Guide to choosing the right water filter
The essential criteria for choosing the right water filter
The market for water filters has grown significantly in recent years.
Between serious brands that test their products according to international standards, and those that rely on marketing, it becomes difficult to tell the difference.
Here are the 4 essential criteria to check before buying a water filter, based on the studies in our blog and our 2025 comparative analyses.
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Check the manufacturer's traceability
First and foremost, ask yourself, "Who actually makes this filter?"
A transparent brand always mentions the name and country of its factory. If this information is absent, replaced by a vague "developed in Europe", trust is compromised.
Case in point: Coldstream (UK) and Doulton (UK) clearly publish their production sites and certifications.
On the other hand, Star Filter, Weeplow and Monderma are not transparent about their factories and subcontractors.
Read also:
- 2025 Coldstream vs Doulton vs Generic Filters Comparison - The Real Differences Between a Tested Filter and a Declared Equivalent Filter | Pure Filters
- The KLT Filtration Factory
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Evaluate Factory Quality and Certifications
A reliable factory is not just an assembly workshop: it is a certified test center, capable of guaranteeing the traceability and repeatability of the tests.
Look for the following labels:
- ISO/IEC 17025: international accreditation of testing laboratories.
- NSF / IAPMO: certification of microbiological and chemical performance.
Examples of good practices:
- Coldstream FTO+: comprehensive testing to NSF/ANSI P231, 42, 53 and 401, by Envirotek and IAPMO (ISO 17025).
- Doulton Ultrasterasyl: equivalent, reproducible and audited assays for more than 40 years.
Avoid: Brands simply refer to "SGS tests" without specifying the site (e.g. SGS Xiamen): this laboratory is not ISO/IEC 17025 accredited, and its reports are internal.
Related: Pure Filters vs. Ultimate Star Filter
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Analyze the transparency of lab tests
A reliable filter publishes all of its tests, including:
- The name of the laboratory,
- The ISO/IEC 17025 accreditation number,
- The standards followed (NSF/ANSI or EPA).
If a report says "For internal research only", it means that the test is not certifying. This is an internal assessment, not internationally recognised.
In practice:
- Envirotek / IAPMO (Coldstream) or BCS Labs (Weeplow USA) tests are ISO/IEC 17025 recognized.
- SGS Xiamen (Star Filter / Weeplow China) tests are not.
To find out more: What is ISO/IEC 17025 and why is it important?
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Look for independent certifications
NSF/ANSI certifications are the global gold standard for filtration. They guarantee a performance measured over time, with independent supervision and audits.
What they cover:
- NSF/ANSI 42: Chlorine, Taste, and Odor.
- NSF/ANSI 53: heavy metals, PFAS, pesticides.
- NSF/ANSI 401: Emerging Contaminants (Pharmaceutical Residues).
- NSF/ANSI P231: bacteria, viruses, and protozoa.
A filter certified according to these standards can treat microbiologically unsafe water, such as river or rainwater. On the other hand, a non-certified filter remains limited to water that is already drinkable.
Read also: Everything you need to know about NSF P231 certification and its importance for water filtration
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In summary: 3 simple questions before buying
Before investing in a water filter, ask yourself these three questions:
- Does the brand clearly state who makes the product?
- Is the laboratory cited ISO/IEC 17025 accredited?
- Does the filter have an official certification (NSF, IAPMO, ISO)?
If one of these answers is no, be careful: a product that is not very transparent about its origin or its tests offers no real guarantee of its performance.
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Conclusion
Choosing a good water filter is above all about choosing a transparent and traceable brand, which publishes its data, cites its laboratories and proves its performance.
NSF/ANSI and ISO/IEC 17025 certifications are the only reliable benchmarks to ensure the quality and safety of filtered water.
Conversely, unaudited or anonymous internal tests, even "signed by SGS", do not provide any official guarantee.
Read more: 2025 Coldstream, Doulton, Star Filter, Weeplow, Monderma comparison - the real difference.
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Sources and Verifications
This guide is based on verifiable data from public and independent sources:
– Coldstream FTO+ tests (ISO/IEC 17025 certified laboratories)
– ISO/IEC 17025 – Testing and Calibration Laboratories (ISO.org)
– Why ISO 17025 is essential (Ideagen)
– NSF International Organization – Filter Performance Standards
Pure Filters undertakes to disseminate only verified, transparent information that complies with international technical standards.