Performance evaluation of an anoxic/oxic activated sludge system: Effects of mean cell residence time and anoxic hydraulic retention time

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Abstract
A laboratory investigation has been undertaken to asses the effects of two operating parameters, mean cell residence time (MCRT) and anoxic hydraulic retention time (HRT), on the performance of an anoxic/oxic activated sludge system. The performance of the system was evaluated in terms of its COD, nitrogen, and biomass characteristics. An activated sludge system is capable of producing a better effluent, in terms of COD and nitrogen characteristics, when it is operated in an anoxic/oxic fashion. A longer MCRT and an adequate anoxic HRT are desirable in the operation of an anoxic/oxic activated sludge system. For the wastewater used in this investigation, the anoxic/oxic unit was capable of producing an effluent with the following characteristics when it was operated at MCRT = 20 days, total system HRT = 10 h, and anoxic HRT = 3–5 h: COD = 15 mg/L; VSS = 10 mg/L; TKN = 1.30 mg/L; NH3− N = 0.60 mg/L; and NO2+ NO3− N = 5.0 mg/L. A uniform distribution of biomass is achievable in an anoxic/oxic activated sludge system because of the intensive recirculation/convection maintained. The provision of an anoxic zone in the aeration tank promotes a rapid adsorption of feed COD into the biomass without an immediate utilization for cell synthesis. This, in turn, results in a high microbial activity and a lower observed biomass yield in the system. A tertiary treatment efficiency is achievable in an anoxic/oxic activated sludge system with only secondary treatment operations and costs. A conventional activated sludge system can be easily upgraded by converting to the anoxic/oxic operation with minor process modifications.