Recycled water: active management of treated wastewater

Evaluation of the minimum flow of treated wastewater to be discharged into the natural environment to maintain the vital functions of the watercourse.

The practice of reusing treated wastewater from wastewater treatment plants can have an impact on the receiving environment. Water withdrawn for irrigation or any other use is no longer returned to the watercourse. This can have an ecological and hydrological impact. With this in mind, Action 1, carried out in the Saint-Drézéry area, aims to assess the repercussions on the fauna, flora and low-water flow of the Pradas watercourse, a tributary of the Bérange.

Thus, the aim is to carry out an initial assessment of these watercourses by taking a census of the fauna (macroinvertebrates) and flora, as well as monitoring the flow of the watercourses by measuring variations in water height.

This action took place over the 2018-2021 period to characterize the impact of the implemented REUT practice.

Results:

The results indicate a poor initial ecological status (2018) with regard to invertebrate fauna at all sampling points. Nevertheless, the "Bérange amont" shows a "médiocre" ecological status for the first time in 2021, indicating an improvement in living conditions compared to June 2018.

Furthermore, hydrological monitoring shows that the Bérange only really seems to subsist through the presence of an underground water table with a resurgence into the watercourse. Le Pradas only flows into the Bérange thanks to the discharge from the Saint-Drézéry STEU, otherwise the Pradas stream would not exist in times of drought. It's a natural dry stream.

Creation of a biodiversity area dedicated to local flora/fauna and pollinating insects and serving as an ecological corridor for riparian birds, irrigated with treated wastewater.

What if treated wastewater enabled the development of a biodiversity area? This is what we wanted to demonstrate through Action 2 - water for honey - in the Saint-Drézéry area. In 2020, a refuge plot for local flora and fauna is being invested and irrigated with treated wastewater from the plant, the quality of which is measured and monitored.

To achieve this, an initial assessment of the plant and animal species within the plot was carried out to determine the species to be planted and seeded necessary to maintain and produce biodiversity. This parcel is an ecological corridor, a space of connection and passage between reservoirs of biodiversity, offering favorable conditions for the development of riparian birds in particular.

To make this project feasible, an application for prefectoral authorization was made and approved on November 28, 2019. To ensure that the project is profitable, beehives have been installed on the plot. Each year, a faunistic and floristic inventory is carried out to assess the impact of the project on biodiversity.

Results:

To date, according to the analysis of phytosociological surveys, the floristic cortège of the plot is close to that of wasteland and vegetation accompanying crops. In addition, hedgerow growth, monitored on a weekly basis during the irrigation season, has gone well.

Creation of a plantation for the production of plant biomass irrigated with treated wastewater, to be valorized within the Metropole's boilers.

The objective of this action carried out in Saint-Drézéry is the irrigation of a plot dedicated to the production of biomass (or lignocellulosic crop) with treated wastewater from the plant. To this end, an application for prefectoral authorization was submitted and approved on November 28, 2019, following monitoring and characterization of the treated wastewater leaving the plant.

Results:

The species selected for the biomass production project is Willow, which is non-invasive and has no impact on the adjoining biodiversity plot. Irrigation began in the 1st half of 2020. The plant species are not yet mature enough to be mowed and sent for trial combustion in one of the Montpellier Metropole's boilers. Nevertheless, their growth is very satisfactory.

Evaluation of irrigable agricultural potential with treated wastewater and governance approach around this new resource.

This action took place in the Fabrègues territory from 2018 to 2021. The aim is to identify agricultural plots potentially irrigable by treated wastewater from the Fabrègues plant, in consultation with local agricultural stakeholders. A study was carried out to assess potential users' interest in this resource, social acceptance, the distribution of water needs between users, the cost of water, etc.

Results:

The field study highlights a varied agricultural territory with different water needs (alfalfa, vines, market gardening). However, the majority of farmers do not currently use irrigation, except for market gardening. The social acceptability study carried out shows a certain reluctance towards recycled water resources.

This raises the question of its authorization in the specifications for organic farming, which has not yet been discussed. Finally, the cost of water, which depends on investment according to the volume reused, is not competitive enough when compared with the BRL resource present on the territory.

Evaluation of the irrigable potential of green spaces with treated wastewater and governance approach around this new resource.

This action, which ran from 2018 to 2021, is a feasibility study primarily targeting the watering of green spaces in the town of Fabrègues. It is being carried out in collaboration with the Metropole's technical services, to reflect together on the proper governance of this water, its use and sharing, and its cost.

Results:

The technical services of Fabrègues are very interested in replacing the resource used to date with recycled water. Nevertheless, the volume used of around 5,000 m3/year for manual irrigation is low compared with the investments planned for the reuse of treated wastewater, even though the water leaving the Fabrègues plant is of very good quality (A) and requires no additional treatment. In this sense, the cost of water would be too high.

Mutualization of uses (green spaces and agriculture) would improve the profitability of the project, but there is currently no opportunity for agricultural irrigation in the Fabrègues commune.

Identification of health risks associated with possible fire-fighting uses of treated urban wastewater.

In the summer months, the Hérault département is often hit by violent fires, for which thousands of liters of drinking water are used. In conjunction with the Syndicat départemental d'incendie et de secours (SDIS 34), a study is being carried out into the reuse of treated wastewater as a substitute for drinking water in fire-fighting. It is taking place at several stations (Baillargues, Saint-Drézéry) from 2019 to 2021.

The SDIS 34 is very sensitive to the issue of preserving resources and saving water. The use of treated wastewater to fight fires is a real solution, but its use - in the project phase - is not then regulated. Consequently, a sanitary risk study is being carried out with tests on different categories of water (drinking, river, cistern, treated wastewater) in order to assess the ENT risks associated with spraying, and compare the impact of using treated wastewater with regard to other resources.

Results:

The results demonstrate the absence of ENT risk when using Class A treated wastewater for firefighting with aerosol risk under conditions of means of control to be put in place.

However, storage may appear to be a factor in degrading the quality of treated wastewater or any other type of water, even drinking water, for example through the presence of biofilm on tank walls or contaminated stagnant water at the bottom of the tank.

Identification of the potential use of treated wastewater for hydrocleaning activities and analysis of the health risks associated with the use of this water.

Nowadays, wastewater pipes are flushed and cleaned using drinking water. This action aims to characterize the ENT health risk, like action 6, when using treated wastewater for hydrocleaning, a new practice authorized by the decree of March 10, 2022 on the uses and conditions for reusing treated wastewater. This action is being carried out at the Baillargues and Fabrègues plants over the period 2020 to 2021.

Results:

The study demonstrates that the use of treated wastewater of quality A does not present a risk when it is used, leading to the dispersion of aerosols if the conditions of the means of control to be put in place are respected. However, as soon as it passes through the hydrocleaner tank, it degrades. This observation is valid not only for TME but also for drinking water.

Assessment of changes in the quality of stored treated wastewater over time and determination of conditions for maintaining a quality suitable for use.

As part of actions 2 - water for honey and 3 - water for energy, a storage tank supplying the irrigation system is being set up at the Saint-Drézéry plant for the period from 2020 to 2021. The aim of this action is to assess the residence time of treated wastewater in the tank, to ensure good quality of the resource to be reused.

Results:

The results do not highlight any significant degradation over a 4-day period. Further analysis over a longer period would enable us to characterize the effects of degradation and the conditions to be put in place in order to limit it.