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.