Separate collection and biological waste treatment in the EuropeanCommunity

Werner Bidlingmaier*, Jean-Michel Sidaine & E.K. PapadimitriouBauhaus-Universität Weimar, Fakut€at Bauingenieurwesen, Condraystrasse, D-99423 Weimar, Germany(*author for correspondence, e-mail: waste@bauing.uni-weimar.de)

1. Introduction

It is obligatory for all member states to adopt theguidelines of the EU in their national law. Ofspecial importance are the ordinances for landfill-ing (1999/31/CE) and incineration (2000/76/CE).Great discrepancies exist with both the stipulationof the limit values and the realisation of a modernwaste management.

On a first view Europe can be divided in threegroups:

Group 1: The national conditions and prescrip-tions have been worked out before theEU guidelines were published. Ger-many, Austria, Denmark, Luxem-bourg, the Netherlands and Belgium(Flanders) belong to this first group.

Group 2: The EU standards have been adoptedas a target in the national law, therealisation already converted: France,Italy, Sweden, England and Finland

Group 3: The EU standards are acknowledgedas a future target, the realisation is onthe way: Greece, Ireland, Portugal andSpain.

2. Quantities of organic waste

An enormous quantity of organic waste is pro-duced annually within the EU (Figure 1). By far,agriculture is the biggest contributor in organicrejects, followed by yard and forestry waste, sew-age sludge waste water treatment plant (SWWTP),food processing waste and organic fraction ofmunicipal solid waste (OFMSW). The vast

majority of this waste stream can be subjected toeither anaerobic digestion (AD) or compostingprocessing or both. It is therefore obvious that thepotential role of these two biological treatmentmethods in a pan-European waste managementplan will be important.

3. Separation/collection of organic waste

In as much as the quality of a feedstock influencesthe quality of the resulting end-product, separa-tion is a crucial element in implementing AD orcomposting. Agricultural waste, yard waste, foodprocessing waste, forest and forest product resi-dues, and SWWTP are usually homogeneousmaterials with low level of physical impurities.Thus, the need to separate does not really apply tosuch wastes. One might express concerns about thepresence of xenobiotics or heavy metals in suchmaterials. These substances are absorbed ontosolid particles (e.g., pesticides in yard and agri-cultural residues) or diluted in the liquid phase,and therefore they are not prone to physical sep-aration or removal.

Organic waste separation is linked inexorablywith the biological treatment of OFMSW, and itsimportance is reflected in the repeated failuresassociated with un-sorted MSW compostingoperations throughout the last 40 years. As aresult, in countries such as Germany, Switzerland,Denmark, and The Netherlands, source separationschemes have been widely adopted (IEA 1996).The degree of efficiency of the separation stagedefines, to a certain extent, both the diversion rateof biodegradable organics and the quality of the

Reviews in Environmental Science & Bio/Technology (2004) 3: 307–320 � Springer 2005DOI: 10.1007/s11157-004-2334-1

compost produced. In addition, the separationstrategy affects the economics of the overall man-agement system.

The role of AD and composting in MSWmanagement may be exemplified by working outthe theoretical recovery rate for the total biode-gradable fraction of MSW. This might be as highas 52%, assuming an average concentration of30% for food, vegetable and fruit waste, 12% forgarden waste, and 10% for non recyclable biode-gradable paper (Table 1). This figure mightbecome higher with the introduction of biode-gradable polymers in every day life activities.

The separation of OFMSW may be carried outeithermanually by the end-users/consumers (sourceseparation) or mechanically at a central facilitywhich is usually an integral part of a composting orAD plant. Source separation usually takes place byusing two separate bins; one for the biodegradablefraction and the other for the recyclables, inert, andhazardous materials. A variation of this system

arises by, in addition, incorporating source-sepa-ration of household, hazardous material such asbatteries, solvents, pesticides, and cleaning agents.There might also be central collection points forglass and paper collection.

The selection of a collection/separation systemmay be based on criteria such as separation effi-ciency, suitability of feedstock for biological pro-cessing, potential quality of end product, and, ofcourse, the cost involved.

The cost increases with the number of binsused, i.e., with the degree of source separation. Onthe other hand source separation leads in a lesscontaminated feedstock which in its turns favoursa better marketable end-product. Feedstock orig-inated from un-sorted MSW contains 30–40%, byweight, contaminating material (e.g., heavy met-als) and impurities such as plastic film, brokenglass, etc. On the other hand, a two-bin system(i.e., biodegradables source separation) yields afeedstock of which the foreign polluting matercontent is in the range of 2–12% by weight (ORCA1992). The impact of different separation methodson the heavy metal content of compost is shown inTable 2 of which the data clearly dictate the ben-eficial effect of source separation. It, is however,expected that the one-bin system will carry onserving densely populated areas where installationof infrastructure is not easily performed.

The most frequently reported disadvantage ofthe two-bin system is the low level of comfort itoffers. This applies to cases where fairly longperiods (usually once per fortnight) involved incollecting the putrescibles which, especially duringthe warm period of the year, release odours andattract vermin. This situation might be amelio-rated by disposing of in the ‘‘green’’ bin non-recyclable paper (e.g., spoiled, or coated paper)thus increasing the structural properties of the‘‘biowaste’’. Such a practice has been found not toaffect negatively the composting process (e.g., bycausing a prolonged duration owing to lignocel-lulosic ingredients), or the compost quality. Nor itaffected recyclable paper collection rates (Boelenset al. 1995). Beneficial effects from this practicehave also been reported with regard to AD oper-ation which resulted a higher methane yield,despite the general belief in cellulose retarding ADprocesses (Baeten and Verstraete 1993).

The ‘‘Technical Directive for municipal solidwaste’’ (Technische Anleitung Siedlungsabfall

0

200

400

600

800

1000

1200A

gric

ultu

ral

was

te

SW

WT

P

OF

MS

W

Yard

and

fore

sty

was

te

Foo

dpr

oces

sing

was

te

Mill

ion

tons

per

yea

r

Figure 1. Annual production of various organic waste streamsin the EU territory.

Table 1. Average MSW composition in EU

Fraction Concentration

(% by weight)

Paper 25–35

Plastic 7–10

Ferrous metals 3–5

Non ferrous metals 0.5–2

Glass 5–10

Ceramic 1–2

Food, vegetable and fruits 25–35

Yard waste 10–15

Hazardous waste 1–2

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1993) regulated collection and treatment of bio-waste at first. The Directive regulated also theapplication of the compost by the LAGA M10(1995). Compost plants in Germany have to bebuilt housed in a structure (i.e., enclosed) to pre-vent odour problems.

4. The Recycling of wastes from packaging

The recycling of wastes from packaging has beenintroduced in all countries of the EU except ofGreece. The organisation and the standard ofdistribution, however, varies (Tables 3 and 4).

The above mentioned results can be explainedwith the different financing means, which areavailable for the competent authorities. The feespaid for the same product are presented inTable 5. Here are enormous differences shownwhich can mount up to a ratio of 1–60 (e.g., a glassbottle: Portugal: 0.52 Euro · 10)3, Austria: 30.52Euro · 10)3).

5. Biological waste treatment

The politic of European Countries regarding therecycling of organic wastes can be classified inthree categories (Figure 2):

� Austria, Belgium (Flanders), Denmark,Germany, Luxembourg and the Netherlands(also Switzerland as a country that does notbelong to the EU) have by far realised theirpolitical task for a collection and treatment oforganic wastes. Within the last 3 years clim-bed the connection quota for separate collec-tion in the Netherlands from zero to 95%. TheEU countries of this first category recycle 85%of the momentarily separately collected andtreated organic waste fraction in Europe viacomposting. The digestion plays a secondaryrole for the moment.

� Belgium (Wallonia), Finland, France, Italy,Sweden and Great Britain belong to thesecond category. Norway, also, as a countrythat does not belong to the EU, can be

Table 3. Organisation of the recycling of wastes from packaging material

Countries Performance of collection and treatment

(household packing)

Organised/financed by:

Belgium Municipalities/territorial entity Fost Plus

Denmark Municipalities/territorial entity Industry

Germany DSD+other private organisations Industry (DSD + Garantiegeber)

Finland Municipalities/territorial entity PYR

France Municipalities/territorial entity Eco-Emballages, Adelphe

Great Britain Municipalities/territorial entity Industry

Ireland Municipalities/territorial entity Repak

Italy Municipalities/territorial entity CONAI

Luxembourg Municipalities/territorial entity Valorlux

Netherlands Municipalities/territorial entity Industry

Austria ARGEV + other private organisations Contracting partner (material-specific)

Portugal Municipalities/territorial entity Ponto Verde + contracting partner

(material-specific)

Spain Municipalities/territorial entity Eco-embalajes

Sweden Contracting partner (material-specific) Contracting partner (material-specific)

Table 2. Impact of OFMSW separation methods on heavy metal content of compost (in mg/kg dry weight)

Separation Zn Pb Cd Cr Cu Ni

Non; un-sorted MSW 1510 513 5.5 71 274 45

Mechanical at central facility 510–770 356–484 1.8–2.3 22–30 173–252 23–33

Source separation of OFMSW 290 87 1 49 47 22

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added to this group. For the time being theyare creating and realising the political andhousekeeping frame conditions for compo-sting.

� Countries where a politic for recycling and/orseparate collection of the organic fraction isneither enforced nor planned, like Greece,Ireland, Spain and Portugal belong to thethird category.

As a summary the political frame conditionsin Europe show a distinct tendency towards theseparate collection and recycling of organicwastes (Table 6). The promotion of home com-posting is in several countries a component ofwaste policy.

Figure 2 shows the actual amount of theorganic waste quantities which are collected sepa-

rately in the EU. More than 80% fall to thecountries of Denmark, Germany, Belgium, TheNetherlands and Austria.

5.1. Composting

When we look into the past, the history of com-posting is a history of problems. The followingproblems have been discussed:

� 1965 glass in compost� 1970 plastic in compost� 1975 heavy metals in compost� 1978 PCBs in compost� 1984 odour emissions from composting plants� 1989 dioxin in compost� 1990 germs in the exhaust air� 1996 control of the process

Table 4. Systems, collection in practice and results

Countries Distribution of the collection 1999 Collection system Results (1997)

Material

recycling %

of the potential

Energetic

recycling %

of the potential

Recycling

total %

of the potential

Belgium Nearly area-wide Fetch system, except glass 62.3 No data 62.3%

Denmark Area-wide Varying 48.7 38.0 86.7

Germany Area-wide Mainly fetch system

Bring system for glass

and paper

78.3 2.3 80.5

Finland Only in towns Bring system 41.8 12.2 54.1

France Under construction

(40–50% connected)

Varying 41.0 14.5 55.5

Great Britain Some towns and areas Mainly bring system 31.3 3.2 34.5

Ireland Under construction Bring system No data No data 14.8

Italy Mainly in the

North of Italy

Varying 29.6 2.2 31.8

Luxembourg Mainly fetch system

Bring system for

glass and paper

Netherlands Area-wide for glass,

paper and cardboard

Mainly bring system 55.2 22.4 77.6

Austria Area-wide Mainly fetch system

Bring system for glass

and paper

64.8 4.8 69.6

Portugal Under construction Mainly bring system k.A.

Spain Under construction Mainly bring system 34.4 1.6 36.0

Sweden Area-wide Bring system 57.9 7.2 65.1

EU-11 Total 46.3 6.3 52.6

Following: ‘‘Composting in the European Union’’, DHV, Amersfoort, 1997 (up-dated and supplemented).

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Every single problem mentioned above is enoughto prevent composting. Especially odour isbecoming an increasingly sensitive issue as people

move closer to existing treatment plants. Whensites for new facilities are proposed, the potentialfor odour is invariably one of the first concernsraised by local residents. Therefore, odours havebeen rated as the primary concern of the publicrelative to implementation of composting facilities.Designers must be conscious of this fact and befamiliar with odour generation and odour control.

Perhaps the most important fallacy in compo-sting research and implementation is thediscussion of composting systems by consideringphysical attributes and material handling aspectswhile neglecting process microbiology. Conversely,composting is a tolerant process involving rapidmicrobial succession rates. A different approachhas been attempted by distinguishing two groups

Germany50%

Luxembourg0%

Sweden3%

Denmark5%

France4%

Great Britain3%

Austria11%

Netherlands18%

Italy2%

Belgium3%

Finland1%

Figure 2. Collected and composted organic waste in the EU(2002) Total amount: approx. 12 million tons.

Table 6. Status of separate collection and composting in Europe

Country Separate Collection = SC Home Composting = HC Composting of Mixed Waste = MW

Belgium (FL) SC HC –

Denmark SC HC –

Germany SC HC –

Finland SC HC (MW)

France SC HC MW

Greece – – MW

Great Britain SC HC

Ireland – – MW

Italy SC – MW

Luxembourg SC HC –

Netherlands SC HC –

Norway SC HC –

Austria SC HC

Portugal – – MW

Spain – – MW

Sweden SC HC –

Switzerland SC HC –

Table 5. Comparison of fees for the ‘‘Green Dot’’ (Grüner Punkt, Germany)

kg Fee for different materials in Euro · 10)3

Austria Germany Belgium Luxembourg Portugal Spain France

Glass bottle (1 l) 0.35 30.52 28.46 6.77 5.99 0.52 2.40 0.75

Tetra pack (1 l) 0.027 5.47 25.28 6.14 5.69 0.27 2.25 2.99

PET bottle (1 l) 0.03 32.90 45.12 10.44 8.59 1.20 3.53 3.47

Aluminium can (33 cl) 0.015 6.92 13.65 2.40 2.00 0.52 0.76 0.45

Iron can (33 cl) 0.03 11.97 11.61 1.74 1.24 0.52 0.93 0.42

Cardboard 1 202.76 190.64 37.68 31.23 9.98 15.47 74.09

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of composting processes; the first encompassespractices which facilitate an optimum microbialecosystem management while the second ones donot do so.

The following terms are defined and are relatedto oxygen and temperature control havingassumed that oxygen and temperature are theprimary process control parameters:

� automatic feedback: oxygen and temperatureevolution is followed continuously and, byusing a computer, aeration is commenced tosatisfy predetermined set points (for oxygencontent and temperature levels);

� manual feedback: oxygen and temperaturereadings are periodically taken by personnelto adjust temperature and/or oxygen at presetvalues.

� initial condition: oxygen and/or temperaturelevel is regulated within a quite broad range ofvalues by adjusting, at the onset of a compo-sting process, aeration rate or pile size. Theterm ‘‘adjusting’’ implies very little or no time-course intervention; and

� default condition: there is no deliberate processcontrol. In other words, the nature of spon-taneous self heating ‘‘controls’’ the process.

The term ‘‘open’’ applies to the configurationwhich allows the material to come into contactwith common surrounding air (e.g., windrow, pile,or bay arrangements). Subsequently, in the case ofa ‘‘container’’ system, the composting materialinterfaces only with the head space available in thereactor, as the rest of the material’s surfaces comeinto contact with the reactors walls (e.g., theso-called in-vessel or reactor systems). Both openand container systems might be enclosed in a shedor a building.

A robust microbial ecosystem managementsustains a high process rate, and, thus, entails,among others, the following certain economic andpractical benefits:

(1) reduced capital and operating costs;(2) minimisation of material handling;(3) odour prevention at its source; and(4) a better stabilised compost production

(Finstein 1992; Miller 1993).

That way, factors causing a composting projectto fail, such as poor public acceptability, andlimited compost marketability are restricted from

occurring (Panter et al. 1996). It might, conse-quently, be claimed that composting practicesaiming at supporting a wealth microbial ecosystemare the most preferable ones. Nonetheless, differ-ent circumstances introduce different constrains,and, therefore, a case sensitive approach shouldalways be adopted. Table 7 provides an overviewof pros and cons of some of the most frequentlyused composting systems.

As indicated by Tables 7 and 8, Windrowcomposting might be a good option for managingyard or any other seasonal waste stream at a re-gion with a high land availability. Such a facilityshould be located sufficiently far away from in-habitated areas to prevent odour complains. Aer-ated static pile (ASP) or aerated pile with turning,assuming forced pressure aeration, might be agood compromise between cost and efficiency butthe odour potential is still present. Negative pres-sure aeration ASP is outclassed by positive pres-sure aeration ASP which is more efficient andimplies a lower cost. Finally, container systemswith automatic temperature and/or oxygen controlrepresent the state of the art with regard to pro-cesses efficiency and health and safety standardscompliance, however, at a higher cost.

A block diagram describing the various unitprocesses and material flow of a composting plantis provided in Figure 3. It should be pointed outthat both the degree of pre- and posttreatmentdepends on the quality of the waste arriving at afacility. Even with source separated waste though,a certain degree of pretreatment is needed to re-move unwanted material such as plastic, ferrousand non-ferrous metals etc. The particle size andnutrients balance adjustment almost invariablyhave to be practised.

The term ‘‘active’’ phase covers the processingcourse during which high temperatures areattained by virtue of biodegradable matter’sabundance. This phase is followed by the‘‘stabilisation’’ phase at the end of which thecomposting material reaches near-ambient tem-peratures. Last, the curing phase may or may nottake place depending on compost quality stan-dards to be fulfilled.

5.2. Anaerobic digestion

AD systems can be divided into two generic cate-gories, namely those of high-solids digestion

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Table 7. Advantages and disadvantages of common composting processes

OPEN TYPE CONTAINER TYPE

Windrow Aerated static pile (ASP) Air recirculation Air once through

(preset frequency of

material turning -i.e.

initial condition)

negative pressure

(control with blowers

set on timer is

assumed - i.e., initial

condition control mode)

positive pressure

(temperature feedback

control is assumed -i.e.

automatic feedback control

mode)

(Temperature and

oxygen feedback

control -i.e.

automatic feedback

control mode)

(Temperature and

oxygen feedback

control -i.e.

automatic feedback

control mode)

Advantages

Low cost option

Simple to operate

Acceptable compost

quality

Useful in conjunction

in combination with

positive pressure

Effective heat removal

Low land requirements

Faster decomposition

Less prone to odour

generation than

negative pressure systems

Reduced amounts

of exhaust air

High rate

composting

Off-gas attainment

Complies with high

health and safety

standards

High rate

composting

Off-gas attainment

Complies with high

health and safety

standards

Disadvantages

Low level process

control

High land

requirements

Odour causing

Generates dust

Difficult to operate

(e.g. duct clogging)

throughout the

material

Odour problems

Leachate generation

Indoors, it causes

snowing or training,

and excessive

amounts of off-gas

to be treated

Material stratification

Slow decmposition

More costly than

positive pressure

aeration

High capacity blower

needed

Odour problems

Leachate problems

possible

Indoors, it causes snowing,

or raining, and excessive

amounts of off-gas

to be treated

Material stratification

Skilful staff is

needed

High investment

and operating

costs

Need to treat

leachate from the

condensation

chamber

Skilful staff is needed

High investment

and operating

costs

Need to treat

leachate from the

condensation

chamber

More exhaust

off-gas to be

handled

Table 8. Composting system versus detention time (The Composting Council 1994)

Process Type Windrow

(control initial condition,

plus water replenishment)

ASP (automatic feedback-

temperature control,

plus moisture replenishment)

Aerated Pile

(control as in ASP,

plus turning)

Container

(control as in ASP)

Processing phase Duration

Active 16–40 days 16–30 days 14–21 days 4–15 days

Stabilisation-

temperature

decreasing

30–60 days 30–60 days 21–60 days 21–45 days

Curing up to 8 months

(turned)

1–3 moths

(static aerated)

1–2 months (turned,

aerated, water added)

1–2 months

(turned aerated

water added)

Total time 2–12 months 2–6 months 1.5–6 months 1–4 months

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(HSD) and low-solids digestion (LSD) systems.The former implies a dry matter concentrationwithin the reactor in the range of 20–40% whilethe latter one of less than 20% (usually 3–15%).

LSD is mostly operated at a continuous mode toenhance process stability. On the other hand, HSDprocesses may be either continuous or batch. Afurther distinction may be drawn based on whetherthe material in the digester is completely mixed ornot (Figures 4 and 5). Last, a digestion process canbe described as being a one-step or two-step process

depending on whether methanogenesis takes placein a distinct reactor, separately from hydrolysis andacidification. Of those two stages the latter maytake place either in a HSD or a LSD reactorwhereas the former is a LSD process.

LSD systems are mainly applied to liquid wastetreatment, although the stirred tank reactor(CSTR) method has been applied to solid wastetoo. A Batch HSD system (i.e., the BIOCEL)results in excessively long processing times com-pared to the DRANCO (plug flow) and VAL-

Figure 3. Flow chart composting

314

Figure 4. Flow chart AD.

315

ORGA (mixing dynamics between stirred and plugflow) techniques (Table 9). HSD systems sufferfrom the less number of drawbacks compared toLAD and two-step methods. Two-step systemsmay have a lower retention time than one-stepones (Table 10), but result in high amounts ofeffluent which need to be processed. The latterapplies to CSTR too. Conversely, this is not thecase with HSD, as the effluent quantity is so lowthat it can easily be reapplied onto the digestedmass (van Santen et al. 1997). CSTR is energydemanding as a result of heating, and pumpingoperations. It also implies, similar to two-phasesystems, a high capital cost stemming form thebigger tanks size and the higher number of unitprocesses involved. The latter makes, in addition, aprocess more complex to operate. The advocatedhigher methane yield per solids unit mass of LSDis, apparently, overweighed by the smaller energydemands of HSD as a result of the high heatstoring capacity of a high-solids mass. Care shouldbe taken, however, to keep total solids concen-tration lower than 40% as above that level inhi-

bition of methane production commences(Kayhanian et al. 1991). Finally it might be con-cluded from the aforementioned that HSD seemsthe most appropriate for treating municipal andother organic solid waste.

Regarding the best temperature operatingregion (i.e., mesophilic vs. thermophilic), generallyspeaking it might be said that mesophilic levelspromote higher process stability at a low heatingdemand (Van Santent et al. 1997). However, HSDsystems can sustain thermophilic temperatures atvery low or nil heating requirements. A thermo-philic temperature is said to be more conducive topathogens inactivation and weed seed destructionas well, despite the lack of conclusive evidence(Brinkman et al. 1997).

5.3. Composting vs. anaerobic digestion

An emphatic opinion can not be expressed withregard to the supremacy of either composting orAD. Rather, when having to choose betweenthose two processes, a case dependent approachshould be followed. Such an approach ought toinvolve a decision-making procedure based oneconomic, and environmental (including social)values. Consequently, in this section a generalcommentary is attempted by referring to the fol-lowing aspects:

(1) process robustness;(2) waste stream amenability;(3) environmental impact; and(4) economic cost.

Figure 5. Schematic classification of AD systems.

Table 9. Typical data on operational parameters and methane yield in representative AD processes

AD process OLRa HRTb TSc (%) Temperature CH4 yield Reference

LSD (days) (�C) (l/kg VS)

CSTRd 2–7 kg COD/m3 10–20 7–20 30–55 120–300 Baeten and Verstraete (1993)

AFe 1–5 kg COD/m3 d 1–2 5 mainly

mesophilic

Metcalf & Eddy (1991)

HSD

VALORGA 18–20 VS/m3 d 9 30 60 220 Begouen et al. (1988)

BIOCEL 2–3 VS/m3 d 60–90 30 35 270 Ten Brummeler et al. (1988)

DRANCO 15–20 VS/m3 d 15–20 30–40 55 235 Van Meenen and Verstraete (1988)

Two-stepf 8–40 kg VS/m3 d 2–7 10–30 30–37 195–290 Baeten and Verstraete (1993)

(a) OLR: organic loading rate; (b) HRT: Hydraulic retention time; (c) TS: total solids; (d) CSTR: continuous stirred tank reactor; (e)anaerobic filter; (f) all figures but the one of methane yield refer to the 1st step.

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5.3.1. Process robustnessFundamentally speaking, AD robustness is limitedby the very nature of its microbiology. First,anaerobic metabolism takes place at lower ratesthan the aerobic one. Second, microbial successionand, thus, recovery in the case of a system’s failureare generally slower in AD than in composting(Finstein et al. 1980). The latter, in contrast towhat happens in AD with respect to methane-forming bacteria, does not depend on any specia-lised group of microorganisms, and, in addition, itinvolves a diverse microbial population of whichthe component genera and/or species exhibit opti-mum activity under a wide array of environmentalconditions. However, process engineering in ADhas produced systems which generally can workwithout frequent process inconsistency problems.

The theoretical assumption that composting ismore rapid than AD is not reflected in real pro-cessing times however. A typical VS reduction in a2–3 weeks period of AD processing lies in therange of 60–75%. Composting, seems to performat a slower rate, although figures of VS reductionas high as 73% in 16 days can be achieved. Thissituation might be attributed, among others, to

inappropriate process control and engineeringwhich generate conditions inhibitive to aerobicdegradation and favourable to anaerobic metab-olism. In fact, the latter has been found responsi-ble for a substantial portion of the overallmetabolic activity during composting (Tseng et al.1995; Atkinson et al. 1996). Hence, it would not beunreasonable to infer that composting biodegra-dative efficiency, unlike that of AD systems, hasnot been realised in practice at a high degree.

5.3.2. Waste stream amenabilityRegarding waste stream amenability, both pro-cesses may be applied to any organic waste pro-vided that certain physical and chemicalconditioning aspects of the starting material aretaken care of. It should be mentioned neverthelessthat composting can generally cope better withlignaceous waste (e.g., wood residues). On theother hand, the need for structural material in ADis minimum and not so important as in compo-sting. Composting possesses an advantage overAD when it comes to on-farm, or back yard wastemanagement practices.

Table 10. Comparison between LSD, HSD, and two-step systems

AD process

category

Advantages Disadvantages

LSD (CSTR) High methane yield/solids unit mass

More suitable for slurries and sludges

Influent pretreatment is required

Lower energy production rate per reactor unit volume

than HSD

Effluent processing is needed

Digestate’s dewatering is needed

Energy consuming

Large capital cost

HSD Higher organic loading rates than CSTR

Very small quantities of water needed

High energy production rate per reactor

unit volume than

Minimum to nil heat inputs

No effluents to be treated

Batch systems are very cheap

Batch systems produce digestate demanding

intensive treatment

Slightly lower methane yield per solids mass unit

Two-phase More suitable for complex wastewaters

containing recalcitrant compounds

High volumetric conversion rates

May immobilise heavy metals

Complex to operate

Effluent processing is needed

Digestate needs further stabilisation

Doubtfully efficient with regard to sanitisation

Large capital cost

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5.3.3. Environmental impactEnvironmental impact is taken to mean any posi-tive or negative influence, with regard to environ-ment, which arises form implementing AD, orcomposting. Both process facilitate carbon andother nutrients cycling. Composting alone mayresult in a mature product while digestates usuallyare less stabilised and sanitised, and necessitatefurther aerobic treatment (Brinkman et al. 1997).On the other hand AD usually preserves nitrogencontent and produces a digestate with a higherfertilising value than aerobic composts.

Resources conservation is included among theenvironmental impact factors. AD has a greaterpotential recovery capacity as it may recover bothenergy and material. As a rule of thumb, an ADplant can have a net energy yield equivalent to 100–150 kW he/tonne feedstock whereas in compostingan input of 30–35 kW he/tonne feedstock is needed(IEA 1996). Energy extraction from composting inthe form of hot water has ranged between 4 and 10.9MJ/kg input TS (Thorstrup 1985; Verstraete et al.1985).However, the low form of that energy and thedifficulties associated with its extraction has notencouraged further work on this topic.

Air emissions is another important parameterat play. Odour potential is low with AD, as pro-cessing takes place in air-tight containers and thebiogas is stored before being utilised. Composting,however, gives rise to odour problems. In a recentstudy (De Baere and Kaendler 1997) compostingof a digestate resulted in 7-fold and a 200-foldreduction in volatile compounds and volatile or-ganic compounds, respectively, in comparison tothe emissions which would have arisen if compo-sting had not been preceded by AD. On the otherhand digestates are malodorous themselves. Withregard to greenhouse gases CH4 and CO2, Tufdrup(1994) hinted that there is a reduction in CO2 of130 kg CO2 /m

3 biomass owing to deceased ferti-liser manufacturing and fossil fuels use. However,the paucity of data on this topic, especially withregard to CH4 and N2O emissions, does not renderany further remarks possible.

Dust and bioaerosols have an equal potentialto be produced at the pretreatment lines of bothAD and composting plants. However, in examin-ing the emissions from the actual processes, opencomposting may generate high-germ emissions atthe work place (Fischer 1996). This is not the casefor the well-contained digesting material.

Liquid emissions are not so important in con-sidering environmental aspects because anyleachate or wastewater produced (the latter arisingfrom AD only) can easily be treated by usingwastewater treatment technology. Rather, theproduction of such effluents is of an economicimportance.

5.3.4. Economic costCost has always been a key factor in waste man-agement decision-making. Unfortunately, detailedstudies on this field are also lacking. In Figure 6,the cost for MSW processing by composting andAD with nominal plant capacities is graphed forthe country of The Netherlands (IEA 1996). It isobvious from that figure that composting ischeaper than AD. The same is suggested by thedata of Baeten and Vestraete (1993) who statedthat both investment and operating cost for AD isalways greater than that of composting in Europe,the operating cost of container composting andAD being almost the same however. The cost of anAD plant is expected to be reaching higher levels ifan aerobic postprocessing stage is included. Nev-ertheless, in a currently environmentally mindedatmosphere, it is anticipated that motivationmeasures, such as better prices and tariffs, wouldrender AD economically more competitive.

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0

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