Over the course of single day, an average American household sends an entire tank of hot water down the drain—and all of the energy it took to warm it ends up in the sewer. But now it's becoming more common to build a loop to bring that heat back inside.
Sewers are hot: on average, the water inside is around 70 degrees. Along with whatever's flushed down toilets, pipes are filled with hot water from dishwashers and laundry machines, and showers. So when "sewage recovery systems" capture the heat from that water and circulate it back through houses—cleaned and in separate pipes, to keep the poop away—it's possible to use it as a renewable source of energy.
"Really it's one of the simplest alternative energy systems ever," says Lynn Mueller, CEO and founder of International Wastewater Systems. "We move heat from the sewer into the building or into hot water, or wherever it's needed." On a hot day, the system can work in reverse and use sewage to dissipate heat, so air conditioners can run less.
In a year, an estimated 350 billion kilowatt hours of energy is wasted from heating water, creating a massive carbon footprint and wasting money. "We can recapture some of the $40 billion a year that is put down the drain," Mueller says. "It's truly the ultimate renewable energy, because you can recapture it, use it, and then use it again the next day. It's the same energy just being recycled day after day and you can't wear it out."
Larger systems can serve whole neighborhoods. In December, the company converted a university in Scotland from natural gas to sewage heat recovery. On the coldest days of the year, the school may still turn on its boilers. But the sewer can supply 95% of the heat that the campus uses. The company is building other district energy systems around the world.
It also works on a smaller scale. In a single apartment building, as long as there are at least 50 units, one of the company's products can catch energy as it leaves the building and feed it back to the next person who wants to take a shower or do a load of laundry.
Other companies offering different methods, like a bath insert that recycles heat while you're taking a shower.
The systems are a straightforward way to trim carbon footprints, and Mueller believes they make sense in every city and home. "We have to reduce our dependency on fossil fuels, so if the first thing we just start doing is reusing the 30% of the energy we throw every day that's a very good start," says Mueller. "It's simple, it's easy, it's ubiquitous. So I see it as being one of the easiest things you can do to help."