Abstract
Advanced onsite wastewater treatment systems (OWTS) designed to remove nitrogen from residential wastewater play an important role in protecting environmental and public health. Nevertheless, the microbial processes involved in treatment produce greenhouse gases (GHGs) that contribute to global climate change, including CO2, CH4, N2O. We measured GHG emissions from 27 advanced N-removal OWTS in the towns of Jamestown and Charlestown, Rhode Island, USA, and assessed differences in flux based on OWTS technology, home occupancy (year-round vs. seasonal), and zone within the system (oxic vs. anoxic/hypoxic). We also investigated the relationship between flux and wastewater properties. Flux values for CO2, CH4, and N2O ranged from −0.44 to 61.8, −0.0029 to 25.3, and −0.02 to 0.23 μmol GHG m−2 s−1, respectively. CO2 and N2O flux varied among technologies, whereas occupancy pattern did not significantly impact any GHG fluxes. CO2 and CH4 – but not N2O – flux was significantly higher in the anoxic/hypoxic zone than in the oxic zone. Greenhouse gas fluxes in the oxic zone were not related to any wastewater properties. CO2 and CH4 flux from the anoxic/hypoxic zone peaked at ~22-23 °C, and was negatively correlated with dissolved oxygen levels, the latter suggesting that CO2 and CH4 flux result primarily from anaerobic respiration. Ammonium concentration and CH4 flux were positively correlated, likely due to inhibition of CH4 oxidation by NH4+. N2O flux in the anoxic/hypoxic zone was not correlated to any wastewater property. We estimate that advanced N-removal OWTS contribute 262 g CO2 equivalents capita−1 day−1, slightly lower than emissions from conventional OWTS. Our results suggest that technology influences CO2 and N2O flux and zone influences CO2 and CH4 flux, while occupancy pattern does not appear to impact GHG flux. Manipulating wastewater properties, such as temperature and dissolved oxygen, may help mitigate GHG emissions from these systems.
Highlights
We quantified the flux of CO2, CH4, and N2O from five different advanced N-removal onsite wastewater treatment systems (OWTS) technologies in the towns of Charlestown and Jamestown, Rhode Island, USA: SeptiTech® Series D, Orenco Advantex® AX20, Orenco Advantex® RX30, BioMicrobics MicroFAST®, and Norweco Singulair®
greenhouse gases (GHGs) emissions result from microbial processes that respond to environmental conditions and availability of nutrients, electron acceptors, and organic C, and we assessed the relationship between gas emissions and effluent temperature, pH, and dissolved oxygen (DO), as well as effluent 5-day biochemical oxygen demand (BOD5; a proxy for organic C), ammonium, nitrate, and total N concentration
Advanced N-removal OWTS rely on microbial processes for wastewater treatment, and inevitably emit GHGs
Summary
We quantified the flux of CO2, CH4, and N2O from five different advanced N-removal OWTS technologies in the towns of Charlestown and Jamestown, Rhode Island, USA: SeptiTech® Series D, Orenco Advantex® AX20, Orenco Advantex® RX30, BioMicrobics MicroFAST®, and Norweco Singulair®. We measured GHG flux in the summer and fall of 2016, and summer and winter of 2018. Because differences in home occupancy pattern may drive differences in microbial activity and the GHGs produced by this activity, we assessed the relationship between home occupancy patterns and GHG emissions by sampling systems used year-round and systems only used during the summer season. GHG emissions result from microbial processes that respond to environmental conditions and availability of nutrients, electron acceptors, and organic C, and we assessed the relationship between gas emissions and effluent temperature, pH, and dissolved oxygen (DO), as well as effluent 5-day biochemical oxygen demand (BOD5; a proxy for organic C), ammonium, nitrate, and total N concentration. We quantified emissions per capita in terms of CO2 equivalents to allow for comparison with other types of wastewater treatment
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