It's all too easy to forget the underwater rivers of wastewater running below our feet, through pipes to wastewater treatment plants.

We usually think of this water, which drains from our dishwaters, showers, and, yes, toilets, as carrying just filth -- but it also flushes a great deal of heat from homes and businesses. In cold months, sewage is about 60 degrees, and in summer it can exceed 75 degrees, reports Andrew Maykuth of the Philadelphia Inquirer.

And why let that heat go to waste? To avoid doing so, Philadelphia-based NovaThermal Energy launched its first US pilot project in Philadelphia on April 12 that will harvest heat from still-warm sewage to heat buildings.

The technology is essentially a geothermal heat pump that extracts and concentrates heat as the sewage travels through the pump. In the winter, pipes carry this heat through the walls of buildings; in the summer, another set of pipes carries the heat away.

The critical piece of NovaThermal's tech is primarily scatological: the company holds a patent on a filtering device that removes -- ahem -- "larger debris" that could damage the pump from entering the system.

NovaThermal Energy already has its "wastewater geothermal energy" units installed in China, where they power the Southern Beijing Train Station and the municipal building for the province of Shanxi, among other buildings. And they plan to launch projects in New Jersey and Texas within the year.

Tapping into a constant resource: sewage's heat [Philadelphia Inquirer]

Photo: Sustainable Sanitation/Flickr

This post was originally published on Smartplanet.com