A Membrane Bio Reactor (MBR) is sewage treatment plant based upon the robust activated sludge process where a membrane is used to separate the final effluent from the mixed liquor.

The membrane is suspended in the mixed liquor and the product is drawn through the membrane with either a pump or in some circumstances a siphon arrangement.

The membrane has a very small pore size of less than 0.1 microns which means that the effluent is of an outstanding quality and is typically less than 2mg/l BOD and 1mg/l suspended solids. Unlike conventional works where the works has to be operational for several days before the final effluent achieves
“discharge consent” for carbonaceous treatment only an MBR will achieve this consent within 24 hours. The outstanding effluent quality enables the effluent to be re-used or may permit discharge into sensitive receiving waters.

The final effluent is better than a conventional activated sludge plant comprising Final Settling Tanks, Tertiary Sand Filters and Ultraviolet Disinfection in terms of suspended solids and BOD.

These items of plant require considerable area and it comes as no surprise that an MBR requires significantly less land than a conventional nitrifying works. In some cases only half the land area is required.

Membrane BioReactors are particularly attractive when treating high strength wastes as the membrane area is determined on the hydraulic throughput and not on the biological load. When comparing a typical effluent with a high strength waste, the aeration tank will increase as the biological load increases the membrane area and consequently a significant part of the capital cost remains fixed. An MBR can be used to relieve overloading of existing sewage works. For example if an MBR was installed to treat high strength sludge liquors rather than return these to the works inlet the biological load on the aeration system can be greatly reduced. Alternatively if the Final settlement Tanks were replaced with by an MBR the existing works could operate at higher MLSS and higher throughput could be achieved.

Typically an MBR operates with a Mixed Liquor concentration of 8,000 to 15,000mg/l and consequently operates at a much longer “Sludge Age” or Solids Residence Time (SRT) than a conventional works which results in a significant reduction in sludge production. In some cases sludge production can be 50% of a conventional activated sludge process and the sludge produced has a lower organic content.

This long “Sludge Age” also ensures that the effluent is always fully nitrified. De-nitrification can be achieved with the construction of an anoxic zone before the aeration tank which will also reduce the aeration requirement and consequently the power absorbed. The long sludge age results in a stable sludge with no offensive odours.

The membrane is a “barrier” which prevents the loss of the biomass within the aeration tank so nature of the biomass and its ability to settle is not a concern when operating a MBR. The biomass can not be lost in the event that the sludge does not settle thereby preventing pollution in the receiving waters.

This high mixed liquor concentration serves as a buffer and the final effluent quality is maintained even if there are significant variations in the feed BOD. Furthermore if the biomass receives a shock load resulting in partial destruction of some of the biomass the MBR will continue to produce the high quality effluent and as the bacteria are retained by the membrane the process will recover faster than a conventional works.

A MBR can operate successfully where there are significant seasonal variations in load, for example at a tourist resort. Either the sludge age can be increased with reducing biological load in winter or the MLSS concentration can be allowed to increase as the load increases.

Whilst no single treatment process is the answer to all the problems a MBR can be an attractive alternative to conventional processes for all sizes of plant when land available for construction of a sewage works is restricted, sludge production should be minimised, where high quality effluent is demanded or when the feed conditions are variable.