Achieving　partial　denitrification　(PD)　by　using　fermentation　products　extracted　from　waste　activated　sludge　(WAS)　rather　than　commercial　organic　matters　is　a　promising　approach　for　providing　nitrite　for　anammox,　while　sludge　reduction　could　also　be　realized　by　WAS　reutilization.　This　study　proposed　an　In-situ　Sludge　Fermentation　coupled　with　Partial　Denitrification　(ISFPD)　system　and　explored　its　performance　under　different　conditions,　including　initial　pH,　nitrate　concentrations,　and　organic　matters.　Results　showed　that　nitrite　production　increased　with　the　elevation　of　initial　pH　(from　6　to　9),　and　the　highest　nitrate-to-nitrite　transformation　ratio　(NTR)　reached　77%　at　initial　pH　9.　The　PD　rates　and　NTR　were　observed　to　be　minimally　influenced　by　initial　nitrate　concentrations.　Acetate　was　preferred　by　denitrifying　bacteria,　while　macromolecules　such　as　proteins　necessitated　be　hydrolyzed　to　be　suitable　for　further　utilization.　The　insights　gained　through　this　study　paved　the　way　for　efficient　nitrite　production　and　sustainable　WAS　reutilization　in　harmony.　©　2024　Elsevier　Ltd