The treatment of activated sludge in membrane bioreactors (MBR) is an exceptionally compact and efficient method for purifying biological water. Membrane bioreactors are employed when wastewater treatment plants need to be highly compact due to limited space and/or when a very high quality of effluents is required, or when it is reused as service water, e.g., for irrigation.

In this method of activated sludge treatment, the sludge is not separated in the secondary clarifier and recirculated; instead, it is retained in the aeration tank through membrane filters. The biomass concentration in the aeration tank is increased three to four times, making the tank sizes small.