Partial nitritation (PN) is the controllable bottleneck of the combined PN-anaerobic ammonium oxidation process, because it has a low nitrogen conversion rate. In this study, a "co-culture and screening" technology was developed to start up PN, and a dual dissolved oxygen-hydraulic retention time (DO-HRT) control strategy was developed to regulate the NH4+-N/NO2--N ratio. The results showed that PN could be successfully started up and it had a high nitrogen loading rate and ammonium conversion rate (9.42 kg-N m(-3) day(-1) and 4.74 kg-N m(-3) day(-1)), respectively. A response surface methodology (RSM) with a central composite design (CCD) was used to optimize the parameters of DO and HRT. When DO and HRT satisfied the relationships 0.66-0.5 x DO <= HRT <= 0.79-0.53 x DO and 0.5 mg L-1 <= DO <= 0.75 mg L-1, the performance of PN was excellent with an NH4+-N/NO2--N ratio ranging from 1:1.04 to 1:1.47, an ammonium conversion efficiency of 53.6-62.1% and a nitrite accumulation efficiency greater than 90%. The excellent performance of PN process was attributed to the high specific activity of sludge (3.69 g-N g(-1) VSS day(-1)), the predominance of ammonia-oxidizing bacteria, Nitrosomonas, and the inhibition of nitrite-oxidizing bacteria, Nitrospira. (C) 2015 Elsevier Ltd. All rights reserved.