Nitrifying biomass on ring-shaped carriers was modified to nitritating one in a relatively short period of time (37 days) by limiting the air supply,changing the aeration regime,shortening the hydraulic retention time and increasing free ammonia (FA) concentration in the moving-bed biofilm reactor (MBBR).The most efficient strategy for the development and maintenance of nitritating biofilm was found to be the inhibition of nitrifying activity by higher FA concentrations (up to 6.5 mg/L) in the process.Reject water from sludge treatment from the Tallinn Wastewater Treatment Plant was used as substrate in the MBBR.The performance of high-surfaced biocarriers taken from the nitritating activity MBBR was further studied in batch tests to investigate nitritation and nitrification kinetics with various FA concentrations and temperatures.The maximum nitrite accumulation ratio (96.6％) expressed as the percentage of NO2--N/NOx--N was achieved for FA concentration of 70 mg/L at 36℃.Under the same conditions the specific nitrite oxidation rate achieved was 30 times lower than the specific nitrite formation rate.It was demonstrated that in the biofilm system,inhibition by FA combined with the optimization of the main control parameters is a good strategy to achieve nitdtating activity and suppress nitrification.