Soil signals

Sewage sludge from urban wastewater treatment plants contains various nutrients that benefit food crops, and, consequently, sludge has been used as a fertiliser across Europe for many decades. However, sludge can also contain a range of pollutants, including heavy metals and organic contaminants. These can accumulate in the environment and put both soil/ecosystem health and human health at risk.

To monitor and assess the impact of sewage sludge application, field trials have been carried out in Sweden since 1981. Overall, these studies do not show that sewage sludge application presents any significant risk to ecosystem or human health (Naturvårdsverket, 2015, Kirchmann et al., 2017, VA SYD, 2020).The findings include:

• Sludge application has increased crop yields by an average of 7%.
• Heavy metal quantities in sludge have reduced significantly over recent decades, with metals such as mercury, cadmium, copper, lead and zinc decreasing by an average of 85%.
• Sludge application has not significantly affected heavy metal levels in soils or in the crops grown in soil.
• The concentrations of certain organic contaminants, such as PFAS (per- and polyfluoroalkyl substances) and brominated flame retardants, increase with repeat application. These contaminants were also found in earthworms from the same locations. Grains grown in these soils did not have detectable contaminant levels. 
• Overall, it was concluded that organic contaminant levels do not put ecosystem or human health at risk.

Nonetheless, there are opportunities to reuse sewage sludge more sustainably. One option is to extract key nutrients from sludge rather than simply applying sludge to land (Anderson et al., 2021). This would enable other contaminants to be removed from the sludge matrix; however, the preferred long-term solution is preventing these contaminants from being released into sewers in the first place.

Importantly, it should be noted that this case study is specific to Sweden. Different risk levels may be present elsewhere, depending on the local characteristics and the nature of the sludge applied. Other available research identifies risks related to sewage sludge spreading; hence, risk assessment should be applied at a local level to improve understanding of its potential impacts. Sludge of industrial origin may also present specific risks, as mentioned in the cross-cutting story on PFAS.