A sewer has recently become the heat source for a Paris swimming pool.

According to Reuters, the Aspirant Dunand pool features heat pumps that recycle residual warmth from showers, dishwashers and washing machines in its sewage pipes.

Suez, a French waste and water group, already operates a dozen of these heating systems at pools throughout the country, located in schools, apartment buildings, and administrative buildings.

Here’s how the whole system works: France has approximately 400,000 kilometers of sewage networks with water temperatures that range from 55-68F throughout the year. When the sewage pipes are lined with stainless steel, Suez recuperates 4-8 degrees of the sewer warmth and boosts it to about 50 degrees for use in hot water. Suez reports that the costs range from 200,000-1 million euros, and the projects pay for themselves within an 8-9-year period.

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