Retrofitting municipal sewage treatment plants using an innovative membrane-bioreactor system

Most of the municipal sewage treatment plants (MSTPs) being operated in Korea were designed for wastewater with average concentrations of municipal sewage. However, they are in fact being operated at low organic loading. Thus, most of the secondary small-scale sewage treatment plant cannot satisfy the effluent standard. An innovative ultrafiltration membrane-bioreactor (MBR) system was proposed to retrofit the existing MSTP. The objective of this study was to investigate the feasibility of applying ultrafiltration MBR to the conventional activated sludge process in order to improve the treatment efficiency of the plant. The tested membrane was a UF of 50 kDa and an effective surface area per module was 8.4m(2). The UF was operated with about 2m/s of the crossflow velocity. In preliminary experiments, the MBR was more effective than the direct ultrafiltration of sewage for the enhancement of effluent quality and the preventing fouling. In the crossflow membrane-bioreactor (CFMBR) system, the COD concentration of the effluent was maintained at less than 10 mg/L, regardless of the influent COD concentration. The total nitrogen and total phosphorus of the effluent were 5 mg/L and 1 mg/L, respectively. The CFMBR system could completely remove pin-flocs which occurred in the sedimentation tank due to the low organic loading. The water qualities of the effluent obtained from all the experiments in this study were satisfied to the guidelines for the effluent discharge. Stable operation of a CFMBR, as judged by the effluent COD, SS and T-N concentrations, was achieved with low organic wastewater for 40 days without chemical washing.