Gravity Filters: The Real Differences Between Tested Brands and Marketing Copy

Understand what Coldstream, Doulton, Monderma, Weeplow, Star Filter, and Phoenix filters are really worth.

A market flooded with "gravity" filters

Gravity water filters are very popular. Their principle is simple: purify water without electricity, using a slow and natural filtration system.

But between serious brands, which test according to international standards, and those that communicate on non-certified "internal tests", the difference is considerable.

This article draws on official and verifiable laboratory reports, as well as international certifications (NSF, IAPMO, ISO/IEC 17025) to provide an update. On the other hand, this article compares six major manufacturers, based on official, independent laboratory reports

• Coldstream (KLT Filtration, UK)
• Doulton / British Berkefeld (UK)
• Monderma Black Filter (China / USA)
• Weeplow (USA / China)
• Star Filter (France / China)
• Phoenix (India)

Why NSF/ANSI Standards Make a Difference

NSF/ANSI standards are the global benchmark for evaluating the performance of water filters. They define the protocols, test conditions and reduction thresholds to be achieved to ensure the safety of drinking water.

• NSF/ANSI 42 Cosmetic TestsTaste, Odor, Chlorine

• NSF/ANSI 53 Health TestingHeavy Metals, PFAS, Pesticides

• NSF/ANSI 401 Emerging TestsDrugs, Hormones, Caffeine

• NSF/ANSI P231 Microbiological TestingBacteria and Virus Reduction

• NSF/ANSI/CAN 61 Components in Contact with Drinking Water – Health Effects (Material Extraction Tests). 

• NSF/ANSI/CAN600 Toxicological reference standard that defines the health criteria used in particular by NSF 61/60 (it is not a product certification on its own).

Product certifications are issued by NSF or IAPMO. ISO/IEC 17025 accreditation (by ANAB, etc.) attests to the competence of the laboratory that performs the tests, not to a product certification.

What the 2025 tests really show

New tests released in 2025 by some brands (including Star Filter, Weeplow, and Monderma) reveal a clear distinction between exploratory testing and prescriptive certifications.

Many brands mention "SGS", "ISO" or "laboratory tests", without specifying that these tests are non-certifying and often carried out outside the international framework.

SGS Xiamen and Wessling France laboratories do carry out real analyses, but their reports relate to water quality or internal research, they do not constitute performance certifications according to NSF/IAPMO standards. Their scope is limited to research or internal control, without global normative recognition.

Detailed analysis by brand

Coldstream (KLT Filtration, United Kingdom)

Coldstream is one of the few European manufacturers whose filters have been tested to the standards NSF/ANSI 42, 53 and P231 by IAPMO R&T/Envirotek (ISO/IEC 17025 accredited laboratory). The tests show in particular a viral reduction > 99.99% (rotavirus) up to 3,000 L, as well as a very high effectiveness on heavy metals, pesticides, pharmaceutical residues, etc.

The Coldstream FTO+ filters were tested according to the microbiological protocol (NSF/ANSI P231) for use on untreated water (surface, rain, well), and they held reductions ≥ 99.99% over 3,000 L for viruses (e.g. rotavirus), in addition to bacteria. They are not designed for industrial or highly polluted water.

Nitrates & fluorine: how to read the Coldstream FTO+ results

• In the first ~500 L, independent tests show about 90% reduction in fluorine (F⁻) and more than 60% reduction in nitrates (NO₃⁻).
• As expected for highly soluble ions, the efficiency decreases with media wear: towards the end of the cycle (~3,000 L), ≈ 4.55% reduction is observed on fluorine and ≈ 29.63% on nitrates.

This is why, beyond microbiology (bacteria/viruses/cysts, maintained up to 3,000 L), we recommend replacing filters at 6 months or 1,500 L when the reduction of chemical contaminants (fluorine, nitrates, PFAS, etc.) is a key criterion.

Doulton / British Berkefeld (Ultrasterasyl, United Kingdom)

Doulton/British Berkefeld filters are tested and certified on 42/53/372/401 (depending on models), with excellent results in chemistry (metals, PFAS, pharmaceuticals) and bacterial microbiology.

Doulton says its filters can make untreated natural water (lakes, rivers, rain) drinkable, but are not designed for industrial or highly polluted water. They are reliable, durable and recognized filters on a global scale.

Weeplow (United States / China)

Contrary to what its communication suggests, Weeplow has not carried out any tests in France. Microbiological analyses were performed by BCS Laboratories (Gainesville, Florida, USA), an ISO/IEC 17025:2017 accredited laboratory by ANAB (L2422). These assays show a 99.9 to 99.99% reduction in bacteria (E. coli, Enterococcus, coliforms) over a volume of approximately 300 liters, according to EPA 1600 and APHA 9222 protocols.

However, the report states: "The data obtained may not be representative or indicative of a real-life process."

The chemical analyses were conducted by SGS Xiamen (China) according to GB/T 5750 methods and SGS internal procedures (not ISO 17025 accredited).

The reports shall be marked by: "For scientific research and internal quality control only. Not for product certification."

In summary : the tests are real but scattered (microbiology in the United States, chemistry in China), and none have been conducted in France. The brand has no official NSF/IAPMO certification, only one-off tests on limited volumes.

Star Filter (France / China)

The first tests of Star Filter (Wessling France, 2024) focused on heavy metals and chemical pollutants. In 2025, the brand has published twelve new SGS Xiamen (China) reports, presented as "endurance tests" on 3,000 to 3,200 liters. The results show reductions of more than 99% for heavy metals, PFAS, bacteria and viruses (coliphagous MS2).

However, these reports specify that they were carried out "for internal research purposes" and "without certification value."

SGS Xiamen is not ISO/IEC 17025 accredited, but only CMA (China Metrology Accreditation), which limits the scope of its results to China.

The protocols used (GB/T 5750, EPA 537.1, EPA 1602) are water testing methods, not filter performance standards (such as NSF/ANSI P231). No independent supervision accompanied these trials.

Another test published by Star Filter on a Coldstream FTO+ (CF163W) was performed on 100 L, with an influent viral concentration of 4.2×10⁵ PFU/mL. As a benchmark, the EPA protocol (Guide Standard 1987) — the basis of NSF/ANSI P231 — provides for a viral challenge of 10⁷ PFU/L, or 1×10⁴ PFU/mL. The SGS test applied to Coldstream is therefore ~42× heavier than this protocol benchmark, and ~17× higher than the concentration used in the KLT/Coldstream tests (2.4642×10⁷ PFU/L), which showed a 99.99% viral reduction up > 3,000 L.

Note: The "EPA Method 1602" cited by SGS is a coliphage enumeration method (SAL). It does not set an influent concentration to evaluate the performance of a filter and should not be confused with the EPA 1987 protocol that underpins NSF/ANSI P231. (Methodological clarification text, without new figures.)

Finally, 4.2×10⁵ PFU/mL (4.2×10⁸ PFU/L) exceeds the high range frequently reported for raw wastewater (10⁶ PFU/L), which limits comparability with domestic use.

In summary : the Star Filter tests are real but not certified, carried out in a non-ISO/IEC 17025 accredited laboratory, with favorable experimental conditions for their filters and extremes for Coldstream, making comparisons impossible.

Phoenix (India)

Phoenix's tests were carried out by Atom Testing Laboratory (Chennai, India), which is ISO/IEC 17025 accredited. The protocols refer to NSF/ANSI standards 42, 53 and P231, but the volumes tested are low (<400 L).

The results are good in the short term, but no evidence of endurance or full certification has been published.

Monderma Black Filter (China / USA)

Tests of Monderma were conducted by NSF International (USA), GDDCM (China) and NACC (China Academy of Sciences). They cover material safety (NSF/ANSI Standard 42 – Material Extraction) and chemical compliance (GB/T 5750).

No microbiological performance data are provided. The filters are therefore safe for drinking water, but not validated for advanced filtration.

Monderma Imperial GF (USA)

The Monderma case: strong promises, but comprehensive reports difficult to find

For some time now, Monderma has been promoting several filters for gravity-fed systems, including so-called "Black Filters" or "Imperial GF" cartridges, announced with a 12-month service life.

These products are advertised as cost-effective, compatible with different systems, and capable of reducing a wide range of contaminants.

The problem is not that a ceramic or activated carbon cartridge exists. This type of technology is well known in the field of filtration.

The problem lies elsewhere: when a filter announces a 12-month lifespan, very broad performance and a direct comparison with other serious technologies on the market, the consumer must be able to consult the complete laboratory reports.

However, to date, we have not found on the Monderma website the complete reports that would allow us to precisely verify:

• the exact cartridge model tested ;
• the laboratory that carried out the tests ;
• the protocol used ;
• the number of filters tested ;
• the total volume tested ;
• the inlet and outlet concentrations ;
• the results at the end of capacity ;
• the exact match between the filter sold and the filter tested ;
• the technical justification for the advertised duration of 12 months.

Without these elements, it is impossible for a customer, a reseller or a professional to seriously verify the performance advertised.

12 months with activated charcoal: A promise that requires solid evidence

A filter containing activated carbon can be effective in reducing certain organic compounds, chlorine, certain tastes, odors, and some chemical contaminants.

But activated carbon works by adsorption. It retains certain substances until they are gradually saturated. Its lifespan therefore depends on many factors:

• the quality of the water at the entrance ;
• the actual level of contamination ;
• frequency of use ;
• the filtered volume ;
• storage conditions ;
• periods of non-use ;
• temperature ;
• the possible stagnation of water ;
• maintenance of the system.

In this context, Announcing 12 months of use for a cartridge incorporating activated carbon must be supported by solid, legible and complete data.

At Pure Filters, we have chosen a more conservative approach: Coldstream FTO+ cartridges are recommended for 6 months of use.

This duration is less spectacular from a marketing point of view, but it corresponds to a logic of responsibility. In drinking water filtration, it is better to announce a prudent and reliable duration than to promise a flattering longevity without making the evidence fully accessible.

Colloidal silver: a formulation that deserves clarification

Some cards associated with Monderma cartridges or their variations mention the presence of colloidal silver.

This wording should be taken with caution.

In the field of ceramic filters, we generally speak of silver-impregnated ceramics, with a bacteriostatic function. The aim is to limit bacterial proliferation in the filter media itself, in particular to help protect the ceramic from internal microbiological development.

This is not the same as suggesting that colloidal silver is released into water or acts as a disinfectant in drinking water.

The distinction is important.

Serious technical wording should clearly explain:

• what form of money is used ;
• where it is in the filter ;
• if it is integrated into ceramics ;
• if it is intended to protect the filter itself ;
• whether or not there is a measured release in the filtered water ;
• which tests demonstrate the safety of use.

Again, without complete documentation, the consumer is left with a promise that is difficult to verify.

Compatibility does not mean equivalent performance

Another important point is compatibility.

Many cartridges are presented as compatible with standard gravity filtration systems: Berkey, British Berkefeld, Weeplow, Pure Filters or other stainless steel tanks.

But a cartridge that physically "fits" into a tank is not automatically equivalent to a cartridge tested in a specific system.

Mechanical compatibility does not prove:

• the same filtration performance ;
• the same safety of use ;
• the same long-term consistency ;
• the same reduction in contaminants ;
• the same build quality ;
• the same traceability ;
• the same real lifespan.

This is exactly why comprehensive laboratory reports are a must.

A product sheet is not enough. A promise is not enough. A "tested to NSF/ANSI" statement is not sufficient if the full report is not accessible and verifiable.

Based on our tutorial "How to choose the right water filter", we give this cartridge a score of 7/20.

This rating does not mean that the technology announced is necessarily bad. Above all, it reflects a lack of verifiable guarantees: complete laboratory reports are not published, the announced duration of use seems very ambitious for a cartridge containing activated carbon, and certain technical formulations, particularly around silver, remain insufficiently clear.

In drinking water filtration, a commercial promise is never a substitute for complete, accessible and verifiable tests.

Interpretation: what it means for the consumer

Test Type	What it proves	What this doesn't prove
Chemical Safety (NSF 42 / GB 5750)	The filter does not release harmful substances	Its real effectiveness in the long term
Spot tests (SGS, Atom, Wessling)	Short-term initial efficacy	Endurance, stability, certification
Certifications NSF/IAPMO (Coldstream, Doulton)	Verified, auditable and traceable performance	/

Technical Ranking 2025

Category	Brands	Reliability
NSF/IAPMO certified	Coldstream, Doulton	Comprehensive, audited, traceable tests
Tested without certification	Star Filter, Weeplow, Phoenix	Spot tests, local protocols
Tested for safety only	Monderma Black Filter	Safe filter, no proof of performance

Interpretation and advice to consumers

This comparison highlights an essential distinction: A filter can be "tested" without being "certified".

Coldstream and Doulton filters are based on certified normative tests, carried out by ISO/IEC 17025 accredited laboratories, while brands such as Star Filter or Weeplow communicate on non-normative internal tests, sometimes carried out in laboratories without international accreditation.

For the user, this means that a "tested" filter does not necessarily offer guarantees of performance in real conditions.

Only recognized product certifications (NSF/IAPMO) ensure audited and traceable performance. Tests carried out in an ISO/IEC 17025 accredited laboratory attest to the competence of the laboratory, but do not constitute product certification.

Spot tests performed outside of the normative framework (e.g., EPA/GB analytical methods applicable to water, but without a performance protocol such as NSF/ANSI 42/53/401/P231) are not comparable to an audited product certification (NSF or IAPMO) performed in ISO/IEC 17025 accredited laboratories.

Conclusion

The Coldstream FTO+ filters and Doulton Ultrasterasyl remain the only solutions tested, audited and certified according to international standards. They guarantee complete purification of natural or untreated water.

The other brands (Star Filter, Weeplow, Monderma, Phoenix) do carry out tests, but their tests have no certifying value. Their results should therefore be considered as exploratory data, useful for information purposes but not comparable to international certification.

Notes : 

This article was written using data and test reports available as of October 31, 2025.

The information presented here comes from public sources, independent laboratory reports and technical data sheets provided by the manufacturers.

Readers wishing to consult the complete tests can refer directly to the official documents of the laboratories mentioned (NSF, IAPMO, Envirotek, Wessling, SGS, etc.).