Shower drains and dirty dishwater and laundry water could be on the cutting edge of energy efficiency and recovery.

Around the world, and more recently in the U.S., cities are realizing that the water leaving our homes and offices—specifically, warm and hot wastewater—is an astoundingly powerful source of energy. One estimate is that Americans flush 350 billion kilowatt-hours of energy into the sewers each year—roughly enough to power 30 million U.S. homes. Cities are taking notice, and taking steps to install sewage heat recovery systems to get a piece of that energy resource.

"I never thought I'd be saying the words that 'Sewage heat recovery is the coolest thing,'" said Jessie Israel, resource recovery manager at King County's Wastewater Treatment Division.

A Hot Resource

The technology is simple. Wastewater, which consists of what gets flushed down toilets but is mixed with millions of gallons of hot water from showers, dishwashers, washing machines, and more, maintains a fairly constant temperature as it travels through sewers to the treatment plant—typically about 60°F (15.6° C), though this varies by geography and season.

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 all operates as a closed-loop system, meaning that the dirty water never touches the clean water. But the warmth of the sewage water helps heat the water that is then used in showers, washing machines, dishwashers, or even in radiators to help heat buildings.

The trick of the system is that it takes a lot less energy to heat 60°F water than to heat cold water. And in the summer, buildings with sewage heat recovery systems can reverse their heat pumps and use the 60°F-sewage to dissipate excess building heat, and reduce a building's air-conditioning costs. This is exactly the way that geothermal heat pumps can help reduce both building heating and cooling costs by tapping into the moderate, constant heat of the Earth. But geothermal systems require expensive and disruptive digging deep beneath buildings to pipe up the Earth's heat. In contrast, "sewer-thermal" systems harvest the man-made heat from shallow underground pipe systems that already have been built.

The first big wastewater heat recovery system in North America is in Vancouver, British Columbia, providing 70 percent of the energy needs to the community known as the Olympic Village. The former industrial waterfront site, renovated for the 2010 games and now a residential and mixed-use enclave, aspires to be one of the greenest communities in the world, in part by recycling wastewater heat. The finger-like exhaust stacks of the facility, False Creek Energy Centre, are designed as public artwork, topped with LED lights that glow blue at times of low energy use and red when demand is high. (Related Photos: "London Leaps Hurdles in Green Olympic Games Bid")

With sewage heat recovery systems also in operation in Tokyo and in Oslo, Norway, as well as in the million-square-foot Beijing South Railway Station, the United States now has begun experimenting with this unlikely source of energy. (Related Quiz: "What You Don't Know About World Energy")

In Chicago, where the Metropolitan Water Reclamation District had been scrambling to cut costs, a sewage heat recovery system went online in May. The $175,000 system, designed by a University of Chicago professor, was funded 50-50 by the district and the private nonprofit Illinois Clean Energy Community Foundation. "We've cut our energy costs for heating and cooling [at the plant] by 50 percent," since it was installed May 7 at the James C. Kirie Water Reclamation Plant, said Catherine O'Connor, district director of engineering. The Chicago system captures its energy from effluent, or treated wastewater, rather than raw sewage. (Related Interactive Map: "The Global Electricity Mix")

Even in midwinter in Chicago, the water leaving the treatment plant is a relatively constant 55°F (13°C), even if the air outside is significantly below freezing. The system should pay for itself within just three years, O'Connor said. And heating and cooling Kirie's buildings uses only 2 percent of the potential energy in the effluent, so there's plenty more savings to be had, by using the wastewater heat in nearby buildings—not just in the water treatment facility.

Spreading the Word

The success of fledgling sewer heat recovery systems has won newfound respect for the role wastewater can play in addressing the world's energy challenge. "Not a lot of people shake your hand when you're in the sewage business," jokes Lynn Mueller, president of International Wastewater Heat Exchange Systems, based outside Vancouver. "But everybody I've talked to in the past two years recognizes the amount of heat that goes down the sewer. It should be recovered."

Many local governments are eager to give it a try. When King County, Washington, which includes Seattle, held an informational session for developers, "we were standing room only," Israel said. "Two years ago, nobody knew about sewage heat recovery. Now . . . when I'm at meetings, people come up to me and say, 'Hey, sewage heat recovery. Tell me about that.' " While no shovels have hit the ground yet, there's a lot of interest from real estate developers to tap into Seattle's 250 miles (402 kilometers) of pipes filled with warm water.

To turn that resource into something useful is more difficult, though. The county is still trying to figure out what exactly it means to offer a developer access to its sewers and what sorts of paperwork that would entail. Developers say they need more accurate data about the temperatures of the system and how they fluctuate from place to place and from hour to hour.

One way to simplify the process would be to capture the wastewater energy circulating beneath and inside individual Seattle buildings, without venturing into constructing a system that would involve the public infrastructure beneath city streets.

That's where Mueller comes in. His system is designed for installation in individual buildings, rather than on or near city-owned property, so it captures the heat from wastewater going down drains even before it leaves the building. At this point, the wastewater heat is an even greater resource; because of building insulation, the wastewater is about 70°F (21°C) "I always thought to myself, 50 percent of a building's energy goes to heating and cooling, and almost 50 percent goes down the drain." His company installed a system at a group of townhouses in Vancouver that reduced their energy usage by 75 percent and helped the development earn LEED Platinum, the highest rating in the internationally recognized certification system administered by the U.S. Green Building Council.

But sewage heat recovery is progressing in baby steps rather than major strides. For example, Brainerd, Minnesota, a town of 13,000 two hours north of Minneapolis, has measured the temperature of its sewage outflow in an attempt to quantify how much energy it has at its disposal, but the project's now sitting dormant. No million-square-foot developments here—yet.

Israel says the industry will need to advance several more demonstration projects to show that the technology can work in many different geographies and environments. And she and other advocates of the technology will have to do a lot more work to educate the public about the surprising potential of dirty water as a source of clean energy.