Paper bags are less expensive to process and can increase the quality of the compost.

The presence of foreign matter (inorganic contaminants) affects the quality of the compost, especially its appearance, and therefore its marketing.  More specifically, plastic bags have an impact on the quality of the compost, for the removal of plastic fragments is imperfect, and even certified compostable plastic bags  can leave up to 10% of residuals (after 84 days of composting).

  • Scientific advances in Germany in measuring foreign matter content in composts have found that as little as 0.25% plastic residue is visually objectionable (Verband der Humus- und Erdenwirtschaft (German Council of Humus & Soil Producers), 2014).
  • The removal of plastic bags is a solvable engineering exercise, though it adds some operational costs in the form of additional upfront processing needed (mechanical and/or manual pre-sorting), residual screening and disposal (potentially higher reject rate).
  • Partially degraded compostable plastic bags can’t be distinguished from other plastic in compost and will thus be treated as rejects.

Careful consideration should be given to the acceptance of compostable plastic products, and preference should be given to paper products (MEO, 2012. Guideline for Composting Facilities and Compost Use in Ontario):

  • Fully compostable, paper bags generate no visible residue.
  • Easily recognizable, paper bags do not cause confusion among consumers.
  • Absorbing liquids, paper bags stick to organic matter and do not fly away in a composting site.

Most Canadian provinces base their compost quality requirements on The Guidelines for Compost Quality, published by the Canadian Council of Ministers of the Environment (CCME, 2005):

  • A specification has been established to limit the presence of the foreign matter, in relation to the visibility of these “contaminants”; the acceptability threshold for users was set at 25mm.
  • The standard CAN / BNQ 0413-200, developed by the Bureau de normalization du Québec  (BNQ 2005 Organic amendments – Composts) states limits regarding the number, the  size and the dry weight % of foreign matters in the compost (AA grade, A and B).
  • In terms of foreign matter content, the Ontario Compost Quality Standards (MEO, 2012) consider that plastic cannot exceed 0.5% (calculated in a dry weight basis).  A similar criteria is also applicable in the province of Québec for « unrestricted use » compost (MDDEFP, 2012. Guide sur le recyclage des MRF).

It is possible for a certified plastic product not to disintegrate or biodegrade in a biomethanisation/anaerobic digestion process and for plastic residue to be found in the digester sludge:

  • If the sludge is used right away in agriculture, it is possible for the plastic product not to biodegrade or disintegrate into the soil;
  • If the sludge is stabilized through composting, it is still possible for the plastic product not to disintegrate as quickly through the process (shorter than usual because the active phase is already completed).

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