Determining the Amount of Plastic and Compostable Plastic in Compost “Overs”

Conducted by Integrated Waste Management Consulting - July, 2012


As more and more food scraps diversion programs come on line, compostable plastics are seen as helping to facilitate increased participation and increased capture rate of these programs. The city of Portland, OR and similar jurisdictions implementing source-separated food scraps collection programs must make choices and policy decisions as to what types of materials are allowable and which are not. Most food scraps collection programs, due to the desire to maximize diversion from landfills, tend to be fairly inclusive in terms of materials accepted. For example, while we broadly categorize these programs as “food scraps” collection programs, most of these programs also allow food-soiled paper and food related paper products (e.g. pizza boxes) in addition to food scraps. In addition many of these emerging programs allow or encourage “compostable” food service ware (i.e., plates, cups, forks, etc.) or bags as part of the program. There is emerging information from Europe, Canada, and Washington state2, that programs that allow compostable bags (primarily as in-home bin liners) can increase participation in food scraps collection programs. In addition many commercial food scraps programs allow and encourage “wet-strength” cardboard that is commonly used to package produce and similar materials as part of the “food scraps” stream.


The objective of this study was to analyze a random sampling of “overs” at selected composting facilities to determine if compostable plastics account for more than 10 percent of the overs and to characterize the type and amount of remaining plastic residue. This work will help to quantify the composition of overs which are ordinary plastics and which are “compostable plastics”.


Based on IWMC’s, experience the amount of all plastics removed from the overs at both facilities was comparable and fairly typical of composting facilities handling food scraps from relatively new collection programs. 

The amounts of compostable plastics recovered ranged from 2% to 8%. However it is clear, based on third-party laboratory analysis, that the composition of the plastic was overwhelmingly “conventional” plastics (polyethylene, polypropylene, etc.). 

While a small amount of compostable plastic was recovered in the overs, it appears to be less than 4% of the total material analyzed by AMC, potentially indicating that that truly compostable plastics are breaking down significantly during the composting process at these two sites. The study did not have any information on the “upstream” volume of compostable plastics; it is not possible to draw a definitive conclusion. 

Download the full report here.

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