Design and operation considerations for wastewater treatment using a flat submerged membrane bioreactor

Abstract Combining a flat submerged membrane with a biological reactor was investigated for synthetic wastewater treatment of high concentration. The increase of suction pressure as a result of fouling of the submerged membrane, nitrification and chemical oxygen demand (COD) removal efficiencies were monitored during 4–9 months operation. In order to find a proper geometry of the reactor with respect to the long-term stability of membrane filtration, three types of membrane bioreactor with a different ratio of cross-sectional area of the downcomer to the riser were operated and the time profiles of suction pressure increase were compared. While maintaining a filtration flux at 12–16 l m −2 h −1 , a higher ratio of downcomer to riser area gave a better filtration stability of submerged membrane over several months of operation. A sharp increase of suction pressure was also observed when sludge bulking occurred during normal operation. Recovery of initial pure water flux for the membrane, which has been used over 6 months, was possible after 96 h of chemical cleaning. Under operating conditions with 12–16 h hydraulic retention time and 8000–16 000 mg l −1 sludge concentration in the bioreactor, COD removal efficiency was above 98% for 900–1600 mg l −1 influent COD and nitrification above 95% with 50–600 mg l −1 total nitrogen fed to the membrane bioreactor.