We are marking World Soil Day by sharing The James Hutton Institute’s latest assessment that suggests limiting the application of sewage sludge to agricultural land in Scotland (and the rest of the UK) until we can better mitigate potential risks to soil health. This independent research by risk modelling expert Professor Rupert Hough, investigates the risk that the application of contaminated sewage sludge poses to our agricultural soils. The report finds that microplastics, many organic contaminants and a number of pharmaceutical and personal care products—all currently applied to agricultural land in Scotland, England, and Wales—have significant potential to cause harm to our soil ecosystems.
This threat to agricultural soil health is a threat to our natural environment, food security and reinforces that sewage sludge application is providing a direct pathway for contaminants to enter our food and water supply. We support the James Hutton Institute’s recommendations for the Scottish and other UK Governments to take a more precautionary approach to agricultural use of sewage sludge, including limiting the use of treated sewage sludge (biosolids) as a fertiliser until proven to be a clean, safe resource.
Sewage sludge applied to agricultural land is a cocktail of potentially harmful contaminants.
Treated sewage sludge, also known as ‘biosolids’, are the by-product of wastewater treatment. 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 [1]. 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 and there is currently no requirement to monitor or remove these unregulated pollutants from biosolids. Alarmingly, many of the unregulated persistent contaminants applied to our soils are known to have negative outcomes for human health (e.g. PFOA [2]) or influence the reproduction, growth and survival of soil organisms [3], [4].
Our soil is a vital living ecosystem.
The air we breathe, food we eat and nature we enjoy are all dependent on healthy and functioning soils [5], [6]. The highly diverse network of organisms living within our soils, for example bacteria, fungi and earthworms, are particularly important for maintaining soil structure, water purification and nutrient cycling, all key determinants of soil health. Soil organisms break down organic material and release essential nutrients like nitrogen back into the soil in plant-available forms [6]. Under normal healthy circumstances, soil biota can help break down some harmful organic pollutants, transforming them into less harmful organic compounds [7]. However, the repeated application of sewage sludge risks overloading these organisms with harmful pollutants, disrupting the function of this ecosystem.
Sewage sludge can impact the health of our soils.
This latest research takes a holistic approach to better understand the impact of sewage sludge application on Scotland (and the rest of the UK’s) soil health, considering the recent rise in awareness around the abundance, persistence and harm associated with unregulated contaminants [9]. The report focusses on the impact of microplastics, organic chemicals (including PFAS, “forever chemicals”) and Pharmaceutical and Personal Care Products (PPCPs), all of which are found within biosolids and have potential harmful effects on soil that are understudied.
The James Hutton Institute find 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 to build up in our soils and enter our environment and water supply.
- A number of chemical contaminants are expected to exceed the safe limit for soil biota. In particular, PFAS demonstrated strong potential to accumulate in soil and pose long-term environmental risks.
- Almost half of the PPCPs tested posed a notable risk to soil health after a single application.
- The cocktail effect is likely to compound all these risks. The report emphasises that, while the impact of these chemicals is modelled on an individual basis, the reality is that a complex mixture of chemicals is highly likely to exacerbate any risks to soil or human health.
The report author Professor Rupert Hough commented:
“This new assessment builds on existing and growing scientific evidence around the risks that contaminated sewage sludge poses to the environment. Impacts on soil biota, when directly exposed to contaminants of concern, are more acute compared to impacts on humans exposed via the food chain. Further research into the long-term fate of unregulated contaminants in agricultural soils and measures to limit their release into the environment is needed.”
A more precautionary approach is needed to safeguard our soils.
Overall, the James Hutton Institute have recommended that these findings merit a more precautionary approach to sewage sludge application, which they note may require limiting the application of biosolids until more evidence is available. The report supports improved wastewater treatment, encompassing a broader range of contaminants, and the development of alternative treatment technologies. Furthermore, the James Hutton Institute recommend more research on known unregulated and emerging contaminants and regular monitoring of the levels of these contaminants in biosolids.
Fidra support all these recommendations and call on the Scottish and other UK Governments to further regulatory efforts to mitigate harmful contamination of our soils from contaminated sewage sludge. This includes working on upstream solutions to limit the entry of contaminants to our wastewater supply and future proofing our 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] UK Assured Biosolids, “Biosolids Assurance Scheme.” Accessed: Nov. 29, 2024. [Online]. Available: https://assuredbiosolids.co.uk/
[2] K. Li, P. Gao, P. Xiang, X. Zhang, X. Cui, and L. Q. Ma, “Molecular mechanisms of PFOA-induced toxicity in animals and humans: Implications for health risks,” Environ Int, vol. 99, pp. 43–54, Feb. 2017, doi: 10.1016/j.envint.2016.11.014.
[3] E. Lahive, A. Walton, A. A. Horton, D. J. Spurgeon, and C. Svendsen, “Microplastic particles reduce reproduction in the terrestrial worm Enchytraeus crypticus in a soil exposure,” Environmental Pollution, vol. 255, p. 113174, Dec. 2019, doi: 10.1016/j.envpol.2019.113174.
[4] D. Cao, X. Wang, X. Luo, G. Liu, and H. Zheng, “Effects of polystyrene microplastics on the fitness of earthworms in an agricultural soil,” IOP Conf Ser Earth Environ Sci, vol. 61, p. 012148, Apr. 2017, doi: 10.1088/1755-1315/61/1/012148.
[5] J. Lehmann, D. A. Bossio, I. Kögel-Knabner, and M. C. Rillig, “The concept and future prospects of soil health,” Nature Reviews Earth & Environment 2020 1:10, vol. 1, no. 10, pp. 544–553, Aug. 2020, doi: 10.1038/s43017-020-0080-8.
[6] P. Pereira, I. Bogunovic, M. Muñoz-Rojas, and E. C. Brevik, “Soil ecosystem services, sustainability, valuation and management,” Curr Opin Environ Sci Health, vol. 5, pp. 7–13, Oct. 2018, doi: 10.1016/J.COESH.2017.12.003.
[7] E. Barrios, “Soil biota, ecosystem services and land productivity,” Ecological Economics, vol. 64, no. 2, pp. 269–285, Dec. 2007, doi: 10.1016/J.ECOLECON.2007.03.004.
[8] J. R. Hanson, J. L. Macalady, D. Harris, and K. M. Scow, “Linking toluene degradation with specific microbial populations in soil,” Appl Environ Microbiol, vol. 65, no. 12, pp. 5403–5408, 1999, doi: 10.1128/AEM.65.12.5403-5408.1999/ASSET/4DA79DC7-FA1E-4DF2-80A9-C61139351E34/ASSETS/GRAPHIC/AM1290743004.JPEG.
[9] Fidra, “World Soil Day: How does sewage sludge impact soil health?” Accessed: Nov. 20, 2024. [Online]. Available: https://www.fidra.org.uk/news/world-soil-day-sewage-sludge/