms in theoriginal substrate is three orders of magnitude higher than the number of thermophilicorganisms, but the activity of primary decomposers induces a temperature rise.

3.4.2. Thermophilic Phase (35–65◦C)

Organisms adapted to higher temperatures get a competitive advantage and gradually, andat the end, almost entirely replace the mesophilic flora. Previously flourishing mesophilicorganisms die off and are eventually degraded by the succeeding thermophilic organisms,along with the remaining, easily degradable substrate. The decomposition continues tobe fast, and accelerates until a temperature of about 62◦C is reached. Thermophilic fungido have growth maxima between 35 and 55◦C, while higher temperature usually inhibitsfungal growth. Thermotolerant and thermophilic bacteria and actinobacteria are known to

7


0

1

2

3

−10 0 10 20 30 40 50 60 70

Temperature in °C

Gen

erat

ions

per

h

Psychrophile

Mesophile

Thermophile

Fig. 3.2. Temperature range of psychrotolerant, mesophilic, and thermophilic organisms, and theirgeneration time.

remain active also at higher temperatures. Despite the destruction of most microorganismsbeyond 65◦C, the temperature may rise further and may exceed 80◦C. It is probable thatthis final temperature rise is not due to microbial activity, but rather is the effect ofabiotic exothermic reactions in which temperature-stable enzymes of actinobacteria mightbe involved. The temperature range of psychrotolerant, mesophilic, and thermophilicorganisms and their generation times are shown in Fig. 3.2.

The same temperatures are not reached in all zones of a compost pile; thus, it isimportant that, through regular turning, every part of the substrate is moved to the central,hottest part of the pile. From a microbiological point of view, four major zones may beidentified within a pile (as shown in Fig. 3.3). The outer zone is the coolest, and wellsupplied with oxygen; the inner zone is poorly supplied with oxygen; the lower zone is hot,

outer zone

upper zone

inner zone lower zone

Fig. 3.3. Cross section of a compost windrow (major zones and convection stream are indicated).

8

and well supplied with oxygen; while the upper zone is the hottest zone, and usually fairlywell supplied with oxygen.

The thermophilic phase is important for hygienization. Human and plant pathogensare destroyed; weed seeds and insect larvae are killed. Not only the temperature dur-ing the thermophilic phase, but also the presence of a very specific flora dominated byactinobacteria, are important for hygienization through the production of antibiotics. Thedisadvantage of temperatures exceeding 70◦C is that most mesophiles are killed, and thusthe recovery is retarded after the temperature peak. This may, however, be avoided byappropriate measures for recolonization.

3.4.3. Cooling Phase (Second Mesophilic Phase)

When the activity of the thermophilic organisms ceases due to exhaustion of substrates, thetemperature starts to decrease. Mesophilic organisms recolonize the substrate, either origi-nating from surviving spores, through spread from protected microniches, or from externalinoculation. While in the starting phase organisms with the ability to degrade sugars,oligosaccharides and proteins dominate, the second mesophilic phase is characterized byan increasing number of organisms that degrade starch or cellulose. Among them are bothbacteria and fungi.

3.4.4. Maturation Phase

During the maturation phase, the quality of the substrate declines, and in several successivesteps the composition of the microbial community is entirely altered. Usually, the propor-tion of fungi increases, while bacterial numbers decline. Compounds that are not furtherdegradable, such as lignin–humus complexes, are formed and become predominant.

9

La méthanisation

Qu’est-ce que la méthanisation ?

La méthanisation est un procédé biologiquede transformation de la matière organique enbiogaz, par l’action de bactéries.

Le procédé se déroule en plusieurs étapes,avec des bactéries adaptées à chaque étape,exactement comme pour la digestion desaliments :

• l’hydrolyse, qui transforme les moléculescomplexes (ce l lu lose , l i p ides ,protéines…) en molécules plus simples(acides gras…) ;

• l’acidogénèse, qui transforme cesacides en acide acétique, en gazcarbonique et en hydrogène ;

• la méthanogénèse, qui transformel’acide acétique en méthane et gazcarbonique, et le gaz carbonique etl’hydrogène en méthane.

D’où viennent ces bactéries ?

Il s’agit de bactéries que l’on retrouve àl’état naturel : par exemple dans lesintestins, au fond des marais, dans la vase,le fumier… Ce sont ces bactéries quipermettent la digestion des aliments, etelles vivent sans oxygène : on parle de« digestion anaérobie ».

Il n’y a aucun besoin d’ajouter des bactéries :elles sont déjà présentes dans les déjectionsanimales et se développent spontanémentlorsque les conditions sont remplies.

Que peut-on digérer ?

Le produits « digestibles » sont les lisiers,fumiers , les végétaux , des déchets del’agro-alimentaire, des boues, des déchetsde cuisine, des graisses , et d’une façongénérale tout produit… digestible !

Un digesteur fonctionne comme un estomacou le rumen d’une vache. Il peut digérer de lapaille, par exemple, si elle est renduedigestible en ajoutant de l’ammoniaque (quel’on trouve dans les déjections).

Photo SCHMACK

Fiche n° 2. TECHNIQUES DE METHANISATION

A quoi ressemble une unité deméthanisation ?

Le digesteur est constitué d’un réservoirétanche, en béton ou en acier, où lesmatières à digérer séjournent plusieurssemaines : c’est le digesteur. C’est le cœur duprocédé, ou plus exactement son estomac.

La méthanisation se déroule classiquement à37°C (mode dit « mésophile »), et pluslargement entre 20 et 6O°C. Pour que lesbactéries colonisent l’ensemble de la masseen fermentat ion, i l est nécessaired’homogénéiser le produit.

Un digesteur est donc généralement chaufféet brassé.

Quels sont les principaux types dedigesteurs ?

Les digesteurs-fosse : ce sont desfosses à lisier classiques, couvertespar une membrane étanche etthermiquement isolante. Photo SOLAGRO

Les digesteurs silos : ce sont des cuvesverticales, en acier ou béton. Photo SOLAGRO

Les digesteurs horizontaux, en acier. PhotoSOLAGRO

Substrat àtraiter

Mélangeur

Digestat

Biogaz

Stockagebiogaz

Produitdigéré

Module technique

Elec.

Le digesteur fosse est équipé d’un pilier central, quisupporte une structure en bois sur laquelle estposée une membrane isolante. Photo Agrikomp

Fiche n° 3. TECHNIQUES DE METHANISATION

Alimentation et extraction

L e s d i g e s t e u r s« modernes » sont àfonctionnement continu : leproduit est introduit de façonrégulière par exemple parp o m p a g e ( s u b s t r a t sliquides), ou par trémie etpompe hacheuse (fumiers)voi r pompe à béton(substrats solides).

Une quantité de matièreéquivalente est extraite parsurverse.

Brassage

Le brassage estessentiel : il permetd’homogénéiser lesubstrat, et d’éviter laf o r m a t i o n d ’ u nchapeau de massesolidifiée à la surface,ce qui empêcheraitl ’ é v a c u a t i o n d ubiogaz.

Il existe différentest e c h n i q u e s d eb r a s s a g e , q u ip e u v e n t ê t r ecomplémentaires.

Préfossed’alimentation avecbrassage. Photo SOLAGRO

Extraction du digestat par pompage et tonne à lisier.Photo SOLAGRO

Introduction de substrat solide par trémie. PhotoSOLAGRO

Mélangeur à grandes pales sur axe incliné. Photo

Agrikomp.

Agitateur sur axe àhauteur réglable. Photo Solagro.

Mélangeur à grandes pales sur axe horizontal.Photo Agrikomp

Fiche n° 4. LE BIOGAZ

Qu’est-ce que le biogaz ?

La méthanisation produit du biogaz,contenant environ 60% de méthane et 40%de gaz carbonique. Le méthane est leprincipal constituant du gaz naturel (gaz deLacq, de Groningue, d’Algérie…).

1 m3 de biogaz possède un pouvoir calorifiqued’environ 6 kWh soit l’équivalent énergétiquede 0,6 litre de fioul.

Combien produit-on de biogaz ?

La production de biogaz varie entre 15 m3 partonne de lisier, à 50 m3 par tonne de fumier.

Elle dépend de la teneur en matièresdigestibles , qui représentent souvent lamoitié des matières sèches d’un substrat. 1tonne de matière digérée à 100% produit 500m3 de méthane.

1 tonnede…

m3 d ebiogaz

Equivalentl i t re defioul

KWhélec.

Lisier 16 11 30

Fumier 60 35 100

Paille 220 120 350

Graisse 450 350 1000

Faut-il stocker le biogaz ?

Il faut 1.000 m 3 (ballon souple par exemple)pour stocker l’équivalent de 700 litres de fioul.Le stockage représente généralementquelques heures de fonctionnement dudigesteur, car sinon son coût devient prohibitif.

Le stockage sous pression permettrait deréduire ce volume, mais la compression estcoûteuse et consomme de l’énergie. Cetteoption est réservée aux installations trèsimportantes.

Le biogaz doit être utilisé pratiquement au furet à mesure de sa production.

Une installation de méthanisation possède engénéral une capacité tampon, qui correspondà quelques heures de production :

• Soit intégrée au digesteur ou à lafosse de stockage du digestat

• Soit en ouvrage séparé (ballon souple)

Le biogaz est-il dangereux ?

Le biogaz est explosif, corrosif et toxique(présence d’hydrogène sulfuré), et unminimum de précautions doit être pris pouréviter la dégradation rapide des matériels etles risques pour les personnes : appareilsélectriques adaptés près de la « zone gaz »,surveillance des fuites, matériaux noncorrosifs.

Lorsque ces précautionsé lémen ta i r es son tprises, le risque est trèsfaible.

Les quantités d’énergies t o c k é e s s o n tcomparables à cellesqui sont stockées dansune cuve de fiouldomestique.

Le stockage sous une légère surpressionempêche toute infiltration d’air, et donc touteformation de mélange explosif.

En cas de fuite et d’incendie, une flamme sedéveloppe au point de fuite mais le biogazn’explose pas.

Soupape desécurité. PhotoAGRIKOMP

Exemple de stockage de gaz intégré au digesteur.Photo AGRIKOMP

Fiche n° 5. LA VALORISATION DU BIOGAZ

Quelles sont les utilisations dubiogaz ?

Le biogaz peut être utilisé pour produire, parexemple :

• de l’eau chaude (chaudière) ;

• de l’air chaud (brûleur en veine d’air ourécupération sur gaz d’échappementmoteur ou chaudière) ;

• de l’électricité par moteur. Le rendementd’un moteur est d’environ 30%. 1 m3 debiogaz produit 2 kWh électrique.

• la cogénération consiste à utiliser lachaleur du moteur pour produire del’eau chaude. Pour 1 kWh électrique, onpeut récupérer 1,5 kWh de chaleur.

L’utilisation la plus fréquente du biogaz est lacogénération : production d’électricité etrécupération de la chaleur pour chauffer ledigesteur et les bâtiments voisins : cesconsommations de chaleur sont en générallimitées, et on n’utilise que rarement la totalitéde la chaleur disponible.

Quels types de moteur utiliser ?

• Moteurs à gaz : fonctionnent à 100%biogaz, investissement e t coûtsd’entretien plus élevés, rendement plusfaible (compter 25% pour un moteur demoins de 50 kWél)

• Moteur dual-fioul : moteurs dieselfonctionnant avec un mélange 90%biogaz et 10% fioul. Meilleur rendement,investissement et coûts d’exploitationplus faible, mais achat de fioul. Pourcertains moteurs, on peut envisager deremplacer le fioul par de l’huilecarburant.

Faut-il chauffer le digesteur ?

Les bactéries travaillent à la mêmetempérature que le corps humain, autour de37°C. Les digesteurs doivent être isolésthermiquement.

Une partie du biogaz (environ 20 à 30%) estutilisée pour maintenir cette température. Engénéral, le chauffage s’effectue par unéchangeur à l’intérieur du digesteur.

Chauffage par réseau de tubes ; agitation par unmalaxeur mécanique. Photo AGRIKOMPGroupe électrogène Gaz. Photo SOLAGRO

Fiche n° 6. LE DIGESTAT

Quelles sont les propriétés duproduit digéré ?

Le produit digéré - ou digestat - contient lamatière organique non biodégradable(lignine…), les matières minérales (azote…) etl’eau.

Il peut être stocké et manipulé sans odeursnauséabondes par rapport à un produit nontraité.

Sa valeur fertilisante n’est pas dégradée.Seule la fraction rapidement putrescible de lamatière organique est transformée en gaz, lafraction ligneuse qui contribue à laformation de l’humus n’est pas attaquée.

L’azote est majoritairement sous formed’ammoniaque : plus facile à gérer quel’azote organique, mais aussi plus volatile. Lavalorisation agronomique doit donc tenircompte de ces propriétés pour en tirer lemeilleur parti.

La méthanisation n’est pas un moyen de« détruire » l’azote, mais un procédéconservatif de l’azote et qui constitue un outilpour améliorer la gestion de l’azote.

Est-il intéressant de tamiser ledigestat ?

Si l’on tamise le digestat, on obtient un produitsolide qui contient l’essentiel de la matièreorganique stable, et une bonne part duphosphore.

Ce produit est utilisable comme amendementde fond (restauration de l’humus, relargageprogressif de faibles quantités d’azote).

La fraction liquide contient au contraire peu dematière organique, mais l’essentiel del’ammoniac. Elle est utilisable comme engraisliquide (effet fertilisant immédiat), enremplacement des engrais minéraux azotés.

Photo SOLAGRO

Fiche n° 7. DONNEES ECONOMIQUES

Quel est l’investissement pour uneunité de méthanisation ?

Pour des exploitations produisant plus de300 t de matières sèches (MS) par an(déjections animales + co-substrats), avec desteneurs en matière sèche à partir de 10-12%(valeur considérée comme minimum pour laréalisation d’une étude de faisabilité dans lesconditions actuelles), l’investissement estd’environ 700 € par t de MS.

On compte environ 300 à 400 Euro par m3 ded i g e s t e u r , p o u r l e poste« méthanisation ».

L’investissement pour un groupeélectrogène adapté au biogaz est del’ordre de 1.200 à 1.800 Euro par kWinstallé.

Ajouter 20 à 30% de coûtssupplémentaires divers.

Attention, il ne s’agit que d’ordres degrandeur. Les investissements peuvent varierconsidérablement d’un projet à l’autre, parexemple en fonction de la configuration deslieux (distances de canalisations), de la teneuren matière sèche.

Quels sont les coûts defonctionnement ?

La conduite nécessite généralement entre 10et 30 minutes par jour.

L’entretien représente environ 2 à 3% del’investissement hors groupe électrogène, plusenviron 5 à 10% de l’investissement dugroupe électrogène.

Globalement, le coût d’exploitation est del’ordre de 5 à 10 € / m3 de produit à digérer,ou encore de 50 à 90 € / MWh (1 MWh =1.000 kWh).

Quelles sont les recettes ?

En France, la loi fait obligation au distributeurd’acheter l’électricité produite à partir dubiogaz. Le producteur peut souscrire :

• Soit un contrat d’achat « petitesinstallations » pour des puissances demoins de 36 kVA. Le prix d’achat est

égal au prix de vente : environ75 € / MWh pour un tarif de base.

• Soit un contrat d’achat « biogaz deméthanisation, pour des installationsde puissance supérieure. Le tarif estalors d’environ 50 € / MWh.

Quelle est la rentabilité d’une unitéde méthanisation ?

La rentabi l i té des instal lat ions deméthanisation repose, en Allemagne par

exemple, sur les prix d’achat de l’électricité.

Le développement du biogaz à la ferme enFrance nécessite donc des complémentsde revenus pour ces installations :

Valorisation aussi poussée que possiblede la chaleur (qui représente l’équivalent,en quantité, de l’électricité produite).

Co-digestion , c’est-à-dire traitementcombiné des déjections d’élevage, et derésidus de l ’agro-al imentaire avecrémunération de l’agriculteur pour uneprestation de traitement.

Soutien public

Les subventions

Les études peuvent être subventionnées(jusqu’à 70% par l ’ADEME), et lesinvestissements bénéficient de subventionsmaximales de 60% à 75%, dont 30% del’ADEME.

Le surcoût peut être minimisé en fonction ducontexte local, par exemple en intégrant l’unitéde méthanisation dans la mise aux normes del’exploitation.

Dans les condit ions actuelles, nousconsidérons qu’il est possible de rentabiliserune installation de biogaz à la ferme en 10ans, à condition de disposer de 300 tonnesde matières sèches, dont une partie desubstrats payants.

Une telle installation couterait 200.000 €, etbénificierait normalement de 50% desubventions. La puissance installée serait de35 kW, l’électricité est alors vendue à77 € / MWh.

Fiche n° 8. QUELS CO-SUBSTRATS ?

Une condition nécessaire

Dans le contexte actuel, il est extrêmementrare qu’un projet de biogaz agricole puisseêtre rentabilisé si celui-ci ne traite pas desco-substrats (de 10 à 30% du tonnagedisponible sur l’exploitation).

Les co-substrats apportent des recettesnécessaires à l’équilibre financier :

• grâce à une production plus importantede biogaz, qui génère des recettes devente d’énergie ;

• grâce aux redevances de traitementpayées par les producteurs de déchets.

Quels sont les co-substratsenvisageables ?

Une installation de biogaz agricole peut traiterdes déchets organiques propres :

• Tontes de gazon et feuilles provenantdes espaces verts des communes etdes ménages.

• Déchets de l’industrie agro-alimentaire.

• Résidus d’assainissement (boues,matières de vidange, graisses destations d’épuration…).

• Déchets de restauration collective,huiles de friture, eaux grasses…

À quelles conditions peut-onajouter des co-substrats ?

Ces co-substrats doivent être compatiblesavec l’installation :

• Au plan technique : vérifier que lesubstrat envisagé n’implique pas dec o n t r a i n t e s s u p p l é m e n t a i r e s(chargement, tri, pompabilité…).

• Au plan agronomique : actualisation dubilan de fertilisation.

• Au plan réglementaire : statut dusubstrat, règlement sanitaire, pland’épandage, statut de l’installation auregard de la loi sur les ICPE(Instal lat ions Classées pour laProtection de l’Environnement)… Il estconseillé de se rapprocher des autoritéscompétentes (par exemple la directionvétérinaire).

Quel est le montant de laredevance ?

Il varie selon le type de substrat et selon lecontexte local.

Les déchets organiques propres et facilementdigestibles, qui sont les plus intéressants surles plans technique, agronomique etréglementaire, sont souvent épandus sansdifficulté particulière, ou compostés pour uncoût modéré. Le « prix du marché » peut doncêtre bas, de l’ordre de 10 à 30 € par tonne.

A contrario, les substrats dont le « prix dumarché » est élevé, de l’ordre de 50 à 100 €par tonne, sont souvent des substrats quiprésentent une difficulté.

Si le potentiel local de déchets organiquespeut sembler élevé, celui-ci se réduit souventfortement lorsque l’on veut concilier à la foisles intérêts économiques, techniques,agronomiques et réglementaires.

Quels substrats privilégier ?

Lors des études préalables, il est préférablede miser sur des produits végétaux propres, àfort contenu énergétique : déchets vertsexempts de branches et d’impuretés, déchetsvégétaux d’entreprises agro-alimentaires,huiles de friture…

Fiche n° 9. ETAT DES LIEUX EN FRANCE ET EN EUROPE

La méthanisation en général

La méthanisation de déchets organiques estappliquée depuis plus d’un siècle pour traiterdifférents produits : boues d’épuration, lisierset fumiers, effluents industriels, déchetsmunicipaux organiques… L’objectif est deréduire les odeurs, la teneur en matièreorganique très fermentescibles, les germespathogènes, et de produire de l’énergie.

Le biogaz « à la ferme » en France

En France, la méthanisation a été développéepour des installations à l’échelle del’exploitation agricole dans les années 30, puisdans les années 1939-1945 (pénuried’énergie), à nouveau ensuite en 1956-1957(crise de Suez). A la fin des années 1970(crises du pétrole), le biogaz « à la ferme » aété à nouveau développé. Un programme dediffusion a permis la réalisation d’une centained’installations.

A partir de 1985, le contre-choc pétrolier aconduit à l ’abandon de nombreusesinstallations dont tout l’équilibre économiqueétait bâti sur la perspective d’un fioul cher. Deplus, beaucoup d’installations étaient desprototypes qui ont connu des difficultés demise au point.

Celles-ci auraient pu être surmontées si leprogramme s’était poursuivi. Mais les pouvoirspublics ont arrêté toute action dans cedomaine, dans le cadre de la réductiondrast ique de sout ien aux énergiesrenouvelables qui a suivi le contre-chocpétrolier.

Le biogaz « à la ferme » en Europe

Dans d’autres pays d’Europe, cet effort anéanmoins été poursuivi. Il a trouvé un nouvelélan avec la prise en compte des problèmesd’environnement.

Aujourd’hui, le biogaz « à la ferme » sedéveloppe dans d’autres pays d’Europe :Autriche, Danemark, Suisse, mais surtoutAllemagne (près d’une centaine d’installationsnouvelles par an en moyenne depuis 10 ans).

Cet essor est permis par plusieurs facteursdont les principaux sont les suivants :

• le biogaz « à la ferme » bénéficie detarifs d’achat d’électricité élevés (90 à100 € /MWh ; le tarif en Allemagne a été

révisé à la hausse en 2004, il peutatteindre 200 €/MWh) ;

• un grand nombre d’installations traitedes déchets (de collectivités locales, dusecteur agro-industriel…) avec les lisiers,et bénéficient de redevances detraitement qui permettent d’améliorerl’équilibre économique.

Adapter les « modèles »européens

La méthanisation s’est développée dans cesdifférents pays, dans des contextes qui leursont propres.

La diffusion de ces techniques en France doittenir compte de la différence de contexte, caril ne suffit pas de les transposer telles quelles.

Le « biogaz agricole » devrait être relancé àl’initiative des pouvoirs publics. Le Plan Climatadopté en Juillet 2004, prévoit en effet desoutenir ces opérations. L’ADEME (Agence del’Environnement et la Maîtrise de l’Energie)sera chargée pour cela, de lancer un appel àpropositions auprès des Régions.

Fiche n° 10. MONTER UN PROJET DE METHANISATION

Autoconstruction ou clés-en-mains ?

En pratique, une installation de méthanisationest toujours réalisée en part ie enautoconstruction, et en partie avec du matérielclés-en-mains.

Certains composants ne peuvent évidemmentpas être fabriqués par l’agriculteur (groupe decogénération), et inversement il est plusjudicieux que certains équipements soientréalisés par l ’agriculteur (tranchées,aménagements).

La part d’autoconstruction qui peut êtreassurée par l’agriculteur dépend de la manièredont celui-ci compte son temps. Au tarif de lamain d’œuvre agricole, l’autoconstructionapporte peu d’économie significative.

L’agriculteur peut également travailler « enrégie » sous la responsabilité de l’ensemblier.Il est important dans ce cas de préciser lesresponsabilités de chacun.

Indispensable : l’ensemblier

Nous considérons qu’il est impératif, dans lecontexte actuel en France, de recourir à unensemblier, car celui-ci :

• Dispose d’un savoir-faire capitalisé parde nombreuses expériences.

• Dispose de compétences multiples(é lec t r i c i té , é lec t romécan ique,tuyauter ie, génie civi l…) quel ’agr icul teur ne possède pasforcément.

• Assure la responsabilité globale del’installation.

Le préalable : les études

Avant de demander un devis à un ensemblier,l’agriculteur devra rassembler les élémentspermettant à celui-ci d’établir son offre :

• Nature, quantité, saisonnalité desproduits à traiter, teneurs en matièresèche et en matière organique.

• Bilan des consommations d’énergiesur le site, saisonnalité.

• Implantation des ouvrages, réseauxde canalisations.

• Existance ou non d’une préfosse et defosses de stockage ; volumes,équipements (pompes).

L’agriculteur devra faire appel à un bureaud’étude spécialisé pour l’assister lors desétudes préalables, lors de la passation desmarchés, ou en assistance lors de laréalisation des travaux.

L’offre commercialed’équipements

La méthanisation « à la ferme » estaujourd’hui bien maîtrisée sur le plantechnique.

Cependant, les constructeurs ou fournisseursde matériels spécifiques sont des entreprisesétrangères (Allemagne, Suisse, Autriche…) quitravaillent sur un marché national, voirerégional (Bavière par exemple). Certainsconstructeurs commencent à s’implanter enFrance.

Microbiologie de la digestion anaérobie 1

M I C R O B I O L O G I E D E L A D I G E S T I O N A N A E R O BI E

LA THEORIE DE LA METHANISATION (DIGESTION ANAEROBIE)

La méthanisation est un processus biologique naturel qui permet de convertir la matière organique (glucides, lipides, protéines) en éléments simples (CH4, CO2, NH3 et H2S) grâce à l’action de bactéries anaérobies. Cette digestion anaérobie, processus biologique complexe, peut être décrit en quatre phases de dégradation : l’hydrolyse, l’acidogénèse, l’acétogénèse et la méthanogénèse. Chaque phase fait intervenir un groupe de bactéries particulières (figure 1). Toutes les molécules qui ne seront pas dégradées par cette voie pour produire du biogaz (lignine par exemple) et les déchets de ces réactions anaérobies composeront le digestat.

FIGURE 1 :SCHEMA RECAPITULATIF DE LA METHANISATION


Chaque groupe de bactéries possède des caractéristiques spécifiques (pH, température, sensibilité { des composés particuliers…). Nous allons présenter rapidement les propriétés des populations bactériennes de chacune des phases de la méthanisation.

L’HYDROLYSE

Les bactéries hydrolytiques dégradent la matière organique fraiche (les polymères) en fragments solubles (monomères). Ces bactéries produisent des exo-enzymes qui vont dégrader les polymères de la matière organique. Les vitesses de dégradation dépendent des substrats. (figure 2).

Ainsi il est important de bien brasser le milieu afin d’homogénéiser la solution et d’optimiser l’hydrolyse. Cette étape est fondamentale pour la suite de la réaction car elle permet de fractionner la matière organique et donc de faciliter l’action des bactéries qui interviennent par la suite.

TABLEAU 1 : CARACTERISTIQUES DES BACTERIES HYDROLYTIQUES

Bactéries hydrolytiques

caractéristiques bactéries relativement résistantes, tolérantes à O2,

production d’exo-enzymes gamme de pH optimal [4,5 – 6,3]

temps de division quelques heures (reproduction rapide) sensibilité lignine (pas dégradable, ralenti la réaction)

L’ACIDOGENESE

Les bactéries acidogènes dégradent les molécules simples de matière organique (monomères) en acides et en alcools. Les acides synthétisés sont des Acides Gras Volatils (AGV). Ces AGV sont des acides avec une chaine carbonée plus ou moins longue (de 2 à 10 atomes de carbone en général).

TABLEAU 2 : CARACTERISTIQUES DES BACTERIES ACIDOGENES

Bactéries acidogènes

caractéristiques bactéries sensibles à O2, participent en général

également { l’hydrolyse gamme de pH optimal [4,5 – 6,3]

temps de division quelques heures (reproduction rapide) sensibilité H2S, NH3, sels, antibiotiques

FIGURE 2 : VITESSE DE DEGRADATION PAR LES BACTERIES HYDROLYTIQUES EN FONTION DU SUBSTRAT


L’ACETOGENESE

Les bactéries acétogènes vont transformer les AGV en acide acétique (CH3-COOH) et en H2 et CO2. Ces molécules vont ensuite servir de substrat aux bactéries méthanogènes. Les bactéries acétogènes produisent de l’H2 mais leur activité est inhibée par un excès de H2 dans le milieu. Ainsi, la symbiose de ces bactéries avec les bactéries consommatrices d’H2 (méthanogènes notamment) est indispensable pour garantir une bonne activité bactérienne dans le digesteur. Les acétogènes et les méthanogènes vivent fixées les unes aux autres, une agitation rapide risque de détruire ce lien d’où la recommandation d’un brassage lent.

TABLEAU 3 : CARACTERISTIQUES DES BACTERIES ACETOGENES

Bactéries acétogènes

caractéristiques bactéries relativement fragiles, sensibles à O2,

production de H2 gamme de pH optimal [6,8 – 7,5]

temps de division quelques jours (1-4 jours ; reproduction lente)

sensibilité H2 en excès, H2S, NH3, sels, antibiotiques, variations de

température

LA METHANOGENESE

La méthanogénèse constitue la dernière étape de dégradation anaérobie de la matière organique. Les microorganismes qui réalisent cette étape sont des archaebactéries, microorganismes proches des bactéries, on les appelle bactéries méthanogènes par abus de langage. On distingue deux groupes de bactéries méthanogènes, les hydrogénotrophes (qui utilisent H2 et CO2) et les acétoclastes (qui utilisent l’acide acétique). Dans les deux cas, elles vont réduire leur substrat en méthane (CH4). La composition moyenne du biogaz ainsi produit est un mélange de CH4 (~60%) et de CO2 (~40%) avec des traces de H2S, de NH3 et de H2.

Ces bactéries méthanogènes ont comme particularité de se développer en condition anaérobie stricte, elles sont totalement intolérantes { l’O2. Elles ont également besoin de nickel (Ni) pour se développer. Ainsi il est important de garantir une absence totale d’O2 dans le digesteur ainsi qu’un apport en Ni satisfaisant pour leur croissance (en général la quantité de Ni contenue dans la matière organique entrant dans le digesteur est suffisante).

TABLEAU 4 : CARACTERISTIQUES DES BACTERIES METHANOGENES

Bactéries méthanogènes

caractéristiques archaebactéries très fragiles, très sensibles à O2, besoin

de Ni, plusieurs substrats possibles gamme de pH optimal [6,8 – 7,5]

temps de division quelques jours (5-15 jours ; reproduction lente) sensibilité O2, variations de pH et température, Cu, sels


CONSEILS PRATIQUES POUR ASSURER LA STABILITE DE L’UNITE DE METHANISATION

Comme nous venons de le voir, l’activité bactérienne mise en jeu lors de la digestion anaérobie est très complexe et très sensible, il est donc important de mettre en œuvre des pratiques pour maintenir dans le digesteur des conditions favorables aux bactéries et donc maintenir un bon fonctionnement de l’unité de méthanisation.

Les paramètres suivis sont souvent de nature économique (directement ou indirectement) ou agronomique comme le taux de charge du digesteur, le temps de séjour dans le digesteur, le potentiel méthanogène du substrat, la quantité de biogaz produit ou encore la qualité du digestat. Mais ces paramètres ne permettent pas d’appréhender l’aspect biologique dans le digesteur. Il est important de suivre quelques paramètres d’ordre biologique afin de prévenir et de maitriser les perturbations possibles du process, comme les intoxications des bactéries. Nous allons présenter quelques exemples d’intoxication ainsi que les solutions possibles pour y remédier.

INTOXICATION AUX AGV : ACIDOSE

L’acidose correspond { la chute du pH dans le digesteur, entrainant un disfonctionnement des bactéries méthanogènes et donc un diminution de la production de biogaz. L’acidose peut être due { un substrat trop abondant et trop fermentescible (forte activité d’hydrolyse et d’acidogénèse, donc accumulation d’AGV) ou { une inhibition des bactéries acétogènes ou méthanogènes (variation de température, présence d’antibiotique, d’H2S, de métaux lourds…).

Les conséquences de cette intoxication sont la baisse du pH, la diminution de la quantité de biogaz produite et la perte de qualité du biogaz (augmentation de la concentration en CO2). Lorsque l’on détecte ces problèmes, il est déj{ trop tard, le disfonctionnement est installé. Certains paramètres peuvent permettre de prévenir l’intoxication :

L’augmentation de la pression partielle d’H2 dans le biogaz, l’H2 étant un inhibiteur des bactéries acétogènes, son augmentation traduit un début de disfonctionnement. Cette mesure peut se faire { l’aide d’une sonde { H2, cependant des erreurs de mesure peuvent se produire à cause de l’H2S et du NH3, des recherches sont en cours pour améliorer les sondes (capteur à membrane spécifique à H2).

La diminution de l’alcalinité totale (pouvoir tampon) du milieu. Pour se faire, on peut réaliser la mesure du TIC (Carbone Inorganique Total), celle-ci est encore aujourd’hui difficilement réalisable sur site, mais de nouveaux appareils de mesure sont développés.

L’accumulation d’AGV et le changement de composition (de plus en plus d’AGV de grande taille). On peut suivre deux indicateurs, la concentration totale en AGV et le rapport acide acétique (deux carbones) sur acide propionique (trois carbones). Mais ces mesures nécessite du matériel de laboratoire, donc difficilement réalisable sur site.

Quand l’acidose est détectée, l’arrêt de l’apport de substrat, le mélange du milieu pour favoriser l’activité des bactéries et l’ajout d’une base (NaHCO3 par exemple) peuvent permettre au système de redevenir fonctionnel.

Remarque : l’acidose est souvent l’étape finale des autres intoxications. Un disfonctionnement d’une étape va entrainer une accumulation d’AGV, donc une chute du pH et l’acidose.


INTOXICATION A L’AMMONIAC (NH3) : ALCALOSE

Comme nous l’avons vu précédemment, le NH3 est toxique pour les bactéries acidogènes et acétogènes. Par contre l’hydrolyse et la méthanogénèse sont encore fonctionnelles tant que du substrat est disponible pour ces bactéries. Ainsi on observe dans le digesteur une accumulation des produits de l’hydrolyse (acides aminés, acides gras…) et une diminution de la quantité de biogaz produit (mais même qualité).

Pour détecter cette intoxication, on peut mesurer la concentration en NH3 dans le digesteur. Les méthodes jusqu’alors disponibles étaient réalisables hors site et nécessitaient un matériel de laboratoire (titrage automatique) ou bien n’étaient pas pratiques (test colorimétrique). Une nouvelle technologie venue d’Allemagne permet de simplifier la manipulation et de la réaliser sur site. Cette méthode est simple et robuste mais utilise des produits dangereux (soude concentrée et oxydant fort), elle nécessite donc une attention particulière et une protection lors de la manipulation.

Cet excès de NH3 dans le digesteur peut être la conséquence d’un substrat en entrée trop riche en protéine (forte teneur en N C/N < 30) comme les lisiers ou les fumiers de volaille par exemple. Pour remédier à ce problème il faut apporter un substrat moins riche en protéine et fortement fermentescible (C/N > 30) pour relancer l’activité des bactéries.

INTOXICATION AU DIHYDROGENE DE SOUFFRE (H2S)

Tout comme le NH3, le H2S a un effet inhibiteur sur l’acétogénèse et l’acidogénèse. Les conséquences sur l’activité bactérienne sont donc semblables à une intoxication au NH3, on observe une accumulation des produits de l’hydrolyse et une diminution de la quantité de biogaz produit.

Pour essayer de détecter cette intoxication avant qu’il ne soit trop tard, on peut suivre la composition du biogaz et en particulier la concentration en H2S. Des sondes existent pour réaliser de telles mesures en continue dans le biogaz.

La forte présence de protéines dans le substrat est, comme pour le NH3, responsable de cette intoxication. En effet, les protéines contiennent du souffre, en faible quantité dans certains acides aminées (cystéine et méthionine). Une attention particulière doit donc être portée à la qualité du substrat pour éviter cette intoxication.

INTOXICATION A L’OXYGENE (O2)

L’O2 inhibe quasiment toutes les phases de la méthanisation mis { part l’hydrolyse qui peut persister en milieu aérobie. Ainsi on observe une accumulation des produits de l’hydrolyse et un arrêt de la production de CH4.

La principale source d’O2 dans le digesteur est l’introduction par les substrats poreux (comme la paille par exemple) ou par des erreurs de manipulation qui entrainent une entrée d’air dans le digesteur. La présence d’oxygène peut se détecter facilement par un capteur d’O2 dans le digesteur (mesures en continue). Afin de palier { ce problème il est préférable d’utiliser des substrats denses, et dans le cas des pailles, il est recommander de la défibrer avant (pas de soucis avec la paille piétinée par les animaux présente dans les fumiers).


INTOXICATIONS AUX METAUX LOURDS (CUIVRE, ZINC…) ET AUX ANTIBIOTIQUES, DESINFECTANTS…

Ces intoxications sont causées par des substances initialement présentes dans le substrat, contaminations : aux métaux lourds dans les lisiers, aux médicaments animaux (pour traiter les mammites par exemple) dans les déjections, aux désinfectants de la salle de traite dans les eaux de lavage… Dans tous les cas, ces intoxications entrainent l’inhibition des bactéries acidogènes, acétogènes et méthanogènes, et donc une diminution de la production de biogaz et une accumulation d’AGV, ce qui entraine une baisse du pH et donc un risque d’acidose (cf. ci-dessus).

Le facteur déterminant pour ces intoxications est donc la qualité des substrats introduits dans le digesteur, il est donc primordial de bien connaitre ces substrats et donc de les faire analyser en laboratoire pour éviter ensuite toute déconvenue qui aurait pu être évitable.

CAS PARTICULIER DE LA CARENCE EN NI

Les bactéries méthanogènes ont besoin de Ni pour se développer. Hors le Ni peut faire défaut dans le substrat, il faut donc connaitre la composition des substrats pour apporter la « bonne ration » aux bactéries méthanogènes. Une carence en Ni entraine l’inhibition de la méthanogénèse donc la diminution de la production de méthane ainsi que l’augmentation de la concentration en AGV, donc la diminution du pH et un risque d’acidose.

Certains « remèdes miracles » pour apporter du Ni au milieu existent, mais leur efficacité n’est pas encore prouvée.

Cependant, il est important de noter que ce problème intervient surtout en Allemagne où les cultures énergétiques sont fortement utilisées comme substrat pour la méthanisation, or ces cultures sont généralement pauvres en Ni, d’où les carences observées. En France, compte tenu de la diversité et de la qualité des substrats utilisés dans les unités de méthanisation, ce problème ne devrait pas se poser.

CONCLUSION

La digestion anaérobie fait intervenir un très grand nombre de bactéries et de réactions biologiques, il s’agit donc d’un processus très complexe. De plus il est impossible de généraliser toutes les approches car chaque digesteur est unique compte tenu du process mis en jeu, des substrats utilisés, des populations bactériennes en présence…

Le principal conseil que l’on peut apporter est d’avoir une bonne connaissance de tous les substrats que l’on introduit dans le digesteur (attention aux substrats ne provenant pas de l’exploitation) pour éviter toute contamination facilement évitable (métaux lourds, médicaments, désinfectants antibiotiques…). Ensuite il est également conseillé de réaliser un suivi de certains paramètres biologiques afin de prévenir les intoxications possibles et donc la perte de productivité de l’unité de méthanisation. On peut préconiser un suivi sous la forme suivante :


Suivi quotidien ou continu de la température, du pH, des teneurs en CH4, CO2, H2S, O2 ,H2 et NH3 si substrat à risques (riche en protéines)

Suivi hebdomadaire ou bihebdomadaire du pouvoir tampon (TIC) et des teneurs en AGV pour prévenir les variations de pH

De plus, il est préconisé d’isoler les animaux malades pour éviter les contaminations médicamenteuses, de faire attention aux concentrations en sels (KCl, NaCl…) lorsqu’on utilise des boues de STEP et d’éviter au maximum d’introduire des substrats riches en lignine et des matières inertes dans le digesteur (plastique, verre, silice…). Enfin, il est important de maintenir une température constante dans le digesteur car certaines bactéries sont sensibles aux variations de température.

Microbiologie de la Digestion

AnaérobiePartie Théorique

Digestion Anaérobie ?

• Processus biologique complexe de conversion de

la matière organique en:

CH4, CO2, NH3, H2S

Equation générale: (Buswell & Müller, 1952)

CcHhOoNnSs + y H2O à x CH4 + n NH3 + s H2S + (c-x) CO2

SUCRES: C6H12O6 à 3CO2 + 3 CH4

LIPIDES: C12H24O6 + 3 H2O à 4.5 CO2 + 7.5 CH4

PROTEINES: C13H25O7N3S à 6.5 CO2 + 6.5 CH4 + 3 NH3 + H2S

Substrats Production théorique

de biogaz

Nl/kg DOM CH4 (%) CO2 (%)

hydrates de carbone 746 50 50Lipides 1390 72 28Proteines 800 60 40

Composition théorique

de biogaz Protéines: CO2 se lie à NH3 et

H2S reste principalement en

phase liquide à CH4 » 60%

Solubilité des gaz dans l’eau

Digestion Anaérobie ?

• Processus biologique complexe qui peut

être décrit en 4 phases de dégradation :

1. Hydrolyse

2. Acidogénèse

3. Acétogénèse

4. Méthanogénèse

La Digestion Anaérobie

Polymères complexes

HydrolyseHydrol

pH

4.5 – 6.3

temps O2

heures

Caractéristiques

Monomères, Dimères, a.a., ac. grasAcidogénèse

Hydrol

4.5 – 6.3 heures

AGV, CO2, H2, AlcoolesAGV, coolcoolcool

Acide Acétique

Acetogénèse 6.8 – 7.5 1-4 jours

CH4, CO2

Méthanogénèseénès 6.8 – 7.5 5-15 jours

Les facteurs limitants et d’inhibition

exo-enzymes

Monomères, Dimères, a.a., ac. gras


HydrolyseHydrol lignine, mélange

Acidogénèse

Hydrol

H2S, NH3, sels,

antibiotiques

AGV, CO2, H2, AlcoolsAGV, coolcoolcool

Acide Acétique

Acetogénèse Excès H2, H2S, NH3,

sels, antibiotiques, T°

CH4, CO2

Méthanogénèse O2, pH, T°, Cu, sels,

carence en Ni Etroite association/actions concertées !

1 HYDROLYSE• Exoenzymes produits par des bactéries anaérobies

facultatives ou obligatoires dégradent les substrats

insolubles (polymères: cellulose, hémicellulose, amidon,

protéines, lipides) en fragments solubles (monomères)

• Les bactéries anaérobies facultatives consomment l’O2

dissout dans l’eau et causent de ce fait une réduction du

potentiel redox nécessaire au développment des bactéries

anaérobies strictes.

• Glucides à qqus heures

• Protides et Lipides à qqus heures à qqus jours

• Lignocellulose à qqus semaines

• Lignine à ‘ indigestible’

qq j

Bactéries hydrolysantesGenre Species Description

Bacteroides uniformis immobiles, Gram-neg, bâtonnets

acidifaciens

vulgatus

ruminicola

Lactobacilus pentosus immobiles, Gram-pos, bâtonnets

plantarum endospores

Propioni-bacterium microaerophilium immobiles, Gram-pos, bâtonnets

propionicus spores

cyclohexanicum

Sphingomonas subterranea présentes dans les sédiments profonds

Sporobacterium olearium

Megaspheara elsdenii rumen

Bifidobacterium

X

X

2 Acidogénèse

2

Organic fraction

Carbohydrates Proteins Lipids

Monosaccharides Amino Acids Long Chain Fatty Acids

HAc

HPr, HBu, HVa,…

CH4

Volatile Fatty Acids

(VFA)

HVa: valeric acid (C5)

HBu: butyric acid (C4)

HPr: propionic acid (C3)

HAc: acetic acid (C2)

Anaerobic

Digestion

Substrate H2OInorganic

Bactéries acidogènesLa majorité des acidogènes participent aussi à l’hydrolyse !

Genres: Clostridium, Ruminococcus, Paenibacillus

1. Clostridium: grand groupe diversifié

Clostridium tetani (tétanos) Clostridium botulinum (botulisme) ?

Bactéries acidogènes

2. Ruminococcus: Glucides à Acetate, Formate, Succinate, Lactate

EthOH, H2, CO22 2

3. Paenibacillus: Glucides à Acetate, Formate, Lactate, Propionate

3. Acétogénèse

Les acétogènes produisent obligatoirement H2

Inhibition si forte pression partielle en H2

Symbiose nécessaire avec des bactéries consommatrices d’H2

3 Acetogénèse

2

Bactéries homoacetogènes réduisent l’H2 et CO2 en Acétate

Bactéries acetogènes

• Symbiose obligatoire avec des bactéries consommatrices de H2

• Régénération = 84 h

• Acides organiques, Alcooles, a.a. à Acetate, CO2, H2

4. MéthanogénèseAcétoclastiques Hydrogénotrophes

Bactéries methanogènes

• Bactéries primitives = Archaea

• Possèdent le co-facteur F420 (transporteur d’H2, autofluorescent)

• Principaux = Methanobacterium, Methanospirillum, Methanosarcina

• Archaea methanogènes =

• Anaerobie stricte !!!

• Substrats = acetate, formate, methanol/H2, H2/CO2

• Nikel dépendantes

Methanosaeta

Methanosarcina

Bactérie méthanogène

en symbiose avec le

protiste Nyctotherus

ovalis (dehydrogenase

coenzyme F420)

Bactéries methanogènes

1

CO2

H2

acide acétique

METHANOGENESE

bactéries hydrognotrophes

bactéries acétoclatiques CH4

CO2

CH4

bactéries methyltrophiquesmethanol CH4

Compétition Bactérienne

• Bactéries réduisant les sulfates

•Réduisent le SO42- en H2S en utilisant l’acétate et H2

•à consomment les deux substrats principaux de la méthanogenèse

• Il faut maintenir un ratio Substrat/ SO42- > 3

Desulfovibrio vulgaris

•Desulfobacterales

•Desulfovibrionales

•Syntrophobacterales

•Thermodesulfobacteria

Compétition Bactérienne

• Bactéries homoacétogènes

•Réduisent H2 et CO2 en Acétate

•à consomment l’H2

•àcompétition avec les méthanogènes hydrogénotrophes (!)

•à favorisent les méthanogènes acétoclastiques (=acétotrophes)

• Digesteurs agricoles

• Passé: CH4 70% viendrait AcAc et 30% CO2 + H2 = CH4

• Depuis Klocke 2007 et 2008 l’inverse a été démontré !

Partie Pratique

Biogaz à la ferme

Les paramètres utiles à suivre

et les bonnes pratiques pour assurer une

biométhanisation stable

La Biométhanisation à la fermeCH4, H2

CO2, H2S, H2O

Substrats = Matière organique :

•Déjections animales

•Production végétale

•Sous-produit de l’agro-

alimentaire, ménages,…

Digestats :

•MO non digérée

•N, P, K

•Odeur réduite

A. Quel substrat ? (qualité, digestibilité)

Quel approvisionnement ? OLR = mS x [DOM] / Vr (DOM kg/m3j)

Quel temps de séjour ? HRT = Vr / Vs (j)

B. Phénomènes d’indigestion ? Acidose, Intoxication NH3, métaux lourds, antibiotiques, sels, T°

C. Les digestats peuvent-ils

encore produire du CH4 de

manière rentable ?

Vs (m3/j)

ms (kg/j) VD (m3/j)

mD (kg/j)

Vg (m3/j)

%CH4 %CO2

Vr (m3)

T°, pH

Mélange

Et la BIOLOGIE

?

La Digestion Anaérobie


HydrolyseHydrol

pH

4.5 – 6.3

temps O2

heures

Caractéristiques

Monomères, Dimères, a.a., ac. grasAcidogénèse

Hydrol

4.5 – 6.3 heures

AGV, CO2, H2, AlcolesAGV, colecolecole

Acide Acétique

Acetogénèse 6.8 – 7.5 1-4 jours

CH4, CO2

Méthanogénèseénès 6.8 – 7.5 5-15 jours

Les facteurs limitants et d’inhibition

exo-enzymes

Monomères, Dimères, a.a., ac. gras


HydrolyseHydrol lignine, mélange

Acidogénèse

Hydrol

H2S, NH3, sels,

antibiotiques

AGV, CO2, H2, AlcoolsAGV, coolcoolcool

Acide Acétique

Acetogénèse Excès H2, H2S, NH3,

sels, antibiotiques, T°

CH4, CO2

MéthanogénèseénèsO2, pH, T°, Cu, sels ,

carence NiEtroite association/actions concertées !

Perturbations du processus• Intoxication due aux AGV (Acidose, pH<7)

• Intoxication due à NH3 (NH4>3kg/m3 dig.)

• Intoxication due à H2S (H2S>50 mg/l dig.)

• Intoxication due à l’O2 (O2>0.1 mg/l dig.)

• Intoxication due aux métaux lourds

(Cu, Zn, Cr, Pb, Fe, Cd)

• Intoxication due aux antibiotiques/désinfectants etc…

– Que se passe-t-il ?

– Origine ?

– Comment les détecter ?

– Comment y remédier ?

Acidose

• Que se passe-t-il ?

• Origine

• Comment la détecter à temps ?

• Comment y remédier ?

Bact. Acetogènes

Bact. Methanogènes

Acidose : Que se passe-t-il ?

Polymères Complexes

Carbohydrates, Lipids, Proteins, Ac nucléiques

Hydrolysis

monomères, dimères

sucres, acides gras branchés, a.a.,

Acidogenesis

AGV : Ac., Prop., But,

Alcools, Acetone, CO2, H2Acetogenesis

Acetate H2, CO2

Methanogenesis

CH4 + CO2

Accumulation:

• H2 et CO2

• acides organiques

Chute:

•pH

•CH4, m3 biogaz

Signes annonciateurs

Bact. AcidogènesBact. Acidog

Bact.Hydrolytiques

Acidose : Origine ?

• Alimentation excessive

• Substrats trop fermentescibles

• Inhibition des ACETOGENES par:

– Antibiotiques, désinfectant

– T°

– H2S (protéines)

– Sels (STEP)

• Inhibition des METHANOGENES par:

– T°

– Cu, Zn, Cr, Pb, …

– O2 (introduit avec les substrats)

– Carence en Ni

Bourbes de vinification (riche en sucres solubles)

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700 800 900

heures

ga

s (

m3/t

)

CH4 (m3/t FM)

CO2 (m3/t FM)

Acidose : Comment la détecter ?

• Chute de production de biogaz

• Perte de qualité du biogaz (CH4 < 50%, CO2 > 50%)

• pH < 7

• Déplacement de l’H2S vers la phase gazeuse

1. Paramètres communément disponibles sur site

SOUVENT IL EST DEJA TROP TARD !!!

Acidose : Comment la détecter ?

2. Paramètres plus sûrs car précurseurs de l’acidose

• Augmentation de la pression partielle en H2

• Chute de l’alcalinité totale = pouvoir tampon

= amortisseur d’acidité

• Accumulation des AGV et modification de la balance en AGV (proprotion d’Ac Ac chute alors que Prop, But, Val augmentent)

Pression partielle en HYDROGENE (H2)

• Le plus précoce = H2 puisqu’il

inhibe le processus

• H2 est quasi insoluble dans l’eau et

se retrouve donc dans la phase

gazeuse

Þ mesurer la pression partielle en H2

dans le biogaz 1ppm < H2 < 100

ppm

• Détecteur à H2 à prix abordable

existe aujourd’hui sur le marché

• Attention aux interférences avec

H2S et NH3 Þ capteur spécifique

Þ phase de développement

Projet INTERREG IV A OPTIBIOGAZ

Suivi de l’Hydrogène

0

50

100

150

200

250

300

350

400

0

500

1000

1500

2000

2500

3000

09/08 11/08 13/08 15/08 17/08 19/08 21/08 23/08 25/08 27/08 29/08

CH

4, C

O2

(%)

an

d H

2(p

pm

) co

nce

ntr

atio

n, b

ioga

s p

rod

uct

ion

ra

te (

mL/

min

)

an

d f

ee

din

g r

ate

(g

/10

0L)

H2S

co

nce

ntr

atio

n (

pp

m)

Date

H2S biogas rate

H2 feeding rate

CH4 CO2

Double feeding rate Temperature decrease

Suivi de l’Hydrogène

Hydrogen

-100.0

100.0

300.0

500.0

700.0

900.0

1100.0

1300.0

1500.0

2011-0

4-0

7 0

0:0

0

2011-0

4-1

7 0

0:0

0

2011-0

4-2

7 0

0:0

0

2011-0

5-0

7 0

0:0

0

2011-0

5-1

7 0

0:0

0

2011-0

5-2

7 0

0:0

0

2011-0

6-0

6 0

0:0

0

2011-0

6-1

6 0

0:0

0

2011-0

6-2

6 0

0:0

0

Date

H2

[p

pm

]

Pilot 1 Pilot 2 Pilot 3 Pilot 4

Chute de l’alcalinité totale

Source: Melanie HECHT, unpublished

• Alcalinité totale = pouvoir tampon = “amortisseur” d’acidité

• Principal acteur = CO2 = carbone inorganique (TIC ou TAC)

• Tampon d'acide carbonique (H2CO3) et de hydrogénocarbonate (HCO3

-) maintient le pH entre 7,35 et 7,45.

6.5 10.4

AGVTIC

pH

Comment mesurer l’alcalinité totale (TIC) ?

www.hach-lange.de

TAC=20ml

V× M TAC× 250

TIC: Total Inorganic Carbonate (mg/kg)

V: volume of sample (mL)

MTAC : amount of 0.05 M sulfuric acid im mL

TIC

Problèmes:

•Réalisé en laboratoire

•Forte production de mousse (CO2)

•Utilisation H2SO4 [conc]

•Sonde pH pour solution organique

Titrateur automatique

Comment mesurer l’alcalinité totale (TIC) sur site?

http://www.biogaspro.de/index.html

Le QUANTOFIX ou l’AGRO-LISIER pourraient être adaptés

Coopagri Bretagne, mars 1995

TIC

CO2 + H2O « H2CO3

Accumulation des AGV tot et modification de la

composition en AGV

• Lorsque l’Acetogenèse est inhibée par l’excès d’H2, les AGV de taille supérieure à l’acétate (C2) s’accumulent

• Le premier à s’accumuler est l’Ac propionique (C3)

Dans la pratique il n’y a pas de

valeur standard pour les AGVtot

Chaque digesteur est unique !

• Spectre en AGV

• C2 Ac Acétique CH3-COOH

• C3 Ac Propionique CH3-CH2-COOH

• C3 Ac Lactique CH3-CHOH-COOH

• C4 Ac Butyrique CH3-(CH2)2-COOH

• C5 Ac Valérique CH3-(CH2)3-COOH

• C6 Ac Caproïque CH3-(CH2)4-COOH

• C8 Ac Caprylique CH3-(CH2)6-COOH

• C10 Ac Caprique CH3-(CH2)8-COOH

• Extraction par entraînement à la vapeur et titration (AGV tot), Chromatographie

• Les premiers à s’accumuler sont les AGV de taille supérieure à l’acetate. Accumulation du propionique au détriment de l’acétique

• Les AGV branchés (isoBut, isoVal) sont plus difficilement transformés en Acétate

• [Ac Acétique] / [Ac propionique] ³ 3 est OK10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0 37.5 40.0

1.00

2.00

3.00

4.50µS

min

–L

acti

c a

cid

-A

ceti

c a

cid

-P

rop

ion

ic a

cid

-i-

Bu

tyri

c a

cid

-n

-Bu

tyri

c a

cid

0.00

Acidose : Comment y remédier ?

• Stopper l’alimentation !!!

• Mélanger pour favoriser les échanges entre bactéries

• Diluer ?

• Ajout de NaHCO3 (CaO, CaCO3)

•Réaction rapide

•Augmentation du pH et de la capacité tampon des digestats

•Faire un test préliminaire (10 litres de digestat + incrément de 1 g de

NaHCO3 jusqu’à un pH de 7.8 puis extrapoler au volume du digesteur)

•Un stock de sécurité de 500 kg est recommandable

• Dévier le digestat acidifié vers un méthaniseur à lit fixe


• Intoxication due à NH3 (NH4+ > 3kg/m3 dig.)

• Intoxication due à H2S (H2S > 50 mg/l dig.)

• Intoxication due à l’O2 (O2 >0.1 mg/l dig.)





– Origine ?



Intoxication à NH3


• Origine



Intoxication à NH3 : Que se passe-t-il ?•Excès de protéines dans la ration

•Accumulation d’NH3 dans le

digestat à pH augmente !

•Effet inhibiteur principal de NH3

sur les Acétogènes et les

Acidogènes

•L’Hydrolyse et la Méthanogenèse

fonctionnent

•Accumulation de monomères,

AGV de grande taille, acides

aminés, sucres, alcools, acétone,…

•CH4/CO2 reste favorable !!!

•Nm3 biogaz chute



Hydrolysis



Acidogenesis



Acetate H2, CO2

Methanogenesis

CH4 + CO2

• NH3 > 3 kg/m3 de digestat

• Les bactéries montrent en

général une bonne capacité

d’adaptation

• Si le pH ou la T° augmente le

N-NH3 devient encore plus

toxique (fragilité thermophile)

Intoxication à NH3 : Origine ?

• Substrats riches en protéines

• Rapport C/N < 30

• Gluten, protéines animales,

lisiers, fumier de volaille,…

NH3 + H2O à NH4+ + OH-

C’est la forme N-NH3 qui est toxique !

Intoxication à NH3 : Comment la détecter ?

www.hach-lange.de

Titrateur automatique Méthodes colorimétriques

Problèmes:

•Préparation de l’échantillon

•Forte dilution nécessaire

Hors site

Comment mesurer NH3 sur site ?

•Ajout d’une base en excès

•Et d’un oxydant fort NaOCl

•NH4+ est converti en NH3 qui est a

son tour oxydé en NCl3 insoluble

•On mesure le volume de NCl3

produit par déplacement de la

colonne d’eau

•Pro: Simple et robuste

•Con: NaOH [conc] et oxydant fort

à PROTECTION

N-NH3

NH3+NaOCl à NH2Cl+NaOH

NH2Cl+NaOCl à NHCl2+NaOH

NHCl2+NaOCl à NCl3+NaOH

Le QUANTOFIX ou l’AGRO-LISIER pourraient être adaptés

Coopagri Bretagne, mars 1995

No

laboratoire

Désignation

de

l’échantillon

pH Total des

acides gras

volatils en

mg/kg dans

la matière

telle quelle

Acide

acétique

en mg/kg

dans la

matière

telle quelle

Acide

propionique

en mg/kg

dans la

matière telle

quelle

Acide

isobutyrique

en mg/kg

dans la

matière telle

quelle

Acide

butyrique

en mg/kg

dans la

matière telle

quelle

Acide

isovalérique

en mg/kg

dans la

matière telle

quelle

Acide

valérique

en mg/kg

dans la

matière

telle quelle

Acide

caproïque

en mg/kg

dans la

matière telle

quelle

1726/09 Digesteur 1 7,9 9.433 3.906 4.418 314 71 593 119 12

1727/09 Digesteur 2 8,0 1.597 187 1.390 10 2 8 - -

1728/09 Digesteur 3 8,1 1.860 165 1.671 14 1 9 - -

Exemple d’intoxication à NH3 Intoxication à NH3 : Que faire ?• Peu de recommendation dans la litterature !

• Stopper l’alimentation avec le substrat fautif riche en

protéines et source du NH3

• Acidifier le digestat (AcAc), Diluer ??

• Réalimenter prudemment avec un substrat fortement

fermentescible pour retrouver un rapport C/N favorable = 30 :

à production d’acide

à chute du pH

à NH3 passe sous forme NH4+

à toxicité diminue

à L’Acetogenèse et l’Acidogenèse reprennent









– Origine ?



Intoxication à H2S


• Origine





Hydrolysis



Acidogenesis



Acetate H2, CO2

Methanogenesis

CH4 + CO2

Intoxication à H2S : Que se passe-t-il ?

•Excès de protéines dans la ration

•Accumulation d’ H2S dans le digestat

• Effet inhibiteur principal de H2S sur les

Méthanogènes hydro-génotrophes, moins

sur les acétoclastiques, mais aussi sur les

Acidogènes et Acétogènes

•L’Hydrolyse et la Méthanogenèse

fonctionnent malgré tout

•Accumulation de monomères, AGV de

grande taille, acides aminés, sucres,

alcools, acétone,…


•Nm3 biogaz chute

• H2S > 50 mg/l de digestat

• H2S ± 2 000 ppm biogaz

• Si le pH chute H2S devient

encore plus toxique (HS- à H2S)

Intoxication à H2S : Origine ?

• Substrats riches en protéines

• Kératine = corne, peau, poils,

plume…

• protéines animales, lisiers,

fumier, colza et autres

crucifères (Brassicaceae)

• Vitamines (biotine, thiamine,

coenzyme A)

C’est la forme H2S qui est toxique !

coenzyme A

Méthionine Cystéine

-s-s-

Structure spatiale des protéines

Intoxication à H2S : Exemple

Plumes de poulet : Production cumulée de CH4 et de CO2

(MATIERE FRAICHE)

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

0 5 10 15 20 25 30 35 40

Temps [Jours]

Pro

du

ctio

n c

um

ulé

e d

e C

H4

[Nm

³/ t

FM

]

CH4

CO2

• Plumes: faible digestibilité et risque de corrosion pour les digesteurs et les

groupes de cogénération. (cornes, poils, plumes, colza, …)

Plumes :

MS : 26,5 %

MOS : 95,6 %


•Nm3 biogaz chute

Intoxication à H2S : Comment la détecter ?

Analyseur de gaz sur site

Capteur électrochimique

Rappel:

•L’intoxication à l’ H2S provoque

principalement l’inhibition des

Acétogènes:

à Accumulation des AGV

à Chute du pH

à HS- à H2S

à Déplacement vers la phase

gazeuse

à Détection accrue d’ H2S dans

le biogaz

CORROSION !!!700 ppm à Mort Immédiate ©

Intoxication à H2S : Que faire ?• Peu de recommendation dans la litterature !

• Stopper l’alimentation avec le substrat fautif riche en

protéines soufrées, repos

• Reprendre l’alimentation prudemment avec un substrat

fortement fermentescible pour retrouver un rapport C/N

favorable = 30 :

à production d’acide

à chute du pH

à H2S est déplacé vers la phase gazeuse

à Insufflation d’O2 dans le biogaz

à 2 H2S + O2 àS2 + 2 H2O (fleur de soufre, élémentaire)

à L’Acetogenèse et l’Acidogenèse reprennent









– Origine ?



Intoxication à O2

• Que se passe-t-il ?– O2 > 0.1 mg/l digestat

– Seul l’Hydrolyse fonctionne bien

– Accumulation de monomères

• Origine– Introduction avec les substrats (pailles)

– Désulfurisation biologique (O2)

• Comment la détecter ?– Capteur O2 (biogaz)

– Nm3 Biogaz chute

– CH4 chute

• Comment y remédier ?– Substrats denses

– Contrôle de la désulfurisation biologique

Hydrolyse

Acidogénèse

Hydrololololololyse

Acetogénèse

Méthanogénèse

di ?









– Origine ?



Intoxication aux métaux lourds Intoxication aux métaux lourds

• Que se passe-t-il ?– La méthanogenèse est inhibée

– Les autres phases fonctionnent

• Origine– Cu > 50 mg/l

– Zn > 150 mg/l

– Cr > 100 mg/l

• Comment la détecter ?– CH4 chute, AGV augmentent, pH chute = Acidose

– Dosage des métaux lourds en laboratoire

– ICP-MS (Inductively Coupled Plasma - MS)

• Comment y remédier ?– Eviter les substrats d’origine inconnue

– NaHCO3

Hydrolyse

Acidogénèse

Hydrolyse

Acetogénèse

Méthanogénèse









– Origine ?



Hydrolyse

Acidogénèse

Hydrololololololololyse

Acetogénèse

Intoxication aux AB désinfectants, etc…• Que se passe-t-il ?

– Acidogenèse et Acétogenèse inhibées

– Hydrolyse et Méthanogenèse fonctionnent

• Origine– Médication animale (mammite)

– Bactériostatiques (élevage porcin)

– Désinfectants (salle de traite)

– Poubelle “verte” (AB à usage humain)

• Comment la détecter ?– CH4 CO2 biogaz chutent (Acidose ou Intox NH3)

– Monomères augmentent

– Dosage des xénobiotiques en laboratoire

– HPLC

• Comment y remédier ?– Eviter les substrats d’origine inconnue

– Séparer les animaux malades du troupeau

– Stockage 2-3 semaines, la plupart sont dégradés

Méthanogénèse

INTERROGATION

Prenez un quart de feuille svp !Substrat

à digestion

rapide

Acides

organiques CH4

CO2

1. Les substrats rapidement hydrolysés

Pomme de terre

Mélasse, Fruits

Céréales en grain

Bourbes, Racines

Biogaz

Rq: Même effet si on suralimente le digesteur


de biogaz




de biogaz

Substrat

à digestion

« régulée » Acides

organiques

CH4

CO2

2. Les substrats hydrolysés « idéalement »

Cellulose

Ensilage de maïs et de céréales immatures, fumier

(apport en bactéries méthanogènes), …

Biogaz


de biogaz




de biogaz

Substrat

à digestion

lente Acides

organiques

CH4

CO2

3. Les substrats lentement hydrolysés

et fortement méthanogènes

de part leur composition

Graisses (insolubles dans l’eau)

Huiles

Boues de laiterie, fromagerie, chocolaterie Biogaz


de biogaz




de biogaz

Protéines

Acides

organiques

CH4

CO2

4. Les substrats riches en protéines

fortement méthanogènes

Déchets d’abattoire

Touteau de Colza

Protéines riches en Méthionine et Cystéine Biogaz

Si Excès à NH3 et H2S


de biogaz




de biogaz

Digestion en 2 étapes

• On surralimente l’Hydrolyseur

• Production rapide et importante d’Acides organiques

• Inhibition des Acétogènes et Méthanogènes

• Production de CO2 (80%) et d’H2 (à20%)

Ac org.

Hydrolyseur Méthaniseur

CO2 H2

H2S

CH4 =70%

• Dégradation de molécules récalcitrantes (ulvanes)

• Substrats fortement dégradables présentant un risque d’acidose (fruits, légumes, racines, …)

• Fumier, ensilages

Diagnostic ?

Ration:

1500 kg/j de céréales

750 kg/j de contenu de pense de vache

750 kg/j graisses de flottation

Analyse du digestat:

FOS: 17.300 mg/l

TAC: 24.800 mg/l

FOS/TAC 0,70

pH 7,95

Ntot: 10 g/l

N-NH3: 7.9 g/l

Biogaz:

CH4: 58%

CO2: 41%

Faible production

Echantillon pH

AGV

totaux

en

mg/kg

de MS

Acide

acétique

en mg/kg

de MS

Acide

propionique

en mg/kg

de MS

Acide

isobutyriqu

e en mg/kg

de MS

Acide

butyrique

en mg/kg

de MS

Acide

isovalériqu

e en mg/kg

de MS

Acide

valérique

en mg/kg

de MS

Acide

caproïque

en mg/kg

de MS

Digesteur 1 7.9 17300 6206 10018 514 71 593 119 12

Digesteur 2 8.0 1597 187 1390 10 2 8 - -

Digesteur 3 8.1 1860 165 1671 14 1 9 - -

Diagnostic ?

Biogaz:

CH4: 48%

CO2: 50%

Faible production

Echantillon pH

AGV

totaux

en

mg/kg

de MS

Acide

acétique

en mg/kg

de MS

Acide

propionique

en mg/kg

de MS

Acide

isobutyrique

en mg/kg

de MS

Acide

butyrique

en mg/kg

de MS

Acide

isovalérique

en mg/kg

de MS

Acide

valérique

en mg/kg

de MS

Acide

caproïque

en mg/kg

de MS

Digesteur 5.5 14 883 5 050 2 327 270 3 693 540 1 428 1 575

Conclusions• Connaissance des substrats

• Suivi quotidien de la T°, CH4, CO2, H2S, O2, H2

• Suivi hebdomadaire du pouvoir tampon (TIC)

• Suivi NH3 si substrats à risques

• Isoler les effluents des animaux malades

• Attention aux métaux lourds sous forme soluble (Cu, Zn)

• Attention au sels (KCl, NaCl) à STEP

• Attention aux cosmétiques (SiH4, siloxanes)

• Contrôle rapproché de l’insuflation d’O2

• Chaque digesteur est unique !

CH4

CO2

Références:

Dieter Deublein and Angelika Steinhauser. 2008. Biogas from

wastes and Renewable resources. Wiley-VCH Verlag GmbH & Co

KGaA, Weinheim, Germany. 443 pp. (ISBN 978-3-527-31841-4)

Michael H. Gerardi. 2003. The microbiology of anaerobic

digesters. Wiley-Interscience, John Wiley & Sons, Inc., Hoboken,

New Jersey. 177 pp. (ISBN 0-471-20693-8)

Documents

1/cours/MIB 4228 Solid W… · UNIVERSITE DE YAOUNDE I FACULTE DES SCIENCES THE UNIVERSITY OF YAOUNDE I FACULTY OF SCIENCE DEPARTEMENT DE MICROBIOLOGIE DEPARTMENT OF …