In side stream membrane bioreactor technology, the filtration modules are outside the aerobic tank, hence the name side-stream configuration. Like the immersed or submerged configuration, the aeration system is also used to clean and supply oxygen to the bacteria that degrade the organic compounds. The biomass is either pumped directly through several membrane modules in series and back to the bioreactor or the biomass is pumped to a bank of modules, from which a second pump circulates the biomass through the modules in series. Cleaning and soaking of the membranes can be undertaken in situ with the use of an installed cleaning tank, pump, and pipework. The quality of the final product is such that it can be reused in process applications due to the filtration capacity of the micro- and ultrafiltration membranes.
Usually, the external/side stream configuration is used for smaller scale and higher strength applications; the main advantage that the external/side stream configuration shows is the possibility to design and size the tank and the membrane separately, with practical advantages for the operation and the maintenance of the unit. As in other membrane processes, a shear over the membrane surface is needed to prevent or limit fouling; the external/side stream configuration provides this shear using a pumping system, while the internal/submerged configuration provides the shear through aeration in the bioreactor, and there is an energy requirement to promote the shear by pumping. In this configuration fouling is more consistent due to the higher fluxes involved.