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Anaerobic Digestion Anaerobic Digestion Series i Anaerobic Digestion: Basic Processes for Biogas Fact Sheet FS-994 Production October 2014 A Biological Process Microbes at Work Anaerobic digestion is a process in which anaerobic Harnessing microbes to work for us is not a new bacteria break down or "digest" organic material in concept. We do it every day in numerous activities the absence of oxygen to produce "biogas" as a by- from making beer to cleaning oil spills. In each product of their metabolism. The process occurs case, the biological needs of the microorganisms naturally in water-logged soils, deep water bodies, must be met to encourage rapid growth. In and in the digestive systems of termites and large anaerobic digestion, the biological needs of the animals (that includes you)1. organisms include: In anaerobic digesters, naturally-occurring biological A) food energy (carbon source) that can be processes are exploited in an engineered system to degraded; treat and dispose of waste materials, stabilize end B) moisture; and products, destroy pathogens, and generate biogas, C) a hospitable environment in terms of a valuable product (Figure 1). Biogas produced in temperature, a lack of oxygen, and a neutral anaerobic digesters consists of methane (50%– pH. 80%), carbon dioxide (20%–50%), and trace levels of other gases such as hydrogen, carbon monoxide, 2 nitrogen, water vapor, and hydrogen sulfide . Microbial Processes Anaerobic digestion occurs in four processes and is carried out by various groups of bacteria working together. Figure 2 shows a general schematic of the processes involved in producing biogas from raw organic feedstocks. The basic processes are described below. Hydrolysis (or liquification): Microbial breakdown of the input feedstocks (such as corn, grain, sawdust, food waste, or manure) results in simple sugars, Figure 1. An anaerobic digester operating on an Oregon fatty acids, and amino acids, which are then dairy farm. (Photo credit: Dan Sullivan)3. available to other bacteria. For more information on this and other topics visit the University of Maryland Extension website at www.extension.umd.edu 1 Acidogenesis: Bacteria transform the products of Methanogenesis: Methane-producing microbes hydrolysis into short chain volatile acids, ketones, utilize acetic acid, hydrogen, and carbon dioxide to alcohols, hydrogen, and carbon dioxide. methane. Acetogenesis: Microbial conversion of organic acids While we discuss the process in steps, in reality, all into acetic acid, ammonia, hydrogen, and carbon of these steps are occurring simultaneously, with dioxide. different groups of bacteria working together to transform biomass into biogas. Figure 2. Schematic of microbial processes in anaerobic digestion and the stepwise progression of these processes. (Diagram adapted from Clarke Energy20) Additional groups of bacteria reduce the hydrogen made of reinforced concrete, steel or fiberglass with toxicity by converting hydrogen into ammonia, a gas-tight cover to capture the biogas. The plug hydrogen sulfide, and methane5. A majority of the flow digester normally functions without internal bacteria involved in the process are facultative agitation. This type of digester works well with a anaerobes, which means they can survive in the scrape manure management system with little presence of oxygen but do not use it. Methanogens, bedding and no sand. In theory, the daily input of are obligate anaerobes, which means they can only manure in a plug flow digester functions as a daily live and grow in the absence of oxygen. The stages plug, advancing towards the outlet whenever new of anaerobic digestion function in a dynamic manure is subsequently added. In reality, the equilibrium whereby if one reaction is inhibited, it manure may not remain as a plug and more liquid components of the manure may flow through the will hinder the subsequent reactions6. digester at a faster rate with some settling or Digester Types floating of some solids within the digester. There are two general classifications of digesters, Complete-mix digester: A complete-mix digesters is based on the flow through the digester vessel. typically an above- or below-ground tank that is part of a manure management system to handle Plug-flow digester: A plug flow digester vessel is a manure containing 2 to 10 percent solids. The long, narrow, insulated and possibly heated vessel digester is mixed mechanically or with gas injection For more information on this and other topics visit the University of Maryland Extension website at www.extension.umd.edu 2 to keep the solids suspended. This maximizes biological activity for digestion of volatile solids, methane production and odor reduction. Biogas from either type of digester can be used to run an engine generator set to produce electrical power. Some waste heat from the engine may be used to heat the digester. A Little Bit About Microbes and Their Environment Temperature Figure 3. Methane production occurs more quickly in Anaerobic digestion can occur in three different digesters operated in the thermophilic range temperature ranges, with the most reactors using compared to the mesophilic range (Diagram the second and third ranges: credit: Michael Gerardi)4 7,8 1) psychrophilic (41-68°F) , pH and Alkalinity 2) mesophilic (77-95°F), and 3) thermophilic (122-140°F) conditions4. In addition to temperature, pH and alkalinity affect the anaerobic digestion process. Alkalinity is necessary for pH control and acts as a buffer against Bacteria, and thus biogas production, can be acid formation during acidogenesis. Methane- inhibited at temperatures between 104-122°F4. The producing bacteria (methanogens) are sensitive to microbial communities present in one temperature pH change and operate optimally between a pH of range are not the same as the ones occurring in 6.8 and 7.24. The production and consumption of another temperature range, and thus decreases in volatile acids and the carbon dioxide content in the biogas production can occur when temperature biogas affect the pH4. Decreases in alkalinity can ranges are not maintained9. In general, for optimal occur when organic acids accumulate in the digester biogas production, the anaerobic digester should be due to failure to convert the acids into methane 10 kept at a constant temperature . In most areas of during methanogensis or the introduction of large the United States, anaerobic digesters require quantities of organic acids through the digester insulation, heating, or a combination of both. feed source4. A pH buffer, such as sodium bicarbonate can be added to maintain a consistent Typically, digesters operated in the mesophilic pH, or co-digestion with a substance with a high range are larger than those operated in the higher buffering capacity, such as dairy manure, can be temperatures of the thermophilic range because used to buffer pH changes from adding substances the decomposition process takes more time in the with higher organic loadings, such as food waste. lower temperature regime (Figure 3). The mesophilic process is less sensitive to upset or Additional Environmental Considerations change in operating regimen compared to thermophilic digesters, which have shorter Final environmental considerations include the retention times than those operated in the water/solids ratio, carbon/nitrogen ratio, agitation mesophilic range but tend to be more sensitive in the digester, particle size of the material, and retention time. It may be necessary to add water to changes in feed materials or temperature1. For more information on this and other topics visit the University of Maryland Extension website at www.extension.umd.edu 3 the feed material if it is too dry or if the nitrogen precipitated phosphorus will “re-appear”. There is content is high. A carbon/nitrogen ratio of 20:1 to no real loss of phosphorus in the digester. 30:1 is preferred. The solids content entering an anaerobic digester Occasional mixing or agitation of the digesting can range from 7% to 13%. Once digested, the material can aid the digestion process. Antibiotics in slurry becomes more of a soupy mixture composed livestock feed have been known to inhibit the of more fine solids suspended in the liquid. This anaerobic bacteria in digesters. composition makes it possible to use the waste as a liquid (agitating may be necessary) or to further Digester Inputs and Outputs liquefy the effluent leaving the digester by The nitrogen in manure enters the digester mostly separating the solids and using these solids as in the forms of ammonium and organic nitrogen. In bedding in the dairy barn or selling as compost the digester, organic nitrogen is converted to material. The remaining liquid can then still be ammonium. Therefore, the effluent will have applied to the fields as an agricultural fertilizer. higher levels of ammonium than the influent In a plug flow digester, the conversion efficiency of manure but similar total nitrogen levels (organic volatile solids to biogas can approximate 70%, nitrogen + ammonium). If field-applied, the making it more efficient than many other energy ammonium is available to volatilize if the material is conversions. However, when electrical generation not incorporated into the soil. However, because of is added into the system, and the biogas is not used the lower solids content, digester effluent will directly for heating purposes, the system efficiency infiltrate better than the raw pre-digestion waste can decrease to approximately 20%-30%, as the stream, resulting in less volatilization with digester engine generator set converting the methane in the effluent compared to un-digested manure biogas to electricity has efficiencies of 25-40%. application. In our temperate climate, the anaerobic digester is The main difference between the fertilizer value of only slightly exothermic, meaning the heat released anaerobically digested waste and either undigested by the microorganisms through their metabolic or aerobically digested waste lies in the form of processes is not large, and therefore, the digester ammonia (NH3) or, more precisely, the ammonium will need extremely good insulation, additional + ion (NH4 ).
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