In a pioneering study published in the Journal of Water Process Engineering, authors Anna Volkova, Jessica Subirats, Marta Turull, Víctor Matamoros, and Mònica Escolà Casas examined a more sustainable way to manage the problematic leftovers of modern water purification. As cities increasingly turn to reverse osmosis to recycle wastewater, they are left with a highly concentrated, pollutant-rich liquid known as brine. This byproduct contains high levels of salts, nutrients, and contaminants of emerging concern, such as pharmaceuticals, which are difficult to remove and often end up back in the environment. The researchers tested whether nature-based solutions, specifically constructed wetlands using innovative materials like biochar and coke, could act as a final polish for this concentrated waste.

The team discovered that wetlands built with biochar were remarkably effective, removing more than 90 percent of almost every emerging contaminant they monitored. This held true even in a high-pollution scenario designed to mimic the intense discharge from brand-new purification membranes. In comparison, traditional sand filters and coke-based systems only managed to remove about 73 percent of these chemicals under the same heavy loads. While all the tested wetland configurations were successful at removing more than 60 percent of common nutrients like nitrogen and phosphorous, biochar was the only material that could reliably handle the complex cocktail of medicines and personal care products found in the brine.

The secret to the superior performance of biochar lies in its ability to support a thriving, specialized microbial community. The study revealed that biochar filters became a home for specific types of bacteria, such as Xanthobacter and Flavobacterium, which are known for their unique ability to degrade persistent organic pollutants. Interestingly, the researchers found that simple adsorption, where pollutants just stick to the surface of the filter, accounted for less than 35 percent of the removal for most chemicals. This means the bacteria and natural chemical reactions within the biochar were doing the heavy lifting by actually breaking the pollutants down. Coke and sand filters, which were less efficient, were found to be populated by general bacteria that lacked these specialized degradation skills.

Efficiency was not the only benefit identified in the study; the biochar-based approach also offers a dramatic reduction in operational expenses. Traditional advanced chemical treatments are often energy-intensive and rely on expensive chemical additives. Nature-based constructed wetlands, however, utilize gravity and natural biological activity, leading to daily running costs that are 90 to 98 percent lower than high-tech alternatives. This makes it a highly attractive option for water reuse stations that need to manage brine on-site without breaking the budget. By mimicking natural processes, these systems transform a difficult waste product into water that, with some minor adjustments, could be used for irrigation in water-scarce regions.

The study also addressed how the age of purification equipment affects waste, finding that new membranes produce brine with chemical levels up to 40 times higher than older ones. Despite these wild fluctuations in pollution levels, the biochar wetlands remained resilient and maintained their high removal rates. This practical reliability is essential for real-world applications where conditions are rarely constant. While some parameters like salt levels still require management before the water can be used for farming, the study provides a robust proof-of-concept for a circular water economy. By using a simple, low-cost amendment like biochar, artificial wetlands can be turned into powerful tools for cleaning up the most challenging industrial effluents.

Source: Volkova, A., Subirats, J., Turull, M., Matamoros, V., & Escolà Casas, M. (2026). Assessment of bioelectrochemical materials and aeration in constructed wetlands for reducing contaminants of emerging concern from reverse osmosis brine. Journal of Water Process Engineering, 87, 109973.