WWT/Process Areas/TBT-WWT-012

Final Settlement Tank Safety

Water & Wastewater Treatment › Process Areas › Final Settlement Tank Safety

Presenter		Project
Location		Date

Final settlement tanks (FSTs) are large circular or rectangular tanks where treated effluent is clarified.
The rotating scraper mechanism sweeps the tank floor continuously, creating entanglement and crush hazards.
Bridge structures spanning the tank carry the scraper drive mechanism and provide the only access to the centre.
The tank perimeter walls are typically 3 to 5 metres above the water surface with limited edge protection.
Falls into the tank cause drowning; the settled sludge layer at the base traps anyone who enters the water.
Hydrogen sulphide can accumulate at the water surface in warm weather, especially in covered tanks.
Access onto the bridge or into the tank for maintenance requires a permit to work and LOTO isolation.
Rotating bridge mechanisms operate slowly but with enormous torque; they crush without stopping.
Handrails on tank perimeters and bridge structures must be maintained and inspected regularly.
The Confined Spaces Regulations 1997 may apply to covered tanks or work below the water surface.

Drowning risk	Falls from tank perimeters and bridges into deep water with settled sludge traps workers and prevents self-rescue.
Scraper entanglement	Rotating scraper mechanisms crush and entangle workers who enter the tank without proper isolation and LOTO.
Gas hazards	Hydrogen sulphide accumulating at the water surface in warm weather or covered tanks causes rapid unconsciousness.

Don't

Isolate and lock off the rotating scraper mechanism before entering the tank area.
Wear a personal flotation device when working on tank perimeters and bridges.
Use a permit to work for all access onto tank bridges or into the tank.
Monitor for hydrogen sulphide at the water surface, especially in warm weather.
Maintain handrails and edge protection around the full tank perimeter.
Position rescue equipment including throw lines at the tank edge.
Brief the team on the drowning risk and scraper isolation procedure.
Keep the bridge walkway clean, non-slip, and free of tools and debris.
Treat covered or enclosed tanks as confined spaces requiring full entry procedures.
Inspect tank edge protection and bridge handrails regularly for damage.

DON'T enter the tank without isolating and locking off the scraper mechanism.
DON'T work on tank perimeters without a personal flotation device.
DON'T lean over tank edges without secure handrails and edge protection.
DON'T access the tank bridge without a valid permit to work.
DON'T ignore hydrogen sulphide odour near the tank surface; monitor gas levels.
DON'T walk on scraper arms or mechanisms even when the system appears stopped.
DON'T remove handrails or edge protection from tank perimeters for access.
DON'T leave tools or materials on the bridge walkway creating trip hazards.
DON'T assume the scraper is isolated because it appears stationary.
DON'T enter covered tanks without confined space entry procedures and gas monitoring.

See also: Wastewater Treatment Works Safety Awareness | Drowning Risk in Process Tanks

Generate professional Risk Assessment and Method Statements in minutes. 10 document formats, site-specific content, instant Word download.

Toolbox Talk Record