Simultaneous Nitrification and Denitrification through Low-Do Operation

Single-tank simultaneous nitrification and denitrification (SNdN) processes can potentially eliminate the need for separate tanks and for recycling mixed liquor from oxic nitrifying zones to the typically upstream anoxic zones for denitrification. However, the conditions involved are more susceptible to sludge bulking by excessive growth of filamentous bacteria. This study was aimed at identifying the suitable operation conditions, such as sludge retention time (SRT), DO and cyclic aeration, to avoid bulking and promote nutrient (N and P) removal in SNdN systems. The cyclic aeration was carried out by alternating the aeration between a higher rate for 1 h and a lower (or zero) rate for 30 min. In different experiments, the DO (HDO) was 0.4, 0.6, 0.8, or 2.0 mg/L during the period of higher aeration and the DO (LDO) was 0.0 or 0.2 mg/L during the period of lower aeration rate. Shortening SRT was shown to significantly improve the sludge settling. Compared to constant-DO aeration, the cyclic aeration produced better-settling sludge and more complete N and P removal. For N removal, the advantage resulted from the more ready availability of nitrate and nitrite (generated by nitrification during the HDO period) for denitrification (during the LDO period). For P removal, the advantage of cyclic aeration came from the development of a higher population of polyphosphate-accumulating organisms, as indicated by the higher P contents in the sludge solids of the cyclically aerated systems. Nitrite shunt was also observed to occur in the low-DO systems. Higher ratios of NO2−/NO3− were found in the systems of lower HDO (and, to less dependency, higher LDO), suggesting that the nitrite shunt took place mainly because of the disrupted nitrification at low DO. The study results indicated that the HDO employed should be kept reasonably high (∼0.8 mg/L) or the HDO period prolonged, to promote adequate nitrification, and the LDO kept low (≤ 0.2 mg/L) to achieve more complete denitrification and higher phosphorus removal. The NAD(P)H fluorescence profiles in these bioreactors were also monitored using an online fluorometer. The findings in the laboratory systems find strong support from the results obtained in full-scale plant implementation.