A new pH-based procedure to model toxic effects on nitrifiers in activated sludge

A combination of a titration experiment and a biokinetic parameter estimation procedure is proposed as an experimental tool to study the kinetics of NH4+-oxidizing bacteria in activated sludge. The method was used to quantify the effect of low concentrations of a toxic compound on the maximum substrate removal capacity and the substrate affinity constant (KNH) of NH4+-oxidizing bacteria in activated sludge samples. Experiments in the presence of increasing concentrations of a toxic compound (CN−, 3,5-DCP, Cu2+ and phenol) were performed with nitrifying activated sludge samples obtained from two full-scale wastewater treatment plants. The repeatability of the proposed procedure was found to be sufficient to deduce trends in the behavior of the NH4+-N-oxidizing bacteria based on one series of experiments with increasing toxicant concentrations. The experimental results showed that the two sludge samples reacted completely differently in the presence of a certain concentration of the same toxic compound. For phenol, the shape of the titration curves did not correspond any longer to a simple Monod model. In this case, titration curves could be described by a model including both nitrification inhibition by phenol and degradation of the phenol by heterotrophic bacteria.