The effects of inlet gas concentration and gas flow-rate on the NO elimination process were studied in a laboratory-scale biofilter and the removal process is discussed. Various pollutant inlet concentrations and gas flow-rates were tested in the biofilter. The results showed that the removal efficiency increased with increase in the empty bed residence time (EBRT) and finally reached a steady state. The mass transfer process became the limiting step of NO removal when the EBRT was less than 2 min. A removal efficiency of similar to 73% was obtained in the biofilter at an inlet concentration of 107.1 mg m(-3). The removal efficiency decreased with increase in inlet concentration, whereas the removal capacity increased. The removal process consisted of chemical oxidation and bio-nitrification in the biofilter. The bioprocess mainly contributed to the removal. (c) 2006 Society of Chemical Industry.