Waste water can be used either before, or after it passes through a wastewater treatment plant (WWTP).
Wastewater used before treatment
Wastewater used after treatment
When the WWTP is located at altitude, it is often difficult to find close-by a waterway or a water body which allows the wastewater to be sufficiently diluted. It is therefore necessary to rout it to a more important waterway. The head between the outlet of the WWTP and the waterway can thus be exploited by means of a turbine.
Note that if the volumes of water are important, it is also possible to operate a turbine with the head of a few meters that is often found between the outlet of the WWTP and the downstream waterway.
Example of implementation: La Douve II Leysin (click here to know more).
In both cases, there is no environmental impact, and no authorization is required.
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