Plastic pollution is a persistent global problem affecting the environment. Compostable plastic materials are considered a viable alternative to fossil-fuel-based products as they are perceived to degrade much faster. As Australian jurisdictions phase out single-use plastic use and compostable substitutes are increasing, it is important that their ultimate disposal in compost does not add to existing plastic pollution.
Plastics claiming to be compostable can be certified to comply with Australian and international standards for disintegration and degradation. Disintegration is a physical fracturing of plastic material into smaller pieces and test methods measure compliance with standards as a percentage of mass loss under pilot-scale compost conditions. Degradation is a chemical decomposition of polymers into CO2, water, and biomass and test methods measure standards compliance as a percentage of evolved CO2 under bench-scale lab conditions. Test methods are similar for both industrial and home compostable plastic certification, but methods poorly reflect actual home compost environments. Studies show that some compostable plastics may pass certification tests but fail to disintegrate as expected in applied settings.
Outdoor, household scale compost trials were conducted to measure disintegration of certified home compostable plastic dog waste bags in mixtures containing dog faeces and sawdust. In two experiments using 25L test compost bins, compostable plastic dog waste bags failed to meet disintegration standards, regardless of compost treatment. A third trial in progress uses 140L test vessels containing dog faeces, food waste, sawdust and both industrial and home certified compostable dog waste bag samples. Preliminary results from the current trial will be discussed as well as suggestions for improving test methods and reproducibility of future studies on the fate of compostable plastics in home compost environments.