Tài liệu Establish an anaerobic batch system by using guideline VDI 4630 and determine the biogas yield of different substrates in food processing villages

VNU UNIVERSITY OF SCIENCE - TECHNICAL UNIVERSITY OF DRESDEN --------------------------------- ® Bui Dieu Linh ESTABLISH AN ANAEROBIC BATCH SYSTEM BY USING GUIDELINE VDI 4630 AND DETERMINE THE BIOGAS YIELD OF DIFFERENT SUBSTRATES IN FOOD PROCESSING VILLAGES MASTER THESIS Hanoi - 2011 VNU UNIVERSITY OF SCIENCE - TECHNICAL UNIVERSITY OF DRESDEN --------------------------------- ® Bui Dieu Linh ESTABLISH AN ANAEROBIC BATCH SYSTEM BY USING GUIDELINE VDI 4630 AND DETERMINE THE BIOGAS YIELD OF DIFFERENT SUBSTRATES IN FOOD PROCESSING VILLAGES Major: Waste Management and Contaminated Site Treatment Code: MASTER THESIS SUPERVISORS: ASSOC. PROF. DR. NGUYEN THI DIEM TRANG PROF. DR. RER. NAT. DR. H.C. PETER WERNER Hanoi - 2011 Topic Establish an anaerobic batch system by using guideline VDI 4630 and determine the biogas yield of different substrates in food processing villages Task The first task is to establish an anaerobic batch system in Environmental Chemistry laboratory, Faculty of Chemistry, Hanoi University of Natural Sciences, Hanoi National University by using German guideline - VDI 4630. With this system, different inocula from various sources around Hanoi will be assessed their quality to choose the good one for next experiments with substrates. Reference substrate is sodium acetate. Substrates as wastes (cassava residues, rice residues, water hyacinth, pig manure) in Dai Lam village - a craft village of wine production and pig breeding will be sampled, prepared and fermented in free-oxygen environment. The processing parameters are biogas production, biogas composition (CH4 and CO2), pH - value, TS (total solids), VS (volatile solids), COD (chemical oxygen demand). The biogas yield (per amount of substrate, per VS of inoculum, per COD of substrate) and the degradability of different substrates will be evaluated. Objective The aim of this thesis is from learning the methods of guideline VDI 4630 to establish in practice an anaerobic batch system in the conditions of a Vietnamese laboratory. Then it is to control this system to investigate the quality of inocula, the fermentability/ the biogas potential/ the specific biogas activity of different organic wastes from food processing and livestock of a Vietnamese craft village. It is also close tied to the one objective of education and technology transfer of INHAND project (project funded by the Federal Ministry for Education and Research of Germany - BMBF; with the project coordinator Institute of Waste Management and Contaminated Site Treatment, Dresden University of Technology) about Integrated management of water, wastewater, waste and energy in craft villages in Vietnam.    I Contents Contents Contents ..........………………………………………………………………. II Abbreviations..………………………………………………………………. IV List of figures....……………………………………………………………... V List of tables .……………………………………………………………....... VII Acknowledgements.………………………………………………………..... IX 1. Introduction………………………………………………………………. 1 2. Theoretical Basics………………………………………………………… 4 2.1. Basics of Anaerobic Digestion………………………………………... 4 2.1.1. The four stages of decomposition………………………………… 4 2.1.2. Influence Factors of Digestion……………………………………. 7 2.1.3. Biogas potential of different substrates…………………………… 13 2.2. End products of fermentation…………………………………………. 15 3. Materials and methods…………………………………………………… 17 3.1. VDI survey about batchtests for biogas yield determination…………. 17 3.1.1. Scope and purpose of Fermentation batchtests…………………… 17 3.1.2. Methods of VDI 4630 in detecting the biogas yield……………… 17 3.1.3. Method of VDI 4630 in approaching the batchtests………………. 19 3.2. Analysis method ……………………………………………………… 19 3.2.1. Sampling…………………………………………………………... 19 3.2.2. Sample preparation………………………………………………... 23 3.2.3. Determination of parameters: TS, VS, COD, and CH4 and CO2   II Contents content in biogas………………………………………………….. 25 3.3. Experimental procedure………………………………………………. 30 3.3.1. Test condition and batch apparatus……………………………….. 30 3.3.2. Method to calculate the biogas production, the biogas yield, the   biogas composition, the degree of degradation…………………… 37 4. Evaluation and discussion of the batch experiments…………………... 41 4.1. Activity potential of different inocula………………………………… 41 4.2. Biogas production and biogas composition of different substrates…... 47 4.2.1. Biogas yield (mlN/gVSinoculum, mlN/gCODsubstrate, mlN/ g substrate)… 47 4.2.1.1. Biogas yield of trial 6 with inoculum 5………………………... 47 4.2.1.2. Biogas yield of trial 7 with inoculum 5………………………... 50 4.2.1.3. Biogas yield review of different substrates with inoculum 5….. 54 4.2.1.4. Biogas yield review of different substrates with inoculum 6….. 56 4.2.2. Comparison of biogas yields with theory and literature…………... 61 4.2.3. Biogas composition (CH4 and CO2 content)……………………… 64 4.3. Residue of biogas production…………………………………………. 66 4.3.1. Degree of degradation of volatile solids…………………………... 66 4.3.2. Degree of degradation of total solids……………………………... 69 4.3.3. Degree of degradation of COD total……………………………… 71 4.4. Error analyis………………………………………............................ 77 5. Conclusion…………………........................................................................ 80 References…………………............................................................................ 83 Annex- Data records....................................................................................... I Statement under oath...................................................................................... XI III Abbreviations Abbreviations abs absolute B Blank sample C/ CR Cassava residues sample COD Chemical oxygen demand Inoc. Inoculum lN standard liter, volume under normal condition mlN standard milliliter, volume under normal condition P/ PM Pig manure sample R Reference sample RR Rice residues sample SA Sodium acetate SLR Sludge loading rate or sludge loading ratio TS Total solids VS Volatile solids W/ WH Water hyacinth sample   IV List of Figures List of Figures Figure 2-1: Four stages of anaerobic degradation (WEILAND, 2003).......... 4 Figure 3-1: Test apparatus according to DIN EN ISO 11734: Gas volume measurement with a gas pressure measurement instrument (VDI 4630)...…………………………………………………... 18 Figure 3-2: Gas volume measurement with a gas pressure measurement   instrument (S. Meier, 2009)......................................................... 18 Figure 3-3: Map showing position of Dai Lam village…………………...... 21 Figure 3-4: Water hyacinth at main sewer…................................................. 21 Figure 3-5: Rice residues and cassava residues sampling at households…... 22 Figure 3-6: Pig manure at a small swine farm of a household..……………. 22 Figure 3-7: Homogenizing cassava residues by blender…………………… 23 Figure 3-8: Homogenizing water hyacinth by blender……………………... 24 Figure 3-9: Filling the bottles by weighing method………………………... 30 Figure 3-10: Closing bottles with silicone stoppers......................................... 31 Figure 3-11: Creating the vacuum inside the bottles........................................ 31 Figure 3-12: Lovibond conditioning cabinet.................................................... 32 Figure 3-13: K2000 Pressure table - EXTECH manometer 407910 ............ 32 Figure 3-14: MultiLabP4.................................................................................. 33 Figure 3-15: MEMMERT drying cabinet......................................................... 33 Figure 3-16: TDW muffle furnace................................................................... 33 V List of Figures Figure 3-17: Lovibond ET 108 and MERK spectroquant TR320 block digesters....................................................................................... 31 Figure 3-18: PI 722N instrument in measuring COD..................................... 34 Figure 3-19: Gas chromatograph Shimadzu GC-2010..................................... 34 Figure 4-1: Biogas yield without blank on different inocula [mlN Biogas/ g COD sodium acetate]................................................................... 44 Figure 4-2: Biogas yield with blank on different inocula [mlN Biogas/ g COD sodium acetate]................................................................... 45 Figure 4-3: Biogas yield with blank on inoculum 5 of trial 6 [mlN Biogas/ g VS Inoculum].............................................................................. 48 Figure 4-4: Biogas yield without blank on inoculum 5 of trial 6 [mlN Biogas/ g COD substrate]............................................................ 49 Figure 4-5: Biogas yield with blank on inoculum 5 of trial 7 [mlN Biogas/ g VS Inoculum].............................................................................. 52 Figure 4-6: Biogas yield without blank on inoculum 5 of trial 7 [mlN Biogas/ g COD substrate]............................................................ 52 Figure 4-7: Biogas yield with blank on inoculum 5 of trial 6-7 [mlN Biogas/ g VS Inoculum].............................................................. 54 Figure 4-8: Biogas yield without blank on inoculum 5 of trial 6-7 [mlN Biogas/ g COD substrate]............................................................ 55 Figure 4-9: Biogas yield with blank on inoculum 6 of trial 9 [mlN Biogas/ g VS Inoculum].............................................................................. 58 Figure 4-10: Biogas yield with blank on inoculum 6 of trial 9 [mlN Biogas/ g   COD substrate]............................................................................ 59 Figure 4-11: The corrected biogas composition of different samples.............. 65 VI List of Tables List of Tables Table 2-1: Average composition of biogas (FNR, 2005)............................. Table 2-2: Factors influencing the anaerobic degradation (WEILAND, 2001) modified........................................................................... 8 Table 2-3: Micro-nutrients for the anaerobic degradation........................... 10 Table 2-4: Inhibitory concentrations of various elements (WESSELAK, 2009) 11 Table 2-5: Inhibitory concentrations of various heavy metals (WESSELAK, 2009).................................................................. Table 2-6: 12 Biogas composition and yield of different groups of substances (Biogas Guide 2006)................................................. 13 Table 2-7: Properties of renewable resources (FNR, 2005)......................... 14 Table 3-1: Description about sources of six inocula................................... 20 Table 3-2: Description of the batchtests for investigating the quality of six inoculums............................................................................................. Table 3-3: Table 4-1: 37 Description of the experimental approach and parameters in trials investigating quality of six inocula................................... Table 4-2: 36 Description of the batchtests for investigating the biogas potential of different substrates..................................................   6 42 Summary of biogas yield without blank and SLR on different inocula........................................................................................ 46 Table 4-3: Description of the experimental approach and parameters in trial 6 47 Table 4-4: Summary of biogas yield and SLR of trial 6.............................. 49 Table 4-5: Description of the experimental approach and parameters in trial 7 51 VII List of Tables   Table 4-6: Summary of biogas yield and SLR of trial 7.............................. 53 Table 4-7: Description of the experimental approach and parameters in trial 9 56 Table 4-8. Summary of biogas yield and SLR of trial 9.............................. 60 Table 4-9: Comparison of biogas yields with theory................................... 62 Table 4-10: Comparison biogas yields of water hyacinth samples with literature. 63 Table 4-11: Biogas composition of different samples in trials 6, 7, 9........... 64 Table 4-12: Volatile solids of different samples............................................ 66 Table 4-13: Degree of degradation of volatile solids..................................... 68 Table 4-14: Total solids content of different samples................................... 69 Table 4-15: Degree of degradation of total solids.......................................... 70 Table 4-16: Chemical oxygen demand of different samples.......................... 72 Table 4-17: Degree of degradation of COD................................................... 73 Table 4-18: Degree of degradation of COD................................................... 75 Table 4-19: Degree of degradation of COD................................................... 76 VIII Acknowledgements Acknowledgements Special thanks I would like to say to my supervisor Prof. Dr. Nguyen Thi Diem Trang for giving me the opportunity to make this thesis at Faculty of Chemistry, Hanoi University of Science in the Double-Degree-Program between the Hanoi National University and the Dresden University of Technology. Many thanks also to my German supervisor Prof. Dr. Peter Werner. Special thanks also to Msc. Tran Thi Nguyet (from Institute of Waste Management and Contaminated Site Treatment, Dresden University of Technology) and Dipl.-Ing. Sebastian Meier (from Institute for Water Quality and Waste Management, Leibniz University Hannover) – team members of INHAND project who enthusiastically supervised me during the course of this thesis. Thanks to the help in inocula collecting of Prof. Dr. Nguyen Viet Anh, from the Centre for Environmental Engineering of Towns and Industrial areas (CEETIA), Hanoi University of Civil Engineering and the help in gas composition analysis of Mr. Thai Ha Vinh, from Monitoring and Environmental Analysis Department, Monitoring and Analysis of Working Environment Station, National Institute of Labor Protection. Thanks also to the help in sample preparation of the staff (Mrs. Nguyen Thi Diem Huong, etc) in the laboratory of Monitoring Centre for Natural Resources and Environment of Bac Ninh province. Thanks to the students of the Environmental Chemistry laboratory (Thang, Cham, Thao, Lan, etc) for their friendship with me during the experimental period time. Special thanks also to my family members for their great support in all the time of this master course!   IX Introduction 1. Introduction In view of the challenges in the global and regional energy markets and facing the needs for global climate protection as well as the increasing efforts in developing rural areas, the utilization of renewable energies has to be advanced. In this context, the usage of biogas plays an exceptional role as it is a highly flexible fuel with respect to a wide range of input substrates. Biogas also offers various options in providing and using energy on a local, regional and global scale. Vietnam is no exception to the trend biogas applications to replace fossil energy. Biogas production has been studied and applied long in Vietnam, but until 2003, it became the real attention when the Ministry of Agriculture and Rural Development collaborated with the Netherlands Development Organization to build renewable energy project, the “Support Project to the Biogas Program for the Animal Husbandry Sector in Some Provinces of Vietnam.”. By the end of 2008, the project has supported construction of over 56,000 household biogas plants, provided training for 500 provincial and district technicians, 700 biogas mason teams, and organized thousands of promotion workshops and trainings for biogas users. Up to now, the project has become a national program “Biogas program for the Animal Husbandry Sector of Vietnam” that supports the implementation of household biogas digesters throughout Vietnam. By the end of 2012, the team aims to complete 164,000 biogas plants (including large plants) in 58 provinces throughout Vietnam and gain 1.5-3 tradable emission rights per year per digester(1). This program raises the effective movement of production technology and application of biological energy, reduces environmental pollution in rural areas, creates jobs and improves living standard for Vietnamese farmers and minimizes the greenhouse effects. Along with that, the World Bank is also currently funding an array of manure management demonstration projects in Vietnam, ranging from small household -scale systems to village-scale systems(2).                                                              (1) http://biogas.org.vn/vietnam/ (2) http://www.globalmethane.org/documents/ag_vietnam_res_assessment.pdf Bui Dieu Linh 1 Introduction The studies(3) of biological energy have been simultaneous developing, as well as Prof. Dr. Bui Van Ga – director of Da Nang University and his colleagues at the Research Center for Environmental Protection (Da Nang University) with research focused on biogas refining and motor applications, or the teams from Hanoi Polytechnic University and Ho Chi Minh Polytechnic University with research on completed conversion of gasoline and diesel to biogas fuel running engines ... Therefore, Vietnamese farmers now are utilizing the produced biogas not only in cooking, lighting, heating but also generating electricity for their own farms by their own capital. The only pity is that the farmers can not sell surplus generated power into general grid. Besides, the Bio-Gas Project in the framework of the "Go Green Green Journey" by Toyota Vietnam (TMV) in collaboration with the General Department of Environment and Ministry of Education and Training has been installing 500 generator powered by bio-gas in households, farms, small and medium enterprises from 2008 up to 2012. Based on those, biological energy has developed at a larger scale, focusing on the factories, farms, producing biogas from waste water of tapioca starch plants, seafood processing plants, rubber production plants, etc. Up to now, Vietnam has achieved 11 CDM(4) Projects(5) in validation, with the credit period from 2009/2010 to the end of 2020/2030, in the field of waste/ waste water treatment and biogas capture (mainly in tapioca starch sector) with the parties involved as limited companies, corporations from Japan(4), France(1), Netherlands (5), Germany(1).                                                                                                                                                                                      (3) http://www.dongcobiogas.vn   (4) CDM (Clean development mechanism) is the one of three market-based mechanisms in Kyoto Protocol which requires developed countries to reduce green house gas (GHG) emission. CDM enables emission reduction projects, like biogas capture projects, in developing countries to get CER (certified emission reduction) credits which can be traded with developed countries to meet part of their emission reduction commitments.   (5) http://noccop.org.vn Bui Dieu Linh 2 Introduction The development of biogas production and application with all the domestic potential and overseas support has been bringing great benefits for Vietnam’s economic development and environmental improvements. For that, contributing to the studies of biogas in Vietnam, this thesis focused on the method of German guideline (VDI 4630) to assess the potential of recovery of organic waste in a Vietnamese food processing village by biogas production. Firstly, it is to establish an anaerobic batch system for biogas production in the laboratory in the North of Vietnam. Then, experiments with this system were set up to investigate the quality of inocula from different sources around Hanoi. And with chosen inocula, substrates as organic waste of Dai Lam village - a wine production and pig breeding village in Bac Ninh province were assessed. Parameters of process were pH, temperature, TS, VS, and COD. Biogas potential and the fermentability of these substrates were evaluated and interpreted by using VDI- 4630 guideline.   Bui Dieu Linh 3 Theoretical Basics 2. Theoretical Basics 2.1. Basics of Anaerobic Digestion 2.1.1. The four stages of decomposition The methane fermentation process comprises four stages that organic materials being degraded by anaerobic microorganisms in the absence of oxygen. The degradation of high molecular weight starting substrates such as carbohydrates, fats and protein via low molecular weight compounds (fatty acids and alcohols) to methane, which is the main component of biogas. Figure 1 shows a degradation process which is described below: Stage 1 Hydrolytic phase Stage 2 Acidogenic Phase Stage 3 Stage 4 Acetogenenic Phase   Methanogenic Phase   H2 / CO2 Biomass Polysacharides Proteins Fats Sugars Amino acids Fatty acids Biogas CH4 / CO2 Carboxylic acids Alcohols Hydrolytic Bacteria Fermentative Fermentative Bacteria Bacteria Acetate Acetogenic Bacteria Methanogenic Bacteria Figure 2-1: Four stages of anaerobic degradation (WEILAND, 2003) In the first stage (hydrolysis), facultative anaerobic microorganisms hydrolyze the biomass with the aid of extracellular enzymes (exoenzymes(1)) to low molecular weight components. Here, the organic substance is transferred by addition or temporary storage of water molecules in a dissolved form. The facultative anaerobic                                                              (1)  Exoenzymes are synthesized by the bacteria-consuming enzymes, which serve to break up nutrients. Bui Dieu Linh  4 Theoretical Basics bacteria which were created to consume the remaining dissolved oxygen present with low required redox potential (less than - 330 mV) are the obligatory anaerobic bacteria (methanogens). The rate of hydrolysis is determined by the substrate composition, temperature, pH value (optimum pH = 6, (BISCHOFSBERGER, et al., 2005)), and the concentration of microorganisms. For example, sugar and hemicelluloses are hydrolyzed very well, whereas pectin and lignin are difficult to hydrolyze. Therefore, the higher the proportion of shares in the easily cleavable substrate is, the faster the digestion process of the bacterial cells run. The hydrolysis is therefore generally regarded as rate-limiting step of anaerobic (and aerobic) degradation (ROEDIGER et al., 1990). Subsequently, the hydrolysis products in the second stage of fermentation (acidification) fermented intracellularly by the bacteria, the bacteria excrete mainly carboxylic acids, ethanol, ammonia, hydrogen sulphide and carbon dioxide. The acidogenic bacteria here have a large tolerance, so that the pH value for the acidification of carbohydrates may fall below 4.0 (BISCHOFSBERGER et al., 2005). The degradation of the hydrolysis and acidification can be inhibited by its own metabolic products. One hand, the bacteria produce only insufficiently dissolved substrate exoenzymes, on the other hand, these exoenzymes are sensitive to pH values less than 6.5 (ROEDIGER et al., 1990). Because methanogenic bacteria produce methane only from acetic acid, hydrogen and carbon dioxide in the third stage (acetogenesis), the carboxylic acid and alcohol of the second stage can be converted to acetic acid, carbon dioxide, water and hydrogen. Ammonia is mineralized to ammonium. In the fourth stage (methanogenesis) methanogenic bacteria convert acetic acid, hydrogen and carbon dioxide to methane. The methane formation is carried out for 70% of the acetic acid degradation (also called acetate-degrading, see equation 2-1) and about 30% of the conversion of carbon dioxide and hydrogen (ROEDIGER et al., 1990). See equation 2-2. CH3-COO- + H2O Æ CH4 + HCO3- Bui Dieu Linh  5 (Equation 2-1) Theoretical Basics 4 H2 + CO2 Æ CH4 + 2 H2O (Equation 2-2) The methane formation can also occur with a few methanogenic bacteria from formic acid, methanol and methyl amine (CH3NH3) (BISCHOFSBERGER et al., 2005). For methanogenesis, the optimum pH is 7 and the optimum temperature is at 35 ° C and 55 ° C (ROEDIGER et al., 1990). There are 13 species of methane-forming bacteria, of which 11 species have their optimum working environment in the mesophilic range and one type in the thermophilic environment. Another type is thermo-tolerant and can operate in mesophilic and thermophilic range. Throughout the fermentation process, hydrogen inhibits the acetogenic bacteria. The close symbiosis between the acetogenic bacteria and the methanogenic bacteria is an important prerequisite, since methanogenic bacteria prevent the inhibitory effects of excess hydrogen, by converting the hydrogen with carbon dioxide to methane (see equation 2-2). The symbiosis should be adjusted by the mixing process in the reactor with registered shear forces, in order not to destroy it. After a trouble-free decomposition of organic substrates, there is an energy-rich gas mixture which consists mainly of methane and carbon dioxide. An average biogas composition is given in Table 2-1. Table 2-1: Average composition of biogas (FNR, 2005) Ingredients Concentration Methane(CH4) 50 – 75 Vol.-% Carbon dioxide (CO2) 25 – 45 Vol.-% Water (H2O) 2 – 7 Vol.-% (20 – 40 °C) Hydrogen sulfide (H2S) 20 – 20000 ppm Nitrogen (N2) < 2 Vol.-% Oxygen (O2) < 2 Vol.-% Hydrogen (H2) < 1 Vol.-% Bui Dieu Linh  6 Theoretical Basics 2.1.2. Influence Factors of Digestion Anaerobic degradation processes involve a number of factors that determine the growth and activity of microorganisms significantly. We distinguish effects due to operational factors (temperature, mixing, residence time, organic loading rate) and the influence of substrate components (C: N: P ratio, pH - value, concentration of inhibitory substances and nutrients, etc). Some of these factors influence each other and are mutually dependent. Selected factors are briefly explained in the following sections. Mixing Optimum mixing of the reactor is set to achieve sufficient contact between bacteria and fermentation substrate. Temperature and concentration differences are compensated in the fermenter by mixing. Without or with insufficient mixing, the mass of bacteria would decrease and easily demolished at the bottom to rise up the fermentation and form a floating layer through which the gas outlet would escape more difficult. On the contrary, with vigorous mixing, there is the danger to those living in small communities, symbiosis acetogenic and methanogenic bacteria communities are affected by too high shear forces (GRONAU et al., 2007). Temperature The maximum metabolic rate and the maximum growth rate of organism is a function of the temperature. For example, different optimal temperature possesses involved anaerobic microorganisms. Therefore, a classification of organisms is based on temperature ranges, micro-organisms can be distinguished with psychrophilic (< 25°C), mesophilic (25 - 40°C) and thermophilic (45 - 60°C) (BISCHOFSBERGER et al., 2005). In industrial processes there are mainly mesophilic or thermophilic microorganisms. The choice of the temperature range depends on factors such as the substrate properties, the hygiene degree, or the residence. Since mesophilic fermentation processes have a greater diversity of methanogens (12 species), they are shown against thermophilic fermentation processes (only two species of methane bacteria) as a stable. It should be noted that Bui Dieu Linh  7 Theoretical Basics the species Methanosarcina barkeri(2) are both the mesophilic and thermophilic methane bacteria (DORNACK, 2001). Since methane bacteria are very temperaturesensitive, and are adversely affected even at low temperature, a constant temperature control of anaerobic reactors is necessary (ROEDIGER et al., 1990). With extreme temperature fluctuations, the bacteria respond with lower metabolic rates and reproductive performance (DORNACK, 2001). pH value The pH - value arises from the metabolic products of naturally present microorganisms and the different self-regulating buffer system. The predominant organisms in the degradation phase species have different optimal pH. During hydrolysis and acid-forming, bacteria find their pH optimum at 4.5 to 6.3, methane bacteria thrive best in a very narrow pH - 6.8 to 7.5 (FNR, 2005). A shift in the pH value out of the narrow range from 6.6 to 8.0 can be rough interference to substrate degradation, result in the bacterial growth rate and metabolism (FAULSTICH, 1995). For example, the stable of hydrolyzing substrates can be inhibited by an excess of lime, because the required optimum pH is not created by the buffering action of the lime. The consequence is lower gas yields. Factors influencing the anaerobic degradation are summarized in table 2-2 below. Table 2-2: Factors influencing the anaerobic degradation (WEILAND, 2001) modified Hydrolysis / acidification Methane fermentation pH 5.2 – 6.3 6.7 – 7.5 C: N – ratio 10 – 45 20 – 30 Solids < 40% TS < 30% TS Nutrient Requirements - C: N: P: S 500 : 15 : 5 : 3 600 : 15 : 5 : 3 Red-ox potential +400 – (-)300 mV < -250 mV Trace elements No special Claims essential: Ni, Co, Mo, Se                                                              (2)  http://microbewiki.kenyon.edu/index.php/Methanosarcina_barkeri    Bui Dieu Linh  8 Theoretical Basics Ratio of nutrient elements The C/N ratio and the C/N/P/S ratio can be used as an indicator of the optimal supply of carbon and nutrients. If the C/N ratio is too high then the used carbon is not completely eliminated and therefore the methane potential not fully exploited. This ratio is shifted to nitrogen, there is the danger of the formation of ammonia which is toxic in too high on the bacteria. The C/N/P/S ratio is used to draw conclusions about the nutrient supply of the bacteria. Trace elements For the growth and survival of the bacteria, trace elements that do not exceed certain levels should also be required (see Table 2-3). The trace elements must present in dissolved form. The natural sources of entry for the trace elements in agricultural biogas plants are agricultural soils, etc, which can be distinguished between natural and anthropogenic influences such as pedogenesis fertilizer, emissions from industry and traffic. The incorporation of trace elements in plant and animal feces is based on feeding and additives and other input sources. Lacking of trace elements, so as at too high concentrations, a delay occurs that can lead to the stoppage of the anaerobic degradation. Bui Dieu Linh  9