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What the real deal is with compostable bags

With World Environment Day 2018 focused on “beating plastic pollution” – it’s important to know what the real deal is with compostable bags.

Compostable plastics are being used more and more in New Zealand but most of them will still end up in landfills – Waste Engineer Chris Purchas explains why.

“There is currently no way to collect or process compostable plastic bags from consumers or households across New Zealand, so it is likely many will end up going to landfill. Internationally, composting sites typically treat compostable bags the same way as other plastics (i.e. a contaminant that requires removal). This means they are removed and landfilled.

While we have commercial composting operations around New Zealand most focus on processing green waste (leaves, lawn clippings, branches). Their focus is on producing a high quality product and they will remove anything that could be seen as a contaminant. The key risk for processors is that compostable plastics may not be fully composted during the commercial process, i.e. fragments of plastic end up reducing the value of the end product.

If compostable materials end up in landfill, they will breakdown – but very slowly. In the meantime, they have the potential to become windblown litter (like plastic bags), and take up valuable space in the landfill.”

Does this mean I can add compostable bags to my green bin, or compost them at home?

“For green bins, it depends on the provider. Some companies supplying green bins may not accept compostable bags – so check whether yours does. They will need to make sure that the processing facility is able and willing to accept compostable plastics.

At home, you may be able to compost your own bags with your regular compost. But, bear in mind if you’re using compostable dog waste bags, you shouldn’t be adding dog faeces to your compost as it may add diseases to your compost. It is likely to take a long time for the material to completely break down - until it does you may find small fragments of plastic in your compost/garden. There are no standards for home composting because it is difficult to be sure how each home composter is running their compost system. Standards for compostable plastics assume a carefully managed composting process, with regular turning of material and the composting mass reaching relatively high temperatures. This is different to most home compost piles that are relatively cold and not turned regularly.

If you’re just using the composting bag to line your food scraps bucket or container – do you really need that extra layer? An alternative could be a sheet or two of newspaper.”
So when is it good to use compostable materials?

“Compostable bags or other products make a bit more sense where there is an easy way to capture the material after use, and a suitable processing option available. A good example is using compostable plates and utensils at festivals with dedicated collection bins, but only where there is an agreement with a commercial composter who is happy to process the material. An alternative in this context is reusable materials, e.g. plates with a communal washing area or cups for hot drinks.”

- Chris Purchas is a specialist waste engineer and Tonkin + Taylor’s Waste Sector Director.

Ends

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