Validation of a new model for the sizing of denitrification reactors, by testing full scale plants

A new deterministic model for the calculation of the Specific Denitrification Rate (SDNR), to be used for the design of pre-denitrification reactors, was tested on eight full scale activated sludge plants of different capacity. The model represents the SDNR at 20°C (SDNR20°C) as a function of the sludge loading in the denitrification reactor (F:MDEN) and the residual dissolved oxygen in the denitrification reactor (DODEN). The results proved the ability of the new model in calculating the SDNR20°C. The model shows a little lower degree of adaptability for small-size plants.. SDNR20°C proved a very strong sensitivity to DODEN, mainly in correspondence of low dissolved oxygen (DO) concentrations (less than 2 mg L-1). The sensitivity decreases at greater DO values, but with a progressively less marked gradient, up to become weak only at DO concentrations greater than 0.4-0.5 mg L-1, which are however rarely found in full-scale plants. DO concentrations measured in real-scale facilities are mostly in the range 0.20.4 mg L-1 and are mainly due to the mixed-liquor recycle. These concentrations cause severe adverse effects on the kinetics of nitrogen removal, and consequently on the denitrification performance. Thus, minimizing DO in the pre-denitrification reactor is relevant. The sensitivity of SDNR20°C to F:MDEN resulted much less important, as it is characterized by a growing linear behaviour with a very low slope.