In April, treated Denver wastewater flows into the South Platte River. In what may be the largest of its kind in the United States, Denver will use excess energy from sewage to heat and cool a new agricultural, art, and education center. Hart Van Denburg/CPR News hide caption

In April, treated Denver wastewater flows into the South Platte River. In what may be the largest of its kind in the United States, Denver will use excess energy from sewage to heat and cool a new agricultural, art, and education center.

There is a secret treasure house of clean energy hidden in the sewers, and people are increasingly trying to use it in the fight against climate change.

The U.S. Department of Energy estimates that Americans flush enough energy into the sewer every year to power approximately 30 million homes. The source is usually daily necessities in the household. Think about hot showers, washing machines and sinks. Continuously evolving technology makes it easier to use mainly warm water.

According to Enwave, a Canadian energy company preparing to operate the system, Denver is currently building what may be the largest sewer heat recovery project in North America.

Over the next few years, a $1 billion renovation will turn this 250-acre site into an arts, education, and agricultural center, home to the National Western Stock Show and rodeos. The transformed National Western Center will include approximately 1 million square feet of new indoor space, all of which will be heated and cooled by the energy of the sewer pipes below.

Brad Buchanan, chief executive officer in charge of the redevelopment, said the project has changed his view of the best location for real estate. Large-scale sewer infrastructure has long hindered development. Now he imagines that they might be sought as a way to save energy costs and avoid greenhouse gas emissions.

The National Western Center estimates that the project will help it avoid the carbon equivalent of driving an ordinary gasoline-powered car 250 times a year around the equator.

Buchanan said: "It will be interesting to see if people start to pay attention not only to the location of light rail lines or good schools, but also to the distances around large sewer pipes."

The National Western Center negotiated the "permanent" heat in the sewers under its construction site. It hopes to save energy costs and avoid emissions from climate warming. Hart Van Denburg/CPR News hide caption

The National Western Center negotiated the "permanent" heat in the sewers under its construction site. It hopes to save energy costs and avoid emissions from climate warming.

How to mine sewer heat

The technology for collecting heat from the sewer is not complicated.

At the National West Center, construction crews have completed a pit that exposes the main sewer. Local officials said that regardless of the weather outside, the wastewater inside would maintain a mild temperature of 55 to 75 degrees throughout the year. This consistent temperature can be used to heat and cool buildings on the ground.

The key is a huge heat pump, which will be housed in a central factory on the campus. The working principle of this equipment is similar to that of a reversible air conditioner. In winter, it transfers energy from sewage to the clean water circuit connecting the building, adding heat to the indoor space. The process can then be reversed to stay cool in the summer.

And to answer an obvious question: No, untreated sewage will never be exposed to the air, so people living in buildings will not be affected by the smell of sewers.

Shanti Pless, a research engineer at the National Renewable Energy Laboratory in Golden, Colorado, said this technology opens up a broad world of "renewable heat extraction."

"With the advent of large heat pumps, we can cost-effectively use, for example, 70-degree waste water to heat our buildings and hot water systems," Pless said.

The staff works in the National West Center. Denver's sewage treatment district now hopes to attract other developers to arrange for the use of sewage heat to heat and cool buildings. Hart Van Denburg/CPR News hide caption

The staff works in the National West Center. Denver's sewage treatment district now hopes to attract other developers to arrange for the use of sewage heat to heat and cool buildings.

Rethink how we heat and cool buildings

Pless said the biggest obstacle is not technology-it is helping developers rethink the size of their heating and cooling systems. Sewer heat recovery is usually best suited as the core of a regional-scale energy system, where a central factory provides energy for the entire community or office building.

Denver itself shows how this centralization strategy has fallen out of favor in the United States.

A 141-year-old steam heating system heats many of the city’s downtown buildings and is powered by a central natural gas plant operated by Xcel Energy. Since the cost of improving the system has fallen on customers, many building operators have disconnected from steam services and switched to smaller independent heating and cooling systems.

Nevertheless, Pless said that district energy usually makes sense, but it requires careful study of the energy needs and resources of a particular area. NREL has developed tools to help the community simulate the feasibility of heat recovery systems. He said the National Western Center could also help open a path to the sewer.

"This is a good local example of how we brought this idea to other parts of the country," Pless said.

Sewer energy gold rush?

If sewer energy becomes popular, one of the reasons may be the potential benefits of sewage treatment areas.

That's because warm sewage can cause its own environmental problems. In Denver, wastewater is usually hotter than the South Platte River, which is the final destination after passing through the treatment plant. This "thermal pollution" can harm local plants and wildlife.

Mickey Conway, Metro Wastewater District Manager, said that cooling towers can be built to cool the treated sewage, but the National Western Center project can show a better alternative. New houses and buildings can collect this heat, cool the water, while avoiding emissions that cause climate warming, and potentially reduce energy bills.

"This is a perfect storm of a win-win situation," Conway said.

Metro Wastewater has produced an exquisite brochure that uses sewer heat to attract other developers. Conway said there are no reliable plans yet, but the area has "discussed with Denver and Thornton," a subway suburb in Denver.

At the same time, the National Western Center has taken action to protect its supply in the event of a sewer hot gold rush. The project’s partners, the City and County of Denver, exercised a three-year option for exclusive access to energy in pipelines running through the campus.

Project CEO Buchanan said that this is equivalent to a new environmental resource. His development projects have sewer heat rights, not mineral rights or water rights.

"We protect it because we always rely on this energy," he said.