Potentially hazardous agents in land-applied sewage sludge: human health risk assessment

This report on the risks to human health is part 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).


4. Conclusions

Under good practice (e.g. adherence to the Safe Sludge Matrix) and normal circumstances, none of the potentially hazardous agents assessed posed significant health risks. Under realistic worst-case circumstances, a number of potentially hazardous agents were estimated to pose risk to health for human receptors. These are listed in Table 4‑1 along with possible mitigation suggestions. It should be noted that these potential risks have been identified as part of a theoretical mathematical exercise with any mitigation (e.g. by taking a precautionary approach to minimise or eliminate potential risks) only necessary if the predicted exposures and associated risks were found to be real.

Table 4‑1 – List of potentially hazardous agents for which a (semi-)quantitative estimate of risk was possible
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 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

This analysis has focussed on emerging contaminants and issues, and as such it was not possible to assess a proportion of the potentially hazardous agents (semi-)quantitively in a meaningful way. There is very little known about microplastics in the terrestrial environment, and while they are likely present in sewage sludge, it is not possible to determine if people will end up being exposed to them or what the consequences of this exposure might be. Similarly, while the majority of pathogens potentially present in sewage sludge have been shown to pose minimal risk to human receptors, the significance of sewage sludge as a reservoir of antibiotic resistance is poorly understood.

Overall, the impacts of sewage sludge (when used correctly) on physical health outcomes is likely to be minimal and not significantly different to other organic soil amendments (e.g. animal manures). Impacts associated with quality of life and well-being (such as annoyance from malodour) can, however, be much more acute. These impacts are not exclusive to sewage sludge but are associated with a range of industrial and agricultural practices. In our opinion, physical health risks have been by and large managed through improvements in sludge treatment processes as well as best practice; however, the impacts on well-being have been largely ignored.

It should be noted that 'emerging' potentially hazardous agents is an ever-changing situation. The main hazards of concern today are likely to be superseded in the future. Since the 2008 report (SNIFFER 2008) we have seen the focus move from inorganic contaminants towards organics and pharmaceuticals, as well as 'novel' pathogens and antimicrobial resistance. Over the next 10 years, a different set of hazards are likely to become priority. Protecting public and environmental health is an on-going process.

Contact

Email: gary.gray@gov.scot

Back to top