What Are Compostable Bags Made Of and How Do You Dispose Of Them Correctly?

what are compostable bags made of

Compostable bags aim to provide a more eco-friendly option for carrying goods and collecting household food waste.

A compostable bag is an alternative to the more notorious plastic bag, which we now clearly know has a disastrous impact on the environment both in its raw material extraction and afterlife when its discarded.

The idea of a compostable bag is that it will naturally break down into organic nutrients once it’s been used. These nutrients will then be used to enrich soils with zero harmful side effects for the environment.

This seems logical as most compostable bags are made from plant-based materials, which makes it a much more sustainable way to deal with food waste disposal and reduce our reliance on plastic. Despite this good news for the environment, it’s not always so simple in reality.

This blog will be talking specifically about compostable bags, not biodegradable ones. There is confusion over these terms, especially for the consumer, but I’ll hopefully makes these much clearer for you by the time you’ve reached the bottom of this post.

So, let’s find out what compostable bags are, what they are made from and how to deal with them.

What are compostable bags?

Let’s get the first thing clear:

A compostable bag is not the same as a biodegradable bag.

Here’s why.

The term compostable means that the product will completely break down into its organic constituents after a certain amount of time, enriching the soil with nutrients and leaving no visible signs or toxic residues.

To be labelled as ‘compostable’, a product must be certified by one of independent bodies (more on this shortly) as proof.

natural environment with steam trees plants
A compostable bag must break down within a certain time leaving no visible signs or harmful chemicals in the environment. Source: Tinyecohomelife

On the other hand, the term biodegradable does not have an official, regulated definition. The word ‘biodegradable’ means that the product can be broken down by microorganisms. This doesn’t necessarily mean that the biodegradable product is eco-friendly.

After all, plastic will biodegrade and break down in the natural world, but as you know, this is not good for the environment and ecosystems at large.  

These discrepancies do cause confusion and it’s easy to see why. What we do know about a compostable bag is that it will biodegrade within a certain time frame and be beneficial for our all-important soil.

Compostable bags are now used as a replacement for plastic alternatives. The two main uses of a compostable bag are to hold shopping in and to line a kitchen food waste bin. Once food waste has been collected, the bag and food inside can then be put into your bigger food waste bin for collection from the local council in the UK (not all councils may offer this).

Hopefully your local council gives you free compostable food liners like mine does. If not you can buy compostable bin bags at a very reasonable price from places such as andkeep.com, a plastic-free, ethically focussed website.

Many shops and supermarkets, such as Co-op, now offer compostable bags to carry your shopping and goods home. Once home, these compostable bags can then be reused to line food waste caddies.

How do I know if a bag is compostable?

New compostable carrier bag from the Co-op with the Seedling logo
New compostable carrier bag from the Co-op with the Seedling logo. Source: Co-op

A compostable bag will be officially certified with one or numerous logos as proof of its compostability. This is similar in a way that an organic product has to be independently certified.

The most important logo to look out for is the Seedling logo (a registered trademark of European Bioplastics), as well as the alphanumeric mark: EN13432.

EN13432 is a European standard developed in 2000 to help tackle packaging waste. Certifications and logo marks, such as the Seedling logo, all adhere to the EN13432 standard.

Anything certified as meeting the EN13432 standard conforms to all the compostable criteria outlined and is suitable for industrial composting.

A product certified with EN13432 will decompose into organic matter, carbon dioxide and water within 12 weeks. During this time at least 90% of the original material must have fragmented into pieces less than 2mm in size and there should be no harmful substances left in the soil or negative impacts on the environment.

EN13432 compostable bags have a close alignment with food waste collection and commercial composting facilities, including here in the UK.

All food waste caddy liners suitable for UK use will have the EN13432 accreditation (if you buy a compostable bag from the supermarket, make sure it also says this before you use it for food waste collection).

The commercial aspect to this is important as it means such bags are not suitable for home compost piles. Instead, they must be treated in a industrial composter – another bone of confusion.  

There are other logos and certifications, such as:

  • OK Compost – commercial composting (meets EN13432), can be put in your compost bin for collection (check local council rules)
  • OK Compost Home – suitable for home composting, can also be put in your compost bin for collection (check local council rules)
  • Seedling logo – commercial composting (meets EN13432), can be put in your compost bin for collection (check local council rules)
  • BPI Compostable – North American compostable standard
compost logos on food waste caddy liner bag
The logos on one of my food waste caddy liner bags. Marked with: OK Compost, EN13432, Seedling logo and BPI Compostable logo

What are compostable bags made from?

Now we have a better understanding of what makes a bag compostable and how it’s certified, what are compostable bags made from?

Compostable bags are made from natural, plant-based materials, including potato starch, corn starch and other vegetable matter.

Ensuring a compostable bag is made from plant-based materials means that it will break down at roughly the same rate as the food waste it collects inside.

Any ink used on the compostable bag must also comply with the criteria. Generally, the ink used on these bags are water-based and safe for the soil.

Potato and corn starch make for good raw materials for a compostable bag because of their high starch levels which provide strength for the bags, as well as being a natural material. These varieties are different from the potatoes and corn you may eat, however. Special super high starch varieties are grown, which are largely inedible.

In the EU, around 1% of all corn grown is this material-providing variety. This vegetable matter that goes into making plant-based materials, including compostable bags, has a name: Bioplastics.

What are bioplastics? Are they bad for the environment?

As soon as you hear the word ‘plastic’, alarm bells may start to ring. These are distinct though, let’s explain.

Bioplastics are plant-based or microorganism-based plastic.

A plastic in the technical sense is a synthetic material made from polymers, which are molecules made from many repeating units.

The major difference comes from the material sourcing and production. Bioplastics are 100% plant-based whereas ‘traditional’ plastic is fossil fuel-based and made from petroleum.

Although they do have similar characteristics, bioplastics have a number of benefits.

Firstly, these plant-based bioplastics are manufactured into a number of things, including bags, that are compostable in line with EN13432 in an industrial composter.

They also have a lower carbon footprint and are produced in a more energy efficient manner compared to petroleum-based plastics.

Have a more in-depth read here to see if bioplastics are good for the environment.

pototoes growing in a field for bioplastic use
Potatoes like these are grown purely for Bioplastic use. Source: Potatoworldmagazine

Concerns over bioplastics

There are questions asked over the sustainability of bioplastics, however. Questions such as: How much land is being used? Should crops be grown for plastic instead of food? How much water is needed?

There are differences between different types of bioplastics even. When comparing corn and potato starch for example, corn requires around 40% more land use than potatoes to produce the same amount of starch, as well as requiring irrigation. Potato starch on the other hand takes up less space and requires only natural rainfall.

Although bioplastics are compostable, they may cause similar problems to petroleum-based plastics if they leak out into the environment. This is because the bioplastic needs specific conditions, found in industrial composting facilities, to break down the chains of polymers that have been created.

If just left to their own device in the environment, a compostable bag may not always decompose as you would think or like.

This study published in 2019 found that a compostable bag did completely break down in seawater after 3 months but the same compostable bag was still present when buried in soil after 27 months. Not maybe what you’d expect. So how do you deal with them?

Can you put compostable bags in compost? What to do with them?

In simple terms there are two types of composting options for you:

  • At home, garden composting
  • Industrial composting

A compostable bag that is certified with the EN13432 standard is not suitable for a garden or home composter. Such bags, which constitute the majority of compostable bags, are only suitable for industrial composting facilities.

This is because the majority of current compostable bags require conditions to break down the bioplastics that are simply not achievable in most home compost piles, such as temperature around 150oC.

A compostable bag needs the right levels of five key conditions:

  • Heat
  • Moisture
  • Microbes
  • Oxygen
  • Time

The correct conditions just cannot be met by garden compost heaps that remain at low temperatures with non-optimal moisture levels.

Some compostable bags can be composted at home, but only certain types. These will be accredited and labelled with certain logos, such as OK Compost Home, to make it clear.

ok compost home logo tuv
Only specific compostable bags with OK Compost Home logo are suitable for garden compost piles

How do I dispose of a compostable bag?

If the bag is marked with OK Compost Home then you can add the bag along with your food waste into your garden compost pile.

If the bag doesn’t distinguish that it’s suitable for home composting, then it must be sent to an industrial composting facility.

This is where the local council comes in useful with the disposal.

For most of us in the UK, it looks a little bit like this: Collect all food waste in a kitchen caddy lined with a compostable bag (marked EN13432), when it’s full dispose the compostable bag into your garden/food waste bin, put this bin outside for collection by the local council when it’s the right time of the week or month.

The only other option is for a compostable bag to go into the general bin that generally makes its way to landfill (last resort) or to an energy recovery facility.

You can now get compostable dog poo bags that can be composted at home or sent to a facility. This is a fantastic way to reduce a lot of plastic use.

Do compostable bags break down in landfill?

This all depends on the conditions of the landfill.

Compostable bags are not intended for landfill and in an ideal world they wouldn’t get disposed here.

If the five composting conditions outlined above aren’t met sufficiently, a bioplastic-based bag may persist in landfill for some time in the same way a fossil-fuel based plastic bag would. It will decompose eventually but unclear after how many months or, more likely, years.

How long do compostable bags take to decompose?

As set out in the EN13432 standard, a compostable bag in an industrial composter must degrade down after 12 weeks (where 90% of the material is in pieces less than 2mm) and must be completely composted after six months.

A compostable bag will break down into three main parts:

  • Biomass
  • Water
  • Carbon dioxide

Microorganisms do the incredible job of composting natural materials into these parts. They work in the same way that a composting toilet works. These tiny living organisms consume the organic food matter, resulting in a nutrient rich soil called humus. Microorganisms require oxygen and water to do this process effectively and breath out carbon dioxide in turn, just like we do. Out in the wild, many other animals and microorganisms help along the way too.

When adequate oxygen supply isn’t present, different microorganisms take over. Instead of releasing carbon dioxide, these ones release methane and aren’t as efficient at breaking down the waste. This is what can happen in landfill sites where there isn’t much oxygen about, resulting in bad smells alongside the more the more potent greenhouse gas methane.  

industrial composting facility in crewe
An industrial composting facility in Crewe, UK. Source: Wastewise

If you’re composting a suitable bag in your own garden compost pile it’s likely to take a little longer than the industrial equivalent. In your home heap, it can take anywhere from a few months up to a year or more depending on the compost conditions.

When a compostable bag doesn’t reach an industrial composter, or it isn’t suitable for home composting, it can take a lot longer to decompose.

A study by Napper and Thompson, published in 2019, tested out numerous bag types in three different conditions: open air, buried under soil and submerged in seawater over a 3 year period.

In the marine environment, the compostable bag completely degraded after 3 months but it didn’t get on so well in the other environments. In the open air, the compostable bag fragmented into little bits after 9 months, but it was still in one piece when buried in soil for two and a quarter years, although it had lost most of its strength.

In the experiment, other bags, such as ones labelled by the manufacturer as ‘biodegradable’ and ‘OXO-degradable’, were still present in both soil and water after three years and could even hold shopping!

Is a compostable bag the same as a biodegradable bag?

No, a compostable bag is not the same as a biodegradable bag.

As shown in Napper and Thompson’s study, a compostable bag and one labelled as biodegradable break down very differently in the environment.  

Anything labelled as biodegradable should be treated with caution. It sounds like it should be good for the environment, but in reality it’s rarely the case.  

Biodegradable is a broad, flimsy term with very few regulations meaning producers can use the term without many repercussions, causing great confusing for people at large.

This differs significantly to an item labelled ‘compostable’, which is tightly regulated and must meet specific compostable criteria as identified in the EN13432 standard.

Some biodegradable bags are indeed made from plant-based materials whereas others are fossil fuel-based with a number of additives to make it breakdown in the environment at a faster rate. Just what they are breaking down into (and whether this is good for the environment – highly unlikely) is another question completely.  

Although a compostable bag is biodegradable, a biodegradable bag is not necessarily compostable.

a compostable bag for food waste caddy liner
A biodegradable bag is not the same as a compostable bag like this food waste caddy liner. Source: Tinyecohomelife

There’s no doubt that bioplastic-based compostable bags are much better for the environment that traditional plastic predecessors. As you may have gathered, it’s rare that a material contains perfect eco-friendly credentials without having some kind of impact, whether this is taking up land space or requiring certain conditions to benefit the environment.

Compostable bags are a more sustainable step in the right direction, causing less harm to the environment and having a smaller carbon footprint.

A compostable bag is 100% plant-based, which means it will break down at the same rate as the food waste is contains inside. I hope it won’t be too long until all compostable bags can be used in home compost piles.

If you don’t get free compostable bags from your local council you can buy compostable food bin liners from andkeep.com.


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