[GreenYes] Are compostable plastics good?

[SPENDELOW.Peter.H@deq.state.or.us]

Recent postings here have praised certain types of plastics as being
compostable or implied that it would be good if all garbage was putrescible.
While I think we all support composting, we need to evaluate whether, in
specific cases such as plastics, making compostable plastics is good or bad
environmentally speaking.  I don't have the answer, but on first
consideration, the "bad" seems to outweigh the "good". 

Here are some potential negatives:

1) If the plastic decomposes aerobically, it will add to the carbon dioxide
in the atmosphere, increasing our greenhouse gas problem.  If it does not
decompose, it will sequester that carbon dioxide so that it does not
contribute to global warming.

2) If the plastic decomposes anaerobically, as most would in a landfill, it
will produce methane.  As other have pointed out in the "bioreactive
landfill" debate and the proposal regarding yard debris landfilling in
Peoria, much of the methane generated in landfills is not captured by the
landfill gas collection systems, and escapes to the atmosphere.
Nonbiodegradable plastic would not produce this methane.  Methane is 21
times worse than carbon dioxide as a greenhouse gas.

3) If the plastic is recycled into another product, that other product would
probably not have the same durability as it would be more subject to
degradation.

4) What type of compost would be produced by the plastic?  Not the nice
compost you get out of yard debris.  One of the main values of yard debris
compost is how it affects soil structure.  I don't think that composted
plastic would have anything to add to soil structure, and in fact could be
detrimental.  Also, most plastics would not add any nutritive value at all
to the compost, if the plastic has no nitrogen or other valuable nutrients.

Here are some positives:

1) If the plastic is littered, it will degrade eventually rather than remain
on or near the surface of the ground.

2) If used for things like grass trimming bags or food waste collection
bags, the bag can be composted with the other material, and not have to be
removed and discarded.

Anyone else have thoughts on this subject?

Peter Spendelow
Oregon Department of Environmental Quality
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