Nitrogen removal via nitrite has recently gained a lot of interest because it results in significant savings in both aeration costs and COD (chemical oxygen demand) requirements for denitrification, when compared to the conventional biological nitrogen removal via nitrate. The effectiveness of two different control strategies to achieve the nitrite pathway in systems with sludge retention has been experimentally demonstrated: (i) control of aerobic phase length, with which aeration is terminated as soon as ammonia is completely oxidised; (ii) operation at low DO setpoints in the aerobic phase. These strategies have been extensively studied in nitrifying reactors and are currently applied in real systems achieving biological carbon, nitrogen and phosphorus removal. In this work, we aim to demonstrate, through modelling and simulation, that the competition between nitrite reducers and nitrite oxidisers for nitrite, rather than kinetic selection plays a major role in NOB washout. Moreover, the results show that the occurrence of simultaneous nitrification and denitrification under “aerobic” conditions is very helpful for the nitrite pathway obtainment and for a more efficient COD utilisation.

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