The nitrification and nitrite accumulation were studied in various free ammonia (FA) concentration and temperature combinations. Both ammonia oxidation rate and nitrite oxidation rate increased significantly with the increase in temperature from 10 to 30 degrees C. The increase in the substrate oxidation rate with temperature was relatively faster for ammonia than nitrite. Nitrite accumulation started to occur at 20 degrees C when the specific oxidation rate of ammonia showed a higher value than that of nitrite. Subsequently, the nitrite accumulation continuously increased with the increasing temperature from 20 to 30 degrees C. The broad range in FA concentration exhibited negligible effect on the specific substrate utilization rate as well as the relative nitrite accumulation. On the basis of Arrhenius equation, the activation energies of ammonia oxidation were 87.1 and 38.6 U mol(-1) in the temperature ranges 10-20 degrees C and 20-30 degrees C, respectively. However for the nitrite oxidation, the activation energy was obtained as a constant value of 34.2 kJ mol(-1) in the temperature range of 10-30 degrees C. (c) 2007 Elsevier Ltd. All rights reserved.