Nitrous Oxide Emissions from Wastewater Treatment and Water Reclamation Plants in Southern California

Nitrous oxide (N 2 O) is a long‐lived and potent greenhouse gas produced during microbial nitrification and denitrification. In developed countries, centralized water reclamation plants often use these processes for N removal before effluent is used for irrigation or discharged to surface water, thus making this treatment a potentially large source of N 2 O in urban areas. In the arid but densely populated southwestern United States, water reclamation for irrigation is an important alternative to long‐distance water importation. We measured N 2 O concentrations and fluxes from several wastewater treatment processes in urban southern California. We found that N removal during water reclamation may lead to in situ N 2 O emission rates that are three or more times greater than traditional treatment processes (C oxidation only). In the water reclamation plants tested, N 2 O production was a greater percentage of total N removed (1.2%) than traditional treatment processes (C oxidation only) (0.4%). We also measured stable isotope ratios (δ 15 N and δ 18 O) of emitted N 2 O and found distinct δ 15 N signatures of N 2 O from denitrification (0.0 ± 4.0‰) and nitrification reactors (−24.5 ± 2.2‰), respectively. These isotope data confirm that both nitrification and denitrification contribute to N 2 O emissions within the same treatment plant. Our estimates indicate that N 2 O emissions from biological N removal for water reclamation may be several orders of magnitude greater than N 2 O emissions from agricultural activities in highly urbanized southern California. Our results suggest that wastewater treatment that includes biological nitrogen removal can significantly increase urban N 2 O emissions.