Clean water thanks to the sun
The contamination of water resources by organic micropollutants is a growing global concern, posing significant challenges to 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 water at extremely low concentrations (micrograms, or even nanograms, per liter), but even at these concentrations, their impact on aquatic ecosystems and public health is well established.
Global warming is exacerbating the situation, as temperature fluctuations, changes in hydrological patterns, and extreme weather events can affect the movement of these substances and lead to an increase in their concentration in water bodies.
Conventional wastewater treatment technologies used in wastewater treatment plants (WWTPs) may prove insufficient to remove these substances. Wastewater treatment plants therefore contribute to the release of these substances into the environment.
Given this reality, it is imperative to develop new water treatment processes capable of effectively removing organic micropollutants. Innovative approaches—such as the use of advanced oxidation technologies (AOT), activated carbon adsorption, or membrane separation—are needed to address the growing challenge of micropollutant contamination.
Advanced oxidation technologies
Advanced treatment technologies (ATTs), such as ozonation and photo-oxidation processes, have the advantage of non-selectively breaking down organic contaminants—whether biotic (bacteria, pathogens) or abiotic (pesticides, pharmaceuticals)—and are therefore ideally suited to addressing the challenges posed by micropollutants.
These processes 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 various organic substances. This process leads to the breakdown of pollutants into carbon dioxide, water, and salts; this is referred to as mineralization.
Among AOTs, certain processes convert light energy into chemical energy to oxidize and degrade organic molecules—these are referred to as photo-oxidative processes. In the case of heterogeneous photocatalysis, photons are absorbed by a photosensitive material such as photocatalysts (e.g., titanium dioxide, zinc oxide). They induce the formation of charges on the catalyst’s surface, which initiate the production of radical species via redox processes.
Photo-oxidation technologies are expected to harness sunlight to break down contaminants. "Solar photoreactor" systems are still being developed in laboratories. The goal is to optimize efficiency and to determine how to minimize environmental and energy costs (during operation).
For example, research has been conducted to evaluate the capabilities of solar photoreactors for the decontamination of wastewater from hospitals (pharmaceuticals), agricultural effluents (biocide residues), groundwater remediation (solvent residues such as trichloroethylene), as well as the treatment of wastewater for agricultural (irrigation) or industrial uses.
To consider the deployment of these technologies, it is necessary to improve the performance of solar photoreactors and optimize the use of solar energy.
The available solar energy for photooxidation
The use of solar energy is indeed a key issue in today’s global climate, energy, and environmental context, as it is essential for ensuring the energy transition. To this end, efforts are being made to implement sustainable technologies that are cost-effective to operate, powered by solar energy.
This solar resource is variable (due to clouds, the day-night cycle, and the seasons, among other factors). When it comes to generating electricity (through photovoltaics), this poses a challenge, as it is costly to store the electricity generated until it is needed.
In contrast, in water treatment, contaminants can be stored through adsorption on carbon columns or in wastewater retention basins until the sun comes out.
Thus, when developing solar-powered water purification systems, their operational capacity is designed to cover the entire year, or their capacity is optimized to meet specific needs—such as seasonal demands in tourist areas.
Finally, solar radiation is divided into three main wavelength ranges: ultraviolet, visible, and infrared radiation. The photocatalysts currently available on the market have limitations in terms of their ability to absorb the solar spectrum. Today, only the ultraviolet range—which accounts for just 5% of the solar spectrum—can be utilized for photocatalysis in water treatment.
For the past three decades, research has been conducted to improve the performance of photosensitive materials, with the aim of increasing their photoconversion efficiency and their ability to absorb visible light (45% of the solar spectrum).
In this context, the challenges now lie in expanding the capacity of existing systems, improving water quality, and reducing the energy costs of the facilities.
To this end, the future of advanced oxidation technologies lies in combining them with other processes: biological processes (to remove “biorecalcitrant” pollutants, i.e., those that are not biodegradable), membrane processes (to remove small-sized pollutants not filtered by membranes), or even the solar thermodynamic cycle (to thermally activate catalysts).
The Aquireuse Project
Our Aquireuse project explores a treatment process that is unique in France, involving an initial stage of solar photocatalysis followed by infiltration into soil rich in organic matter, which helps break down pollutants.
In fact, for certain applications—such as recharging a groundwater aquifer with treated wastewater to serve as a source for drinking water production—the water must be free of micropollutants.
Recharging groundwater with treated wastewater is a practice that is still unfamiliar in France but is more widespread in places such as Australia and California. In particular, it helps combat a phenomenon that is becoming increasingly common in coastal areas: “saltwater intrusion.” When groundwater levels along the coast drop due to excessive extraction, seawater seeps in and contaminates freshwater resources, as the resulting saline water becomes unfit for human consumption.
In the Aquireuse project, effluent from a wastewater treatment plant is used to feed a pilot solar photocatalysis system, where an initial stage of total or partial degradation of micropollutants takes place. The treated effluent is then sent for infiltration into sediments, where the organic matter in the soil helps refine the treatment by continuing to degrade the micropollutants and byproducts resulting from solar photocatalysis.
The initial results are very promising: a large proportion of the micropollutants are completely degraded after passing through the treatment process. These results are currently being published.
Such a system, which combines a sustainable process with a nature-based solution, is an example of a circular economy approach to water treatment.
Gael Plantard, Professor of Materials Chemistry, University of Perpignan and Julie Mendret, Associate Professor, HDR, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.