Modelling the combined inhibition and time-dependent inactivation effects of toluene on ammonia and nitrite oxidation using a nitrifying sludge

Abstract

The simultaneous oxidation of ammonia, nitrite and toluene using a nitrifying sludge was evaluated and modelled. The experimental data showed that the sole instantaneous inhibition effect due to the presence of toluene was not sufficient to explain the severe decrease in the nitrification activity during the cultures. In addition, prior cell exposure to toluene provoked an important – even complete – inactivation of the nitrifying process. The main contribution was thus to design and integrate an adjustment function in the reaction kinetics to account for the cell inactivation mechanism that occurs during an ongoing culture but is initiated during previous cell exposure to toluene; the use of an adjustment function was a requirement indicated by a previous study using the same nitrifying sludge but in presence of benzene. This inactivation factor varies according to a first-order kinetic model that is time-dependent and enhances the dynamic characteristics of the modelling approach. The model was validated by batch experiments with variable initial toluene concentrations (0 to 20mgCL−1) and additional inactivation batch assays, where cells were first exposed to 0 to 50mg of tolueneCL−1, washed and then reused in a culture medium without toluene. Changes observed in the ammonium, nitrite, nitrate and toluene concentrations agreed well between the model and experimental data with a single set of kinetic parameters in which confidence intervals and sensitivity were also analysed. The proposed model may be used as a predictive tool to avoid critical conditions for nitrification processes where cells are repeatedly or sequentially exposed to hydrocarbons, such as in a SBR system.