Using sewers to heat homes

All the water you flush from your toilet and all the greywater from your shower and sinks leaves your house warm, and it warms the sewer pipes as it travels to the processing facility.  Now the StarTribune reports that a suggestion has been made to recapture that heat:

"There's a significant amount of energy, literally enough to heat hundreds of homes within the streets of the city of Brainerd," Peter Nelson of Hidden Fuels told the Minnesota Public Radio News...

Hidden Fuels would rely on technology already in use. A heat pump will circulate water from which energy can be extracted to heat or cool buildings, similar to how geothermal heating and cooling systems work.

Officials in Canada used a similar system during the Vancouver winter Olympics. But it didn't have the challenges of using an existing sewer. What Nelson and others want to do in Brainerd is new because the water's full of waste...

Earl Wolleat, director of buildings and grounds for the school district, said he's confident Hidden Fuels can pull it off, but it will take years before it's cost effective.