[A comparison between a submerged membrane bioreactor and a conventional activated sludge process].

A comparison between a submerged membrane bioreactor (MBR) and a conventional activated sludge process (CAS) was carried out under similar operational conditions. MBR demonstrated a more stable and excellent effluent quality than CAS. Its effluent COD concentration was 55.5 mg/L on average, much lower than that of CAS (79.7 mg/L). Soluble microbial products accumulated in the MBR during the first 120 days in operation due to membrane interception of macromolecules, but these accumulated substances were degraded at last with microbial acclimation. No similar phenomenon was observed in the CAS system. Compositions of the CAS effluent, MBR supernatant and membrane permeate were found quite different. In the CAS effluent and MBR supernatant, both macromolecules with MW > 60,000 and small molecules with MW < 3,000 were dominant and macromolecules had a much larger occupation in the MBR supernatant. In the membrane permeate, however, small molecules with MW < 3,000 were the major component. The relatively small floc size in the MBR was proved favorable to improve oxygen transfer rate.