| Enhanced biological phosphorus removal(EBPR)has been widely applied in wastewater treatment plants to control phosphorus discharge to receiving water bodies.However long-term operation of this process is frequently confronted by process instability and the reason responsible for deterioration of EBPR is still not clear.The objective of the present study was to investigate the effects of influent carbon source change and nitrite addition on phosphorus removal efficiency of two EBPR sludge acclimated with acetic and propionic acid as sole influent carbon source,respectively.EBPR sludge was first acclimated in two anaerobic-aerobic SBRs fed with acetic(SBR-1)and propionic acid(SBR-2)as sole influent carbon source,respectively.High phosphorus removal rate of 98%could be achieved after an acclimation period of 22 d and 6 d and steady-state was successfully established for both reactors as indicated by high phosphorus removal rate and TP content in the activated sludge after 30 days of acclimation.Characteristic metabolic behavior typical to PAOs was observed for both sludges,which showed nearly complete phosphorus removal when influent phosphorus concentration was not more than 18 mg/L.A comparative study was conducted to elucidate the effects of carbon source change on phosphorus removal efficiency of both EBPR sludges.Transient deterioration of EBPR was observed on the acetate-acclimated PAO when influent carbon source shifted from acetate to propionate but it was fully recovered and adapted well to propionate after 4 days of operation.Phosphorus removal efficiency of acetate acclimated PAO was not influenced when influent carbon source was changed from acetate to butyric or lactic acids.Meanwhile phosphorus removal efficiency of propionate acclimated PAO remained highly stable when influent carbon source was changed from propionate to acetate,butyric or lactic acids.These results indicated that use of propionic acid as influent carbon source would be helpful for maintaining the stability of EBPR process.Although excellent phosphorus removal efficiency remained for both SBR-1 and SBR-2 when they were fed with acetic,propionic,butyric or lactic acids,there was a significant difference in the uptake rate by either EBPR sludges.Generally acetate and propionate could be consumed more quickly than butyrate and lactate by EBPR sludges.Regardless of the organic acid fed to the reactors,PHA content synthesized during anaerobic phase was higher than total amount of the carbon source in the influent.Chemical analysis demonstrated that metabolites derived from decomposition of glycogen could be used by PAOs for PHA synthesis.The effects of nitrite addition on phosphorus removal efficiency of both EBPR sludge was investigated at different pH values.Results showed that significant inhibitory effects were observed on either acetate or propionate-fed EBPR sludge when certain amount of nitrite was added to the influent.Lower pH value may lead to stronger inhibition on the phosphorus removal activity.Toxicity of nitrite was also influenced by the mode of addition.For example,it would be more toxic to EBPR activity when it was added at the start of anaerobic phase while less toxic when added at the start of aerobic phase.Reduced PHA synthesis might explain the strong inhibitory effects on phosphorus removal efficiency of both EBPR caused by the presence of nitrite.Propionate-fed PAOs was more sensitive while acetate-fed PAOs was more tolerant to the inhibitory effects caused by nitrite since the later showed higher denitrifying activity.Therefore use of acetic acid as influent carbon source would be helpful to for maintaining process stability for those systems with nitrite accumulation... |
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