Abstract
Lignocellulose-amended, layered soil treatment areas (STAs) remove nitrogen (N) passively from wastewater by sequential nitrification and denitrification. As wastewater percolates through the STA, the top sand layer promotes nitrification, and the lower, lignocellulos-amended sand layer promotes heterotrophic denitrification. Layered STAs can remove large amounts of N from wastewater, which may increase their emissions of CO2, N2O, and CH4 to the atmosphere. We measured greenhouse gas (GHG) flux from sawdust-amended (Experimental) and sand-only (Control) STAs installed in three homes in southeastern Massachusetts, USA. The Experimental STAs did not emit significantly more GHGs to the atmosphere than Control STAs receiving the same wastewater inputs, and both Control and Experimental STAs emitted more CO2 and N2O – but not CH4 – than soil not treating wastewater. Median (range) flux (μmol m−2 s−1) for all homes for the Control STAs was 7.6 (0.8–23.0), 0.0001 (−0.0004–0.004), and 0.0008 (0–0.02) for CO2, CH4 and N2O, respectively, whereas values for the Experimental STAs were 6.6 (0.3–24.3), 0 (−0.0005–0.005), and 0.0004 (0–0.02) for CO2, CH4 and N2O, respectively. Despite the absence of differences in flux between Control and Experimental STAs, the Experimental STA had significantly higher subsurface GHG levels than the Control STA, suggesting microbial consumption of excess gas levels near the ground surface in the Experimental STA. The flux of GHGs from Experimental and Control STAs was controlled chiefly by temperature, soil moisture, and subsurface GHG concentrations. Total emissions (gCO2e capita−1 day−1) were higher than those reported by others for conventional STAs, with mean values ranging from 0 to 1835 for septic tanks, and from 30 to 1938 for STAs. Our results suggest that, despite a higher capacity to remove N from wastewater, layered STAs may have limited impact on air quality compared to conventional STAs.
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