Anaerobic Digestion

Although each of the alternative sludge digestion and composting processes have its own degree of metabolic complexity and operational peculiarities, anaerobic systems are widely regarded as the most difficult to maintain given the interlinked sensitivity of their biochemical pathways. Furthermore, where the aerobic options (i.e., aerobic digestion and composting) involve only an oxidative breakdown of the sludge solids and a single separation (liquid-solid) step, the anaerobic digestion process entails a sequential series of complex metabolic pathways and concluding separations of gas, liquid, and solid products. As a result, this digestion strategy has historically tended to require more attention and care while being more prone to upset. Conversely, though, there are a number of potential benefits to be gained with this technology. Most notably, the anaerobic digestion process tends to have a far lower energy demand (since it does not require aeration); at the same time, it can be used to generate a useful, energy-rich by-product in the form of methane gas that is created reductively by these metabolic reactions.

The underlying biochemical mechanism with anaerobic digestion is that of fermentation rather than the respiration reactions maintained in most aerobic wastewater treatment systems, as discussed previously. In either case, these options again involve a balanced set of oxidative and reductive (i.e., redox) reactions that are linked to electron donor and acceptor compound conversions, respectively. With fermentation reactions, the electron donor and acceptor compounds are typically both organic in form, although there are also critical pathways in which inorganic hydrogen and carbon dioxide gas species can fill the associated roles (Zehnder, 1988). Yet another unique feature of fermentation is that of a metabolic flexibility which allows a single organic compound to fill both roles, not only accepting electrons but also serving as the electron donor. For example, during the fermentative anaerobic transformation of acetate, this single substrate can simultaneously be oxidized to CO2 (CH3COO~ + 2H2O ! 2CO2 + 4e+ + 7H+) and reduced to methane, CH4 (CH3COO~ + 4e+ + 9H+ ! 2CH4 + 2H2O).

Figure 16.40 provides an overview of the three progressive metabolic steps that take place during the complete anaerobic digestion process: hydrolysis, acidogenesis, and methanogenesis. During the first, hydrolysis step, macromolecular organic compounds entering the system are transformed hydrolytically from large, and in many instances solid-phase macromolecular materials (e.g., cellulose, grease, protein, microbial cells) into their smaller, soluble building blocks (e.g., amino acids released from protein, carbohydrates from polysaccharides, fatty acids from lipids and fats). During subsequent fermentation maintained by the anaerobic digestion process, one fraction of these hydrolyzed organics will eventually be oxidized to carbon dioxide, another fraction will be reduc-tively converted to methane, and a third, comparatively smaller fraction will be assimilated anabolically into a new anaerobic cell mass.

The two fermentation reactions following hydrolysis sequentially are acidogenesis and methanogenosis. Acidogenesis involves the formation of acid products by which

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