Sewage Sludge in Farmlands: A Gateway to Soil Microplastic Pollution?

This study examines the accumulation of microplastics in agricultural soils, an emerging concern linked to the widespread application of sewage sludge. In the UK, 87% of sludge is disposed of through this route. To investigate this potential pathway for microplastic transfer to soils, we analysed dewatered-anaerobically digested sludge from a local wastewater treatment plant and adjacent fields with varied sludge usage histories, including control fields with no sludge application.

Utilising fluorescent microscopy, Fourier-transformed infrared and Raman micro-spectroscopies, we detected significant microplastic presence in sludge samples (2900 ± 1400 particles/g DW and 1200 ± 400 fibres/g DW), predominantly polyethene, polyester, polypropylene, polystyrene, polyvinyl chloride and polyamide, sized between 20-500 µm. Preliminary results revealed elevated microplastic levels in fields that ceased sludge usage a decade ago in the topsoil (62 ± 33 particles/g DW), compared to control fields (8 ± 4 particles/g DW), with pronounced weathering effects on the surfaces of the microplastics.

The study also discusses differences in microplastic concentrations detected by different analytical methods and is one of the first to investigate microplastic retention in soil after sludge disposal as dependent on soil characteristics.  These insights into microplastic fate in soil post-sludge disposal are crucial for enhancing environmental risk assessments and supporting the development of evidence-based policy revisions for sustainable land management.