Biodegradable Surfactants Explained: What OECD 301 Testing Really Measures

Almost every surfactant supplier describes its products as biodegradable, and almost every brand brief now asks for "biodegradable surfactants". Both sides usually mean a pass in one of the OECD 301 tests. Very few people on either side have read the guideline. This post explains what the tests measure, what a pass does and does not prove, and how to turn a supplier's certificate into a claim that will hold up.

It builds on our earlier post on sustainable formulation with biodegradable surfactants and RSPO palm derivatives, which covers the raw-material side. This one is about the test.

Three words that get confused

Primary biodegradation means the surfactant molecule has been changed enough to lose its surfactant properties. The molecule is no longer a surfactant, but its fragments may still be there.

Ultimate biodegradation means the molecule has been broken down to carbon dioxide, water, mineral salts, and new microbial biomass. This is what most people picture when they hear "biodegradable", and it is the only thing OECD 301 measures.

Ready biodegradability is a regulatory shorthand: a substance that passes one of the OECD 301 tests, which are deliberately harsh, is assumed to degrade rapidly in most aerobic environments. A substance that fails may still degrade, just not fast enough or not under those conditions. That is tested separately under OECD 302, "inherent biodegradability", which uses friendlier conditions and is a much weaker claim.

When a supplier says "biodegradable" without qualification, the first question is which of these they mean.

What an OECD 301 test does

All six methods share a design. The test substance is the only significant source of organic carbon in a mineral medium, it is inoculated with microorganisms from a source such as activated sludge from a sewage treatment plant, and the mixture is held at a controlled temperature in the dark for 28 days. The test then tracks how much of the substance has been mineralised, using one of three measurements:

• Dissolved organic carbon (DOC) removal, for substances that dissolve in water.
• Carbon dioxide produced, compared with the theoretical maximum (ThCO2) if all the carbon were mineralised.
• Oxygen consumed, compared with the theoretical oxygen demand (ThOD) of full mineralisation.

The six methods, with their pass levels, are:

• 301 A, DOC Die-Away: measures DOC removal. Pass level 70%.
• 301 B, CO2 Evolution (Modified Sturm): measures CO2 produced. Pass level 60% of ThCO2.
• 301 C, Modified MITI (I): measures oxygen consumed. Pass level 60% of ThOD.
• 301 D, Closed Bottle: measures oxygen consumed. Pass level 60% of ThOD.
• 301 E, Modified OECD Screening: measures DOC removal. Pass level 70%.
• 301 F, Manometric Respirometry: measures oxygen consumed. Pass level 60% of ThOD.

The respirometric pass level is lower than the DOC pass level because some of the carbon from the test substance is built into new microbial cells rather than released as CO2, so 60% mineralisation implies near-complete degradation.

For surfactants, methods B, D, and F are the most common choices. DOC methods struggle with surfactants because the molecules adsorb onto the inoculum and the glassware, which reads as removal without any degradation.

The 10-day window

A pass is not just reaching 60% or 70% at some point within 28 days. The guideline requires the pass level to be reached within a 10-day window that starts when biodegradation reaches 10%, and the window has to close before day 28. A substance that creeps to 65% on day 27 after a slow start has not passed.

Two exceptions are written into the guideline itself. The 10-day window does not apply to the MITI method (301 C), and a 14-day window is acceptable in the Closed Bottle method (301 D) when the number of bottles needed to resolve a 10-day window makes the test impractical.

There is a third, practical exception. Commercial surfactants are not single molecules. A sodium laureth sulfate or a cocamidopropyl betaine is a mixture of chain lengths and, often, of related structures, and the components degrade at different rates. The 10-day window was designed for single substances, and regulators including the European Chemicals Agency accept that it may not be applied when assessing such mixtures, provided the 28-day pass level is met. A supplier certificate that says "readily biodegradable, 10-day window not applicable, mixture" is therefore normal, not evasive.

What a pass proves

A ready biodegradability pass is a strong result. The test uses a low inoculum concentration, no acclimatised microorganisms, and a substance that is the only food source. A substance that mineralises to 60% under those conditions within a 10-day window will, in the guideline's own words, be assumed to degrade rapidly in the environment. For a surfactant, it is the single most useful piece of environmental data a supplier can provide.

What a pass does not prove

This is the part that gets lost between the supplier's certificate and the brand's label.

• It says nothing about anaerobic conditions. Sediments, some sewage sludge treatments, and landfill are anaerobic. Anaerobic biodegradability is a separate test (ISO 11734 / OECD 311), and several surfactants that pass OECD 301 do poorly there.
• It says nothing about the marine environment. OECD 306 covers seawater, and rates there are typically far slower.
• It says nothing about the metabolites. Ultimate biodegradation is measured as carbon balance, not as the fate of every fragment. Intermediate breakdown products can be more toxic than the parent substance, which is why ecotoxicity testing sits alongside biodegradability rather than being replaced by it.
• It was run on the surfactant, not on your product. A finished shampoo contains preservatives, fragrance, polymers, silicones, and colourants. Some of those are not readily biodegradable, and a formula-level claim needs a formula-level assessment.
• It does not test the packaging. A readily biodegradable formula in a non-recyclable bottle is a formula claim, not a product claim. Our post on compostable vs recyclable packaging claims covers that side.
• It says nothing about the raw material's origin. Biodegradability and sustainable sourcing are different questions. A palm-derived surfactant and a petrochemical one can both pass OECD 301; only one of them can carry RSPO certification. Our post on RSPO mass balance vs segregated sourcing explains that side.

How regulators use the test

EU detergents. Regulation (EC) No 648/2004 requires every surfactant used in a detergent to be ultimately biodegradable, with the test methods and pass levels drawn from OECD 301 (60% for respirometric methods and 70% for DOC methods). Derogations were possible for industrial and institutional products in narrow cases. Its replacement, Regulation (EU) 2026/405, was published in March 2026 and applies in full from 23 September 2029. It removes the derogation route entirely, extends biodegradability requirements to water-soluble films such as those used in laundry capsules, and introduces a digital product passport. A private label laundry detergent launching in Europe over the next three years should be formulated to the new regulation, not the old one.

EU cosmetics. Regulation 1223/2009 does not require cosmetic surfactants to be biodegradable, but any "biodegradable" claim on a cosmetic must satisfy the common criteria for claims under Regulation (EU) 655/2013: truthful, evidenced, and not misleading. A supplier certificate for one ingredient does not evidence a claim about the whole formula.

Ecolabels. Schemes such as the EU Ecolabel and Nordic Swan go further than the base regulations, typically requiring aerobic ready biodegradability and anaerobic biodegradability for surfactants, and restricting or scoring other ingredients. If a brand's target is an ecolabel, the surfactant selection is more constrained than "passes OECD 301".

Malaysia and ASEAN. The ASEAN Cosmetic Directive does not impose biodegradability requirements on surfactants, so the claim is governed by general advertising rules and by the destination market's requirements for export.

How to read a supplier certificate

A useful biodegradability certificate for a surfactant states:

1. The method, by number. "OECD 301 B" is a test; "OECD biodegradable" is not.
2. The result, as a percentage at day 28, and ideally the value at the end of the 10-day window.
3. Whether the 10-day window was applied, and if not, why.
4. The test substance, as supplied or as active matter, since commercial surfactants are diluted.
5. The laboratory and date. Test data older than the current product specification should be questioned.
6. Anaerobic and ecotoxicity data, if the brand's claim or ecolabel needs them.

If any of these are missing, ask. Reputable suppliers have the data; the certificate that arrives without it is usually a summary written by a sales team rather than by the lab. This sits inside the broader raw material supplier validation process that an ISO 22716 factory runs on every ingredient.

The common surfactants, honestly

Most of the surfactants used in mainstream personal care pass OECD 301 in their commercial grades: sodium laureth sulfate, sodium lauryl sulfate, cocamidopropyl betaine, alkyl polyglucosides, sodium cocoyl isethionate, sodium lauroyl sarcosinate, and the amino-acid based glutamates and glycinates among them. The "sulfate-free" and "natural" labels are marketing categories, not biodegradability categories, and a sulfate-free formula is not automatically more biodegradable than a sulfate one. Where products differ more is in the supporting ingredients: some silicones, some film-forming polymers, some chelating agents, and some fragrance materials are the components that fail, and those are where a formula-level review earns its keep. Our surfactant science 101 post explains how we select cleansing agents on performance and mildness first.

What this means for a brief

If your brief says "biodegradable", decide what you want to be able to say on the label and to whom. "Formulated with readily biodegradable surfactants" is a claim we can evidence with supplier data for every surfactant in the formula. "Biodegradable formula" is a claim that needs a formula-level assessment and, in the EU, a claims file. "Ecolabel-certified" is a project with its own ingredient rules. Each is achievable; each costs a different amount of development time.

Our sustainability page describes the wastewater, palm sourcing, and packaging commitments that sit around the formulation work. To discuss a biodegradability claim for a specific product, contact our team with the claim you want to make and the markets where you want to make it.