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Water Treatment Transformation Products: What the New EU Guidance Means
27th April 2026
Blog by Kevin O’Brien, Senior Regulatory Affairs Manager Environmental Fate at Vali Consulting
Purpose – Why this topic matters
From April 2026, EU regulations will require the assessment of how substances behave during drinking water treatment. This represents an important evolution in regulatory science, as it goes beyond evaluating the original substance and its environmental metabolites alone.
During drinking water treatment, substances may be transformed into new compounds. These transformation products can differ significantly in their chemical and toxicological properties, with potential implications for human health and environmental safety.
Understanding and addressing these transformation products is now essential for compliance, robust risk assessment, and the protection of drinking water quality
Background – The scientific and regulatory context
Current regulatory frameworks already require that substances do not pose a risk to human or animal health via drinking water exposure.
Until now, assessments have primarily focused on environmental fate processes occurring before water abstraction.
The new guidance closes an important gap by recognising that drinking water treatment processes—such as chlorination or ozonation—may not fully remove substances but instead transform them into new chemical entities.
This updated framework will become mandatory for plant protection products and biocides submitted from April 2026 onward.
Scientific process – How we address the challenge
The guidance follows a structured, step-by-step approach to determine whether further assessment is required.
Step 1: Exposure assessment
The first step is to evaluate whether residues can reach drinking water sources.
A key regulatory trigger is 0.1 µg/L. If predicted concentrations remain below this threshold, no further assessment is required.
If the threshold is exceeded, the assessment continues and surface water exposure estimates are refined using dilution factors.
Step 2: Transformation during treatment
The next step evaluates what happens during drinking water treatment.
Relevant processes include chlorination, ozonation, UV disinfection, and filtration.
The assessment begins with available literature data and predictive models such as QSAR to evaluate substance stability. If stability is demonstrated, experimental testing may not be required.
If uncertainty remains, experimental studies are conducted.
Testing typically starts at elevated concentrations to identify potential transformation products and is then repeated at environmentally relevant levels.
Only transformation products exceeding 0.075 µg/L at relevant conditions are taken forward for further assessment.
Step 3: Toxicity assessment
If transformation products are identified, their toxicological relevance must be evaluated.
- Tier 1: focuses on genotoxicity. In the absence of data, a conservative approach is applied.
- Tier 2: assesses general toxicity, often using the Threshold of Toxicological Concern (TTC) approach.
- Tier 3: includes more detailed studies where uncertainty remains.
Implications for Industry
This new guidance increases both scientific and regulatory complexity.
Early exposure modelling becomes critical, as it determines whether further assessment of transformation products is triggered.
Testing requirements are also becoming more demanding. Simulating drinking water treatment processes and identifying unknown compounds requires advanced analytical methodologies and robust experimental design.
In some cases, entirely new substances or transformation products may need to be assessed. This can involve additional analytical work as well as toxicological studies.
Timelines and regulatory outcomes may be impacted if risks are identified during the assessment process.
Early planning and integrated strategies therefore play a key role in reducing uncertainty and avoiding delays.
Conclusion
The new guidance reflects a more complete and realistic view of environmental risk assessment.
It ensures that drinking water safety is evaluated across the full lifecycle of a substance, including transformation processes during water treatment.
By addressing transformation products early in the development process, it is possible to support regulatory compliance while strengthening the scientific basis for protecting human health.
At the same time, this approach reinforces responsible innovation and long-term environmental protection.
Download the whitepaper to learn more about the 2026 requirements.