June 2, 2020 | Composting

Fate Of Compostable Products During Industrial Composting Of SSO

Debate in the Netherlands about acceptance of compostable products in source separated organics based on disintegration rate in current processing infrastructure led to research trials at full-scale facility. Part I.

Maarten van der Zee and Karin Molenveld

A recent study at Wageningen University & Research in the Netherlands analyzed the fate of compostable packaging in an industrial composting plant. The central aim of the study, “The fate of (compostable) plastic products in a full scale industrial organic waste treatment facility,” was to gather more empirical data on whether the disintegration rate of compostable products is sufficient to be compatible with current organic waste treatment practices in the Netherlands. In an industrial organic waste treatment trial, nine different compostable plastic products were tested: organic waste collection bags, plant pots, tea bags, coffee pads, coffee capsules, and fruit labels. The authors studied how compostable plastics behave in the current Dutch treatment system for source separated municipal organic waste (GFT) and came to the conclusion that compostable products can be processed well with GFT.

For several years in the Netherlands — similar to the United States — there has been debate about the acceptance of compostable (packaging) products in GFT. The debate centers around whether the disintegration rate of compostable products that are certified according to the current standard in Europe (EN 13432) would be sufficient to be compatible with current GFT treatment practices in the Netherlands that focus more and more on high throughput of GFT and corresponding short composting cycles (low residence times). Organic waste treatment companies (organized in the Vereniging Afvalbedrijven (VA)) and the companies producing compostable plastics (organized as Holland Bioplastics (HB)), joined forces in defining the research questions that could provide clarity on this matter and advised the project set-up. Wageningen Food & Biobased Research, commissioned by the Dutch Ministry of Economic Affairs and Climate Policy (EZK), independently carried out the research from February to October 2019.

Composting Trials

The full-scale organic waste treatment trial was performed at Valor, Sint Oedenrode, which is one of the 21 facilities that were treating GFT in the Netherlands at the time of the research (one closed in Spring 2020). About half of the composting installations have facilities for anaerobic digestion that they can use combined with composting (as a pretreatment).

The trial consisted of two separate parts:

  • Part 1: Following selected compostable products during the organic waste treatment process and identifying in which (residual) fractions the products would likely end up. No compostable products where contained separately in mesh bags.
  • Part 2: Evaluating the disintegration of selected compostable products under the regular operation conditions and timeframe (in relation to the results obtained with laboratory testing required for certification according to EN13432). Compostable products and GFT were contained in mesh bags.

This Valor organic waste treatment facility was chosen for multiple reasons:

  • Annually processes about 45,000 metric tons of GFT. The organic waste input is therefore representative for that of other GFT treatment facilities in the Netherlands.
  • Runs a batch process (in well controlled “tunnels” of 650 m3, net processing volume about 450 m3), which is large enough to be representative of commercial organic waste treatment, and at the same time practical for placing and recovering test samples.
  • Duration of a single organic waste treatment cycle is 10 to 12 days, which is on the short side, but in accordance with common practice in the Netherlands.
  • Regular process does not have a separation step before the organic waste enters the composting unit, which means that all (plastic) products present in the organic waste will undergo the composting process.
  • After each sieving step of a composting cycle, the residual fractions (i.e. 10-40 mm, and >40 mm) are usually recirculated and composted again in the next cycle. If not completely disintegrated in the first cycle, products will have a longer residence time in the process. A flow chart of the process is shown.
  • Staff has previous experience with composting trials with compostable materials.
  • Facility could make one composting tunnel (and staff) available and dedicated to the trial in the desired testing period.

A number of compostable plastic test products were selected to investigate the fate of compostable products during organic waste treatment in a commercial industrial facility. Important criteria for the selection include:

  • Diversity in type of plastic, i.e. flexible film, rigid products, nonwovens, labels, etc.
  • Diversity in base material, i.e. starch-based, PLA-based, etc.
  • Commercially available (or at least close to market)
  • Demonstrated compostability, i.e. certified according to EN 13432 (or equivalent)
  • Expected co-benefit of the product or application for the organic waste treatment process, i.e., products that could increase separate collection of GFT and divert typical compostable waste from landfill or incineration, and reduce contamination of GFT with plastics (e.g. compostable versions of products typically disposed of in the GFT bin).

Full-Scale Trials — Mix of GFT And Products

In Part 1, the test products were mixed in a pile (approx. 8 m3) of recently collected GFT. Most of the organic waste collection bags were added as empty bags separated from the roll, but 5% were prefilled with GFT and closed with a knot to simulate its use in practice.

Most of the plant pots were added as empty pots (as received), but 10% to 15% were prefilled with commercial potting soil to simulate how they would likely end up in GFT. Tea bags, coffee capsules and coffee pads were received in a used state, i.e., they had undergone the hot water and/or pressure treatment in their usual application device. About half of the number of fruit labels were applied on the outside of orange and banana peels. The other half was added as a string of labels still on the roll.

The mixture of test products with GFT was left in a pile overnight, and processed the following day. According to normal operational procedures at Valor, the GFT mixture was pretreated with a Komptech Crambo 5000 low speed waste shredder for additional mixing and size reduction (<60 mm). This shredded mixture was blended further with structure material (i.e., the >40 mm residual stream from an earlier composting cycle) in order to obtain the right density, etc., during introduction in the composting tunnel. As this was insufficient mass to fill the whole composting tunnel, it was placed in a specific and marked section of the tunnel, in between sections filled with regular GFT (not supplemented with test products).

The organic waste treatment tunnel was operated according to the usual practice, i.e. composting for 11 days with active aeration from below and spraying moisture from above. Within 12 hours, the temperature rose to above 60°C, and was maintained at that level for at least 48 hours for hygienation. The requirements for Keurcompost is a minimum of 3 days above 50°C. Both runs in the trial fulfilled that requirement.

Full-Scale Trials — Mesh Bag Containment

In Part 2 of the full-scale trials, the test products were mixed with recently collected GFT, from which large plastic impurities were removed manually and put in mesh bags (approx. 50 liter, mesh 2 mm). The next day, the filled mesh bags were placed in duplicate at different representative positions among the regular GFT during the customary loading of the composting tunnel (the same tunnel used for Part 1 of the trial. One set of mesh bags was used to mark the beginning of the section with test products; the other set were deposited at the end of the section. The tunnels were operated according to the usual practice as described in Part 1.

After the first organic waste treatment cycle (approx. 11 days), the mesh bags were recovered when the tunnels were unloaded and opened for visual inspection of the products and photographed. The content of each mesh bag was subsequently mixed with some fresh organic waste (and additional water to bring the moisture content to normal levels) and the mesh bags were refilled with the mixture. These bags were placed again at different representative positions during the customary loading of the tunnel for a second waste treatment cycle of 11 days. After this second run, the contents of each bag were dried and fractionated in order to facilitate the recovery of all test products (fragments) by handpicking.

Part II of this article discusses the results of the full-scale trials. It also includes findings from Wageningen University & Research’s detailed analysis of the composition of the current contamination of source separated municipal organic waste (GFT) by conventional plastics.

Maarten van der Zee, PhD, is a senior scientist and project manager at Wageningen Food and Biobased Research. He has been working for more than 25 years in the field of biodegradable and/or biobased products. Karin Molenveld is a senior scientist and expertise leader at Wageningen Food and Biobased Research. She focuses on developing new biobased materials and products examining specific end-of-life options for these products including organic, mechanical and chemical recycling.

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