With development of old filtration methods and their evolution to membrane processes, using of semipermeable membranes in waste water treatment industries has been increases significantly, so that with combination of ordinary biotreatment processes with membranes, new methods have been developed. Sizes of these filters are in the range of 0.02-0.08 micrometer and ordinary settling tanks, disinfection and filtration have been replaced with combined processes. This combined biologic process is known as MBR.

Most important applications of MBR method are using of treated effluent in large buildings, in-house waste water treatment in low people societies and treatment of industrial effluent.

MBR advantages are:

1- Larger volumetric loading less hydraulic residence time at the reactor(4-8 hr)

2- Increasing of cellular residence time and producing less sludge

3- Operation in the case of low DO and possibility of simultaneous nitrification and denitrification

4- Highest quality of treated waste water in turbidity, bacteria, TSS and BOD

5- Lower required area due to elimination of settling tank, chlorination, and all filtration steps and decreasing aeration tank.

6- Possibility of upgrading of in-use active sludge plants