Analysis: Compostability is influenced not just by what a product is made from, but by how it is made, which greatly influences how it behaves at end-of-life
Walk down any supermarket aisle and the word 'compostable' appears everywhere; from coffee cups, to food packaging, to cutlery, and bin liners. It sounds reassuring – a promise that these products will quietly disappear back into nature once we’re done with them. In reality, whether something truly composts depends on what it is made from, how it is manufactured, and what happens to it after we throw it away.
A compostable material must be able to break down into carbon dioxide, water and biomass within a defined time period, and do so without leaving behind harmful residues that damage soil or plant growth. Crucially, this breakdown has to happen under specific conditions. Without those conditions, even certified compostable materials may persist far longer than expected.
One of the most common misunderstandings is the belief that compostable items will decompose away naturally if they are littered, buried in soil or placed into a garden compost heap. The majority of compostable plastics are designed for industrial composting facilities, which operate at elevated temperatures – typically around 55 - 60°C – with carefully controlled moisture and oxygen levels.
From RTÉ Archives, the world's first biodegradable computer in 2008
These conditions accelerate chemical and biological processes that simply do not occur in the natural environment or in home compost bins. Outside these systems, compostable products may fragment into smaller pieces, but fragmentation is not the same as full biodegradation.
This confusion is compounded by the way the term "biodegradable" is used. Biodegradable only means that a material can, at some point, be broken down by microorganisms. Compostable materials, by contrast, are expected to meet much stricter criteria. They must physically disintegrate within weeks, chemically biodegrade within months, and leave compost that is safe to use. A product that biodegrades eventually but fails on any of these points should not be described as compostable.
Many of the compostable plastics now on the market are made from polylactic acid, or PLA. PLA is derived from renewable resources such as corn or sugar beet and is often presented as a greener alternative to conventional, fossil fuel-derived plastics. Chemically, PLA can biodegrade, but it does not do so easily. The first step in its breakdown is driven not by microbes, but by heat and water.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
From RTÉ Radio 1's Countrywide, seashore solutions to plastic pollution in Cork
These conditions cause the long polymer chains that give the material its strength to break into much shorter fragments. Only once this has happened can microorganisms convert the material into carbon dioxide, water and biomass. Without the sufficient levels of heat and moisture, this process slows dramatically, highlighting just how strongly composting outcomes depend on the material itself and its underlying properties.
Compostability is influenced not just by what a product is made from, but by how it is made. In recent research from our Grain-4-Lab research team, we examined PLA items produced using two common manufacturing methods: injection moulding, which is widely used for mass-produced packaging, and 3D printing. When these items were placed in identical industrial composting conditions, they behaved very differently. The 3D-printed PLA objects disintegrated far more rapidly than their injection-moulded counterparts, with over 90% of the material breaking down within 12 weeks. The injection-moulded items fell well short of this threshold.
The reason lies in structure rather than chemistry. 3D-printed objects typically have rougher surfaces and more internal pathways where water can penetrate. This increases the surface area available for degradation and allows the initial breakdown to happen more quickly. Injection-moulded plastics, by contrast, tend to be denser and smoother, which slows this early stage. Two products made from the same "compostable" plastic can therefore behave very differently at end-of-life simply because they were manufactured in different ways.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
From RTÉ Radio 1's Drivetime, report finds just 28% of plastic packing in Irish homes is recycled
This matters because composting in the real world does not always look like composting in the laboratory. Industrial facilities are designed primarily to deal with food and garden waste, not plastics. If compostable items do not break down quickly enough, they may be screened out as contamination and sent to landfill or incineration. This is one reason why some composting facilities are reluctant to accept compostable plastics at all, despite their certification.
Composting is also not always the most sustainable option available. Mechanical recycling, where plastics are reprocessed into new products, can in some cases have a lower environmental footprint, particularly if it reduces the need to produce new material.
Our work shows that PLA can be mechanically recycled several times before it loses its usefulness, and that recycled PLA can still break down effectively under industrial composting conditions later on. Designing products with reuse, recycling and composting in mind – rather than treating composting as the default solution – may deliver better overall environmental outcomes.
For consumers, all of this makes the compostable label a more complicated signal than it first appears.
When you see it on a product, it is worth asking whether the item is certified to recognised standards, whether industrial composting is available where you live, and whether your local waste collection actually accepts compostable plastics. If the answer to that last question is no, then even a well-designed compostable product may end up being landfilled or burned.
In short, compostable is not a promise that something will disappear. It is a conditional claim – and the conditions matter.
Follow RTÉ Brainstorm on WhatsApp and Instagram for more stories and updates
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
