This　study　presents　a　new　method　for　energy-efficient　wastewater　treatment　that　synergizes　the　partial-denitrification,　anammox,　and　in-situ　fermentation　(SPDAF)　processes　in　an　up-flow　reactor.　Nitrate-containing　wastewater　and　actual　domestic　sewage　were　fed　into　this　SPDAF　system,　which　was　operated　for　180　days　without　the　addition　of　external　carbon　sources　and　aeration.　The　total　inorganic　nitrogen　(TIN)　removal　efficiency　reached　93.1%　with　a　low　C/N　ratio　of　1.6,　a　NO3--N/NH4+-N　ratio　of　1.13　and　a　TIN　concentration　of　92.5　mg　N/L.　The　contribution　of　anammox　to　nitrogen　removal　accounted　for　95.6%.　Batch　tests　demonstrated　that　the　partial-denitrification　process　was　able　to　use　organics　from　either　the　influent　or　those　produced　by　fermentation,　thus　providing　nitrite　for　anammox.　Significantly,　fermentation　played　a　key　role　in　using　the　slowly　biodegradable　organics　and　provided　adequate　electron　donor　for　partial-denitrification.　Metagenomic　sequencing　analysis　showed　that　the　genera　related　to　partial-denitrification,　anammox,　and　fermentation　bacteria　were　coexisted　in　this　SPDAF　system.　The　key　functional　genes　of　anammox　bacteria　(Hzs,　3986　hits;　Hdh,　2804　hits)　were　highly　detected　in　this　study.　The　abundances　of　cytoplasmic　nitrate　reductase　(58　706　hits)　and　periplasmic　nitrate　reductase　(70　540　hits)　were　much　higher　than　copper　nitrite　reductase　(16　436　hits)　and　cytochrome　cd(1)　nitrite　reductase　(14　264　hits),　potentially　contributing　to　the　occurrence　of　partial-denitrification.　Moreover,　different　abundances　of　genes　involved　in　fermentation　metabolism　suggested　that　fermentation　likely　generated　easily　biodegradable　organics　for　partial-denitrification.