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Wastewater treatment is carried out using a combination of physical, chemical and biochemical processes. Of these three process types, the most important are the biochemical processes, achieved by harnessing specific bacteria contained in large reactors.
In membrane bioreactors (MBRs), membrane separation is used to retain the bacteria (or 'biomass') in the reactors, generating a clarified product.
Biological treatment is used for removing organic carbon and nutrients (N and P), and can be aerobic, anoxic or anaerobic according to whether or not oxygen is present and its chemical form (molecular oxygen or oxyanion).
Aerobic processes are the mostly widely employed, most commonly as the conventional activated sludge (CAS) process. The CAS is the most prevalent example of a 'suspended growth' biological process.
MBR processes can be configured as either immersed or sidestream, with the membrane used to retain the mixed liquor within the biological treatment stage while providing enhanced clarification. Membranes can also be employed as a separate 'polishing' step downstream of a conventional biological process, rather than integrated with it as an MBR.
Media processes in wastewater treatment employ pieces of material (media) onto which biofilms can form. The bacteria in the biofilms then provide the biological treatment.
Media processes are examples of 'fixed film' processes, where the media act as substrata (i.e. support) for the biofilm.
A ‘membrane-aerated biofilm reactor’ (MABR) is an aerobic biological treatment process where the membrane is both the aerator − delivering oxygen to the system − and the substratum for the fixed film. This configuration differs from the MBR, which is a suspended growth process employing the membrane for filtering the mixed liquor.
