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

This is the project summary of the research project undertaken by the James Hutton Institute on the impacts on human health and environment arising from the spreading of sewage sludge to land (CR/2016/23).

5. Quantitative risk assessment

Table 5‑1 summarises the main findings of the (semi-)quantitative risk assessment. The study suggests that malodour is potentially the highest priority issue to tackle. Reduction in odour exposure should improve well-being in affected communities as well as possibly influencing perceptions of risk.

The other nine risks flagged are all organic and/or pharmaceutical chemical agents. Concentrations of these can be reduced or minimised through process controls, and even under worst-case scenario conditions they pose low risks to receptors. It might be sensible to undertake some monitoring work to keep a watching brief on these agents.

It was not possible to fully assess a number of (high profile) agents including microplastics and anti-microbial resistance. Further data gathering may be required in order to provide a more complete assessment.

Table 5‑1 Summary of the outcomes of the (semi-)quantitative risk assessment
Potentially hazardous agent Outome Magnitude of risk (worst case) Uncertainty Possible mitigation
Malodour Annoyance Medium High
  • Distance to residents from spreading operations >3km
  • Avoid weather conditions that are conducive for exposure (wind speed >6 m s-1; 50% night time cloud cover; partial daytime solar radiation)
  • Application rate <20 t ha-1
  • Avoid or restrict the use of lime treated sludge
Nonylphenol HQ>1 Low Medium
  • AD may reduce concentrations but evidence compounded by ready transformation of NP2EO to NP
Nonylphenol diethoxylate HQ>1 Low High
PBDE-99 HQ>1 Low Medium
  • Partially broken down by AD
PBDE-209 HQ>1 Low Medium
Benzothiazole HQ>1 Low-Medium High
  • Should be removed during aerobic treatment
Triclocarban HQ>1 Low Medium
  • Partial removal by thermal hydrolysis
Cyclomethicone 5 HQ>1 Low Medium
  • Cycolmethicone 5 readily degraded by aerobic treatment and AD
  • Cyclomethicone 6 more likely to remain in sludges
Cyclomethicone 6 HQ>1 Low Medium
Atenolol HQ>1 Low High – Very High
  • Aerobic waste water treatment followed by anaerobic digestion
Chemical exposures (general) HQ>1 Low - Medium Medium – Very High
  • Any risks could be further attenuated by restricting sewage sludge use to pasture/forage crops
  • Extremely precautionary approach

ADAS. 2001. The Safe Sludge Matrix: Guidelines for the Application of Sewage Sludge to Agricultural Land. 3rd Edition, ADAS, Gleadthorpes.


Email: gary.gray@gov.scot

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