Greenhouse gas emissions from lignocellulose-amended soil treatment areas for removal of nitrogen from wastewater.

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.

Structure of greenhouse gas-consuming microbial communities in surface soils of a nitrogen-removing experimental drainfield.

Septic systems represent a source of greenhouse gases generated by microbial processes as wastewater constituents are degraded. Both aerobic and anerobic wastewater transformation processes can generate nitrous oxide and methane, both of which are potent greenhouse gases (GHGs). To understand how microbial communities in the surface soils above shallow drainfields contribute to methane and nitrous oxide consumption, we measured greenhouse gas surface flux and below-ground concentrations and compared them to the microbial communities present using functional genes pmoA and nosZ. These genes encode portions of particulate methane monooxygenase and nitrous oxide reductase, respectively, serving as a potential sink for the respective greenhouse gases. We assessed the surface soils above three drainfields served by a single household: an experimental layered passive N-reducing drainfield, a control conventional drainfield, and a reserve drainfield not in use but otherwise identical to the control. We found that neither GHG flux, below-ground concentration or soil properties varied among drainfield types, nor did methane oxidizing and nitrous oxide reducing communities vary by drainfield type. We found differences in pmoA and nosZ communities based on depth from the soil surface, and differences in nosZ communities based on whether the sample came from the rhizosphere or surrounding bulk soils. Type I methanotrophs (Gammaproteobacteria) were more abundant in the upper and middle portions of the soil above the drainfield. In general, we found no relationship in community composition for either gene based on GHG flux or below-ground concentration or soil properties (bulk density, organic matter, above-ground biomass). This is the first study to assess these communities in the surface soils above an experimental working drainfield, and more research is needed to understand the dynamics of greenhouse gas production and consumption in these systems.

Comparison of N2O Emissions and Gene Abundances between Wastewater Nitrogen Removal Systems.

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 (NO), a potent greenhouse gas, needs to be evaluated to assess their environmental impact. We quantified and compared NO 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 ( = 3 per type) using cavity ring-down spectroscopy. We also used quantitative polymerase chain reaction to determine the abundance of genes from nitrifying () and denitrifying () microorganisms that may be producing NO in these systems. Nitrous oxide fluxes ranged from -4 × 10 to 3 × 10 µmol NO m s and in general followed the order: centralized WWTP > Advantex > SeptiTech > FAST. In contrast, when NO emissions were normalized by population served and area of treatment tanks, all systems had overlapping ranges. In general, the emissions of NO accounted for a small fraction (<1%) of N removed. There was no significant relationship between the abundance of or genes and NO emissions. This preliminary analysis highlights the need to evaluate NO emissions from wastewater systems as a wider range of technologies are adopted. A better understanding of the mechanisms of NO emissions will also allow us to better manage systems to minimize emissions.