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Reuse in rural areas is specific from the point of view of uses, since it is mainly in this context that the agricultural reuse of treated wastewater is practised. The other uses likely to be present in rural areas, whether domestic or at the level of a community (watering of green spaces, cleaning of roads, etc.), are not specific to the rural area and will be dealt with under the research theme "urban reuse".

Another specificity of rural areas is the relative weakness of the human, technical and financial means of the authorities in charge of sanitation in these territories. The infrastructures of the reuse system must take these elements into account, in particular the needs for operation, maintenance and monitoring of microbiological parameters. On the other hand, land pressure is lower than in the city, which makes it possible to envisage the creation of irrigated areas near the treatment plants, but also to mobilise nature-based solutions for treatment. The latter are generally more energy-efficient and require less maintenance, but have a larger land area than more intensive solutions.

Agricultural reuse of treated wastewater faces a number of challenges, such as:

• Adaptation to regulations, since the European regulation governing practices at the EU level will be applicable from 26 June 2023 and will significantly modify the conditions compared to the pre-existing French regulation;
• Technical efficiency, as we have seen for the production of treated wastewater, but also for their use: storage (because the volumes produced may not correspond to the needs) and irrigation equipment must be adapted to this new matrix that is treated wastewater;
• Agronomic, since one of the interests of agricultural reuse is to provide water and nutrients. It is a question of maximising this supply, while adapting fertilisation practices according to the needs of the crops so that excess nutrients do not end up in the environment;
• Environmental, in addition to the virtuous objective of saving water, which is only achieved in cases where conventional resources are replaced by treated wastewater, even though the latter did not otherwise play a role in supporting the environment. This raises the question of the transfer of waste pollutants present in wastewater (nutrients, pathogens, micropollutants, salts). These transfers concern plants, the soil and even the hydrological system. This also raises the question of preserving the quality of the soil in the long term, both in terms of its physical properties and the maintenance of biodiversity;
• Economic, as the balance of the cost of water for end-users seems particularly complicated to achieve in comparison with other conventional water sources, as shown by the high number of aborted or abandoned projects (CEREMA, 2020^*);
• Organisational, since it involves associating several actors of different natures (local authorities, private individuals, the agricultural world, consumers) in order to manage the associated risks and to agree on the relevance of this solution for the final consumer or the management of the rural territory.

The INRAE reuse network produces knowledge through its four research themes (Territories, Risks, Sectors and Actors) that can be used to meet the challenges of reuse in rural areas. Several projects are already in line with this theme. The challenge for 2021 will be to link them so that they feed each other, identify gaps and make the knowledge produced available.

Contact: Nassim Ait-Mouheb, Rémi Lombard-Latune and Stéphanie Prost Boucle (see details on the contact page).

^*Réutilisation des eaux usées traitées - Le panorama français, CEREMA, 2020