Estimation of the active nitrifying biomass in activated sludge

An in situ nitrifier mass estimation technique was developed to estimate the nitrifier population within an activated sludge sample using dominant cultures of nitrifying organisms. This technique was applied to activated sludge samples from the Milton, Ontario Wastewater Treatment Plant and the results were compared to nitrifier population estimates calculated using the IAWPRC (now IAWQ) Activated Sludge Model No. 1 (ASM1). Several sets of nitrification model parameters were used, including values calculated in this investigation from dominant cultures of nitrifier organisms as well as the ASM1 default estimates. The trends in active nitrifier population measured by the in situ mass estimation technique (MET) were predicted by the model simulations when the parameter estimates developed in this study were used. There was good agreement and no statistical difference existed between the model estimates and the experimentally determined masses. The current ASM1 default parameters did not predict the trends in the active population as measured by MET.