Oxo-biodegradable Plastics in Composting
Most polyolefin short use-life and other film products will not enter composting units and so compostability of such materials is not an issue. Plastic bags for collecting and transporting compostable material are the exception of course (please see below). However, there is the mistaken impression that biodegradability and compostability are the same: they are not. Biodegradability refers to the consumption of material by microorganisms. Compostability refers to material that undergoes degradation by biological processes during the composting process to yield carbon dioxide, water, inorganic materials and biomass at a rate consistent with other known compostable materials, leaving no distinguishable or toxic residue. Since managed composting is the only disposal route for plastics for which a standard performance specification has been developed, this standard has often been inappropriately used as the determinant of environmental acceptability. There are numerous situations, soil contact conditions for example, in which biodegradation is an important mechanism for the recovery of value from used/discarded plastics. Compostability is not involved.
Another problem with the mistaken idea that compostability is invariably an asset is that the term can have several different meanings: home composting [not considered here], composting which involves a closed-container maturation period, and outdoor windrow composting. An additional problem is that neither of the composting standards (see below) provides a reasonable facsimile to full-scale composting conditions. Nevertheless, EPI’s TDPA™ - based polyolefins can provide products that compost nicely in commercial composting facilities.
Plastics that are permitted to enter commercial composting plants must have no deleterious effects on the composting operation, must undergo biodegradation themselves, and must not leave a harmful or toxic residue at the end of the composting procedure. EPI commissioned two composting studies, one in Austria and one in the UK for TDPA™ - PE and for TDPA™ PP respectively. The results may be summarized as follows:
- An extensive commercial-scale composting trial was carried out by Professor B. Raninger (Leoben University) using the municipal composting plant at Vienna Neustadt in Austria. This plant serves a population of about 100,000 people and typically treats about 10,000 tonnes of mixed household and garden waste annually. Composting occurs in two stages: an in-vessel, forced aeration ‘tunnel’ process for 2 weeks followed by an outdoor, open pile composting stage. PE bags modified by TDPA™ were included in the composting stream at a realistic level (1.1 wt%) for bags in a commercial composter. After 6 months of composting, the final compost produced was of excellent quality and met all the requirements of the Austrian Compost Quality Seal. Test results at the BVA labs (Linz, Austria) showed that the compost with EPI compostable sacks showed minimum or no trace of heavy metals and passed the plant tolerance and seeds and propagules tests ( Austrian Standard On 2023). Eco-toxicity tests carried out by Organic Waste Systems (OWS Belgium) according to DIN V 54900-3, ON S 2200 & ON S 2023 indicated absolutely no toxic or harmful by-products. There were no negative effects in the crest test, the summer barley plant growth test, the daphnia test and the earthworm test.
- Another composting study was carried for EPI (Europe) by CalRecovery (Europe) Ltd. In this case the shredded municipal green waste input to an open windrow composting plant located in Leeds, UK contained 1 wt% of a TDPA™ PP film produced for short term packaging in garden centers and intended to be composted after use. After 13.5 weeks composting, with occasional turning, the plastic was completely disintegrated to the point where only under well-lit conditions could extremely small film fragments be distinguished. The resulting compost was found to satisfy the disintegration criterion of EN 13432. The plastic film did not interfere with the composting process, did not give rise to any eco-toxicity effects in the final compost, and did not have any negative impacts on the quality of the final compost.
- The results of other composting trials in Canada, USA and Germany are summarized in ref. 1, page 459. It has been shown repeatedly that EPI’s oxo-biodegradable plastics compost well and produce good quality composts although they do not mineralize quickly enough to meet the arbitrary time limits in the standards ASTM D 6400 and EN 13432. Of course, neither do oak leaves nor paper. This is because the test methods in those standards derive from the BOD test that had originally been developed to evaluate the environmental persistence of synthetic detergents in inland waterways during the 1950s. Indeed, EN 13432 and EN 14046 require that as-produced plastics must mineralize to 90% within 6 months at 58oC. This is a ridiculously high rate of biodegradation that fails to meet the requirement of the European Waste Framework Directive which requires that organic waste be reclaimed. There should be as much ‘unconverted’ biomass and humic material as possible in the compost because this is what imparts the nutritive value in horticultural and agricultural applications of compost. If all the carbon in the compost bags has been converted to carbon dioxide during composting, then there is no recovery and a resource will have been wasted. There is no method of measuring biomass formation in either EN 13432 or in ASTM D 6400, although ASTM has long recognized that organic material in composting operations need not mineralize fully until long after the compost has been utilized as soil conditioner/fertilizer.
References
- G. Scott and D. M. Wiles, “Degradable hydrocarbon polymers in waste and litter control,” in Degradable Polymers: Principles and Applications, 2nd ed. G. Scott (ed.) Dordrecht, Kluwer Academic Publishers, pp. 454-457, 2002.