This renewable energy project will make you love that dirty water
By Coleman Cornelius | Photography by Matthew Staver | July 1, 2022
AT THE NATIONAL WESTERN CENTER, an unparalleled system is mining dirty water for clean energy. It’s the largest sewer-heat recovery project in North America.
You’re not alone if you read the word “sewer” and thought, “Wait, what?”
Yes, this green energy relies on raw sewage from thousands of homes and businesses in Denver – a great gush of wastewater expelled from dishwashers, washing machines, sinks, showers, tubs, and toilets. Sewage often is associated with its fecal content, but it contains something far more relevant to sustainable energy. That’s heat. Consider: An 8-minute shower typically uses a whopping 20 gallons of water at roughly 105 degrees Fahrenheit. With each load of laundry, a high-efficiency washing machine could gulp 13 gallons of water at up to 130 degrees. And, with each cycle, a dishwasher might use 4 gallons of water at 140 degrees. That’s a lot of water – and a lot of heat – down the drain.
In fact, the U.S. Department of Energy estimates that Americans send the equivalent of 350 billion kilowatt-hours of energy down our drains each year – enough to power about 32 million U.S. homes.
“It really is just wasted heat,” said Leslie Fangman, a civil engineer and vice president of corporate development for CenTrio. CenTrio is part of a consortium called EAS Energy Partners, which was selected by the National Western Center Authority and the city and county of Denver to finance, design, build, operate, and maintain the sewer-heat recovery system.
The sewer-heat recovery system extracts heat from wastewater in the wintertime and uses it to warm buildings. In the summertime, the system reverses and rejects heat to cool buildings. Illustration: National Western Center Authority
The project relies on technology that is more than a decade old but has not been widely adopted, largely because of infrastructure complexities and high upfront costs. Yet, the concept is straightforward: During wintertime, extract heat from sewage and recycle it to warm a network of buildings, called an energy district; during summertime, use the same system to reject heat and cool the buildings. In so doing, dramatically reduce use of natural gas and electricity, which power furnaces and air conditioners.
After several years of planning and construction, the sewer-heat recovery system is poised to become a highlight of sustainability at the National Western Center. The center comprises 250 acres near I-25 and I-70 in north Denver. It is a $1 billion redevelopment, transforming the historic grounds of the National Western Stock Show into a year-round site for entertainment, education, and innovation. CSU Spur is the center’s educational anchor, with three new buildings dedicated to public education, research, and community outreach around the critical topics of food, water, and animal and human health.
“When I first heard about this system, I remember thinking, ‘Holy cow, there’s a lot of thermal energy capacity that’s going downstream that we could capture,’” said Brad Buchanan, chief executive officer of the National Western Center Authority, which contracted with EAS Energy Partners to build the sewer-heat recovery system. “It really grabbed my attention because we decided to hold a very high bar for sustainability. It seemed to be the perfect fit if we were really going to walk the talk of reducing carbon emissions.”
Leslie Fangman, vice president of corporate development for CenTrio, has led project development on behalf of a business consortium hired for the job.
The heat recovery system took 18 months to design and build. It is projected to fill 90 percent of heating and cooling needs in seven buildings encompassing more than 1 million square feet. That makes the system the largest of its kind in North America. With buildout of the National Western Center’s initial phases, the heat recovery system is expected to save 2,600 metric tons of carbon dioxide per year – equivalent to eliminating 6.6 million vehicle miles from roadways. And it has capacity to expand even beyond the center’s first planned phases of construction.
The system began operating in April. For now, it serves the Vida and Terra buildings on the CSU Spur campus, as well as the nearby HW Hutchison Family Stockyards Event Center; they are the first new buildings at the National Western Center. Soon, the Hydro building will open at CSU Spur, becoming the fourth building in the energy district.
“It’s a great way to recover resources that we usually think about as waste,” said Jocelyn Hittle, who has led development of CSU Spur for the Colorado State University System. “I love the idea of Spur being able to help advance the state of the art by using nascent technology that is novel at this scale.”
The system diverts sewage from a 72-inch pipeline that runs along the western border of the National Western Center. The pipeline carries wastewater from tens of thousands of homes and businesses to the Robert W. Hite Treatment Facility, which is operated by Metro Water Recovery on the city’s northern edge. It is the largest wastewater treatment facility in the Rocky Mountain West.
The side stream of dirty water enters the Central Utility Plant at the National Western Center and runs through a grinding system to break down solids before wastewater goes through a heat exchanger and then flushes back to the sewer. During cold months, an industrial plate-and-frame heat exchanger draws warmth from the dirty water and transfers it to clean water that constantly circulates through the energy district in a closed loop. Clean water never touches dirty water as it runs through this “ambient loop.” When warm water arrives at each building, equipment again transfers heat – this time, from the ambient loop to a forced-air system, which then cycles warmth through building air. Back at the Central Utility Plant, dirty water returns to the sewer; it is enclosed in pipes, so the sewage does not emit odors.
During warm months, the process reverses: The system extracts heat from air in district buildings and transfers it to the ambient loop, then on to sewage – thereby rejecting heat from the energy district. Wastewater again runs to the Hite Treatment Facility, while cool, clean water runs into the energy district. At each building, cooler temperatures then are pulled from the ambient loop and cycled through building air.
In both cases, heat pumps are needed to extract and exchange thermal energy, and water is the medium sharing that energy. In this way, the sewer-heat recovery system may warm or cool buildings. If clean water in the ambient loop isn’t the desired temperature, boilers give it a boost in cold months, and cooling towers reduce it in hot months.
“You wouldn’t even be aware it exists, but the system really is revolutionary. It really represents a lot of the city’s goals toward resiliency, and it’s a great example of how we can do something creatively and innovatively,” said Mike Bouchard, program director for the Mayor’s Office of the National Western Center. The city and county of Denver owns National Western Center land and several center facilities; the office spearheaded the procurement process for the heat recovery system, coordinated efforts with center partners, and constructed the ambient loop.
The Delgany Interceptor pipes run along the border of the National Western Center and convey sewage to a treatment plant. The pipes were above ground, shown at left (Photo: Metro Water Recovery). The pipes were replaced and buried, above right, providing a prime chance to build a new green energy system.
The Delgany Interceptor pipes run along the border of the National Western Center and convey sewage to a treatment plant. The pipes were above ground, shown at top (Photo: Metro Water Recovery). The pipes were replaced and buried (bottom) providing a prime chance to build a new green energy system.
The system saves significant energy in part because sewage maintains a fairly constant temperature, typically ranging between 55 degrees and 75 degrees throughout the year. That means the source already is close to ideal building temperatures, said René Moffet, who managed system engineering and design for AECOM Technical Services Inc., another of the EAS Energy Partners. Saunders Construction of Denver built the system as part of the partnership.
“This system is something we can take a lot of pride in,” Moffet said. “It’s awesome – especially with a project that’s the first of this scope in North America. A lot of people are watching this to see how it will go.”
Top left: Wastewater enters the Central Utility Plant and runs through a heat exchanger, which extracts thermal energy and transfers it to clean water that flows to new buildings in the energy district. Bottom left: If water is not warm or cool enough, boilers or cooling towers adjust water temperature. Right: A closed pipeline of clean water, called an ambient loop, is the medium conveying thermal energy.
Top: Wastewater enters the Central Utility Plant and runs through a heat exchanger, which extracts thermal energy and transfers it to clean water that flows to new buildings in the energy district. Middle: If water is not warm or cool enough, boilers or cooling towers adjust water temperature. Bottom: A closed pipeline of clean water, called an ambient loop, is the medium conveying thermal energy.
The sewer-heat recovery system cost $34 million, financed through a public-private partnership spanning 40 years. At the end of that period, total system costs are expected to be slightly above those of conventional systems, Buchanan, of the National Western Center Authority, said. However, those costs would decrease if the system were expanded to additional construction at the site or if partners were able to capitalize on potential carbon offsets, he said.
“I’m an evangelist for this system,” Buchanan said. “It will be a substantial difference maker with carbon reduction, and it’s pretty easy to get excited about that.”
Brad Buchanan, CEO of the National Western Center Authority, says he is an evangelist for the system.
The concept emerged in 2015 with Jim McQuarrie, former director of technology and innovation for Metro Water Recovery, Denver’s wastewater utility. The utility pursues sustainability and cost savings at the energy-water nexus. It also has a significant issue to manage: To meet state and federal regulations, effluent – or treated wastewater – must be a sufficiently low temperature, especially during cold months, before it can be discharged to the South Platte River. The guidelines are designed to avoid disrupting river ecology. Cooling effluent is a costly and energy-intensive undertaking, so Metro Water Recovery sought an environmentally sustainable way to do it – one that might have benefits well beyond regulatory compliance.
An opportunity arose during master planning for the National Western Center. Among stakeholder objectives was burial of the Delgany Interceptor sewer lines – two pipes, both 6 feet in diameter, that run along the South Platte River on the west side of the National Western Center. The pipes carry Denver sewage to the Hite Treatment Facility. For years, they were above ground – an eyesore that blocked access to the river. McQuarrie and other leaders thought site redevelopment offered a chance to replace and bury the interceptor lines, while fulfilling additional goals: It would be an ideal time to install a landmark renewable energy project, which would save carbon emissions and reduce wastewater temperatures to help meet effluent guidelines; meantime, pipeline burial would open the riverfront for new trails, open space, and National Western Center programming.
The new system cuts “thermal pollution” in effluent and contributes to Denver’s climate goals, making it a model for utilities and municipalities nationwide, said Blair Wisdom, who succeeded McQuarrie as director of technology and innovation at Metro Water Recovery. “It’s really a recycling concept that addresses single-use heat,” Wisdom said. “Denver and the state are recognizing that a lot of greenhouse gas emissions are from people heating and cooling their built environments, and that includes household water.”
The project, which involved dozens of National Western Center stakeholders, also demonstrates the power of collaboration, noted McQuarrie, who now leads water projects for Tetra Tech, a global engineering firm. “One of the most striking things about this whole project is the impact that can be created when people partner together and work toward a common goal,” McQuarrie said. “Something like this requires people to think big and challenge themselves about whether adhering to traditional past practices is truly the best thing for future generations.”
The utility plant is an unobtrusive building containing leading-edge technology.
Early in the planning process, McQuarrie discussed the concept of a sewer-heat recovery system with Ken Carlson, a Colorado State University professor who served as McQuarrie’s adviser as he attained a master’s degree in civil and environmental engineering. Carlson is director of CSU’s Center for Energy Water Sustainability and is an expert on water recycling technologies. He agreed the heat recovery system might work well at the National Western Center; the two pitched the idea to the CSU System, which, in turn, took it to a larger leadership group. Carlson then asked six undergraduates to study the concept – a move that fit well with CSU Spur’s educational goals.
The students – calling themselves “the Sustainulators” – evaluated sewer-heat recovery systems as part of a senior design project, a capstone for CSU students in civil and environmental engineering. During 2015-2016, with the guidance of senior research manager Asma Hanif, the student team gathered reams of data; their meetings, site visits, and final report generated information and enthusiasm leading into formal planning for the heat recovery system. In fact, the CSU team recommended pipeline burial and system installation much like that later accomplished.
Natalie Thompson led the student team. In May 2016, she earned a bachelor’s degree in environmental engineering, with a minor in global environmental sustainability, and went on to attain a master’s degree at the University of Cincinnati. The CSU project heightened her interest in designing water and wastewater systems, Thompson said. Now, she’s doing that engineering work as part of international development projects throughout Uganda.
“Our project was such an exciting time to see how you can incorporate sustainability into design, while also making a space more beautiful,” Thompson wrote in an email sent from Kampala, Uganda. “This project opened my eyes to heat recovery, which makes so much sense when thinking about all the hot water we use in America. It made me see that we should not view wastewater as a waste, but as an opportunity. That really shifted my perspective as someone who has always been inspired by sustainability.”
Natalie Thompson, kneeling, led a CSU student engineering team that studied the sewer-heat recovery system; she’s now working on international water projects
Viewing wastewater as an opportunity – and, specifically, as an important source of thermal energy, nutrients, and fresh water – is at the core of a principle called “One Water.” The theory holds that water has value in all its forms and may be managed through integrated systems and technologies that together improve water quality, access, and sustainability on an increasingly thirsty planet. The sewer-heat recovery system at the National Western Center exemplifies the One Water concept, and university students and researchers will continue to study the system and its benefits, Hittle said. In the forthcoming Hydro building at CSU Spur, researchers with CSU’s One Water Solutions Institute also will advance the One Water idea by testing new technologies for the treatment and use of wastewater, stormwater, and roof runoff.
The combination of big ideas and technical challenges inspired the engineering students who first evaluated the sewer-heat recovery system, Thompson said. “It really ignited my passion for working with communities, understanding needs, and then designing,” she wrote. “I love the idea that sustainability is not just a buzzword, but a lifetime of serving a community.”