The discharge limit of ammonia-nitrogen in the domestic effluent is typically subjected to the local, state and federal regulations and standards. Thus, nutrients removal forms an essential operation of domestic sewage treatment. However, the conventional activated-sludge process employed in the domestic sewage treatment plants are not able to meet the regulations, as nitrification is inhibited due to insufficient solids retention time and sludge age of the biomass. The purpose of this study is to determine the degradation rate of ammonia in the modified activated-sludge system through a modified, laboratory scale biological reactor. To achieve this, a modified biological reactor with pre-installed continuous narrow baffle walls operating at extended aeration (SRT = 40 days) was designed to intentionally create a number of complete-mix activated-sludge zones. In addition, a pre-anoxic compartment was also provided prior to the aerobic chamber for denitrification purpose. In order to compare the performance, a control reactor (SRT = 2 days) operating at conventional aeration was created to simulate the conventional activated-sludge process in the domestic STP. After the acclimatization period, the influent ammonia was increased gradually by approximately 15 mg/l and the corresponding effluent ammonia level was monitored. The specific substrate removal rate (kg ammonia removed per day / kg MLVSS) was plotted against the effluent ammonia (mg/l) to determine the slope of the graph, which corresponded to the nitrification kinetics (k) of the reactor. Based on the analysis, the nitrification kinetic of the control and modified reactor was observed to be 0.0823 and 10.321, respectively. The nitrification kinetic of the control reactor was considered as negligible due to the conventional activated-sludge process, which inhibits the growth of nitrifiers. In contrast, the modified reactor has achieved nitrification kinetic of 10.32, due to the narrow baffle walls that increased the overall retention time. © 2011 Published by Elsevier Ltd.