Anaerobic Digestion And Biodegradation

Anaerobic digestion is widely used in wastewater treatment to degrade the solids from primary and secondary treatment. Anaerobic processes are also applied in the treatment of industrial effluents and in bioremediation of contaminated soil and ground water. Although environ mental microorganisms have been functioning and serving the needs of humans for a very long time, the commercialization and engineering of this aspect of bioprocess technology was started after the discovery of microorganisms. Wastewater treatment is the second oldest bioprocess technology after food processing.

Anaerobic degradation processes depend on the electron acceptors that are present. Stouthamer (13) points out that when several electron acceptors are present, the microorganism selects the electron acceptor that yields the largest amount of energy by repressing the formation of reductase enzymes for the other electron acceptors. Table 6 gives the standard free energy changes for oxidation of acetate with oxygen, nitrate, sulfate, and water as electron acceptors, respectively. When oxygen is present, aerobic processes are observed and anaerobic processes are generally absent unless there are spatial regions or time periods where oxygen is absent. Nitrate is the electron acceptor of choice when oxygen is absent, and when nitrate and oxygen are both absent, sulfate is the favored electron acceptor. Methane production, which occurs when water is the electron acceptor, is generally inhibited if sulfate is present. Stouthamer points out that other electron acceptors may participate, including other oxidized forms of nitrogen and fumarate.

Many bacteria have been found that can use nitrate as the terminal electron acceptor. The first product of nitrate respiration or nitrate reduction is formation of nitrite, which can be further used as an electron acceptor by many microorganisms. In denitrification, the end product of further reduction is nitrogen gas; however, in other cases the end product is ammonia. Stouthamer (13) reviewed the en-zymology and bioenergetics of nitrate reduction.

Sulfate-reducing bacteria include Desulfovibrio, Desul-fobacterium, Desulfotomaculum, Desulfococcus, Desulfos-arcina, Desulfomonile, Desulfonema, Desulfoarculus, De-sulfobulbus, Desulfobotulus, and Desulfobacula (19). For these genera, many species have been described in the literature (13,14,19). These organisms have great diversity in biochemical properties, and they can completely oxidize a variety of organic substrates.

Methanogenic bacteria are widely found under anaerobic conditions in nature. Methane is produced in soil, landfills, anaerobic digesters, rumens of animals, and many other places. In anaerobic digestion, methanogenic bacteria are found together with hydrolytic fermentative microorganisms and syntrophic acetogenic bacteria. The hydrolytic fermentative organisms, which include eubac-

Table 5. Estimated Values of me and gmax for Selected Experiments

me

gmax

Organism

Substrate

Product

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