NITRITE EFFECT ON THE PHOSPHORUS UPTAKE ACTIVITY OF PHOSPHATE ACCUMULATING ORGANISMS (PAO) IN PILOT-SCALE SBR AND MBR REACTORS

Batch tests, performed to investigate the effect of nitrite on the P-uptake activity of biomass grown in pilot-scale SBR and MBR systems, showed that the nitrite has an inhibitory but reversible effect on the aerobic P-uptake activities of the SBR and the MBR biomass. The inhibitory effect was found more pronounced on the SBR biomass, in which the aerobic Puptake was inhibited by 65 % and 37 % at 10 mgNO2-N/l for the SBR and the MBR respectively. For the anoxic P-uptake activities, both the SBR and the MBR biomass were found capable of using nitrite as electron acceptor as effectively as nitrate. Moreover, for the SBR biomass the anoxic P-uptake rate using nitrite was found even higher (60%) than the Puptake rate with nitrate. From an operational point of view, nitrite inhibition can be circumvented by preventing nitrite buildup in BNR systems, or by enhancing the denitrifying PAOs to proceed over the nitrite route instead of the nitrate. From a modeling point of view, the current mechanistic models require appropriate extensions to describe these various effects of nitrite. Hence, an extension of the ASM2d model has been provided. Last, until the underlying mechanism of nitrite inhibition is resolved, experimental determination of the nitrite effect is advised.