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[GreenYes] Are degradable plastics zero-waste?



To the GreenYes list:

The GreenYes List has a lot of zero-waste advocates as members, so I thought to an appropriate place to raise this issue.

There is a conference in Minnesota this Thursday and Friday (April 6-7) that will look at bio-based plastics. Bio-based plastics are plastics made using feedstocks from renewable organic molecules such as starches and sugars. A prominent bio-based plastic is PLA (Polylactic Acid), which is formed by fermenting sugars from corn to form lactic acid, and then polymerizing the lactic acid into a plastic.

I have been seeing discussion from some people indicating that use of bio-based degradable plastics is in keeping with zero waste principals, because when you are done with the items made from PLA, you can compost them back into carbon dioxide and water.

I am hoping to spark some discussion on whether composting PLA is really zero waste. When I look at the web site of the Zero Waste Alliance (http://www.zerowaste.org/case.htm), I see that their Zero Waste goal lists "zero waste of resources" as its first goal, followed by zero waste to landfill and zero waste of other things (hazardous waste, emissions, etc).

Composting PLA might meet the "zero waste to landfill" criteria if you don't look at the solid wastes created in the production of the PLA, but how well does it fare on "zero waste of resources" issue? As far as I can tell, it fares poorly. When you compost PLA, you get nothing useful out of it - only carbon dioxide and water. You don't get any significant amount of "compost". Compost is valuable product for restoring organic matter to soils, and consists mainly of complex organic molecules derived from partially-decomposed lignins from wood and other plant matter. When you compost leaves, grass, and wood, you get "compost". Put PLA in the compost pile though and my understanding is that the PLA will decompose almost completely into carbon dioxide and water, and not add anything appreciable to the compost.

Thus, when you compost PLA, you get nothing of value out of it. All of the work and energy that went into growing the corn, milling and isolating the sugar and starches, fermenting them to produce lactic acid, and then polymerizing them to form PLA - all of that is lost. All of these inputs require fossil fuel - to produce the fertilizer, run the tractor, and do the chemistry to produce PLA. For that matter, based on Cargill-Dow's life cycle analysis, it currently takes almost as much fossil fuel to produce PLA as it does to produce common commodity-grade plastics, and it takes more fossil fuel energy that the amount of petroleum embodied in an equivalent amount of polyethylene plastic. Cargill-Dow is working to reduce this fossil fuel requirement, but they are not there yet.

In contrast, recycling or reusing the PLA should save considerable resources. PLA is a thermoplastic, and it should take a lot less energy to make a PLA item out of recycled PLA than it does to make it out of corn sugars. Even burning PLA as a fuel should at least save the energy embodied in the plastic itself. PLA should burn very cleanly in a large industrial boiler.

The only time I can see value in composting PLA is when PLA is used in a manner that makes the composting of other materials easier, or where no recycling or energy-recovery option is present. Examples include PLA bags used for collecting food waste or yard debris for composting, or possibly plates and utensils used in schools and institutions that compost their food waste.

In contrast, making water bottles out of PLA and then composting those water bottles doesn't seem very "zero waste" to me.

So - all you zero waste advocates out there - what do you think about how degradable plastics fit in with the Zero Waste vision?

Peter Spendelow
Oregon Department of Environmental Quality
503-229-5253



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