Clean Water Thanks to the Sun
The contamination of water resources by organic micropollutants is a growing concern worldwide, 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 trace concentrations in water (micrograms, or even nanograms, per liter), but even at these concentrations, their impact on aquatic ecosystems and public health has been proven.
Global warming is exacerbating the situation, as temperature fluctuations, changes in hydrological patterns, and extreme weather events can affect the mobility 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 processes (ATPs), such as ozonation and photooxidation, have the advantage of non-selectively destroying organic contaminants—whether biotic (bacteria, pathogens) or abiotic (pesticides, pharmaceuticals)—and are therefore ideally suited to addressing the issue of 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 degradation of pollutants into carbon dioxide, water, and salts; this is known as mineralization.
Among TOA processes, some convert light energy into chemical energy to oxidize and degrade organic molecules—these are known as photooxidative 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 make it possible to use sunlight to break down contaminants. “Solar photoreactor”-type 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
Harnessing solar energy is indeed a major challenge in the current global climate, energy, and environmental context, as it is essential to ensuring the energy transition. To this end, efforts are being made to implement sustainable technologies that are cost-effective to operate using solar energy.
This solar resource is variable (due to clouds, the day-night cycle, and the seasons, among other factors). When trying to generate electricity (through photovoltaics), this poses a challenge, because it is costly to store the electricity generated until it is needed.
In water treatment, however, contaminants can be stored by adsorption on activated carbon columns or in wastewater retention basins until the sun comes out.
Thus, when developing solar-powered water purification systems, their operating capacity is designed to meet annual demand, or their capacity is optimized to meet specific needs—such as seasonal demand 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 absorption of the solar spectrum. Today, only the ultraviolet range—which accounts for just 5% of the solar spectrum—can be used for photocatalysis in water treatment.
For three decades, studies have been conducted to improve the performance of photosensitive materials, with the goal of increasing their photoconversion efficiencies and their ability to absorb visible radiation (45% of the solar spectrum).
In this context, the challenges now are to expand the capacity of existing systems, improve water quality, and reduce 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 pollutants that are 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, which involves an initial stage of solar photocatalysis, followed by infiltration into soil rich in organic matter, which helps break down pollutants.
In fact, for certain uses—such as replenishing 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 unknown 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 withdrawals, seawater seeps in and contaminates freshwater resources, rendering the water unsuitable for human consumption.
In the Aquireuse project, effluent from a wastewater treatment plant is used to feed a pilot-scale 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, University 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.