Going Beyond the Call of Duty to Manage Greenhouse Gas Emissions

The Metro Wastewater Reclamation District (Metro District) in Denver, Colorado, is known for their innovative, forward thinking – employing biogas for digester heating as well as for onsite electricity generation at the Robert W. Hite Treatment Facility (RWHTF). The RWHTF is owned and operated by the Metro District and receives wastewater from various regions in the Denver Metro area including Denver, Arvada, Aurora, Brighton, Lakewood, Thornton, and Westminster. The facility serves approximately 1.8 million people across 715-square miles receiving an annual average daily influent flow of 148 million gallons per day (mgd). Nearly 100 percent of sewage sludge is treated to Class B biosolids for land application to farmland. This environmentally conscious mindset, as well as evolving greenhouse gas (GHG) emissions regulations, has led the Metro District to pursue a deeper understanding of their GHG emissions sources and offsets at the RWHTF and identify potential opportunities to further offset emissions while optimizing treatment of the wastewater.To quantify and track sources of GHG emissions at the RWHTF, a GHG emissions inventory was developed by the Metro District and Carollo Engineers, Inc. Of the original six GHGs that have been prioritized for regulatory and inventory purposes, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are relevant to wastewater treatment emissions/offsets and are the focus of this study.The GHG Inventory verified that purchased electricity and offsite chemical production were the primary contributors to GHG emissions in 2015. Approximately 40 percent of total emissions generated by the RWHTF operations were attributed to purchased electricity, which is typical for wastewater treatment facilities (ranging between 40-60 percent). While this is not a regulated source of GHG emissions, it represents another area for potential improvements and cost savings. Off-site chemical production proved to be a significant source of carbon dioxide equivalent (CO2e) emissions from the facility, accounting for approximately 27 percent of the total emissions. Reduction in CO2e emissions are possible through the use of alternative chemicals, such as peracetic acid for disinfection in place of sodium hypochlorite, aqua ammonia, and sodium bisulfite. This tool has provided the Metro District the ability to quantitatively show a reduction in GHG emissions and is being used to support planning decisions.The RWHTF has taken several measures to offset GHG emissions – equal to approximately 50,000 metric tons of CO2e, which is equivalent to removing over 10,500 passenger vehicles from the roads for an entire year.