Aerators Enhance Aerobic Digestion Process

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Courtesy of S&N Airoflo Inc.

The city of Eden,N.C.,operates an extended aeration process designed for an average flow rate of 13.5 mgd with a mixed liquor suspended solids concentration that typically checks in about 6,000 mg/l.

This high biomass concentration is necessary to provide a well-settling sludge and good effluent quality. Further, the plant influent is approximately 40% domestic wastewater and 60% industrial wastewater. The industrial component is dominated by textile facilities.The Eden Wastewater Treatment Plant (WWTP) generates about 20,000 lb per day of waste activated sludge solids, which are aerobically digested prior to storage in sludge lagoons and ultimate land application.

For many years, the facility has had difficulty maintaining adequate dissolved oxygen levels and adequate mixing in its aerobic digester. However, this all changed when the plant installed four new floating brush rotor aerators in May 2004.

Aerobic digestion is a sludge stabilization process designed to convert waste activated sludge solids to carbon dioxide,water, residual cell debris and other end products.

The oxidation of cellular material is referred to as endogenous respiration and is typically the dominant reaction in aerobic digestion.The detention time in the digester is normally about 20 days to achieve satisfactory solids stabilization.

Additionally, the primary functions of an aerobic digester are to reduce odors, reduce biodegradable solids content, to prepare the sludge for ultimate disposal on land consistent with Federal 503 regulations.Aerobic digestion stabilizes the sludge solids more rapidly than anaerobic digestion, even though a less complete breakdown of cells is typically achieved. Oxygen can be supplied by mechanical aerators or by diffused aeration.

At the Eden WWTP, vertical surface aerators have been used for many years. Plant personnel have had serious
concerns because of operating problems as well as poor process performance. Prior to installation of the new floating brush rotor aerators, the plant had 145-hp of vertical aeration equipment in the aerobic digester.The vertical aeration equipment consisted of three 40-hp units and one 25-hp unit.

In aerobic digestion, it is desirable to maintain at least 1.0 mg/l dissolved oxygen level to provide an efficient aerobic environment.Typical dissolved oxygen concentrations were 0.0 mg/l, which led to odor problems and poor aerobic digester performance.

In addition, it is extremely important to provide adequate mixing to keep the waste solids in suspension.Typically, one-third of the aerobic digester was never mixed, creating dead zones detrimental to aerobic digestion. Moreover, it usually required two to three days of settling to facilitate efficient decanting of supernatant prior to wasting sludge to the sludge storage lagoons. Consequently, the aerobic digestion process performed below expectations and created operating problems for the WWTP staff.

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