In the jargon, it is called the “Réut”. This technology consists in making the dirty water usable as soon as it leaves the treatment plants. A circular water economy that raises hope, but which alone will not solve the problem of drought.
France is in lack of water. After long months without precipitation, and in anticipation of a possible new summer of drought, the authorities invited the French, from February, to be vigilant about their water consumption. So why do we flush with drinking water? And why aren’t the streets all over the country washed with treated wastewater? In France, “less than 1% of treated wastewater is reused”noted the Minister of Ecological Transition on Sunday February 26 in an interview with the JDD.
Christophe Béchu thus opened a debate on the possible “delay” of France in this matter. Franceinfo has looked into the subject by deciphering the ideas received on the reuse of treated wastewater – the “Reut” in the jargon – which is only one solution among others in the face of drought.
No, not all treated wastewater is wasted
Just because we don’t use it doesn’t mean that the 99% of wastewater treated in treatment plants is useless. As the drought lengthens the low water period (when the level of a watercourse is at its lowest), or even dries up rivers, lakes and ponds, “animal and plant species suffer from the drop in flow, which contributes to the erosion of biodiversity”notes Catherine Franck-Neel, project director on the management of hydrosystems at the center of expertise of the Ministry of Ecological Transition (Cerema).
Because the lower the flow rate of a river, the higher its pollution rate. Faced with this observation, the treated wastewater, discharged at the end of its journey into these waterways, rivers and soils, contributes to “healthy ecosystems”. It is the principle of “low water support”explains the specialist.
This treated water is also part of the “little water cycle” by feeding environments from which we will come to draw later. Reusing these waters without giving them the opportunity to accomplish this mission would be “maladaptation”points out Catherine Franck-Neel. “The Réut is generally not relevant in the continental zone. On the other hand, it can be done without regret in coastal zones, where the treatment plants discharge their effluents into the sea or the ocean, which constitutes a loss of ‘pure water”continues the author of a detailed report by Cerema (in PDF) on the development of this technology in France.
In this document dated 2020, the expert pointed out, logically, that “the departments with the most cases in operation and in project are located in the coastal zone (Somme, Manche, Loire-Atlantique, Charente-Maritime, Landes and Vendée, Hérault, Var, Pyrénées-Orientales).”
No, Réut is not desirable everywhere
To reuse or not is a real problem, including at the seaside. The answer is “contextual and territorial”, according to Nassim Ait Mouheb, researcher at Inrae and coordinator of the Réut experimental platform for the irrigation of Murviel-lès-Montpellier (Hérault). The expert cites, for example, the case of Noirmoutier, in Vendée.
On the peninsula, the reuse of treated wastewater has made it possible, since 1981, to cultivate a famous potato, without having a water table of drinking water underfoot. “Many factors are taken into account to determine the feasibility of the Réut in a territory, whether technical or economic factors”he insists, pointing “the ratio between the volume of water available and the need identified in a given territory”.
The size of local treatment plants also matters. “The greater the volume of water treated, the more interesting it will be.” But here again, other questions arise, such as that of uses. “Is there agricultural land nearby? Are we talking about urban use? Should a watercourse be supported? Is the cost of any additional treatment depending on the intended use and the routing of water remains interesting?”, Nassim Ait Mouheb list. “Pulling pipes over kilometers, carrying out analyses… It’s very expensive. We won’t invest to water one hectare. But to water 300, it’s different.”
Currently, Réut’s biggest project in France, the Jourdain program, is in Les Sables-d’Olonne, still in Vendée. On the phone, a Vendée Eau manager lists the benefits: “We do not have significant groundwater and have 94% surface reserve thanks to 13 dams in the east of the department. However, in the west, in summer, water consumption is multiplied by two, and more still on the coast, where tourists flock.”
Eventually, the project will transport part of the water treated by the Sables-d’Olonne wastewater treatment plant to a refining station, before bringing it up, by pipeline, to the Jaunay reservoir, ‘where it can be picked up again, head to the taps of around twenty Vendée municipalities.
No, recycling does not dispense with saving water
In the case of seaside towns where demand varies significantly with the seasons, the Réut is a great tool. But it should never be an excuse to dispense with efforts, insist the experts interviewed by franceinfo. A specialist in its use in agriculture, Nassim Ait Mouheb believes that it must accompany “a reflection on the maintenance of water in the soil and the evolution towards agroecological practices”. For the researcher, it is not necessary “not believe that the Réut will allow us to continue with very water-intensive crops”. An idea that does not flow naturally, if we are to believe a recent study by Cieau, according to which 67% of French people consider it necessary to invest in technologies that allow the same comfort of water use to be maintained.
In Murviel-lès-Montpellier, Nassim Ait Mouheb’s teams are testing the Réut on a few hectares of vines. “Until then, the vines did not need irrigation in the region. In an increasingly arid climate, this has become a real question”, explains the researcher. Everywhere, water needs increase with heat and drought.
Catherine Franck-Neel’s 2020 report (discussed above) determines that on of the 58 cases of reuse of treated wastewater in urban wastewater treatment plants, the most widespread use concerns the watering of crops (34), followed by golf courses (15). This is the case of greens of Sainte-Maxime, in the Var. While the department has been experiencing restrictions since February, does the Réut not have other priorities? “When there is a conflict of use, all the local actors must sit around the table to determine what will be privileged in times of crisis”, explains the specialist. In 2020, of the 25 cases of Réut planned, nine were intended for the watering of courses (against two for agriculture).
No, it’s not like drinking sewer water
Supplying our taps, irrigating crops… “Each use of water, each receiving environment, corresponds to a level of treatment and precise regulations”, recalls Nassim Ait Mouheb. Since 2010, French law has governed the Réut for the watering of green spaces and agricultural irrigation. And this year, new thresholds, adopted at European level, come into force.
In the wake of the Varenne de l’eau, in February 2022, France gradually opened up to new uses for the reuse of treated wastewater: cleaning roads, fighting fires, cleaning networks or yet the artificial recharge of the aquifers has been authorized by decree for a year. With, here again, treatments, pathogen thresholds, conditions… The water that waters the tomato plant of a market gardener in the south of France, for example, has nothing to do with that which comes to wash the trucks of an agribusiness cooperative in the COtes-d’Armor.
If the technique makes it possible to go as far as potability, “undesirable characteristics in one case may turn out to be useful in another,” explains Nassim Ait Mouheb. Nitrogen, for example, is not welcome in water discharged into natural environments. But for the irrigation of the vineyard, “water with a certain level of nitrogen will make it possible to limit the use of fertilizers and fertilisers”, he illustrates. A “tailor-made” approach that ensures the quality and harmlessness of reused water.
On the other hand, if you are annoyed to flush with drinking water every day, the Réut cannot do much for you, explains Julie Mendret, lecturer at the University of Montpellier and specialist in wastewater treatment. “There is a tendency to confuse the reuse of treated wastewater and the recovery of gray water (which comes from showers, bathtubs, sinks, etc.) and which consists of using the same water twice.”
To use non-potable water, he The building must have a double network to guarantee that the two types of water do not meet. “In new buildings, you can decide on a separate circuit, but this is not the case in the majority of homes at the moment.”