In an effort to develop a more planet friendly lifestyle, one thing I’ve been doing for the last year or two is buying compostable or degradable bin liners.
I don’t actually compost at home, so the bin liners go straight to landfill, but I thought that at least they’d still break down a bit more quickly and be better for the environment.
I was wrong.
As it turns out, degradable or compostable bags don’t break down any quicker than normal plastic bags in landfill. There may still be a small environmental benefit to buying them, but it isn’t what I thought, so I feel a bit like I’ve fallen into some kind of green marketing scam.
I also know that I’m not the only one to have held this misconception, so today I’m going to explain what the deal is with degradable and compostable plastic bags. In the process of doing this research, I also found out a bit about recycling plastic bags, so I’ll talk about that too.
Degradable plastic bags break down into little bits more quickly than normal plastic bags do, when exposed to UV light . As you can imagine, this probably won’t happen in a pile of landfill .
It might happen in the ocean though, which is good news for turtles and birds that could otherwise eat or get tangled up in the plastic. But tiny pieces of plastic floating around in the ocean are also pretty bad, as I’ve written about before.
Compostable bags break down through the action of bacteria into CO2 and a soil-like substance called ‘humus’, which can be used as a fertiliser. This is good – the plastic is not just breaking up into little bits, it’s actually turning back into something more like soil. But this process requires really specific conditions – temperature, oxygen content, moisture content – which are hard to maintain in your compost heap at home [3, 4]. So these compostable bags will really only turn to compost in a commercial composting facility. I haven’t been able to find any of these in Canberra.
What happens if you send these compostable bags to landfill? Well, if you can’t compost it at home, it’s certainly not going to happen in landfills, which are very anoxic environments. These bags will take just as long to degrade in landfill as any other plastic bag would.
However, there is still one minor benefit of compostable bags to regular ones. Compostable bags are made from plants, not petroleum. When the plants are growing, they take in CO2 from the atmosphere, and so if the bags don’t break down, that carbon is effectively stored in the plastic . I suspect that wouldn’t be a very large amount of carbon in the scheme of things though. And then there’s the ethical question of: should we be using farmland to produce throw-away convenience products like plastic bags, instead of producing food to feed an ever growing population ?
Recycling PLASTIC bags
Yes, plastic bags can be recycled, but NOT in the curb-side recycling bin.
In Canberra we have ‘co-mingled’ recycling – cans, jars, paper and cardboard all go in the same bin, and these are sorted at recycling facilities by machinery. But the trick is, if you put plastic bags in with this stuff, they can get tangled up in the machines, and damage them [5, 6].
Instead, you can take soft plastic like shopping bags to dedicated bins at supermarkets (there’s one at Woolworths in Dickson).
- ‘Degradable’ bags just break down more quickly into little bits when exposed to UV light. This won’t happen in landfills, and anyway, little bits of plastic are bad.
- ‘Compostable’ bags can only be composted in a commercial composting facility. This also won’t happen in landfills.
- Plastic bags can be recycled, but only in dedicated bins, usually located at supermarkets.
P.S. The term “biodegradable” is a bit ambiguous. In some things I’ve read, it’s used to mean “degradable” and in other things I’ve read it’s used to mean “compostable”.
 O’Brine, T. and Thompson, R.C. (2010) Degradation of plastic carrier bags in the marine environment. Marine Pollution Bulletin 60: 2279-2283.
 Greene, K.L. and Tonjes, D.J. (2014) Degradeable plastics and their potential for affecting solid waste systems. In: Brebbia, C.A., Passerini, G. and Itoh, H. (eds.), Waste Management and The Environment VII. WIT Transactions on Ecology and The Environment 180: 91-102.
 Vaverková M., Adamcová D., Zloch J. How do degradable/biodegradable plastic materials decompose in home composting environment? (2014) Journal of Ecological Engineering, 15 (4), pp. 82-89.
 Kale, G. et al. (2007) Compostability of bioplastic packaging materials: an overview. Macromolecular Bioscience 7:255-277
Images from woolworths.com.au, amazon.co.uk and istockphoto.com