Last updated April 4, 2018 at 3:32 pm
Scientists flag plastics concern over organic composts.
Organic fertilisers and compost may be significant source of microplastic particles (MMPs) entering the terrestrial food chain, German researchers have discovered.
In a paper published in the journal Science Advances, a team led by Nicolas Weithmann from the University of Bayreuth reports finding as many as 895 millimetre-size plastic particles per kilogram in material sold as organic fertiliser for commercial and household use.
And while concentrations of MMPs in the oceans are the focus of extensive research to determine both origin and effect, Weithmann and his colleagues conclude “organic fertilisers from biowaste fermentation and composting, as applied in agriculture and gardening worldwide, are a neglected source of microplastic in the environment”.
Plastics in commercial fertiliser products
To make their findings, the scientists looked at the processes and products coming from a range of commercial organic fertiliser plants. Included in the sample was a biowaste composting plant that treats household waste and plant clippings aerobically, and two biowaste digesters that use an anaerobic process to break down mainly household waste. A dedicated crop digester that processed no household material was used as a control.
MMP load in the finished products varied considerably, and seemed to be affected by the pre-treatments the raw waste underwent.
Fertiliser produced by the composting plant contained only 20 to 24 particles per kilogram, while material from the digesters yielded between 146 and 895 particles. The specialist crop digester contained none.
Weithmann and colleagues slate the differences home to the way the initial waste is handled. Before entering the composting plant, material is pressed through sieves of different grades, metal is removed, and a manual sort completed. In contrast, it is fed directly into the digester, with sieving done at the end of the process.
The source of the plastics
The researchers suggest that the MMPs enter the organic waste flow directly from households, even though – in Germany as in many other countries – separate collection bins are provided for organic garbage.
“Theoretically, a pure organic fraction very suitable to composting/biogas fermentation should be obtained,” they write.
“However, in practice, most biowaste contains contaminants, often including plastics.”
They note that material collected from commercial properties tends to be less contaminated, but they single out food industries, which often throw out unsold wrapped products, which allows plastic particles to creep into biowaste reserves.
They also note that the majority of particles found were “secondary” – that is, the products of the breakdown of large pieces of plastic – rather than micro-units deliberately included in the make-up of items such as toothpastes and cleaning products.
And although the MMP concentrations in even the most heavily contaminated fertilisers were far lower than those found in some seawater samples, the scientists still flagged them as cause for concern.
How and if the MMPs accumulate in soil and lower lifeforms, they note, remains unknown, as are any possible risks associated with their accumulation further along the food chain.
“Hence,” they conclude, “further studies on the possible consequences and impacts of MPP contamination of fertilisers originating from biowaste treatment plants for soil quality and soil life forms are necessary before any risk assessment can be undertaken.”