
This fragment of bamboo cane is biodegradable, but has not composted - despite spending several months in an industrial composting facility.
It would have degraded better had it been more finely shredded, since larger items take much longer to decompose.
Biodegradable vs Compostable
To the general public, these two terms may seem largely synonymous, however there are formally accepted differences in decomposition conditions and time frames.
Both are very similarly explained in the US FTC green guidelines, except "biodegradable" can also be applicable to the natural environment, whereas "compostable" implies more deliberate and controlled disposal methods.more
More specifically, biodegradable plastics should "decompose into elements found in nature within a reasonably short period of time", whereas compostable plastics should "break down into, or otherwise become part of, usable compost (e.g., soil-conditioning material)."
These US guidelines state that the time needed for composting should be comparable to the other materials being composted, i.e., general green waste. And for natural biodegradation, the process should be complete in less than 12 months unless stated otherwise.
Caveat Emptor: Although compostable items are obviously biodegradable in (hot) compost heaps, this may not be true in other (cooler) environments. Conversely, items that can successfully biodegrade in nature might not actually decompose fast enough to meet current compostability standards.
ISO and ASTM definitions
Although there are generally accepted international definitions for the three main classes of degradable plastics, they are routinely confused:
- Degradable - These materials break down in the environment, but will not necessarily return to natural by-products. Non-biological processes (e.g. hydrolysis, oxidation, UV exposure, mechanical weathering) cause significant changes in the polymer's chemical structure and a loss of physical properties. The process usually generates micro-plastics and was historically termed "perishing".
- Biodegradable - Naturally occurring microorganisms return these materials to nature under ambient environmental conditions. The material should be fully assimilated, and leave no residues in the natural environment. High temperatures are not essential, but the process can take more time in cold conditions.
- Compostable - Microbes return these materials to nature more rapidly, but only at controlled temperatures. Compostable plastics degrade to carbon dioxide, water and inorganic compounds without leaving visually distinguishable or toxic residues. (Methane is produced in anaerobic conditions.)
A Standards Based Approach

Items marked with this logo are only suitable for industrial composting at high temperature.
A test certificate should be available from the manufacturer, ideally stating the material and any thickness limitations.
A growing number of test protocols are used to assess biodegradability and compostability, but the two most commonly encountered are ISO/EN 13432 and the US equivalent ASTM D6400.more These focus on composting, which is considered the most aggressive environment for promoting biodegradation.
According to these standards, a product can be classed as industrially compostable if under such conditions 90% of the article disintegrates into fragments, less than 2mm, within 12 weeks, and 90% is converted into CO2 and water by the end of the following 12 weeks.more
Although there are currently no internationally accepted standards for home composting, the French standard NT T 51–800 is widely used in Europe. This requires a product to undergo a similar level of degradation, but within 12 months at about 25°C.
Uncontrolled biodegradability in ambient natural environments can be assessed by various soil and aquatic test standards, but these are less commonly invoked since testing can take much longer. (Microbial activity in soil can be significantly lower than in compost, and aquatic conditions are even less favourable.)
For example, ISO 17556:2019 (Aerobic Biodegradability In Soil), is conducted at 20-28°C and can take up to 2 years to complete. Although products can be tested intact, to speed things up, the standard recommends that the specimen (and a cellulose reference) are used in finely powdered form. This is because the form of the sample makes no difference to its intrinsic biodegradability, only to the time it takes.more
It is important to emphasise that biodegradability standards like ISO 17556 are essentially material tests, whereas compostability standards such as ISO 13432 are product tests. The two are therefore not interchangeable.more
A common pitfall is the use of materials described as "compostable". In most cases this simply means that thin films have been tested as product, with the thickest one to pass being selected. (This maximum compostable thickness is usually stated on the test certificate. A comprehensive list of certified materials and their maximum thicknesses is available from https://www.tuv-at.be/green-marks/certified-products/.more An approved list of materials and products can also be found at www.bpi.org.)
When products are made from a certified material, compostability is only assured if it remains below the maximum thickness. It is wholly unsafe to assume that simply using a "compostable" material automatically makes a product compostable.

Compostable and biodegradable plastics will contaminate the post-consumer waste-stream. They should not be sent for recycling.
Disposal Issues
Although biodegradable and compostable plastics are generally seen as desirable ways of reducing waste, consumers need to be aware that they are not compatible with other types of recycled plastics, and separate disposal is required.
The most suitable option is collection for industrial composting, an approach that seems a particularly attractive way to dispose of food-contaminated packaging and garbage bags, neither of which could easily be recycled by other means.
However, such facilities are not available to everyone, and discounting landfill and incineration, home composting is becoming an increasingly popular alternative. more
The Reality of Home Composting

This photograph was submitted to the Big Composting Experiment and shows a bag with a TUV home compost label. Despite this endorsement, the bag has not lived up to expectations.more
Also note the undegraded twigs in the background. According to the present international standards, these are not actually compostable! more
Between November 2019 and November 2021, a "Big Composting Experiment" was conducted in the UK. About 900 citizen-scientists evaluated products marked "home compostable" in their own domestic composting bins. Most participants (~85%) had prior home composting experience and many submitted photographic evidence to document their results.
Of 1300 items tested, only about 30% were reported to have fully biodegraded, with another 10% being reduced to sub-2mm fragments. These results were similar regardless of the composting system used (principally closed outdoor bins), and there were no significant regional differences.
Surprisingly, the composting duration also made little difference to the results (mostly 6-12 months); Either an item showed significant biodegradation within a few months, or none at all.
Wipes, food trays and tea bags were reported to have composted well (60-70%), whereas produce bags, films and cutlery were less successful (30-50%). Coffee pods were not widely tested (13 instances), and only one was reported to have fully biodegraded.

Photograph submitted to the Big Composting Experiment showing a coffee pod after attempted home composting.
(Maybe the material was only suitable for industrial disposal, or maybe the walls exceeded the rated compostable thickness of the material used?)
Significantly, items that displayed compostable certification marks fared no better than those that didn't. According to the report "the majority [of items] did not fully disintegrate, including 60% of those that were certified home compostable."
Although the experiment was somewhat ad hoc, and subject to considerable variation in home composting methods, the reported results were largely consistent. As an explanation for the low levels of success, the authors of the report suggested the current testing standards are not adequately representing the reality of home composting.
They also noted that there is considerable confusion over labelling, with many consumers making mistakes. Based on participants' photos, at least 14% of items tested were actually marked as only suitable for industrial composting, and it appeared that a significant number of products were not clearly marked at all.
[In additional to the authors' conclusions, manufacturers are also probably guilty of mislabelling some products, and there are certainly cases where products made from certified materials are themselves erroneously assumed to be compostable. It is very easy to overlook the fine print that states the maximum compostable thickness, and then to make a product that exceeds it.]
Finally, the authors of the report pointed out that even if all products were fully compostable and correctly labelled, very little plastic waste could actually be disposed of through small-scale home composting facilities. Therefore, until alternative disposal routes become more effective, they concluded that compostable plastics are not currently a realistic option for a sustainable circular economy.more