Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants

International guidance for estimating emissions of the greenhouse gas, nitrous oxide (N2O), from biological nutrient removal (BNR) wastewater systems is presently inadequate. This study has adopted a rigorous mass balance approach to provide comprehensive N2O emission and formation results from seven full-scale BNR wastewater treatment plants (WWTP). N2O formation was shown to be always positive, yet highly variable across the seven plants. The calculated range of N2O generation was 0.006–0.253 kgN2O–N per kgN denitrified (average: 0.035 ± 0.027). This paper investigated the possible mechanisms of N2O formation, rather than the locality of emissions. Higher N2O generation was shown to generally correspond with higher nitrite concentrations, but with many competing and parallel nitrogen transformation reactions occurring, it was very difficult to clearly identify the predominant mechanism of N2O production. The WWTPs designed and operated for low effluent TN (i.e. <10 mgN L−1) had lower and less variable N2O generation factors than plants that only achieved partial denitrification.