[en] The exploitation of municipal purified sewage and drain waters of the city of La Chaux-de-Fonds for energy production requires complex and varied developments, in the domains of both civil engineering, and hydraulic equipment; this explains why, beyond some generic investigations, this source of energy has not been exploited so far. In fact, the purified water must first be pumped uphill over a little summit, before its hydraulic energy can be exploited downhill to the Doubs River. A number of engineering works in sequence must be achieved, beyond installing the electro-mechanic turbine device. The key data in this project are the following: (i) nominal electric power: 1532 kW (excluding pumping power requirement of about 400 kW); (ii) net hydraulic head: 380 m (excluding preliminary pumping height of 65 m); (iii) annual production: 4,178,830 kWh/yr (net of energy requirement for pumping); (iv) cost price: 0.13 CHF/kWh. The planned exploitation mode gives priority to the exploitation at the times of peak electricity demand, by the construction of a retention basin. This cost price is significantly lower in comparison to the other renewable energy technologies available in Switzerland. Chances of achievement are excellent; one electric utility has already expressed its interest in project buyback, in view of a short-term execution.