The end products of anaerobic degradation are gases, mostly methane (CH4), carbon dioxide (CO2), and small quantities of hydrogen sulfide (H2S) and hydrogen (H2). The process involves two distinct stages: acid fermentation and methane fermentation.
In acid fermentation, the extracellular enzymes of a group of heterogenous and anaerobic bacteria hydrolyze complex organic waste components (proteins, lipids, and carbohydrates) to yield small soluble products. These simple, soluble compounds (e.g., triglycerides, fatty acids, amino acids, and sugars) are further subjected, by the bacteria, to fermentation, ^-oxidations, and other metabolic processes that lead to the formation of simple organic compounds, mainly short-chain (volatile) acids (e.g., acetic [CH3COOHj, propionic [CH3CH2COOHj, butyric [CH3-CH2-CH2-COOHj) and alcohols. In the acid fermentation stage, no COD or BOD reduction is realized since this stage merely converts complex organic molecules to short-chain fatty acids, alcohols, and new bacterial cells, which exert an oxygen demand.
In the second stage, short-chain fatty acids (other than acetate) are converted to acetate, hydrogen gas, and carbon dioxide—a process referred to as acetogenesis. Subsequently, several species of strictly anaerobic bacteria bring about methanogenesis—a process in which hydrogen produces methane from acetate and carbon dioxide reduction. In this stage, the stabilization of the organic material truly occurs. Figure 7.28.1 shows the two stages of anaerobic treatment as sequential processes; however, both stages occur simultaneously and synchronously in an active, well-buffered system.
The main concern of a wastewater treatment facility in operating an anaerobic system is that the various bacterial species function in a balanced and sequential way (Forster 1985). Hence, although other types of microorganisms may be present in the reactors, attention is fo-cussed mostly on the bacteria.
The major groupings of bacteria, as numbered in Figure 7.28.1, and the reactions they mediate are as follows (Pavlostathis and Giraldo-Gomez 1991): (1) fermentative bacteria, (2) hydrogen-producing acetogenic bacteria, (3) hydrogen-consuming acetogenic bacteria, (4) carbon-dioxide-reducing methanogens, and (5) aceticlastic methano-gens. Two common genera of aceticlastic methanogens are Methanothiix and Methanosarcina; and species from the Methanobacterium group are commonly known to produce methane by hydrogen reduction of carbon dioxide.
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