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 <i>in situ</i> 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, <i>p =</i> 0.0015) with temperature. Interestingly, N₂O-N yield was also positively correlated with mixed-liquor suspended solid (MLSS) concentration (ρ = 0.41, <i>p</i> = 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.