Ammonia (NH4+-N) removal pathways were investigated in a potable water treatment biofilter that simultaneously removes manganese (Mn) and NH4+-N. The results indicated a significant loss of nitrogen in the biofilter. Both the completely autotrophic nitrogen removal over nitrite (CANON) process and nitrification were more likely to contribute to NH4+-N removal. Moreover, the model calculation results demonstrated that the CANON process contributed significantly to the removal of NH4+-N. For influent NH4+-N levels of 1.030 and 1.749 mg/L, the CANON process contribution was about 48.5% and 46.6%, respectively. The most important finding was that anaerobic ammonia oxidation (ANAMMOX) bacteria were detectable in the biofilter. It is interesting that the CANON process was effective even for such low NH4+-N concentrations. (C) 2014 Elsevier Ltd. All rights reserved.