Hi lamojai,
Can u give more information, which type of reactor u have, ananerobic fermentation tank may be UASB reactor or UAFBR or it may be aanerobic tank without methane generation.
For aerobic system C:N

ratio is 100:5:1 whereas for anaerobic system this ratio varies from 600:5:1( Metcalf and Eddy, 2003, 5th edition, page 1005). SO biological phosphorus requirements are for each 600 mg of rbCOD removal, ther will be 1 mg of biological phosphorus removal. There are various other factors invoved such as biomass concentration, P content of sludges. This biological P is consumed by anaerobic bacteria and with sludge wastage this can be removed. For anaerobic sludges, P content of as high as 10% of VSS were reported ( Malnou et. al., 1984, Water Sci and Technology)
So Phosphorus is consumed by the microorganisms and afterwards it has been removed by means of sludge wastage.
So my point is it is more related to SRT rather than hydraulic retention time(HRT) or residence time. For conventional treatment systems, at HRT of as low as 6 hrs, biological phosphorus removal has been reported.
Give us more data and you will definitely get reliable answer