Aerobic Sludge Digestion

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Wastewater treatment plants produce organic sludge as wastewater is treated; this sludge must be further treated before ultimate disposal. Sludges are generated from primary settling tanks, which are used to remove settable, particulate solids, and from secondary clarifiers (settling basins), which are used to remove excess biomass production generated in secondary biological treatment units.

Disposal of sludges from wastewater treatment processes is a costly and difficult problem. The processes used in sludge disposal include: (1) reduction in sludge volume, primarily by removal of water, which constitutes 97–98% of the sludge; (2) reduction of the volatile (organic) content of the sludge, which eliminates nuisance conditions by reducing putrescibility and reduces threats to human health by reducing levels of microorganisms; and (3) ultimate disposal of the residues.

Aerobic sludge digestion is one process that may be used to reduce both the organic content and the volume of the sludge. Under aerobic conditions, a large portion of the organic matter in sludge may be oxidized biologically by microorganisms to carbon dioxide and water. The process results in approximately 50% reduction in solids content. Aerobic sludge digestion facilities may be designed for batch or continuous flow operations. In batch operations, sludge is added to a reaction tank while the contents are continuously aerated. Once the tank is filled, the sludges are aerated for two to three weeks, depending on the types of sludge. After aeration is discontinued, the solids and liquids are separated. Solids at concentrations of 2–45 are removed, and the clarified liquid supernatant is decanted and recycled to the wastewater treatment plant. 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. 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.

Disadvantages include: (1) higher operating costs, especially energy costs; (2) highly sensitive to ambient temperature (operation at temperatures below 59°F [15°C]) may require excessive retention times to achieve stabilization; if heating is required, aerobic digestion may not be cost-effective); (3) no useful byproduct such as methane gas that is produced in anaerobic digestion; (4) variability in the ability to dewater to reduce sludge volume; (5) less reduction in volatile solids; and (6) unfavorable economics for larger wastewater treatment plants.

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Gaudy Jr., A. F., and E. T. Gaudy. Microbiology for Environmental Scientists and Engineers. New York: McGraw-Hill, 1980.

Peavy, H. S., D. R. Rowe, and G. Tchobanoglous. Environmental Engineering. New York: McGraw-Hill, 1985.