A batch feed study using nitrifiers that had been continuously acclimated under a low-ammonia environment showed that a sudden change of ammonia concentration resulted in sluggish physiological adaptation and biochemical reaction of nitrifiers (i.e., indicated by the parameter specific oxygen utilization rate). When the one-stage continuous-stirred tank reactor (CSTR) system was maintained at a short microbial cell residence time and a high volumetric loading rate, an accumulation of nitrite occurred. Under such circumstances, ammonia and nitrite oxidation both limit overall nitrification at different stages of the process. Batch studies with biomass respectively removed from the front and rear reactors (i.e... high-ammonia and low-ammonia growth environments) of a two-stage CSTR system showed that the estimated kinetic parameters for nitrifiers with the low-ammonia growth environment were 0.3-0.8-fold lower than those for nitrifiers with the high-ammonia growth environment, possibly leading to inaccurate model simulation results. Accordingly, biomass removed from a CSTR system that had been operated continuously to grow bacteria under a high-substrate environment should be loaded into the batch reactor if the batch reactor method is to be used to estimate kinetic parameters. (c) 2007 Elsevier Ltd. All rights reserved.