Conventional nitrification/denitrification process is gradually being replaced with partial-nitritation/anammox (PN/A) processes due to its installation and running cost. However, high ammonia-oxidizing bacteria (AOB) and anaerobic ammonia-oxidizing (anammox) bacteria activity as well as optimum out-selection of nitrite-oxidizing bacteria (NOB) are necessary to achieving efficient PN/A process. Consequently, to enhance PN process via nitrifying bacteria enrichment/out-selection within psychrophilic environment, a novel pH-DO (dissolved oxygen) control strategy was proposed and the response of PN, kinetics, AOB enrichment, and NOB out-selection efficiency was investigated during start-up and long-term operation. With DO of 0.7 mg/L and pH of 7.5–7.9, quick start-up of the PN process was established within 34d as NO 2 −-N accumulation ratio (NAR) reached 90.08 ± 1.4%. Again, when NLR was elevated to 0.8 kg/m3·d (400mgNH 4 +-N/L), DO curtailed to 0.2 mg/L, pH maintained at 7.7 and free ammonium at 6.5 mg/L, NAR and NH 4 +-N removal rate could still reach 97.04 ± 2.4% and 97.84 ± 1.5%, respectively. After optimum control factors had been established, real nitrogen-rich-mine-wastewater was fed (DO, 0.2 mg/L, pH, 8.9, and free ammonia, 6.5 mg/L) and NAR and NH 4 +-N removal rate reached was 97.33 ± 0.5% and 97.76 ± 1.1%, respectively. Estimated kinetic parameters including maximum degradation rate (Vmax = 1.58/d), half-rate constant (Km = 33.8 mg/L), and inhibition constant (Ki = 201.6 mg/L) suggested that inhibition on NH 4 +-N oxidation was most feasible at higher concentration of NH 4 +-N. To elucidate biological mechanisms, 16S rRNA high-throughput revealed that AOB (Nitrosomonas) enrichment had increased from 0.08% to 49% whereas NOB (Nitrospira) abundance reduced from 1% to 0.034%, indicating pH-DO control efficiently enriched AOB and out-selected NOB. Conversely, when influent NH 4 +-N was curtailed to about 200 mg/L and free ammonia concentration maintained at 6.5 mg/L, the population of AOB was observably reduced by 6% within a period of 14 days, indicating control strategies including pH-DO control and substrate availability were the key factors which substantially influenced and promoted the activities and growth of AOBs in the present SBR. Schematic of the sequencing batch reactor, performance and microbial community evolution: (1) Influent feed tank; (2) Feed pump; (3) Effluent pump; (4) Effluent tank; (5) Mechanical stirrer; (6) Air distribution disc; (7) Air flow meter; (8) Air pump; (9) Timer; (10) Automatic control system; (11) Dissolved oxygen monitor; (12) DO probe; (13) pH probe (14) pH controller; (15) Peristaltic pump; (16) Alkaline solution for pH adjustment. Unlabelled Image • Rapid and high-rate partial nitritation was achieved within 34d. • Performance of NAR and ANR were satisfactory as efficiency of both exceeded 97%. • pH-DO control was the major driver to out-select NOB and promote AOB activities. • With DO of 0.2 mg/L and pH of 7.7, Nitrosomonas (AOB) increased from 0.08% to 49%. • Kinetics revealed that inhibition on NH 4 +-oxidation was feasible at higher NH 4 conc. [ABSTRACT FROM AUTHOR]