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A couple stores I frequent now use, or are starting to use, plastic bags and cups that say they're compostable. Are these safe to add to my compost pile or are they just more environmentally friendly for landfills?

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"Safe" is a loaded word. What risks are you concerned about? Or are you looking for a list of risks associated with compostable plastics? –  bstpierre Aug 21 '12 at 21:25
@bstpierre A list of potential risks should do. I've tried to keep my garden as organic as I can so one of the concerns I had was if adding this would negate my other efforts or not. –  Brian Surowiec Aug 21 '12 at 21:41
See also How do I deal with 'compostable' bags? –  THelper Feb 20 '14 at 10:30

4 Answers 4

I suspect the answer is still no, they're not suitable for your compost heap. There is currently a programme in action at Imperial College in London which is making good progress on producing plastic bags and the like made from degradable polymer, which should be compostable.

However, most 'biodegradable' plastic bags currently are polylactide based, and these need high temperatures of the sort achieved in industrial facilities to ensure breakdown rather than what you might achieve in your compost heap. The current crop of these plastics otherwise requires sunlight and air to break down - placing in landfill or in the middle of a compost heap means the breakdown process will be slow or non existent. This polylactide product has metals added which help it to biodegrade, but according to a Swedish study recently, the process is also halted by cool or cold, damp conditions. Since that description applies to most of the Northern hemisphere during the winter, 'biodegradable' in this case appears to be a very loose term indeed.

Given optimum conditions, polylactide based plastics should, in theory, biodegrade within 3 years - in reality, that is not what's happening because they're not in optimum conditions. There is also a big question mark over the metals they contain and what happens to those within the environment, never mind your compost heap. So on balance, I'd not recommend putting them in your compost heap unless you are sure they are polymer based and, so far as I'm aware and have been able to ascertain, these won't be available for another 5 years or so, unless someone out here has already perfected the process and has marketed them.

UPDATE 14th September: sorry only just seen your comment that the cup you have is made from PLA - PLA is polylactide, not polymer (vegetable origin) so I wouldn't be putting it on my compost heap. Although you could try one, I'd love to know if/how long it takes to degrade, and whether you have fragments of plastic in its place rather than a whole cup. Which is what I'm betting will happen, no idea how long that'll take, I suspect 3-5 years, but it'd be interesting to see.

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The cup I have right now says it's made from PLA and that they are "100% renewable, BPI certified compostable and ASTM compliant". It's a GreenStripe Cold Cup from Eco-Products. –  Brian Surowiec Aug 23 '12 at 17:09

Unfortunately industry has done a poor job of educating the public about compostable 'plastics'. It all started with Sun Chips who have since eliminated their 'PLA'/Compostable chip bag campaign. PLA/Ingeo/Earthfirst or other PLA 'compostable' plastics do not compost in a residential compost pile and must be commercially composted. While Europe has done a great job of establishing hundreds of commercial composting facilities to deal with the deluge of compostable 'plastics' the US has fallen short in this area and, to date there are few commercial composting facilities available to the public.

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Unless it is made from something organic like corn or hemp, I would say no. Plastic (other than from corn, hemp, etc) is made from petroleum, and those plastics advertised as biodegradable or compostable are simply petroleum processed in such a way that it breaks down faster than regular plastic. When petroleum-based plastic breaks down, it does not turn into something organic, it's just smaller bits of petroleum. All petroleum-based plastics will break down in this manner eventually, and they will build up in the environment - a good example of this is the 'Great Pacific Garbage Patch' - creating a toxic environment for local flora and fauna.

In short, unless it's made completely from materials you would put in your compost in their raw form, I wouldn't add it in it's processed form. I wouldn't add crude oil to my compost, so I wouldn't dream of adding plastic made from crude oil to my compost either, especially with it's links to cancer and other nasties.

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Your home compost will likely never get hot enough to compost the plastic. These are intended for industrial operations.

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