Impact of direct greenhouse gas emissions on the carbon footprint of water reclamation processes employing nitrification–denitrification

Water reclamation has the potential to reduce water supply demands from aquifers and more energy-intensive water production methods (e.g., seawater desalination). However, water reclamation via biological nitrification–denitrification is also associated with the direct emission of the greenhouse gases (GHGs) CO2, N2O, and CH4. We quantified these direct emissions from the nitrification–denitrification reactors of a water reclamation plant in Southern California, and measured the 14C content of the CO2 to distinguish between short- and long-lived carbon. The total emissions were 1.5 (±0.2) g-fossil CO2 m−3 of wastewater treated, 0.5 (±0.1) g-CO2-eq of CH4 m−3, and 1.8 (±0.5) g-CO2-eq of N2O m−3, for a total of 3.9 (±0.5) g-CO2-eqm−3. This demonstrated that water reclamation can be a source of GHGs from long lived carbon, and thus a candidate for GHG reduction credit. From the 14C measurements, we found that between 11.4% and 15.1% of the CO2 directly emitted was derived from fossil sources, which challenges past assumptions that the direct CO2 emissions from water reclamation contain only modern carbon. A comparison of our direct emission measurements with estimates of indirect emissions from several water production methods, however, showed that the direct emissions from water reclamation constitute only a small fraction of the plant's total GHG footprint. •Direct greenhouse gas emissions were measured at a wastewater reclamation plant.•These greenhouse gas emissions amounted to 3.9 (±0.5) g-CO2-eqm−3 of wastewater.•14C analysis of the CO2 emissions was conducted to determine the fossil component.•11.4% to 15.1% of the emitted CO2 was derived from fossil sources.