Fluidized-bed biofilm nitritation and denitritation reactors (FBBNR and FBBDR) were operated to eliminate the high concentrations of nitrogen by nitritation and denitritation process. The dissolved oxygen (DO) concentration was varied from 1.5 to 2.5 g/m(3) at the top of the reactor throughout the experiment. NH4-N conversion and NO2-N accumulation in the nitritation reactor effluent was over 90 and 65%, respectively. The average NH4-N removal efficiency was 99.2 and 90.1% at the NLR of 0.9 and 1.2 kg NH4-N/m(3) day, respectively. Increasing the NLR from 1.1 to 1.2 kg NH4-N/m(3) day decreased the NH4-N elimination approximately two-fold while NH4-N conversion to NO2-N differences were negligible. The NO2-N/NOx-N ratios corresponded to 0.74, 0.73, 0.72, and 0.69, respectively, indicating the occurrence of partial nitrification. An average free ammonia concentration in the FBBNR was high enough to inhibit nitrite oxidizers selectively, and it seems to be a determining factor for NO2-N accumulation in the process.