“Pool” and “sewer” are two words that most folks would rather not see used in the same sentence together.

Ever.

Yet in Paris, the city’s labyrinthine sewer network — all roughly 1,500 miles of it — and swimming pools are simpatico in some instances, with the former giving the latter a remarkable assist in the energy-savings department.

A 25-kilometer indoor swimming pool in the 14th arrondissement, Piscine Aspirant Dunand, is the latest public natatorium (yes, there are others) to be warmed by residual heat captured from previously heated domestic greywater — think: tepid wastewater generated by showers, dishwashers and washing machines — through an innovative heat recovery scheme that slashes the pool’s energy consumption in half.

And just to be clear, it’s just the heat that’s recovered, not the smell.

As reported by Reuters, the Aspirant Dunand pool joins a dozen or so public facilities that already tap into and recycle excess sewer heat. This includes a couple of other municipal pools along with a handful of schools, public housing blocks and government buildings located across the French capital city and beyond.

Like with these previous installations that turn toasty sewer pipes into renewable heat sources, the pool-warming project at Piscine Aspirant Dunand is the work of Paris-based global waste and wastewater firm, Suez. Reuters notes that the company is building three to four sewer heat exchanger/pump systems per year at costs ranging from 200,000 euros for a swimming pool to 1 million euros ($1.12 million) for an apartment building. Generally, the systems pay for themselves within eight to nine years.

Piscine Aspirant Dunand In addition to sewer-heated Piscine Aspirant Dunand (pictured above), the historic Piscine de la Butte aux Cailles in the city's 13th arrondissement harnesses heat generated by data center servers. (Photo: City of Paris)

Writes Reuters:

France has some 400,000 kilometers of sewage networks, where the water temperature typically ranges between 13 degrees Celsius (55.4°F) in winter and 20 degrees (68°F) in summer.

With a stainless steel lining on the inside of sewage pipes, Suez recuperates 4 to 8 degrees of that warmth and boosts it to about 50 degrees for use in space heaters or as hot water. The technology is similar to low-temperature geothermal energy, already widely used in some countries to heat individual homes.

What’s more, the standard temperature in some public Parisian pools, Piscine Aspirant Dunand included, will be lowered from 28 to 26 degrees Celsius (roughly 82 to 79 degrees Fahrenheit) for additional energy savings according to French daily newspaper 20 Minutes.

“Obviously, there will be no contact between the waste water and the pond [pool] water" assures Celia Blauel, an energy and sustainable development-minded assistant to climate change-combating Paris Mayor Anne Hidalgo. Under Hidalgo's leadership, the city has vowed to source 30 percent of its overall energy needs from renewables — recycled sewer heat, included — by the year 2020.

Located on the Left Bank near Observatoire de Paris, Montparnasse and the Catacombs, the subterranean Piscine Aspirant Dunand — just one of 39 municipal swimming pools in Paris — was built in 1982 and welcomes about 130,000 freestylers, backstrokers and general splashers-about annually. It’s unclear if the savings gained by heating the pool with wastewater will be passed on to the Speedo-clad patrons themselves, who pay 3 euros a head to use the five-lane pool.

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.