MBRs were designed with the separation device located external to the reactor and relied on high trans-membrane pressure (TMP) to maintain filtration. With the membrane directly immersed in the bioreactor, submerged MBR systems are usually preferred to side stream configuration, especially for domestic wastewater treatment. The submerged configuration relies on coarse bubble aeration to produce mixing and limit fouling. The energy demand of the submerged system can be up to 2 orders of magnitude lower than that of the side stream systems and submerged systems operate at a lower flux, demanding more membrane area.In submerged configurations, aeration is considered as one of the major parameters in process performance both hydraulic and biological.
Aeration maintains solids in suspension, scours the membrane surface and provides oxygen to the biomass, leading to a better biodegradability and cell synthesis.