The growing interest in the concept of membrane biology for industrial wastewater treatment is based on the fact that, by using it, several tasks can be solved at the same time and several very advantageous aspects for biological wastewater treatment ensue. With membrane techniques, unlike conventionally used sedimentation, freely selectable operationally safe retention and upgrading of biomass with significantly less space is possible. Beyond that, and in addition to solid-particle and germfree processes, the wastewater and substrate residence times can be decoupled.