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

International guidance for estimating emissions of the greenhouse gas, nitrous oxide (N(2)O), from biological nutrient removal (BNR) wastewater systems is presently inadequate. This study has adopted a rigorous mass balance approach to provide comprehensive N(2)O emission and formation results from seven full-scale BNR wastewater treatment plants (WWTP). N(2)O formation was shown to be always positive, yet highly variable across the seven plants. The calculated range of N(2)O generation was 0.006-0.253 kgN(2)O-Nper kgN denitrified (average: 0.035+/-0.027). This paper investigated the possible mechanisms of N(2)O formation, rather than the locality of emissions. Higher N(2)O 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 N(2)O production. The WWTPs designed and operated for low effluent TN (i.e. <10 mgN L(-1)) had lower and less variable N(2)O generation factors than plants that only achieved partial denitrification.