Biogas Technology

As previously mentioned, many different organic sources may be used in biogas generation. Although most of the natural organic material, except lignin (Sleat and Mah 1987; Petersson and Wellinger 2009), can be used as a biogas precursor, the most important sources are those processes that are directly involved with waste management, mainly sewage sludge, landfills, agriculture wastes, manure, and industrial food wastes (Fig . 9.1).

The anaerobic digestion process, responsible for biogas production, is carried out in a closed reactor called a digester. Different designs of this reactor type have been studied and optimized to suit industrial operations, but the final configuration of the digester depends on many factors, viz., characteristics and concentration of the raw organic feed, the scale of application and energy requirements, among others (Chynoweth 1987).

Inside the digester, a mixed consortium of both anaerobic and facultative microorganisms will carry out the transformation of organic compounds into methane. This process, illustrated in three stages, as hydrolysis, acid forming and methanogenesis, is shown in Fig. 9.2. In the first stage, complex insoluble polymers (proteins, lipids and carbohydrates), are converted to simple soluble compounds (aminoacids, fatty acids and monosaccharides). The transformation of these complex substrates is carried out by anaerobic or facultative hydrolytic bacteria. During the acid forming stage, both anaerobic and facultative bacteria will degrade the simple substrates, originated in the hydrolysis step, through a series of fermentative processes. The main products

Biogas Bacteria Methane

Fig. 9.1 Schematic of biogas production, sludge storage and supply unit. After pre-conditioning the raw material (organic waste), microbial conversion of organic matter to methane and other useful end-product is achieved in the biogas plant, under well-optimized environmental conditions. This process is anaerobic and is quintessentially carried out by the action of various groups of anaerobic bacteria

Fig. 9.1 Schematic of biogas production, sludge storage and supply unit. After pre-conditioning the raw material (organic waste), microbial conversion of organic matter to methane and other useful end-product is achieved in the biogas plant, under well-optimized environmental conditions. This process is anaerobic and is quintessentially carried out by the action of various groups of anaerobic bacteria of fermentation are organic acids, alcohols, carbon dioxide, hydrogen, and compounds originated from proteins degradation, such as sulfides and ammonia. However, not all the organic acids can be used as substrate by methanogenic bacteria, and hence they have to be degraded into other compounds like acetate (Gerardi 2003). Acetogenic microorganisms are involved in the production of acetate from organic acids and alcohols (Eq. 9.1). Acetate can also be produced from the reaction of carbon dioxide and hydrogen originated from previous steps (Eq. 9.2).

Organic acids + alcohols ® Acetate (9.1)

Finally, the methanogenic bacteria will use these final products in order to generate methane, using different paths [Eqs. 9.3-9.5].

Fig. 9.2 Different stages of the anaerobic digestion (Gerardi 2003). The transformation of these complex substrates to methane is carried out by anaerobic or facultative hydrolytic bacteria. The methane thus produced can be burned to produce both heat and electricity, usually with a reciprocating engine

Methanogenesis _i_
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