A MBBR system is based on ...

A MBBR system is based on fixed film growth on the media and hence is not like an activated sludge system where you control the sludge age through return activated sludge and waste activated sludge. This actually poses a challenge to configure the system for A2O where you would normally recycle the sludge and the nitrified effluent back to an anoxic reactor and/or anaerobic reactor.  So the option is to install media in the anaerobic tank and the anoxic tank to retain sufficient biomass in these reactors.  However for effective P removal you need to carry the phosphate uptake bacteria from the anaerobic tank to the aerobic zone, which will not happen if they are attached to the media. The anoxic zone can however function as it is not necessary for the biomass to be exposed to aerated conditions. MBBR then relies on the existence of anaerobic, anoxic and aerobic conditions all in the aerobic reactor as you would find in a bio-filter for improved P removal which is sloughed off and carried through to the clarifier.  I do not believe this is as effective as a suspended growth system for P removal.  SRT is also then dependent on the organic loading and growth conditions for biomass growth and sloughing of the biomass layers. Hopefully researchers can offer another perspective as I do like the MBBR concept.