Oxygen Transfer Studies in the Activated Sludge Process

The major goals of this research were to determine the impact of mean cell residence time (MCRT) on the alpha coefficient (a) and how low dissolved oxygen (DO) concentrations in the aeration basin affect effluent quality. Two, bench-scale activated sludge reactors were operated for 8 weeks. Daily, 18 L of synthetic wastewater were prepared for each reactor by mixing tap water with bacto-peptone,sodium bicarbonate, ammonium sulfate, and other salts. Parameters measured on the influent and effluent included: chemical oxygen demand, total nitrogen, ammonia nitrogen, alkalinity, total phosphorus, and pH. Other effluent parameters monitored included: nitrite, nitrate, oxygen transfer coefficient (K(L)a), alpha coefficient and beta coefficient (beta), and suspended solids. Reactor 1 (R-1) was operated at MCRTs of 20.3 +/- 1.5 and 14.1 +/- 0.5 days; and Reactor 2 (R-2) at 17.7 +/- 2.3 and 12.8 +/- 0.8 days, respectively, during the first and second runs. A non-linear regression program was used to determine KLa. Water quality analyses were performed in accordance to HACH procedures. R-1 performed better than R-2 achieving average ammonia removals of 91.2% and 98.1% during the first and second runs; while R-2 achieved ammonia removals of 69.2% and 25.3% respectively during the first and second runs. Alpha coefficients for each run increased as MCRT increased. Oxygen uptake rates were much higher in R-2 than for R-1 and increased as MCRT increased. The nitrogenous oxygen demand of the synthetic wastewater fed to R-2 was approximately 15 g/day compared to 1.6 g/day for R-1 while the same air rate was supplied to each reactor resulting in low DO concentrations and a nitrite buildup in R-2.