Disintegration of certified compostable plastic bags in outdoor household composting conditions

Picking up dog faeces with single use plastic bags and disposing in landfill is a common practice which ultimately harms the environment. Compostable plastic dog waste bags may help to divert these wastes from landfill and recover dog faeces as a compost feedstock, though little is known about how certified home compostable plastics behave in real world home compost systems. This study investigated the disintegration of commercially available certified home compostable plastic bags in outdoor home composts containing dog faeces. Two pilot trials (25 L) and one household trial (160 L) were conducted over 7, 15, and 9.5 months, respectively. Thermophilic temperatures were reached in all trials while moisture and pH were within optimal ranges for well managed compost systems. Bags showed statistically significant differences in disintegration. Based on final mass, none of the tested bags met the Australian Standard AS5810 minimum disintegration requirements of 90 % mass loss of plastic fragments >2 mm, with average mass change of certified home compostable bags ranging from +1.51 to -81.28 %. All certified industrial compostable bags showed an average mass increase of 10.90-35.04 % during composting. However, time series images of plastic fragmentation indicated some bags fully disintegrated and revealed residual biofilm that may have affected mass change data. Microplastic fragments < 2mm and macroplastic fragments >5 mm were recovered in all composts. Due to the potential risks of using home compost contaminated with microplastics in household gardens, dog owners should avoid including compostable plastic bags in their home composts.