Environmental Science and Engineering
Evaluation of Greenhouse Gas (GHG) Emissions from Septic Systems
Publication Date: July 2010
Cooperating Institution: University of California - Davis
Principal Investigator Jeanie Darby
Project Budget: $76,637
Project Identifier: DEC1R09
This study determined the emission rates of greenhouse gases (GHGs) from individual onsite septic systems used for the management of domestic wastewater. A static flux chamber method was used to measure the emission rates of methane, carbon dioxide, and nitrous oxide gases from eight septic tanks and two soil dispersal systems. A technique developed for the measurement of gas flow and concentration at clean-out ports was used to determine the mass flow of gases moving through the household drainage and vent system. There was general agreement in the methane emission rates for the flux chamber and vent system methods. Several sources of variability in the emission rates were also identified.
The septic tank was the primary source of methane, whereas the soil dispersal system was the principal source of carbon dioxide and nitrous oxide emissions. Methane concentrations from the soil dispersal system were found to be near ambient concentrations, similarly negligible amounts of nitrous oxide were found in the septic tank. All emissions originating in the soil dispersal system were discharged through the building vent as a result of natural, wind-induced flow. The gaseous emission rate data were determined to be geometrically distributed. The geometric mean and standard deviation (sg) of the total atmospheric emission rates for methane, carbon dioxide, and nitrous oxide based on samples from the vent system were estimated to be 10.7 (sg = 1.65), 335 (sg = 2.13), and 0.20 (sg = 3.62) g/capita•d, respectively. The corresponding total anthropogenic CO2 equivalence (CO2e) of the GHG emissions to the atmosphere, is about 0.1 tonne CO2e/capita•yr.