BACKGROUNDThis work evaluated the kinetics of stepwise ammonia oxidation, separately accounting for ammonia oxidizers (AOB) and nitrite oxidizers (NOB) in an enriched nitrifying microbial culture. This culture was sustained in a fill and draw reactor fed with 50mgNL(-1) of ammonia together with limited organic substrate. Batch experiments were started with the enriched biomass and the same ammonia feeding to monitor profiles of oxygen uptake rate and resulting nitrogen fractions. RESULTSA model structured to define organic carbon removal and nitrification was calibrated with the experimental profiles. Modeling identified a sequential nitrification mechanism with different maximum specific growth rates of 1.4 day(-1) for AOB and 0.65 day(-1) for NOB, with two distinct nitrifying communities. It could not be calibrated with a single phase overall nitrification approach as in most activated sludge models. CONCLUSIONOxygen uptake rate measurements highlighted the potential merit of an enriched culture by identifying measurable metabolic responses for all biomass fractions. (c) 2013 Society of Chemical Industry