Free nitrous acid inhibition of biological phosphorus removal in integrated fixed-film activated sludge (IFAS) system

• DPAO/PAO were selected in IFAS system with different FNA levels during anoxic stage.
• DPAO in att. and sus. biomass adapted to higher FNA level, aerobic P uptake did not.
• NA inhibition threshold for DPAO was four times higher than the adapted conc.
• FNA inhibition of aerobic P uptake in biofilm was 20% of that in suspended biomass.
• The attached biomass could moderate the FNA inhibition of P removal in IFAS system.

Free nitrous acid (FNA) inhibition of anoxic and aerobic phosphorous removal in an integrated fixed-film activated sludge (IFAS) system with different FNA adaptation was investigated. A bench scale sequencing batch reactor (SBR) with plastic media was operated in an anaerobic/anoxic/aerobic sequence. During the anoxic period, nitrite was fed into the reactor at different concentrations to select for biomass adapted to 0.06 and 0.4 μg HNO2-N L−1 of FNA during anoxic stage in Phase I and II, respectively. Long term anoxic/aerobic phosphorus removal was achieved in the IFAS reactor in both phases. In Phase I, aerobic phosphorous uptake was inhibited at higher level compared with anoxic phosphorus uptake. In Phase II, DPAO in both suspended and attached forms could adapt and were not inhibited at FNA level four times higher than the adapted concentration. The PAO’s aerobic activity in Phase II, did not show significant adaptation and was inhibited at slightly lower level compared with that in Phase I. The FNA inhibition of aerobic phosphorous uptake rate in attached biomass was 20% of that in suspended forms. In batch testes with the FNA level was raised to three times the adapted concentration, the contribution of attached biomass to overall anoxic and aerobic phosphorus uptake increased by 20% and 39%, respectively. The attached biomass could allow an IFAS system to be less inhibited and better maintain phosphorous removal at sudden FNA inhibition events.