Model for nitrification in activated sludge process.

A model for the simulation of nitrification in activated sludge is presented and calculations are compared with experimental results from different sources. The role of protozoa and the influence of reactor type and of diurnal variations in flow and strength of the sewage are considered. Favorable conditions for nitrification are low rates of sludge wasting, high temperatures, plug-flow-type reactors and constant operational conditions. Protozoa preying on bacteria decrease the maximum allowable load at which nitrification still occurs. The hydraulic detention time in the aeration tank does not directly affect the degree of nitrification but short detention times may cause failure of the settling tank to separate the sludge from the effluent and hence increase the rate of sludge wasting. Under variable flow conditions, however, a long detention time is favorable for nitrification.