Do compostable zip bags actually compost?

The European specification is EN 13432. It asks for at least 90 per cent disintegration after twelve weeks, meaning no more than a tenth of the original dry mass is left behind on a two millimetre sieve, and at least 90 per cent biodegradation, measured as carbon dioxide given off, within six months. It adds a heavy metals limit and an ecotoxicity test, so the compost that comes out at the end has to grow plants 1.

The American specification is ASTM D6400, certified in North America through the Biodegradable Products Institute. BPI runs the disintegration test at 58 degrees Celsius, plus or minus two, for up to 84 days, and the biodegradation test for a minimum of six months. It also caps total organic fluorine at 100 parts per million, which keeps fluorinated treatments out of certified food packaging 2.

Both describe the same kind of place: a managed plant, running hot and wet and aerated for a defined period. Neither says anything about a garden heap, a hedge, a landfill or the sea.

Home composting has its own schemes and its own numbers. TÜV AUSTRIA’s OK compost HOME asks for at least 90 per cent degradation within twelve months at ambient temperature 1. BPI’s home scheme, built on the French standard NF T51-800, runs disintegration at 25 degrees Celsius, plus or minus two, for up to 180 days, and biodegradation for a minimum of twelve months 2. Twice the time, at less than half the temperature. A bag can hold one certificate and fail the other.

The home schemes are more closely related than their names suggest. TÜV AUSTRIA states that OK compost HOME is not based on a standard but is the basis for several: the Australian AS 5810 in 2010, the French NF T51-800 in 2015 and the European EN 17427 in 2022 3. So an AS 5810 mark comes from the same lineage of conditions.

Outside a laboratory the evidence thins. The Big Compost Experiment, a citizen science study run from University College London and published in Frontiers in Sustainability in 2022, gathered 1,307 item results from 902 UK households composting at home. Sixty per cent of the items certified as home compostable were reported with remains still clearly visible at the end 4. The results are self-reported and the heaps uncontrolled, which is the point rather than a flaw: an uncontrolled heap is what a home compostable bag actually meets.

Temperature, first and mostly. European Bioplastics gives 50 to 70 degrees Celsius as typical for industrial composting, and says certified compostable products convert within six to twelve weeks under those conditions 5. The BPI disintegration test sits inside that range at 58 degrees 2. Heat is the variable the timescales are built on.

The home test runs at 25 degrees for a reason. European Bioplastics describes home composting as taking place at lower and less consistent temperatures with longer degradation times 6. NatureWorks, which makes the polylactic acid sold as Ingeo, splits composting into two steps: moisture and heat break the polymer chains apart, then microorganisms consume the smaller fragments 7. Take the heat out of the first step and the second one waits.

A plant also supplies aeration, a managed moisture level and a controlled residence time. A heap supplies whatever the weather did that month.

This is the question the mark cannot answer, and in the United States the numbers are sobering. BioCycle’s 2023 nationwide survey identified 200 full-scale food waste composting facilities in the entire country. Of those, 142 accept compostable food-contact packaging, and certified compostable liner bags were accepted by 93 of the 141 answering that question 8. Acceptance is rising, from 49 of 103 respondents in 2018 8, but the base is small.

Getting the bag to one of those plants is a separate problem. The same survey year counted 14.9 million American households with access to municipally supported food waste collection, through 400 programmes across 710 communities. Of the curbside programmes, 145, or 56 per cent, do not accept compostable packaging at all 9. For most American households the honest answer is no: not because the certificate is wrong, but because no route runs from the kitchen to a plant that would honour it.

England is about to collect far more food waste and has already ruled on the material. Under Simpler Recycling, collectors must take food waste from all households from 31/03/2026, weekly and free of charge, and the guidance lists what councils do not need to collect: any plastic packaging or non-packaging items labelled compostable or biodegradable 10. NatureWorks says the quiet part too, telling readers that industrial composting facilities are limited and may not be available near you 7.

Two polymers do most of the work in compostable film. PLA is polylactic acid, made by fermenting plant sugars to lactic acid and polymerising the result 7. PBAT is polybutylene adipate terephthalate, a copolyester: BASF’s ecoflex, the original commercial grade, was developed from fossil raw materials and is certified to EN 13432, ASTM D6400 and AS 4736 11. Film is usually a blend, because PLA alone is stiff and PBAT alone is slack.

Both are polymers, and EU law treats them as plastic. The Single-Use Plastics Directive defines plastic as a material consisting of a polymer to which additives may have been added, excluding only natural polymers that have not been chemically modified, and its recitals place bio-based and biodegradable plastics inside that definition 12. A bag made of PLA and PBAT is a plastic bag with a different end-of-life route. Describing it as plastic-free is not a shade of emphasis, it is the reverse of the legal definition.

That does not make the material equivalent to polyethylene: a film a plant can process is a different proposition from one it screens out and sends to landfill. The claim that fails is the one about what the bag is made of, not the one about what a plant can do with it.

The Seedling is a registered trademark of European Bioplastics and certifies conformity with EN 13432. Its guidelines are specific: on a certified product the logo always has to be shown with a valid registration number in the form 7PXXXX printed below it, because that number ties the item to a live certificate. Both certifiers, DIN CERTCO and TÜV AUSTRIA, run databases where the holder and the validity can be checked 13.

There is one exception, and it is the one worth carrying around. For advertising purposes only, the guidelines allow the Seedling to be used without the number 13. A number-free Seedling in a web banner is within the rules. On the packet, it is not what the scheme asks a certified product to carry.

BPI works from the other end, publishing a catalogue of certified items searchable by product and by company, down to the item number 14. OK compost INDUSTRIAL is TÜV AUSTRIA’s scheme against EN 13432, with biodegradation, disintegration, ecotoxicity and heavy metal content as the four required tests 15, and OK compost HOME is a separate licence. In every case the verifiable thing is a licence held by a named company, not the picture.

There are jobs this material does that nothing else does as well. European Bioplastics makes the structural argument: where a plastic item is mixed with biowaste, mechanical recycling is not feasible for either, and a compostable item lets the whole mixture go to organic recycling instead of disposal 5. Caddy liners where the district accepts them, service ware at a venue that controls its own waste stream, and packaging that reaches a plant soiled beyond recycling are all cases where the certificate does real work.

A zip bag in a drawer is a harder case, because of what it is compared against. Set against another single-use bag bound for landfill, the certified compostable one has an end-of-life route the other lacks, where that route exists. Set against a washable box, the comparison is not about end of life at all.

• Which certificate it holds, industrial or home. Separate schemes, different temperatures, and one does not imply the other 1 2.
• The number beside the mark. A Seedling with no 7PXXXX below it on the pack, or a BPI claim you cannot find in the catalogue, is a claim rather than a certificate 13 14.
• Whether the closure is covered. Certification applies to the item as sold, inks and additives included, so a zip track is part of what should have been tested.
• What your own collector accepts, in writing. Of the American curbside programmes surveyed in 2023, 56 per cent do not take compostable packaging 9, and English councils are not required to collect it at all 10.
• What the film is made of. If the pack says plastic-free and the material is PLA or PBAT, the two statements on that pack do not agree 12.

California has turned the home compostable half of this into law. A product may not be labelled home compostable there unless its manufacturer holds OK compost HOME certification for it 16. When a legislature names one specific certificate to stop a phrase drifting, the phrase had been drifting. The useful question is not whether the bag can compost. It is who, within reach of your kitchen, is going to compost it.