How Compostable are Co-op's Carrier Bags?

Co-op carrier bag

Back in May of 2021, the Co-op supermarket chain here in the UK rolled out compostable carrier bags across all its stores. 

As you can see, every bag proudly advertises itself as compostable rather than recyclable plastic. This claim got me thinking, how truely compostable and how truely safe are these bags? For example, are they compostable in household compost piles or only industrial composting units? Are the decomposed remains actually safe for my soil? Safe for my vegetables growing in that soil? Safe for my family eating those vegetables? And safe for the wider environment? So many questions.

I began seeking answers by reading the bag itself.

Co-op carrier bag
 
If I'm interpreting the 'OK Compost' little symbol properly, with HOME promenently displayed, it seems to indicate you can indeed compost these bags in your home compost pile. Interesting. We will return to this symbol again below, but for now let's look at another.
 
I noticed too this MATER-BI symbol with its clever little 'M' stylised like a discarded orange peel. A graphics designer put a lot of thought into that one.
 
Anyway, back to the point. Googling this, I discovered that MATER-BI is NOVAMONT's 'family of biodegradable and compostable bioplastics'. Nice! Now I've got something to get my teeth into. According to their website, MATER-BI is 'biodegradable and compostable according to the European standard UNI EN13432'. This correlates with yet another rather cryptic symbol on the bag itself.
 
Co-op carrier bag
 
What are the requirements of this EN13432 standard? In other words, what tests must the product pass to conform to this standard? 
 
I discovered the following. During industrial composting, at least 90% of the organic material must degrade into carbon dioxide and water within six months. Additionally, the material must stay within strict thresholds for heavy metals and pass a series of ecotoxicity tests to ensure the composted product is both safe for people and the environment. 

Well this sounds all quite encouraging except for one catch. This standard is only relevant to composting within an industrial unit. It turns out, there are no international standards that specify the requirements needed for a material to be labelled as compostable at home.

This takes us back to 'OK Compost HOME' from earlier. It turns out, this symbol is awarded by TÜV AUSTRIA, a certification body that has the authority to award the seedling logo to products that comply with good old EN13432. 
 
TÜV AUSTRIA recognised that consumers desire products that are assured as home compostable, which led them to develop 'OK Compost HOME'. It is not an internationally recognised standard, but it does offer some assurance that a product with this logo can fully biodegrade in a home garden compost pile. At this time, it is the best assurance we have... I find this a rather surprising, and disconcerting, truth.

My unease is heightened further by the fact the TÜV AUSTRIA website is less than transparent regarding what tests they actually carry out to ensure a material is worthy of the 'OK Compost HOME' symbol. Their FAQ section contains a short blurb about tests related to industrial composting standards and that's it. Really disappointing.
 
Due to this dead-end, I then took a different approach. I reviewed scholarly articles about the safety of MATER-BI. This search also threw up some worrying results. A phytotoxicity study in Italy discovered that leachates from MATER-BI negatively impact seed germination. Specifically, in this study, MATER-BI caused the seed's radicle (first embryonic root of the seed) to grow abnormally. I also found another review paper of bioplastics more generally, which concluded that 'effects induced by bio- polymers may, at least, be in the same magnitude as conventional plastics'.
 
Alarm bells are ringing in my head. I think it suffices to say, I'm not adding Co-op's compostable carrier bags to my compost bin any time soon. I'll need to relegate them to general waste.
 
What a shame. I think these types of products, and the standards regulating them, just need a few more years of development. I'll need to console myself by remaining patient.



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