Think organic compost is clean? Think again

Commercially prepared organic composts may contain significant quantities of microplastic particles and represent unknown risks to the environment.

That’s the finding reached by a team of German scientists, who analysed the plastic content of composts and fertilisers produced by biowaste technologies and sold for either farm or household use.

In a paper published in the journal Science Advances, a team led by Nicolas Weithmann from the University of Bayreuth reports the results of analyses conducted on the end-products of household and garden waste turned into fertiliser. The team looked at material produced by an aerobic composting plant and by two anaerobic biowaste digesters.

A plant that processes only crop waste was used as a control.

The results were varied, but disturbing, especially for users who assume that organic composts are by definition pure and uncontaminated. Fertiliser produced by the composting plant contained 20 to 24 particles per kilogram, while material from the digesters yielded between 146 and 895 particles. The specialist crop digester contained none.

Most, if not all, of the microplastics were the result of the breakdown of larger pieces, rather than the type deliberately included in consumer items such as cleaning products and toothpastes.

Weithmann and colleagues suggest that the initial sources of the particles are household collection bins. In Germany, as in other nations, each residence has a separate garbage container for organic waste.

This, write the scientists, should in theory result in a clean flow of raw material into composting facilities. “However,” they note, “in practice, most biowaste contains contaminants, often including plastics.”

Commercial waste tended to be less contaminated, they found, except that produced (ironically enough) by the food industry, which had a nasty habit of throwing away unwanted stuff still wrapped in plastic.

Levels of microplastics in finished fertilisers correlated with the treatment of the raw material, the researchers found. Material going into the aerobic composter was subjected to several cleaning processes, including metal removal, hand-sorting and sieving. Material going into the anaerobic digesters, in contrast, had little pre-treatment, with actions to remove impurities occurring once fermentation was complete.

Even at its worst, the level of contamination in the compost is orders of magnitude less than that found in some seawater samples. Nevertheless, conclude the scientists, it still comprises “a neglected source of microplastic in the environment”.