The issue of climate change has led to an increased emphasis on sustainable practices in almost every facet of our lives. For water utilities, this has increased scrutiny on energy use. Although traditionally viewed solely in financial terms, energy use is also the primary source of greenhouse gas (GHG) emissions from water utilities. The emerging concern over GHG emissions coincides with potential federal legislation and regulation by the Environmental Protection Agency (EPA). In order for water utilities to determine their GHG emissions, guides and tools must be made readily available. Information to educate water utilities about their GHG emissions is often scattered and calculation tools are not publically available for utilities in the United States.
The main objective of this research was to develop an accounting tool to facilitate water utilities in calculating their GHG emissions. This tool will allow a water utility to create a GHG emissions baseline and assist in meeting any emissions reduction goals. More specifically, this research project focused on four sub-objectives: (i) to create an Excel-based program to serve as the shell of the GHG emissions accounting tool, (ii) to develop energy prediction equations for different portions of the water production process, (iii) to include the water-energy nexus in the accounting tool, and (iv) to test the program using real data at various water utilities.
A thorough literature review was conducted to determine all available data and equations that pertained to the GHG emissions of water utilities. This information was used to create a GHG emissions accounting tool that was designed to be flexible enough for use by a wide range of utility sizes, treatment processes, and locations in the United States. Energy prediction equations were developed for the raw water collection and finished water distribution phases of a water utility. A prediction equation for the treatment processes was not able to be developed with the current data set; therefore, literature data were utilized for energy prediction purposes in that phase. These prediction tools as well as a water-energy nexus evaluation were included in the program. The survey data obtained to form energy prediction equations had an average energy use of 3.1 kWh/1000 gallons.
The GHG emissions accounting tool was tested at seven water utilities in Georgia, North Carolina, and South Carolina. The average carbon inventory of the seven utilities was 1240 kg carbon dioxide equivalents (CO2-eq.)/MG. Two of the utilities tested exceeded the EPA reporting rule threshold of 25,000 metric tons of CO2-eq./yr. Assuming an average carbon inventory of 1240 kg CO2-eq./MG, water utilities with a flow rate higher than 55.2 MGD would also exceed the reporting rule limit. These values vary greatly when utilizing different electrical grids. When using the highest and lowest GHG emitting EPA subregions, the seven utilities tested had an average carbon inventory ranging from 550 to 2190 kg CO2-eq./MG. The flow rate required to exceed the EPA reporting rule threshold ranged from 31.3 to 123.5 MGD when using the previously stated carbon inventory averages. The main source of the GHG emissions within a utility is pumping because the raw water collection and finished water distribution phases can account for 75% of the carbon inventory. The carbon footprints of the seven utilities compared favorably to literature data. The main source for the carbon footprints of all seven utilities were Scope 2 (electricity-based) emissions, which accounted for at least 80% of the total emissions. The water-energy nexus evaluation showed that the water consumed in generating electricity for all seven water utilities was less than one percent of the total average production from each utility
