After pretreatment (depending upon the feedstock, vide infra), the biomass is subjected to biochemical transformation carried out by microorganisms (methanoar – chaea) in the absence of oxygen, that is, anaerobic digestion. The bacteria that do the work can be divided into three groups: (1) those that carry out the fermentation, (2) the acetogenic bacteria, and (3) the methanogenic bacteria. This mixed community of microbes carry out four essential steps in the anaerobic digestion of organic matter (Liu et al. 2010):
1. Hydrolysis. The feedstocks used for anaerobic digestion may vary widely in their chemical composition, from LCB to animal proteins and lipids. In order to liquify or solubilize the material for digestion, proteins must be broken down into amino acids, carbohydrates to sugars, and lipids to glycerol and fatty acids (Figure 8.35). This is all carried out by the diversity of enzymes present in microorganisms. This hydrolytic solubilization is the rate-determining step of the anaerobic digestion process.
2. Acidogenesis. In this step, the water-soluble hydrolysis products (sugars, amino acids, etc.) are fermented into a variety of smaller molecules, including ethanol, acetate, and low-molecular-weight (C3 and C4) fatty acids.
3. Acetogenesis. At this stage, acetogenic bacteria take butanoate, propanonate, lactate [CH3CH(OH)CO- ], succinate (-O2CCH2CH2CO-), and ethanol and
FIGURE 8.35 The various hydrolysis pathways of anaerobic digestion.
convert these substrates into acetate, H2, and CO2, the substrates for conversion into methane.
4. Methanogenesis. The final stage of anaerobic digestion takes place as meth – anogenic archaea cleave acetate to form methane and CO2 and use hydrogen gas and bicarbonate to generate additional methane (Willey et al. 2013).
The biochemistry of methanogenesis is another interesting example of nature’s use of metals. The greatly oversimplified summary is that acetate is cleaved to provide CO2 followed by reduction of CO2 into CH4 with hydrogen gas, all of which is catalyzed by metalloenzymes (Figure 8.36). Methyl coenzyme reductase is a nickel-containing enzyme common to all methanogenic pathways, for example, and all methanoarchaeal F420-dependent hydrogenases contain iron-sulfur clusters and nickel, with some also containing selenium (recall the discussion of hydrogenases in Section 188.8.131.52) (Ferry 2002). An abbreviated catalytic scheme in which these hydrogenases catalyze the transformation of CO2 to CH4 is given in Figure 8.37 with the key to the biomolecular structures in Figure 8.38. In step (1), methanofuran (a) is formylated with CO2 to generate formylmethanofuran (b), which then transfers the formyl group to tetrahydromethanopterin [H4MPT, (c)] in step (2), regenerating methanofuran. The formylated H4MPT (d) can then undergo an intramolecular ring closure (step 3) to form the dihydroimidazolium intermediate (e) in step (4). Reduction of this iminium ion takes place stereospecifically, presumably by hydride transfer from an F420-dependent enzyme. The next step, (5), is another F420-dependent catalysis to reductively cleave the N-C bond in f and set up the methane precursor (g). This compound delivers a methyl group to coenzyme M in step (6). A cobalt – porphyrin cofactor is an intermediary in the regeneration of H4MPT and transfer of the methyl group to coenzyme B (not shown in Figure 8.37). Oxidative coupling of coenzymes M and B with reduction of the methyl group to methane completes the methanogenesis cycle (step 7) (Ferry 2002).
The product—biogas—is mostly a mixture of methane and CO2 with other minor components such as hydrogen sulfide, ammonia, oxygen, nitrogen, and so on with other minor components such as hydrogen sulfide, ammonia, oxygen, nitrogen, and so on. CO2 makes up the vast majority of the biogas balance, thus reducing its energy value. Siloxanes (oligomeric Si-O-Si compounds) may be present in minute amounts in landfill gas from MSW disposal, since silicon-containing compounds are present in a large number of personal care products, pharmaceuticals, and so on (Mota et al. 2011).