When we think of sewage, we usually think about what we flush down the toilet. But that’s only part of a wastewater system – there’s also water from showers, washing machines, and dishwashers that contains something that can generate sustainable energy: Heat.
The U.S. Department of Energy estimates that Americans send the equivalent of 350 billion kilowatt-hours of energy down the drain each year – enough to power about 32 million homes.
The CSU Spur campus at the National Western Center is part of the largest sewer-heat recovery project in North America, tapping into Denver’s dirty water to turn it into clean energy to power furnaces and air conditioners.
The concept is straightforward: In winter, extract heat from sewage and recycle it to warm buildings; in summer, reject heat into sewage and cool the buildings, dramatically reducing demand for natural gas and electricity.
The National Western Center’s heat recovery system is projected to fill 90% of the heating and cooling needs in seven buildings encompassing more than 1 million square feet at buildout. It is also expected to save 2,600 metric tons of carbon dioxide per year – equivalent to eliminating 6.6 million vehicle miles from roadways.
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. When the final CSU Spur building, Hydro, opens in 2023, it will be the fourth building in the energy district.
How it works
The system starts with the two 72-inch pipelines that carry wastewater from tens of thousands of homes and businesses to the Robert W. Hite Treatment Facility, the largest wastewater treatment facility in the Rocky Mountain West. Some of the wastewater is diverted from the pipelines on the western border of the National Western Center to a central utility plant for processing and on to the heat recovery system.
During cold months, an industrial plate-and-frame heat exchanger draws warmth from the dirty water and transfers it to clean water circulating through the energy district in a closed loop. When warm water arrives at each building, equipment again transfers heat – this time, from the ambient loop to a forced-air heating system. Back at the central utility plant, dirty water returns to the sewer.
During warm months, the process reverses: The system transfers heat from buildings to the wastewater and replaces it with air cooled by the fresh-water loop.
The system saves significant energy in part because sewage maintains a fairly constant temperature, between 55 degrees and 75 degrees, throughout the year – already close to ideal building temperatures. 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.
Early in the planning process, Jim McQuarrie, former director of technology and innovation for Denver’s wastewater utility, discussed the concept of a sewer-heat recovery system with Ken Carlson, director of CSU’s Center for Energy Water Sustainability.
In 2016, as part of the system the CSU team recommended burying the massive sewer pipes that had blocked access to the South Platte River for decades.
A longer version of this article originally appeared in STATE Magazine, Summer 2022