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 relies on official and verifiable laboratory reports, as well as international certifications (NSF, IAPMO, ISO/IEC 17025) to take stock. 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 that must 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 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 worldwide 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+ filtersHave been tested according to the microbiological protocol (NSF/ANSI P231 base) for use on untreated water (surface, rain, well), and have maintained 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 they are not designed for industrial or highly polluted water. They are reliable, durable and recognized worldwide.

Weeplow (United States / China)

Weeplow publishes five downloadable laboratory reports on its website: bacteria, viruses, PFAS, pesticides and heavy metals. This is more than many market players, and this availability is worth noting. However, a detailed analysis of these documents makes it possible to specify their real scope.

Where were the tests carried out ?

The site weeplow.com displays the mention "Laboratory tested in France" and refers to analyses carried out by laboratories "in the United States and Europe" (found on the "Independent laboratory tests" page, accessed on August 26, 2026). However, the five published reports come from three laboratories: BCS Laboratories in Gainesville (Florida, United States) for bacterial microbiology, Qalitex Inc. in Irvine (California, United States) for PFAS, and SGS-CSTC Xiamen (China) for viruses, pesticides and heavy metals. None of the reports made available come from a laboratory located in France or elsewhere in Europe.

Bacterial Microbiology: The BCS Report (United States, February 2023)

The most comprehensive report in the dossier is that of BCS Laboratories, an ISO/IEC 17025:2017 accredited laboratory by ANAB (certificate L2422), whose accreditation logos appear on the report. The protocol is clearly described: three units tested, 75 liters per day for four days, with challenge water in accordance with the NSF P231 standard. The last measurement point is at 300 liters — this is the maximum volume tested on all the documentation published by the brand.

The detailed results per unit are worth reading carefully. On bacteria (E. coli, total coliforms, E. faecalis), unit D achieves reductions of Log 4.2 to 4.7. Units E and F are between Log 2.2 and Log 3.5 depending on the strain and measurement point. To put these numbers in perspective, the NSF/ANSI P231 microbiological framework, which defines the "water purifier" category, requires a Log 6 reduction on bacteria. Expressed as percentages, results of "99.9%" and "99.9999%" may seem close; in reality, three orders of magnitude separate them.

The BCS report also contains this reservation, formulated by the laboratory itself:

"The data obtained may not be representative or indicative of a real-life process and/or application."

Viruses: the SGS Xiamen report (China, August 2024)

Weeplow has been publishing a virus report (SGS Xiamen, ref. XMF24-0006475-01) on the MS2 coliphage since 2024, with a measured reduction of 99.9999%. The report specifies that the measurement was carried out "at the start-up of the service life" — i.e., at the start of the filter life, at a single measurement point. No data are published on the maintenance of this performance over time or in volume. The method cited (EPA 1602) is a method for enumerating coliphages; it does not constitute a filter performance protocol within the meaning of NSF/ANSI P231.

PFAS: The Qalitex Report (United States, August 2024)

The PFAS report (Qalitex Inc., Irvine, California, Ref. 9145253 V1, August 23, 2024) is issued in the name of OLCAM LLC, domiciled at the same Miami address as indicated for Weeplow on the SGS reports. It covers seven compounds (PFBS, PFDA, PFHxA, PFNA, PFOA, PFOS, PFSxS), all measured "at the start-up" — at the start of the service life, at a single measuring point. The method indicated for each compound is "In-house" : The report does not refer to EPA Method 537.1 or EPA Method 533, which are the reference analytical methods for PFAS in water, and does not include any accreditation marks. The one-page document does not describe the test protocol.

A reading note is in order on the rates displayed: the report indicates a reduction of ">99.9%" for each compound, starting with an input concentration of 1.00 μg/L and an output concentration of less than 0.002 μg/L. The calculation based on these values — (1.00 − 0.002) / 1.00 — corresponds to a reduction greater than 99.8%. The rate of ">99.9%" displayed would assume an output concentration of less than 0.001 μg/L, which is not documented in the report.

Pesticides and Heavy Metals: SGS Xiamen Reports (2024)

The heavy metals report (ref. XMF24-0002261-01, June 2024) measures the reduction of thirteen elements, also "at the start-up of the service life". The methods combine the Chinese Ministry of Health's sanitary standard, GB/T 5750 and EPA 200.8 analytical methods, with reference to NSF/ANSI standards 42 and 53 for doping conditions. The pesticide report (ref. XMF24-000026-01, February 2024) covers six molecules, measured at start-up, according to an internal laboratory method ("SGS Inhouse Method").

The scope of SMS reports, as specified by the reports themselves

Each SMS report concludes with a reference to use. The one in the virus report is as follows:

"Unless otherwise stated the results shown in this report refer only to the items tested. Within the territory of the People's Republic of China, this test report shall only be used for scientific research, technical research and development, teaching, and internal quality control. »

The heavy metals and pesticides reports include an equivalent mention. In other words, the laboratory itself limits the use of these documents to scientific research, development, teaching and internal quality control. These reports do not constitute a product performance certification within the meaning of the NSF or IAPMO standards. It should also be noted that, unlike the BCS report, which displays its ANAB and ilac-MRA accreditation marks, the SGS reports provided do not bear any CNAS or ILAC accreditation mark: they are therefore not issued under the guise of accreditation, which limits their recognition outside the framework described by the laboratory.

Announced shelf life and volumes tested

Weeplow announces a capacity of up to 22,000 liters per pair of cartridges. The brand itself specifies on its website that it is a "theoretical life if certain conditions are met". The published reports put this figure into perspective: the maximum volume tested, all reports combined, is 300 liters (BCS report), with SGS reports having only one measurement point at start-up. There is no published data documenting filtration performance between 300 and 22,000 liters.

Settings with no published report

As of the date of consultation (August 26, 2026), no reports are published by the brand regarding pharmaceutical residues or nitrates.

In summary : Weeplow publishes real reports, including one serious bacterial report carried out in an accredited laboratory (BCS, up to 300 litres), and four reports – PFAS, viruses, pesticides, heavy metals – measured only at the start of the lifespan, carried out using internal methods or outside the accreditation coverage. The brand does not have any NSF or IAPMO performance certification. The gap between the maximum volume tested (300 litres) and the advertised capacity (22,000 litres, described as theoretical by the brand itself) remains the main point that the consumer should keep in mind when comparing.

Star Filter (France / China)

The first tests of Star Filter (Wessling France, 2024) focused on heavy metals and chemical pollutants. In 2025, the brand 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 analysis methods, not filter performance standards (such as NSF/ANSI P231). No independent oversight accompanied these tests.

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 the NSF/ANSI P231 standard — predicts 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 1987 EPA protocol that underlies NSF/ANSI P231. (Methodological clarification text, no new figures included.)

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 performed 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)

Monderma's tests were conducted by NSF International (USA), GDDCM (China) and NACC (China Academy of Sciences). They focus on material safety (NSF/ANSI Standard 42 – Material Extraction) and chemical compliance (GB/T 5750).

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

MondermaImperial 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 growth 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 listing is not enough. A promise is not enough. A "tested to NSF/ANSI" statement is not enough 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 some 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 laboratory's competence, 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+ filtersAnd 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 only but not comparable to international certification.

Notes :

This article was written based on data and test reports available as of October 31, 2025and updated on August 26, 2026 (Weeplow section)."

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.).