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I know that you need industrial facilities to compost compostable plastics within a reasonable timeframe. But how does this process work exactly? How long does it take? Is it simply a matter of doing hot composting and maintaining high temperatures? Are any extra resources (e.g. water or energy) needed? Is the end product ready-to-be used compost?

I suspect the exact method may depend on the type of compostable plastic. I'm mainly interested in composting PLA-based plastic since they form the majority of all compostable plastics.

EDIT: I've learned that here in The Netherlands compostable plastic isn't recycled, but incinerated instead. Perhaps this is the same elsewhere and no compostable plastic is recycled?

  • The reason that compostable plastic isn't recycled? 1) When mixed with other compostible materials it's hard to distinguish from 'real' plastics, so it's picked out and handled like ordinary plastics. This may change when we start using more compostable plastics. 2) I'm not aware of large volume compostable plastic waste streams that can be handled separately. – Jan Doggen Oct 26 '15 at 11:12
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The point of compostable plastics is that they can be processed in the same way as the rest of the garbage stream they are mixed in with.

If a fast food outlet uses 100% compostable packing and cups etc, then they can compost everything in their bins without having to separate. The process start off with a fine shedding and mixing of everything, so if the plastic is still only 50% composted at the end of the process, it will keep braking down when the compost is used.

Another example is by giving out compostable plastic bags so as to make it more likely that people will put their food garbage in the compostable collection bin.

In some areas they are now composting all garbage that cannot be separated to reduce its size before the compost gets dumped – the “compost” that is produced is only good for landfill, but at least it takes up a lot less landfill than the uncomposted garbage.

Also when compostable plastic gets into the environment, at least it does not remain there for tens of thousands of years.

  • Thank you for your answer. You've listed several things that 'are possible' and you mentioned landfills, but I'm not interested in both. I would like to know how a state-of-the-art large industrial composting facility that handles compostable plastic actually processes the plastic, preferably with sources/references – THelper Mar 7 '14 at 13:36
  • Also you are incorrect when you say that "they can be processed in the same way as the rest of the garbage stream they are mixed in with". You can't recycle compostable plastics with traditional plastics, because it will lower the quality of the traditional plastic. Also, where I live industrial composting facilities filter out and incinerate compostable plastics because they take too long to fully decompose. – THelper Nov 28 '15 at 9:43
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I found information about composting conditions on this website:

The conditions PLA needs to degrade are exposure to temperatures greater than 140°F (60°C) and relative humidity greater than 90%, for approximately 60 to 80 days.

This is confirmed in this paper where it also says that:

...once exposed to the proper combination of oxygen, moisture and naturally occurring organisms, it will break down into carbon dioxide, water and a small amount of nontoxic waste.... Some studies have shown that it can take as long as 15 months for PLA to start to decompose, even in a controlled composting environment

So far I haven't been able to find information about an industrial composting facility that properly composts PLA, so I'm starting to think this isn't done anywhere.

EDIT (4th apr 2016): A Dutch television program asked several professional recycling companies what they do with compostable plastics like PLA. Most companies said they filter out all plastics, including compostables. Only 1 company (Attero in the city of Venlo) said they compost PLA plastic along with other organic materials. For Dutch speakers, you can view the video here: http://programma.vara.nl/groenlicht/media/355207

  • I wonder if we ever solve this - the requirements for "compostable plastic" contradict. We don't want it to start degrading during its 'usage' lifetime, yet we want it to break down quickly in the compost heap. Look e.g. at the water soluble 'plastics' used for packaging disk washer tablets: they become sticky when you leave them exposed to the (moisture in the) air. – Jan Doggen Oct 26 '15 at 11:17

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