“To avoid shortages of fresh water in urban areas, there are alternative solutions”

Ihe anthropogenic water cycle, or “small water cycle”, refers to the route that water takes from the point of collection in the river or from the groundwater table until it is discharged into the natural environment. . This path is made up of the following steps: raw water abstraction, water purification in specific plants, drinking water storage in reservoirs or water towers, drinking water distribution to all users through the water networks, the collection of wastewater by unitary networks (wastewater and rainwater collected together) or separate (with a network reserved for wastewater and another for rainwater), the treatment wastewater in treatment plants (collective sanitation, which concerns 85% of users) and, finally, discharge into the natural environment.

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In recent decades, this “small water cycle” has responded effectively to public health issues, since access to safe drinking water prevents a large number of diseases. In addition, wastewater treatment limits the degradation of the receiving environment and preserves biodiversity. Nevertheless, is this system still relevant when the water resource becomes scarce and the stakes become multiple? Contrary to the large water cycle, the “small water cycle” has until now been rather understood as a linear system where each stage follows one another up to the receiving environment. Rethinking this water cycle with issues of circularity, by adopting the principles of the circular economy, appears today as a necessity.

In urbanized areas, the distribution of drinking water and the occasional flow of rainwater can be considered as the only two incoming flows of this “small water cycle”. The outgoing flows are treated urban wastewater and runoff water. Rethinking the current scheme requires questioning these flows, in particular by optimizing the management of the “storage basins” used to regulate the circulation of these flows in the urban and peri-urban area. Urban wastewater − an average of 150 liters per inhabitant per day in mainland France − represents around 4 billion cubic meters of water per year for mainland France, a quantity greater than the needs for agricultural irrigation in mainland France. The reuse of this urban wastewater − which has relatively low salinity and limited treatment needs, therefore low energy consumption, to make it quality water for defined use − has many advantages compared to that of brackish and marine water .

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