Modelling the simultaneous effects of organic carbon and ammonium on two-step nitrification within a downward flow biofilm reactor

Abstract Process modelling is an approximated description of the reality, and choosing appropriate kinetics forms is crucial for model accuracy. It has been long observed that the inhibition effect from organic carbon on nitrification does not change gradually as the organic carbon level changes, which is not capable to be adequately reflected by traditional Monod type kinetics. Therefore, with a purpose to address this issue from modelling perspectives, this study combines the inhibition effects from organic carbon and ammonium on two-step nitrification and incorporates them into a modified ASM1 (Activated Sludge Model No.1) based two-step nitrification framework using two different forms: one in traditional Monod type and one in Logistic type. The two modified models demonstrate improvements in model prediction from both temporal and spatial perspectives, with the Logistic type model even better than the Monod type model. Simulated bacterial depth profiles of the biofilm reactor correspond to lab observations in that heterotrophs dominate the upstream layers where nitrification is inhibited, while ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) dominate the downstream layers where the inhibition on nitrification is relieved. A direct implication for process control is that higher media porosity and longer biofilm length can work in favor of nitrification, but higher hydraulic loadings and higher ratio of organic carbon over ammonium in the influent will be undesirable for nitrification. Overall, a necessity is proven in this study to use appropriate numerical forms to describe the combined inhibition effects of organic carbon and ammonium on two-step nitrification.