Mitigating Methane Emissions from Septic Systems
On-site (septic) systems are estimated by the EPA to make up 76% of wastewater sector methane emissions in the US, trapping heat in the atmosphere over 20 times more effectively than carbon dioxide and contributing to poor air quality . A compost biofilter system my help to mitigate the effects.
Alan Hodges, Celeste Hancock,
Concerns with climate change have led to efforts to reduce greenhouse gas emissions (GHGs). Methane has been identified as a GHG that is over twenty times more effective at trapping heat in the atmosphere than carbon dioxide. Using assumptions developed by the Intergovernmental Panel on Climate Change (IPCC), the U.S. Environmental Protection Agency GHG inventory (2009) estimated that 76 percent of wastewater sector methane emissions in the United States are from onsite (septic) systems. This is due to the large number of individual septic systems in use and the anaerobic conditions present in septic tanks.
In addition, methane contributes to the formation of NH4NO3, which is a major component of particulate matter less than 2.5 microns (PM2.5). PM2.5 is an important air contaminant that contributes to the poor air quality that occurs in Cache Valley and in other areas of Utah during winter inversions.
A project funded by the Water Environment Research Foundation (Evaluation of Greenhouse Gas Emissions from Septic Systems, 2010) concluded that study is needed to develop technologies for the control of GHG emissions from on-site wastewater systems. In this project we are investigating the potential effectiveness of mitigating the impacts of methane produced in septic tanks by collecting the methane and treating it in a compost biofilter system where the methane can be converted to carbon dioxide, which can then be used by plants growing on the compost.
After preliminary methane degradation feasibility was observed, in FY15/16 we focused on the development of modeling temperature effects on methane degradation. Bench-scale, batch-operated (Figure 2) systems were temperature controlled and the effect of temperature on methane degradation was measured. These data were then used to create an Arrhenius rate correction for temperature change (Figure 3). Additionally, a commercially sized septic system was identified to explore the feasibility of applying a compost biofiltration system for the reduction of methane at high methane loadings.
Methane removal was observed in both continuous flow and batch operated reactors. Continuously fed reactors showed an average methane removal rate of 142 g CH4 m-3 d-1. A predictive model for methane reduction in Logan City was developed (Figure 4) that will be tested with field scale monitoring in FY16/17.
Looking to the Future
Future research will focus on two objectives:
- Test the temperature based methane reduction model developed in FY15/16 in the field. Field scale biofilters will be built and monitored for methane removal and ambient temperature at both a residential and commercial septic system.
- Use metagenomic analysis to determine the change in methanotroph populations through time while under methane-enriched conditions.
Arrhenius Plot for temperature correction of methane removal rate.
Logan City compost based methane removal model
Benefits to Utah
The project will provide direct benefit to the State of Utah, especially the Cache Valley area, by targeting an environmental source of methane for reduction. This can potentially reduce the amount of methane that is a precursor for the formation of PM2.5, as well as GHG, in areas of Utah where air quality problems exist and septic systems are commonly used for on-site wastewater treatment
Conference Poster Presentation:
Hodges, Alan, Hancock, Celeste, Sims, Ronald, and Sims, Judith. 2016. Reduction of Greenhouse Gas Emissions from Septic Tanks via Compost Biofiltration. Annual Conference, Institute of Biological Engineering, Greenville, SC, April 7–9.