A new study analysing the microplastic content in treated organic waste material - including compost and its biodegradable packaging - has found each kilogram of waste material contained thousands of tiny pieces of plastics, invisible to the naked eye.
The research team from Griffith University's Australian Rivers Institute assessed the abundance, characteristics and size ranges of microplastics in compost samples collected from 11 composting facilities across Victoria.
They found every kilogram of compost contained between 1,500 to 16,000 microplastic particles, most of which were in microscopic sizes, ranging from 20 to 500 micrometres.
"Microplastics are quietly infiltrating our soil through sources we often consider sustainable, such as composting," lead author Dr Shima Ziajahromi said.
"Composting is widely promoted as an environmentally friendly solution to organic waste management, reducing landfill burdens while enriching the soil with valuable nutrients.
"Unfortunately, we found that every kilogram of compost in our analysis was contaminated with seven to 760 micrograms of microplastics."
The most prevalent type of microplastics the team found during sampling was microfragments and microfibres, which were typically from larger plastic materials and fabric items such as clothing.
To identify the source of these microplastics, the team also analysed compostable bin bags - marketed as 'biodegradable' - and found a high similarity between physical and chemical characteristics of some microfragments and the compostable bags.
"So, we suspect the origin of those fragments are compostable bags used to place food and garden waste into," Dr Ziajahromi said.
"As an additional concern, microplastics can degrade further into nanoplastics, which are small enough to be absorbed by plant roots and enter agricultural products.
"This raises significant food safety concerns as microplastics and associated chemical additives could ultimately pose risks to human health through the consumption of contaminated crops."
Dr Shima Ziajahromi
Dr Ziajahromi added current Australian policies relating to waste and plastics primarily focused on waste diversion and did not adequately address the risks of emerging contaminants such as microplastics in compost.
"Despite growing international evidence that compost can introduce significant amounts of microplastics into soil, there are no national standards in Australia regulating microplastics in processed organics," she said.
"The absence of clear guidelines leaves composting facilities, waste processors, and end users vulnerable to unintended plastic pollution.
"To protect our soil ecosystem, we must implement effective monitoring and source control for microplastics.
"Without action, composting may become an environmental crisis rather than a solution to it."
'Microplastic in Australian processed organics: Abundance, characteristics and potential transport to soil ecosystem' has been published in Journal of Environmental Management.
