Effects of the DO of Nitrification Return Flow on the Nitrogen Removal of A/A-MBR Combined Process

Abstract

In this study, the traditional A/A/O process was combined with MBR to create the A/A-MBR combined process. A 2.0 m3/h pilot scale study on the combined process was carried out. The influence mechanism of dissolved oxygen (DO) in the return flow on the A/A-MBR combined process was analyzed and the suitable dissolved oxygen concentration in MBR was further optimized. The results show that the combined process has the characteristics of rapid start and stable operation. Its COD, NH4+-N, TN and TP removals were 82.1%~92.4%, 93.0~98.3%, 48.4~70.7% and 93.8~97.9%, respectively. The return flow with high concentration of DO in MBR seriously affected the denitrification rate. The DO concentration of return flow increased gradually from 2.0 mg/L to 5.0 mg/L, the denitrification rate continuously decreased, the denitrification rate in the first stage decreased from 2.52 mg NO3--N/(gVSS•h) to 0.34mg NO3--N/(gVSS•h). When the DO of nitrification return liquid ascended to 5 mg/L, the denitrification ability of activated sludge was severely inhibited, and its denitrification activity was even lost. DO were controlled between 4.0±0.5 mg/L by adjusting the aeration rate in the MBR, effluent TN could be stably maintained between 10.82-13.94 mg/L with 62.6% average removal rate. The effluent COD, NH4+-N, TN and TP stably qualified to t criteria of the first level A of China’ s "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant" (GB18918-2002). If the effective control of membrane fouling could be ensured, the DO in the MBR was controlled as much as possible fewer than 4.0 mg/L. This could decrease the inhibition of denitrification by high DO from the return flow and insure that effluent TN achieved the discharge standard.