O-rings for drinking water: certifications and materials

Sealing drinking water is not a chemistry problem first — it is a regulatory one. An O-ring that touches potable water must not leach harmful substances, must not support microbial growth, and must not change the water's taste, smell or appearance. That is a different and stricter regime than the food-contact rules (FDA, EC 1935/2004, USP Class VI, 3-A) that govern food and pharma. This guide explains the real potable-water approvals, what each one actually certifies, which country needs which — and why EPDM is the elastomer you will reach for nine times out of ten.

Why drinking water has its own rulebook

A food-grade certificate does not make a seal fit for drinking water. Food-contact approvals assume short, intermittent contact with food. Drinking-water seals sit in permanent contact with water that a person will drink for years, often warm and chlorinated. So the drinking-water schemes test three things food rules largely ignore: the migration of substances into water over long exposure, the growth of bacteria and biofilm on the elastomer surface, and effects on odour, flavour and turbidity. A seal can be perfectly FDA-compliant and still fail a drinking-water test on microbial growth alone.

United Kingdom — WRAS and BS 6920

In the UK the recognised benchmark is WRAS approval, based on the test methodology of BS 6920 (Suitability of non-metallic products for use in contact with water intended for human consumption). BS 6920 assesses the effect on water quality: odour and flavour, appearance, growth of aquatic micro-organisms, and the extraction of metals and other substances. A WRAS-approved material certificate is valid for five years and is tied to a specific compound and colour — not to a generic material family. Practical consequence: an O-ring is only WRAS-approved if its exact compound is listed, so always ask for the approval reference, not just the words 'WRAS EPDM'.

Germany — KTW-BWGL and DVGW W270

Germany runs the strictest dual requirement in Europe, and it changed recently. Germany replaced its older drinking-water elastomer assessment (the former UBA Elastomer Guideline) with the broader UBA criteria document KTW-BWGL — the German Environment Agency's assessment basis for plastics and other organic materials in contact with drinking water. The KTW-BWGL took effect in March 2021, and elastomers were brought into its scope from March 2022, with a transition period for existing O-ring approvals running to 2025/2026. It governs the chemical/migration side — the positive list of permitted starting substances and the migration limits.

On its own that is not enough. The second, separate requirement is DVGW W270, which tests microbial growth — how much the material itself feeds biofilm in water. A German potable-water elastomer needs both: KTW-BWGL for migration and W270 for microbiology. Suppliers often write this as 'KTW-BWGL + W270', and you should treat a quote that mentions only one of them as incomplete.

USA and Canada — NSF/ANSI 61 and NSF/ANSI 372 (lead-free)

For North America the controlling standard is NSF/ANSI/CAN 61, which evaluates contaminants and impurities that leach from drinking-water system components into the water. It is the standard plumbing inspectors and many municipalities require. A closely related standard, NSF/ANSI/CAN 372, certifies the 'lead-free' requirement (a weighted-average lead content of no more than 0.25 % on wetted surfaces, aligned with the US Safe Drinking Water Act). For non-metallic seals the lead limit is rarely the binding constraint, but specifying NSF 61 — and 372 where the buyer demands lead-free documentation — is the norm for any component sold into the US or Canadian water market.

France and the EU positive-list direction

France has its own scheme: ACS (Attestation de Conformité Sanitaire), required for materials in contact with water intended for human consumption. Across the EU the picture is consolidating. The recast Drinking Water Directive (EU) 2020/2184 introduced harmonised European requirements for materials in contact with drinking water, including European positive lists of permitted starting substances and common test methods, intended to replace the patchwork of national schemes over a transition period. For now you still meet national approvals (WRAS, KTW-BWGL/W270, ACS, NSF 61) market by market; over time a single EU framework will sit above them.

Why EPDM is the default potable-water elastomer

EPDM (ethylene propylene diene monomer) is the workhorse of cold and warm drinking water. It has excellent resistance to water, steam and polar media, and it ages well against ozone and UV — exactly the profile a water fitting needs. Its continuous service range is roughly -40 °C to +130 °C, with short-term excursions to about +150 °C, which comfortably covers cold mains water and most domestic hot-water service. Drinking-water EPDM is almost always peroxide-cured rather than sulphur-cured, and formulated without plasticisers, process oils or other extractable additives — precisely the substances that would migrate into the water and fail a WRAS or KTW-BWGL test.

It is also the only one of the common families that carries the full set of potable-water approvals as a matter of course. In our material reference EPDM is the one elastomer listed against WRAS, KTW, DVGW and NSF 61 together — and our engineering note marks it as the single sensible choice for potable water (alongside brake fluid). That is not marketing: it reflects which compounds the industry actually certifies.

Why NBR and FKM are usually wrong for cold potable water

Nitrile (NBR) is the cheapest seal in the catalogue and the instinct is to default to it — but for potable water it is the wrong instinct. NBR is formulated for mineral oil and fuel; its water rating is limited to cold water only, and standard nitrile compounds contain the kind of plasticisers and additives that potable-water schemes are designed to screen out. NBR drinking-water compounds exist but are a specialty, not the norm; reaching for a stock black nitrile O-ring on a water fitting is how you end up with off-taste and a failed certificate.

FKM (Viton) is the opposite mistake — over-specifying. It is a premium high-temperature, chemical-resistant material that costs roughly five times NBR, and it brings no advantage in cold water. Worse, standard FKM is explicitly rated incompatible with hot water and steam, so it is not even a safe choice for the hot side. There are dedicated drinking-water and steam grades, but for ordinary cold or warm potable water FKM is paying premium money for the wrong properties.

Hot water, sanitary and chlorine considerations

Most domestic and commercial hot-water service stays within EPDM's window, so peroxide-cured EPDM remains the default for hot potable water too. Where temperatures climb beyond EPDM — sustained steam, sanitising cycles, or higher-temperature process water — silicone (VMQ, usable from about -60 °C to +200 °C) is an option for static seals thanks to its physiological inertness and wide temperature range, though its weak mechanical strength rules it out of dynamic duty. FKM and FFKM enter only for specific high-temperature or aggressive sanitiser cases, with the appropriate water-contact grade, not as a general default.

Chlorine and chloramine are the slow killers of potable-water seals. Municipal water is disinfected with free chlorine or, increasingly, with chloramine, and both attack elastomers over time — chloramine is the more aggressive of the two toward EPDM. Standard EPDM tolerates normal residual chlorine levels well, but for chloraminated supplies or elevated disinfectant doses, ask specifically for a chlorine/chloramine-resistant EPDM compound. This is one more reason to specify the exact approved compound rather than a generic material name.

Frequently asked questions

Is a food-grade O-ring safe for drinking water?

Not automatically. FDA, EC 1935/2004, USP Class VI and 3-A are food- and pharma-contact approvals; they do not cover the migration-over-time and microbial-growth tests that drinking water requires. For potable water you need a drinking-water approval — WRAS, KTW-BWGL plus DVGW W270, NSF/ANSI 61, or ACS — depending on the market.

Which approval do I need for which country?

As a rule of thumb: WRAS (to BS 6920) for the UK; KTW-BWGL plus DVGW W270 for Germany; NSF/ANSI 61 (and 372 for lead-free documentation) for the USA and Canada; ACS for France. The EU Drinking Water Directive 2020/2184 is harmonising these into a single European framework, but national approvals still apply during the transition.

Do I need DVGW W270 if I already have KTW-BWGL?

Yes, for the German market. KTW-BWGL covers chemical migration; W270 separately tests resistance to microbial growth. They are complementary, not interchangeable — a compliant German drinking-water elastomer holds both.

How to proceed

For ordinary cold and warm drinking water, specify peroxide-cured EPDM with the approval your market requires, and ask for the specific approved compound reference rather than a generic 'WRAS EPDM'. If you know the dimensions, our configurator gets you to the right ring in seconds; check chlorine or chloramine exposure against the chemical-compatibility table, convert any inch sizes with the dimension converter, and if a seal has already failed, work back through the failure analysis. If a certificate question is still open after that, write to Filip — but most of the time you can settle the material choice yourself in a few minutes.