Scoping (un)regulated organic substances in aqueous effluents for circular water reuse: a Dutch case study

In many parts of the world, including the Netherlands, the idea of turning used wastewater into fresh drinking water or irrigation for crops is moving from a desperate last resort to a necessary strategy. As freshwater sources become scarcer, treating sewage to a high standard and putting it back into the water cycle offers a way to secure the future of our water supply. However, before this water can be trusted, scientists must ensure it is free from invisible dangers. The primary concern is not the water itself, but the thousands of organic chemicals that flow through our pipes. These include residues from medicines, pesticides, personal care products, and industrial materials. Unlike bacteria, which are relatively easy to kill, many of these chemicals are designed to be tough. Some are persistent, meaning they do not break down easily in nature; others are mobile, meaning they travel quickly through soil and water; and some are toxic even in tiny amounts. When these substances slip through treatment plants, they can contaminate groundwater and crops, posing long-term risks to human health and ecosystems. The challenge for regulators is to keep up with this chemical soup, ensuring that the water we reuse is safe.

A recent study by researchers at Wageningen University and Utrecht University takes a hard look at how well current laws protect against these risks in the Netherlands. The team combined a chemical analysis of water leaving Dutch wastewater treatment plants with a deep dive into the legal rules governing those chemicals. They examined data from 332 treatment plants collected over a decade, from 2015 to 2025, to see exactly what was coming out of the pipes. Their goal was to determine if the existing European Union and Dutch regulations were actually covering the organic substances found in this water, or if dangerous chemicals were slipping through the cracks.

The researchers found a significant gap between the chemicals present in the water and the laws meant to control them. Out of the 554 unique substances detected in the treated effluent, a large portion—325 of them—were not covered by any of the specific water regulations the team analyzed. This means that for more than half of the chemicals found in the water, there were no specific legal limits or monitoring requirements in place to ensure they were safe for reuse. The most common group of unregulated substances were pesticides and their breakdown products, followed closely by pharmaceuticals. The study also highlighted that even when a substance is banned for one use, it might still be allowed for another, allowing it to enter the water system legally. For instance, a chemical might be prohibited as a pesticide but permitted as an ingredient in industrial plastics or household products, eventually finding its way into the sewer system and out into the environment.

A major focus of the research was on a specific type of chemical hazard known as PMT or vPvM. These are substances that are Persistent (they last a long time), Mobile (they move easily through water and soil), and Toxic (they are harmful). Once these chemicals enter the water system, they are incredibly difficult to remove with standard treatment methods, making them a persistent threat to water quality. The study revealed that a notable percentage of the organic substances found in Dutch effluent fit these dangerous profiles. While some pesticides and biocides were successfully classified as PMT or vPvM because they had been tested for these traits, many other chemicals, particularly industrial compounds and pharmaceuticals, lacked the necessary data to be classified at all. Without this data, regulators cannot determine if these substances are safe, leaving them in a legal gray area where they are neither banned nor monitored.

To address these gaps, the researchers explored the idea of grouping chemicals together rather than regulating them one by one. Currently, laws often treat each chemical as a unique individual, requiring a separate, time-consuming safety assessment for every single substance. The study suggests that chemicals with similar structures or behaviors could be regulated as a group. For example, if a family of chemicals all share a specific chemical structure that makes them persistent and mobile, the entire group could be restricted together. This approach, known as "one substance, one assessment," would allow regulators to use existing safety data from one chemical to protect against a whole class of similar chemicals. The study found that many of the unregulated substances in Dutch wastewater could be organized into such groups, including families of plastic additives and pesticide breakdown products. This method would not only speed up the regulatory process but also prevent "regrettable substitution," where a banned chemical is simply replaced by a slightly different one that is just as harmful.

The findings point to a clear need for change in how water safety is managed. The researchers argue that relying on current, fragmented laws is insufficient for the complex reality of modern wastewater. They suggest that enforcement of existing rules needs to be stricter, particularly regarding substances that are already prohibited but still appear in the sewer system. Furthermore, the scope of regulations must be expanded to specifically target the risks associated with water reuse, rather than just general pollution. By adopting a "one substance, one assessment" approach and using grouping strategies, regulators could close the loopholes that allow dangerous chemicals to persist in the water cycle. This would not only make water reuse safer and more efficient but also ensure that the water we rely on for our future is truly clean. The study concludes that while the science of identifying these chemicals is advancing, the legal framework must evolve just as quickly to keep pace, ensuring that the promise of circular water reuse does not come at the cost of public health.