Obtaining　desirable　partial　nitritation　(PN)　is　crucial　for　successful　application　of　the　combined　PN　and　anammox　process.　In　this　study,　the　partial　nitritation　and　simultaneously　phosphorus　removal　(PNSPR)(1)　granular　sludge　reactor　treating　low-strength　domestic　sewage　was　rapidly　started　up　in　67　days　through　seeding　denitrifying　phosphorus　removal　(DPR)(2)　sludge.　The　nitrite/ammonium　ratio　in　effluent　was　approximately　1　and　the　nitrite　accumulation　rate　(NAR)　was　more　than　95%,　about　93%　of　orthophosphate　was　removed.　The　DPR　sludge　rich　in　phosphate　accumulating　organisms　(PAOs)　with　few　nitrifying　bacteria　could　promote　the　achievement　of　PNSPR.　Quantitative　microbial　analysis　showed　that　the　ammonium　oxidizing　bacteria　(AOB)　gene　ratio　in　sludge　increased　from　0.21%　to　3.43%,　while　nitrite　oxidizing　bacteria　(NOB)　gradually　decreased　to　0.07%.　The　average　particle　size　of　sludge　increased　from　114　to　421　mu　m,　indicating　the　formation　of　PNSPR　granules.　The　high　phosphorus　content　in　sludge　and　phosphorus　removal　facilitated　rapid　granulation.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.