Evolving technology is making it possible to turn sewage wastewater into energy that can heat and cool large buildings. The largest such project in the U.S. is under construction in Denver.

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RACHEL MARTIN, HOST:

A secret weapon against climate change might be hiding in sewers. The underground pipes contain excess energy that could heat and cool buildings. The concept is called sewer heat recovery. One of the largest such projects in the U.S. is underway in Denver. Sam Brasch of Colorado Public Radio reports.

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SAM BRASCH, BYLINE: At a construction site in north Denver, Katie Hegarty oversees a crew inside a recently dug pit. At the bottom is a sewer pipe. It's one of the main arteries transporting the city's wastewater.

KATIE HEGARTY: We still call it wastewater, but it's not wasted. We're using it to recover heat that can both heat and cool this campus.

BRASCH: That campus will soon host the National Western Stock Show and Rodeo. It's an old West event getting new West digs. A massive remodel is adding an arena, offices and classrooms. And all that new space will be kept comfy with sewer heat.

HEGARTY: I've built all sorts of different projects. Nothing like this before.

BRASCH: It's no mystery where the excess heat comes from. Any time you take a shower or wash the dishes, you're literally sending energy down the drain. The Department of Energy estimates the total amount could power about 30 million U.S. homes for a whole year. By recovering some of it, buildings can avoid climate warming emissions. And there's another benefit, too.

WILLIAM CAVANAUGH: After the wastewater is collected and treated, we return it to the river and recharge the river at this location.

BRASCH: William Cavanaugh is with the Metro Wastewater Reclamation District. We're outside its massive treatment plants, where waterfalls of clean sewage cascade into Denver's South Platte River. Cavanaugh says the water is warmer than the river itself and that thermal pollution can harm aquatic life. But the sewer heat system will actually cool the wastewater.

CAVANAUGH: That's correct. We're removing thermal energy. And so that'll help us move towards our goals of reducing the temperature in our effluent.

BRASCH: So to recap, the project will cut carbon emissions and improve river health. It's a win-win. So what's making this possible now? Shanti Pless is with the National Renewable Energy Laboratory. And he says one reason is improvements to heat pumps, which are basically reversible air conditioners.

SHANTI PLESS: With the advent of large-scale heat pumps, we can now cost effectively, you know, use, you know, say, 70-degree wastewater to heat our buildings and our hot water systems.

BRASCH: Pless says the technology is opening up a whole new world of renewable heat mining. But those systems tend to work best for whole neighborhoods or districts, not individual homes or buildings. He says the Denver project could prove the upfront investment is worth it.

PLESS: The National Western project has been a great local example for us to take that idea to the to the rest of the country.

BRASCH: But the people behind the project also worry it could trigger a kind of sewer heat gold rush. Brad Buchanan is the CEO of the National Western Center. And he says his plans for the campus came together.

BRAD BUCHANAN: We have to answer the question, how do we protect these sewer thermal energy rights?

BRASCH: The result was an agreement guaranteeing the campus exclusive access to energy inside nearby sewer pipes. Buchanan says that has him thinking about the whole future of real estate.

BUCHANAN: If folks sort of start to look at not just where light rail lines are located or good schools are located, but what's the proximity to a large sanitary sewer line running through Denver somewhere?

BRASCH: Because if you choose the right place, a wealth of sewer heat could be all yours. For NPR News, I'm Sam Brasch in Denver.

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Tags: Series: Environment And Energy Collaborative