Septic Tanks Produce Fewer Greenhouse Gases Than Estimated

A recent study published in

Environmental Science and Technology reports that the actual rate of greenhouse gas production from septic tanks is half what has previously been estimated. One-fifth of the U.S. population uses residential septic tanks to treat wastewater; during this process, micro-organisms decompose the matter and produce greenhouse gases such as methane, carbon dioxide (CO₂), and nitrous oxide (N₂O). Estimates of 0.23 metric tons of carbon dioxide equivalents per user per year, made by the Intergovernmental Panel on Climate Change (IPCC), are based on how quickly microbes convert organic matter into methane. According to study participant Harold Leverenz, P.E., Ph.D, a post-doctoral researcher at the University of California–Davis, IPCC “assumes all organic matter entering the tank is converted under anaerobic conditions. [But the team] found that only a portion of the influent organic matter actually undergoes anaerobic decomposition.” Further, “the team found that dissolved organic matter which exits the tank in the effluent is converted under aerobic conditions to carbon dioxide,” which causes less methane and more CO₂ production, reducing the overall CO₂ equivalent produced.