ARTICLE pubs.acs.org/est
Methane, Carbon Dioxide, and Nitrous Oxide Emissions from Septic Tank Systems Libia R. Diaz-Valbuena,† Harold L. Leverenz,† Christopher D. Cappa,† George Tchobanoglous,† William R. Horwath,‡ and Jeannie L. Darby*,† † ‡
Department of Civil and Environmental Engineering, University of California, Davis, California 95616, United States Department of Land, Air, and Water Resources, University of California, Davis, California 95616, United States
bS Supporting Information ABSTRACT: Emissions of CH4, CO2, and N2O from conventional septic tank systems are known to occur, but there is a dearth of information as to the extent. Mass emission rates of CH4, CO2, and N2O, as measured with a modified flux chamber approach in eight septic tank systems, were determined to be 11, 33.3, and 0.005 g capita-1 day-1, respectively, in this research. Existing greenhouse gas (GHG) emission models based on BOD (biochemical oxygen demand) loading have estimated methane emissions to be as high as 27.1 g CH4 capita-1 day-1, more than twice the value measured in our study, and concluded that septic tanks are potentially significant sources of GHGs due to the large number of systems currently in use. Based on the measured CH4 emission value, a revised CH4 conversion factor of 0.22 (compared to 0.5) for use in the emissions models is suggested. Emission rates of CH4, CO2, and N2O were also determined from measurements of gas concentrations and flow rates in the septic vent system and were found to be 10.7, 335, and 0.2 g capita-1day-1, respectively. The excellent agreement in the CH4 emission rates between the flux chamber and the vent values indicates the dominant CH4 source is the septic tank.
’ INTRODUCTION In the U.S., approximately 20% of the population relies on onsite septic systems to treat their wastewater.1 Such systems are most commonly composed of a septic tank and a soil infiltration system for effluent dispersal. A portion of the influent organic matter in septic tanks is converted into methane (CH4) and carbon dioxide (CO2).2 Given the large numbers in use, septic tanks have the potential to be significant sources of greenhouse gas (GHG) emissions.3,4 It has been reported that the total GHG emissions (given in CO2-equivalents assuming a 100 year time horizon) from septic tanks could be as large as 0.23 tonne CO2e capita-1 year-1, which considers only CH4 emissions.1,4 However, the data on which these inventories are based are not tied directly to measurements but instead are based on assumptions as to conversion rates of the influent organic matter. Three distinctive layers are usually found within septic tanks: the top scum layer is formed from grease, oil, and buoyant particles; settled solids create the bottom sludge layer; in the middle, clarified water flows between the scum and sludge forming the clear liquid layer.2 It is within the sludge layer that methane gas production occurs, resulting from biological processes known as acidogenesis and methanogenesis.5 The anaerobic oxidation process can be described as a two-stage process: r 2011 American Chemical Society
the first stage is identified as waste conversion (acetogenesis, acidogenesis), in which complex organics are first hydrolyzed and then fermented into simple organic compounds (e.g., hydrogen and carbon dioxide) and volatile fatty acids (e.g., acetate) by facultative bacteria known as acetogens and acidogens.5 After the organic matter has been converted to simpler compounds, waste stabilization (methanogenesis) occurs, where the acids are synthesized by methanogens into methane and carbon dioxide.5 The two major mechanisms of methane formation are the breakdown of acetic acid and the reduction of carbon dioxide. These processes are influenced by temperature, organic loading, and solids residence time,5 although direct measurements of CH4 emissions from septic tanks are limited. For example, in 1910 Kinnicutt et al.6 reported measurements of CH4 emissions from community septic tanks in Lawrence, MA, observing higher emissions in the summer than in the winter, with an average value of 10 g CH4 capita-1 day-1; however, the methodology used to perform the measurements is not described. Winneberger7 Received: October 25, 2010 Accepted: February 10, 2011 Revised: February 2, 2011 Published: March 07, 2011 2741
dx.doi.org/10.1021/es1036095 | Environ. Sci. Technol. 2011, 45, 2741–2747
Environmental Science & Technology
ARTICLE
Table 1. Description of the Sites Selected for Gas Sampling gas sample locations
site
septic tank volumeb
number of
years since septic tank
year system
scum
(m3)
occupants
pumped
built
thickness(cm) c
sludge thickness effluent (cm)
filter
septic
venting
soil dispersal
tank
system
system
x
x
x
x
1
5.5
2
3
unknown
