Although　the　treatment　of　sludge　with　free　nitrous　acid　can　effectively　recover　short　chain　fatty　acids,　the　feasibility　of　sequential　nitrite　reduction　and　methane　recovery　without　acidic　pH　adjustment　is　still　scarcely　studied.　Therefore,　this　study　aimed　to　provide　insights　into　the　effect　of　nitrite　at　different　levels　on　nitrite　reduction　and　methane　production.　The　results　showed　that　the　nitrite　concentrations　of　100,　200,　400　and　800　mg/L　were　completely　reduced　in　1,　2,　2　and　4　days,　respectively.　The　nitrite　reduction　process　stimulated　the　fermentation　of　sludge　to　produce　more　organic　matters,　which　served　as　electron　donors　for　denitrification.　With　the　nitrite　concentrations　increasing　from　200　to　800　mg/L　methane　production　decreased　from　128.7　to　0　mg/L　at　the　digestion　time　of　15　d.　The　toxicity　of　nitrite　to　methanogenic　microorganisms　and　the　nitrite　reduction　process　competing　with　methanogens　for　carbon　sources　may　lead　to　the　inhibition　of　methane　production　by　excessive　nitrite.　Moreover,　the　methane　production　reached　184.4　mL　with　100　mg/L　nitrite　reduction,　which　was　increased　by　83.2　%　compared　with　that　without　nitrite　addition　(101.1　mL).　Nitrite　reduction　stimulated　hydrolysis　without　negatively　impacting　acetogenesis,　thereby　providing　more　substrates　for　subsequent　methanogenesis.　Model-based　analysis　indicated　that　nitrite　reduction　enhanced　the　maximum　methane　yield　and　methane　production　rate,　aligning　with　the　aforementioned　analysis.　16S　rRNA　analysis　unraveled　that　the　bacterial　abundance　associated　with　hydrolysis　increased.　This　anaerobic　digestion　technique　driven　by　nitrite　reduction　is　both　environmentally　and　economically　attractive　for　increasing　methane　production.　©　2025