Assessment of a two-step nitrification model for activated sludge

The objective of developing empirical models with well determined parameters is to obtain a better understanding of the nitrification of activated sludge. The D-optimal experimental design criterion and nonlinear regression analysis were used to obtain precise parameter estimates. Our experiment consisted of a series of 14 batch runs where activated sludge was spiked with ammonium chloride and sodium nitrite individually and in combination. Specific respiration rate, the ratio of respiration rate to mixed liquor suspended solids concentration, was used as the model response variable. Empirical models for specific respiration rate were specified as piecewise, Monod type functions of ammonium and nitrite concentration. Statistical methods were used to evaluate the effects of nonlinearity and parameter correlation on the models. The effect of experimental design on model specification and on parameter estimation is discussed.