As in cities everywhere, thousands of gallons of heated water drain into Paris’ sewage system every day from showers, dishwashers, and washing machines. Throughout France’s 400,000 kilometers of sewage networks, the average water temperature is 13 degree Celsius during the winter and 20 degrees in summer. This domestic grey water naturally transfers its heat to the pipes that it flows through, which then dissipates in the underground tunnels, of no use whatsoever to the residents above.
However, a new stainless steel lining can recapture between four and eight degrees of this warmth. The system, developed by French waste and water group Suez, can boost this to 50 degrees and pump water to where it is needed. The technology is similar to that of low-temperature geothermal energy.
For the Piscine Aspirant Dunand, the heat comes from a sewer 300 meters away, bringing the water temperature of the pool and showers to a comfortable 26 degrees, and will cut electrical energy consumption in half. It remains to be seen whether the economic gains from this will be passed on to swimmers.
Of course, at no point do the water from the sewer and the water from the pool come into contact, and it is only the heat that is transferred, not the smell!
Suez has a 30-year contract and has already put in place a dozen systems across France, including swimming pools, at 200,000 euros each, and the heating systems of schools, apartment blocks, and administrative buildings, where the cost was closer to one million euros. Projects pay for themselves within eight to nine years unsubsidized.
The potential here is huge: wherever there are sewers, there’s a heat source. In Paris, 2,400 kilometers of sewers await exploitation and the city’s other 38 municipal swimming pools seem like a great place to start.
Initiatives like this and the one at the Piscine de la Butte aux Cailles, which now uses heat recaptured from a computer data center, in conjunction with a soon-to-be-introduced policy of pool temperature reduction from 28 to 26 degrees, are one element of the city administration’s plan to increase renewable energy use to 30 per cent by 2020.