With approximately 110,000 people, the City of Gresham (immediately east of Portland) is Oregon’s fourth largest City. Its wastewater treatment plant, located near the Columbia River, has the capacity to treat 20 million gallons of sewage per day, and averages 13 million gallons. That’s a capacity equal to 30 Olympic size swimming pools every day.
Wastewater treatment is an energy-intensive process, so like many cities across the nation, Gresham’s plant has historically been its largest consumer of electricity from municipal operations, using over 6 million kilowatt hours per year. It has also been the City’s largest producer of greenhouse gas emissions.
After nearly a decade of planning and implementing energy efficiency and generation projects, Gresham’s wastewater treatment plant achieved its first net-zero energy consumption day on February 12, 2015. All of the electricity it uses is now produced from onsite renewable power. Approximately 95 percent of it is produced from two cogeneration CAT engines fueled by biogas, which is a by-product of the anaerobic digestion wastewater treatment process that was historically burned and released into the atmosphere. Approximately 5 percent of the energy comes from a 420-kilowatt solar electric system. The production of biogas is enhanced with an innovative fats, oils, and grease (FOG) receiving station that is supplied by private companies that collect FOG from local restaurants, helping to keep Gresham’s sewer pipes clog-free.
Alan Johnston, Senior Engineer with the City of Gresham, states that the innovation goes beyond the technologies at the plant. “The public-private partnerships we have with local FOG hauling companies provides an ideal win-win-win situation. The City and its wastewater utility ratepayers receive a revenue stream in the tip fees we charge the private haulers, and we’re able to double biogas production to produce more renewable energy. The private haulers save money by disposing of FOG with us instead of the landfill, and the environment wins with less material going into the landfill and less greenhouse gas being released into the atmosphere.”
This prolonged effort has turned the City’s biggest energy user into its biggest energy producer. It’s a shining example of a new way of looking at wastewater treatment plants as resource recovery facilities, and can be emulated at plants across the nation.
Generating renewable energy worth over $500,000 annually, along with FOG tip fee revenues of $300,000 per year ($0.08 per gallon), has helped to make Gresham’s investments pencil out financially as well. It’s one of the tools the City is using to keep wastewater utility rates low and predictable.
As the Pacific Northwest’s first energy net zero wastewater treatment plant, and one of only a handful in the United States, Gresham’s plant is getting a lot of attention. As such, it’s helping to promote the use of these reproducible technologies. It is estimated that only 8 percent of wastewater treatment plants in the United States with anaerobic digestion generate electricity or hot water as a renewable energy resource. According to the United States Environmental Protection Agency, up to 400 megawatts of additional renewable electrical power could be generated with the installation of cogeneration units (or other combined heat and power systems) at facilities where it is currently feasible. 400 megawatts of biogas-based renewable energy would prevent approximately 3 million metric tons of carbon dioxide emissions annually, equivalent to the emissions of approximately 596,000 passenger vehicles (U.S. EPA Combined Heat and Power Partnership, 2011).