title: Impact of direct greenhouse gas emissions on the carbon footprint of water reclamation processes employing nitrification-denitrification.
abstract: 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 (14)C 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 (14)C 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.
authors.affiliation: University of Cincinnati, Department of Geology, Cincinnati, OH 45221, United States. Electronic address: andrew.schneider@yale.edu.
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Schneider
authors.affiliation: University of Cincinnati, Department of Geology, Cincinnati, OH 45221, United States; University of Cincinnati, Department of Geography, Cincinnati, OH 45221, United States.
authors.forename: Amy
authors.initials: A
authors.lastname: Townsend-Small
authors.affiliation: Department of Civil and Environmental Engineering, University of California, Irvine, CA 92697-2175, United States.
authors.forename: Diego
authors.initials: D
authors.lastname: Rosso
keywords: Carbon footprint; Direct emissions; Fossil carbon; Greenhouse gases; Radiocarbon; Water reclamation