Modelling nitrification in plant scale activated sludge

Nitrification performance data from an activated sludge plant was simulated using a simple model based on growth requirements of nitrifiers only and a complete-mix flow pattern. The model best accounted for the plant data under high peak flows and ammonia loadings and confirmed the observation that high peak loadings cannot be fully compensated by increasing the safety factor applied to theoretical mean cell residence time. Difficulties in control of dissolved oxygen and MLSS concentration under high transient loads also adversely affected nitrification. Overall ammonia oxidation rates of the sludge, under the imposed nitrogen and carbon loadings, varied from 2.5 down to 1.3 mg.g−1.hr−1 at mean cell residence times of 3 to A and 9 to 10 days respectively. Adaptation of the model to predictive use requires modification to include overall biomass growth, oxygen relationships and sludge recycle arrangement.