Aiming at solving the contradiction of sludge retention time (SRT) between nitrification and dephosphorization

and enhancing the simultaneous nitrogen (N) and phosphorus (P) removal efficiency of the low C/N (P) ratio domestic sewage in the traditional A2/O process

an improved A2/O technique with long SRT was developed. The novel process could select and strengthen the activated sludge

and enrich the denitrifying phosphorus accumulating organisms (DPAO) with long SRT effectively. After washing out nitrite oxidizing bacteria (NOB)

the reactor ran under the condition of SRT=19.6 d

and sludge concentration (MLSS)=5.5 g·L-1

hydraulic retention time (HRT)=8.2 h

sludge return ratio (R)=90%

nitrated liquid reflux ratio (r)=250%

dissolved oxygen (DO)=1.5—0.3 mg·L-1 for the A2/O zone. HRT=4 h

aeration cycle was 1 h

aeration time was 1 min (DO=0.3—0.5 mg·L-1) and settling time was 59 min for the intermittent aeration zone. Average removal rate of COD

NH4+-N

TP and TN reached 88.71%

99.2%

93.77% and 89.52% respectively

nitrosation rate (NO2--N/NOx--N) was up to 97.2%

and DPAO/PAO ratio in sludge was 95.5%

and the final effluent met the first class level A of the GB 18918—2002 standard. It was found that about 72.96% of COD in sewage was utilized by DPAO for PHA production to accumulate P

15.75% of COD was consumed by heterotrophic denitrifying bacteria

and about 41.96% and 31.31% of N were wiped off through phosphorus removal and heterotrophic denitrifying. The discharged sludge was mainly composed of the biomass of DPAO and denitrifying bacteria

accounting for 82.74% and 17.24% respectively. The technique could reduce 58.76% of carbon source consumption and 44.6% of sludge discharge comparing to the traditional nitrogen phosphorus removal pathway.