Millions of dollars’ worth of thermal energy is flowing beneath the streets. Around the globe, a few utilities are tapping into that energy as a smart sustainability strategy. With more than 90 such projects already operating in northern Europe, China, and Canada, the United States is finally entering the market.

This new renewable energy market is sewer heat recovery. In the United States, the King County Wastewater Treatment Division in Seattle is leading the way. The utility is seeking “deep-green real estate developers” and district energy companies to help determine the best way to develop renewable thermal energy resources by tapping into the network of regional sewer lines. In mid-July, county officials sent out a formal request seeking interest and information that included a regional map of the area’s large piping system to test the potential interest and to better understand what might be necessary to encourage real estate developers to use the thermal energy in wastewater.

“We think that this can be a win/win where we are tapping a waste stream and putting it to work for our ratepayers, our communities, and the planet,” says Jessie Israel, manager of the King County Resource Recovery Section. “It requires us to think out of the box as a government and consider technologies that we haven’t previously considered.”

Says Dow Constantine, King County executive, “We have a hidden supply of energy that flows right under our streets—energy that can reduce our reliance on fossil fuels. We want to hear from the private sector about how we might capture this thermal energy to heat and cool and power our buildings.”

Heat from domestic hot water supplies
can be recaptured from city wastewater.

To understand how sewer heat recovery works, think of the large network of wastewater pipes lying under the city as a constantly flowing source of millions of British thermal units (Btu) of energy each day. These Btu are constantly replenished as people take baths and showers, do dishes, and wash laundry. Industrial, commercial, and institutional users such as hospitals and schools contribute as well by sending heated water down their drains. That thermal energy is carried along in the pipes until it reaches the wastewater treatment plant, where systems remove the nutrients and other pollutants in order to clean the water.

In sewer heat recovery, nonclogging filters and pipes with heat exchangers transfer thermal energy from the wastewater stream to water-to-water heat pumps. The relatively stable temperature of the wastewater stream makes these heat pumps very efficient because it takes less energy to heat water that is already warm. Buildings using these systems then have a separate loop of clean water that circulates in the buildings where efficient radiators heat or cool the living and working spaces in the building. This approach is also used to provide domestic hot water for cleaning, showers, and other hot water uses. The same principles that heat water can also be used to cool water by rejecting unwanted heat back into the sewer lines.

Smart developers can tap the network of collection lines near their properties wherever flows are sizable enough to justify the equipment. For example, a project on the drawing boards in Seattle is studying a 500,000-square-foot (46,000 sq m) commercial building located adjacent to a regional sewer line that flows an average of 20 million gallons (76 million liters) per day. Engineers estimate that tapping that pipe could easily heat and cool 3 million square feet (280,000 sq m) of commercial space, with little impact on the sewer line temperature. If the impact on the temperature is limited, more than one project can be installed on a sewer line if there is enough flow and distance between the projects.

King County is not the only American utility investigating such systems. Chicago is testing several different technologies at its Kirie Water Reclamation Plant in Des Plaines, Illinois. Interest has been piqued at several other large utilities, and the process has been considered in smaller markets, including both Moscow, Idaho, and Brainerd, Minnesota. In New Jersey, Hillsborough-based Natural Systems Utilities, a private utility company that master develops sustainable water systems, couples sewer heat recovery strategies with its building and district-scale water reuse systems.

Sewer heat recovery is not a new idea. Numerous cities in northern Europe have systems, including Oslo, Norway, which has been extracting heat from the sewer mains with the Skøyen district heating plant for years. In Vancouver, British Columbia, the Southeast False Creek district, designed to accommodate participants in the 2010 Winter Olympics, has North America’s first operating sewer heat recovery district energy system. Japan’s Kansai Electric Power Company and Osaka City University started quantification tests of sewer heat recovery in March. Manufacturers such as Huber, based in Berching, Germany, offer machinery for the systems, as do specialty firms that design and build sewer heat recovery systems, including International Wastewater Heat Exchange Systems Inc. in Burnaby, British Columbia, and Rabtherm in Zurich, Switzerland.

Recovering resources from waste is as old as agriculture. What is new is that there are new technologies that cost-effectively harvest the energy and nutrients from the pipe networks connected to centralized sewer collection systems and treatment plants. Utilities like King County Wastewater Treatment that embrace these technologies are demonstrating leadership in the rapidly changing world of urban infrastructure.