Onsite wastewater treatment systems in a changing climate: Technology robustness and mechanisms
Nearly 25% of U.S. households rely on onsite wastewater treatment system (OWTS), or septic systems, to renovate wastewater before it is recharged to groundwater. These systems rely on soil processes as the final step in contaminant removal. Reliance on soil microbial, physical and chemical processes, which are sensitive to environmental perturbations (e.g. changes in pH, temperature, moisture, O2, presence of toxins), may result in variable wastewater treatment and release of contaminants to groundwater. ^ The extent of treatment in the soil treatment area (STA; also known as drainfield or leachfield) depends on the volume of unsaturated soil the wastewater must pass through, represented by the vertical separation between the infiltrative surface of the STA and the water table. Reduced treatment may result in greater transport of pathogens, nutrients (N and P), and biochemical oxygen demand (BOD5) to groundwater, jeopardizing public and aquatic ecosystem health. The combined effects of climate change – warmer temperatures and elevated water tables due to sea level rise and increased incidents of extreme precipitation – are expected to diminish the size of the unsaturated treatment area and reduce the availability of O2, both of which are important for the removal of contaminants. This may reduce the ability of soil-based OWTS to treat wastewater, especially in coastal zones with shallow water tables commonly found in southern New England. ^ Shallow narrow STAs are assumed to provide better wastewater renovation and may be more resilient to the effects of climate change than conventional STA. Conventional STAs receive wastewater from the septic tank, where infiltration occurs deeper in the soil profile. The shallow narrow STAs receive pre-treated wastewater from secondary treatment components that allow shallower dispersal of effluent compared to the conventional STA, providing a large volume of soil for treatment. Current understanding of the differences in performance among STA types is rudimentary, and their response to climate change is unknown. I used replicated (n = 3) intact soil mesocosms to measure the performance of two shallow narrow STAs – shallow narrow drainfield (SND) and Geomat® (GEO) – and a conventional pipe and stone (P&S) STA, and their response to climate change. ^ I first evaluated the water quality functions of conventional and shallow narrow STAs under present climate conditions. Between 97.1 and 100% of BOD5, fecal coliform bacteria (FCB) and total P were removed in all STA types. Total N removal averaged 12.0% for P&S, 4.8% for SND, and 5.4% for GEO. All STA types performed similarly for most water quality functions despite differences in carbon and O2 content, input wastewater, dosing regimen, and placement of infiltrative surface within the soil profile. ^ I also examined the mechanisms of N removal within conventional and shallow narrow STAs using a 15N tracer. Nitrogen removal in the STA is attributed to N2 production via heterotrophic denitrification, with little direct evidence to support this. Removal of N in the gas phase was attributable primarily to N2, which had a flux 102 – 103 times larger than N2O in all STAs. The constraints imposed by differences in availability of electron donors and acceptors in different STAs pointed to autotrophic N removal processes (e.g. anaerobic ammonia oxidation, autotrophic denitrification) as playing an important role in N removal in addition to heterotrophic denitrification processes. ^ The impacts of climate change on the STAs were evaluated by raising the water table in the mesocosms 30 cm and increasing the soil temperature 5°C. Greater removal of BOD5 was observed under climate change for all STA types. Release of FCB increased from <1 >(present climate) to up to 20 CFU 100 mL-1 under climate change, likely the result of lower attachment of bacteria in saturated soil and greater transport to groundwater. Climate change resulted in decreased total P removal, from 75-100% under present climate to 66-72%, possibly due to reduction of Fe and Mn oxides involved in the formation of insoluble P-metal complexes. Total N removal increased from 14.2% to 19% for conventional STA, but decreased from 5.6-7.0% to < 3.0% for shallow narrow STAs under climate change. Higher BOD5 removal in the latter may have lowered N removal by limiting carbon availability to microorganisms responsible for heterotrophic denitrification. Climate change is likely to affect contaminant removal in the STA, with the extent of effects depending on the contaminant and type of STA. To mitigate climate change impacts, I suggest that planners, regulators and OWTS designers investigate methods for carbon additions to the STA and reduce reliance on the soil by utilizing more effective and sustainable pre-treatment measures to reduce treatment variability.^
Biogeochemistry|Water resources management|Environmental science
Jennifer Anne Cooper,
"Onsite wastewater treatment systems in a changing climate: Technology robustness and mechanisms"
Dissertations and Master's Theses (Campus Access).