Long-term impacts of sludge spreading on agricultural land (Signal)

Sewage sludge from urban wastewater treatment plants contains organic matter and various nutrients that benefit food crops. Consequently, sludge has been used as a fertiliser across Europe for many decades; this has had positive effects on soil quality, reduced reliance on inorganic fertilisers and promoted the circular use of nutrients. Spreading sewage sludge on agricultural land is regulated in the EU by Directive 86/278/EEC. However, this directive only addresses a few organic pollutants and heavy metals. 

The problem is that sludge may contain a wide range of pollutants. These can accumulate in the environment, and put soil, ecosystem 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. The findings include: 

• Sludge application has increased crop yields by an average of 7%; 

• Heavy metal quantities in sludges have fallen significantly over recent decades. Metals such as mercury, cadmium, copper, lead and zinc decreased by an average of 85%; 

• Sludge application has not significantly affected heavy metal levels in soils or in the crops grown within them; 

• Concentrations of certain organic contaminants, such as per- and polyfluoroalkyl substances (PFAS) 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, in these studies, sewage sludge application does not present any significant risk to ecosystem or human health. 

The findings are in line with research by Pulkrabová et al. (2019), looking into sewage sludge applied on agricultural soils in Czechia since 1996. Sludge application on four different sites tended to increase the levels of PFAS, brominated flame retardants and synthetic musk substances in soil without presenting a direct environmental risk. 

However, other research identifies potential risks related to sewage sludge spreading. Studies have drawn particular attention to the accumulation of microplastics in agricultural soils fertilised with sewage sludge. In this case, microplastics are concentrated during wastewater treatment processes and end up in the soil, compared to cases in which no sludge was applied.  

On the other hand, Swedish studies have shown that the use of sludge over 40 years may result in the same magnitude of microplastic accumulation in soil as atmospheric deposition. In other words, the mass flow of microplastics from sludge use on agricultural soils can be just as moderate —in the range of 3-10g microplastics, per person and per year.  

Despite some evidence of accumulation and threats towards soil organisms, risk assessments for microplastics cannot be performed. This is because there are no effect thresholds available for microplastics in soils to assess potential adverse impacts on soil health.  

In Denmark, Magid et al. (2020)conducted a quantitative environmental risk assessment of both sewage sludge, cattle and pig slurries for metals and organic substances. They showed that the risk to soil organisms from the application of sewage sludge was driven by phthalates and triclocarban substances, although underlying assumptions and the use of safety factors bring uncertainties to the results. Nevertheless, the study concluded that sewage sludge in Denmark did not pose a higher risk to soil organisms than pig slurry. 

More recently, the Joint Research Centre found that significant risks to both humans and soil organisms may originate from a relatively small set of pollutants when present in concentrations levels typically documented for sewage sludge. These priority contaminants — for example, polychlorinated dibenzofuran and dioxins, polycyclic aromatic hydrocarbons, long-chain PFAS and short and mid-chain polychlorinated paraffins — are persistent in soils and have bioaccumulative and toxic properties. 

These results imply that different risk levels may be present across Europe, depending on the local characteristics and the nature of the sludge applied. Hence, risk assessments should be applied at local levels to improve understanding of potential impacts. Levels of substances of concern in soil should be monitored to prevent risks arising from long-term applications.  

There are opportunities to recover nutrients from sewage sludge even if it is contaminated. One option is to extract key nutrients from sludge and apply those to land instead. This would enable other contaminants to be removed from the sludge matrix. However, the preferred long-term solution is to prevent these contaminants from being released into sewers in the first place as far as is possible. 

Sludge of industrial origin may also present specific risks, as mentioned in the cross-cutting story on PFAS of the Zero pollution monitoring assessment. 

Other relevant indicators and signals

Population connected to at least secondary wastewater treatment (Indicator)

Progress in the management of contaminated sites (Indicator)

PFAS contamination and soil remediation (Signal)

Impacts of microplastics on health (Signal)