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Does compostable plastic really compost?

Returning to the Big Compost Experiment

As you may have noticed, I’m getting increasingly nutty about compost (currently researching compost loos and humanure – hahaha! watch out for more on that). Now that I’m finally getting the hang of it, I’m beginning to reap the ‘black gold’ rewards. It really it the BEST FUN opening my bin and digging out all that wonderful dark brown crumbly compost that a few months before had been the slimey contents of our food scraps bin.

I emptied out one of my Green Johanna bins (pictured above) just a few weeks ago after having left it for 5 months to mature, with no new additions. It was joyful as always to scoop out all the compost…

But I was also really curious to discover the results of my experiment – my small contribution to UCL’s Big Compost Experiment. Would the ‘compostable’ plastics that I put in when I started the bin off in November still be there, or would they have composted along with the food scraps and garden cuttings?

Drum roll……

And, hmmm…. a mixed picture. Let me run you through it.

The Results

Item 1 – Paper hot drink cup with starch-based plastic lining

Paper hot drinks cup with starch-based plastic lining in Nov ’20. 9 months later it was no longer visible in the compost bin.

This was great news because there was no sign of it at all. Paper and cardboard compost really easily so no surprise about that. The plastic lining of these cups is so thin that we barely notice it, so it makes sense that it composted easily – just as a thin lettuce leaf would perhaps.

Item 2: Hot drink cup lid made of plant-based plastic (PLA)

Left – Intact lid in Nov 20, Right – Condition of lid after 9 months in compost bin

The thicker plastic of the lid was a different story. Pretty much the whole of the lid was still recognisable, although it was now in fragments and the opaque white colour had become mottled in parts. Obviously this item needs quite a lot more time in the compost bin, or different conditions, to decompose effectively. The mottling and fragmentation could be early signs of decomposition, but it seems just as possible that the breaking is simply damage from being squashed in the bin.

Item 3: Salad pot made of plant-based plastic (PLA)

Left – Intact salad pot in Nov 20, Right – Condition of pot after 9 months in compost bin

With this third item, a clear plastic salad pot, there was no visible evidence of decomposition at all. The pot was cracked but was that simply damage from being in the bin?

As an aside though, I also found another pot of the same type that wasn’t one of my official experiment items so I don’t have a record of when I put it in. However, I think it had already had a stint in a bin – perhaps 6 months or more – been pulled out as a ‘reject’ and put into this batch to ‘go again’. So it’s probably had 18 months or so already in the bin. As you can see, it’s warped in places, and is more opaque, so material changes are definitely occurring…but still it has the same weight/strength that it started with. I would guess it still has at least 12 months more to go.

Plant-based plastic salad pot showing signs of decomposition – length of time in bin between 12-18 months

Item 4: Cup made of plant-based plastic (PLA)

Left – Intact cup in Nov 20, Right – Condition of cup after 9 months in compost bin

Finally, the clear plastic cup, and again the only change was breakage that could as easily have been caused by crushing as by any biological/chemical changes.

My results are consistent with the results so far submitted by other home composters who have taken part in the UCL study. The most recent report states that 71% of participants said the compostable items were still visible at the end of their usual time-frame for producing compost (for most people this was 12 months), and only 32% reported that the items were no longer visible.

Conclusions & Questions

So, three out of four of my compostable items haven’t composted…yet. What to conclude from all this? It brings up so many questions doesn’t it? I’ve been looking into some of these and will share what I’ve found out so far…

Will the items compost eventually? They will, but based on progress so far, I think it could take at least another year- probably more. It’s just the same with ordinary organic material – soft and moisture-rich ingredients like fresh grass and leaves compost in no time, harder, drier ingredients like woody stems or fruit stones take longer, and twigs and branches aren’t really worth putting in at all as they take so long. I suspect that it’s the same story with compostable plastic. The thin stuff like t