Earth Flow Composts Biodegradable Plastics in 3 Weeks

Earth Flow Composts Biodegradable Plastics in 3 Weeks

In 2014, Green Mountain Technologies conducted a collaborative research project with Gustavus Adolphus College in Minnesota and World Centric, a manufacturer of compostable serviceware, to test the compostability of World Centric serviceware items in an Earth Flow composting system installed at Gustavus Adolphus. The goal was to provide a real life practical evaluation of how readily serviceware items would compost inside of the Earth Flow composting system.

The Earth Flow is an in-vessel composting system with an automated mixing technology integrated into the vessel. The Earth Flow system provides optimal conditions for hot, aerobic composting. The particular model installed at Gustavus Adolphus was a 24-foot custom vessel Earth Flow (EF-24-CV) with a compost capacity of about 24 cubic yards.

The Earth flow is a continuous-flow composting system. This means that new feedstocks can be added at any time on the load-end of the vessel. Similarly, finished product can be removed from the discharge end of the vessel at any time.

James Dontje, head of the Johnson Center for Environmental Innovation at Gustavus Adolphus, directed and conducted the tests.  James Dontje manages the Earth Flow composting system for the College.

World Centric provided about 38 compostable serviceware sample items from their line for the test.  These included cups, plates, clamshells and cutlery.   Some samples we’re made out of PLA-based compostable plastics.  This included a thick PLA plastic plate designed for reuse.  PLA (polylactic acid) is a thermoplastic resin made from fermented plant starches such as corn or sugar cane.  Other samples were made out of paper or other plant-based fibers.

All samples received both plastic and aluminum identification tags, which were attached to each sample using a hole-punch and a zip-tie.

Compostable Serviceware Samples
Gustavus Earth Flow System

The idea with this test was to place all of the samples at the same time into the active compost process inside of the earth flow and track their progress through the vessel to obtain a sense of how rapidly and completely the items break down and compost inside the vessel.  In other words, the samples were added to a compost mass that was already in progress.

Samples being added to the Earth Flow compost system on Day 1

The tests were conducted over a period of about one month. Compost samples were
collected on six different dates throughout a four-week period: day 1, 10, 16, 20, 23 and

The findings of the tests included:

  • Both the PLA-based plastics and the fiber-based products were completely degraded in the Earth Flow system within about 3 weeks.
  • Only small pieces of the durable PLA plastic plate were found in the compost pile searches.
  • The vast majority of legible labels were found in the pile discharged on Day 23.  This provides evidence that the majority of samples had been transported to the discharge end of the system in about 3 weeks.
  • Of the 50 test samples added to the system with labels, only 14 legible labels were recovered.  This indicates that even the non-biodedgradable plastic tags were significantly degraded in the system.
  • The aluminum tags were also completely dissolved by the compost conditions (low pH was a likely factor).

This evaluation has given us evidence and confidence that compostable serviceware will compost effectively inside of an Earth Flow composting system in about three weeks.  This result assumes that the recipe of inputs overall meets our compost best practices for thermophilic composting.  

We should also emphasize that this test involves a smaller number of compostable plastic items in a relatively large compost mass.  Therefore, there may be a limit on the proportion of compostable plastics that can be added to a recipe and still achieve these rates of decomposition.  We hope to conduct another test soon with a larger proportion of compostable plastics in the mix.  This next test will likely be with our system installed in Hawaii at Full Circle Farm [Information coming soon!].  Stay tuned for the results of that test!

In the meantime, if you would like assistance with your composting project or would like help testing your own compostable products, please contact us at [email protected].

Written by Van Calvez, System Designer, Engineer, Sales & Customer Service

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