In　this　study,　an　up-flow　anaerobic　biofilter　(AF)　was　operated　to　investigate　the　efficiency　of　anaerobic　ammonium　oxidation　(ANAMMOX)　in　treating　low　strength　ammonia　(46.5　mg/L)　at　ambient　temperatures　(20.3-23.2　degrees　C).　Microbial　compositions　and　functional　populations　of　the　upper　(140-190　cm),　middle　(40-140　cm),　and　lower　(0-40　cm)　parts　of　the　biofilter　were　monitored　using　scanning　electron　microscopy,　denaturing　gradient　gel　electrophoresis　(DGGE),　clone　and　sequence.　The　results　show　that　stable　biofilter　performance　was　achieved　with　an　average　nitrogen　removal　rate　of　2.26　kg/(m(3).d)　and　a　total　nitrogen　removal　efficiency　of　75.9%.　Approximately　67%　of　the　ammonia　and　nitrite　disappeared　in　the　middle　part　of　the　biofilter.　The　spherical　bacteria,　similar　to　ANAMMOX　bacteria,　dominated　the　middle　part　of　the　biofilter.　There　were　eight　bacterial　DGGE　bands;　clone　and　sequence　results　showed　that　they　included　Oxalicibacterium　sp.,　Ignavibacterium　album,　Bacterium　rJ15,　Candidatus　Kuenenia　stuttgartiensis,　Hippea　maritima,　Thioprofundum　lithotrophica,　and　Rhodopseudomonas　palustris.　The　genus　of　ANAMMOX　bacterium　remaining　at　constant　levels　in　different　parts　of　the　biofilter　was　identified　as　Candidatus　Kuenenia　stuttgartiensis.　The　AF　bioreactor　maintained　high　activity　due　to　the　ANAMMOX　bacteria＇s　ability　to　adapt　to　ambient　temperature　and　low　matrix　influent　conditions.