In this paper, long-term (>8 month) results of ammonia removal in biofilters was studied. Compost was used as the biofilter medium and activated carbon as an added material. The ammonia removal was normally >95% at influent ammonia concentrations of 20-500 ppmv. According to the test results, the influent ammonia concentration should be < 200 ppmv (0.1570 g-ammonia/kg-media per day) so that the effluent concentration of ammonia is (1.0 mg/m(3) (the emission standard of China), and the biofiltration system can achieve good long-term performance. In the biofiltration system utilized in this study, the shortest retention time that the system could attain was 0.532 min. However, the retention time can be decreased further without decreasing the ammonia removal efficiency. Countercurrent flow is favorable, as it enhances the moisture retention ability of the media. In the bioreactors, ammonia can be converted into the nitrate. The bioreactors have a stratification phenomenon for ammonia removal over the biofiltration depth, which implies that different parts of the bioreactor play different roles in the ammonia removing process.