Comparative study on effect of mechanical mixing and ultrasound on the performance of a single up-flow anaerobic/aerobic/anoxic bioreactor removing CNP from milk processing wastewater

Abstract

In this study, an up-flow anaerobic/anoxic/aerobic sludge bed (UAAASB) bioreactor under two different mixing conditions, ultrasound-based mixing and mechanical mixing, was evaluated for simultaneous removal of nutrients from milk processing wastewater. The process performance of the two conditions was compared. The effects of three independent variables, including HRT (6, 12, 18 h), aeration time ratio (1, 1.5, 2) and MLSS content (4000, 7000, 10000 mg/l) were investigated. The central composite design (CCD) and response surface methodology (RSM) were applied to design the experiments, model and optimize the process. In order to evaluate the performance of both systems, ten responses were monitored. The process showed similar performance in terms of sCOD removal in both systems. 83.76% and 70.11%, respectively, were achieved as TN and TP removal efficiencies in the UAAASB mixed by the ultrasonic irradiation. While in the system with mechanical mixing, the maximum TN and TP removal were obtained 57.87% and 57.47%, respectively. The results proved that the high-frequency ultrasonic irradiation (1.7 MHz) had a positive effect on removing TN and TP, sludge settling velocity and effluent turbidity.