Stringent　wastewater　discharge　standards　require　wastewater　treatment　plants　(WWTPs)　to　focus　on　enhancing　nitrogen　(N)　and　phosphorus　(P)　removal　efficiency.　Increasing　sludge　concentration　by　regulation　of　sludge　retention　time　(SRT)　would　enhance　wastewater　treatment　loads.　However,　phosphorus-accumulating　organisms　(PAOs)　would　be　outcompeted　by　glycogen-accumulating　organisms　(GAOs)　under　long　SRT,　leading　to　a　collapse　of　P　removal.　In　this　work,　pilot-scale　anaerobic-oxic-anoxic　(AOA)　and　anaerobic-anoxic-oxic　(AAO)　systems　with　long　SRT　(30　d)　were　parallelly　established　for　actual　urban　wastewater　treatment.　The　results　indicated　that　sludge　reflux　ratio,　temperature,　and　C/N　ratio　significantly　impact　N　and　P　removal　performance　of　AOA　and　AAO　systems　with　long　SRT,　and　removal　efficiency　of　AOA　system　significantly　exceeded　that　of　AAO　system.　AOA　system　with　long　SRT　achieved　the　optimal　performance　at　sludge　reflux　ratio　of　200%,　temperature　of　25　°C,　and　C/N　ratio　of　8,　with　COD,　NH4+-N,　TN,　and　PO43--P　removal　ratio　of　92.80　±　2.24%,　97.38　±　0.89%,　88.97　±　2.47%,　and　94.33　±　3.27%,　respectively.　In　addition,　compared　to　AAO　system,　AOA　system　could　save　23.08%　of　the　aeration　volume.　This　work　highlighted　that　AOA　system　with　long　SRT　included　multiple　coupled　nitrogen　and　phosphorus　removal　pathways,　such　as　autotrophic/heterotrophic　nitrification,　anoxic/oxic　denitrification,　endogenous　denitrification,　and　denitrifying　phosphorus　removal.　Among　these,　the　synergistic　effect　of　endogenous　denitrification　and　denitrifying　phosphorus　removal　driven　by　internal　carbon　sources　contributed　to　satisfactory　nitrogen　and　phosphorus　removal　efficiency　in　AOA　system　with　long　SRT.　©　2024