Fluctuations in Ammonia-Oxidizing Bacteria Biomass Density as a Potential Driver of Seasonal Variations in N 2 O Emissions from Activated Sludge Nitrification

Wastewater treatment plants (WWTPs) exhibit marked seasonality in N2O emissions. This study aimed at investigating whether the temperature response of the wastewater nitrifying community contributes to this seasonality. NH4+ oxidation and N2O production rates were determined with indigenous activated sludge in the laboratory at the water temperatures measured in situ at the time of sampling (14.5-27.5 °C) under nonlimiting O2 availability (>5 mg L-1 throughout all incubations). The N2O yield, which ranged between 0.004 and 0.028 mol N2O-N/mol NH4+, exhibited a significant negative correlation (ρ = -0.53, p = 0.0015) with temperature. Interestingly, N2O-N yield was also positively correlated with mixed-liquor suspended solid (MLSS) concentration (ρ = 0.41, p = 0.017), a parameter upheld in winter to sustain nitrification rates. This biomass effect was substantiated by subsequent experiments in which N2O yields of activated sludge (MLSS: 2343 ± 39 and 3760 ± 93 mg L-1) were significantly higher (1.6- to 1.9-fold) than their 2-fold dilutions, regardless of temperature. Higher NH2OH levels detected in denser activated sludge during nitrification (peak concentration of 0.25 ± 0.13 μM versus 0.09 ± 0.01 μM of the 2-fold dilution) suggested NH2OH accumulation as a possible mechanistic explanation. These observations suggest that the higher design MLSS for winter performance may contribute to an increase in N2O emissions from nitrogen removal WWTPs.