Desalination in France: OiEau's contribution to the IGEDD report on its potential and limitations

In what context did this reflection on desalination take place?

Y.F.: The initiative was launched by the Ministry for Ecological Transition, which was preparing a report on the state of knowledge on desalination. An official from the Ministry contacted us to gather technical information and feedback, particularly because OiEau offers several training courses dedicated to reverse osmosis, membrane processes and seawater desalination.

From the very first discussions, the framework proved to be very open: there was no specific focus, but rather a desire to gain a general understanding of membrane technologies and their possible uses. The scope of the study was broad, covering seawater desalination, the treatment of highly mineralised or brackish water, wastewater reuse, softening and the reduction of micropollutants.

The report aimed to remain accessible to elected officials, local authority technical services, industrialists and farmers alike. The primary objective was to provide a clear and impartial overview to inform national strategy in a context of increasing pressure on water resources, particularly in certain regions such as Vendée and the overseas territories.

The teams appointed did not initially have in-depth technical expertise, but were tasked with quickly producing a strategic report that would inform public decision-makers and enable them to anticipate future developments. The Ministry therefore expected the working group to provide it with fundamental insights: how osmosis processes work, the differences between seawater desalination and the treatment of highly mineralised water, French and international experience, and energy, environmental and regulatory issues.

It also sought to gain concrete insight into the limitations and conditions for deploying desalination. Finally, the Ministry wanted to be able to identify the levers for action and the points of vigilance necessary for building a coherent French position, taking into account the objectives of sobriety, REUSE and other available solutions.

What diversity of profiles was brought together within this working group?

The working group led by IGEDD deliberately brought together a wide variety of stakeholders in order to combine technical, institutional, operational and scientific approaches. Between November 2024 and summer 2025, more than 160 people were interviewed, representing the entire water ecosystem.

These included government departments, water agencies, local authorities and drinking water management unions, water production and treatment companies, design offices, membrane unit operators, researchers and academics, as well as experts from overseas territories, where desalination is already widely practised. This diversity made it possible to compare strategic visions with the technical realities on the ground, while incorporating international feedback, particularly through visits such as the one to the Barcelona plant.

OiEau was one of the first organisations to be consulted, due to its role as a training provider and its ability to provide an educational overview of membrane-related issues before the Ministry conducted more in-depth investigations with other specialists.

What was OiEau's role within the working group?

Two representatives from OiEau were called upon: myself for the technical aspects related to desalination and membranes, and Mr Éric Tardieu, Director General, for the governance, strategy and public decision-making aspects. Our contribution focused mainly on clarifying the processes and highlighting the current limitations of desalination development in France.

One issue quickly emerged as central: the question of what to do with the concentrate discharged by reverse osmosis units. This is a subject on which we provided specific expertise, based on various feedback reports. We also helped to distinguish between the different types of desalination, as the challenges and constraints are not the same for seawater as for highly mineralised inland water.

Beyond the technological aspect, we highlighted the environmental impact study and the existing regulatory difficulties. Environmental authorisations for brine discharge are currently very limited, and there are no real plans for heavily loaded flows from desalination. This lack of a regulatory framework is a major obstacle that must be addressed if France wishes to consider the controlled development of these solutions.

What are the main technical points highlighted by OiEau?

The first point therefore concerns the management of the concentrate. Today, desalination technology is well known, but the way in which the concentrated flow is treated or safely discharged remains very poorly regulated. We have emphasised the potential impacts, the quantities involved, the environmental constraints and the need for regulatory change.

The second point concerns the energy constraint. Seawater desalination is very energy-intensive, which poses a real challenge in a context where electricity demand is already set to increase significantly, whether for mobility or for the construction of new nuclear reactors. For this reason, the report encourages, above all, the reuse of treated wastewater and the treatment of brackish water, which are less energy-intensive and more realistic in the short term.

We also drew attention to the actual performance of membrane systems. Current yields (often around 80 to 85%) differ significantly from those expected for conventional drinking water plants. If France plans to integrate these technologies, regulatory requirements will need to be adapted to take into account the specific characteristics of membrane systems.

Finally, we emphasised the importance of feedback from overseas territories, where reverse osmosis has been used for a long time. These accumulated skills would benefit from being further promoted at the national level, as the overseas territories constitute a full-scale laboratory for issues related to seawater intake, corrosion, climate risk management and the continuous operation of membrane units.

Which OiEau training courses already cover these issues?

OiEau currently offers two training courses directly related to desalination. The first, course SB021, is specifically devoted to seawater desalination. It covers the entire chain, from collection (including marine drilling techniques) to waste treatment, including pre-treatment, reverse osmosis and the final quality of the water produced.

The second, course SN037, takes a broader look at membrane processes. It covers low-pressure osmosis, wastewater reuse, brackish water treatment and the basics of seawater desalination. Its audience is more diverse, as it is aimed at both industrial players and professionals in the drinking water and sanitation sectors.

A third training course, dedicated more specifically to wastewater reuse (REUSE), is under consideration and could eventually include practical work. Several OiEau teams are already working on this.