ammonium is nitrified mostly to nitrate and most phosphates are removed

during the aerobic period (aeration), where the accumulated nitrate

is completely denitrified during the anoxic period (non-aeration), and phosphorus (P) is taken up.

P is found in wastewater as phosphates (orthophosphates, condensed

phosphates, organic phosphate fractions), and it can be eliminated either by

precipitation and/or adsorption, or by luxury uptake. Luxury uptake of P is accomplished by the introduction of an anaerobic phase in the wastewater treatment line ahead of the aerobic phase and recycling of sludge through the anaerobic and aerobic phase. Exposing mixed liquor to an anaerobic/aerobic sequence selects phosphate accumulating microorganisms (PAOs) due to a competition between PAOs and other aerobic organisms. This competition mechanism is based on a complete anaerobic uptake of the lower fatty acids by the polyP organisms (i.e., PAOs), which assures that in the aerobic phase, no fatty acids are left. The polyP organisms use the stored internal substrate during aerobic conditions while other aerobic organisms are lacking substrate. This process is usually referred to as the enhanced biological phosphorus removal (EBPR) process. EBPR process can be established in MBR treatment unit by operating it in intermittent aeration mode. 

Intermittently aerated MBR can achieve nitrogen and phosphorus removal

by a simultaneous nitrification and denitrification, P-uptake and P-release in

the same reactor in accordance with time cycle of aeration and non-aeration.

However, even though intermittent aeration was successful in removing nitrogen, P removal was difficult to achieve at a higher level  This is probably due to the inhibition by nitrate. In the anaerobic stage, nitrate reduces phosphate release, and in the aerobic stage it diminishes its uptake.

Denitrification has more capability than phosphorus release with respect to

the competition of substrate. This is because nitrate will be utilized as a final electron acceptor in the growth of non-polyP heterotrophs. Thereby, the amount of substrate available for polyP organisms is reduced and hence the removal of phosphorus is lowered. There are some studies that confirm the ability of polyP organisms for denitrification, however, not all PAOs can use nitrate as an electron acceptor.