The　anoxic　zone　serves　as　the　core　functional　unit　in　municipal　wastewater　treatment　plants　(MWWTPs).　Unfortunately,　in　most　cases,　the　downstream　range　of　the　anoxic　zone　is　severely　lacking　in　available　organic　carbon　and　thus　contributes　little　to　the　removal　of　nutrients.　This　undesirable　range　is　termed　the　＂carbon-restricted　anoxic　zone＂,　representing　an　insurmountable　drawback　for　traditional　MWWTPs.　This　study　uncovers　a　previously　overlooked　role　for　the　carbon-restricted　anoxic　zone:　a　hotspot　for　anaerobic　ammonium　oxidation　(anammox).　In　a　continuous-flow　pilot-scale　plant　treating　municipal　wastewater　(55　m(3)/d),　virgin　biocarriers　were　introduced　into　the　carbon-restricted　anoxic　zone　(downstream　25%　of　the　anoxic　zone　with　BOD5　of　5.9　+/-　2.3　mg/L).　During　the　517-day　monitoring,　anammox　bacteria　highly　self-enriched　within　the　biofilms,　with　absolute　and　relative　abundance　reaching　up　to　(9.4　+/-　0.1)　x　10(9)　copies/g-VSS　and　6.17%　(Candidatus　Brocadia),　respectively.　N-15　isotopic　tracing　confirmed　that　anammox　overwhelmingly　dominated　nitrogen　metabolism,　responsible　for　92.5%　of　nitrogen　removal.　Following　this　upgrade,　the　contribution　ratio　of　the　carbon-restricted　anoxic　zone　to　total　nitrogen　removal　increased　from　9.2　+/-　4.1%　to　19.2　+/-　4.2%　(P　＜　0.001),　while　its　N2O　emission　flux　decreased　by　84.5%　(P　＜　0.001).　These　findings　challenge　stereotypes　about　the　carbon-restricted　anoxic　zone　and　highlight　the　multiple　environmental　implications　of　this　newfound　anammox　hotspot.