World Soil Day: How does sewage sludge impact soil health?

Happy World Soil Day

Fidra are celebrating World Soil Day by focusing on how to preserve the health of our soils. Our new project ‘Sewage-free Soils’ highlights the presence of chemical contaminants and microplastics within treated sewage sludge, a by-product of wastewater treatment used as fertiliser on agricultural land.

We are calling for improved monitoring of persistent emerging contaminants and regulations that prevent their accumulation in soils. Fidra calls for legislation that supports the precautionary approach, stopping the agricultural use of treated sewage sludge until proven to be a clean safe resource.

What is sewage sludge?

Treated sewage sludge, also known as biosolids, is a by-product of the wastewater treatment process. When we flush our toilets, unplug our sinks, and finish our weekly laundry – all the wastewater goes to a treatment plant where solids are removed and clean safe water is produced through a complex series of treatment processes. Clean water comes at the expense of an increasingly contaminated solid material or sludge. This sludge is rich in organic matter and nutrients but removal of contaminants from sludge during sludge treatment is challenging.

So, what is in it and why do we use it?

Initially, the sludge contains a mixture of water, organic materials, nutrients, and various contaminants including metals, pathogens, pharmaceuticals, microplastics, PFAS, and bisphenols. The combination of industrial and domestic wastewater at wastewater treatment plants increases the presence of these contaminants.

Due to sludge being rich in organic matter and nutrients such as nitrogen and phosphorus, it can be used as an organic fertiliser. Before application to agricultural land, sewage sludge must undergo treatment such as anaerobic digestion to destroy harmful pathogens and metal contents must be monitored. Current treatment processes, however, can’t remove other contaminants, meaning that unless they are monitored in the same way as metals, they are readily applied to our soils.

Let’s look closer at some of these potential emerging contaminants entering our soils…


PFAS (per- and poly-fluoroalkyl substances) are a group of over 4,700 industrial chemicals, widely used in everyday products from food packaging, toiletries, and non-stick cookware to clothing and carpets. As such, they have a direct route from consumer uses into sewage sludge. Sewage sludge has even been used as an indicator of PFAS use over time in research1.

These chemicals are used largely for their water and oil repellent properties, but these properties are also why they are known as forever chemicals, with some PFAS being so persistent that it takes over 1000 years for them to degrade under typical soil conditions2.

It is not only their persistence in the environment that is worrying, with a continual build up of PFAS in soils where sewage sludge and other industrial bioresources are being applied, but these chemicals threaten the soils overall health too. PFAS have been found to alter microbial community functions3, reducing the biodiversity and connectivity of soil bacteria4.


Bisphenols are one of the most highly produced chemicals in the world, with bisphenol-A (BPA) being the most well studied. These chemicals are used in the manufacture of polycarbonate plastics and epoxy resins, which can be used to line food packaging and many other household items. They are also present in thermal paper which can be recycled to make toilet paper, providing a direct route into wastewater treatment plants and sewage sludge5.

Bisphenol-A (BPA) contamination can interfere with the soils microbiological and biochemical balance, disrupting plant growth and development6.


Microplastics can enter wastewater from a variety of sources, through our washing machines and down our drains – they are prevalent in our everyday life. Although not visible to the naked eye, they accumulate in sewage sludge during wastewater treatment and contaminate our soils when treated sewage sludge is 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 sludge7.

As well as consisting of toxic additives such as brominated flame retardants and BPA, microplastics can act as carriers for other environmental contaminants such as metals, PCBs, dioxins and PAHs8.

Microplastics are easily ingested by soil organisms. Earthworms, an important organism for maintaining soil health, were found to decrease in biomass when introduced to soils containing different types of microplastics (biodegradable polylactic acid), conventional high-density polyethylene (HDPE), and microplastic clothing fibres9. 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 ocean10.


Pharmaceuticals are substances used by humans and livestock for medicinal purposes such as antibiotics and hormones. Some of these are not fully absorbed or broken down after use and are therefore excreted in urine/faeces, eventually entering wastewater treatment plants and sewage sludge or getting released directly into the environment via livestock grazing and manures. The James Hutton Institute found 25 pharmaceuticals and personal care products within sludge11.

The continual release of substances like antibiotics into the environment is of great concern, with elevated concentrations in soil providing the perfect conditions for antibiotic-resistant bacteria. These bacteria are then capable of altering the entire microbial populations sensitivity to antibiotics12.

Current sewage sludge contaminant regulation in the UK

Fidra SfS Regulation Table

Potentially toxic elements in sewage sludge such as metals are monitored and controlled through the UK sludge use in agriculture regulations13, and pathogens are controlled through voluntary measures under the UK biosolids assurance scheme, but synthetic organic chemicals and microplastics are not.

Therefore, without testing for this complex mixture of chemicals and the existing need to establish viable restrictive limits for all contaminants of concern found in sewage sludge, Fidra are calling for the precautionary approach to be adopted until the levels of contaminants of concern found within sewage sludge are proven to be safe (see our Position Paper). Whilst those involved with the sewage sludge supply chain already adhere to the current agricultural use of sludge regulations and biosolids assurance scheme, these guidelines do not currently reflect the modern-day composition of sewage sludge and are, therefore, not fit for purpose.

Read our Soil Health report which explores this topic in more depth alongside other agricultural sources of contaminants!




1Fredriksson, F., Eriksson, U., Karrman, A. and Yeung, L, W, Y. (2022) ‘Per- and polyfluoroalkyl substances (PFAS) in sludge from wastewater treatment plants in Sweden — First findings of novel fluorinated copolymers in Europe including temporal analysis’. Science of the Total Environment, 846.

2 Russell, M.H, Berti, W.R., Szosteck, B. and Buck, R.C. (2008). ‘Investigation of the biodegradation potential of a fluoroacrylate polymer product in aerobic soils.’ Environmental Science and Technology, 42(3), pp. 800–807.

3 Wu, J., Ding, F., Shen, Z., Hua, Z. and Gu, L. (2022). ‘Linking microbiomes with per- and poly- fluoroalkyl substances (PFASs) in soil ecosystems: Microbial community assembly, stability, and trophic phylosymbiosis.’ Chemosphere, 305, pp. 135403.

4 Cao, L., Xu, W., Wan, Z., Li, G. and Zhang, F. (2022). ‘Occurrence of pfas and its effect on soil bacteria at a fire-training area using PFOS-restricted aqueous film-forming foams.’ iScience, 25(4), p. 104084.

5 Zhang, Z., Velly, M., Rhind, S., Kyle, C., Hough, R., Duff, E. and Mckenzie, C. (2015). ‘A study on temporal trends and estimates of fate of Bisphenol A in agricultural soils after sewage sludge amendment.’ The Science of the Total Environment, 515, pp. 1-11.

6 Zaborowska, M., Wyszkowska, J., Borowik, A., and Kucharski, J. (2021). ‘Bisphenol A – A Dangerous Pollutant Distorting the Biological Properties of Soil.’ International Journal of Molecular Sciences, 22(23), pp. 12753.

7 K. W. Magnusson, C. (2014) “Screening of Microplastic Particles in and Down- Stream of a Wastewater Treatment Plant.” (Swedish Environmental Research Institute, Stockholm, Sweden)

8 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.

9 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.

10 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.

11 Scottish Government (2018) The impacts on human health and environment arising from the spreading of sewage sludge to land CR/2016/23: research reports (

12 Cycon, M., Mrozik, A. and Piotrowska-Seget, Z. (2019) ‘Antibiotics in the Soil Environment—Degradation and Their Impact on Microbial Activity and Diversity’. Frontiers in Microbiology, 10.

13 The Sludge (Use in Agriculture) Regulations (1989) (