Denitrification kinetics in biological N and P removal activated sludge systems treating municipal wastewaters

The denitrification kinetics at 12, 20 and 30 degrees C in nitrification de nitrification biological excess phosphorus removal (NDBEPR) systems were de lineated in batch tests on sludge harvested from laboratory scale M/UCT sys tems. In some investigations, it was found that the P release and uptake we re confined exclusively (>95%) to the anaerobic and aerobic reactors respec tively and the observed P removal conformed to the BEPR model of Wentzel er al. In these investigations, due to an absence of anoxic P uptake (substan tiated by PHB measurements), it could be inferred that the phosphate accumu lating organisms (PAOs) did not significantly contribute to the denitrifica tion. The ordinary heterotrophic organism (OHO) and PAO groups were separat ed with the aid of the BEPR model of Wentzel er ai. Ascribing the denitrifi cation to the OHO group performing this process, the specific rates of deni trification associated with the utilization of slowly biodegradable COD (SB COD) in the primary (K-2') and secondary (K-3') anoxic reactors were calcul ated and compared with the rates in ND systems (K-2 and K-3). In other inve stigations it was found that P release and uptake were not confined exclusi vely to the anaerobic and aerobic reactors respectively and the observed P removal was only about 60% of that expected from the BEPR model of Wentzel et al. In these investigations significant P uptake under anoxic conditions was observed so the PAOs may have been involved with the denitrification. However, the denitrification rates were calculated as before by attributing it exclusively to the OHOs. Widely varying K-2' rates were observed at 20 degrees C, ranging from 0.071 to 0.335 mgNO(3)-N/(mgAHVSS.d). The variation in K' rates is mainly due to widely varying OHO active fraction estimates for NDBEPR systems. (C) 1999 IAWQ Published by Elsevier Science Ltd. All ri ghts reserved.