Occurrence　of　anaerobic　ammonium　oxidation　(anammox)　in　marine　and　freshwater　systems　has　greatly　changed　our　understanding　of　global　nitrogen　(N)　cycle　and　promoted　the　investigation　of　the　role　and　ecological　features　of　anammox　in　anthropogenic　ecosystems.　This　study　focused　on　the　spatio-temporal　abundance,　activity,　and　biodiversity　of　anammox　bacteria　in　full-scale　municipal　wastewater　treatment　plants　(WWTPs)　via　traditional　nitrification/denitrification　route　with　low-strength　ammonium　loading.　The　anammox　bacteria　were　detected　in　all　the　treatment　units　at　the　five　WWTPs　tested,　even　in　aerobic　zones　(dissolved　oxygen　＞2　mg　L-1)　with　abundance　of　10(5)-10(7)　hydrazine　synthase　(his)　gene　copies　The　(15)(N)-isotope　tracing　technology　revealed　that　the　anammox　rates　in　WWTPs　ranged　from　0.08　to　0.36　mu　mol　N　g(-1)　h(-1)　in　winter　and　0.12-1.20　mu　mol　N　g(-1)　h(-1)　in　summer　with　contributions　of　2.05-6.86%　and　1.71-7.26%　to　N-2　production,　respectively.　The　diversity　of　anammox　bacteria　in　WWTPs　was　distributed　over　only　two　genera,　Brocadia　and　Kuenenia.　Additionally,　the　exploration　of　potential　interspecies　relationships　indicated　that　ammonia　oxidation　bacteria　(AOB)　was　the　major　nitrite-substrate　producer　for　anammox　during　nitrification,　while　Nitrospira,　a　nitrite　oxidation　bacteria　(NOB),　was　the　potential　major　competitor　for　nitrite.　These　results　suggested　the　contribution　of　N-removal　by　the　widespread　of　anammox　has　been　overlooked　in　traditional　municipal　WWTPs,　and　the　ecological　habitats　of　anammox　bacteria　in　anthropogenic　ecosystems　are　much　more　abundant　than　previously　assumed.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.