Effect of pH, substrate and free nitrous acid concentrations on ammonium oxidation rate

► The activity of AOB is half-inhibited at pH values 6 and 10. ► Growth rate of AOB in activated sludge reactors depends on TAN concentration. ► Different kinetic parameter values are obtained for different AOB species. ► The most appropriate kinetic expression for AOB growth has been established. Respirometric techniques have been used to determine the effect of pH, free nitrous acid (FNA) and substrate concentration on the activity of the ammonium oxidizing bacteria (AOB) present in an activated sludge reactor. With this aim, bacterial activity has been measured at different pH values (ranging from 6.2 to 9.7), total ammonium nitrogen concentrations (ranging from 0.1 to 10mgTANL−1) and total nitrite concentrations (ranging from 3 to 43mgNO2–NL−1). According to the results obtained, the most appropriate kinetic expression for the growth of AOB in activated sludge reactors has been established. Substrate half saturation constant and FNA and pH inhibition constants have been obtained by adjusting model predictions to experimental results. Different kinetic parameter values and different Monod terms should be used to model the growth of AOB in activated sludge processes and SHARON reactors due to the different AOB species that predominate in both systems.