Case Study: Bretton Water Treatment Works
Case Study Summary
Support Services Provider: Severn Access & Rescue Services
Services Provided: NC4 Confined Space Entry and Cleaning Services
Project Location: Bretton Water Treatment Works, Bretton, Cambridgeshire, UK
Task Summary
The task involved Severn Access & Rescue Services supporting a third-party contractor in the cleaning and removal of built-up silt from a pulsator chamber, which plays a critical role in the water purification process. Over time, sludge and organic matter had settled within the pulsator, requiring manual removal to restore process efficiency.
Due to the condemnation of the fixed access ladder, safe access and egress were achieved by lowering operatives approximately 6 metres into the chamber using a mechanical winch and tripod system operated by trained personnel.
Confined space entry services and specialist rope access services were therefore provided in the completion of the task. A confined space rescue team also had to wait on standby in the event of an emergency situation.
Safe System of Work (SSOW)
Confined Space Risk Classification
The primary objective was the installation of acoustic monitoring equipment within the internal cells of the bridge structure. These areas were classified as Complex Confined Spaces (NCX), due to their restricted entry and exit points, environmental hazards, and the potential need for rapid emergency response.
- Depth and single point of vertical access/egress.
- Potential for toxic gases, particularly Hydrogen Sulphide (H₂S), which can be generated from the breakdown of organic matter present in accumulated silt and sludge.
- Possibility of oxygen depletion.
- Emergency response requirements, including standby rescue and breathing apparatus.
Although pre-entry atmospheric testing confirmed a clean atmosphere, the nature of the task (disturbing stagnant, anaerobic silt) presented a credible risk of releasing H₂S or other hazardous gases during the operation. For this reason, the NC4 classification was applied, and appropriate control measures implemented.
Personnel Involved
- Entrants (2 personnel)
- Top-man (Entry Controller)
- Rescue Team (3 personnel, including one first aider)
- Appointed Supervisor / Confined Space Manager
Control Measures & Methodology
1. Pre-Entry Documentation
- Permit to Work (PTW) issued
- Point of Work Safety Assessment (POWSA) completed
- Confined Space Entry Log initiated
- Toolbox Talk delivered to the full team
- Risk of toxic gas release from sludge disturbance discussed in the briefing
2. Equipment Used
- Tripod with mechanical winch for controlled vertical access
- Fall arrest block as backup
- Aztec pick & pivot system for safe edge handling
- MSA Altair 4X gas monitors for atmospheric testing
- SCBA (Self-Contained Breathing Apparatus) sets were kept on standby due to the risk of atmospheric change, specifically potential H₂S release during sludge agitation
- All entrants wore ELSA (Emergency Life Support Apparatus) to enable self-rescue in the event of sudden atmospheric contamination
- Oxygen therapy unit, defibrillator, and medical bag were present topside
- PPE included harnesses, helmets with chin straps, chemical-resistant coveralls, gloves, and boots
3. Atmospheric Monitoring
- Pre-entry checks confirmed:
- Safe oxygen levels
- No detectable H₂S, CO, or LEL presence
- Atmospheric testing was repeated every 20 minutes
- Continuous monitoring maintained throughout the entry period
- Any reading deviation would have triggered immediate evacuation and escalation
4. Entry Control
- Entrants signed in/out on the Confined Space Entry Log
- Entry and exit were controlled solely via the winch system
- Top-man monitored entrants at all times and logged communications and timings
- 20-minute check-ins maintained via hardline comms and visual confirmation
Rescue Plan (Standby Rescue)
5. Rescue Team On Site
- Confined space-trained personnel stationed topside
- SCBA sets ready for immediate deployment in case of hazardous atmosphere
- Rescue team prepared to initiate immediate extraction using the primary tripod and winch system
- First aider present with full medical kit, oxygen therapy, and defibrillator
6. Rescue Procedure
- Distress signal or comms failure triggers emergency response
- Entry controller alerts confined space supervisor and rescue team
- Rescue operative dons SCBA and enters space if needed
- Casualty secured using rescue harness
- Casualty raised via winch system
- First aid administered; emergency services contacted if required
7. Post-Task Actions
- Debrief conducted with operational team
- Final atmospheric reading take for history records.
- Confined space closed, signed off by Confined Space Manager
- Equipment cleaned, inspected, and stored
- Entry records, atmospheric logs, and rescue readiness documentation filed for audit
Conclusion
This NC4 confined space task at Bretton Water Treatment Works was executed safely and effectively under strict controls. The potential for Hydrogen Sulphide release from disturbed organic silt justified the high-risk classification and the presence of SCBA on standby.
Entrants were equipped with ELSA sets to ensure immediate self-rescue capability. A thorough SSOW, supported by detailed documentation and a competent on-site rescue team, ensured the task was completed without incident.
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