Microplastic Elimination

Compost and digestate returned to agricultural soil are generally contaminated with more or less plastic. Over time, this builds up as microplastic contamination with no natural degradation pathway. Pyrolysis can break this cycle: under controlled conditions, the plastic in the feedstock is fully eliminated while most of the carbon and nutrients are preserved in the biochar and pyrolysis oil. We proved the concept and demonstrated that under controlled conditions no new contaminants are created.

Pyrolytic Elimination of Plastic Contamination

Organic recycling, such as composting and anaerobic digestion of biowaste, is essential for closing nutrient cycles in agriculture. But biowaste is more and more contaminated with plastic. Despite sorting and sieving, residual plastic enters compost and digestate at levels that, applied year after year, accumulate as microplastics in agricultural soils. There is no natural degradation pathway for most of those synthetic polymers when applied to soil. The contamination is long-term and growing.

Pyrolysis offers a solution. At temperatures above 520°C, synthetic polymers devolatilise completely - they are thermally cracked into syngas that combust cleanly. A large part of the biomass carbon and most nutrients survive. In principle, pyrolysing plastic-contaminated biowaste can produce EBC-certifiable biochar suitable for soil application, turning a waste management liability into a carbon sink.

The Ithaka Institute investigated this systematically through the CoPyKu project (2021–24), funded by the Swiss Federal Office for the Environment. Hilber et al. (2024) produced biochars from wood residues obtained from sieving biowaste-derived digestate, artificially enriched with 10% mixed plastic waste, and pyrolysed at 450°C and 600°C. At 600°C, all biochars met EBC contaminant thresholds for heavy metals, PAH, and organic pollutants - demonstrating that pyrolysis of plastic-contaminated biowaste can produce biochar suitable for agricultural use. While at 450°C, NMR spectroscopy detected residual plastic signatures in the biochar, at 600 °C no plastic residues were found in the biochar, confirming that a minimum pyrolysis temperature is critical.

We further asked whether pyrolytic plastic elimination works in practice without creating new problems. Co-pyrolysis of biomass and plastic could, in theory, generate chlorinated organic contaminants - dioxins, furans, PCBs - particularly when PVC or other chlorine-containing polymers are present in the waste stream. The companion study by Grafmüller et al. (2024) addressed the chlorine question directly. The work showed that under controlled pyrolysis conditions, the presence of chlorine-containing plastic precursors in the feedstock does not increase the risk of dioxin or furan formation in the biochar. This finding removed the main regulatory concern blocking the co-pyrolysis pathway and provided the evidence base for the EBC's feedstock rules regarding plastic contamination limits.

Together, these results establish pyrolysis as a viable route to break the plastic-to-soil cycle: biowaste that cannot be safely composted or digested because of plastic contamination can be pyrolysed into clean, certified biochar - recycling the carbon and nutrients while eliminating the plastic and generating a permanent carbon sink.