Wastewater for irrigation: “Projects that take time to complete”

Nearly 400 hectares of potatoes in Noirmoutier, 750 hectares of cereals and corn near Clermont-Ferrand, 80 hectares of vineyards in Narbonne… Across France, some farmers irrigate their crops with wastewater. This is called "Reut," for reuse of treated wastewater. The practice, which is not new, is also observed in industry (particularly for cleaning), on roads, watering green spaces, and even golf courses. "This allows for large volumes of water to be available year-round, to meet agricultural needs and cope with periods of drought," observes Olivia Martin, director of the Water Plan project at the Center for Studies and Expertise on Risks, the Environment, Mobility, and Development (Cerema).
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Reuse can, under certain conditions, be a way to preserve an increasingly scarce resource. Usually, wastewater is treated in a treatment plant and then released into the environment, most often into rivers, where it contributes to the water cycle. The water we consume is then taken from this same environment. "Reuse has the effect of replacing and reducing abstraction ," explains Olivia Martin. "But if it's to deprive environments that need this resource, it won't be worthwhile. We have to look at it on a case-by-case basis." The risk would indeed be to further lower the level of rivers that are already dry.
Reut has been in the spotlight with climate change and successive droughts: while it is estimated that only 1% of wastewater is reused in France, this proportion rises to 10% in Italy and 14% in Spain. "Today, it has become a political issue, with a lot of communication," analyzes Rémi Lombard-Latune, a researcher specializing in REUT at the French National Institute for Agricultural, Food and Environmental Research (INRAE).
In 2019, the water conference aimed to triple the "volume of unconventional water used by 2025" . In 2023, the water plan provided for 1,000 Reut projects by 2027 and a tenfold increase in volumes by 2030. In October, according to the Ministry of Ecological Transition , there were only 136 treatment plants in operation, the majority of which were for agricultural use, seven authorized but not yet in operation, and 24 whose applications were being processed. This figure is not exhaustive, since it only records data from sanitation plants managed by local authorities and does not include private projects, carried out by industrialists or campsites, for example.
"Since 2023, many communities have looked into the issue. But these projects often take time to come to fruition. It is necessary to analyze the technical feasibility, the health and environmental risks, the agreement of users... The study phase is long before obtaining authorization," according to Olivia Martin. Another difficulty is the cost, often much higher than conventional water when you take into account the water treatment, the analyses to be carried out, as well as the network to be set up to pump and reroute the vital liquid to farms. "20 % of the cost of water is linked to treatment, and 80 % to transport," points out Rémi Lombard-Latune.
The required quality of reused water is not the same for all uses, nor for all crops. A higher level of quality is required for lettuce, for example, compared to potatoes or vines. A 2020 European regulation, confirmed by a 2023 decree aimed at regulating practices, has strengthened the expected health thresholds for agricultural irrigation. "The required quality levels are achievable, but come at disproportionate costs. From an economic, organizational, and environmental point of view, since the more extensive the treatments, the more energy they consume," explains the researcher. This makes agricultural irrigation projects in rural areas, where treatment plants are small, less profitable and more complicated.
It remains to be seen whether the consumer, at the end of the chain, is ready to buy tomatoes irrigated with water from their toilet flush. "The closer the drop of treated water is to us, the more worried we are," according to Philippe Vervier, founder of a research firm specializing in social acceptability. In other words, it is easier for a consumer to support the reuse of wastewater for cleaning pipes than for market gardening... "It's a question that we have often brought up, without really knowing how people would react. The shield of acceptability has almost become a political argument for not going there ," says Benjamin Noury, author of a thesis on the social acceptability of wastewater reuse. It's not just the acceptability of the citizen, but that of the users : farmers, municipalities, elected officials..." For the expert, summarizing the subject to that of acceptability makes it possible to avoid tackling the causes of opposition. "Sometimes, the issue is not the quality of the water, but the fairness of this access to water." An increasingly central question.
Libération