In order to study the effects of anoxic condition on release of phosphorus in an enhanced system for phosphorus removal with granular sludge

batch tests were carried out using acetate and propionate(carbon molar ratio=2∶1)as carbon source and adding different amount of NO-3-N(0

20

30

50 and 75 mg·L-1

pH=8.0)and NO-2-N(10

20

40 and 60 mg·L-1

pH=7.0

7.5 and 8.0)in initial anaerobic stage.The results showed that addition of NO-3-N and NO-2-N had no obvious inhibition on release of phosphorus.Absorption of fatty acids(VFAs)

phosphorus release and denitrification took place simultaneously

and absorption rate increased while release rate of phosphorus decreased because of competition between denitrifying bacteria and phosphorus accumulating microbial granule(PAOs)for carbon source.The results of batch tests at various NO-2-N concentration and at different pH showed that FNA

rather nitrite

had a strong stimulation effect on P-release and the rate of P-release increased by 4 times when FNA concentration was higher than 0.004 mg HNO2-N·L-1.Meanwhile

FNA had obvious suppression on absorption of VFA and synthesis of PHA

and the absorption rate and synthesis mass decreased by 53% and 70%

respectively.Based on analysis of energy generation and metabolism of PAOs as well as toxic action of nitrite

the mechanism stimulating P-release of FNA was

in presence of free nitrous acid

that PAOs was required to release more energy to provide adequate proton motive force(pmf).