Spreading of sewage sludge to land - impacts on human health and environment (CR/2016/23): project summary

1. Background

The recycling of treated sewage sludge to agricultural land has been an important part of Scotland's circular economy for many decades. Sewage sludges are a valuable source of nitrogen and phosphorus, organic matter, major and minor plant nutrients, and can provide long-term benefits to soil structure and fertility. As such, the recycling of sewage sludge to land is recognised as being the best practicable environmental option by the European Union (EU) and UK Government in most circumstances.

It has long been recognised that sewage sludge can contain agents that are potentially harmful to human and environmental health, including heavy metals and pathogens. Measures have been put in place to control against these risks (e.g. ADAS 2001). While these 'traditional' contaminants are well controlled, more recent concerns have been raised regarding 'emerging' issues such as the presence of pharmaceuticals, 'novel' organic compounds, and (anti-microbial) resistance in sewage sludges. Similarly, while 'traditional' controls have focussed on potentially hazardous agents with potential to cause physical health outcomes, little has been done to safeguard against outcomes associated with well-being such as nuisance caused by odour and transportation operations.

To address these emerging issues, the Scottish Government Rural & Environmental Science and Analytical Services commissioned The James Hutton Institute along with RSK ADAS and AquaEnviro to undertake a contemporary health risk analysis incorporating both physical and social aspects of the risk profile of sewage sludge.

The scope was limited to human health and well-being outcomes associated with non-occupational exposures from the use of sewage sludge in agriculture in Scotland. The following activities were undertaken:

• A review of sewage sludge production and use in Scotland, including potential system controls (report; Section 2 below)
• A workshop with communities affected by the land-application of sewage sludge to understand more fully the social aspects of health and well-being outcomes associated with sewage sludge use in agriculture (report; Section 3 below)
• A review and associated in-situ field measurements to characterise and understand the odour generation potential of different sewage sludge products (report; Section 4 below)
• A (semi-)quantitative risk assessment to identify and assess a wide range of (emerging) potentially hazardous agents present in sewage sludge, with risk estimates for both physical and well-being outcomes (report; Section 5 below)

1.1. Headline findings

• Due to existing controls, the majority of 'traditional' hazards (heavy metals, pathogens) pose minimal risks to human health and well-being.
• The majority of 'emerging' hazards (59 hazards quantitatively assessed: primarily organic pollutants, pharmaceutical and personal care products,) also posed minimal risks to human health and well-being in most situations.
• There is no evidence that prions posed human health risks due to spread via sewage sludge
• Ten potentially harmful agents (nine organic and pharmaceuticals plus malodour) were identified as posing a potential risk to human health and well-being under 'plausible worst case' situations.
• The impact of malodour on well-being seems to heighten sensitivities and perceptions of risks associated with land-spreading practices (not limited to sewage sludge, but other malodourous practices too).
• Malodour from sewage sludge can be managed through reductions in the use of lime-treated sludge, as well as operational constraints such as avoiding spreading when wind speeds exceed 6 m s^-1 and ensuring a buffer distance to the nearest residents.
• The nine organic and pharmaceutical agents can be partially controlled through sludge treatment processes such as anaerobic digestion and/or thermal hydrolysis (Table 5‑1).
• Due to data/knowledge limitations, it was not possible to fully assess a number of potentially hazardous agents; including microplastics and all 'emerging' pathogens.
• A watching brief should be maintained to assess new information on the hazards included in this study, as well as to identify any further potential hazards as they emerge.