Partial　denitrification　(PD,　nitrate　→　nitrite)　is　a　promising　process　for　the　hazardous　nitrate　removal　by　producing　nitrite　for　Anammox.　In　this　study,　the　startup　and　performance　of　PD　using　slowly　biodegradable　organic　matter　in　primary　sludge　was　explored　by　combining　with　in-situ　hydrolytic　acidification　(HA).　Results　showed　that　efficient　PD　was　established　with　61.3%　nitrite　production　at　an　influent　nitrate　level　of　50　mg/L,　with　a　simultaneous　23.1%　reduction　in　volatile　sludge　mass.　Efficient　electron　donors　including　acetate　(13.2%),　dissolved　saccharide　(11.9%),　and　intracellular　poly-hydroxyalkanoates　(22.5%)　were　generated　from　sludge　HA,　jointly　promoting　desirable　nitrite　production.　Microbial　analysis　revealed　that　adding　primary　sludge　significantly　increased　community　diversity;　however,　the　specific　genera　Dechloromonas　(11.9%)　and　Thauera　(10.5%)　remained　stably　enriched　to　facilitate　the　efficient　sludge　reduction　and　nitrite　production.　These　findings　provide　a　novel　strategy　for　simultaneously　treating　primary　sludge,　nitrate　contaminant,　and　domestic　wastewater　using　a　HAPD　and　Anammox　process.　©　2020　Elsevier　Ltd