Clean water from the sun

The contamination of water resources by organic micropollutants is a growing global concern, posing significant challenges for water quality and human health.

Gael Plantard, University of Perpignan and Julie Mendret, University of Montpellier

These organic micropollutants, such as pesticides, pharmaceuticals and persistent organic compounds, are often detected in minute concentrations in water (micrograms, or even nanograms, per liter), but even at these concentrations their impact on aquatic ecosystems and public health is proven.

Global warming exacerbates the situation, as temperature variations, changes in hydrological regimes and extreme weather events can affect the mobility of these substances and lead to an increase in their concentration in water reservoirs.

Conventional wastewater treatment technologies used in wastewater treatment plants (WWTPs) may be insufficient to eliminate these substances. Treatment plants therefore contribute to the dispersion of these substances in the environment.

Faced with this reality, it is becoming imperative to develop new water treatment processes capable of effectively eliminating organic micropollutants. Innovative approaches - for example, the use of advanced oxidation technologies (TOA), activated carbon adsorption or membrane separation - are needed to meet the growing challenge of micropollutant contamination.

Advanced oxidation technologies

TOA processes, such as ozonation and photo-oxidation, have the advantage of non-selective destruction of organic contaminants, whether biotic (bacteria, pathogens) or abiotic (pesticides, pharmaceuticals), and are therefore ideally suited to the problem of micropollutants.

They involve the production of highly reactive chemical species (known as "radicals" or "hydroxyl radicals"), which are capable of breaking the carbon-carbon bonds that make up the various organic substances. This process leads to the degradation of pollutants in the form of carbon dioxide, water and salts: this is known as mineralization.

Some TOA processes convert light energy into chemical energy to oxidize and degrade organic molecules - these are known as photo-oxidative processes. In heterogeneous photocatalysis, photons are captured by a photosensitive material such as a photocatalyst (titanium dioxide or zinc oxide, for example). They induce the formation of charges on the surface of the catalyst, which initiate the production of radical species via oxidation-reduction processes.

Photo-oxidation technologies should make it possible to harness sunlight to degrade contaminants. Solar photoreactor" installations are still under development in the laboratory. The aim is to optimize yields, and also to see how to achieve the lowest possible environmental and energy (operating) costs.

For example, research has been carried out to assess the capabilities of solar photoreactors for the decontamination of hospital wastewater(pharmaceuticals), agricultural effluents(biocide residues), groundwater remediation(solvent residues such as trichloroethylene), as well as wastewater treatment for agricultural (irrigation) and industrial uses.

In order to deploy these technologies, we need to step up the performance of solar photoreactors and optimize the use of solar resources.

The solar resource available for photo-oxidation

In today's global climate, energy and environmental context,harnessing the sun's energy resources is a major challenge for the energy transition. To achieve this, we're looking to implement sustainable, low-energy-cost technologies that use solar energy.

This solar resource is variable (due to clouds, alternating days and nights, seasons, etc.). When it comes to producing electricity (photovoltaics), this is a pitfall, as it's costly to store the electricity produced until it's needed.

For water treatment, on the other hand, contaminants can be stored by adsorption on carbon columns or in wastewater retention basins, waiting for the sun to shine.

For example, when developing solar water purification plants, we design their operating capacity on a year-round basis, or optimize their capacity to meet specific needs - seasonal, for example, in tourist areas.

Finally, solar radiation is divided into three major wavelength families: ultraviolet, visible and infrared. The photo-catalysts currently available on the market have limitations in terms of absorption of the solar spectrum. Today, only the ultraviolet range - which represents just 5% of the solar spectrum - is exploitable for photocatalysis applied to water treatment.

For the past three decades, research has been carried out to improve the performance of photosensitive materials, with the aim of increasing photo-conversion yields and their ability to absorb visible radiation (45% of the solar spectrum).

Against this backdrop, the challenges are to increase the capacity of existing plants, improve water quality and reduce energy costs.

To achieve this, the future of advanced oxidation technologies lies in coupling with other processes: biological processes (to eliminate "biorecalcitrant" pollutants, i.e. those that cannot be biologically degraded), membrane processes (to eliminate small pollutants that cannot be filtered by membranes), or even with the solar thermodynamic cycle (to thermally activate catalysts).

The Aquireuse project

Our Aquireuse project explores a treatment process unique in France, based on an initial stage of solar photocatalysis, followed by infiltration into soil rich in organic matter, which helps to break down pollution.

In fact, for certain uses, such as recharging a water table with treated wastewater to serve as a reserve for drinking water production, the water must be free of micropollutants.

Recharging groundwater with treated wastewater is still an unknown practice in France, but is more widespread in Australia and California, for example. In particular, it helps to combat a phenomenon that is becoming widespread in coastal areas, the "salt-water rise". When groundwater levels in coastal areas drop as a result of over-intensive abstraction, seawater seeps in and contaminates freshwater resources, making the salty water unfit for human consumption.

In the Aquireuse project, effluent from a wastewater treatment plant is used to feed a solar photocatalysis pilot plant, where a first stage of total or partial degradation of micropollutants takes place. The treated effluent is then sent for infiltration into sediments, where the soil's organic matter helps refine the treatment by further degrading the micropollutants and by-products of solar photocatalysis.

Initial results are very promising: a large proportion of micropollutants are completely degraded after passing through the treatment process. These results are currently being published.

Such a process, combining a sustainable process and a nature-based solution, is an example of the circular economy in water treatment.

Gael Plantard, University Professor of Materials Chemistry, University of Perpignan and Julie Mendret, Senior Lecturer, HDR, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.