In an MBR system, the membranes are submerged in an aerated biological reactor. The membranes have porosities ranging from 0.035 microns to 0.4 microns, which is considered between micro and ultrafiltration. This level of filtration allows for high quality effluent to be drawn through the membranes and eliminates the sedimentation and filtration processes typically used for wastewater treatment.

Because the need for sedimentation is eliminated, the biological process can operate at a much higher mixed liquor concentration. This dramatically reduces the process tankage required and allows many existing plants to be upgraded without adding new tanks. To provide optimal aeration and scour around the membranes, the mixed liquor is typically kept in the 1.0-1.2% solids range, which is 4 times that of a conventional plant.

It’s generally acknowledged that membrane bioreactors have a number of advantages over other wastewater technologies although this is not to say they are always the right choice and there are a number of factors to take into consideration. But, in general terms, MBRs:

• High-quality effluent: The small pore size (<0.5 µm) of the membrane means that the treated effluent is of very high clarity and significantly reduced pathogen concentration compared with the conventional process. MBR processes provide a substantially clarified and disinfected effluent of high enough quality to be discharged to sensitive receiving bodies or to be reclaimed for applications such as urban irrigation, utilities or toilet flushing. It is also of high enough quality for feeding directly to a reverse osmosis process.
• The retention of the solids in the reactor and the increase in SRT to generate higher biomass solids concentrations impact on the plant footprint. The increased concentrations mean that the same total mass of solids is contained in a smaller volume, so the footprint is smaller.

• Uncoupled SRT & HRT– significant advantage of the membrane bioreactor process is that the biological solids (mixed liquor or sludge) are completely retained in the bioreactor. This means the solids retention time (SRT) in the bioreactor can be controlled independently from the hydraulic retention time (HRT). This is different from the conventional process where the flocculant solids (‘flocs’) that make up the biomass have
  to be allowed to grow in size to the point where they can be settled out in the secondary clarifier. In a conventional process, the HRT and SRT are therefore coupled, since the floc size and therefore its settle-ability relate to the HRT: increasing the HRT increases the amount of time the flocs have to grow in the bioreactor, which then increases their settle-ability The SRT is a key parameter in biological wastewater treatment. Increasing
  the SRT (or the length of time the biological solids are retained in the bioreactor) increases the concentration of biological solids.
• Better biotreatment – The longer solids retention times tend to provide better biotreatment overall. The conditions encourage the development of the slower-growing micro-organisms, specifically nitrifiers. MBRs are especially effective at the biological  removal of ammonia (‘nitrification)