Among the various types of tertiary sewage treatment technologies, only MBR membrane technology can fully integrate the three functions of biological treatment, solid-liquid separation (secondary sedimentation tank) and tertiary treatment. Because the solid-liquid separation effect of the MBR membrane is much better than that of the secondary sedimentation tank (normally, the effluent SS of the ultrafiltration membrane is close to 0), membranes effectively retain all bacteria and macromolecular organics in the biological system. Due to the membrane retention properties, those bacteria with a slower digestion rate are enriched having longer retention time in sludge which in turn can improve removal rate of organics, nitrogen and phosphorus.