WWT/Process Areas/TBT-WWT-013

Storm Tank and CSO Safety

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Storm Tank and CSO Safety

Toolbox Talk Record

Ref: TBT-WWT-013  |  Issue: 1  |  Date: March 2026
PresenterProject
LocationDate

What?

  • Storm tanks and combined sewer overflows (CSOs) hold diluted sewage during heavy rainfall events.
  • These structures present confined space, drowning, and biological hazards to workers.
  • Water levels can rise rapidly and without warning during rainfall upstream in the catchment.
  • Hydrogen sulphide (H2S) and methane may be present, particularly after prolonged dry weather.
  • Surfaces inside storm tanks are often covered in biofilm making them extremely slippery.
  • Storm tanks may contain debris, sharps, and industrial contaminants from the sewer network.
  • Entry into storm tanks and CSO chambers requires a confined space permit and rescue plan.
  • Real-time weather monitoring and flow data should inform risk assessments for these structures.
  • Workers must be vaccinated against hepatitis A and tetanus before entering sewage-related areas.
  • Drowning remains the leading cause of fatality in water industry confined space incidents.

Why?

Drowning riskStorm tanks can fill rapidly during rainfall — water levels may rise metres within minutes, trapping workers inside.
Toxic atmosphereH2S and methane accumulate in enclosed sewage structures and can cause unconsciousness or death within seconds.
Biological hazardsContact with raw sewage exposes workers to leptospirosis, hepatitis, and other serious infections.
Legal dutyThe Confined Spaces Regulations 1997 and CDM 2015 require employers to control these risks with robust safe systems of work.
Do Don't
  • Treat every storm tank and CSO chamber as a confined space
  • Check weather forecasts and upstream flow data before planning entry
  • Complete atmospheric monitoring for H2S, methane, and oxygen levels
  • Ensure a full rescue plan and trained rescue team are in place
  • Wear appropriate PPE including waterproof coveralls and safety boots
  • Use non-slip footwear and maintain three points of contact on ladders
  • Keep continuous communication with the top person at all times
  • Monitor water levels throughout the entry and set trigger alarms
  • Ensure all workers are up to date with hepatitis A and tetanus vaccinations
  • Exit immediately if weather conditions deteriorate or water levels rise
  • DON'T enter a storm tank or CSO without a confined space permit
  • DON'T rely on weather being dry — upstream rain can arrive quickly
  • DON'T work alone inside any storm water or sewage structure
  • DON'T ignore continuous gas monitoring alarms at any time
  • DON'T touch your face, eat, or drink until you have fully decontaminated
  • DON'T enter without checking the current flow and tank level status
  • DON'T remove PPE while inside the structure, even if conditions seem safe
  • DON'T attempt rescue without breathing apparatus and a trained team
  • DON'T store tools or materials where they could be swept away by rising water
  • DON'T assume the structure is safe because it was empty an hour ago

See also: Hydrogen Sulphide (H2S) on WwTW | Confined Space Entry Procedures
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