1 FEASIBILITY STUDY OF STAND-ALONE SMALL-SCALE DIGESTION SYSTEMS TO PRODUCE BIOGAS …kth.diva-portal.org/smash/get/diva2:678792/FULLTEXT01.pdf · 2013-12-13 · II Master of Science

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FEASIBILITY STUDY OF STAND-ALONE SMALL-SCALE DIGESTION

SYSTEMS TO PRODUCE BIOGAS FOR LOCAL USE

MARIE JANET EUSTASIE

Master of Science Thesis

Stockholm, Sweden Year 2012

FEASIBILITY STUDY OF STAND-ALONE SMALL-SCALE

DIGESTION SYSTEMS TO PRODUCE BIOGAS FOR LOCAL

USE

MARIE JANET EUSTASIE

MSc Thesis Year 2012

Department of Energy Technology

Division of Heat and Power Technology

Royal Institute of Technology

100 44 Stockholm, Sweden

II

Master of Science Thesis, EGI 2012: 042MSC EKV891

Feasibility study of stand-alone small-scale digestion

systems to produce biogas for local use

Marie Janet Eustasie

Approved

Examiner

Prof. Professor Torsten H. Fransson

Supervisor

Dr. Anders Malmquist

Commissioner

Prof. Professor Torsten H. Fransson

Contact person

Dr. Anders Malmquist

ABSTRACT

The purpose of this project was to carry out a feasibility study of implementing a small-scale biogas

plant on KTH campus using food wastes from the neighbouring restaurants and KTH kitchens as

input substrate, where the biogas produced could be used in the Energy laboratory, to run the micro

turbine, as an example. Considering that KTH has 10 departments and that each department has its

own kitchen, the amount of food wastes that can be recuperated for input to the plant is around 225

kg per day for a normal working week, requiring a 10m3 biogas plant.

This report also relates to the potential implementation of small-scale biogas plants on farms in

Mauritius using manure and crop endings as input substrate as a solution to the waste management

problems the farmers are currently facing as well as using the biogas to provide energy for cooking and

heating.

Based on the quantity of food wastes available from the neighbouring restaurants, the sizing of the

proposed biogas plant was calculated and the best technical and commercial options were considered.

An experimental biogas plant was setup on the campus and the planning and logistics involved were

studied as well as the potential quantity and quality of gas produced from food wastes. The optimal

biogas production for a 5 m3 capacity biogas plant and processing food wastes is 6 to 10 m3 of biogas

per day composed of about 68-72% methane. But, since the experiment was run for only a two-week

period, the optimum biogas production could not be reached. Several operational and biological

problems were encountered during the operation of the plant. However, the output of the experiment

is positive as the logistics required for the setting up and running of a biogas plant on campus or

elsewhere including the technicality of such a plant and the human resources requirements have been

deduced from the lessons learned.

Keywords: Anaerobic digestion, Food wastes, Manure, Mauritius, Biogas, Small scale plants

MSc thesis: Feasibility study of stand-alone small-scale digestion systems to produce biogas for local use Marie Janet Eustasie

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ACKNOWLEDGEMENTS

I would like to express my greatest gratitude to the Assoc. Prof. Andrew Martin, SEE Worldwide

Program Director and Prof Torsten Fransson, Chair, Heat and Power Technology, KTH as well as Dr

Dinesh Surroop, Local Supervisor, University of Mauritius and Prof Roumila Mohee, Dean of the

faculty of Engineering, University of Mauritius for believing in me and giving me this extraordinary

opportunity to complete my Master’s thesis at the distinguished university of KTH.

I would also like to acknowledge and thank Dr Anders Malmquist, Deputy Project Manager, and

Supervisor at KTH for his helpful discussions, advice and support during the execution of this work. I

appreciate his time and support in directing me towards my goals.

I would not have gone through this enriching journey if not for Mr Tord Magnusson, who sponsored

this scholarship to whom I owe my appreciation for this once-in-a-lifetime experience.

The project has obtained its financial support from InnoEnergy and the Division of Heat and Power at

KTH through their Lighthouse Project Explore Polygeneration. This project made it possible to bring

the test unit to KTH and to perform real biogas production tests through the experimental setup.

Next, I would like to extend my sincere thanks to Mr. Göran Holmberg, Akademiska hus, Mr. Claes

Henningsson, CAMPUS AB who have helped me in acquiring the necessary resources for the setup of

the experimental biogas plant on LTH grounds as well as Prof. Anna Schnürer, Dept. of Microbiology,

Swedish University of Agricultural Sciences, Uppsala for her precious help in the understanding of the

‘microbiological process’ in the experimental setup.

The Livestock division of the Agricultural Research and Extension Unit (AREU), Mauritius has been

very helpful in supplying the needful data for the basis of my project for which I am truly grateful.

Mr Gustav Rogstrand, Researcher, JTI Swedish Institute of Agricultural and Environmental

Engineering and his team have helped me a lot in the setup and operation of the experimental biogas

plant.

My thanks also go to Mr Jeevan Jayasuriya, Lecturer at the Heat & Power division at KTH who played

a leading role in expanding the Distance based Sustainability Energy Engineering (DSEE) Programme

to Mauritius and giving us a golden opportunity to follow the Sustainable Energy based Master

programme KTH.

Throughout my life my parents, my brothers Francois and Jerry, and my sister Shirley have always

played an important role in supporting me. Through these past months, they also encouraged and

supported me with kindness and I thank them tenderly.

In closing, I would like to thank Erwin for his support, encouragement and patience during this period.


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TABLE OF CONTENTS

ABSTRACT .............................................................................................................................................. II

ACKNOWLEDGEMENTS .................................................................................................................... III

TABLE OF CONTENTS ....................................................................................................................... IV

LIST OF FIGURES ................................................................................................................................... V

LIST OF TABLES ................................................................................................................................... VI

NOMENCLATURE .............................................................................................................................. VII

1 INTRODUCTION ....................................................................................................................... 1

1.1 BIOGAS ................................................................................................................................................................. 1 1.2 THE BIOGAS PROCESS .................................................................................................................................. 1 1.3 BIOGAS PRODUCTION AT KTH ................................................................................................................ 2 1.4 BIOGAS IN MAURITIUS ................................................................................................................................. 2

2 OBJECTIVES ...............................................................................................................................3

3 METHOD OF ATTACK ..............................................................................................................4

4 LITERATURE REVIEW .............................................................................................................5

4.1 PRINCIPLES OF BIOGAS TECHNOLOGY .............................................................................................. 5 4.2 FACTORS AFFECTING THE ANAEROBIC DIGESTION PROCESS .............................................. 5 4.3 ANAEROBIC DIGESTION SYSTEMS ........................................................................................................ 8 4.4 PRE-TREATMENT STEPS .............................................................................................................................. 9 4.5 BENEFITS OF BIOGAS ................................................................................................................................ 10 4.6 POTENTIAL ENERGY USES OF BIOGAS ............................................................................................ 10 4.7 THE DIGESTED RESIDUAL PRODUCT (BIO-MANURE) .............................................................. 11 4.8 HOW BIOGAS IS USED IN MAURITIUS AT PRESENT .................................................................... 12

5 EXPERIMENT SETUP ............................................................................................................. 13

5.1 AIM OF THE EXPERIMENT ...................................................................................................................... 13 5.2 AMOUNT OF WASTES TO BE DIGESTED ........................................................................................... 13 5.3 DESIGN AND SIZING OF THE BIOGAS PLANT ............................................................................... 13 5.4 CHOICE OF BIOGAS PLANT AND JUSTIFICATIONS ..................................................................... 19 5.5 LOGISTICS: PLANNING THE BIOGAS PLANT .................................................................................. 19 5.6 EQUIPMENT SETUP ..................................................................................................................................... 21 5.7 RESULTS AND DISCUSSIONS ................................................................................................................... 24 5.8 LESSONS LEARNED AND FUTURE WORKS ...................................................................................... 29 5.9 MEASURES TO IMPROVE PROCESS ENERGY EFFICIENCY ...................................................... 32

6 PRODUCTION POTENTIAL OF BIOGAS (HEAT & POWER) FROM FARMS IN MAURITIUS ............................................................................................................................... 34

6.1 ESTIMATE OF CATTLE, PIG AND POULTRY MANURE................................................................ 34 6.2 BIOGAS PRODUCTION POTENTIAL .................................................................................................... 37 6.3 POWER PRODUCTION POTENTIAL ..................................................................................................... 37

7 BIOGAS PRODUCTION TECHNOLOGIES .......................................................................... 39

7.1 PRE-FEASIBILITY STUDY .......................................................................................................................... 39 7.2 TYPES OF DIGESTERS AVAILABLE ...................................................................................................... 39 7.3 SUITABLE TECHNOLOGY FOR FARMS IN MAURITIUS ............................................................... 41 7.4 APPLICATIONS OF BIOGAS IN MAURITIUS ...................................................................................... 42

8 ECOMOMIC ANALYSIS ........................................................................................................... 43

9 POLICY AND LEGAL FRAMEWORK ..................................................................................... 44

10 CONCLUSIONS ........................................................................................................................ 46

11 LESSONS LEARNED AND FUTURE WORK ........................................................................ 47

12 REFERENCES........................................................................................................................... 48


V

LIST OF FIGURES

Figure 1.1: The General Anaerobic Digestion Process (adapted from http://www.anaerobic-

digesters.com)

Figure 4.1: Simplified anaerobic digestion of organic matter [Gujer, W., and Zehnder, A. J. B., 1983]

Figure 4.2: General scheme of a common biogas plant with continuously stirred tank reactor (CSTR) in

Europe [Institut für Energetik und Umwelt et al., 2006]

Figure 4.3: Benefits of Biogas from the anaerobic digestion of organic wastes

Figure 4.4: Potential uses of biogas and digestate from anaerobic digestion

Figure 4.5: Slurry tanker, Schouten, New Zealand (http://www.schoutenmachines.co.nz)

Figure 5.1: Philippine BioDigester by: Gerardo P. Baron, December 2004, (Tarlac City, Philippines)

[http://www.habmigern2003.info/biogas/Baron-digester/Baron-digester.htm]

Figure 5.2: Completed PE digester prior to final installation of lid and wrapping with insulation

[http://biorealis.com]

Figure 5.3: Small and medium scale digester biogas plants, by BioTech, India [http://www.biotech-

india.org]

Figure 5.4: Containerised biogas plant by BioBowser, Australia

[http://www.srela.com.au/biobowser.php]

Figure 5.5: Components of the BioBowser containerised biogas plant


Figure 5.6: The Research biogas production unit at JTI, Swedish Institute of Agricultural and

Environmental Engineering

Figure 5.7: Proposed site for the biogas plant setup

Figure 5.8 : Main components of the mobile biogas plant ( Source: JTI)

Figure 5.9: Disperator® grinder

Figure 5.10 : Amount of biogas produced as a percentage of gas storage bag volume

Figure 5.11 : Amount of biogas produced 16th June 2012 to 21st June 2012

Figure 5.12 : % of Methane in the biogas produced

Figure 5.13 : Percentage of carbon dioxide in the biogas produced

Figure 5.14 : Percentage of oxygen in the biogas produced

Figure 5.15: Upgrading of biogas (IEA Bioenergy, 2006)

Figure 6.1: Number of cattle, goats, sheep and pigs by type of breeder as at June 2011, Mauritius

[AREU, 2011]

Figure 6.2: Location of clusters of small scale poultry farms in Mauritius [AREU, 2011]

Figure 7.1: Horizontal Digester [T. Fischer & A. Krieg, 2002]

Figure 7.2: Standard Digester in Agriculture [T. Fischer & A. Krieg, 2002]

Figure 7.3: Upright Large Digester [http://www.biotech-india.org]

Figure 7.4: Geo-membrane biogas plant [http://www.biotech-india.org]


VI

LIST OF TABLES

Table 4.1: Theoretical biogas and methane production from carbohydrates, fats and proteins [Buswell

&Neave, 1930]

Table 4.2: Average characteristics of different manures and their biological methane potentials (BMP)

[1) Viljavuuspalvelu, 2004; 2) Steineck et al., 1999; 3) KTBL, 2010; 4) Ministerium für Ernährung,

Landwirtschaft, Forsten und Fischerei Mecklenburg-Vorpommern, 2004; 5) Institut für

Energetik und Umwelt et al.,2006; 6) Edström, 2011]

Table 5.1: Survey of restaurant wastes

Table 5.2: Mobile biogas plant basic data (Source: JTI)

Table 5.3: Operation of the biogas plant

Table 5.4: Results of analysis of feed substrate (sample taken on the 13th June 2012)

Table 5.5: Results of analysis of digestate samples at the beginning and at the end of the experiment

Table 6.1: Number of breeders and livestock/ poultry status by geographical district as at June 2011

[AREU, 2011]

Table 6.2: Standard live-weight values of animal husbandry [Werner U., Stohr U., Hees N., 1989]

Table 6.3: Biogas production from different feedstock

Table 6.4: Average production potential of biogas power and heat in Mauritius from farm manure.


VII

NOMENCLATURE

Subscripts

Ntot Total Nitrogen

Abbreviations

°C degrees Celcius

A Amperes

AREU Agricultural Research and Extension Unit

BMP Biological Methane Potentials

CH4 methane

CHP Combined Heat & Power

DIY Do-It-Yourself

DM Dry Matter

DMC Dry Matter Content

EC European Community

EUR Euro

GEF Global Environment Facility

GHG Green House Gas

Gkg Giga kilogram

Gm3 Giga cubic metres

GWh Giga Watt Hour

h/d hours/ day

HRT Hydraulic Retention Time

hr or h hour

IPP Independent Power Producers

JTI Swedish Institute of Agricultural and Environmental Engineering

KTH Kungsliga Tekniska Hogskolan (Royal Institute of Technology)

kg Kilogram

kW Kilowatts

kWh Kilo Watt Hour

LPG Liquefied Petroleum Gas

m3/t cubic metres per tonne

MID Maurice Ile Durable (Mauritius Renewable Island)

min minutes

MW Mega Watt

Nm3 Normal Cubic Metres

No. Number

OLR Organic Loading Rate

PLC Programmable Logic Controller

PN Pressure Number

PPE Personal Protective Equipment

Prof. Professor

Ref. Reference

TS Total Solids

tFM tonnes of Fresh Matter

tVS tonnes of Volatile Solids

UNDP United Nations Development Programme

VS Volatile Solids


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1 INTRODUCTION

This project has been chosen to be able to demonstrate the feasibility of the implementation of a

small-scale anaerobic digester at KTH using food-wastes from KTH kitchens and neighbouring

restaurants as substrate and using the biogas produced in the energy laboratory. This project will also

cover the feasibility of implementing small-farm-scale anaerobic digesters as a means of farm waste

management as well as a source of renewable and carbon free energy in terms of biogas in Mauritius.

The anaerobic digestion of biomass to produce biogas is said to be a model in choosing the best

alternative sources of energy for rural areas using the reasoning that it is cheap and it can be locally

produced and used. Also, the biogas produced can be used for a number of purposes such as heating,

lighting, fuel for cooking, local or on-the-grid electric power generation.

1.1 BIOGAS

Raw biogas is a colourless mixture of methane (60-70%), carbon dioxide (20-30%), and trace amounts

of hydrogen sulphide depending on the conditions of production and on the origin of the input

substrates. The biogas produced is usually saturated with water vapour.

1.2 THE BIOGAS PROCESS

Biogas is generated when organic material (manure, food wastes, yard wastes, sludge from sewage

treatment plants, slaughterhouse waste, crop residues, etc.) is decomposed by micro-organisms in

anaerobic (i.e. oxygen-free) conditions. Biogas is also produced in natural environments where the

availability of oxygen is limited, such as in swamplands and in the stomachs of ruminants. Anaerobic

decomposition also takes place in landfills and different types of biogas plants. The remaining residue,

digestate, contains plant nutrients such as nitrogen, phosphorus and potassium, preserved from the

substrates used and is a very good natural fertilizer which can be used in agriculture.

Figure 1.1: The General Anaerobic Digestion Process (adapted from http://www.anaerobic-

digesters.com)

This biogas can be used as it is for cooking, heating or it can be upgraded (pressurised and removal of

CO2 and H2S) and used to provide electricity or used as vehicle fuel.


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1.3 BIOGAS PRODUCTION AT KTH

In line with the plan of developing the polygeneration mobile and flexible energy conversion unit that

can function during extreme conditions, the small-scale biogas plant would be studied as an option for

energy production. The biogas produced in the biogas plant would be used to test equipment such as

the micro turbine in the energy laboratory.

Also, it must be noted that it is just a matter of time for Swedish regulations to impose the segregation

of organic wastes in institutions such as schools, restaurants, hospitals, etc as well as at home.

This project will also open the opportunities for future collaboration of KTH to other stakeholders in

the biogas production field

1.4 BIOGAS IN MAURITIUS

The importance of energy in the development of Mauritius as a Small Island cannot be over-

emphasized. Energy represents a hub around which the island’s industrialisation and development

revolves. Any change in the energy supply chain in time results into serious economic and social

crunch. The role that energy plays in the production sustainability of industrial activities and in the

elevation of the standard of living of the people is significant [Sambo, A. S. 2005].

The introduction of farm-scale anaerobic digesters for the production of biogas from manure is a

perfect fit to the Maurice Ile Durable (MID) or Mauritius Sustainable Island project. One of the thrusts

of the project is to make Mauritius less energy dependent on fossil fuels with a target autonomy of

65% by 2028, or a target 20% of our energy need from such renewable energy by 2025 through

increased renewable energy use and the efficient use of energy in general [Ministry of Public Utilities

Mauritius, 2008].

There exists at present a considerable number of small-scale to medium-scale poultry and pig farms as

well as an increasing number of cattle farms in Mauritius. There are serious disposal problems and

environmental pollution arising from the significant quantities of manure from these farms.

Furthermore, the biogas potential from this sector would be 16 GWh/year (Table 5.4).

The current waste management practices consist of letting the manure dry and used as natural fertilizer

or further composted to be used as soil conditioner.

Therefore, there is a need to promote the biogas technology in Mauritius considering the large

populations of poultry, pig and cattle farms and the related agricultural activities. This technology

would provide an effective use of the farm manure and effluents and also reduce the environmental

pollution caused by the disposal of manure.

Furthermore, these stand-alone remote systems can be engineered to meet the client demand without

being connected to the grid. Often, farms in Mauritius are located in remote and inaccessible areas the

extension to the electricity grid is not a cost effective solution and sometimes not technically possible.

Therefore, these systems are most promising for the energy production in such cases.

We should not forget to point out that although the potential for generating biogas energy from wastes

in Mauritius is substantial, due to the Solid waste management policies in place in Mauritius, it is

important to maintain a good coordination with the concerned authorities and corresponding policies.

Integrated farming is currently being exploited in Mauritius and the benefits will be able to bring a

solution to the energy issues at the local level and probably as part of distributed energy systems. The

magnitude if biogas production from animal waste will be quantified for more comprehensive analysis

in this report.

Further in this report, I will also introduce the two cases where biogas is recovered and used as energy

in Mauritius at present. These two cases are at the St Martin wastewater treatment plant and at the

Mare Chicose landfill.


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2 OBJECTIVES

This project is aimed at the following as it major objectives:

Demonstrate how we can use low-cost technology digesters and bring engineering to it to improve

its efficiency so that it can be affordable and easily sustainable by the users.

The interest of my project is to study the possibility of producing biogas from kitchen and

restaurant wastes from KTH to be used in the Energy Engineering Laboratory

Also, to relate this experiment to the possibility of treating farm and crop wastes from small farms

in Mauritius to produce biogas to be locally consumed for cooking and heating

To create awareness among farmers in Mauritius related to the potential of generating biogas from

farm wastes and also generating a bio-fertiliser.


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3 METHOD OF ATTACK

Initially, a work plan to schedule the project work was implemented.

According to the work plan, the background research work of the anaerobic digestion process

including the literature review and consultations with local biogas producers and equipment suppliers

was undertaken.

A comparison of the different available technologies was studied and the best option for the farm-scale

digester in the Mauritian context is to be chosen.

Apply engineering knowledge to the proposed digester to improve the efficiency and to get a better

quality of biogas produced.

Study of the different legal regulations and permits required for the setting up of an anaerobic digester

as well as the health and safety requirements prevailing locally and how it applies to the Mauritian

context.

The setup of a digester on the KTH university grounds would allow:

a) Verification of compliance to existing legal and safety requirements

a) Identification of source and amount of biomass available for testing

b) Determination of requirements/needs of biogas energy in the KTH Energy laboratory

c) Preparation of a bill of materials required for the digester

If time permits, a simulation of the engineering and energy controls on a low-technology digester

would be carried out. First, familiarisation with the modelling software would be essential. Aspen V7.2

is the modelling platform base for the modelling of industrial and well as chemical processes. Creating

a digester model and setting up control parameters are the main focal areas during familiarizing.

A economic analysis of the process for a farm- scale digester will be carried out, taking into account

the re-use of the biogas for the process itself as well as any other indirect costs inferred, to be able to

determine the most economical end uses of the biogas produced for a stand-alone system.

At the end, I will propose the way-forward and recommendations for further studies and produce a

report that addresses the targets.


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4 LITERATURE REVIEW

4.1 PRINCIPLES OF BIOGAS TECHNOLOGY

The anaerobic digestion can be described as the biological degradation and stabilisation of

biodegradable organic matter in specialised plants under controlled conditions. It is based on the

formation of methane-rich gas and a nutrient –rich digestate through the microbial activity in oxygen

free conditions. The anaerobic degradation is achieved through several parallel and subsequent steps,

with each step having a certain consortium of active micro-organisms.

CARBOHYDRATES PROTEINS LIPIDS

SIMPLE SUGARS, AMINO ACIDS LCFA, ALCOHOLS

INTERMEDIARY PRODUCTS (VFA)

ACETATE HYDROGEN

METHANE

NH4

HydrolysisHydrolysis

AcidogenesisAcidogenesis

AcetogenesisAcetogenesis

MethanogenesisMethanogenesis

ORGANIC MATTER & WATER

BIOGAS (CH4 + CO2)

+ DIGESTATE

BIO-FERTILIZER

Figure 4.1: Simplified anaerobic digestion of organic matter [Gujer, W., and Zehnder, A. J. B., 1983]

4.2 FACTORS AFFECTING THE ANAEROBIC DIGESTION PROCESS

Temperature and pH

The bacterial growth in the digester is influenced by the temperature of the process. The higher the

temperature, the higher the rate of microbial growth as well as the chemical and enzymatic reactions in

the process until the optimal temperature is reached.

For the digestion of manure as a heterogeneous substrate, mesophilic and thermophilic processes are

the most common according to the bacterial cultures required for the digestion of such materials.

The outdoor temperatures also affect the process. The gas quantity used will depend on the climatic

conditions outside. Also, in a very cold climate a bigger heat exchanger will be required to maintain the

process temperature and the heat losses will be much higher too. The optimum pH for the enzymatic

work is 6.0. This pH is maintained through the buffering capacity of the raw materials.


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Carbon to nitrogen ratio (C/N ratio)

The C/N ratio indicates the comparative amounts of carbon and nitrogen present in the feed

substrate. Nitrogen is an important nutrient for the development of bacteria in the process and will

thus affect the yield of biogas. Therefore, a high C/N ratio would suggest a deficiency in nitrogen

whereas a low C/N ratio would indicate the build-up of nitrogen in the system, an excess of which

would form ammonia disturbing the pH stability of the system. An optimum C/N ratio of 20 – 30 is

recommended for anaerobic digestion (Sadi, 2010).

Volatile fatty Acids (VFA)

The concentration on VFA formed in the intermediate processes affects the stability of the digestion

process. Process instability will lead to the accumulation of VFA in the digester which may lead to a

drop in pH value. VFA accumulation indicates either an organic overload or inhibition of the

methanogenic bacterial culture due to the influence of other factors.

Ammonia

Proteins in the feed substrate are the main source of ammonia in the system. Ammonia is normally

encountered in the system as a gas. High concentrations of this gas in the unionised form would inhibit

the digestion process. The amount of ammonia would depend on the input substrate to the process.

Animal slurries are an example of substrate with a high amount of ammonia originating from urine.

[Teodorita A.S., 2008]

Inhibition and hydrogen partial pressure

The major inhibitor in the biogas process is the formation of ammonium-ammonia which is toxic for

the active process bacteria. The rate of formation is directly proportional to the increase in temperature

and pH. An accommodation time is required by the bacteria to get used to high concentrations of

ammonia but the concentrations should increase step by step.

Other inhibitors for the process include oxygen, contaminants such as heavy metals, nitrates, sulphates,

disinfective compounds, etc.

Technical and operational factors

Mixing: Mixing improves the contact between the substrate and the bacteria as well as a homogeneous

mixture and constant temperature throughout the process. It also enables the release of biogas bubbles

to the gas collection system. If not properly mixed, the production of biogas will be reduced and result

in an unstabilised digestate. Foaming may also occur with insufficient mixing. Optimisation of mixing

is important as mixers are often the most energy consuming components.

Hydraulic Retention Time (HRT): HRT represents the average time the raw materials spend in the biogas

process which is given by the relation of reactor volume and the volume of daily feed [S. Luostarinen,

A. Normak, M. Edström et al.; 2011]. The HRT depends on the source of raw material fed to the

digester. For the digestion of manure, a HRT of 20-30 days is normally required. Kitchen wastes rich

in starch containing vegetable residues will require a shorter HRT.

Organic Loading Rate (OLR): OLR describes the quantity of feed material to be treated to the process at

a given time, that is, the amount of organic material (VS) in daily feed divided by the reactor volume [S.

Luostarinen, A. Normak, M. Edström et al; 2011]. All biogas processes have a threshold OLR above

which there are technical and microbiological limitations.


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Feed Substrates to the digestion process

Basically most of all organic materials can be digested giving different yields of biogas depending on

the carbohydrates, protein and fat contents. As far as agricultural manure and plant biomass are

concerned, all can be fed to biogas plants. Food wastes and sewage sludge also are good inputs to the

biogas plants.

Table 4.1: Theoretical biogas and methane production from carbohydrates, fats and proteins [Buswell

&Neave, 1930]

Substrate Biogas

(m3/t)

Methane

(m3/t)

Methane content

(%)

Carbohydrates 830 415 50.0

Fats 1444 1014 70.2

Proteins 793 504 63.6

Manure as a feed material to biogas plants

Manure is a good raw material for biogas plants for the following reasons:

The production of manure is continuous and therefore continuously available

manure contains all the nutrients required by the anaerobic bacteria, and

it has high buffering capacity.

However, the high nitrogen content of poultry manure may require dilution with fresh or purified

water or co-digestion with other less nitrogen-rich materials or some other specific technology in order

to avoid inhibition by nitrogen.

Depending on the quantity and characteristics of the manure fed to the plant as well as the plant

design, the manure can be either digested alone or co-digested with other raw materials. Different

manure types will yield different amounts of methane depending on animal feeding and housing

solutions, manure TS content and the bedding material used, among other factors.

The table below gives a summary of the different manure types and their biogas potentials.

Table 4.2: Average characteristics of different manures and their biological methane potentials (BMP)

[1) Viljavuuspalvelu, 2004; 2) Steineck et al., 1999; 3) KTBL, 2010; 4) Ministerium für Ernährung,

Landwirtschaft, Forsten und Fischerei Mecklenburg-Vorpommern, 2004; 5) Institut für Energetik und

Umwelt et al.,2006; 6) Edström, 2011]

Manure TS

(%)

VS

(% of TS)

Ntot

(% of TS)

BMP

(m3/tVS

added)

BMP

(m3/tFM

added)

Ref.

Cow, liquid 5-14 75-85 3-6 120-300 10-20 1-5

Cow, solid 17-25 68-85 1.1-3.4 126-250 24-55 1-5

Pig, liquid 4-10 75-86 6-18 180-490 12-24 1-5

Pig, solid 20-34 75-81 2.4-5.2 162-270 33-39 1-5

Poultry,

solid

32-65 63-80 3.1-5.4 150-300 42-156 1.3-6


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4.3 ANAEROBIC DIGESTION SYSTEMS

There are various alternatives for the design of anaerobic digestion systems governed by the

construction of the digester as well as the process technology. The type of system installation and the

management of the plant dictate the efficiency of the latter. Simpler (single –stage) reactors, although

easily designed, are less efficient and require constant monitoring. On the other hand, complex

automated multi-stage systems are programmed to detect errors and send warning signals. These are

more efficient although more costly [Luostarinen, Normak & Edström, 2011].

The engineering and planning for a particular digester will depend on several factors such as the scale

of the plant as well as the raw material fed to the plant (biodegradability, VS & TS content), the

quantity of input substrate, the simplicity desired, heat use, the intended use of the digestate

(pasteurisation or not), local circumstances, economic factors (investment and operation costs) as well

as the use of produced biogas.

Generally, agricultural biogas plants are categorised in to 3 scales by size:

Household digesters (6 m3 – 10m3 capacity plants)

Farm-scale plants (50 m3-5000m3)

Centralised biogas plants (> 5000m3)

For the purpose of this project, we will focus on farm-scale biogas plants. These are classified into a

further 3 categories depending on the capacities of the CHP units [Institut für Energetik und Umwelt

et al., 2006]:

Small scale ≤ 70 kW

Medium scale 70–150 kW

Large scale 150–500 kW

The small to medium scale would be applicable on single farms, while medium to large scale would

most likely be of farm cooperatives.

Figure 4.2: General scheme of a common biogas plant with continuously stirred tank reactor (CSTR) in

Europe [Institut für Energetik und Umwelt et al., 2006]


9

Based on the data given above, for each individual farm, the rough technical design of the plant can be

performed to calculate the following [Fischer T. and Krieg A., 2001]:

Gas prognosis

Digester size

CHP size ( combined Heat and Power station – micro turbine/ gas engine size)

Flow sheet

Layout design

Cost assessment

The next step would be to design the operation of the plant, also based of the data above [Fischer T.

and Krieg A., 2001]:

Mesophilic or thermophilic process temperatures

One or two- stage digestion process

Type of mixing

Mode of feed input

Type of heat input

The results of this planning will determine which of the 3 major types of digesters will be most

appropriate for that particular case.

4.4 PRE-TREATMENT STEPS

The initial treatment of substrate differs at different plants, depending on the material to be digested,

as well as the final use of biogas and digestate produced. Sometimes more than one pre-treatment step

is involved.

Milling/ crushing/ removal of grit and non-renewables

Blending of the manure with re-circulating sludge before the heat exchanger permits the manure to be

heated before entering the digester.

Grinding or milling the input substrate decreases the size particle thus increasing the digestibility of

waste that is hard to break down.

Thickening of materials with a small content of dry solids by for example, centrifuging.

Mixing of the digesting fluid permits a uniform fluid and constant temperature and prevents scum and

grit accumulations.

Sanitisation

For material of animal origin, such as waste from a slaughterhouse, food waste and manure, digestion

is preceded by a sanitation or hygienisation stage, which usually involves heating the material to 70°C

for one hour (1774/2002/EC) or sterilisation (133 °C, 3 bar 20 min; 1774/2002/EC). This treatment

ensures the elimination of pathogens and also loosens the bond structures resulting in more

degradation and thus more biogas produced.

Start-up Inoculum

When a biogas reactor is started up, microorganisms from the inoculant need time to adjust to the

substrate that the specific biogas plant is going to treat. In a biogas plant, both the substrate and the

environment will differ from the original environment and it is important for the organisms to adapt to


10

enable a stable process. During this adjustment period, the organisms in the inoculant that are best able

to survive in the new environment will grow and become established.

Microorganisms that are added via the new substrate may also play a role in the process. The more the

environment from which the inoculant is taken differs from the environment in the digestion tank, the

longer the start-up period will be. To achieve a quick and reliable start-up of the process, it is best if a

microbial community is established already from the beginning, based on adaptation to a similar

substrate. One way to achieve this is to start the process using digestion tank contents from an already

operating process that uses a similar substrate.

4.5 BENEFITS OF BIOGAS

Biogas production by anaerobic digestion clearly have the following benefits for owners / operators:

Figure 4.3: Benefits of Biogas from the anaerobic digestion of organic wastes

4.6 POTENTIAL ENERGY USES OF BIOGAS

Direct combustion of the biogas is still the most energy efficient option. The biogas can be directly

used for cooking, heating, cleaning, drying and hot water. The biogas generated will sufficiently reduce

the regular consumption of other cooking fuels such as LPG.

Biogas can also be used to generate electricity using selected technologies (through the Otto or diesel

engines, (both internal combustion engines). Before the feeding of biogas to generation sets, the gas

has to be passed through a gas scrubber to remove unwanted particles, gases, moisture etc. In the

diesel engine, commonly known as the dual fuel engine, diesel or another oil-based fuel is used for the

ignition of the turbine as the heat of compression of biogas is not sufficient for the ignition of the

engine. In the Otto gas engine, only biogas is used as fuel. They have spark plugs for ignition and a

Biogas

Great potential for use of

various waste streams thus avoiding

treatment costs

Cooking fuel

Suitable for stand alone

decentralised farms

Production of heat and electricity

An appropriate

technology to exploit energy potential from

wet organic waste

Sustainable raw material for chemicals

Alternative fuel for vehicles

Versatile

energy carrier

Decreased dependency

on

natural gas imports


11

gas/air mixing system for providing a combustible mixture to the engine. Otto engines are designed

using a lean burn technology, including turbocharger, with a surplus of air for improving efficiency and

reducing emissions. The externally fired gas turbine is a novel technology under development for small

and medium scale heat and power supply systems.

In the northern part of the Europe, for example, in Sweden or Germany, biogas is also used as a

vehicle fuel and can be distributed to the public natural gas network, after upgrading the biogas.

Figures show that the number of vehicles using biogas as fuel in Sweden in 2010 was 30 100 cars, 1

400 buses and 500 trucks [Energigas Sverige, 2011]. This biogas is from the anaerobic digestion at

wastewater treatment plants or from biogas plants using food wastes as feed substrate.

In a more innovative technology approach, energy from the biogas can be stored in fuel cells by

passing the biogas through a reformer upstream.

ANAEROBIC DIGESTERMANURE/ FOOD AND AMENITY WASTE BIOMETHANE

DIGESTATE

COOKING GAS

ELECTRICITY

VEHICLE FUEL

HEATING

BIOGAS

Figure 4.4: Potential uses of biogas and digestate from anaerobic digestion

4.7 THE DIGESTED RESIDUAL PRODUCT (BIO-MANURE)

The by-products of the anaerobic digestion process are the solid and liquid bio-fertilisers, which

normally come out as a slurry. This slurry can be applied to the arable land as it is by using a special

slurry tanker as shown in the plate below.

Figure 4.5: Slurry tanker, Schouten, New Zealand (http://www.schoutenmachines.co.nz)


12

The slurry can also be separated into solid and liquid fertilisers using different processes such as

centrifugation. But for small and medium digesters, this is not economically viable as such processes

are quite complicated and costly. The solid fertilizer could be packed and eventually sold to the public

so that the beneficiary could recover a part of the project costs.

4.8 HOW BIOGAS IS USED IN MAURITIUS AT PRESENT

Example 1: Biogas Production for the Ecological and Economic Treatment of Cattle Waste

Bio-digesting is a technology which has not taken off within the community of farmers in Mauritius.

But some attempts have been made for its adoption with the support of UNDP-GEF small grants

programme and it seems to be quite a success.

The project comprised of the installation of infrastructure for a pollution-free environment as well as

an economical system of milk production through the setting up of digesters for the treatment of

animal waste generated by the breeding activity of 13 farmers who are members of the Livestock

Keepers Association. Each farmer owns around 6-7 cows as a small backyard farm at their residence

generating about 876 tons of manure per year thus producing about 129000 cubic metres of biogas per

year, or 217 kg of gas per day capable of replacing four units of 12 kg LPG bottles per month (as

stated by one of the families benefitting from this project). The environment also benefitted from this

project as it represents a carbon dioxide emissions reduction of 530 tons. [Biogas Production for the

Ecological and Economic Treatment of Cattle Waste; http://sgp.undp.org/]

This project promoted the use of renewable energy and improved the living conditions of rural animal

keepers by ensuring a cleaner and more hygienic environment, while at the same time encouraging the

use of natural methods for agriculture through treatment of animal waste for use as fertilizer. The

animal waste will be washed and directed into a digester, which will treat the wastes to produce biogas

energy for cooking and water-heating purposes. The effluents from the digester will be used as

fertiliser and for irrigation of the fields where fodder and other crops will be grown.

Example 2: St Martin Municipal Wastewater Treatment Plant

The St Martin Wastewater Treatment Plant treats 70,000 cubic metres of wastewater per day through

the anaerobic direction of the wastewater. The plant is operated by the German company. Berlinwasser

International AG. The biogas produced during the digestion of the sludge (feed of 250-280 m3/day, of

solid content of about 25% and Volatile Solids content of about 70%) is converted into electrical

energy which is then used for the operation of the plant. The biogas production yield is typically 2500-

2600 Nm3/day. 25% of the electricity needs is provided from the biogas produced, using a combined

heat and power plant [Roumeela Mohee & Acknez mudhoo, 2006].

Example 3: Mare Chicose landfill, Mauritius

Two spark ignition engines of 1MW and 2 MW are installed at the landfill site use biogas, namely

methane from the anaerobic digestion of wastes from the landfill to generate green electricity to be

used in the office and for the leachate treatment as well as provide electricity to the village of Mare

Chicose, representing some 20,000 households. This project converts energy from a source that was

previously thought of as waste. It is estimated that the peak generation of biogas from the landfill will

be around 2013 to 2015. The digestate produced can be used as cover material in the landfill cells.

http://sgp.undp.org/


13

5 EXPERIMENT SETUP

5.1 AIM OF THE EXPERIMENT

The aim of the on-campus experiment is to determine the amount of biogas that can be obtained from

food wastes, the logistics behind and to be able to use the biogas in the Heat and Power Laboratory.

5.2 AMOUNT OF WASTES TO BE DIGESTED

A survey of available wastes was performed in the three (3) restaurants in the vicinity of the KTH

University. The three restaurants in the survey were the ‘Brazilia restaurant’, the ‘Restaurant Q’, the

‘Restaurant Sisters n Bro’. The results of the survey are shown in the Table 7.1 below.

Table 5.1: Survey of restaurant wastes

Question Restaurant Brazilia Restaurant Q Restaurant Sisters n Bro*

How much raw/

cooked organic wastes

does the restaurant

generate per day?

The wastes are not

segregated. But at least 2

big bags of wastes

weighing approximately

25 kg each are thrown to

the trash bin every day,

mostly composed of

organic wastes for

around 300 to 350

servings per day.

The restaurant is already

in a recycling

programme. So the

wastes from the dishes

are collected by biogas

producing company.

Also, there are no raw

wastes produced at the

restaurant as the raw

vegetables and meat are

delivered already

processed.

They produce as much as 2 big

bags of kitchen and restaurant

wastes per day, also

representing 40-50 kg per day

for around 450 servings.

*The restaurant management later

stated that it has started a recycling

programme and would not be able to

supply the wastes for the experiment

What is being done

with the used oil/fat?

The used oil is sent for

recycling


recycling


recycling

Therefore, we can assume that we will have around 40-50 kg/ day of restaurant wastes from the

Restaurant Brazilia for the experimental biogas plant setup.

5.3 DESIGN AND SIZING OF THE BIOGAS PLANT

Before designing and setting up the digester there are important factors to consider:

Amount of biogas/ amount of input material – is it a good substrate?

Gas quality

Temperature control (process is temperature sensitive, the temperature should be quite stable

during the digestion process)

Operating parameters

One or two stages of digestion

Continuous or batch process

Choice of materials for the digester and auxiliaries

Leakage control (leakage of biogas from the process or gas storage is dangerous and is a strong

greenhouse gas.

Sanitisation: depends on substrate and use of digestate, as well as prevailing regulations

Permissions required for setting up of plant: permissions for spreading of manure as well as

permissions for the location of plant as there are boundaries and buffer areas to be respected

CO2 and H2S amounts in the biogas produced


14

The following biogas plant design and sizing is based on the generation of 100 kg of restaurant wastes

per day.

Calculation of the reactor volume:

There are two ways of getting to a first estimate of the size of the digester - organic loading rate and

hydraulic retention time (HRT).

As a rule of thumb, the Organic Loading Rate should be about 3.5kg VS/m3 of reactor

Assuming federate : 100kg of fresh (raw) feedstock per day

Given TS% : 20% (% Dry Matter Content per wet weight (DMC/wet weight))

TS : 20 kg

Given VS% : 90% of TS (Volatile Solid per kg of Dry Matter (VS/kg DM))

VS : 18 kg

If 3.5kg VS requires : 1 m3 of reactor volume

Then 18 kg VS : 5 m3 effective volume

Reactor volume required = 5.0 m3

Or

Assuming 22 days of HRT at thermophilic temperature of 55°C

Input substrate : 100 kg of raw kitchen waste + ~100 litres of water (to bring the VS% down to 10% of total mass of input substrate)

: 200 litres

Volume of reactor : 22 x 200 litres =4.4 m3

Applying a safety factor of 10%:

Reactor volume required = 5.0 m3

Design of plant:

The plant design is based on several factors:

The amount of wastes

The cost of equipment installation and running of the plant (considering that Mauritian farmers do

not have the facility to implement a ‘state-of-the-art’ plant.).

The plant should be able to be stand alone, that is, suitable for de-centralised locations, as would be the

case of most farms in Mauritius. The plant should be able to be easily operated, so that a farmer can

operate the plant without requiring any particular skills.

Option 1: Do-it-yourself (DIY) biogas plant

There are several designs of easy DIY plants for small scale plants, usually backyard or domestic biogas

plants. The highlights of this design include:

Flexibility to be used for either continuous flow or plug-flow processes.

The small compact design requires a small site area

Inexpensive design materials. Bladder made of inexpensive tarpaulin can be used, which is tougher,

more durable and safer than polyethylene(PE) for example.


15

Clean and sanitary

Versatile: By raising the top section of the bladder, a suction (vacuum) effect may be created to

extract gas. Conversely, by pressing down or applying weight on the top of the bladder, gas

pressure is increased or adjusted.

Simple to operate and functional.

Examples of some DIY plants are shown below:

Figure 5.1: Philippine BioDigester by: Gerardo P. Baron, December 2004, (Tarlac City, Philippines)

[http://www.habmigern2003.info/biogas/Baron-digester/Baron-digester.htm]

http://www.habmigern2003.info/biogas/Baron-digester/Baron-digester.htm


16

Figure 5.2: Completed PE digester prior to final installation of lid and wrapping with insulation

[http://biorealis.com]

http://biorealis.com/


17

Option 2: Low-cost available ready-made digesters

Figure 5.3: Small and medium scale digester biogas plants, by BioTech, India [http://www.biotech-

india.org]

These digesters are made of fibreglass lined polyvinylchloride (PVC) material which is relatively cheap

and impervious to methane gas. The image to the right shows the biogas plant complete with

upgrading equipment and CHP. The cost of installation is slightly on the high side but the operation is

simple and economically beneficial.

Option 3: Containerised biogas plant

The advantages of this type of plant are that it is transportable from one site to another. All the

components are integrated in the system. The plant is very compact and does not require much land

area. The commercialised containerised plants are manufactured according to international standards

concerning the gas storage tank as well as safety of the equipment.

Figure 5.4: Containerised biogas plant by BioBowser, Australia


This biogas plant is also complete with the waste processing tank and crusher as well as the upgrading

unit as shown in the figure below.

http://www.biotech-india.org/


http://www.srela.com.au/biobowser.php


18

Figure 5.5: Components of the BioBowser containerised biogas plant


Substrates to the JTI mobile biogas plant include, but are not restricted to:

• Sewage sludge

• sludge from grease

• Various waste products from food industry

• Sorted food waste

• Slaughterhouse residues

• Sludge from pulp

• Slurry

• Plant Residues

• Energy crops

Possible process testing with the JTI mobile biogas plant include:

• Co-digestion with the new substrate

• Digestion at higher TS and higher biological load

• Evaluate two-stage anaerobic digestion

• Test the effect of different retention times

• Mesophilic v.s. thermophilic digestion

• Sanitation by thermophilic digestion v.s. pasteurization

• Studies of digestate qualifying small

• Component Tests at different process conditions

• Try on biogas production at plants that are not currently producing biogas

• Demonstration and training



19

Figure 5.6: The Research biogas production unit at JTI, Swedish Institute of Agricultural and

Environmental Engineering

5.4 CHOICE OF BIOGAS PLANT AND JUSTIFICATIONS

The do-it-yourself option was not chosen on the basis of the high equipment and material

standard requirements in Sweden as well as the requirements of safety of the handling of explosive

material on campus.

BioTech India sent a quotation for the treatment of 100 kg of restaurant wastes by proposing a 5

m3 digester including the biogas generation system, pre digester, slurry loop system at an

equipment cost price of EUR 12 450, without costs of transportation or installation. But the main

problem was the delivery time of the equipment which was 60 days as from manufacture to

delivery at local port. They were also reluctant to give additional information on the plant

proposed.

BioBowser would have been the most interesting technical choice, but the price would have been

even higher than the Indian technology.

The JTI containerised unit is available for rental for the months of May and June and is a viable

option for the on-campus experiment set up. The plant has already obtained the certifications for

the gas storage tank and the approvals from the fire departments for previous research projects.

By elimination, the JTI biogas plant was chosen.

5.5 LOGISTICS: PLANNING THE BIOGAS PLANT

Feedstock:

According to the Researcher at JTI, Mr Gustav Rogstrand, an average of 80kg per day of wastes would

be required tofeed the plant for the process to run correctly. For a 2-week running period, the

collection of the wastes should be started 2 weeks prior to the start-up of the plant. The waste has to

be covered to avoid rodents and birds invasion.

Gas volume produced

The gas produced, according to Gustav, would be 6-10m3/day. A 200-litre gas storage balloon tank

would be supplied with the equipment but another tank for transportation of biogas for use in the

laboratory would be required.


20

A certified gas storage balloon tank of 500 litres on the market is around EUR 620 (Schanflex APS,

Denmark) [http://www.schanflex.dk].

Site requirements and preparation:

Surface area

The require surface area would include space for the containerised digester, 8ft container for the

grinder, area for the gas storage tank (placed in trailer) as well as a buffer zone of 2 metres around the

plant. The site should also respect the required 10 metres from any building structure recommended by

the Fire Department. The area should be a flat levelled surface so that the weight sensors in the tanks

would give the correct data.

Surrounding environment

The area should be fenced with limited access to authorised persons only. For safety reasons, the area

around the flaring equipment should be clear. The branches around the mobile plant had to be cut.

Electricity access points/ Power supply available

One 63A outlet and one 32A outlet within a reasonable distance from the mobile plant would be

required for the running of the plant as well as a 16A outlet for the grinder equipment. But as long as

the grinder is not running simultaneously with the motors and pumps, the 32A outlet can be used.

Water access

Access to water is essential for the preparation of the feedstock to the plant and to ensure a clean site

at all times.

Disposal/ use of digestate

About 250 litres of digestate will be removed from the digester each day. This digestate is too much to

be added to the yard composting piles at this time of the year. Therefore, the digestate will be carted

away by registered sewage carriers every 2-4 days.

Odour control

The collection of wastes will start 2 weeks before the start of operations. The wastes will be kept in

covered bins. The site should be at a reasonable distance from buildings to prevent any odour

inconveniences. Nevertheless, notices have been put at the site as well as at the entrances of the

nearest buildings.

Personal Protective Equipment

All necessary PPE for the handling of wastes and biogas would be provided.

Approval from local authorities

Facilities Management

A meeting was held with the facilities management (Mr Göran Holmberg , Akademiska Hus) to get

approval for the proposed site which is the compost site of the KTH university. The site was chosen

as it fits all the above requirements. They would clear the site before the delivery of equipment. They

will also supply electricity and water at the site.

http://www.schanflex.dk/


21

Figure 5.7: Proposed site for the biogas plant setup

Fire department

The Fire department was contacted and briefed about the project. They required a full application for

the handling of flammable materials. This type of application generally takes 3 weeks to be processed.

5.6 EQUIPMENT SETUP

The JTI mobile biogas plant main components

Figure 5.8 : Main components of the mobile biogas plant ( Source: JTI)


22

The table below shows the functions and capacities of the main components of the mobile biogas unit.

Table 5.2: Mobile biogas plant basic data (Source: JTI)

Function Capacity

Digester volume 5 m3 (6 m3 total incl gas volume)

Processing temperature 30 - 70 °C

Residence time 10 - 50 days

Process capacity 100 - 500 kg wet weight/ day, 0.5 to 25 Nm3 of biogas/ day (65/35)

Hygienisation capacity 50-600 kg / round (pasteurization - max 80 °C)

Decomposition Selectable (on board or external)

Heating system Electric boiler on board 26 kW

Valve Drive Automatic compressor on board

Emergency gas treatment Gas monitoring unit and automated flare on board

Gas volume measurement Continuous online flow rate measurement

Gas quality measurement Continuous online - CH4, CO2, O2 and H2S

PLC Management Fully automatic with remote management and logging via 3G

Training

Training on the operation of the plant was provided by JTI as well as what to do in case of alarm or

dysfunction of the equipment or process. Safety procedures were also provided.

Inoculum

Inoculum could be obtained at any running active bio-digester, but preferably if food wastes would be

digested, inoculum from a similar plant running under same conditions would be best, else any

wastewater treatment plant using the anaerobic digestion process would do. The amount of inoculum

should be equal to the reactor volume of the proposed digester.

Collection and storage of raw material (restaurant/ kitchen wastes)

The restaurant does not segregate its wastes. The waste was collected twice per week using a mobile

bin (on wheels). The waste was stored in large covered bins at the experiment site before the organic

part could be separated and fed to the digester.

However, it was observed that the waste collected from the restaurants over the first two weeks were

not usable. Therefore, food waste was provided by a waste management company, RagnSells, who own

digesters themselves. The waste was delivered in a two-tonne container. The wastes are food wastes

from a supermarket and had to be separated from plastic containment. The inorganic part was kept in

a separate bin for collection by registered waste carrier.

Feeding new substrate to the digester

The feedstock would be fed to the grinder using a bucket to the top board, which is then scraped using

a rubber-pallet knife. When the grinder is switched on,

water will be added to the feedstock at a desired

flowrate to reach the required dilution to 10% DMC

prior to being pumped to the buffer tank.

The Disperator® grinder has a metallic feeding table to

prevent any metallic objects such as a fork can enter

the grinder. The machine grinds all kitchen wastes

including bones. The only kitchen waste that should

not be fed to the grinder, as per supplier’s

recommendations, is fish-skin for its rubber-ish and

slippery properties. Figure 5.9: Disperator® grinder


23

The first feed was done on Tuesday 5th June 2012. It took 29 minutes to feed the biogas unit (reception

tank) with 139.2 kg waste and 85.65 litres of water. The grinder was set to mix the waste with 3 litres of

water per minute. Later that same day, a second batch of waste was grinded. The grinder was set to

mix the waste with 2 litres of water per minute. It took 4 minutes to feed the reception tank with 35.6

kg waste and 8 litres of water.

A second feed was prepared on Tuesday 19th June 2012. The grinder was run for a period of 22

minutes. During that time 131.8 kg of waste consisting mainly of bread went through the grinder with

a water mix of 42.86 litres.

Total amount of feed slurry during the experiment = 450 kg of feed substrate

Day-to day operation

Table 5.3: Operation of the biogas plant

SN Description Frequency Responsibility

1 Fill the digester with inoculum (provided by a

digestion plant in Uppsala treating 100% food

wastes at the same thermophilic conditions so

that the inoculum will quickly adjust to the

new substrate environment).

First day of operation

KTH – rental of

a sewage truck

2 The inoculum is heated to a constant

temperature of 55°C


JTI/ Operator

3 Fill the space above digestate in the digester

with nitrogen to ensure anaerobic conditions

during the process (displacing any oxygen that

could be present)


JTI/ Operator

4 Run a process to understand the procedure

(part of training)

Every day or 2 times daily

ensuring an interval of at least

10 hours between the loadings

Operator

5 Separate organic wastes from other wastes, if

necessary

Every four days Operator

6 Test a sample of raw waste to find VS and DS

content to find the appropriate loading rate

Once at early stage

7 Cut wastes to a size to fit the entrance slot of

the grinder

Weigh the wastes before feeding the grinder,

where water is added to form a slurry which is

pumped to the buffer tank


8 Test for DMC and VS of the input slurry to

ensure proper dilution


9 Sampling of digestate to test for pH,

ammonium ammonia, VS, TS, DMC, VFA

As and when necessary Operator

10 Ensure disposal of digestate by registered

sewage truck

Every 2 to 4 days, or as and

when necessary

Operator

11 Take daily readings from the software to

ensure proper processing conditions are

respected.

At least once daily Operator

12 Emergency shutdown of the flaring equipment

in case of thunderstorm (the equipment works

with static electricity). This cannot be done on

a remote basis for safety reasons.

When alarm is activated or

when forecasted

thunderstorms.

Standby-operator


24

5.7 RESULTS AND DISCUSSIONS

The analysis of the input slurry, after grinding and addition of water gave the following results:

Table 5.4: Results of analysis of feed substrate (sample taken on the 13th June 2012)

Parameter Result

pH 6.85

TS (%) 11.19

VS (%) 10.41

NH4-N (g/L) 0.47

If 3.5kg VS requires = 1 m3 of reactor volume

Then 5 m3 of reactor volume = 3.5 x 5 = 17.5 kg of VS

From the analysis of feed substrate, VS% = 10.41%

First day feed = 139.2 + 35.6 = 174.8 kg of feed substrate

Total VS fed = 10.41% x 174.8 = 18.2 kg of VS

This shows that the feed substrate consistency was a bit high on VS content. This allows for more

water to be added to the raw waste. A water mix of 3 litres per minute to the grinder was appropriate

to bring the volatile solids content to about 3.5 kg per cubic metre of reactor volume.

The amount of feed to the reactor was programmed and recorded throughout the experiment. The

amount of biogas produced was also recorded throughout. The gas produced was sampled at regular

intervals and analysed to note the quality of gas produced (%carbon dioxide and %methane). The

graphs below show the results obtained from the experiment.

Figure 5.10 : Amount of biogas produced as a percentage of gas storage bag volume

0

10

20

30

40

50

60

70

80

90

100

Dat

e/t

ime

20

12

/06

/04

21

:17

20

12

/06

/05

03

:57

20

12

/06

/05

11

:49

20

12

/06

/06

00

:54

20

12

/06

/06

13

:59

20

12

/06

/07

03

:04

20

12

/06

/07

16

:09

20

12

/06

/08

05

:14

20

12

/06

/08

18

:19

20

12

/06

/09

07

:24

20

12

/06

/09

20

:29

20

12

/06

/10

09

:34

20

12

/06

/10

22

:39

20

12

/06

/11

11

:44

20

12

/06

/12

00

:49

20

12

/06

/12

13

:54

20

12

/06

/13

02

:59

20

12

/06

/13

16

:04

20

12

/06

/14

05

:09

20

12

/06

/14

18

:14

20

12

/06

/15

07

:19

20

12

/06

/15

20

:24

20

12

/06

/16

09

:29

20

12

/06

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BIOGAS PRODUCED AS A PERCENTAGE OF 0.6M3 GAS STORAGE BAG


25

The above graph (Figure 5.10) shows the variation in biogas production, here shown as a percentage of

the biogas retention tank of volume 0.6 m3. At the start of the experiment, the variations in the amount

of gas produced were due to a faulty level control and indicator measuring the level of gas in the

balloon tank, which was fixed on the 06th June.

The amount of gas produced was then constant until the 9th June where a sudden drop on the volume

of gas produced is observed. The reason behind this may be that the digester started off with a full

tank of inoculum (representing a feed that was already digested and then undergoing a second-stage

digestion) generating lesser and lesser amounts of gas while the fresh feed substrate kicked in and

allowing for the bacterial culture to acclimatise.

After that episode, the amount of gas produced undergone a series of wild variations. This may be due

to the fact that a large amount of substrate was fed to the digester once per day which would have

been too much for the bacteria. The feed amount and frequency was thereafter changed to observe the

difference. The feed amount was reduced by 3 and the frequency programmed to be every 4 hours.

The amount and quality of gas eventually improved and stabilised as it can be seen from the graph as

from the 15th June onwards (encircled in red in Figure 5.10). This is shown in the Figure 5.11 below.

Figure 5.11 : Amount of biogas produced 16th June 2012 to 21st June 2012

Each peak shows that the gas storage tank has reached 75% of its capacity, where the system is

programmed to empty the gas storage balloon up to 25% of its capacity. Therefore each peak

represents 50% of the total capacity, that is 3 m3 of gas produced. It can be seen from the graph by

counting the peaks (98 in all) that during the experiment from 0:00 on the 16th June to 23:59 on the

21st June 2012 that 29.4 m3 of biogas was produced or, on average 0.2m3/h. The gas produced was

flared.

Also, the dip of gas production on the 20th June, as seen from the graph above can be explained by the

fact that 0.6 m3 of gas was stored in a separate balloon tank for off-site experimentation (for use in an

internal combustion engine, Volvo engine V20 through the air inlet in combination with gasoline input

as well as without gasoline input.)

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in g

as

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rag

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an

k (

0.6

m3)

Amount of biogas produced 16th June 2012 to 21st June 2012


26

Figure 5.12 : % of Methane in the biogas produced

The graph shows the quality of the biogas produced, defined by the percentage of methane which will

give the calorific value of the said gas.

As mentioned before, during the first days of operation, the digester was digesting mainly the inoculum

used to start the process, actually representing the second-stage digestion of the substrate given that

the inoculum was taken from an operation anaerobic digestion plant. This explains the high methane

content of up to 74%. The dip in the quality of gas observed on the 11th June may be explained by the

fact that the feeding rate was not adequate for the microbial culture present in the system and may

have caused the process to turn sour, meaning that the bacteria in the system were dying.

Our attempts to revive the microbial culture (specially the methane producing bacteria which are more

sensitive to process changes such as feed rate or temperature changes) and boost the digestion process

which was achieved by varying the feed rate and reducing the process temperature to 51°C proved

successful as the percentage of methane in the gas increased back to an average of 67%. The raw

substrate was fed to the digester at a rate of 30 kg every 3 to 4 hours interval. The process temperature

was maintained at 51°C as proposed by Prof. Anna Schnürer, Swedish University of Agricultural

Sciences, Uppsala. (personal communication).

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:15% m

eth

ane

in b

ioga

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%CH4 in Biogas produced


27

Figure 5.13 : Percentage of carbon dioxide in the biogas produced

The one major rise in carbon dioxide level in the gas observed on the 11th June corresponds to the

time the level of methane dropped drastically also consistent to the time the process turned sour.

Figure 5.14 : Percentage of oxygen in the biogas produced

The oxygen levels during the digestion process were near zero confirming anaerobic conditions of

digestion.

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% O

2 in

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e b

ioga

s p

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% O2 in the biogas produced


28

Table 5.5: Results of analysis of digestate samples at the beginning and at the end of the experiment

Parameter Digestate 1* Digestate 2*

pH 6.95 6.95

TS (%) 3.41

VS (%) 2.42

NH4-N (g/L) 2.01 2.72

Alkalinity (g CaCO3/L)

12.5 10.2

VFA (g/L)

-Acetate 1.02 3.89

-Propionate _ 1.19

-Butyrate _ 0.26

-Valerate _ 0.38

*Note: Digestate 1 sample collected on the 13th June 2012 (inoculum sample)

Digestate 2 sample collected on the 21st June 2012

This experiment setup has been an excellent learning opportunity for me as well as other interested

master students. In this experiment the logistics required and operation requirements to run small

to medium scale digesters have been defined and further discussed in section 5.8 of this report.

Moreover, ways to improve the process energy requirements and biogas production have been

deduced and further discussed in section 5.9.

The method of collection of wastes from the restaurant was inappropriate as the waste was not

segregated at source and the bin was quite heavy and difficult to pull/push all the way from the

restaurant to the plant site. Moreover, the waste collected was observed to be only approximately 10%

organic waste, not recoverable from the waste bags.

On starting up, 5 to 10 hours of pre-heating the inoculum back to 55°C is necessary to stimulate the

active bacteria to the process conditions as during the transportation of inoculum from Uppsala to

KTH, the temperature of the digestate decreased to ambient temperature thus slowing down the

bacterial activity.

The valves have to remain always open when a recipe is not running. If there is active substrate in the

pipe between two closed valves, the pressure will build up due to methane formation and this can

explode at the weakest point of the pipe, that is, at the valves. Even the stainless steel pressure pipes

used in the mobile unit would not sustain the pressure build-up with the gas formation.

Storage time for digested manure (digestate) should be short as possible as the digestion process

continues even after the coming out of the digester. The methane produced is not monitored and

could be dangerous.

Since the biogas plant was run on food wastes for only two weeks, it was impossible to see whether the

substrate will cause any problems on the long run such as

Ammonium nitrogen formation

Foam/ scum formation in the digester

CO2 increase in biogas


29

H2S level increase

The most common problems encountered during the 2 weeks running period were the high pressure

and high levels in the tanks as well as programmed procedures which did not engage in.

An energy analysis of the plant and process was not possible due to several limitations including the

following:

This plant was designed and meant for research work such that the auxiliaries (pumps, mixers,

motors) are able to handle the substrates used in large scale processes and it has to be able to

mimic an actual scale plant, and to give clients improvement ideas for their plant. Normally, this

research unit runs in parallel with the full-scale unit. Also, such centrifuge pumps for example, are

not available in small sizes to fit a 5 cubic metre digester. The plant was not engineered to be

energy efficient. There are variable frequency converters on the various motors on the plant that

can dial down the actual capacity of the components and bring down the energy consumption of

the motors according to the load, but nevertheless, an energy analysis of the plant itself will not be

realistic. However, a theoretical extrapolation can be made for the energy analysis.

Another limitation for energy analysis on this plant would be the limited running period of the

digester for the experiment. The process will not reach the maximum biogas potential in a period

of 14 days of operation, or in this case 7 days of operation with new substrate. A minimum of 3

retention times is required (HRT approx. 21 days at 55°C). For this period we can consider only 65

– 75 % of maximum yield of biogas after steady state. What will happen is that for say, the first ten

days of operation, the biogas produced would be mainly the leftover gas from the inoculum. The

remaining days, the biogas will be produced both from the inoculum and the new feedstock.

This is a 4.5 million SEK, high investment plant, with all equipment and piping in stainless steel.

Mixers installed are meant for hard-to-mix substrates. There are extra valves and other auxiliaries

that are meant for research only, over and above what is required for the normal operation of a

digester. The piping is 100 mm piping that are used to mimic the full scale processes. If 1 inch or

3 inch piping were used, for example, sewage sludge would be stuck at some point and would

require dismantling and cleaning. Also, for rural areas, where safety is not much of a big issue, PE

or clay digesters are often used, as for example China or Philippines.

Possibility of using the biogas produced in the KTH energy laboratory (eg to run the micro-

turbine or investigate other uses or energy storage possibilities) will be considers as future work, when

the permanent biogas plant is set up.

5.8 LESSONS LEARNED AND FUTURE WORKS

The KTH Energy Engineering department intends to exploit this technology in the future to provide

energy to the energy laboratory. But, the quantity of biogas produced from the experiment (recorded

4-6 m3 per day during the peak period) is not enough to run any of the equipment in the lab, given that

both the microturbine and the gas turbine require about 6 m3 of biogas at 65% methane to run an hour

period which is not enough even to reach the optimum power of the turbine.

The solution would be either to have a larger biogas plant to produce about 45 m3/day of biogas to

run the micro turbine for 8 h/day which requires a 30 m3 capacity plant or to run the micro turbine

twice per week and have a biogas plant to produce 45 m3 of biogas for 3-4 days which will require a

biogas plant of about 10 m3 capacity.


30

Food wastes having an OLR of around 3.5kgVS/ m3 digestate, we would then require around 200 kg

of food wastes per day to feed a 10m3 biogas plant.

Origin and sustainability/ continuity of feed substrate:

The waste coming from the adjacent restaurants is not an option unless they are willing to cooperate

on the segregation of the wastes at source. The organic portion of the wastes from the restaurant is not

what was expected. The restaurants use more already-processed foods so that the waste consists mainly

of cans and plastic bags rather than vegetable peelings and food remains. The waste from the

restaurant was not usable.

For the collaboration of restaurants to be success, the wastes should be segregated at source, or

through the installation of an ‘under-the-sink grinder’.

Assuming that:

KTH has 10 ‘schools’ each having its own kitchen/ dining room

Each kitchen produces on average 30 kg of raw organic wastes on a normal working day and

15 kg of wastes over a normal weekend

During summer holidays and vacation periods (2 months every 12 months), a kitchen would

produce around 5kg of organic wastes per day.

So that,

For a normal working week, the amount of wastes generated would be ((30x5) + 15) x 10 kitchens

= 1550 kg/week

= 221 kg/ day, which is more than enough as per OLR required.

There will be a couple of ‘low-waste’ months for which an additional or alternative source of food

wastes should be found.


31

Logistics

Collection of food wastes

For the sustainability of this project, the participation of students and staff members is required. First

of all, segregation of wastes at source is important to facilitate the transport and feeding to the plant. It

will also prevent any damage of the equipment in the plant.

The person responsible for collecting the wastes from the kitchen should understand that organic

wastes go to a special bin (maybe giving it a colour code). Also, there should be a responsible person

(may be on a rotation basis) who will ensure that the waste is transported to the plant.

An alternative collection solution would be placing domestic grinders (similar to the grinder used for

the experiment setup) under the kitchen sinks where only organic wastes would be fed to the grinder.

The slurry produced would be pumped to a buffer tank. This solution involves less manipulation and

less human resources.

Location of the plant

The proposed site for the future permanent biogas plant should be nearer to the energy department

(just outside the department or on the fourth floor where the ‘polygeneration mobile plant’ will be

placed) to avoid transportation of the wastes and transportation of the biogas as the regulations for

these transportations are strict for hygiene and safety purposes.

Design of the plant

Fibreglass reinforced PVC biogas plants

This technology was studied and proposed for the experiment but was rejected due to the long delivery

time (Refer to section 5.4). The technical details of the plant are:

Size of plant – 15 m3 for 200 kg of wastes per day

Treatment capacity of the plant - 200 Kg of organic waste,

Material of construction: Fibreglass coated PVC for its impermeability to methane

Expected gas generation - 15 m3 and 20 m3 every day, which is sufficient to replace 8 - 10 kg of

LPG or for the generation of 23 kWh of electricity.

Organic fertilizer output - 100 litres per day

Area required for the installation of the plant is 60-80 m2. The plant can be installed above the ground

level or 95 % below ground level.

The main civil works relating to the installation of the plant are unloading of materials, excavation of

pit, housing of digesters, construction of platforms for fixing pre digesters and gas collectors, initial

feeding of the plant with cow dung and fixing of gas connection lines from the plant to the utilization

point as well as provision for feeding drainage/ waste water line to the plant.

The price of the unit is EUR 21 450, excluding freight and civil works costs. The return on investment

of such a plant is 2-3 years, depending on the utilization of the output from the plant, that is, amount

and quality of biogas produced and whether the digestate will be used or sold, or disposed of.

Geo-membrane technology

Geo membrane biogas plants are made of specially developed synthetic materials to reduce the

installation time. These plants are suitable for all types of lands and capable of installation in roof tops

of the buildings. The significant features of such a plant are its relatively small surface area requirement

for installation, less technical skills required for installation and operation of the plant and the

possibility to fold and ease of transport.


32

The biogas plant is supplied with an integrated gas scrubber, safety flaring device, pressure boosting

system, repairing material, excess water remover, geo membrane reactor and connection pipes for the

reactor (Bio-Tech Ltd, India). This technology is much less expensive for sure.

However, the geo-membrane technology proposed is not EU certified yet. Therefore, if this option is

chosen, KTH and Supplier have to apply for EU certification to ensure that the technology satisfies

the safety and supervisory requirements. These applications take about 6 months to process, given all

the required information is made available.

Set-backs

Several set-backs have however been noted and solutions will have to be studied. It has been noted

that during vacation and summer holidays, the availability of wastes from the kitchen will be reduced

to about 15 kg/ day. A make-up substrate source would have to be found; else the digester could be

operated on a reduced input substrate. But the latter solution will have a negative impact on the

bacterial culture as well as the biogas yield.

The participation of all stakeholders is essential for the proper functioning of the system proposed in

this thesis. The sustainability of the system will depend on the continuity of feeding material and also a

team of trained operators willing to maintain and run the plant as well as experiment the plant and

process.

5.9 MEASURES TO IMPROVE PROCESS ENERGY EFFICIENCY

Stirring in the pre-digestion tank/ buffer tank

An alternative mixing method can be by passive stirring, experimented by applying fluid pressure

differentials or thermal convective streams inside the tank during the addiction of fresh substrate as

well as by the rise of gas bubbles through the liquid, not requiring any mechanical, hydraulic nor

pneumatic means for mixing. This reduces the energy input but is not quite efficient.

Digestate generated

Heat can be recuperated by sending the digestate from the digester to a heat exchanger before end

storage, to send back heat to the system.

Heat from hygienisation tank

Depending on the chosen temperature of digestion, the heat from the substrate from the hygienisation

tank (which may be at 55°C or 70°C depending on the sanitisation method chosen) will be

recuperated.

2nd stage digestion

From the experiment, it has been observed that the digestion of an already digested digestate produces

a better quality biogas with 70-74% methane content. However, it has to be noted that the amount of

gas produced then would be about 25% of that of a fresh substrate. An economic and financial analysis

should be made to whether these measures can be implemented for a small-scale biogas plant.

Flow by gravity instead of pumping

The flow of substrate from the buffer tank to the hygienisation tank and from the hygienisation tank to

the reaction tank could be carried out by gravity by having the tanks outlet and inlet pipes at different

levels.


33

Upgrading and compressing the biogas

Depending on the use of biogas produced, the

latter should be treated to fulfil the requirements

of gas applications (gas engines, boilers, fuel

cells, vehicles etc.), increase the heating value of

the gas and standardise the gas.

Upgrading involves removal of carbon dioxide

and hydrogen sulphide, water and other trace

gases.

The most common methods for desulfurisation and removal of carbon dioxide can be achieved through absorption processes (water scrubbing or chemical absorption suing solvents) and brings up the methane content to about 97%.

Using other renewable power inputs

Using solar or wind power to heat up the process at the hygienisation tank and thermophilic digestion

processes can be investigated to study its technical and economic feasibility.

Figure 5.15: Upgrading of biogas (IEA Bioenergy, 2006)


34

6 PRODUCTION POTENTIAL OF BIOGAS (HEAT & POWER) FROM

FARMS IN MAURITIUS

6.1 ESTIMATE OF CATTLE, PIG AND POULTRY MANURE

The farmers/farms have been grouped as per districts. Due to larger number of farms in Flacq district,

it has been divided in 2 zones. Please note that 4 farms have been classified outside small farmers,

namely AREU, SURAT, PARKER and PRISON. SURAT and PARKER are 2 large dairy farms

recently in operation as from 2008/2009.

The figures related to broiler poultry (Figure 6.1) are mostly those of small and medium farms. The

corporate sector (INODIS, AVIPRO and others) which caters for more than 75% of total island

production are not included.

The local pig breeding sector produces an annual amount of 25,000 m3 liquid pig wastes awaiting

immediate treatment before being discharge safely in the environment.


35

Table 6.1: Number of breeders and livestock/ poultry status by geographical district as at June 2011 [AREU, 2011]

Animal Cattle Pigs Poultry

District No. of

breeders

No. of

heads

No. of

breeders

No. of

heads

No. of

breeders

Broilers No. of

breeders

Layers

Pamplemousses 148 671 36 956 39 108,775 43 91,800

Riviere du Rempart 232 1,153 41 557 41 91,138 23 53,045

Flacq 199 659 83 3,355 105 102,513 81 36,044

Grand Port 100 341 46 1,723 34 31,052 30 16,086

Savanne 70 527 28 426 72 546,990 62 122,408

Plaine Wilhems 81 496 23 955 38 129,400 34 80,700

Moka 87 561 13 388 38 105,800 17 35,600

Black River/ Port

Louis

101 957 281 14,925 31 150,250 22 17,525

TOTAL 1,018 5,365 551 23,285 398 1,265,918 312 453,208

Figure 6.1: Number of cattle, goats, sheep and pigs by type of breeder as at June 2011, Mauritius [AREU, 2011]

0

5000

10000

15000

20000

25000

30000

Cattle Goats Sheep Pigs

5365

28,132

1899

23,285

184

44

32

0

1047

0

0

0

2 Large commercial farms

Livestock breeding stations and prisons farm

Small breeders


36

The locations of the poultry farms in Mauritius

The Figure 6.2 below shows the location of clusters of small scale poultry farms in Mauritius, June

2011.

Figure 6.2: Location of clusters of small scale poultry farms in Mauritius [AREU, 2011]

Note: group of small scale farms


37

6.2 BIOGAS PRODUCTION POTENTIAL

The farming sector and the agro-food industry are among the main potential areas of biogas

production in Mauritius. For example, the anaerobic digestion of cattle manure to produce biogas

proves to be a promising alternative energy source and will help in regulating the release of the GHG

(methane) in the atmosphere. This treatment of manure addresses these two goals simultaneously as

well as addresses the problem of waste management on the farm.

Manure yield is subject to large variations depending on the substrate itself as well as the housing

intensity and is therefore quite difficult to estimate. Manure yields are therefore either measured or

calculated on a liveweight basis, since there is relatively good correlation between the two methods.

The quantities of manure listed in Table 6.2 are only valid if all of the animals are kept in stables all of

the time and if the stables are designed for catching urine as well as dung, as is mostly the case in

Mauritius. The stated values will be in need of correction in other cases. If cattle are only kept in night

stables, only about one third to one half as much manure can be collected. [Werner U., Stohr U., Hees

N., 1989]

Table 6.2: Standard live-weight values of animal husbandry [Werner U., Stohr U., Hees N., 1989]

Type of manure Liveweight (kg) Daily manure yield as

% liveweight

Manure

(kg/day/head)

Poultry 1.5-2 4.5% 0.0675-0.09

Pig 30-75 2% 0.6-1.5

Cattle 135-800 5% 6.75-40

Considering the average yields obtained from the anaerobic digestion process of volatile solids (VS),

contained in the effluents, expressed as % of total solids (TS), the biogas potential can be evaluated.

These data differ based on the diverse animal species considered [Tricase C, Lombardi M., 2009] as

shown in Table 6.3 below.

Table 6.3: Biogas production from different feedstock

Type of manure Total solids

(TS) (%)

Volatile solids (VS)

(%) of TS

Biogas yield

(m3/kgVS)

Poultry 10-29% 75-77% 0.2-0.4

Pig 2.5-9.7% 60-85% 0.26-0.45

Cattle 6-11% 68-85% 0.20-0.26

6.3 POWER PRODUCTION POTENTIAL

The maximum biogas production potential in Mauritius is determined by combining the data from

Table 6.2 and Table 6.3 (Table 6.4 below). If the biogas were used to produce heat and power by

means of a cogeneration system and considering a low heat value of biogas of around 6.0 kWh/m3

[Tricase C, Lombardi M., 2009], which corresponds to 60% CH4 in its composition, 90% of the energy

content of the fuel can be recovered of which 30% in electricity and 60% in heat, then the power that

can be produced by cattle manure in Mauritius is 16 GWh/year (Table 6.4 below). This confirms the

general rule of thumb that 1 m3 of biogas can generate 1.5 kW of electricity.


38

Table 6.4: Average production potential of biogas power and heat in Mauritius from farm manure.

Type of manure Total quantity of

manure

Gkg/year)

Biogas

(Gm3/year)

Power

(GWh/year)

Heat

(GWh/year)

Poultry 0.049 0.002 3.6 7.2

Poultry* 0.147 0.006 10.8 21.6

Pig 0.006 0.0001 0.18 0.36

Cattle 0.046 0.0007 1.26 2.52

Total 0.248 0.0088 15.84 31.68

*Poultry from the corporate sector (INODIS, AVIPRO and others) which caters for more than 75%

of total island production

The power potential calculated may seem quite small, but the goal of producing biogas from the farm

manure is not to send power to the grid but rather to primarily render the farms self-sufficient in terms

of heat and power or at least reduce the costs of heat and power of these farms.

However, the biogas production from farm manure in Mauritius is far from the calculated potential

representing less than 1%. This may be explained by current policy framework which does not

consider the anaerobic digestion of farm wastes. The policy framework is further described in section 9

of this report.


39

7 BIOGAS PRODUCTION TECHNOLOGIES

7.1 PRE-FEASIBILITY STUDY

The methodology and tools for selection of technology should share the following characteristics:

Low in capital costs

Use local materials whenever possible

Ensure that there is sufficient flow of feed stock to maintain process stability and gas production

Permits local needs to be met in terms of waste management and production of energy and

fertiliser

Are small enough to be affordable by a family or small businesses (farms)

Are user-friendly enough to be run and maintained by non- experts

May involve decentralised renewable energy sources (solar power, man power)

Are flexible so that they can be adapted to fit changing conditions

Avoids unnecessary costs such as transportation costs

It is in harmony with the local cultural traditions (community acceptance)

Nuisances such as noise and odours are controlled

7.2 TYPES OF DIGESTERS AVAILABLE

Anaerobic digesters are classified according to their operation type which can be either batch, semi-

continuous or continuous. When it comes to plant size, anaerobic digestion of organic wastes and

energy crops can be divided as follows [Renewable and Sustainable Energy Reviews, 2010]:

- Horizontal digesters (volume 50-150 m3) suitable for the smallest size plants and well-suited for

treatment of cow manure as well as co-digestion with farm crops due to good mixing conditions. [T.

Fischer & A. Krieg, 2002]

Figure 7.1: Horizontal Digester [T. Fischer & A. Krieg, 2002]

Advantages:

digesting high solids content

high loading rate possible

little short cut flow

automatic sand drain

complete mixing

high digester productivity

Suitable for dry digestion

Disadvantages:

high price

only possible with after digester

limited in size


40

[Source: http://www.biogasregions.org, Train the Trainers Seminar, Wolpertshausen, Germany, 28-29

November 2007, accessed 18.08.12]

- Upright standard agricultural digesters (volume 500-1500 m3). These reactors are fitted with a heating system and mixers. The top of the tank is fitted with a double-membrane gas holder roof. These reactors can treat up to 10,000 m3/ year of feed stock and the hydraulic retention time varies between 30 and 80 days depending on the input substrate. [Sotirios Karellas et al, 2009]

Figure 7.2: Standard Digester in Agriculture [T. Fischer & A. Krieg, 2002]

Advantages:

low-price alternative

simple digester revision

integrated gas holder

Disadvantages:

wind and weather sensitive

not 100% gas-tight

Difficult to indicate the gas filling level

[Uli Werner et al; 1989] - Upright large digester (volume 1000-5000 m3). Here the input substrate is pre-heated and mixed by a central mixer which leads to a much lower hydraulic retention times (20-30 days). These devices can treat up to 90,000 m3/year of feedstock.. [Sotirios Karellas et al, 2009]

Figure 7.3: Upright Large Digester [http://www.biotech-india.org]

http://www.biogasregions.org/


41

- Geo membrane digester. The installation of the low-cost geo-membrane biogas plants is

considered simple and does not require any special skills to be run. These types of plants can be set up

by the farmer him-self or they are readily available on the international market, made in countries such

as India, Thailand or China. They are made with specially developed synthetic materials to reduce the

installation time. These plants are suitable for all types of lands and capable of installation in roof tops

of the buildings. These are commonly used in Thailand, China and India.

Figure 7.4: Geo-membrane biogas plant [http://www.biotech-india.org]

Some of the significant features include:

Compact

Durable

Light weight

Less space requirement for installation

Less technical skill required for installation

Foldable and transportable from one site to another

7.3 SUITABLE TECHNOLOGY FOR FARMS IN MAURITIUS

In Mauritius, the technology of the farm scale digesters should be elaborate but should also remain

simple and easy to use, with the least monitoring possible.

Several factors should be considered:

1. The plant will be de-centralised therefor there is a need for the plant to be self-sufficient in heat

and power. A CHP plant should also be installed.

2. Pre-treatment options of the feed manure to the pre-digestion tank:

Rainfall water runoff separation and recovery system

Sand separator system

Fiber solid screen separator

Composting of fibrous solid

3. HRT of digestion of the different substrates:

Cow dung – approx. 50 days digestion

Poultry dung – approx. 20 days digestion

Food wastes – approx. 20 days digestion

Pig wastes – approx. 20 days digestion

Therefore, it can be said that poultry manure, pig manure and food wastes can be co-digested together.


42

4. Cost of equipment and running costs:

The relief of energy savings as compared to current costs of energy

5. Competition of substrate of the market:

Cow dung is currently dried as utilised as-is in the fields as an organic fertiliser.

Food wastes, for example from hotels, are sold to pig breeders as feed to the pigs.

On the other hand, the pig industry in Mauritius is an explosive issue due to its waste management

in terms of techniques available locally as well as the socio-cultural beliefs nationally. It is clearly a

big issue to be resolved and anaerobic digestion to produce biogas can be considered an

environmentally friendly approach to the problem.

Poultry waste also constitutes a waste management problem that needs to be resolved by all

poultry breeders to prevent any hygiene problems or avian disease

6. Digestion of manure and co-substrates from farms of the same area:

In the case where we are able to implement integrated farm digesters, the size of the biogas plant

would be larger and the yield of biogas greater. The regulations around this type of project have to be

framed in a policy whereby the costs and benefits are equally shared. This policy is further discussed in

section 9 of this report.

As a conclusion, the most suitable digesters for farm wastes in Mauritius would be small to medium

farm scale digesters with the volumes varying anywhere between 5 m3 to 1,500m3. The process

temperature would be mesophilic or possibly thermophilic for the larger ones.

Semi-continuous digesters are suitable to the farms in Mauritius where sludge-like substrate such as

manure are usually added to the process less frequently and in larger proportions.

In my opinion, the horizontal of either flexible-balloon-tank or coated fibreglass tank or coated PVC

tank digesters would be a technically and economically suitable option. These digesters are easy to

construct with easily available materials and easily operated. For medium size digesters the standard

agricultural digester can be considered the most appropriate option.

The work load for the farmer would be approximately 1-2 h/d for small to medium farm scale

digesters, according to several studies in Germany, Sweden and China.

7.4 APPLICATIONS OF BIOGAS IN MAURITIUS

The purpose of installation of a CHP unit is to supply all energy needs on the plant as well as to

provide all the heat and electricity required for the livestock, machinery and administration on the

farm. The surplus of energy can be sold to the public grid or neighbouring farms. The biogas produced

will also serve as heat for cooking and for hot water for local use on the farm.

New markets for the excess biogas produced, applicable to Mauritius, would include:

Suitable fuel to run gas ovens and fires

To heat greenhouses and water heaters

As raw material in manufacturing industrial processes

On the long term, Mauritius could target markets such as vehicle fuel by upgrading and

compressing the biogas or

Store the energy from the biogas via a fuel-cell


43

8 ECOMOMIC ANALYSIS

While governments and private entrepreneurs may be interested in the anaerobic digestion of food and

farm wastes at treatment facilities, they may feel that the cost is a limiting factor. However, there are

many advantages that should be thought of before discounting this technology based on cost.

Considering the final objectives of such a project would be to

Meet customer requirements in terms of waste management and energy

Be competitive on the market

Easy access

Maintain ease of use of equipment, no need of special skills

Finalise costs of energy as compared to current costs of energy

Although the capital investment costs for the implementation of the project may be large, the digestion

food and farm wastes can be quite profitable (referring to both the experiment setup and the study in

Mauritius).

The payback period can be less than three years depending on the desired infrastructure and

technology. There will be a significant amount of savings in energy avoidance due to methane

production. The excess energy can be used for external uses for profit.

Investment costs would include:

Equipment

Grinder at the plant or grinders at the kitchens to convert semi-solid/ solid waste into slurry

Pumps:

Pumping of wastes

Addition of water

Re-circulation pumps

Stirrers

In the preparation/ buffer tank

In the hygienisation tank

In the biogas tank

Valves

Piping

Emergency flaring device

Gas blower

Safety methane gas sensor apparatus for testing leakages in the system

Spreader (Tractor equipped with manure container and spreader)

CHP plant (small plants- the dual fuel engine and larger plants- the Otto engine)?

(Replacement of cooking gas)

(Replacement of heating fuel/ electricity)

(Transportation of wastes to the landfill)

Transportation of wastes to the biogas plant

PLC system (for automation and remote control and monitoring of plant)

This system may not be competitive in the current energy market but versus other stand-alone energy

systems.


44

9 POLICY AND LEGAL FRAMEWORK

Replication and up-scaling of the biogas plant

Reproduction of this project is quite easy. Land for digester setup and community involvement are

prerequisites for the project, which includes sensitizing local communities to segregate waste at

household level, if organic household wastes would be used as substrate. Once these basic

requirements are satisfied, implementation of biogas plants across the island would be made possible.

An additional component necessary for successful enactment in the planning for construction of the

biogas plant would be sizing the plant depending on the amount of waste and purpose of cooking

(commercial or household only), as well as user expectations. [Ministry of Renewable Energy & Public

utilities, 2009]

Farming policy

The animal farming industry has evolved from a traditional mode in the early 70’s to a semi-industrial

approach through the development of Cooperative societies. On the farmer’s point of view, as owner

of a small business, energy costs and waste disposal costs should be considered as it represents a fair

percentage of the total operating costs in the business. This evolution had been made possible through

the introduction of Government funds as well as other financial and technical facilities.

Examples of the incentives given to farmers include:

Infrastructural facilities for production and marketing of the products

Training of farmers

Funding mechanisms such as low-rate loan facilities. In order to meet the political goals set up

regarding the energy and environment policies, a number of finance incentives and measures have

been implemented.

Free consultancy services for farm management from specialised firms

Access to veterinary services

Energy policy

The national targets include achieving significant energy conversion in all sectors of economy in the

short and medium term as well as increase the share of renewable energy on the energy mix to shift

towards cleaner energy, improved energy efficiency and conservation, and improved consumption

pattern.

This project can be an ESCO driven project aiming at energy savings and in turn cost savings in the

agricultural sector.

The government has set up a MID fund to help projects projecting towards sustainability in Mauritius.

These consist of exploitation of natural renewable energy sources available locally, the preservation of

natural resources, programmes to protect the environment through recycling of waste, schemes to

reduce overall energy consumption including grants and soft loans, encourage independent power

producers (IPP) to integrate the power generation system as well as self-made energy alternatives,

conducting research with regards to renewable energy technologies, energy management programmes

both locally and with foreign partners and sensitization campaigns regarding the wise use of energy and

renewable energies.

GHG emissions

The basis of action, according to the treaty of Agenda 21, was based on the fact that all energy sources

need to be used to respect the atmosphere, human health and the environment as a whole.

Promoting the implementation of biogas plants in Mauritius would allow:


45

Expanding access to energy

Stimulating economic growth and empowerment

Increasing the scope and quality of rural services

Reducing environmental degradation and health risks

The alternative of do-nothing with the farm wastes will imply more greenhouse gas emission

(methane), which is posing threat to world climate through global warming. Hence, increasing

emphasis should be laid on decreasing the GHG emissions and substitution of fossil fuels by the

readily available renewable resources to produce energy for local use on farms.

The anaerobic digestion of farm wastes is, in my opinion, the most appropriate solution in order to

achieve renewable energy optimisation as well as further improvement to meet Kyoto Protocol

commitments and beyond to even enable carbon trading and Carbon emission credit.

A number of economic incentives and measures have been implemented to meet the political targets

that have been set up in relation to energy and climate policy.


46

10 CONCLUSIONS

This current study implies a great deal of activities in the field of biogas production in the short term.

The development of biogas plant technologies in small developing countries such as Mauritius or small

communities such as KTH will not only benefit the environment and expand the sustainability of our

energy supply but it also means a step forward in the right direction for the economy in general, for

individual companies, and for the regional development as well.

The motivation of such a project came from the sensibility on individual bodies towards

environmental and waste problems that the agricultural sector is facing, especially in Mauritius, aiming

towards becoming a sustainable island, thus reducing the reliance on fossil fuels via the use of local

sources of renewable energy. Mauritius is well positioned to up-scale the use of anaerobic digestion to

produce energy from biogas generated.

It has also been shown, through literature review and experiment, that with simple materials and basic

technical knowledge, it is possible to construct and run a successful biogas plant, as performed in many

developing countries and Mauritius is no exception. The food grinder was a key unit to the proper

digestion process, providing good water to waste proportion which can be controlled.

The biogas plant setup at the KTH University was a first of its kind. I would furthermore say that the

experiment was a success hoping that the University would consider having a permanent biogas plant

on site for future studies. Such projects would promote energy efficiency studies and investigations.

During only two weeks of operation, including the ups and downs, the plant produced a good amount

of high quality biogas. Throughout the process, focus was given to the stability of the microbial activity

in the reactor and control of the inhibitors such as carbon dioxide formation.

To assess the feasibility of the study, an economic analysis of the setting up of a farm scale anaerobic

digester on a farm, or as part of an integrated farming programme, was considered. It was found that,

depending on the use of digester, that is, whether the digestate generated is discarded, used on the

producing farm or being sold, about 5000 USD can be saved in terms of power and 8500 USD in

terms of heat per year based on the local domestic electricity prices (http://www.ceb.intnet.mu). An

economy of about 10000 USD per year can also be made in terms of LPG gas consumption (or

replacement) based on the current local price of a 12 kg domestic LPG gas cylinder.

The potential is encouraging and the application should be promoted both in Mauritius as well as at

KTH.

http://www.ceb.intnet.mu/


47

11 LESSONS LEARNED AND FUTURE WORK

In this thesis, research has been carried out on small-scale biogas plant (6-10m3 of biogas per day) was

designed and tested. The experiment demonstrated to be technically feasible. However, to gain general

acceptance in the population, we have to prove technical feasibility on the long run as well as the

financial feasibility.

Also, in Mauritius, LPG which is cleaner and readily available is associated with a general improvement

of the living standards of the population. However, this technology can be of value to the remote

islands of Mauritius where farming is practised.

Ways of upgrading and compression of the biogas generated using the biogas itself and thus lower

energy use can be considered as future work. Also, compressed biogas can be easily stored and then

could be tested on a longer run on the microturbine in the KTH Energy laboratory.

Future works would consist of the feasibility study of the use of food wastes for biogas plants in

Mauritius so that the market for the uses for biogas from wastes may be increased.

For the implementation and general acceptance of such a project of implementation of biogas plants in

Mauritius, as well as in KTH for use of biogas in the laboratory, a good promotion and awareness

raising events is essential. Users should be well informed and updated on the technology to ensure

proper maintenance and running of the plant. The misconception of the requirements in running of a

biogas plant may lead to deception and rejection of the concept.


48

12 REFERENCES

Agricultural Research and Extension Unit, Mauritius, 2012, Personal Communication

Buswell &Neave, 1930; Overview of biogas technology, Luostarinen, Normak & Edström

Capareda, Sergio and Mukhtar, Saqib, 2011, "Manure to Energy, Understanding Processes, Principles,

and Jargon." Texas A & M University

Development of an investment decision tool for biogas production from agricultural waste, Renewable

and Sustainable Energy Reviews 2010; 14:1273-82

Edström M, 2012, JTI, Stockholm, Personal Communication

Energigas Sverige, Personal Communication, 2012

Fischer T. and Krieg A., 2001, Planning and construction of Biogas Plants, International Congress,

Renewable Energy Sources in the Verge of XXI Century, Warschau, 10.-11. December, 2001

Fischer T.; Krieg A.; Chae K.J.; Yim S.K.; Choi K.H.; Park W.K. and K.C. Eom; 2002 Farm-Scale

Biogas Plants: Journal of the Korean Organic Waste Recycling Council, Vol. 9, No. 4, S. 136-144

Gujer, W., and Zehnder, A. J. B., 1983, ‘‘Conversion processes in anaerobic digestion.’’ Water Science

and Technology, 15, 127–167

KTBL, 2010. Gasausbeute in landwirtschaftichen Biogasanlagen. Report. 2. überarbeitete Auflage

Institut für Energetik und Umwelt et al., 2006

Long-term Energy strategy and action plan, 2009, Ministry of Renewable Energy & Public Utilities

Ministry of Public Utilities Mauritius, 2008

Persson M., Jönsson O. & Wellinger A.; IEA Bioenergy, Biogas upgrading to vehicle fuel Standards

and grid injection, 2006

Roumeela Mohee & Acknez mudhoo, 2008, Waste-to-Energy, Opportunities & challenges for

developing and transition countries

S. Luostarinen, A. Normak, M. Edström; 2011, Overview of Biogas Technology

Sambo, A. S. 2005. Renewable energy for rural development, The Nigerian perspective. ISESCO:

Science and Technology Vision 1: 12-22

Sotirios Karellas a,*, Ioannis Boukis b, Georgios Kontopoulos c, Development of an investment

decision tool for biogas production from agricultural waste , Elsevier Ltd, 2009

Steineck, S., Gustafson, G., Andersson, A., Tersmeden, M. & Bergström J. 1999. Plant nutrients and

trace elements in livestock wastes in Sweden. Naturvårdsverket, Rapport 5111, Sweden

Technical Advisory Committee, Final Report, 2004

Tricase C, Lombardi M. State of the art and prospects of Italian biogas production from animal

sewage: technical-economic considerations. Renewable Energy 2009; 34: 477-85

Uli Werner,Ulrich Stöhr &Nicolai Hees; Biogas plants in animal husbandry; 1989

Viljavuuspalvelu Ltd. 2011, Manure statistics 2000-2004. Available at: http://www.viljavuuspalvelu.fi

Werner Uli, Stohr Ulrich, Hees Nicolai, 1989, Biogas plants in animal husbandry,

http://www.anaerobic-digesters.com

http://biorealis.com

http://www.biotech-india.org


http://www.lms-lufa.de/upload/39/1254233880_20539_15327.pdf


http://www.schoutenmachines.co.nz


http://www.viljavuuspalvelu.fi/

http://www.anaerobic-digesters.com/

http://biorealis.com/






Dept of Energy Technology

Div of Heat and Power Technology

Royal Institute of Technology

SE-100 44 Stockholm, Sweden

Documents

1 FEASIBILITY STUDY OF STAND-ALONE SMALL-SCALE DIGESTION SYSTEMS TO PRODUCE BIOGAS …kth.diva-portal.org/smash/get/diva2:678792/FULLTEXT01.pdf · 2013-12-13 · II Master of Science