Microbiology of the biogas process Anna Schnürer Åsa Jarvis 1 Authors: Anna Schnürer and Åsa Jarvis Layout and illustrations: Cajsa Lithell, RedCap Design Cover page: Fluorescent methanogens, photo Anna Schnürer ISBN 978-91-576-9546-8 2 Foreword In Sweden and worldwide, there is currently great interest in the biogas process, since it can stabilize and reduce various types of organic waste whilst producing renewable and environmentally friendly energy in the form of biogas. Efficient production of biogas relies on a complex micro- biological process. Controlling the biogas process in an efficient manner to ensure maximum yield requires some advanced knowledge of how microorganisms work and of the microbiology underlying the biogas process. The report ”U2009:03 Microbiological handbook for biogas plants” from Swedish Waste Management was published in 2009 to provide easily accessible literature in Swedish that is specifically written for staff respon- sible for biogas production plants. The handbook was also translated to English and have been used internationally. It has been widely used in training courses and as a separate guide for staff at the plants. This updated version of the handbook includes recent advances in our understanding of the microbiology of the biogas process. Each chapter has also been expan- ded with a number of exercises (with answers) that are intended to be used to support training courses for staff at biogas plants. The guide was compiled by Anna Schnürer (Dept. of Molecular Sciences, Swedish University of Agricultural Sciences, Uppsala) and Åsa Jarvis (Jarvis Biowrite, Uppsala). Both authors have written doctoral dissertations on the subject of the biogas process and its microbiology and have extensive experience in the field. The Swedish version of this handbook was funded by Swedish Waste Management and can be purchased from their web page. 3 Content Introduction 6 1. Microbiology of the biogas process 8 What is required for the function and growth of microorganisms? 8 Environmental factors 12 Degradation of organic compounds 15 Diversity of microorganisms 22 2. The importance of technology to microbiology 25 Start-up of a biogas process 25 Process design 26 Important operating parameters 32 3. Substrates 41 Substrates for biogas production 41 How to choose a substrate for a biogas process 42 Other properties 45 Pre-treatment 46 The importance of different substrate components for the process 48 Important information about various substrates 53 4. Toxicity 62 Inhibition levels 62 Inhibiting substances 63 5. Monitoring and evaluation of the process 71 What should be monitored? 71 Methods for studying the substrate and the process 83 Summary 88 4 6. The digested residues 89 Function and use as fertilizer 89 Plant nutrient value 90 Effects on the soil 91 Quality and Certification 93 Contamination 94 Hygiene 96 Post-digestion and storage 100 Transportation of digestate 101 Environmental benefits and alternative uses 101 What happens in a process malfunction? 103 7. Common problems and solutions 103 Typical problems 105 Corrective measures 108 Additives in the future? 111 Concluding remarks 112 Glossary 113 Literature 117 Questions to the chapters 139 Questions Chapter 1 140 Questions Chapter 2 141 Questions Chapter 3 142 Questions Chapter 4 143 Questions Chapter 5 144 Questions Chapter 6 146 Questions Chapter 7 147 Answers to the questions 149 Answer to the questions to Chapter 1 150 Answer to the questions to Chapter 2 152 Answer to the questions to Chapter 3 154 Answer to the questions to Chapter 4 157 Answer to the questions to Chapter 5 159 Answer to the questions to Chapter 6 161 Answer to the questions to Chapter 7 164 5 Introduction Biogas is formed when organic material, heat or as vehicle fuel. In order to be able to use such as food waste or manure, is degraded biogas for vehicles, the carbon dioxide must first by microorganisms in the absence of oxygen, be separated from the methane. This is called so-called anaerobic (oxygen-free) degradation upgrading. On the combustion of methane, only or digestion. This process differs from compos- carbon dioxide and water are formed. Because the ting, which is also a bio-degradation process, biogas originates from atmospheric sources, the where aerobic conditions (i.e. oxygen) are carbon dioxide that is formed does not contribute required for the organisms to grow and work. to the greenhouse effect, in contrast to when na- tural gas is used as a fuel. Natural gas also consists Biogas is produced spontaneously in environments of methane, but this is a fossil gas extracted from where organic material accumulates with little or bedrock sources. The biogas process, with its no oxygen present, for example in wetlands and diversity of microorganisms, chemical reactions rice paddies, on the seabed or in the stomach of and degradation pathways, may also become ruminants. In recent years, the interest in utilizing important in the future for the production of new this process under controlled conditions has incre- biochemicals. In addition, the methane mole- ased both in Sweden and in other countries and cule can be used as raw material in manufacturing many new biogas plants have been built. Reactor industries. The digester residue can also be used as designs vary, but the basic principle is to allow a raw material for new products. anaerobic microorganisms to degrade the organic The process has been used for energy produc- material which is placed in an air-tight contai- tion since the second half of the 19th century; first ner to prevent oxygen from entering. The biogas in small reactors to provide gas for cooking and formed in this way consists of a mixture of gases, heating for individual households, farms or villa- ges in Asia and Africa, then in increasingly com- mainly methane (CH4) and carbon dioxide (CO2), of which methane is the energy-rich component. mercialized large-scale systems, mainly in Europe. The content of methane varies depending on the In Sweden, the first biogas plants were establis- kind of material that is degraded (digested) in the hed at wastewater treatment plants in the 1940’s process. In addition to biogas, a digestion residue and were expanded in the following decades. is formed, which can be used as a nutrient-rich The purpose of these facilities was primarily to fertilizer and soil conditioner. stabilize and reduce the amount of sludge formed Biogas is an environmentally-friendly fuel that after the aerobic aeration step. The energy crises of can be used for the production of electricity and the 1970’s stimulated the development and use of 6 biogas processes in industry, for example in sugar for the microorganisms to work optimally. This and pulp and paper mills. Biogas also began to book describes the biological process, the im- be recovered from landfills, partly to reduce the portant groups of microorganisms that are active emissions of methane, which is in itself a powerful during the different degradation steps, their nut- greenhouse gas. Large-scale digester plants were ritional and environmental requirements and how built during the 1990’s. In these facilities, many the technology can be adapted to these demands. different materials, such as food wastes, slaughter- Different substrates may require different treat- house waste, fertilizers, etc., are often degraded ments and new substrates are continually being simultaneously in order to extract as much biogas introduced, which may pose new challenges. The as possible. In recent years, interest has also incre- goal of the biogas process is usually to produce as ased in the production of biogas in smaller plants much biogas as possible from a given substrate, but (e.g. at the farm level) from fertilizers and various it is also important to ensure a stable process and plant and food residues. to produce a residue of good quality. This is Common to all biogas processes regardless of discussed in the chapters on substrates, toxici- the design and material (substrate) digested is that ty and the digester residue. In addition, some the degradation occurs step by step in a com- methods for monitoring and controlling the plicated interaction between different anaerobic biogas process are discussed, as well as some of the microorganisms. In order to run a biogas process problems that are commonly encountered and effectively, it is important to understand how this how they can be avoided and remedied. inter- action works and to know what is required Food Waste from cities Waste from farms Nutrients BIOGAS The biogas system is unique from the point of view both of efficient energy production and waste treatment. The nutrient rich residue can be used as a fertilizer. Both large and small scale biogas systems can be constructed and they can be integrated with other bioenergy systems. The biogas process may also provide new bio-chemicals in the future. 7 1. Microbiology of the biogas process A complex microbiological process lies behind The more varied the composition of the orga- the efficient production of biogas. Many nic material, the more components are available different species of microorganisms need to be for growth, and thus allow a greater diversity of active in order for biogas to form. In addition, organisms to grow. However, it is not good if the these organisms have to work closely together. composition varies too much with time because A disturbance of this teamwork results in many of the microorganisms that develop during reduced biogas production and, in the worst the process are specialists, i.e. they grow best on a case scenario, a breakdown of the process. specific substrate. Controlling the biogas process in an efficient In addition to the substrate, the microorganisms manner requires knowledge of the microbio- require a suitable environment in order to thrive logy behind the biogas process and of how and function.