The Effects of Dissolved Oxygen Concentration and Biological Solids Retention Time on Activated Sludge Treatment Performance

A bench scale treatment system with dissolved oxygen (DO) control was used to determine the effects of DO concentration and biological solids retention time (BSRT) on treatment performance using the activated sludge process. The four reactors, operating at BSRTs of 20, 10, 5, and 2 days, were fed settled municipal wastewater collected from the Kuwahee wastewater treatment plant in Knoxville, TN. The DO was maintained at different set points in each reactor ranging from 4.0 to 0.2 mg/L.

Experimental results indicate that carbon treatment performance improved, on average, with increasing BSRT but DO had little effect on carbon oxidation. Sludge volume index (SVI) and effluent suspended solids (ESS) values also indicated that BSRT not DO concentration, affected sludge settling. Complete nitrification occurred in the 20, 10, and 5 day BSRT reactors under excess DO conditions ( ³2.0 mg/L). Nitrification was unaffected at a DO as low as 0.5 mg/L for the two longest BSRTs; however, nitrite buildup occurred in the 5 day BSRT during operation at 0.5 mg/L DO suggesting that nitrite oxidation can limit nitrification when insufficient DO is present. A 2 day BSRT was found to be insufficient for complete nitrification at all DO levels.

Kinetic coefficients for the nitrifiers were determined for Knoxville’s municipal wastewater. The yield, decay coefficient, maximum substrate utilization rate, maximum growth rate, substrate half saturation coefficient, and oxygen half saturation coefficient were found to be 0.33 mg VSS/mg N, 0.17 day-1, 2.2 mg N/mg VSS-day, 0.75 day-1, 0.25 mg/L NH4+, and 0.92 mg/L O2 respectively. These values are within a published range identified in the literature.