In every urban area, heat that humans have generated to shower, wash clothes, cook, and so on flows underground — in the sewers, making them very warm. Today, sewers represent the largest source of heat leakage in buildings. Even toilet water, which is at room temperature, is warm compared to the ground. Sewer air, pipe material (and thus conductivity), surrounding soil type, and other factors also affect the final temperature of waste water, according to Genevieve Tokgoz, Project Engineer in the Research and Innovation Division in the Utility Planning Department of Metro Vancouver, Canada.
A few municipalities have begun to recover some of this energy and to use it to heat and cool buildings. Vancouver used it to heat the athletes’ village for the 2010 Winter Olympics. Paris uses it to provide 10 percent of its energy needs, including heating buildings within about 600 feet of each heat exchange facility. Brainerd, a small city in Minnesota, is building a similar system in partnership with Hidden Fuels, a company based there, and thanks in part to a $45,000 grant from the federal stimulus package.