By Deirdre S. Blanchfield

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Extra info for Environmental Encyclopedia, Fourth Edition volume 1 and 2

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The remaining sludge is a mineralized sludge, with remaining organic materials comprised of cell walls and other cell fragments that are not readily biodegradable. The advantages of using aerobic digestion, as compared to the use of anaerobic digestion, include (1) simplicity of operation and maintenance; (2) lower capital costs; (3) lower levels of biochemical oxygen demand (BOD) and phosphorus in the supernatant; (4) fewer effects from upsets such as the presence of toxic interferences or changes in loading and pH; (5) less odor; (6) nonexplosive; (7) greater reduction in grease and hexane solubles; (8) greater sludge fertilizer value; (9) shorter retention periods; and (10) an effective alternative for small wastewater treatment plants.

In a continuous flow system, an aeration tank is utilized, followed by a settling tank. Aerobic sludge digestion is usually used only for biological sludges from secondary treatment units, in the absence of sludges from primary treatment units. The most commonly used application is for the treatment of sludges wasted from extended aeration systems (which is a modification of the activated sludge system). Since there is no addition of an external food source, the microorganisms must utilize their own cell contents for metabolic purposes in a process called endogenous respiration.

Organisms capable of switching between aerobic and anaerobic existence are found in forms of fungi known as yeasts. The ability of an organism to function both aerobically and anaerobically increases the variety of sites in which it is able to exist and conveys some advantages over organisms with less adaptive potential. Microbial decay activity in nature can occur either aerobically or anaerobically. Aerobic decomposers of compost and other organic substrates are generally preferable because they act more quickly and release fewer noxious odors.

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