Domestic wastewater treatment by a submerged membrane bioreactor with gravitational filtration

This study examined practical performance of a submerged membrane bioreactor with gravitational filtration, using a pilot-scale plant (volume of the bioreactor: 3.12 m3) and raw domestic wastewater. Treated water was filtered through flat microfiltration membrane modules (polyethylene; pore size 0.4 μm) by the pressure head of bulk solution. A combined aerobic/anaerobic (single-sludge) system was used to enhance nitrogen removal. Operation was continued for 491 days. From the 267th to the 324th day of operation, seasonal inflow fluctuation was simulated by increasing inflow by 1.5- to 3-fold. The obtained results were summarized in the following four points. (1) Membrane filtration was able to be continued for the first 371 days without membrane washing and was still stable after the washing. (2) Average removal ratios of BOD, TOC, SS, total nitrogen, total phosphorous and coliform bacteria were 99, 93, 100, 79, 74% and 6-log units, respectively. (3) Experiments on simulated inflow fluctuation showed that the system could cope with a short-term fluctuation of inflow (up to threefold) without deteriorating filtration performances significantly. (4) Average power consumption by the experimental plant was 2.4 kWh per 1 m3 of treated water.