Influence of organic shock loads on the production of N2O in denitrifying phosphorus removal process

•Organic shock loads caused N2O production in denitrifying phosphorus removal.•The main reason for N2O production was due to higher nitrite accumulation.•Organic shock loads affected the amount of PHA synthesis.•The amount of PHA synthesis could affect activities of denitrifying enzymes.•Continuous nitrate addition reduced N2O production under organic shock loads.

In this work, the influences of short-term organic shock loads on N2O production during denitrifying phosphorus removal were investigated by changing the influent COD concentrations (100, 200, 350, and 500 mg/L). After switching the COD concentrations from 200 to 350 or 500 mg/L, N2O-N production amount increased from 1.62% to 7.12% or 3.29% of the TN removal, respectively, while the corresponding effluent phosphorus concentrations increased from 1.84 to 16.55 and 56.08 mg/L, respectively, which were higher than the influent phosphorus concentration (4.93 mg/L). Furthermore, when the COD concentration was decreased to 100 mg/L, N2O-N production amount was only 1.20%. All results suggested that higher organic shock loads increased N2O production. The main reason was that higher organic shock loads increased anaerobic poly-β-hydroxyalkanoates (PHA) synthesis, which resulted in higher nitrite accumulation. The influences of higher organic shock loads on N2O production could be minimized by adopting continuous nitrate addition strategy.