Improvements to the U.S. Wastewater Methane and Nitrous Oxide Emissions Estimates. Working paper

Over the past 2 years, EPA’s estimates of both nitrous oxide and methane emissions from wastewater treatment in the United States have increased by over 100 percent. This increase has been due to the addition of new industrial categories and several significant methodological improvements, including a factor to account for emissions from septic tanks, and an expanded estimate for nitrous oxide from domestic wastewater treatment plant s. These revisions make the U.S. EPA’s estimate one of the most comprehensive estimates of greenhouse gas emissions from wastewater. As a result, US wastewater emission estimates for 1990 have increased from 18.3 Tg CO2 Eq. in 1999 to 36.8 Tg CO2 Eq. in 2001. This paper will describe each of the revisions. First, the paper will discuss the methane emission factor for domestic wastewater, including the impact of septic tanks. Secondly, it will report on experience gathered as a result of developing first-time U.S. emission estimates for significant industry categories from annual production data, wastewater composition and treatment data, as well as input from industry experts. Finally, it will detail the effort to develop a comprehensive compounded estimate of nitrous oxide from wastewater, accounting for direct emissions from the plant processes, as well as for previously unaccounted additional nitrogen loadings from garbage disposals, bath and laundry wastewater