Closed-loop control of ammonium concentration in nitritation: Convenient for reactor operation but also for modeling

A mathematical biofilm model was developed to describe nitritation in aerobic granular reactor operating in continuous mode. The model includes the automatic closed-loop control of ammonium concentration in the effluent. This is integrated in a ratio control strategy to maintain the proportion between the dissolved oxygen (DO) and the total ammonia nitrogen (TAN) concentrations in the reactor effluent at a desired value. The model was validated with a large set of experimental results previously reported in the literature. The model was used to study the effect of DO and TAN setpoints on the achievement of full nitritation, as well as to establish the appropriate required range of the DO/TAN concentration ratio to be applied. Nitritation at 20 °C was tested experimentally and simulated with the model. Additionally, the importance of controlling the TAN concentration was highlighted with different scenarios, in which periodic disturbances were applied mimicking a poor control situation.