The summer of 2022, with its context of intense drought, marked a turning point for the reuse of treated wastewater (Reut) in France. Until now, the subject had never been so present in the public debate. As a reminder, this practice consists of directly reusing the water leaving the treatment plant without going through a return to the natural environment.
In France, the reuse of treated wastewater (Reut) has been regulated since 2010 for watering green spaces and agricultural irrigation, which must meet the thresholds set by the European Union in 2020 by 2023. The 3 August 2022, the European Commission published guidelines to help national authorities and companies concerned to apply these new rules. According to Virginijus Sinkevicius, Commissioner for Environment, Fisheries and Oceans, “it is our duty to stop wasting water and use this resource more efficiently to adapt to climate change and ensure safety and sustainability of our agricultural supply. The guidelines adopted today can help us achieve this and ensure the safe movement, across the European Union, of food products grown with recycled water.”
The advantages of the Reut are numerous: reduction of pressure on resources which represent the main reservoir for the production of drinking water (groundwater, rivers, etc.), improvement of water quality for sensitive activities (swimming, shellfish farming) due to the elimination of wastewater treatment plant discharges, supply of nutrients for agricultural irrigation, etc.
In March 2022, a new decree relating to the uses and conditions of reuse of treated wastewater was published in France: to agricultural irrigation and green spaces are thus added urban uses such as road cleaning, cleaning networks (hydrocleaning) or firefighting… This decree, on the other hand, excludes using it for food purposes, related to personal hygiene and laundry, recreational uses (swimming pools, fountains, etc.) , as well as indoor use in establishments open to the public (health establishments, accommodation for the elderly, nurseries, schools, etc.) and residential premises.
The use of treated wastewater will require prefectural authorization limited to a maximum of five years for their use in the department where they are produced. However, it is to be feared that the fairly restricted scope and the administrative red tape could discourage project promoters.
Despite its undeniable advantages mentioned above, the reuse of wastewater is not the only solution to overcome the lack of water, it is above all a solution dependent on the local context and which therefore implies taking into account when community resource management planning.
It is, for example, very relevant in coastal areas where some wastewater treatment plants discharge their effluents into the sea, which constitutes a loss of fresh water. By practicing Reut, the circular water economy takes place with the secondary benefits of reducing groundwater withdrawals – more vulnerable when their level is low and they are then subject to the penetration of salt water. which can make them unfit for consumption – and the improvement of water quality in bathing or shellfish farming areas.
An emblematic example is now the Jourdain project in Les Sables-d’Olonne, where treated wastewater will eventually be directly diluted in water from a collection point for drinking water.
In continental areas, on the other hand, discharges from wastewater treatment plants sometimes play a significant role in supporting low water levels – i.e. maintaining a minimum flow necessary for the proper functioning of aquatic ecosystems – and the Reut is of less interest in this case, or even could have a negative environmental impact.
If currently around 80 stations practice Reut in France, it could be that the drought experienced during the summer of 2022 encourages the development of this method already widespread in certain countries. In particular, Veolia announced in a press release dated July 25, 2022 its wish to deploy Reut at around a hundred wastewater treatment plants whose consumption volumes justify it (consumption of more than 2,000 m3 of drinking water and/or 5 000 m3 taken directly from the resource), via compact water recycling units. This operation should save 3 million m3 of drinking water, i.e. the average annual consumption of a city of 180,000 inhabitants. Initially, the recycled water will be used for the maintenance of the stations before experimenting with other urban or agricultural uses subject to obtaining authorizations.
However, there are still many obstacles to overcome. The social acceptability of this practice, often misunderstood and subject to preconceptions on the part of the general public, is one of them. Another important obstacle, the Reut will have to find its economic model, certainly requiring financial incentives in order to compete with the river water that farmers take at a price around 10 to 30 euro cents per m3.
After wastewater treatment, the presence of certain salts, mineral and organic pollutants, as well as pathogenic microorganisms is otherwise still possible. The significance of the associated negative impacts on ecosystems and on human health strongly depends on the characteristics of the soil, the plants and the quality of the treated wastewater, as well as agricultural practices. It is therefore important that Reut remains well supervised in order to make it a safe and sustainable practice. This will go through reaching the set thresholds, but also through the implementation of other processes, for example drip irrigation rather than sprinkling.
While the sector is changing rapidly, it appears necessary to conduct scientific studies on the fate of pollutants, viruses and parasites during agricultural irrigation with treated wastewater. In Israel, where Reut has been practiced since the 1980s, now around 80%, a study has shown that pharmaceutical molecules are mainly found in the soil, leaves and roots of plants irrigated with wastewater treated, the fruits and tubers being less contaminated. This study concludes on the interest of improving the treatment channels dedicated to Reut and using it on soils rich in organic matter that promote the degradation of pollutants.
In France, the successful example of Limagne Noire, where cereal crops have been irrigated with treated wastewater from the city of Clermont-Ferrand for several decades, has revealed an absence of contamination linked to pathogens.
Recently, in Murviel-lès-Montpellier, a two-year experiment of drip irrigation of lettuce and leek crops with municipal wastewater treated without and with the addition of fourteen contaminants at a concentration level of 10 ?g/L was conducted under greenhouse culture conditions. The objective was to study their accumulation in the soil and leaves in order to assess the potential health risks. The results revealed a limited accumulation of contaminants in soil and plant leaves, their concentration levels being in the range of 1-30 ng/g and 1-660 ng/g respectively.
Overall, this study confirmed previous reports of minimal risk to human health from consumption of raw leafy green vegetables irrigated with treated domestic wastewater containing residual organic contaminants.
Thus, if the Reut must overcome reluctance and allay certain fears, it nevertheless appears to be a solution for the future in the face of current challenges and sees its usefulness exacerbated by the effects of climate change.
*Julie Mendret, lecturer, HDR, University of Montpellier
