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Heating and Cooling With Sewer Energy


Compiled by Nancy Brannon

You probably know about heat pumps and you may have heard of groundwater heat pumps, which utilize the consistent temperature of groundwater (around 55°F) to heat and cool structures. Now similar technology will be applied at the National Western Center in Denver, Colorado to heat and cool the indoor space with energy from the sewer pipes below. “A secret cache of clean energy is lurking in sewers,” according to NPR reporter Sam Brasch.

“The U.S. Department of Energy estimates Americans wash enough energy down the drain every year to power about 30 million homes,” Brasch wrote. So Enwave, a Canadian energy company, will construct the largest sewer heat-recovery project in North America in the $1 billion remodeling of the National Western Center.

The Center, home to the National Western Stock Show and Rodeo, will be revamped to include about a million square feet of new indoor space, all of which will be heated and cooled with energy from the sewer pipes below. As well as saving money on energy costs, “the National Western Center estimates the project will help it annually avoid the carbon equivalent of driving an average gas-powered car around the equator 250 times.”

How does it work?  “A massive heat pump will be housed in a central plant on the campus. The device works like a reversible air conditioning unit. In the winter, it will transfer energy from the sewage into a clean-water loop connecting the buildings, adding heat to indoor spaces. The process can then be flipped to keep things cool in the summer. The raw effluent is never exposed to the air, so people occupying the buildings won’t get hit with waves of sewer stink,” Brasch explained.

The technology is potentially applicable to many places. “Shanti Pless, a research engineer with the National Renewable Energy Laboratory in Golden, Colo., said the technology opens up a vast world of ‘renewable heat mining. With the advent of large-scale heat pumps, we can cost-effectively use 70-degree wastewater to heat our buildings and our hot-water systems,’ Pless said.”

Sewer heat recovery often works best as the heart of a district-size energy system, where a central plant provides energy to a whole neighborhood or office complex, Brasch explained.

“Pless said district energy often makes sense, but it requires a careful study of the energy needs and resources in a specific area. NREL (National Renewable energy Laboratory) has developed tools to help communities model the viability of heat-recovery systems. He said the National Western Center could also help blaze a trail into the sewers” and set an example for the rest of the country.

Other Benefits
Another potential benefit comes to wastewater districts. Warm sewage water (wastewater) is often hotter than its receiving stream and this thermal pollution can harm stream life, native plants, and other wildlife. It is possible to build cooling towers to chill the treated sewage, said Mickey Conway, Denver Metro Wastewater district manager. But a better alternative is to harvest the heat, cooling the water, and utilizing it for energy, thus reducing sewage treatment costs and energy bills.

Brad Buchanan, CEO of the Western Center project, holds sewer thermal energy development rights and sees sewer thermal energy as a new environmental resource.

Additional Users
However, the technology is not new in 2021 nor exclusive to Enwave. Additional research turned up other companies and places utilizing the technology, as well as earlier articles about the technology.

Huber Technology, also of Denver, Colorado, has developed a system for heat recovery from sewage called ThermWin. “We withdraw sewage from the sewer, screen it, pump it through above-ground Heat Exchangers, and then return it to the sewer.” Read more about their system at:

Veolia is developing methods for transforming wastewater into thermal energy. According to their website: “With the twofold objective of reducing their environmental footprint and reducing their energy expenditure and dependence, more and more cities are turning to renewable sources of energy: hydraulic, wind, geothermal, biomass, etc. Veolia’s Energido solution offers cities the possibility of recovering heat from municipal wastewater. This reduces primary energy consumption, generates energy from a local resource, is available in abundance, and efficiently distributes the energy produced.”

Their system, called Energido, “diverts wastewater to a heat exchanger to transfer the energy it contains to a heat transfer fluid. The recovered calories are sent through a reversible heat pump, which is able to release the energy to supply a heating or cooling network,” according to their website:

In Washington State, King County Wastewater Treatment Division (WTD) is one of the first wastewater utilities in the nation to offer Sewer Heat Recovery (SHR). Private commercial property owners and developers have the opportunity to recover heat energy from WTD sewer pipes for the purpose of heating or cooling their buildings,” according to their website:

In 2014 at the 11th International Conference on Hydroinformatics at CUNY in New York City, Hofman, et al. presented a paper, “Modelling of Thermal Energy Balance in Sewer Systems.” Their model can be used to estimate the recoverable thermal energy and its dynamics. They note that the energy required to provide warm water for showering, bathing, laundry, and dish washing in homes is “10 to 20 times higher than the operational energy required for the water and wastewater treatment and transport.” This water is discharged to the sewer after use, meaning that the sewer is a heat sink in the modern house. “Up to 50% of the energy used in a home is lost in the sewer. Heat recovery in the sewer can be a [viable] option to reduce the use of primary energy sources.

“An important question for heat recovery from sewers is where the most optimum location in the sewer system is to install a recovery system.” Their model predicts the optimum location. “The modeling results and field data show that wastewater contains considerable amounts of thermal energy, which can be harvested.” They conclude that “the sewer heat balance model is an accurate tool to estimate the heat content and recoverable heat from the sewer system.”

In 2012 Informed Infrastructure, a magazine for civil and structural engineers, posted an article, “Sewer Heat Recovery Provides Low-Cost Recycled Energy.” Matteo Luccio writes, “Sewers represent the largest source of heat leakage in buildings.

“In April, Philadelphia [PA]-based NovaThermal Energy announced its first U.S. project to warm a building with heat from sewage. The building, the Southeast Water Pollution Control Facility, belonging to the Philadelphia Water Department, will house the 1 million BTU/hour unit in its basement, where it will directly access and transfer heat from an adjacent sewage channel.”

And in 2012, Rachel Kaufman wrote an article for National Geographic on how “cities worldwide are recognizing sewer heat as an untapped resource that can help cut energy costs.” In the article she explains how household wastewater is typically about 60°F. “In a sewage heat recovery system, a heat pump is used to capture the warmth of wastewater and transfer it to the clean water stream that is entering homes and businesses. It operates as a closed-loop system, meaning that the dirty water never touches the clean water. The warmth of the sewage water then helps heat the water that is used in showers, washing machines, dishwashers, etc.”

So the question for the future is how to expand the technology to recover more of this renewable resource, for both cities and individual households. And what other energy sources, that we now waste, can we turn into productive uses?

Find more information about the National Western Center at:
Brasch, Sam. 2021. “How Your Hot Showers and Toilet Flushes Can Help the Climate.” NPR Morning Edition. May 21.
NREL. Simulation and Optimization of Efficient District Energy Systems.
Hofman, Jan, Martin Bloemendal, Bas Wols, Claudia Agudelo-Vera, Jorge Elias Maxil, Pascal Boderie, Mark Nijman, and Jan Peter Van Der Hoek. 2014. “Modelling of Thermal Energy Balance in Sewer Systems. CUNY Academic Works. International Conference on Hydroinformatics. and
Luccio, Matteo. 2012. “Sewer Heat Recovery Provides Low-Cost Recycled Energy.” Informed Infrastructure. Oct. 24.
Kaufman, Rachel. 2012. “Waste Wattage: Cities Aim to Flush Heat Energy Out of Sewers.” National Geographic News. Dec. 11.

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