Comparison of N2O emissions and gene abundances between wastewater nitrogen removal systems

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Biological nitrogen removal (BNR) systems are increasingly used in the United States in both centralized wastewater treatment plants (WWTPs) and decentralized advanced onsite wastewater treatment systems (OWTS) to reduce N discharged in wastewater effluent. However, the potential for BNR systems to be sources of nitrous oxide (N2O), a potent greenhouse gas, needs to be evaluated to assess their environmental impact. We quantified and compared N2O emissions from BNR systems at a WWTP (Field's Point, Providence, RI) and three types of advanced OWTS (Orenco Advantex AX 20, SeptiTech Series D, and Bio-Microbics MicroFAST) in nine Rhode Island residences (n = 3 per type) using cavity ring-down spectroscopy. We also used quantitative polymerase chain reaction to determine the abundance of genes from nitrifying (amoA) and denitrifying (nosZ) microorganisms that may be producing N2O in these systems. Nitrous oxide fluxes ranged from -4 × 10-3 to 3 × 10-1 mmol N2O m-2 s-1 and in general followed the order: centralized WWTP > Advantex > SeptiTech > FAST. In contrast, when N2O emissions were normalized by population served and area of treatment tanks, all systems had overlapping ranges. In general, the emissions of N2O accounted for a small fraction (< 1%) of N removed. There was no significant relationship between the abundance of nosZ or amoA genes and N2O emissions. This preliminary analysis highlights the need to evaluate N2O emissions from wastewater systems as a wider range of technologies are adopted. A better understanding of the mechanisms of N2O emissions will also allow us to better manage systems to minimize emissions.

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Journal of Environmental Quality