Recent research has unveiled that the application of sewage sludge as a fertiliser is significantly increasing microplastic contamination in agricultural soils.
Biosolids (or treated sewage sludge) are a byproduct of wastewater and sewage treatment processes. Biosolids are applied to agricultural land as a source of crop nutrients and organic matter. In fact, England, Scotland and Wales are one of Europe’s highest agricultural users of biosolids, with 87% of the sewage sludge we produce ending up on agricultural soil [2]. Despite treatment prior to agricultural use, biosolids contain a cocktail of potentially harmful contaminants including microplastics, pharmaceuticals, PFAS (poly or per fluorinated alkyl substances) and chemical flame retardants. Heavy metal contents in biosolids are monitored but there is currently no requirement to monitor or remove these unregulated pollutants from biosolids.
In this study, a range of treated and untreated sludges from industrial and municipal waste streams were applied to long-term experimental plots in Lanarkshire (Scotland) over a four-year period. Measurements of microplastic contents in soils sampled from plots annually over 25 years were undertaken Results revealed that microplastic levels remained relatively unchanged, even 22 years after the last sludge applications, confirming the persistent nature of these pollutants. Different types of microplastics exhibited varying degradation rates. Microfibers from clothing and microfilms from packaging degraded slowly over time, breaking down into smaller microplastics and nanoplastics, which pose additional environmental hazards [3].
Microplastics enter wastewater from a variety of domestic and industrial sources ranging from the use of personal care products, cosmetics and washing machines to tyre plastics road runoff and manufacturing processes. Microplastics are prevalent in our everyday life. Although not visible to the naked eye, they become concentrated in sewage sludge during wastewater treatment and contaminate our soils when biosolids are used as a fertiliser.
Researchers at a Swedish wastewater treatment plant found that, of the microplastics entering the plant, over 99% were captured within the sewage sludge [4].
As well as consisting of toxic additives such as brominated flame retardants and BPA, microplastics can act as carriers for other persistent environmental contaminants such as metals, PCBs, dioxins and PAHs [5].
Microplastics are easily ingested by soil dwelling organisms such as earthworms that maintain soil structure and health. Seed germination and earthworms were found to be impacted by the presence of biodegradable polylactic acid, conventional high-density polyethylene and clothing fibre microplastics in soils [6]. Around 10% of microplastics that are intentionally and unintentionally released into soils originate from agricultural activities, with agricultural soils estimated to receive greater quantities of microplastic pollution than the ocean [7]. Sewage sludge risk analysis by The James Hutton Institute found that microplastics in sewage sludge can pose a risk to soil health after as few as two applications. Repeated application likely provides a pathway for microplastics, alongside additive and adsorbed chemicals, to build up in our soils and enter our waterways and wider environment .
A more precautionary approach is needed to safeguard our soils.
Overall, these findings demonstrate the need for a more precautionary approach to biosolids use as a fertiliser. Microplastics are just one component of the chemical cocktail of contaminants applied to agricultural land in sewage sludge, with pharmaceuticals and organic chemicals (such as PFAS) also posing significant risks to soil health [8]. The James Hutton Institute note that protecting our soils may require limiting the application of biosolids until they can be deemed a clean and safe resource [8]. Fidra call on the Scottish and other UK Governments to further regulatory efforts to mitigate harmful build up of contaminants in our soils from contaminated inputs. This includes working on upstream solutions to limit the entry of contaminants to our wastewater and future proofing our wastewater and sewage management systems. For farmers, cleaner ‘non-industrial’ organic wastes such as manures, digestates and composts are available as alternative circular sources of nutrients and organic matter. See our farmer guidance, farmer survey report and soil health report for more information.
References
[1] Ramage, S. J. F. F., Coull, M., Cooper, P., Campbell, C. D., Prabhu, R., Yates, K., Dawson, L. A., Devalla, S., & Pagaling, E. (2025). Microplastics in agricultural soils following sewage sludge applications: Evidence from a 25-year study. Chemosphere, 376, 144277. https://doi.org/10.1016/j.chemosphere.2025.144277
[2] UK Assured Biosolids, “Biosolids Assurance Scheme.” Accessed: Nov. 29, 2024. [Online]. Available: https://assuredbiosolids.co.uk/
[3] Science for Environment Policy. (2023). Nanoplastics: state of knowledge and environmental and human health impacts. Future Brief 27. Brief produced for the European Commission DG Environment by the Science Communication Unit, UWE Bristol. Available at: https://environment.ec.europa.eu/publications/future-brief-nanoplastics-state-knowledge-and-environmental-and-human-health-impacts-issue-27_en
[4] K. W. Magnusson, C. (2014) “Screening of Microplastic Particles in and Down- Stream of a Wastewater Treatment Plant.” (Swedish Environmental Research Institute, Stockholm, Sweden)
[5] Shi, J., Sanganyado, E., Wang, L., Li, P., Li, X. and Liu, W. (2020). ‘Organic pollutants in sedimentary microplastics from eastern Guangdong: Spatial distribution and source identification.’ Ecotoxicology and Environmental Safety, 193, 110356.
[6] Boots, B., Russell, C.W. and Green, D.S. (2019). ‘Effects of microplastics in soil ecosystems: Above and below ground.’ Environmental Science & Technology, 53(19), pp. 11496–11506.
[7] Nizzetto, L., Futter, M. and Langaas, S. (2016). ‘Are agricultural soils dumps for microplastics of urban origin?’ Environmental Science and Technology, 50(20), pp. 10777–10779.
[8] The James Hutton Institute, “Using new contaminants information to re-assess environmental risks from sewage sludge,” Dec. 2024. Accessed: Jan. 09, 2025. [Online]. Available: https://www.fidra.org.uk/download/james-hutton-institute-re-assessment-of-environmental-risks-from-sewage-sludge/