An　upflow　anaerobic　biofilter　(AF)　was　developed　to　investigate　anaerobic　ammonium-oxidizing　(ANAMMOX)　efficiency　in　treating　low-strength　wastewater　at　ambient　temperature　(15.3-23.2　degrees　C).　Denaturing　gradient　gel　electrophoresis　(DGGE)　and　fluorescence　in　situ　hybridization　were　used　to　investigate　treatment　effects　on　the　microbial　community.　Stepwise　decreases　in　influent　ammonia　concentration　could　help　ANAMMOX　bacteria　selectively　acclimate　to　low-ammonia　conditions.　With　an　influent　ammonia　concentration　of　46.5　mg/L,　the　AF　reactor　obtained　an　average　nitrogen　removal　rate　of　2.26　kg/(m(3)　day),　and　a　removal　efficiency　of　75.9%.　polymerase　chain　reaction-DGGE　results　showed　that　microbial　diversity　in　the　low　matrix　was　greater　than　in　the　high　matrix.　Microbial　community　structures　changed　when　the　influent　ammonia　concentration　decreased.　The　genus　of　functional　ANAMMOX　bacteria　was　Candidatus　Kuenenia　stuttgartiensis,　which　remained　stationary　across　study　phases.　Visual　observation　revealed　that　the　relative　proportions　of　ANAMMOX　bacteria　decreased　from　41.6　to　36.3%　across　three　study　phases.　The　AF　bioreactor　successfully　maintained　high　activity　due　to　the　ANAMMOX　bacteria　adaptation　to　low　temperature　and　substrate　conditions.