Modelling population shifts in a nitrifying biofilm reactor

Abstract : The dynamic reactor behaviour of a nitrifying inverse turbulent bed reactor, operated at varying loading rate, was described with a 1-dimensional two-step nitrification biofilm model, which includes the competition between two different types of ammonium oxidizers, besides nitrite oxidizers. Previously gathered experimental evidence showed that a different loading rate in the reactor resulted in a different nitrifying performance, both on a macroscopic level (in terms of concentrations) and from a microbiological point of view (with respect to the selection of different types of bacteria in the biofilm). Considering the bulk liquid oxygen concentration as the key variable governing the observed population shift, the model was able to describe the experimental data. The influence of microbial growth parameters on the competition dynamics was assessed under both steady state and dynamic conditions. The influence of the endogenous respiration rate and the effect of external mass transfer were analysed. Particular attention was paid to the conditions under which different types of ammonium oxidizers coexist and the related spatial distribution of the biomass in the biofilm.