MODELLING OF THE PERFORMANCE OF A BATCH BIOGAS DIGESTER FED WITH SELECTED TYPES OF SUBSTRATES

Transcription

1 MODELLING OF THE PERFORMANCE OF A BATCH BIOGAS DIGESTER FED WITH SELECTED TYPES OF SUBSTRATES Thesis submitted to the University of Fort Hare, Faculty of Science and Agriculture in fulfilment of the requirements for a Doctor of Philosophy degree in Physics By Patrick Mukumba SUPERVISOR: Dr G. Makaka CO-SUPERVISOR: Dr S. Mamphweli NOVEMBER 213

2 BIOGAS UTILISATION FROM THE 1 m 3 BATCH BIOGAS DIGESTER i

3 DEDICATION Dedicated to my wife, children, mother, father, brothers and sister for all of their love and support during my research period ii

4 ACKNOWLEDGEMENTS My dream has come true, thanks to a massive amount of hard work, support and dedication, partly by me and also by a lot of other people. I am deeply grateful to my supervisors Dr G. Makaka and Dr S. Mamphweli for devoting their time and knowledge to help me to complete this study. I would like to thank Govan Mbeki Research and Development Centre at University of Fort Hare and Eskom for funding my research work. My thanks also go to my wife Paulina, for her support and encouragement and my children Charles, Milton, Allan Kudzaishe and Princess for enduring my absence when they needed me most, and the whole of my big family for the encouragement and support. My father and mother, I would like to thank you for giving me such perseverance for all the studies that I have undertaken from Diploma in Education to PhD studies. My brothers and my sister you are also thanked for your encouragement and support that made me to be focussed throughout my studies. Lastly, I would like to thank my friend, Dr P Tsvara for auditing my document and kept me motivated during the research period. Thank you all, and many others. iii

5 SUMMARY The increasing population and the rapid economic growth in South Africa have led to higher consumption of food resulting in the generation of large amounts of waste. In addition, South Africa has plenty of biomass from cattle, donkeys, horses, goats, pigs, chicken and sheep. However, anaerobic digestion could be an alternative solution for the utilization of these kinds of waste due to its environmental and economic benefits. Therefore, the main focus of the research was design, construct a field batch biogas digester, monitor its performance when fed with cosubstrates and model the methane yield for an optimized mixing ratio. This research is summarised as follows: Chapter 1 provides background information on the research study, defines the research problem, and outlines both the purpose of the investigation and the research design. Research questions [1-1V (section 1.3)] are addressed by means of literature review reflected in chapter 2 and answered in chapter 5. Chapter 2 critically looks at the background of anaerobic digestion which includes hydrolysis, acidogenesis, acetogenesis and methanogenesis. Factors affecting the rate of digestion followed the background of anaerobic digestion. These factors which were looked at include: ph value, temperature, and hydraulic retention time, carbon/nitrogen ratio, ammonium, particle size, water content, agitation and organic loading rate. Section 2.4 looks at biogas digester types that included fixed dome, balloon and floating digesters. The advantages and disadvantages of each of them were also reviewed. Furthermore, feeding methods that include batch and continuous feeding were looked in the section 2.5. Advantages of biogas digesters and biogas composition sections followed this section. Some of the benefits of biogas highlighted in this study were; production of energy for lighting, heat and electricity, improved sanitation, improved indoor air quality and reduction of workload. The last section 2.8 focussed on anaerobic co-digestion. Its advantages highlighted include; ammonia removal, improved methane yield, adjustment of the carbon / nitrogen ratio and improved sanitation. iv

6 Chapter 3 analysed the research methodology of substrates for co-digestion, namely; cow dung, donkey dung, goat dung and horse dung. The co-digestion ratios for the trials were presented. The chemical analysis of the substrates for co-digestion was also looked at. The procedures for chemical analysis for the digester substrates include determination of total solids, volatile solids, ammonia-nitrogen, total alkalinity, calorific value, ph and chemical oxygen demand in the substrates. Similarly, biogas analysis procedure was discussed. The biogas analysis included determination of the percentage composition of methane, carbon dioxide, hydrogen and hydrogen sulphide in biogas. The data storage and retrieving was also discussed. In addition, the instrument, materials and method employed in the determination of the energy values were also discussed in this section. The chapter also discussed performance monitoring of the batch biogas digester. Chapter 4 engages in the design and construction of the 1 m 3 batch biogas digester. A number of equations were used to design the cylindrical surface biogas digester. A number of parameters were considered when designing the digester, these were ease of operation, cost effectiveness and air-tightness, availability of substrates and construction material. After the design of the digester, selection of construction material, site selection, site layout and excavation were done. The digester was constructed in the following steps: construction of the foundation, construction of the body of the digester, construction of the dome of the digester, inside and outside plastering and insertion of the mechanical stirrer. In addition, painting of the inside and outside of the digester with epoxy was also done. Fitting the dome of the digester, plastering and painting the entire digester followed. The digester was fed with cow dung before it was insulated. The parameters measured in cow dung included; total solids, volatile solids, ph, temperature, calorific value and chemical oxygen demand. The cow dung was thoroughly prepared to form a homogenous mixture before being fed into the digester. The biogas produced in the digester was measured using a biogas analyser. The components in biogas analysed include; methane, carbon dioxide, hyrogen and hyrdogen sulphide. The data collected from the biogas analyser and from temperatures was stored in the CR1 data logger. v

7 The digester contents ( cow dung slurry) were removed before insulation of the digester with saw dust. A second single brick wall was constructed and then plastered. The sawdust was later put in the gap between the digester double wall and single wall. The digester was insulated with sawdust to minimise temperature fluctuations. Chapter 5 reports on the research results, and analysis of mono-digestion and co-digestion experiments. It was observed that biogas and slurry temperatures of a biogas digester depended on ambient temperatures. When the biogas digester was not insulated there was an increase in temperature fluctuations as a result of heat transfer from the environment into the digester through the double wall brick structure of the biogas digester. Biogas production was highly affected by temperature fluctuations. The methane producing microbes were very sensitive to these temperature fluctuations. As such insulating the biogas digester with sawdust was seen as a way to minimise the fluctuations but not to prevent them completely, even if the biogas digester is built underground. In the co-digestion of cow dung and donkey dung the highest biogas yield with the highest methane yield was obtained from the mixing ratio of 5% cow dung and 5% donkey dung. However, in the co-digestion of goat dung and horse dung, the highest biogas yield with the highest methane content was obtained from mixing ratio 75% goat dung and 25% horse dung. In addition, in the co-digestion of the four substrates; namely cow dung, donkey dung, goat dung and horse dung, the highest biogas yield was produced from a mixing ratio of 25% cow dung, 25% donkey, 25% goat dung and 25% horse dung. The performance of the batch biogas digester was determined by the kinetics of the anaerobic digestion process. The first order kinetic was used to find the biodegradability of the biogas digester when fed with cow dung, donkey dung, goat dung and horse dung at different mixing ratios. A mixing ratio composed of 25% cow dung, 25% donkey dung, 25% horse dung and 25% goat dung had the highest biogas yield as compared to the other mixtures because the biodegradability value determined by k was highest. vi

8 Chapter 6 reports on the empirical investigation into the methane yield modelling. Mathematical model equations were developed from mixing ratios that produced maximum biogas yields. The multiple regression models were developed for methane yield as a function of the following inputs: ph, COD, NH 4 -N, digester temperature (T D ) and total alkalinity (T A ). The general theoretical equation for the optimized model is expressed by: Y m α α (ph) α (COD) α (NH N) α (T ) α (T ) 4 D 5 A Where: Y m is the output (methane yield and α values are the scaling coefficient). The optimized model equation for donkey dung is expressed by: Y m (pH).4(COD).9162(NH.597(T A ) 4 N).12151(T D ) For cow dung co-digested with donkey dung the model equation is expressed by: Y m (pH).1(COD).232(NH 4 N).168(T ).14(T ) D A Furthermore, the optimized model equation for goat dung co-digested with horse dung is given by: Y m (pH).6(COD).67(NH (T A ) 4 - N).4828(T D ) Finally, the mathematical model equation for co-digestion of the four substrates, namely, cow dung, donkey dung, goat dung and horse dung is expressed as: Y m (pH).557(COD).196(NH.166(T A ) 4 N).12383(T D ) In all the developed mathematical model equations, the input COD and digester temperature (T D ) had positive values which mean that any increase in COD and digester temperature increased methane yield. vii

9 TABLE OF CONTENTS DEDICATION... ii ACKNOWLEDGEMENTS... iii SUMMARY... iv TABLE OF CONTENTS... viii LIST OF FIGURES... xii LIST OF TABLES... xv LIST OF APPENDICES... xvi NOMENCLATURE... xviii CHAPTER BACKGROUND RESEARCH PROBLEM STATEMENT JUSTIFICATION OF THE RESEARCH AIM AND OBJECTIVES RESEARCH QUESTIONS ASSUMPTIONS THESIS SYNOPSIS... 5 CHAPTER INTRODUCTION BACKGROUND TO ANAEROBIC DIGESTION Hydrolysis Acetogenesis Methanogenesis FACTORS AFFECTING BIOGAS PRODUCTION ph-value Temperature Hydraulic retention time (HRT) Redox potential Inocula Carbon/nitrogen (C/N) ratio Ammonium (NH 4 ) Particle size viii

10 Water content Agitation / Mixing Organic loading rate Volatile fatty acids (VFA) BIOGAS DIGESTER TYPES Fixed dome digester Bag digester (balloon digesters) Floating drum digesters FEEDING METHODS ADVANTAGES OF BIOGAS DIGESTERS BIOGAS COMPOSITION ANAEROBIC CO-DIGESTION Merits and de-merits of co-digestion CONCLUSIONS CHAPTER INTRODUCTION SUBSTRATES FOR CO-DIGESTION DETERMINTION OF SUBSTRATE PROPERTIES Determination of total solids Determination of volatile solids Determination of Chemical Oxygen Demand (COD) Determination of ph Determination of Ammonia-Nitrogen Determination of Total Alkalinity Determination of energy values (calorific values) of substrates BIOGAS ANALYSIS Theory of calibration for non-dispersive infrared gas sensors Palladium /Nickel (Pd/Ni) gas sensor theory Data storage CONCLUSION CHAPTER INTRODUCTION DESIGNING A BATCH BIOGAS DIGESTER ix

11 Design parameter Cross-section of a cylindrical batch biogas digester Volume calculation of digester and hydraulic chamber CONSTRUCTION OF A CYLINDRICAL BATCH BIOGAS DIGESTER Selection of construction materials Site selection and layout Excavation Construction of the foundation Construction of the dome Inside plastering and insertion of a mechanical stirrer Plastering the outside of the digester and painting Inserting the curved top of the digester FEEDING OF THE BATCH BIOGAS DIGESTER BEFORE SECOND WALL CONSTRUCTION SECOND WALL CONSTRUCTION AND INSULATION OF THE BIOGAS DIGESTER CONCLUSION CHAPTER INTRODUCTION TEMPERATURE VARIATION AND GAS YIELD Temperature variation and gas yield before insulation of the biogas digester Temperature variation and gas yield (with insulation) CO-DIGESTION Donkey dung Donkey dung and cow dung co-digestion Goat dung and horse dung co-digestion Co-digestion of cow dung, donkey dung, goat dung and horse dung AMMONIUM NITROGEN (NH4-N) AND TOTAL ALKALINITY (T A ) FIELD SCALE DIGESTERS AND LABORATORY SCALE DIGESTERS BIODEGRADABILITY OF THE BIOGAS DIGESTER The First Order Kinetic Model CONCLUSION CHAPTER INTRODUCTION x

12 6.2. MULTIPLE REGRESSION MODEL OPERATION OF THE MATLAB OPTIMISATION MULTIPLE REGRESSION MODEL AND ANALYSIS Mono-digestion Co-digestion CONCLUSION CHAPTER INTRODUCTION CONCLUSION FINDINGS SUMMARY OF MAJOR CONTRIBUTIONS RECOMMENDATIONS REFERENCES APPENDIX A APPENDIX B APPENDIX C xi

13 LIST OF FIGURES Figure 2.1: Four steps of anaerobic digestion (adapted from Jarvis, 24) Figure 2.2: Chinese fixed dome digester (Florentino, 23) Figure 2.3: Balloon digester (Adapted from Fulford, 21)... 2 Figure 2.4: Indian-type digester (Adapted from Florentino, 23) Figure 2.5: Importance of biogas technology (adapted from APCAEM, 27) Figure 3.1: Research design flow chart diagram Figure 3.2: Simplified ph meter circuit diagram Figure 3.3: The digital ph meter and various ph solution tubes Figure 3.4: The calorimeter (CAL2K-ECO) photo Figure 3.5: Filling Station (CAL2K Figure 3.6: Vessel CAL-2K Figure 3.7: Vessel CAL2K-4- GROUP Figure 3.8: Designed batch biogas digester with various sensors positions... 4 Figure 3.9: The data acquisition system Figure 3.1: Basic components of NDIR sensor (Mamphweli, 29) Figure 3.11: Diagram of a Pd/ Ni hydrogen sensor (Mamphweli, 29) Figure 3.12: The CR1 Data Logger Figure 4.1: Cross-section of the cylindrical biogas digester designed Figure 4.2: Geometrical dimensions of the cylindrical shaped biogas digester body Figure 4.3 Sketch showing the cross section of the layout of the digester Figure 4.4: Sketch of the depth and width of the digester foundation/base Figure 4.5: First and second brick work after the foundation Figure 4.6: The biogas digester at a height of 7 mm Figure 4.7: Sketch showing wooden sheet, positions of stone aggregate and reinforced concrete Figure 4.8: Sketch diagram of the digester dome Figure 4.9: The wire mesh curved top dome of the biogas digester Figure 4.1: The reinforced concrete dome fitted with inlet slurry pipe and biogas pipe Figure 4.11: Designed mechanical stirrer with a handle Figure 4.12: Shows inside epoxy painted floor and walls Figure 4.13: Biogas digester with a reinforced concrete dome... 6 Figure 4.14: Preparation slurry for the digester Figure 4.15: Feeding the biogas digester Figure 4.16: Biogas digester with a second wall Figure 4.17: Sawdust insulated biogas digester Figure 5.1: Slurry, ambient and biogas temperatures before insulation (September) Figure 5.2: The variation of slurry temperature in response to ambient temperature Figure 5.3: Biogas yield from the cow dung before the digester was insulated... 7 Figure 5.4: Biogas yield incremental equation before the digester was insulated Figure 5.5: Exponential equation of biogas yield before the digester was insulated Figure 5.6: Variation of slurry temperature in response to ambient temperature after insulation.. 74 Figure 5.7: Biogas yield for cow dung xii

14 Figure 5.8: Relationship between gas yield and ph range for cow dung Figure 5.9: Relationship between gas yield and COD range for cow dung Figure 5.1: Biogas yield and ph values for donkey dung for a retention period of 28 days Figure 5.11: Comparison of biogas yield from cow dung and donkey dung... 8 Figure 5.12: Relationship between gas production and COD for donkey dung Figure 5.13: Biogas production for different mixing ratios cow dung and donkey dung Figure 5.14: Biogas incremental equation from day 4 to 16 for 5% cow and 5% donkey dung 83 Figure 5.15: Exponential equation for 5% cow and 5% donkey dung from day 16 to day Figure 5.16: ph cow dung and donkey dung at different mixing ratios Figure 5.17: The biogas production and ph range for 5% cow dung and 5% donkey dung Figure 5.18: Relationship between gas production and ph for 25% cow and 75% donkey dung Figure 5.19: The biogas production and ph for 75% cow dung and 25% donkey dung Figure 5.2: COD co-digestion of cow dung and donkey dung at different mixing ratios Figure 5.21: Relationship between gas production and COD for 5% cow and 5% donkey dung 88 Figure 5.22: Biogas yield for horse dung and goat dung... 9 Figure 5.23: Biogas yield for the goat dung and horse dung Figure 5.24: Relationship between gas yield and ph for 75% goat dung and 25% horse dung 94 Figure 5.25: The biogas yield and ph for 5% goat dung and 5% horse dung Figure 5.26: Relationship between gas yield and ph for 25% goat dung and 75% horse dung Figure 5.27: COD for co-digestion of goat dung and horse dung at different mixing ratios Figure 5.28: The biogas production and COD for 75% goat dung and 25% horse dung... 9 Figure 5.29: The biogas yield and COD for 25% goat dung and 75% horse dung... 9 Figure 5.3: Biogas yield for 25% cow and 25% donkey dung with 25% goat and 25% horse dung 1 Figure 5.31: Incremental equation for co-digestion of 12.5% cow dung and 12.5% donkey dung with 37.5% horse dung and 37.5% goat dung from 2 to day Figure 5.32: Decay equation for co-digestion of 12.5% cow dung and 12.5% donkey dung with 37.5 horse dung and 37.5% goat dung from 15 to day Figure 5.33: The incremental equation for the co-digestion of 25% cow dung and 25% donkey dung with 25% horse dung and 25% goat dung from 2 to day Figure 5.34: The decay equation for the co-digestion of 25% cow dung and 25% donkey dung with 25% horse dung and 25% goat dung from 13 to day Figure 5.35: The incremental equation for co-digestion of 37.5% cow and 37.5% donkey dung with 12.5% horse and 12.5% goat dung from 2 to day Figure 5.36: The decay equation for the co-digestion of co-digestion of 37.5% cow and37.5% donkey dung with 12.5% horse and 12.5% goat dung from 13 to day Figure 5.37: Variation of ph for different mixing ratios of cow, donkey, goat and horse dung Figure 5.38: The biogas yield and ph for 25% cow, 25% donkey, 25% goat and 25% horse dung... 1 Figure 5.39: The relationship between biogas and ph for 37.5% cow, 37.5% donkey dung codigested with 12.5% goat dung and 12.5% horse dung Figure 5.4: The biogas yield and ph for 12.5% cow and 12.5% donkey dung co-digested with 37.5% goat and 37.5% horse dung Figure 5.41: COD for different mixing ratios for cow dung, donkey dung, goat dung and horse Dung xiii

15 Figure 5.42: The biogas yield and COD 25% cow, 25% donkey, 25% goat and 25% horse dung 11 Figure 5.43: Bo value for 12.5% cow and 12.5% donkey dung co-digested with 37.5% goat and 37.5% horse dung Figure 5.44: Bo value for biogas yield from a mixture of 25% cow dung, 25% donkey dung, 25% goat dung and 25% horse dung Figure 5.45: Bo value for 37.5% cow and 37.5% donkey dung co-digested with 12.5% goat and 12.5% horse dung Figure 5.46: Graph of ln (S/So) against time for 12.5% cow and 12.5% donkey dung co-digested with 7.5% goat and 37.5% horse dung Figure 5.47: Graph of ln (S/So) against time for mixture of 25% cow dung and 25% donkey dung co- digested with 25% goat dung and 25% horse dung Figure 5.48: Graph of ln (S/So) against time for 37.5% cow dung and 37.5% donkey dung codigested with 12.5% goat dung and 12.5% horse dung Figure 6.1: Schematic diagram of a mathematical model Figure 6.2: Experimental and optimized values for donkey dung Figure 6.3: Experimental and optimized values for 5% cow dung and 5% donkey dung Figure 6.4: Experimental and optimized values for 75% goat dung and 25% horse dung Figure 6.5: Mathematical model graph for a mixing ratio of 25% cow dung, 25% donkey dung, 25% horse dung and 25% goat dung xiv

16 LIST OF TABLES Table 2.1: Enzymes utilised during the hydrolysis step of anaerobic digestion (Gerardi, 23) Table 2.2: Examples of products from glucose degradation (Batstonne et al., 22)... 9 Table 2.3: Major acids produced through fermentation processes in anaerobic digesters... 9 Table 2.4: Substrates used by methane-forming bacteria (Gerardi, 23) Table 2.5: Species of methane-forming bacteria and their substrates (Gerardi, 23) Table 2.6: ph and oxygen demands of micro-organisms involved in anaerobic digestion Table 2.7: C/N ratio of some organic materials (Kourik, 1986) Table 2.8: Operating conditions for anaerobic digestion (Engler et al., 1999) Table 2.9: Biogas yield and methane content of different organic compounds (Baserga, 1998) Table 2.1: Biogas Yield on Weight Basis (Nijaguna, 22) Table 2.11: General features of biogas (Deublein and Steinhauser, 28) Table 2.12: Merits and limitations of co-digestion technology (Braun, 22; Wu, 27) Table 3.1: Proportion of cow and donkey dung... 3 Table 3.2: Proportion of horse and goat dung... 3 Table 3.3: Proportion of (cow +donkey dung) and (horse +goat dung)... 3 Table 4.1: Assumptions for volume and geometrical dimensions (REEIN, 212) Table 4.2: Volume calculation of digester and hydraulic chamber... 5 Table 4.3: Parameters and equation for calculating the working volume of the bath biogas digester... 5 Table 5.1: Biogas yield of cow dung before insulation of the digester Table 5.2: Main characterization parameters for cow dung Table 5.3: Biogas composition for cow dung after biogas digester insulation Table 5.4: Substrate characteristics for donkey dung Table 5.5: Biogas composition for donkey dung Table 5.6: Biogas composition of different mixing ratios of cow dung and donkey dung Table 5.7: Substrate characteristics of goat and horse dung... 9 Table 5.8: Composition of biogas in different mixing ratios for goat dung and horse dung Table 5.9: Substrate characteristics for cow dung, donkey dung, goat dung and horse dung Table 5.1: Composition of biogas for different substrates with different mixing ratios Table 5.11: Modelled K values for the biogas digester fed with co-substrates Table 6.1: Model parameters for donkey dung Table 6.2: Multiple regression model and analysis for 5% cow dung and 5% donkey dung Table 6.3: Multiple regression model and analysis for 75% goat dung and 25% horse dung Table 6.4: Multiple regression model for 25% cow, 25% donkey, 25% horse and 25% goat dung xv

17 LIST OF APPENDICES APPENDIX A..16 A. 1 RELATIONSHIP BETWEEN BIOGAS YIELD AND AMMONIUM NITROGEN (NH4-N) FOR COW DUNG AND DONKEY DUNG Figure A. 1 Relationship between biogas yield and NH4-N for cow dung Figure A. 2 Relationship between biogas yield and NH4-N for donkey dung Figure A. 3 The biogas yield and NH4-N for 25% cow dung and 75% donkey dung Figure A. 4 The biogas yield and NH4-N for 5% cow dung and 5% donkey dung A. 2 RELATIONSHIP BETWEEN BIOGAS YIELD AND AMMONIUM NITROGEN FOR GOAT DUNG AND HORSE DUNG 163 Figure A. 6 Relationship between biogas yield and NH4-N for goat dung.163 Figure A. 7 Relationship between biogas yield and NH4-N for horse dung Figure A. 8 The biogas yield and NH4-N for 25% goat dung and 75% horse dung..164 Figure A. 9 The biogas yield and NH4-N for 5% goat dung and 5% horse dung Figure A. 1 The biogas yield and NH4-N for 75% goat dung and 25% horse dung 165 A. 3 RELATIONSHIP BETWEEN BIOGAS YIELD AND AMMONIUM NITROGEN FOR CO- DIGESTION OF COW DUNG, DONKEY DUNG, GOAT DUNG AND HORSE DUNG 166 Figure A. 11 The biogas yield and NH4-N for 12.5% cow and 12.5% donkey dung co-digested with 37.5% goat and 37.5% horse dung Figure A. 12 The biogas yield and NH4-N for 25% cow and 25% donkey dung co-digested with 25% goat and 25% horse dung 166 Figure A. 13 The biogas yield and NH4-N for 37.5% cow and 37.5% donkey dung co-digested with 12.5% and 12.5% horse dung.167 APPENDIX B B. 1 RELATIONSHIP BETWEEN BIOGAS YIELD AND TOTAL ALKALINITY [TA] FOR COW DUNG AND DONKEY DUNG..169 Figure B. 1 Relationship between biogas yield and total alkalinity for cow dung..169 Figure B. 2 Relationship between biogas yield and total alkalinity for donkey dung.169 Figure B. 3 The biogas yield and total alkalinity for 25% cow and 75% donkey dung..17 Figure B. 4 The biogas yield and total alkalinity for 5% cow and 5% donkey dung..17 Figure B. 5 The biogas yield and total alkalinity for 75% cow and 25% donkey dung B. 2 RELATIONSHIP BETWEEN BIOGAS YIELD AND TOTAL ALKALINITY [TA] FOR GOAT DUNG AND HORSE DUNG..171 Figure B. 6 Relationship between biogas yield and total alkalinity for goat dung..171 Figure B. 7 Relationship between biogas yield and total alkalinity for horse dung 172 Figure B. 8 The biogas yield and total alkalinity for 25% goat dung and 75% horse dung 172 Figure B. 9 The biogas yield and total alkalinity for 5% goat dung and 5% horse dung 173 Figure B. 1 The biogas yield and total alkalinity for 75% goat dung and 25% horse dung..173 xvi

18 B. 3 RELATIONSHIP BETWEEN BIOGAS YIELD AND TOTAL ALKALINITY [TA] FOR CO- DIGESTION OF COW DUNG, DONKEY DUNG, GOAT DUNG AND HORSE DUNG..174 Figure B. 11 Relationship between biogas yield and total alkalinity for 12.5% cow dung and 12.5% donkey dung co-digested with 37.5% goat dung and 37.5% horse dung Figure B. 12 Relationship between biogas yield and total alkalinity for 25% cow dung and 25% donkey dung co-digested with 25% goat dung and 25% horse dung.174 Figure B. 13 Relationship between biogas yield and total alkalinity for 37.5% cow dung and 37.5% donkey dung co-digested with 12.5% goat dung and 12.5% horse dung APPENDIX C..176 APPENDIX C. 1: Journal Publications APPENDIX C. 2: Conference Proceedings.176 APPENDIX C. 3: Conference Presentations APPENDIX C. 4: Summary of information 177 xvii

19 NOMENCLATURE SYMBOL AD ALPHA C/N COD CV FVW FW GTZ HRT LR MLR MOS MSW NDIR NH 4 -N OFMSW Pd/Ni SAIRP T A T D TS VFA VS NOMENCLATURE Anaerobic digestion American Public Health Association Carbon nitrogen ratio Chemical oxygen demand Calorific value Fruit and vegetable waste Food waste German technical co-operation Hydraulic retention time Loading rate Multiple regression model Metal crude semi- conductor Municipal solid waste Non dispersive infrared Ammonium nitrogen Organic fraction of municipal solid waste Palladium/Nickel South Africa Integrated Resource Plan Total alkalinity Digester temperature Total solids Volatile fatty acids Volatile solid xviii

20 CHAPTER 1 INTRODUCTION 1.1. BACKGROUND South Africa is the most industrialised country in Africa with abundant, diverse and unexploited energy resources that are yet to be used for improving the livelihood of the vast majority of the population. The country is highly dependent on convectional fuels, non- renewable sources such as coal and oil. This makes it to be amongst the largest emitters of greenhouse gases in the world (Mwakasonda, 27). Coal-fired power plants account for over 9% of the electricity generation in the country while the remainder is generated from hydro, natural gas, nuclear energy, and wind energy, and this accounts for 6% of the total electricity demand. However, these fossil fuels have many negative impact on the environment such as environmental degradation, climate change and human health problems (Elaiyaraju and Partha, 212). After the ratification of United Nations Framework Convention on Climate Change (UNFCCC) and Kyoto Protocol in August 1997 and July 22 respectively, the South African government embarked on numerous projects related to climate change, including projects that have been intended as measures to reduce greenhouse gases (Mwakasonda, 27). Furthermore, the high volatility in oil prices in the recent past resulting in turbulence in energy markets has also compelled the country to look for alternative sources of energy. As a result, the South Africa Integrated Resource Plan (SAIRP) approved by the parliament in 21 sets a target of 4% renewable energy contribution by the year 23 (SAIRP, 21). One measure taken by the country to reduce greenhouse gas emissions was anaerobic digestion by use of biogas digesters in rural areas (Mwakasonda, 27). Anaerobic digestion is the production of biogas consisting mainly methane from organic wastes in complete absence of oxygen by anaerobic microbes such as acidogens, acetogens and methanogens. Biogas results from the microbial degradation of biomass, formed through photosynthesis by solar power in accordance with equation

21 6CO2 6H2O Es C6H12O6 6O2 Where; Es = solar power [1.1] Biogas can be viewed as one of the vehicles to reduce rural poverty and could lead to rural development. The anaerobic digestion produces less greenhouse gases than waste treatment processes such as composting (Walker et al., 29) and land filling (Lou and Nair, 29). The technology is appropriate for recovery of energy as it provides renewable energy and fertilizer. The methane from biogas is a source of renewable energy producing electricity in combined heat and power plants (Clemens et al., 26). The biogas can even be used to complement coal in grid electricity. Compared to other renewable energy sources, such as solar and wind power, the methane component of biogas can be easily stored in bio-bags. Furthermore, the biogas digesters are not prone to theft unlike solar panels and wind turbines. In addition, biogas production would benefit mainly the rural population by providing a clean fuel and reduce energy poverty. According to AGAMA (28) biogas is an overlooked source of fuel in spite of the excitement surrounding the use of bio-fuels as an alternative source of energy. The use of biogas digesters can improve the lives of the people in rural areas in many ways; it reduces deforestation, reduces greenhouse emissions, and controls unpleasant odours from human or animal wastes and reduction of workload and marginalisation of women who collect firewood. Biogas technology, involves the use of digesters that are vessels in which animal waste and other bio-degradable organic materials are broken down into methane, carbon dioxide and hydrocarbons. Biogas digesters are excellent waste disposal systems for wastes such as night soil (human wastes), thereby preventing environmental contamination and spread of pathogenic diseases such as cholera and diarrhoea. The composition of biogas varies depending on the raw materials, the organic load applied, the retention period and temperature. The gas consists mainly of methane, which is generally between 55%-8% (Jemmett, 26). Biogas is about 2% lighter than air and has an ignition temperature in the range of C (Deublein and Steinhauser, 28). 2

22 Numerous studies have been conducted by several researchers in order to optimize biogas yield in anaerobic digestion (Iyagba et al., 29; Li et al., 211; Misi, 21; Mukumba et al., 211; Uzodinma et al., 28). These studies established that using co-substrates in anaerobic digestion system improves the biogas yield due to positive synergisms established in the digestion medium and supply of missing nutrients by the co-substrates. Co-digestion is the simultaneous digestion of a homogenous mixture of two or more substrates (Wu, 27) RESEARCH PROBLEM STATEMENT The previous co-digestion experiments were carried out in laboratories using small containers (batch digesters). It is easier to control variables such as temperature, ph and when using these small scale laboratory digesters. Therefore it is not applicable to apply a mathematical model obtained from a laboratory batch digester to a field biogas digester because the two systems are different. In a field batch system large quantities of wastes are used as compared to laboratory systems where low quantities of wastes are used, for example, 2 ml volume of wastes were used in other studies (Misi, 21), and therefore biogas yields from these two digester systems may not be necessarily the same. In addition, it is also easier to agitate the laboratory batch digesters by shaking the containers once daily for at least five minutes. Gas collection is done easily by use of downward displacement of water acidified to ph 2.5 in a calibrated cylindrical gas holder. The previous co-digestion studies have limitations because no experiments have been carried out using stable field biogas digesters to find the optimum mixing ratios for cow dung, goat dung, donkey dung and horse dung. Lastly, this current study would be the first co-digestion study to operate with a large digester insulated with sawdust in an outdoor setting operating at an average temperature of 3 C. Therefore, this has necessitated the need for the researcher to carry out a study to get reliable data on the performance of field scale batch digesters when fed with cosubstrates because laboratory scale dynamics may be different from field scale dynamics. In addition, the current literature survey revealed that co-digestion of donkey dung with other organic wastes in anaerobic digestion has not been carried out. Hence, the need to investigate the impact of co-digestion of donkey dung with other substrates is crucial. This research is also aimed at developing mathematical models from co-substrates with maximum methane yields. 3

23 The field digester mathematical models are crucial for Eastern Cape Province of South Africa with many biogas substrates mainly from cattle, donkeys, and goats JUSTIFICATION OF THE RESEARCH A number of factors necessitated the need for this research. These included production of clean renewable energy in South Africa, failure of many installed biogas digesters to operate (Mukumba et al., 212), availability of biogas substrates, low biogas yield from existing digesters in the country, lack of knowledge on co-digestion and absence of data on methane quality from field-scale digesters. Therefore, there is no available data on the operation of biogas digesters fed with feedstocks used in this study. The information available in literature is mainly for laboratory scale digesters for which the operating conditions can be easily controlled as opposed to the natural conditions that could prove difficult to control. This researcher designed and constructed a field batch biogas digester and optimised its performance and came up with optimum mixing ratio(s) for maximum methane yield when fed with co-digested substrates. The optimum mixing ratios from the co-mixtures were used to develop mathematical models for optimum methane production. The co-substrates used in the research were cow dung, donkey dung, horse dung and goat dung AIM AND OBJECTIVES The aim of the research was to model the methane yield for optimised mixing ratios in a biogas digester. The research was carried out with the following objectives in mind: i. To design and construct a batch biogas digester. ii. To determine the effect of insulation of the digester on biogas yield. iii. To determine the impact of substrate properties on methane production. iv. To determine the optimum mixing ratios for the methane yield. v. To develop a mathematical model for predicting the methane yield given the substrate properties and digester operating parameters. 4

24 1.5. RESEARCH QUESTIONS The nucleus of this research problem under study can be best articulated by stating the following research questions: i. What are the effects of insulating a batch biogas digester? ii. Does co-digestion improve methane yield? iii. Which mixing ratio of the co-substrates produce the highest methane yield? iv. What are the major factors affecting methane yield? v. In biogas technology, what are the similarities and differences between laboratory and field scale dynamics? 1.6. ASSUMPTIONS The study is based on the following assumptions: A well insulated (stabilised temperature) biogas digester produces a higher yield of biogas. Temperature plays an important role in the bio-methanation process THESIS SYNOPSIS The organisation of the thesis is divided into seven chapters as follows: Chapter 1: This chapter provides the background to the problem, motivation for the research, the research problem statement and aims and objectives of the study, assumptions and thesis synopsis. Chapter 2: This chapter is a synthesis of existing literature on the anaerobic digestion process, as well as an overview of co-digestion. Chapter 3: This chapter presents the methodology employed in carrying out this research. Chapter 4: This chapter deals with the design and construction of a batch biogas digester. Chapter 5: This chapter presents the results of the research. The results are presented and discussed with reference to existing literature. 5

25 Chapter 6: The mathematical model developed for the co-digestion experiments is presented in this chapter. Chapter 7: This chapter gives the conclusions drawn from the co-digestion experiments, design and construction of the biogas digester as well as some perspectives for further investigations. 6

26 CHAPTER 2 LITERATURE REVIEW 2.1. INTRODUCTION This chapter is a synthesis of existing information on biogas digesters. It presents information on the digestion process and the factors affecting biogas production. The various biogas digester types are also presented. Other aspects elaborated in this chapter include feeding methods, uses of biogas, and biogas composition. Anaerobic co-digestion is also presented. Some merits and demerits of anaerobic co-digestion are highlighted as well BACKGROUND TO ANAEROBIC DIGESTION Anaerobic digestion (AD) is a biological process that naturally occurs when bacteria decompose organic matter producing mainly methane (CH 4 ) and carbon dioxide (CO 2 ) in an oxygen-free environment (Arsova, 21). The anaerobic digestion process is divided into four steps as follows: hydrolysis, fermentation (acidogenesis), anaerobic oxidation (acetogenesis) and methanisation (Davidsson, 27; Leksell, 25; Polprasert, 27). Figure 2.1 shows the degradation pathways to produce methane Hydrolysis Hydrolysis is an enzyme mediated stage where insoluble organic compounds such as proteins, fats, lipids and carbohydrates are converted into soluble organic components such as amino acids, fatty acids, monosaccharides, and other simple organic compounds (Yadvika et al., 24). The process is represented by equation 2.1. (C6H1O5)n nh2o nc6h12o6 2.1 Enzymes in the hydrolytic process include cellulase and amylase for degrading carbohydrates into simple sugars (monosaccharides), protease for degrading protein into amino acids and lipase for breaking lipids into fatty acids and glycerol. Hydrolysis is relatively slow and can be limited by dissolution of waste containing large particles with a low surface to volume ratio, for instance lignin (Borggren, 27). 7

27 1. STEP Hydrolysis macromolecule (Hydrolytic phase) 2. STEP Acidification (Acidogene phase) 3. STEP Acetic acid formed (Acetogene phase) 4. STEP Methane formation (Methanogene phase) ph: 5-6 ph: ph: Hydrogen Carbon dioxide Biomass Carbohydrates Proteins Fats Sugars Amino acids Fatty acids Biogas Methane Carbon dioxide Fatty acids (Propionic acid) Alcohols Acetic acid Hydrogen Carbon dioxide Figure 2.1: Four steps of anaerobic digestion (adapted from Jarvis, 24) The overall hydrolysis rate depends on organic material size, shape, surface area, biomass concentration, enzyme production and adsorption (Parawira et al., 24; Boe, 26). Table 2.1 summarizes enzymes utilised during the hydrolysis step of anaerobic digestion. In addition, it gives specific examples of the bacterial genera. Table 2.1: Enzymes utilised during the hydrolysis step of anaerobic digestion (Gerardi, 23). Substrate degraded Exo- Enzyme Example Monomers Polysaccharides Saccharolytic Cellulase Simple sugar Proteins Proteolytic Protease Amino acids Lipids Lipolytic Lipase Fatty acids The degradation of monosaccharides, for example glucose, can manifest in different pathways which lead to the emergence of different products. Table 2.2 shows examples of different products from glucose degradation. The important acid at this stage is acetic acid as it is the 8

28 principal organic acid used as a material by methane forming organisms. At ph greater than 5, the production of volatile fatty acids is increased, whereas at lower ph less than 5, more ethanol is produced and at lower ph less than 4, all processes may cease (Schön, 29). Table 2.2: Examples of products from glucose degradation (Batstone et al., 22) Products Reaction Acetate C6H12O6 2H2O 2CH 3COOH 2CO 2 4H2 Propionate 3C6H12O6 4CH3CH2COOH 2CH3COOH 2CO 2 2H2O Butyrate C6H12O6 CH3CH2CH2COOH 2CO 2 2H2 Lactate C6H12O6 2CH3CHOHCOOH Ethanol C6H12O6 2CH3CH2OH 2CO 2 Table 2.3 shows the major acids produced through fermentation process in anaerobic digestion. Table 2.3: Major acids produced through fermentation processes in anaerobic digesters Name Formula Acetate CH 3 COOH Butanol CH 3 (CH 2 ) 2 CH 2 OH Butyrate CH 3 (CH 2 ) 2 CH 2 COOH 2 Caproic acid CH 3 (CH 2 ) 4 COOH Formate HCOOH Ethanol CH 3 CH 2 OH Lactate CH 3 CHOHCOOH Methanol CH 3 OH Propanol CH 3 CH 2 CH 2 OH Propionate CH 3 CH 3 COOH Succinate HOOCCH 2 CH 2 COOH 9

29 Acetogenesis The third stage of acetic acid formation (acetogenesis) combines the prior acidification with methane formation. The starting substrates are a number of final products from the acidification phase. Examples include, chain fatty acids, propionic acid, polymer substrates (carbohydrates, fats, proteins) and butyric acid. Together with lactic acid, alcohols and glycerol, these substrates are converted by the acetogenic micro-organisms into acetic acid, hydrogen and carbon dioxide (Wiese et al., 29). This process is presented by equations 2.2 to 2.4. CH3CH2COOH 2H2O CH3COOH CO2 3H2 CH3CH2CH2COOH 2H2O 2CH 3COOH 2H2 2CO2 4H2 CH3COOH 2H2O Methanogenesis Methanogens convert the acetate and hydrogen to methane and carbon dioxide. Methanogenic bacteria are divided into three categories. Hydrogenotrophic methanogens use hydrogen to convert carbon dioxide to methane according to equation H2 CO2 CH4 2H2O [2.5] Acetotrophic methanogens split acetate into methane and carbon dioxide. Finally, methylotrophic methanogens produce methane directly from methyl groups, such as methanol, and mono-, di-, and trimethylamines (Gerardi, 23). The largest contributors to methane production in anaerobic digestion are acetotrophic (acetate splitting) methanogens, which generate approximately 7% of methane produced in digesters. In most digesters, less than 3% of the methane originates from reduction of carbon dioxide by hydrogenotrophic methanogens, while the remaining 1-2% of the methane is produced by methylotrophic methanogens (Gerardi, 23). 1

30 Methanogens have a narrow substrate spectrum and are sensitive to the presence of oxygen (Dhaked et al., 21). The reaction that takes place in the process of methane production is called methanisation and is expressed by equations 2.6 to 2.8. CH3COOH CH4 CO2 [2.6] 2CH 3CH2OH CO2 CH4 2CH 3COOH [2.7] CO2 4H2 CH4 H2O [2.8 ] The equations (2.6) to (2.8) show that many products, by-products and intermediate products are produced in the process of digestion of inputs in an anaerobic condition before final product methane is produced. Table 2.4 is a summary of substrates used by methane-forming bacteria, while Table 2.5 shows species of methane-forming bacteria and their substrates. Table 2.4: Substrates used by methane-forming bacteria (Gerardi, 23). Substrate Chemical formula Acetate CH 3 COOH Carbon dioxide CO 2 Carbon monoxide CO Formate HCOOH Hydrogen H 2 Methanol CH 3 OH Methylamine CH 3 NH 2 11

31 Table 2.5: Species of methane-forming bacteria and their substrates (Gerardi, 23). Species Substrate Methanobacterium formicium Carbon dioxide, formate, hydrogen Methanobacterium thermoantotrophicum Hydrogen, carbon dioxide, carbon monoxide Methanococcus frisius Hydrogen, methanol, methylamine Methanococcus mazei Acetate, methanol, methylamine Methanosarcina bakerii Acetate, carbon dioxide, hydrogen, methanol, methylamine 2.3. FACTORS AFFECTING BIOGAS PRODUCTION Parameters within an anaerobic digester that play a key role in the physical environment and efficiency of digestion and biogas production potential include: ph-value, temperature, concentration of solids, hydraulic retention time (HRT), redox potential, volatile solids (VS) and loading rate (LR), inocula, carbon/ nitrogen (C/N) ratio, toxicity, ammonium (NH 4 ), particle size, water content, agitation frequency and volatile fatty acids ph-value The ph value describes whether a substance is chemically acidic, neutral or alkaline. It depends on the amount of free hydroxonium ions per unit volume. Different groups of micro-organisms involved in anaerobic digestion require different ph. The optimum ph for hydrolysis and acidogenesis is between 5.5 and 6.5 (Arshad et al., 211). The ideal ph for methanogens is between 6.8 and 7.6 (Mosey and Fernandes 1989). When the ph is less than 6.1 or greater than 8.3 the biogas yield is decreased (Lay et al., 1997). The ph value is directly related to the input substrates. If the substrate is rich in carbohydrates, a rapid acidification process takes place resulting in a drop in the ph. If the substrate contains more proteins, ammonium forms, resulting in the rise of ph (Gomez et al., 26). Table 2.6, summarizes the ph and oxygen demands of micro-organisms involved in anaerobic digestion. 12

32 Table 2.6: ph and oxygen demands of micro-organisms involved in anaerobic digestion (Deublein and Steinhauser, 28). Process step ph-value Oxygen Tolerance Optima Hydrolysis Tolerant to low oxygen level, in parts: facultative aerobic and strictly anaerobic Acidogenesis Strictly anaerobic Acetogenesis Strictly anaerobic Methanogenesis Strictly anaerobic If ph drops, substrate supply in the biogas digester must be stopped since methane production is low. The common materials used to increase alkalinity are lime, soda ash, ammonia, bicarbonate, sodium hydroxide, or sodium bicarbonate (Deublein and Steinhauser, 28). Alkalinity defines the acid binding capacity of the digestion process. The degree of alkalinity depends on the amount of available alkaline acting ions, mainly carbonates. In biogas systems, total alkalinity is strongly correlated with the carbonate/ bicarbonate, ammonia and phosphate. A high alkalinity is needed to have a stable process (Borggren, 27). Bischofsberger and Keller (25) stated that a drop in ph to 6.4, at an acetic acid concentration of 1, mg/l, reduces the methane concentration by half. The resistance to ph change in digester depends on its buffering capacity (Lin et al., 211). Simultaneous presence of ammonia and bicarbonate in the digester results in the formation of another buffer system (Cecchi et al., 23) Temperature In biogas production temperature of the slurry is an important parameter that affects the rate of biogas production. In nature, methane is formed over a wide temperature range of to 97 C (Dhaked et al., 21). An increase in ambient temperature generally increases the rate of reaction and therefore the rate of biogas production. All the metabolic processes in bacteria are brought about by enzymes. These microbes have the temperature range within which they can strive. When the temperature is not favourable, the 13

33 enzymes may be denatured hence hampering their digestion process. In this regard, bacteria are classified according to their preferred temperature. Psychrophilic bacteria work best between 1 and 2 C, mesophilic bacteria between 2 and 35 C and thermophilic bacteria between 45 and 6 C (Davidsson, 27). Anaerobic digestion is very efficient in the thermophilic range, but rural type digesters without external heating use mesophilic bacteria, as temperatures higher than 35 C are very hard to obtain. For mesophilic bacteria the optimum digestion occurs at about 35 C whereas for the thermophilic bacteria the optimum is 55 C (Marchaim, 1992). Although at mesophilic and thermophilic temperatures the process is well understood, current knowledge on psychrophilic biomethanation is scarce (Dhaked et al., 21). According to Balasubramaniyam et al. (28) anaerobic digestion at psychrophilic temperatures has not been extensively explored as either mesophilic or thermophilic digestion, probably due to little anticipation of the development of economically attractive systems using this technology Hydraulic retention time (HRT) The amount of gas produced depends on the volume of slurry in the biogas digester, being normally about two thirds of digester volume (Fulford, 21). The digester volume is also related to the retention time measured in days and the loading rate, in terms of manure fed per unit liquid volume (Thy et al., 23). The average time spent by the biomass inside a biogas digester before it comes out from the digester is known as the hydraulic retention time (HRT). The process of degradation of biomass requires at least 1-3 days in mesophilic condition while in thermophilic environment the HRT is usually less than ten days (Demetriades, 28). However, shortening the retention time can lead to increase in the volatile fatty acids (VFA) (Kobayashi and Li, 211) Redox potential The redox potential of a digester is a measure of the oxidisability or reducibility of its content. Biogas production only proceeds efficiently in an anaerobic environment. The redox potential must be less than 33 mv (Wiese et al., 29). 14

34 Inocula Biogas production is not possible without a sufficient quantity of biogas microbes. These are often low in number in fresh material. Taking some of the effluent (1 to 3 % of daily input) and putting it back into the digester is a way of inoculating the fresh manure with the active microbial flora. This inoculation of fresh manure can increase gas production by up to 3%. Budiyono et al. (21), used rumen fluid of ruminant animal as an inoculum to increase the biogas production rate from cattle dung at mesophilic condition. The results showed that the rumen fluid inoculated to the digester significantly affected the biogas production. The inoculum caused the biogas production rate and efficiency to increase twice more compared to manure substrate without rumen fluid inoculums (Budiyono et al., 21). However, still to date, data concerning the study of the effect of inoculums content to biogas production rate are very limited. It is often advantageous to seed the biogas digester with active inocula that are preferably low temperature adapted (Dhaked et al., 21) Carbon/nitrogen (C/N) ratio The carbon/nitrogen (C/N) ratio is a measure of the relative amounts of organic carbon and nitrogen present in the substrate. The C/N ratio of waste is determined by its composition. If the C/N ratio of slurry is very high, the nitrogen will be consumed rapidly by methanogens for meeting their protein requirements and will no longer react on the remaining carbon content of the material. Therefore, the waste used as a single substrate will be deficient in nitrogen, which is needed for building up of bacterial communities. As a result, biogas production will be low (Adelekan and Bamgboye, 29). A C/N ratio of 2-3:1 is considered to be optimum for anaerobic digester based on biodegradable organic carbon (Mshandete et al., 24; Yen and Brune, 27). Animal waste such as cattle dung has an average C/N ratio of 24:1. To maintain the C/N level of a material at acceptable levels, materials with a high C/N ratio can be mixed with those with a low C/N ratio. C/N ratios of some commonly used material are presented in Table

35 Table 2.7: C/N ratio of some organic materials (Kourik, 1986). Sample Raw Materials C/N ratio 1 Human excreta 8 2 Chicken manure 1 3 Goat manure 12 4 Pig manure 18 5 Sheep manure 19 6 Cow dung 24 7 Straw (maize) 6 8 Straw(rice) Ammonium (NH 4 ) Microorganisms need some ammonia to form cellular protoplasm for growth and reproduction (Lin et al., 211). Ammonia forms ammonium ions in the substrate and the extent of this depends on the ph value. However, ammonia has an inhibitory effect at concentrations greater than 15 mg/l. The ammonium leads to potassium loss of methanogenic microorganisms and can show reciprocal effects with calcium and sodium ions. In addition, the inhibition by ammonium increases with rising ph value (Deublein and Steinhauser, 28) Particle size The production of biogas is also affected by particle size of the substrate. Too big particle size is problematic for microbes to digest and it can also result in a blockage in the digester. Small particle size gives a large surface area for substrate adsorption and thus allows the increased microbial activity followed by an increase in the production of gas (Yadvika et al., 24) Water content Water is the vital nutrient for micro-organisms life and their activity. The movement of bacteria and activity of extra cellular enzyme are highly determined by the water content in the digester (Nijaguna, 22). Optimum moisture content has to be maintained in the digester and the water content should be kept in the range of 6-95 % (Demetriades, 28). However, the optimum 16

36 water content differs with different input materials, chemical characteristics and bio-degradation rate (Nijaguna, 22) Agitation / Mixing The close contact between micro-organisms and the substrate material is important for an efficient digestion process. This can be achieved in a number of ways. For example, daily feeding of the substrate instead of long interval provides the desired mixing effect. Agitation is critical to the successful operation of an anaerobic process. It provides the following advantages (Schön, 29); Close contact between the raw and digesting sludge and between the micro-organisms and their substrates. Maintenance of a uniform temperature and solids mixture throughout the digester, and prevention of localised accumulation of inhibitory dense substrates. Prevention of scum formation and settlement of dense solids. Encouragement of release of biogas from the sludge in the lower regions of the digester. Poor agitation will lead to stratification within the digester and partially digested sludge being withdrawn (Gray, 24; Eder and Schulz, 26). Installation of certain mixing devices such as propeller, scraper, or piston is also a mechanism for stirring (Yadvika et al., 24). Stirring enhances methane production and reduces the decomposition of solid inside the digesters significantly in thicker manure digestion (Karim et al., 25). Agitation result in higher reaction rates (Cubas et al., 211). The best agitation frequency varies depending on the intended use of the reactor in question (Kobayashi and Li, 211) Organic loading rate The rate at which substrate is supplied to the digester is referred to as organic loading rate. It is usually expressed in terms of kg volatile solids per m 3 per day. The gas production rate in the digester is highly dependent on the organic loading rate (Yadvika et al., 24). 17

37 Volatile fatty acids (VFA) Volatile fatty acid accumulation can lead to drop in ph, which inhibits the activity of the microorganisms. A continual drop in ph can result in biogas digester failure. VFA are needed in small amounts as part of an intermediary step for the metabolic pathway of methane production by the methanogens (Carucci et al., 25). The conclusion was supported by El-Mashad and Zhang, 21) who stated that the VFA is one of the three primary buffer-agents needed for maintaining the ph value, and hence the ammonia concentration, within the desired range. The operating parameters of a typical biogas digester are listed in Table 2.8. Table 2.8: Operating conditions for anaerobic digestion (Engler et al., 1999) Operating parameter Typical value Mesophilic temperature 35 C Thermophilic temperatures 54 C ph 7 8 Alkalinity 2 5 mg/litre (minimum) Retention time 1 3 days Biogas yield.19.5 m 3 /kg VS 2.4. BIOGAS DIGESTER TYPES The most common types of biogas digesters are fixed dome digester, balloon type digester and floating drum digester. The two most familiar types in developing countries are fixed dome and floating drum digesters. The three main digesters are discussed in this section. Their advantages and disadvantages are also presented in this section Fixed dome digester The fixed dome digester is the most popular digester; its archetype was developed in China as early as It is a closed dome shape digester with an immovable, rigid gas-holder and a displacement pit (compensating tank). The biogas produced by methanogenic bacteria in the biogas digester is captured in the gas holder and the slurry is displaced in the compensating tank. When gas is consumed slurry enters back into the digester from the overflow tank. As a result of these movements, a certain degree of mixing is obtained. The more the gas is produced, the 18

38 higher the level at the slurry outlet (Fulford, 21). The fixed dome digester is shown in Figure 2. Biogas collection Slurry inlet pit Removal cover SLURRY Outflow Compensation tank Figure 2.2: Chinese fixed dome digester (Florentino, 23) The fixed dome digester has some advantages which include: relatively cheap and durable, no moving parts and well insulated (Nijaguna, 22). However, the fixed dome digester has disadvantages which include: high technical skills are required for a gas tight construction, special sealant is required for the gasholder, difficult to construct in high water table areas, requires more excavation work and enormous structural strength is required for construction (Nijaguna, 22, Sharma and Giuseppe, 1991) Bag digester (balloon digesters) A balloon digester (bag digester) is a plastic or rubber bag combining the gas holder and digester. This is a plug-flow type reactor. This design was developed in 196s in Taiwan. Gas is collected in the upper part and manure in the lower part. The inlet and outlet are attached to the skin of the bag. The pressure of the biogas is adjustable by laying stones on the bag (FAO, 1996). The advantages of the bag digesters include: low cost, simple technology and easy to clean. However, the disadvantages include: short lifespan, susceptible to physical damage, hard to 19

39 repair, need high quality plastic and difficult to insulate (APCAEM, 27). Figure 2.3, shows a balloon digester. The biogas is collected in the balloon. SLURRY Biogas pipe SLUDGE BIOGAS DIGESTER Figure 2.3: Balloon digester (Adapted from Fulford, 21) Floating drum digesters Floating drum digesters are common in India. The digesters have a moving floating gas-holder, or drum. The gas holder floats either directly in the fermenting slurry or in a separate water jacket. The drum in which the biogas collects has an internal or external guide frame that provides stability and keeps the drum upright. When the biogas is produced the drum moves up, and when the gas is consumed, the gas holder sinks back. The floating drum digesters have advantages which include: the operation of the plant is easy to understand, the gas drum is air tight and there is constant gas pressure as a result of weight of the drum (ISAT and GTZ, 1999). However, it does also have disadvantages which are; steel drum is relatively expensive and needs regular maintenance (priming, painting, and coating) and the effect of low temperature during winter is high (Nijaguna, 22). A floating drum digester is shown in Figure

40 Gas to combustion chamber Slurry Sludge BIOGAS DIGESTER Figure 2.4: Indian-type digester (Adapted from Florentino, 23) 2.5. FEEDING METHODS There are two main feeding methods which are batch and continuous feeding. Each feeding method has its advantages and disadvantages. Batch digesters are filled completely and then emptied completely after a fixed retention time. The disadvantage is that their gas production is not steady with time (Balasubramaniyam et al., 28). Continuous digesters are filled and emptied regularly, normally on daily basis. The substrate to be fed into the digester must be fluid and homogenous. These plants are ideal for rural households. Gas production is constant, and higher than in batch digesters ADVANTAGES OF BIOGAS DIGESTERS Under the right conditions a biogas plant will yield several benefits for the end-users. The main benefits include the following: production of energy for lighting, heat, electricity, improved sanitation (reduction of pathogens, worm eggs and flies), reduction of workload (less firewood collecting), environmental benefits (fertilizers substitution, less greenhouse gas emission) and improved indoor air quality (less smoke and harmful particle emission of a biogas stove compared to wood or dung fuels) (Buysman, 29; GTZ, 27). The advantages of biogas technology are summarised in Figure

41 BIOMETHANIZATION Agricultural waste system Agric-industrial waste system Municipal waste system Energy generation Fertilizer production Hygiene Environmental protection Energy generation Environmental protection Energy generation Hygiene Figure 2.4: Importance of biogas technology (adapted from APCAEM, 27) 2.7. BIOGAS COMPOSITION Gas composition means the content of different components the biogas consists of. The main compounds are methane and carbon dioxide. Gas composition is influenced by the substrates utilised as shown in Table 2.9 and Table 2.1 shows a summary of various aspects related to the feed materials and products of a biogas digester. Table 2.9: Biogas yield and methane content of different organic compounds (Baserga, 1998) Compound Biogas (m 3 /kg of CH 4 Content (%) CO 2 Content (%) DM) Protein Fat Carbohydrates

42 Table 2.1: Biogas Yield on Weight Basis (Nijaguna, 22) Raw material Biogas yield (m 3 /kg of dry matter) Cow dung.34 Chicken manure.31 Swine manure 1.2 Sheep manure Algae.32 Night soil.38 From Table 2.1 it can be observed that swine manure has the best biogas output in terms of volume. Cow dung has a lower biogas yield than sheep manure. Table 2.11 shows the general properties of the biogas. Table 2.11: General features of biogas (Deublein and Steinhauser, 28) Parameter Properties Biogas composition 55 7% methane (CH 4), 3 45% carbon dioxide (CO 2 ) and traces of other gases Energy content kwh m 3 Fuel equivalent.6.65 L oil/m 3 biogas Ignition temperature C Critical pressure ( ) 1 6 Pa Critical temperature 82.5 C Normal density 1.2 kg m 3 Smell Bad eggs 2.8. ANAEROBIC CO-DIGESTION The co-digestion of organic wastes involves mixing of various substrates in varying proportions (Misi, 21). Co-digestion is required to promote ammonia removal during digestion so as to optimise biogas production. 23

43 The mixing of several wastes has positive effects on anaerobic digestion process because it can improve the methane yield, can improve stability and can achieve a better handling of waste (Mata-Alvarez, 23). In addition, such a system is economically more favourable as it combines different streams in one common facility. In the case of the co-digestion of food waste and toilet waste, the low carbon: nitrogen (C/N) ratio and biodegradability content of the toilet waste are compensated by the high values characterising those two parameters for the food waste. Thus, the major problem of ammonia toxicity due to low C/N ratio is avoided and the low biogas yield due to small content of biodegradable matter is increased. It was reported that the combination of whey and poultry manure is capable of maintaining proper carbon: nitrogen (C/N) ratio in the reactor (Desai et al., 1994). A highly buffered system is obtained by co-digestion of solid slaughter house waste, manure, fruit and vegetable waste (Murto et al., 24). Biogas yields of.8-1 m 3 kg -1 volatile solid (VS), were obtained. Anaerobic co-digestion of grass silage, sugar beet tops and oat straw with cow manure was evaluated (Lehtomaki et al., 27). The experiments were carried out in continuously fed laboratory stirred tank reactors. There was high methane yield in a reactor with co-digested substrates as compared to reactors fed with cow dung alone. A series of laboratory experiments were examined in continuously stirred tank reactors at mesophilic conditions, fed continuously with various mixtures of diluted poultry and whey (Gelegenis et al., 27). Biogas yield increased due to co-digestion of substrates. The co-digestion of sugar beet leaves and potatoes showed that the lowest ammonia- nitrogen concentration corresponded to the highest methane production (Parawira et al., 24). Other researchers showed that lower ammonia-nitrogen concentration leads to higher methane production (Misi and Forster, 21). However, their research did not look at optimum mixing ratios for cow dung, donkey dung, goat dung and horse dung. In addition, they carried their experiments in laboratories using laboratory batch reactors. Laboratory experiments were also carried out on co-digestion of cow dung and pig manure at Makerere University (Muyiiya and Kasisira, 29). In the experiments, one and half litre plastic 24

44 bottles were used as reactors. They revealed that 1% pig manure produced a higher mean gas yield of 56.9 (mmh 2 O)/day, than 1% cow dung that produced a mean gas yield of 26.8 (mmh 2 O)/day. The highest mean gas yield of (mmh 2 O)/day was obtained from codigestion of 5% pig manure and 5% cow dung. The higher biogas yield from pig manure was attributed by the presence of native micro flora in the pig manure (Yeole and Ranade, 1992). Conversely, a higher biogas yield is attributed to the low carbon- nitrogen ratio in pig manure (Fulford, 21). Laboratory experiments were also carried out at the University of Birmingham, United Kingdom, using batch co-digestion trials to derive optimum mixtures for co-digestion of poultry manure with molasses, with sheep and goat manure and with thickened waste activated sludge (Misi, 21). Methane yields generally increased with the increased substitution of waste activated sludge with poultry manure. Similarly, it was concluded that using a 15% poultry manure and 85% cow dung enhanced biogas production (Callaghan, 1998). The effects of the mixture ratio on anaerobic co-digestion with fruit and vegetable waste (FVW) and food waste (FW) were investigated by Lin et al., (211). When the influent FVW to FW ratio was 1:1, the highest methane content in biogas reached to 63.8%, and when only FW was fed, the highest methane content of 53.7% was achieved. Therefore, the optimal mix ratio for codigestion of FVW with FW was found to be 1:1. It was concluded that co-digestion improves stability of anaerobic process, and achieves a higher biogas production and organic matter removal efficiency (Lin et al., 211). Co-digestion of cow dung, pig and chicken manures with water hyacinth cow dung mixture in a cone closed gas collector was investigated (Ntengwe et al., 21). The highest biogas yields of 3.8 m 3 /ton-3.88 m 3 /ton were obtained from pig and chicken substrates at a ph range of than cow dung and water hyacinth-cow dung mixture that had biogas yields of.92 m 3 /ton-.64 m 3 /ton at average temperatures of 2-27 C within a HRT of 15 days. However, these codigestion experiments were laboratory based. 25

45 The effect of mixing ratio of slurry on the biogas productivity of wastes from poultry birds, pigs and cattle was investigated (Adelekan and Bamgboye, 29) The study found that poultry and piggery wastes slurries mixed in ratios 3: 1 waste: water produced more biogas than those of 2:1 and 1:1 ratios. For cattle waste, the 2:1 mixing ratio produced the most bogus. It has been established that animal manure is the best co-digestion material for high-fat content wastes due to the high alkalinity of manure, which increases digester resistance to acidification (Murto et al., 24; Gelegenis et al., 27). Additionally, manure has high ammonium levels, which are important in bacterial growth. Co-digesting manure with materials containing 2% fat improves digestion efficiency without an increase in acidity (Mladenovska, 23). All of these previous co-digestion studies were conducted on heated, mixed lab-scale systems of less than 5 L or small, un-replicated pilot-scale systems (Jeyaseelan and Matsuo, 1995; Spajic et al., 29). Additionally, there have been co-digestion studies that have shown that the lab-scale dynamics can be quite different from field-scale applications (Davidsson et al., 28) Merits and de-merits of co-digestion Benefits of anaerobic co-digestion include adjustment of C/N ratio, improved waste management and improved sanitation. Some of the advantages and limitations of anaerobic codigestion are shown in Table Table 2.12: Merits and limitations of co-digestion technology (Braun, 22; Wu, 27) Merits Limits Improve nutrient balance and digestion Pre-treatment requirements are high Additional biogas collection Increased digester effluent Additional fertiliser Increase mixing requirements Renewable biomass Waste water treatment requirement Possible gate fees for waste treatment High utilisation degree requirement Equalization of particulate, floating, settling, Hygiene requirements acidifying through dilution by manure 26

46 2.9. CONCLUSIONS Over the past years researchers on biogas production all over the world have put a great effort in improving methane yield by evaluating all kinds of wastes in order to enhance synergies between different substrates and have shown that performance of the biogas digesters depends on the type and composition of material fed into the digester. In addition, temperature, ph, retention time, inocula type and loading rate affect biogas production. And furthermore, biogas production is also affected by the digester geometry. In South Africa there is little public awareness of the importance of biogas production as an alternative source of energy although there would be an increment in the number of anaerobic digesters in the next future years due to research work on biogas technology. Anaerobic digestion of organic wastes to produce renewable energy in form of biogas could be one of the solutions to eradicate energy poverty especially in the rural areas of South Africa. Hence, experimental studies on how to improve methane production by co-digestion are of significant importance. 27

47 CHAPTER 3 METHODOLODY 3.1. INTRODUCTION This chapter focuses on the methodology that was followed to optimise the performance of the field scale biogas digester fed with co-substrates. The flow diagram below (Figure 3.1) illustrates the sequence of the experimental procedure. COLLECTION OF SUBSTRATES Cow dung Goat dung Horse dung Donkey dung DETERMINATION OF PARAMETERS Total solids (TS) Volatile solids (VS) Chemical oxygen demand (COD) Ammonia-Nitrogen (NH 4 -N) Total alkalinity (T A ) Calorific value (CV) MEASURING INSTRUMENTS Biogas sensors Mass flow meter Pressure gauge Temp sensors ph meter DATA DISPLAY Computer DATA CAPTURING CR1 Data logger DETERMINATION OF BIOGAS COMPOSITION Methane Carbon dioxide Hydrogen Carbon monoxide Figure 3.1: Research design flow chart diagram The cow dung, donkey dung, goat dung and horse dung were collected from University of Fort Hare dairy farm. The substrates were added with water such that the total solids occupied 8% by volume of the fermentation slurry in accordance with the recommendation for better biogas production (Ituen et al., 27). The parameters determined in the substrates were, ph, chemical oxygen demand (COD), total solids (TS), volatile solids (VS), ammonia-nitrogen (NH 4 - N), total 28

48 alkalinity (T A ), temperature (T) and the caloric value (CV). All the analytical determinations were performed according to the standard methods for examination of water and wastewater (APHA, 25). The temperature of the slurry was measured using type-k thermocouples, while the digital ph meter measured the feed, digester slurry and effluent ph. The biogas composition was analysed by the biogas analyser. The data for biogas composition was recorded by a CR1 data logger. A laptop was used to download the data SUBSTRATES FOR CO-DIGESTION The horse dung, goat dung and donkey dung were obtained from the University of Fort Hare Honey Dale trust farm while cow dung was obtained from the University of Fort Hare dairy farm. The horse dung had a lot of vegetable material and was crushed mechanically by a club hammer repeatedly to ensure homogeneity. Before water was added to the substrates, total solids (TS) of each prepared sample were determined (section 3.3.1) to find out the amount of water to be added to the substrates before feeding into the batch digester. The most favourable TS value desired for better biogas production is 8% (Ituen et al., 27) Co-digestion ratios For the co-digestion experiments there were thirteen samples that were prepared. The cow dung and donkey dung were mixed in different ratios as illustrated in Table 3.1, whereas horse dung and goat dung were mixed as shown in Table 3.2. At the start of each digestion trial, collector valves of the batch digester were left open for about 24 hours after feeding to allow the expulsion of any air. 29

49 Table 3.1: Proportion of cow and donkey dung Sample % of cow dung % of donkey dung A 1 B C 5 5 D E 1 Table 3.2: Proportion of horse and goat dung Sample % of horse dung % of goat dung F 1 G H 5 5 I J 1 Table 3.3 shows the mixing ratios of cow dung and donkey dung with horse dung and goat dung. Table 3.3: Proportion of (cow +donkey dung) and (horse +goat dung) Sample % cow % donkey Total % % horse % goat Total % dung dung dung dung K L M DETERMINTION OF SUBSTRATE PROPERTIES Determination of total solids Total solids is the weight of dry material remaining after drying and is also called dry weight or is the portion of a substrate remaining after the elimination of moisture. Different samples of substrates were weighed using digital weighing scales. The weighted samples were placed in a 3

50 heat oven at a temperature of 15 C. The different samples were heated for a period of 24 hours. After heating the samples were reweighted and TS was calculated using the following standard equation (APHA, 25); TS(mg/L) W - W W - W ,, (conversion of g/ml to mg/l) [3.1] Where: W 1 = Weight of dried residue and dish (g) W 2 = Weight of dish (g) W 3 = Weight of wet sample and dish (g) Determination of volatile solids Volatile Solids (VS) is the weight of organic solids burned off when heated to about 6 C in a heat oven or heat furnace or is the portion of dry matter which remains after the elimination of the inorganic portion (raw ashes). The dried residue was weighed and heated in a crucible for two hours at 55 6 C in a furnace. The residue was cooled in a desiccator. The residue was then weighed. The ignition, cooling, desiccating and weighing steps were repeated until the weight change was 5 mg. The final weight was recorded. Volatile matter was determined using standard equation (3.2), (APHA, 25). VS (mg/l) W - W W - W ,, [3.2] Where: W 1 = weight of solids + weight of dish before ignition at 55 C (g) W 2 = weight of solids + weight of dish after ignition at 55 C (g) W 3 = weight of empty dish Determination of Chemical Oxygen Demand (COD) COD is the amount of oxygen required by one litre of sample to change it completely into carbon dioxide and water. Theoretically, 1 gram of COD destroyed gives.35 litres of biogas (Fulford, 21; Martin, 27). 31

51 For the determination of COD the following equipment were used: 25 burette graduated in.1 ml, burette support, 1 ml graduated cylinder, stirring rod, white porcelain evaporating dish, 4.5 inches in diameter, reflux flask, condenser, Erlenmeyer flask, water bath and boiling stones. The following reagents were used to determine COD; Potassium dichromate Cr 2 K 2 O 7 Silver sulphate: powdered Ag 2 SO 4 Concentrated sulphuric acid: H 2 SO 4 18 M Mohr s salt: ferrous ammonium sulphate: Fe (NH 4 ) 2 (SO 4 ) 2.6H 2 O Mercuric sulphate: powdered HgSO 4 Phenanthroline ferrous sulphate indicator solution: ferroin indicator (C 12 H 8 N 2 ) 3 FeSO 4 A 5 ml of sample was taken into a refluxing flask and several boiling stones were added and.1 g of HgSO 4 was added to the solution. A 5 ml of concentrated H 2 SO 4 was also added to the solution. To ensure that HgSO 4 dissolved completely, the solution was swirled slowly while adding sulphuric acid and.1 g of Ag 2 SO 4 was added to this solution. Finally, Cr 2 K 2 O 7 was added. The solution was mixed thoroughly by swirling the flask in a water bath to recover any volatile substances that may have escaped from the liquid state. The flask was then attached to the condenser and further cooling was done and 2 ml of sulphuric acid was added to the solution in the flask continuing cooling and swirling to mix the solution. The solution was refluxed for 1 hour. A blank run (using 5 ml distilled water instead of sample) was simultaneously conducted with the same procedure after cooling. The solution was transferred to an Erlenmeyer flask. The reflux flask was rinsed thrice, pouring the rinsing water to the Erlenmeyer flask. The solution was diluted to 3 ml and 8 drops of Phenanthroline ferrous sulphate was added to the solution as an indicator. The solution was titrated against the Mohr s salt and the titer volume required for the colour change from blue-green to reddish blue was noted. The procedure was repeated for the blank run. Equation 3.3 was used for the calculation of COD value. 32

52 COD (mg/l) Vb Vs V o N [3.3] Where: V b = volume of blank V s V o N = volume of sample = original volume of sample = normality of ferrous ammonium sulphate η = 8 η = milli equivalent weight of oxygen 1 ml/l. (molecular oxygen has a molecular weight of 32 g that is 32 mg and the titrant is.25 N so 32 times.25 is 8) Potassium dichromate acted as a strong oxidising agent and oxidised the organic and inorganic matter in the water. If chlorides were present in the sample they would interfere with the oxidation of the organic matter. To ensure non-interference of chlorides, mercury sulphate was added and formed complex of mercuric chloride. An amount of 1 g of mercury sulphate was required for 1g of chlorides to form complex. Sulphuric acid was added to the mixture so that the mercury completely dissolved. It assisted in oxidizing the nitrogen compounds in the sample and the increased heat accelerated the reaction rate. Silver sulphate catalysed the reaction and also assisted in the oxidation of nitrogen compounds. Mercury sulphate was added first in order to allow the chloride atoms to combine with mercury. If sulphate was added first, the chlorine would bind with the silver. The titer volume of the sample gave the volume of ferrous ammonium sulphate required to react with the excess potassium dichromate in the solution. Similarly, the titer volume for the blank (distilled water) gave the volume of ferrous ammonium sulphate required to react with excess potassium dichromate in the blank Determination of ph ph is a measure of the degree of the acidity or the alkalinity of a solution. The ph of the substrates was measured by a digital ph meter and is calculated as; 33

53 ph - Log H 3.4 Where: [H + ] is the concentration of the hydrogen ions. A ph meter measures the potential difference in milli-volts (mv) between the electrodes and converts it to a display of ph. Figure 3.2 shows the circuit diagram of a ph digital meter. Glass electrode R g I term + mv Amplifier R i - R 1 A/D Converter Microprocessor Reference electrode R 2 Display Figure 3.2: Simplified ph meter circuit diagram The potential difference between the reference electrode and the glass electrode is amplified in the mv amplifier before the analogue/digital (A/D) converter feeds the signal to the microprocessor for result calculation. The glass electrode typically has an inner resistance of 18Ω and the amplifier s input resistance, R i, = 112 Ω. For this reason, the amplifier may send a negligible current through the glass electrode as this will give an error potential and could even disturb the electrode. The terminal or bias current, I tem, is below 1-12 A. When R i is much greater R g, I term = 1-12 A and Rg = 18 Ω, the error introduced can be calculated according to Ohm's Law (V = R I). To attain reliable and consistent results, the amplifier should have a small temperature coefficient, i.e. the influence of temperature variations must be controlled. Figure 3.3 is a photo of a ph meter and various ph solution tubes. 34

54 Digital ph meter Figure 3.3: The digital ph meter and various ph solution tubes Determination of Ammonia-Nitrogen For the determination of ammonia-nitrogen, calgom reagent and nessler s reagent (K 2 HgI 4 ) were used as reagents. A 5 ml measuring cylinder was filled with sample to 25 ml mark and 2 drops of Calgon reagent were added. Calgon reagent was added to prevent development of turbidity. The mixture was stirred and 2 ml Nessler s reagent was also added to the mixture. Ammonia in the sample reacted with Nessler s reagent to produce a yellow-coloured complex in direct proportional to the ammonia concentration. Results were measured in mg/l of ammonianitrogen Determination of Total Alkalinity Alkalinity is a measure of the stability of the digester or is a measure of the capacity of a solution to neutralise a strong acid. It is measured in milligrams of calcium carbonate per litre of slurry. It can be calculated in waste water or slurry samples by titrating the sample water with an acid, using an indicator to determine when the sample ph has dropped to a certain level (the endpoint). For the determination of total alkalinity the following equipment was used; 25 burette graduated in.1 ml Burette support 1 ml graduated cylinder 35

55 Stirring rod White porcelain evaporating dish, 4.5 inches in diameter A 5 ml sample was measured and poured in 1 ml measuring cylinder and 2 drops of mixed indicator (Tashiro indicator) was added to the sample and.2n nitric acid was titrated to the mixture. Colour change was noted at end point, from green to deep pink. The end point is when the entire blue-green colour disappears and the final reading from the burette was recorded. The difference (final initial readings) was the volume of the acid at end point Determination of energy values (calorific values) of substrates The Calorific Values (CV) of various substrates were measured with a calorimeter before being fed into the batch biogas digester. A CAL2K bomb calorimeter was used to determine the calorific values of the substrates. A complete CAL2K system is made up of the following components: calorimeter (CAL2k-1), cooler (CAL2k-2), filling station (CAL2K-3) and vessel (CAL2k-4) The calorimeter (CAL2k-1) Figure 3.4 shows diagram of calorimeter CaL2K-ECO with all major components making up the unit. LID LOCK Pull to open LCD-Display Standard PC Keyboard Figure 3.4: The calorimeter (CAL2K-ECO) photo 36

56 The LCD display of a calorimeter consists of 2 lines as follows: Line number 1: Calorimeter operations and keyboard entry Line number 2: Sample ID and Mass display The lid closure switch is incorporated in the door latch, is for the lid to close securely before determination of the sample is started The Filling Station (CAL2K-3) Oxygen from gas bottles enters the vessel through the filling station by displaying the lever downwards. The filling station is shown in Figure 3.5 Lever Bottle Pressure Gauge Flow Adjuster Oxygen Inlet connection Nozzle Lid Holder Figure 3.5: Filling Station (CAL2K-3) 37

57 The Vessel (CAL2K-4) The sample whose calorific value is to be determined is burned inside the vessel. The complete vessel system is made up of the cap, lid, vessel, and bottom support vessel as shown in Figures 3.6. Cap Lid O -Ring Lid Outside Electrode Sleeve Firing wire Vessel Figure 3.6: Vessel CAL-2K-4 At firing the voltage on the firing capacitors is applied to the firing wire inside the vessel. When the firing wire is heated it glows and this ignites the sample. The firing contact is in the centre of the lid and is a spring contact that is in contact when the lid is closed. The vessel or combustion 38

58 chamber is made out of stainless steel, and tested up to a pressure of 3 atmospheres (42 psi). The oxygen rich environment allows the firing wire to glow and ignition of the sample to take place. The vessel can also be pressurised with compressed air, or inert gas for special applications. As the sample is pressurised and ignited, the released energy is measured as a temperature rise. The vessel CAL2K-4-Group is shown in Figure 3.7. Lid Assembly Lid O -Ring Outside Electrode Centre Electrode Top O -Ring Bottom O- Ring Valve Screw Sleeve Sleeve Figure 3.7: Vessel CAL2K-4- GROUP 3.4. BIOGAS ANALYSIS The biogas composition was analysed using the biogas analyser consisting of Non-Dispersive Infrared (NDIR) sensor for sensing methane and carbon dioxide and Palladium/Nickel (Pd/Ni) sensor for sensing hydrogen and hydrogen sulphide. The data for biogas composition was recorded by a CR1 data logger at a time interval of 2 minutes. The biogas analyser and the CR1 data logger were powdered by a 12V DC battery that was connected to a 2 W 39

59 photovoltaic module. The slurry and ambient temperatures were measured using type K thermocouples connected to the same CR1 data logger as the biogas sensors. The data logger was interfaced to a computer. Figure 3.8 is the schematic diagram of the experimental set-up. Data logger Sensor output COMPUTER Temperature probes 1 and 2 Gas sensors Gas to house Flow meter Inlet pipe Reinforced Concrete dome Inner Brick wall Sawdust 3 cm blade Slurry outlet BIOGAS SLURRY Temperature probe 2 Brick wall Stirrer Temperature probe1 16 cm 2 cm 2 cm Concrete slab Figure 3.8: Designed batch biogas digester with various sensor positions Digester floor Figure 3.9 shows the complete data acquisition system with biogas and temperature sensors. 4

60 CO sensor Flow meter Biogas tube Laptop Hydrophobic filter Gas pump 12V battery CO 2 sensor H 2 sensor Datalogger CH 4 sensor Figure 3.9: The data acquisition system Theory of calibration for non-dispersive infrared gas sensors Non-dispersive Infrared (NDIR) sensors are simple spectroscopic devices often used for gas analysis. The key components are an infrared source, a sample chamber or light tube, a wavelength filter, and an infrared detector. The gas is pumped or diffuses into the sample chamber, and gas concentration is measured electro-optically by its absorption of a specific wavelength in the infrared (IR).The IR light is directed through the sample chamber towards the detector. The detector has an optical filter in front of it that eliminates all light except the wavelength that the selected gas molecules can absorb. Other gas molecules do not absorb light at this wavelength, and do not affect the amount of light reaching the detector. Figure 3.1 shows the basic components of a non-dispersive infrared sensor. 41

61 IR source Gas input Gas output Output voltage I o SIMPLE CELL REFERENCE CELL I ELECTRONICS BOARD L Collimator Band pass filter IR detector Figure 3.1: Basic components of NDIR sensor (Mamphweli, 29) The intensity of IR light that reaches the detector is inversely related to the concentration gas in the chamber. When the concentration in the chamber is zero, the detector will receive the full light intensity. As the concentration increases, the intensity of IR light striking the detector decreases. The Lambert-Beer s law describes the relationship between IR light intensity and gas concentration, and is given as: I I e kp [3.5] Where: I = the intensity of light striking the detector I = the measured intensity of an empty sample chamber k = system dependent constant p = the concentration of the gas to be measured NDIR gas detection properties are; high selectivity- free from cross- interference, sensitive, accurate and no negative memory effects or exposure hysteresis. 42

62 Palladium /Nickel (Pd/Ni) gas sensor theory There are many hydrogen sensors, many of which use palladium metal to trap hydrogen. Finding new materials for trapping hydrogen is still a key to the development of an ideal hydrogen sensor with advantages such as chemical selectivity, reversibility, fast response, sensitivity, durability, small size, simple fabrication, simple control system, and non-contaminating as well as non-poisoning. Hughes and Schubert (1994) have proposed the use of thin film Pd/Ni thin alloys for hydrogen detection so as to achieve some of these advantages. Like palladium and palladium/silver alloy, the electrical resistance of Pd/ Ni thin films is a function of absorbed hydrogen. The Pd/Ni sensors give durable and quick reversible detection of hydrogen at a concentration between.1 and 1% H 2 near 13 kpa and 27 C. Pd/Ni sensors can resist H 2 S poisoning. The Pd/ Ni gas sensor utilised is a patented Pd/ Ni-extended range hydrogen sensor invented by Hughes and Schubert (US patent N ) in This hydrogen sensor marketed and distributed by H 2 Scan TM detects hydrogen concentration from a few parts per million to 1%. The sensor utilises Pd/ Ni thin films to measure hydrogen in low and high ranges. The low level sensor is a Metal oxide Semiconductor (MOS) capacitor with a Pd/ Ni plate on one side of the capacitor. The high level sensor is a meandering Pd/ Ni thin film resistor. There is no cross sensitivity with other combustible gases including natural gas, methane, propane, and butane (Hughes and Schubert, 1994). Figure 3.11 shows the diagram of the Pd/ Ni sensor. FIRST SENSOR TEMPERATURE CONTROL DISPLAY UNIT SECOND SENSOR Figure 3.11: Diagram of a Pd/ Ni hydrogen sensor (Mamphweli, 29) 43

63 Data storage The CR1 allows for multiple measurement and control peripherals and sensor connections. Most commercial sensors can be used with the versatile channels consisting of 16 single-ended or 8 differential analogue inputs (individually configured), 2 pulse counters, 3 switched voltage excitations, and 8 control/digital ports. An RS-232 port and CS I/O port provides the multiple telemetry options for the data logger, via radio, satellite and phone. The data logger uses minimal power that can be supplied by rechargeable batteries or solar panels. It also provides increased on-board memory to store data or additional storage is available by using the CFM1 module with a Compact Flash card. The CR1 has an operating range of -25 o C to 5 o C; an extended range of -55 o C to 85 o C is available. Figure 3.12 shows the CR1 data logger. Analogue inputs SDM connections Figure 3.12: The CR1 Data Logger Power in Switched voltage excitation RS-232 port Peripheral port CS I/O port 3.5. CONCLUSION Biogas quantity and quality depends on the type of substrate used. Some biogas substrates produce more biogas than others. Hence, experimental procedure for the determination of the following substrate properties; TS, VS, COD, ph and calorific value of cow dung, donkey dung, horse dung and goat dung. Biogas consists of the following gases; methane, carbon dioxide, 44

64 hydrogen sulphide, hydrogen and nitrogen. The percentage composition of these gases varies in different wastes. 45

65 CHAPTER 4 DESIGN AND CONSTRUCTION OF THE BATCH BIOGAS DIGESTER 4.1. INTRODUCTION The biogas digester design integrates a number of parameters and there is need to optimise the design parameters for optimal methane production. The construction of an efficient biogas digester requires thorough supervision to meet the fine details of the design specifications. A deviation from the design specifications can result in poor performance of the biogas digester. This chapter focuses on the design and construction of the biogas digester. The chapter is divided into three parts. The first part looks at the design of the biogas digester which involves total solid contents of organic materials, favourable temperature, ph value and the carbon / nitrogen ratio for a good fermentation, and hydraulic retention time. A digester consists of four components, namely; the volume of the collecting chamber, the volume of the gas storage chamber, volume of the fermentation chamber and volume of the sludge layer. These parameters are calculated using design equations that include assumptions for volume and geometrical dimensions. The total volume of the batch digester designed was 1 m 3. The second part looks at the construction of the batch biogas digester. The construction process involved: selection of construction material, site selection and layout, excavation, inserting the mechanical stirrer and inserting the curved top (dome) of the digester. The third part looks at the feeding of the digester with cow dung into the digester before it is insulated. In addition, the biogas yield from cow dung was measured together with slurry, ambient, sawdust and biogas temperatures using the data acquisition system described in chapter 3. Finally, the second wall of the biogas digester was constructed and then the digester was insulated with sawdust. However, the construction process was done ensuring that all the design 46

66 specifications are adhered to. According to Mukumba (28) the success of any biogas digester depends upon the quality of how it has been constructed DESIGNING A BATCH BIOGAS DIGESTER Design parameter Biogas digester design plays a crucial role in digester performance and a number of considerations are taken into account. The following aspects were considered during the design process: durability, air tightness, availability of local materials and easy operation. The design parameters included: Total solid (TS) contents of organic materials. The total solid contained in a substrate is usually used as the material unit to indicate the biogas production rate of the materials. The most favourable TS value is 8% for better biogas production (Ituen et al., 27). Favourable temperature, ph value and carbon/nitrogen ratio for good fermentation. The mesophilic temperature between 25 C to 35 C was chosen in the design. The digester was insulated to keep the temperature within the latter mesophilic range to optimise mesophilic bacterial activity. The ph value selected ranged from 6.8 to 7.8 because the methanogens, prefer a neutral atmosphere with ph between 6.8 and 7.5. The carbon/nitrogen ratio considered ranged from 2:1 to 3:1. Carbon and nitrogen are the main nutrients required by micro-organisms. Therefore, a C/N ratio of 2-3:1 was considered for optimum anaerobic digestion, based on biodegradable organic carbon (Mshandete et al., 24; Yen and Brune, 27). Hence, co-digestion experiments were done to maintain the carbon/nitrogen ratio within the desired range. Hydraulic retention time (HRT) 47

67 For mesophilic digestion where temperature varies from 25 C to 35 C, the HRT was greater than 2 days. In the thermophilic environment HRT is usually less than ten days (Demetrides, 28). Shortening retention time can lead to increase in the volatile fatty acids (VFA) (Kobayashi et al., 211) and this is why mesophilic digestion was considered Cross-section of a cylindrical batch biogas digester A surface cylindrical biogas digester was chosen because it is easy to feed, insulate, clean and easy to construct and remove slurry after every hydraulic retention period. In addition, the batch digester is easy to agitate. Figure 4.1 shows the cross- section of a batch biogas digester. V GA V GA V GB V GB V V GC V GD Figure 4.1: Cross-section of the cylindrical biogas digester designed KEY Volume of gas collecting chamber Volume of gas storage chamber Volume of fermentation chamber The volume of the sludge layer Total volume of the digester, V = V GA = V GB = V GC = V GD = V GA + V GB + V GC + V GD 48

68 Figure 4.2 shows the cylindrical batch digester body with all calculated values indicated and Table 4.1 shows calculated volume and geometrical dimensions of the batch biogas digester. S 1 V 1 =.14 m 3 V GA V GB V f 1 =.24 m D =1.19 m V GC H=.48 m V 3 =.53 m 3 S 2 V GD f 2 =.15 m V 2 =.8 m 3 R=.53 m Figure 4.2: Geometrical dimensions of the cylindrical shaped biogas digester body Table 4.1: Assumptions for volume and geometrical dimensions (REEIN, 212) Assumptions: For volume V GD = 5%V V GB + V GC = 8%V V GD = 15%V V GB =.5 [V GB + V GC + V GD ] k k =.4m 3 /day V =.8 For geometrical dimensions D =1.378 V 1/3 V 1 =.827 D 3 V 2 =.511 D 3 V 3 =.3142 D 3 R 1 =.725 D; R 2 = D f 1 = D/5; f 2 = D/8 S 1 =.911 D 2 ; S 2 =.8345 D 2 49

69 Volume calculation of digester and hydraulic chamber The volume of the digester chamber was calculated using assumptions for volume and geometrical dimensions. In the calculations, a hydraulic retention time (HRT) of 3 days and mesophilic temperature of 3 C were used as mentioned earlier. Table 4.2 shows calculated values of digester chamber from geometrical and volume assumptions. In addition, a detailed feature of the batch biogas digester including the mechanical stirrer and drain pipe is shown in Figure 3.8. Table 4.2: Volume calculation of digester and hydraulic chamber Calculated values of digestive chamber Calculated values of digester chamber f 1 = D/5 =.24 m D = 1.378V 1/3 = 1.19 m R 1 =.725D =.86 m f 2 = D/8 =.15 m V 1 =.827D3 =.14 m 3 R 2 = 1.625D = 1.26 m V 3 =.3142D 3 =.53 m 3 V GA =.5V =.4 m 3 V GD + V GC =.6 m 3 H = (4.3142D 3 ) /3.14D 2 =.48 m Table 4.3 shows a summary of the parameters and calculation of working volume of the batch biogas digester Table 4.3: Parameters and equation for calculating the working volume of the batch biogas digester Parameters Magnitude Temperature 3 C Retention time (HRT) 3 days Concentration of TS 8% Working volume (V GD +V GC ) - Q.HRT.6 m 3 5

70 4.3. CONSTRUCTION OF A CYLINDRICAL BATCH BIOGAS DIGESTER A number of issues were considered during the construction of the biogas digester to ensure nonleakages and minimization of influence of ambient temperatures on the substrate temperature. The construction of the digester was done in the following stages: Selection of construction material Site selection layout and excavation Inserting the mechanical stirrer Inserting the curved top of the digester Second wall construction of the biogas digester Insulation of the biogas digester Selection of construction materials The main building materials for the biogas digester included clinker bricks, sand, concrete stones and Portland cement. The concrete stones were free of soil and organic material. Furthermore, the sand used was clean in order to increase the strength of the digester. The klinker bricks were first soaked in clean water for 5 minutes in order to remove dust and to prevent the bricks from sucking moisture from the mortar thus allowing a strong bonding. Clinker bricks were used in the construction because they offered the following advantages; Acid proof Wear proof Anti-corrosive High compressive strength Low thermal conductivity of.67 W/(m.K) Relatively cheaper than other types of bricks Portland cement was used because it has a low thermal conductivity of.29 W/(m.K) compared to masonry cement with a thermal conductivity of.5 W/(m.K) and epoxy was used for painting the inside of the batch biogas digester because it has a high water proofing and low thermal conductivity of.3 W/(m.K). Sawdust was selected for insulation because of its availability in 51

71 the area and low thermal conductivity of.8 W/ (m.k) compared to dry sand which has a thermal conductivity of W/ (m.k). Therefore, the heat transfer in materials with low thermal conductivity, for example sawdust, is very low Site selection and layout For optimum operation of the biogas digester, the site was close to a water supply. Furthermore, the land slopes gently (to the north) thus allowing easy water drainage. After selection of site, the centre of the digester to be constructed was marked on the ground surface with a steel rod. A string cord of length 53 mm was attached to the fixed steel rod stuck underground. The other end of the cord was attached to marking device (peg). The circumference of the batch biogas digester was marked by rotating the end of the cord in a circular fashion. The cord extended to a length of 53 mm trace to the second circumference. Figure 4.3 shows the sketch layout of the biogas digester. The site layout was again reviewed after site layout to ensure best site selection. Outer wall Inner wall 53cm 23cm 2 cm Wall thickness Brick space Gap for sawdust Figure 4.3 Sketch showing the cross section of the layout of the digester 52

72 Excavation The pit of 5 mm deep was dug. The excavated soil was placed two meters away from the edge of the dug pit to ensure that there was no soil falling inside the pit during the construction process. Furthermore, the pit bottom was levelled Construction of the foundation The digester cylindrical pit of 5 mm deep and radius of 54 mm was filled with concrete. Figure 4.4 shows the sketch (depth and width) of the digester pit. The ratio of cement, sand and stone aggregates used for the mixture were 1: 2: 3. The concrete was rammed to increase strength and was left for 7 days to allow settling. Water was poured twice daily on the concrete slab within the seven days to avoid cracking. The concrete slab was first covered with a black plastic sheet (damp course) in order to prevent the transfer of moisture from the ground to the slurry to keep the water/solids ratio to the measured one. The first course of bricks with mortar was aligned on top of the plastic. The cement to sand ratio for the mortar was 1:3. 54 mm 5 mm Figure 4.4: Sketch of the depth and width of the digester foundation/base A 7 mm high double wall was constructed. The structure was reinforced with a 23 mm brick force after every two courses. The openings for the mechanical stirrer, thermocouples and pressure gauge (for gas pressure) were left on the wall of the digester. Figures 4.5 and 4.6 show different construction stages of the batch biogas digester. 53

73 Damp course Figure 4.5: First and second brick work after the foundation Opening for stirrer Figure 4.6: The biogas digester at a height of 7 mm Construction of the dome The dome of the digester was constructed separately from the main body of the digester. The construction of the dome involved several steps. 54

74 3 mm Step 1 Two sheets of chip wood were glued together to form a single wooden sheet of 3 mm 3 mm. The wooden sheet was placed on a levelled ground. Step 2 The wooden sheet was covered with a sheet of black plastic. A circle of radius 53 mm was marked on the plastic sheet with a pencil and a second circle was also marked. Figure 4.7 shows a clear sketch diagram of wooden sheet, plastic sheet, the first and the second circumferences. 3 mm Chip wood sheet covered with plastic First circumference 16 mm Reinforced concrete Stone aggregate Second circumference Figure 4.7: Sketch showing wooden sheet, positions of stone aggregate and reinforced concrete Step 3 Stone aggregate was placed in the inter-circle on the plastic sheet to form a cone shaped structure with a circular base (diameter) of 16 mm as shown on Figure The stone aggregate had a height of 24 mm from the surface of the wooden sheet. The stone aggregate dome was covered with a black plastic sheet to separate stone aggregate from the reinforced concrete. Figure 4.8 shows the sketch diagram of the digester dome. Step 4 The curved structure of the biogas digester was designed using iron bars. The bottom part of the structure was circular with a diameter of 1 1 mm. It was made up of 12 mm rounded iron bars. 55

75 The iron bars were aligned on the structure to ensure that there were small spaces between the wires. The 23 mm brick force was also part of the wire mesh forming the curved dome structure. The dome wire structure had two layers, the inner (lower) and outer (upper) layers. The inner layer was 24 mm from the surface and while the outer layer was 26 mm. The cone wire mesh was fitted well onto the dome aggregate. A 4 mm slurry inlet pipe with a diameter of 11 mm was inserted into the wire mesh. In addition, another 15 mm slurry inlet pipe was inserted inside the wire mesh and the rest of the pipe was outside the dome. Similarly, a galvanised gas pipe with a thickness of 15 mm and with a length of 3 mm was also inserted. However, the 15 mm of the pipe was within the wire mesh. Furthermore, black sheet on the surface of dome aggregate separated the aggregate underneath from the concrete dome wire mesh. Figure 4.8 shows detailed sketch of the digester dome with gas and slurry pipes. Figure 4.9 shows the constructed wire mesh curved top dome of the biogas digester together with the slurry inlet pipe and biogas outlet pipe. Gas outlet pipe 26 mm 24 mm 54 mm Figure 4.8: Sketch diagram of the digester dome Slurry Inlet pipe Reinforced concrete Stone aggregate Plastic paper Wooden sheet 56

76 Biogas outlet pipe Slurry inlet pipe Wire mesh curved Figure 4.9: The wire mesh curved top dome of the biogas digester Step 5 Pouring concrete on the wire mesh dome was the next step. Concrete mixture was prepared with the ratio of cement, sand and aggregate of 1: 2: 3. The concrete was also rammed. It was ensured that all the dome wires were imbedded into the concrete by adding more concrete. The water was later poured on the concrete dome. This was done in successive 1 days so as to strengthen the dome by preventing formation of cracks. The completed dome structure is shown in Figure mm gas pipe 11 mm slurry inlet Reinforced Concrete dome Figure 4.1: The reinforced concrete dome fitted with inlet slurry pipe and biogas pipe 57

77 Inside plastering and insertion of a mechanical stirrer The first layer of plaster was done before flooring the biogas digester. The designed mechanical stirrer was fitted before the outside plastering of the digester. The mechanical stirrer was designed in accordance with the diameter and height of the batch digester. The blades of the mechanical stirrer were 3 mm in length to ensure that a homogenous mixture is made during stirring. The device was painted with red oxide to prevent rust. The handle was 3 mm long to improve the mechanical advantage of the system. The mechanical stirrer was hollowed to increase strength mass ratio and for easy stirring of the slurry. Figure 4.11 shows the designed mechanical stirrer with a handle. Handle of a mechanical stirrer Rubber seal [3 mm length] Mechanical stirrer blades [3 mm length] Figure 4.11: Designed mechanical stirrer with a handle 58

78 Plastering the outside of the digester and painting. A 11 mm slurry outlet pipe was fitted before outside plastering and painting of the batch digester. Three plastering layers were applied to avoid gas leakages. The thickness of the plaster was 1 mm. The cement to sand ratio for the mortar was 1:3. Epoxy paint was applied on the floor and on the inside walls of the biogas digester to minimise water absorption. Figure 4.12 shows the epoxy painted digester. Epoxy painted walls Figure 4.12: Shows inside epoxy painted floor and walls Inserting the curved top of the digester The reinforced concrete dome was placed on top of the plastered biogas digester with mortar smeared on its top surface to ascertain tight air seal as shown in Figure

79 Slurry inlet pipe Mortar applied Reinforced Concrete dome Figure 4.13: Biogas digester with a reinforced concrete dome An analogue pressure gauge was mounted on top of the batch biogas digester together with the ball valve. The pressure gauge and ball valve were connected to the gas pipe FEEDING OF THE BATCH BIOGAS DIGESTER BEFORE SECOND WALL CONSTRUCTION The biogas digester was first fed with cow dung before insulating with sawdust. This was done to establish the effects of ambient temperatures on biogas and slurry temperatures. The cow dung was collected from the University of Fort Hare Dairy farm. The cow dung was crushed mechanically before being fed into the digester to ensure homogeneity. The cow dung characterization parameters were measured, that is; total solids, volatile solids, chemical oxygen demand, ph, calorific value and temperature. The determination of these parameters has been described in detail in chapter 3 (see section 3.3). After the determination of the total solids, the correct amount of water was added to the cow dung. The mixture was thoroughly stirred to form a homogenous mixture. Figure 4.14 shows the stirring of the cow dung before being fed into the biogas digester, while Figure 4.15 shows feeding of the biogas digester. 6

80 Figure 4.14: Preparation slurry for the digester Figure 4.15: Feeding the biogas digester 4.5. SECOND WALL CONSTRUCTION AND INSULATION OF THE BIOGAS DIGESTER A half brick outer wall was constructed to make the biogas digester two walled as shown in Figure The separation gap for the two walls was 2 mm. 61

81 Inner wall Outer wall Pressure gauge Stirring rod inlet Temp Sensor Figure 4.16: Biogas digester with a second wall A sawdust insulated digester is shown in Figure After plastering of the second wall, dry sawdust was put between the two walls as shown in the same figure. The following data were collected using automated system, that is; ambient and slurry temperatures. The results are presented in chapter 5. 62

82 Data acquisition system Sawdust Pressure gauge Temperature sensor Figure 4.17: Sawdust insulated biogas digester Plastered second wall 4.6. CONCLUSION The 1m 3 batch field biogas digester was designed and constructed successfully. The digester consisted of volume of a collecting chamber, volume of the fermentation chamber and volume of the gas storage chamber. The designed digester was a surface cylindrical biogas digester for easy cleaning and easy feeding unlike underground digesters that not easy to clean. During the construction steps, it was ensured that the desired mixing ratio of cement to water for mortar was followed. The plastering layers both inside and outside the digesters which were done in stages made increased the strength of the digester walls and sealed air spaces on the digester walls. The epoxy paint used was thick and sealed all air spaces that remained after plastering. After fitting the digester dome on the main body of the digester, it was observed the main source of gas leakages was where the dome sat on the main body of the digester. However, correct mixture of water to cement for mortar and correct type of paint overcame the problem. When the batch digester was fed with cow dung before second wall construction, no leakages were found proving that it was a successful surface digester. 63