In the immersed Membrane Bioreactor (iMBR) configuration, the filtration element is installed in either the main bioreactor vessel or in a separate tank. The modules are positioned above the aeration system, fulfilling two functions, the supply of oxygen and the cleaning of the membranes. The membranes can be a flat sheet or tubular or a combination of both and can incorporate an online backwash system which reduces membrane surface fouling by pumping membrane permeate back through the membrane. In systems where the membranes are in a separate tank from the bioreactor, individual trains of membranes can be isolated to undertake cleaning regimes incorporating membrane soaks, however, the biomass must be continuously pumped back to the main reactor to limit mixed liquor suspended solids concentration increases. Additional aeration is also required to provide air scouring to reduce fouling. Where the membranes are installed in the main reactor, membrane modules are removed from the vessel and transferred to an offline cleaning tank. Usually, the internal/submerged configuration is used for larger-scale lower strength applications. To optimize the reactor volume and minimize the production of sludge, submerged membrane bioreactor systems typically operate with mixed liquor suspended solids concentrations comprised between 12000 mg/L and 20000 mg/L, hence they offer good flexibility in the selection of the design Sludge retention time. It is mandatory to take into account that an excessively high content of mixed liquor suspended solids may render the aeration system less effective; the classical solution to this optimization problem is to ensure a concentration of mixed liquor suspended solids which approaches 10.000 mg/L to guarantee a good mass transfer of oxygen with a good permeation flux. This type of solution is widely accepted in larger-scale units, where the internal/submerged configuration is typically used, because of the higher relative cost of the membrane compared to the additional tank volume required.
Immersed MBR has been the preferred configuration due to its low energy consumption level, high biodegradation efficiency, and low fouling rate compared to side stream membrane bioreactors. In addition, iMBR systems can handle higher suspended solids concentrations, while traditional systems work only with suspended solids concentrations between 2.5-3.5, iMBR can handle concentrations between 4-12 g/L, an increase in range of 300%. This type of configuration is adopted in industrial sectors including textile, food & beverage, oil & gas, mining, power generation, pulp & paper.