Removal of pathogens by membrane bioreactor : removal efficiency, mechanisms and influencing factors

A membrane bioreactor (MBR), a unit process consisting of a biological reactor with suspended biomass and a microfiltration membrane, is finding its wide application to replace the conventional activated sludge processes in wastewater treatment. MBRs may remove both bacteria and viruses from wastewater, in addition to its main function of the coupled biological conversion and solid-liquid separation, thereby potentially providing a compact mean of treatment to provide high quality treated sewage effluent for reuse proposes. This research focuses on investigating the performance of a submerged MBR in pathogen removal, the removal mechanisms, and the affecting factors and evaluating the feasibility of utilizing the submerged MBR as a pre-disinfection process. A bench-scale study was conducted using a 0.4-μm hollow-fiber membrane module submerging in an aeration tank. Bacteriophage MS-2 was selected as the indicator due to its small size of 20-25 nm and its high resistance to disinfection. This study demonstrated that the virus removal was contributed by physical filtration by the membrane itself, biomass activity in the aeration tank and biofiltration achieved by the biofilm developed on the membrane surface. The membrane and the suspended biomass individually removed 0.4±0.1 log (or 50 to 68% removed) and 0.8±0.2 log MS-2, respectively. After submerging the membrane into the biomass, the removal increased to 1.5 log in 9 hours (one hydraulic retention time) and it further increased to 3 log in 3 weeks. The roles and contributions of the suspended biomass and the attached biofilm to remove phages are highly dependent on the mixed liquor suspended solid (MLSS) concentration and the solid retention time (SRT). Operating at a lower MLSS level or a longer SRT yields faster and higher phage removal. The observation of effluent flux and trans-membrane pressure gives evidence that phage removal in the MBR is highly susceptible to the biological factors rather than the physical ones