Client Background
The client operates two underground sewerage pumping stations handling stormwater with wastewater flows carrying organic material.
The engineering lead required a DSEAR assessment under the Dangerous Substances and Explosive Atmospheres Regulations 2002 to support a Section 104 adoption agreement with the local water utility.
Adoption submissions face water utility scrutiny and a higher technical bar than internal compliance reviews. This is why we were engaged as a specialist process safety consultancy.
Client Problem
Underground pumping stations are confined spaces with limited natural ventilation.
Organic material in the wastewater decomposes to produce methane, hydrogen sulphide, and carbon monoxide. Methane has a Lower Explosive Limit (LEL) of 5% by volume in air. It rises and pockets at high points. Hydrogen sulphide and carbon monoxide settle low.
Under required flowrates, ventilation prevents flammable atmospheres from forming. The question the assessment must answer is what happens when the flow rate drops and organic loading rises together.
Client Objectives
A full DSEAR assessment for both pumping stations covering:
- Fire and explosion risk identification under normal and abnormal operations
- Compliance against the DSEAR Regulations 2002, BS EN 60079-10-1 (Explosive atmospheres. Classification of areas. Explosive gas atmospheres), and Water UK’s SSG7
- A formal written report to support the Section 104 adoption submission
Insights
Sigma-HSE conducted a comprehensive site assessment considering the likelihood and persistence of explosive atmospheres and potential ignition sources (electrostatic discharges, mechanical sparking, non-ATEX equipment and hot work activities). Sigma-HSE also assessed process interactions, existing control measures, zone classifications for gas, vapour, and dust atmospheres. Lastly, a basis of safety for each process and area was established, covering both normal and abnormal operations.
Assessment Findings
We assessed biogas composition against published thresholds and reviewed ventilation geometry, equipment certification, and each ignition source against its existing control.
Stagnation combined with elevated organic loading is the scenario that drives the basis of safety here. It is where a subsurface flammable atmosphere can develop despite the system reading low risk under normal flow.
Our key findings:
- Submersible pumps carried ATEX certification rated Ex II 2G T4. For Zone 1 (Equipment Category 2G), the gases present require a temperature class of T3 or better, set by the 270°C autoignition temperature of hydrogen sulphide. The T4 rating meets that requirement.
- Vent discharge height was 1 metre, against the SSG7 reference of 4 metres.
- The Basis of Safety was ventilation-driven dilution under normal operating conditions.
Outcomes
The DSEAR report supported the client’s Section 104 adoption submission, with the existing Ex II 2G T4 submersible pumps confirmed suitable.
Getting the zone classification right matters here. Over-classify and the client replaces equipment that did not need replacing. Under-classify and the installed protection no longer matches the hazard.
Where planning permission allows, raising vent discharge to the SSG7 4 m reference is the recommended route.
Earthing and bonding within hazardous areas and a Management of Change procedure were issued as prioritised actions.
A Basis of Safety must be maintained, not just established. As the pumping stations age and operations evolve, Management of Change reviews, hazardous area classification reassessment, and Basis of Safety development are all delivered by Sigma-HSE under one roof. The same specialist team, the same standards, across the lifecycle of the asset.
