It is clear from the fundamental biochemical processes that nitrification of extremely concentrated ammonia loads requires-among others-(1) sufficient alkalinity to buffer acidification and (2) bicarbonate as the substrate for the autotrophic biomass. However, at low pH values the aeration process causes CO(2) stripping and consequently a decrease of the available inorganic carbon. In order to analyse such complex interactions, we suggest in this paper an enhanced version of the widely acknowledged IWA (formerly IAWQ) activated sludge models. These model enlargements comprise an ion-balance for the calculation of the pH value and of dissociation species, a balance of inorganic carbon and a more detailed description of the relevant N-elimination processes and their inhibitions. The model was successfully employed to optimise a treatment strategy for rejection-water and landfill leachate (500-2000 mg ammonia-Nl(-1), COD/N ratio of 0.25-4). Detailed data from two full-scale rejection-water treatment plants were used for systems identification, model calibration and validation. The results suggest that inhibition and limitation by nitrous acid (HNO(2)) and unionised ammonia (NH(3)) have often been overestimated. In this investigation the bicarbonate concentration proved to be crucial for the process. The optimisation of the bicarbonate concentration in the reactor could improve the nitrozation rate up to 100mg NH(4)(+)-Nl(-1)h(-1).