Nitrous oxide (N2O) emissions during real domestic wastewater treatment in an outdoor pilot-scale high rate algae pond

Abstract In order to determine if nitrous oxide (N2O) emissions could affect the sustainability of microalgae-based pond systems, N2O emissions were recorded from an outdoor 900 L pilot high rate algal pond (HRAP) fed primary wastewater over 1 year. The HRAP was mixed using a paddle wheel and operated at a hydraulic retention time (HRT) of 7.5–10 days. Direct N2O emissions ranged from 4.1 to 6400 μg N-N2O·m−2·d−1 (median of 560 μg N-N2O·m−2·d−1, n = 28) at 10 days HRT, and 70–18300 μg N-N2O·m−2·d−1 (median of 4200 μg N-N2O·m−2·d−1, n = 22) at 7.5 days HRT. Using 25–75% of the data, we estimated that HRAPs designed for nitrogen removal operated at 7.5 days HRT (i.e. 9.5 m2·capita-1 required) would generate 12–53 g N2O·capita·yr-1 which is 4–17 fold higher than the default value of 3.2 g N2O·capita·yr-1 given by the Intergovernmental Panel on Climate Change for centralized wastewater treatment plants with controlled nitrification and denitrification steps. When indirect N2O emissions (via nitrogen discharge and ammonia volatilization) are included, a HRAP operated at 7.5 days HRT could generate total emissions equivalent to 21–138 g N2O·capita−1·yr−1. When expressed as a % of the nitrogen input load into the system, the HRAP direct emissions (i.e. 0.13–0.57%) and total (i.e. 0.23–1.5%) where within the range of 0–14.6% reported in the literature for centralized wastewater treatment.