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Membrane bioreactor (MBR) is the combination of a membrane process like microfiltration or ultrafiltration with a suspended growth bioreactor, and is now widely used for municipal and industrial wastewater treatment with plant sizes up to 80,000 population equivalent. 
MBRs are commonly designed for nitrogen removal, using membranes for liquid‐solids separation following the anoxic and aerobic zones instead of conventional clarification.  Membranes can be submersed in the biological reactor or located in a separate stage or compartment.  Low‐pressure membranes (ultrafiltration or microfiltration) are commonly used.  Systems can be pressure driven or vacuum.  All systems use an air scour technique to reduce build up on the membranes.
Membrane materials are either organic polymers or inorganic materials such as ceramics.  They are designed in modular units and are typically configured as either hollow fiber bundles or plate membranes.
For biological nutrient removal applications, the design SRTs and design principles for MBR systems are similar to those used for systems with secondary clarifiers. One of the main differences is that the MBR systems operate at a higher MLSS concentration which results in smaller tanks and smaller space requirements.  In addition, membrane separation provides for greatly reduced TSS in the effluent,
typically below 1.0 mg/L, and hence slightly greater removal of nitrogen and phosphorus.  Operational issues include potential for membrane biofouling and increase pumping costs.