DENVER (AP) — When a massive event center was being developed in Denver, planners had to contend with two existing 6-foot (1.8 meters) wide sewer pipes that emptied into the river, creating an unsightly dilemma. Developers wanted to bury them. The utility said the wastewater needed to vent heat before entering the river.
There, a problem became a solution.
Thermal energy from the sewage now powers a system that heats and cools classrooms, an equestrian center and veterinary hospital at the National Western Center complex.
It’s a recent example of how wastewater flushed down the drain can heat and cool buildings in a sustainable way. Climate experts say sewage is a largely untapped source of energy due to its stable temperature of approximately 70 F (21 C). Wastewater heat recovery systems have already been installed in California, Washington, Colorado, New York and Canada. Pipes that transport sewage are already built, making it a low-cost and widely available resource that reduces the need for polluting energy sources.
There’s no odor since the thermal energy transfer systems keep the wastewater separate from other components.
“Wastewater is the last frontier of sustainable energy,” said Aaron Miller, the eastern regional manager for SHARC Energy, adding: “Even in this current environment where environmental stuff doesn’t really sell, there’s a financial benefit that we can sell to business owners.”
While the technology works in a variety of locations, the Denver complex was uniquely positioned because it’s close to major sewer lines in a low-lying industrial zone. The vast majority of the center’s heating and cooling comes from wastewater heat recovery. During extremely hot or cold weather, cooling towers and boilers are used to fill in the gaps.
“Every city on the planet has a place just like this,” said Brad Buchanan, the center’s CEO. “This is actually a value, a benefit that the bottoms have that the rest of the city doesn’t have.”
How heat from sewage can warm buildings
Extracting the thermal energy starts with the water from toilets, showers and sinks traveling down usual sewage lines before flowing into a tank that is part of the heat recovery system. Heavy solids are separated and the remaining fluid flows through a heat exchanger, a sealed device with stacks of metal plates that can take heat from one source and put it into another.
Thermal energy from the wastewater is transferred to a clean water loop without the liquids coming into contact. The clean water carrying the thermal energy is then sent into a heat pump that can heat or cool rooms, depending on the weather. It can also heat potable water. Once the thermal energy has been extracted, the wastewater flows back into the sewer system and eventually to a water treatment plant.
The heat from the sewage replaces the need for energy from other sources to heat and cool buildings, such as electricity from the grid. Electricity is only needed to run the heat exchanger and pumps that move the water, far more energy efficient than boilers and chillers used in traditional HVAC systems.
Where wastewater heating is being used
Miller said the systems work best in buildings with centralized hot water production, such as apartments, commercial laundromats, car washes and factories. In residential settings, Miller said the technology is best suited for buildings with 50 or more apartment units. The technology works in various climates around the country. Some buildings supplement with traditional HVAC components.
The technology utilizes existing city pipes, which reduces the need for construction compared to some types of renewable energy, said Ania Camargo Cortes, a thermal energy networks expert and board member of the nonprofit HEET (Home Energy Efficiency Team).
“If you can use wastewater, it’s going to be an enormous savings … its billions of kilowatts available to us to use,” said Camargo Cortes.
According to 2005 data from the U.S. Department of Energy, the equivalent of 350 billion kilowatt-hours’ worth of hot water is flushed down drains each year.
In Vancouver, Canada, a wastewater heat recovery system helps supply heat and hot water to 47 buildings served by the False Creek Neighbourhood Energy Utility. In 2025, 60% of the energy the utility generated came from sewage heat recovery, said Mark Schwark, director of water and utilities management at the City of Vancouver.
The future of wastewater heat recovery
Aaron Brown, associate professor of systems engineering at Colorado State University, said he believes use of the wastewater heat recovery systems will grow because it is an efficient, low-carbon system that is relatively easy to install.
Unlike solar or wind power that can vary by weather or time of day, thermal energy from sewage can be available whenever it’s needed, Brown said.
“I think that to decarbonize, we have to think of some innovative solutions. And this is one that is not that complicated as far as the engineering technology, but it’s very effective,” said Brown.
Epic Cleantec, which makes water reuse systems for office and apartment buildings, is expanding into heat recovery after previously focusing on treating water for toilets and irrigation. The company recently installed a wastewater heat recovery system in a high-rise building in San Francisco.
Aaron Tartakovsky, co-founder and CEO of Epic Cleantec, said people have been conditioned to think that wastewater is dirty and should always be discarded, but his company recently launched two beers in collaboration with a brewer made from recycled shower and laundry water to illustrate novel ways to reuse it.
“I think wastewater recovery is going to be a continuously growing thing because it’s something that we’re not taking advantage of,” said Tartakovsky.
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Peterson reported from Denver and O’Malley from Philadelphia.
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