Risk assessment: Most organic pollutants in sewage sludge pose a low risk individually.

However, some substances may still pose a risk. This is the conclusion of a risk assessment of organic pollutants in sewage sludge conducted by the Norwegian Scientific Committee for Food and Environment (VKM).

This work was commissioned by the Norwegian Food Safety Authority (Mattilsynet).

Background

Sewage sludge is the residual product remaining after wastewater is treated in treatment plants. The sludge contains large amounts of organic material and nutrients, such as phosphorus and nitrogen, and is therefore often used as a soil improver and fertilizer on agricultural land. At the same time, it may also contain undesirable substances originating from the wastewater.

Norway has a long tradition of using sewage sludge in agriculture. Today, approximately 50–60 percent of the sludge is utilized in the agricultural sector. This is partly due to high trust in the agricultural sector and a partly decentralized wastewater infrastructure, which makes it necessary to find suitable solutions for managing the sludge produced.

The goal is to increase this share to 70 percent. At the same time, the field is evolving: new sludge-derived products are being developed, and there is growing attention to how their use should be regulated in the future.

Scope of the assessment

VKM has assessed the risks posed by organic contaminants in sewage sludge and sludge-based products used on agricultural land. The assessment covers potential risks to soil-dwelling organisms, aquatic organisms, livestock, and humans through long-term use.

“Most of the organic contaminants we have been able to assess appear to pose low risk under current use. At the same time, the assessment shows that some persistent substances may be problematic over time because they degrade slowly and can accumulate in soil and the environment,”
says Trine Eggen, who led the work at VKM.

Some substances may pose long-term risks in soil

The risk assessment is based on data for more than 1,000 organic contaminants measured in Norwegian sewage sludge. The results show that concentrations vary considerably between treatment plants and regions. Several legacy pollutants, such as PCBs, PAHs, and PBDEs, have declined over time. However, they are still present in sewage sludge and remain relevant because they degrade slowly.

For soil organisms, the assessment showed that current practices may lead to long-term risks for certain substances. After evaluating data quality and relevance, 19 organic substances remained with estimated risk levels above threshold values (levels that may indicate risk). These include certain pharmaceuticals, cosmetic-related substances, plastic-related compounds, PAHs, and PFOS.

VKM found that lower application rates reduce risk but do not eliminate it. For substances that degrade very slowly (persistent substances), annual applications may result in higher cumulative loads than application once every ten years.

Different risk profiles for different sludge products

VKM also assessed various products derived from sewage sludge, including pellets, biochar, and struvite. The results show that the physical form and production process are decisive for risk:

• Struvite (a mineral fertilizer recovered from wastewater) contained very low levels of organic contaminants and generally showed low risk.
• Pellets are expected to have similar or somewhat lower risk than dewatered sludge. They lead to lower peak concentrations in the environment than large single applications, but still contribute to gradual accumulation of persistent substances over time.
• Biochar: Treatment at high temperatures (>600°C) effectively breaks down most organic pollutants. However, extremely stable “forever chemicals,” such as long-chain PFAS, may survive the process. Biochar may act as a filter that binds contaminants in soil, but it is uncertain whether they may later be released as the material weathers or degrades over time.
• Liquid digestate (from biogas plants using sewage sludge): Surface application results in four to six times higher exposure for livestock compared with conventional sludge. This is mainly because the liquid adheres to grass, which animals ingest directly when grazing. Injection below the soil surface, or rapid incorporation into soil, is essential to reduce both direct exposure and runoff to water.

Risk to livestock and humans is generally low

For livestock, calculations showed low levels of individual substances in feed. The highest risk was found for grazing animals, partly because they ingest soil. Delaying grazing after sludge application can reduce exposure.

“For humans, the risk is generally low under current practices, but we do not want organic contaminants to accumulate in agricultural soil over time,”
says Eggen.

Several groups of substances are highlighted as important for food safety, including PFAS, PCBs, dioxins, PAHs, and PBDEs, since exposure levels for these are already higher than desired. Siloxanes, bisphenol A, octocrylene, and certain pharmaceuticals are also of concern because they are considered undesirable or potentially hazardous. Many of these substances are regulated, and a decrease in their concentrations in sewage sludge is expected over time.

Uncertainty

VKM emphasizes that the assessment involves considerable uncertainty:

• We have better data on contaminant levels in sewage sludge than ever before, but many substances remain insufficiently measured or lack adequate analytical methods.
• The calculations are based on models that include simplifications and assumptions, particularly regarding long-term behavior.
• For most substances, experimental data on degradation, transport, and toxicity are very limited.
• The combined effects of multiple substances have not been assessed in detail.
• For some sludge-derived products, especially biochar, there is uncertainty regarding the formation of new contaminants and long-term behavior.

Need for further follow-up

VKM concludes that certain substances should be prioritized for further investigation and monitoring. This applies especially to persistent substances that may accumulate in soil and potentially increase environmental and health burdens over time. This is particularly relevant for PFAS, dioxins, and PCBs, where current background exposure from food for large parts of the population is already close to or above health-based guidance values.

VKM provides independent scientific assessments related to environmental and food safety. VKM does not provide advice or make decisions on risk management.