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.