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.