Despite　the　advantages　of　the　combined　anammox　and　fermentation-driven　denitrification　process　in　nitrogen　removal　and　energy　consumption,　stable　performance　at　decreased　temperatures　remains　a　challenge.　In　this　study,　a　robust　and　high-efficient　nitrogen　removal　efficiency　(95.0-93.1　-86.8-93.4%)　with　desirable　effluent　quality　(3.0-4.1　-7.9-4.9　mg/L)　under　long-term　decreased　temperatures　(30　degrees　C　-＞　25　degrees　C　-＞　20　degrees　C)　was　achieved　in　a　zero-external　carbon　Partial　Nitritation/Anammox　combined　with　in-situ　sludge　Fermentation-Denitrification　process　treating　sewage.　Excellent　sludge　reduction　averaged　at　14.9%　assuming　no　microbial　growth.　Increased　hzsB　mRNA　(2.2-fold)　and　reduced　Ea　(80.9　kJ/mol)　proved　resilient　anammox　to　lower　temperature.　RT-qPCR　tests　revealed　increased　NarG/NirK　(5.1)　and　NarG/NirS　(4.9)　mRNA　at　20　degrees　C,　suggesting　higher　NO3　　-＞　NO2　　over　NO2　　-＞　N2　pathway.　Metagenomics　unraveled　dominant　anammox　bacteria　(Candida-tus_Brocadia,　2.27%),　increased　denitritation　bacteria　containing　more　NarG　(Hyphomicrobium,　0.8%),　fatty　acid　biosynthesis　and　CAZymes　genes.　Enhanced　denitritation　with　recovered　organics　from　sludge　reserved　nitrite　for　anammox　and　facilitated　higher　anammox　contribution　to　N　removal　at　20　degrees　C　(42.4%)　than　30　degrees　C　(39.5%).　This　study　proposed　an　innovative　low-temperature　strategy　for　in-situ　sludge　fermentation,　and　demonstrated　stability　of　advanced　municipal　wastewater　treatment　and　sludge　disposal　through　energy　savings　and　carbon　recovery　under　decreased　temperatures.