Concentration of pollutants especially influent C/N in municipal wastewater is higher in the northern China than in the southern China.Therefore

it is difficult to keep efficient and stable removal of nitrogen and phosphorus by using conventional biological treatment processes because of the limitations and typical water quality in the southern China.In this study

a modified A/O step-feed process was operated at lab-scale to deal with the weak urban sewage and to remove simultaneously biological nitrogen(N) and phosphorus(P).Under the controlled conditions:influent COD/TN 5.16

HRT 8.7 h

SRT 15 d

MLSS 5.66 g·L-1

sludge reflux ratio 75%

the volume ratio of anaerobic/anoxic/aerobic 4∶8∶10

DO 1.0—1.5 mg·L-1 in three aerobic zones

continuous operation with six inflow distribution ratio 150 d

the optimal flow distribution ratio obtained was 20%∶35%∶35%∶10%

and the values of COD，ammonia nitrogen，total nitrogen，total phosphorus in effluent were 33.05 mg·L-1

0.58 mg·L-1

9.26 mg·L-1 and 0.46 mg·L-1

respectively.These values were up to Grade A discharging standard of pollutants for municipal wastewater treatment plant(WWTP)(GB 18918—2002).The majority of COD in raw water was used as carbon source of anaerobic phosphorus release and denitrification processes

and more than 74% of carbon sources was utilized effectively.At the same time

the experiment also found that collaborative control of DO and ORP are not only useful for anaerobic phosphorus release but also for anoxic denitrification and aerobic nitrification.