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Question :
Can oxo-biodegradable plastics be composted?
Answer:
It is important to recognise that compost is an artificial man-devised process. Although composting involves biodegradation it is quite different from biodegradation in the natural environment. By contrast, tests for compostability are designed to satisfy the requirements of manufacturers of bio-based plastics.
For example, International Standards for compostability, (e.g. EN 13432 and ASTM D 6400) require that the carbon in the polymer is “completely consumed” during the composting process. That is - converted to carbon dioxide, minerals and a small amount of dead-cell biomass in 180 days or less. However this is not the way in which nature converts its waste to valuable fertilisers.
In nature, lignocellulose, the most abundant biopolymer, is converted to low molar mass chemicals that are nutrients for micro-organisms. Expelling carbon to the atmosphere in 180 days provides no benefit to the natural environment but instead contributes to the “greenhouse effect”. Most importantly, compost made in this way is not compliant with the EU Waste Framework Directive (1991) which requires that biological recovery is achieved by “Recycling/reclamation organic recovery of organic substances…by spreading on land resulting in benefit to agriculture or ecological improvement including composting and other biological transformation processes” Conversion of carbon to CO2 by this definition is not “recycling, reclamation or recovery” but simply increases the “greenhouse effect”.
The alternatives to the above - windrow and pile composts, like garden compost are not uniformly aerated. As in the case of landfill, they can produce methane, which is over twenty times more effective than carbon dioxide as a “greenhouse gas.” Mechanical turning increases aeration but at the same time it reduces the temperature of the mass below that required to provide consistent sanitisation from pathogenic organisms. Full-scale industrial in-vessel composting plants avoid these difficulties since they normally operate at temperatures between 60oC and 80oC but are expensive to operate. Nature’s prolific oxo-biodegradable lignocellulosic wastes do produce valuable composts.
Although oxo-biodegradable polyethylene bags have been satisfactorily composted in industrial composting facilities5, they do not satisfy the artificial (90% in 180 days) time-scale. The deficiencies in this classification, as described in the previous sections, have been brought to the attention of CEN, ISO and ASTM. The composting standards were developed to match the bio-based plastics, and since bio-based plastics are more expensive than oxo-biodegradable plastics and cannot be re-used or recycled in the waste stream, the “bioplastics” industries rely almost exclusively on the above-mentioned international standards to gain acceptance of their products, and are therefore unwilling to consider science-based ecological arguments.
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