Onsite Wastewarer Treatment Systems in a Changing Climate: Technology Robustness and Mechanisms

Abstract

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.