Methane and biogas are produced naturally during the anaerobic digestion of sewage sludge at wastewater treatment works. Biogas typically contains 60-65% methane and 35-40% carbon dioxide, both of which are hazardous. Methane is flammable and explosive at concentrations between 5% and 15% in air, while carbon dioxide displaces oxygen causing asphyxiation. Biogas is present in digesters, gas holders, pipework, and any enclosed space connected to the digestion process. Strict gas monitoring, zoning, and hot works controls are essential.
Key Hazards
Explosion from methane accumulation finding an ignition source in an enclosed area
Asphyxiation from biogas displacing oxygen in below-ground areas and confined spaces
Fire from biogas leaks near hot works, electrical equipment, or other ignition sources
Carbon dioxide accumulation in low-lying areas causing silent oxygen displacement
Control Measures
Carry a personal multi-gas detector monitoring CH4, CO2, and O2 at all times on the WwTW site.
Maintain ATEX-rated equipment and intrinsically safe tools in all designated gas hazard zones.
Prohibit all ignition sources including mobile phones in designated Zone 1 and Zone 2 gas areas.
Ensure all biogas pipework, valves, and flame arrestors are maintained and tested at regular intervals.
Obtain a specific hot works permit before any flame or spark-producing activity near biogas infrastructure.
Monitor enclosed spaces adjacent to digesters and gas pipework for methane migration continuously.
Ensure forced ventilation is operating in all enclosed plant rooms where biogas may accumulate.
Brief all construction workers on the biogas hazard zones and restrictions before starting work on site.
Report any smell of gas, unusual hissing sounds, or gas detector alarms to the site controller immediately.
Emergency / Rescue
If a gas leak is detected or your monitor alarms, evacuate upwind immediately. Remove all ignition sources. Alert the WwTW site controller and call 999. Do not re-enter until the area is confirmed safe by the gas team.
Remember
Methane is explosive between 5% and 15% in air and can accumulate in any enclosed space on a WwTW.
Carbon dioxide in biogas is heavier than air and collects in pits, chambers, and below-ground areas.
ATEX-rated equipment is mandatory in designated gas hazard zones — standard tools can cause ignition.
Hot works near biogas infrastructure require a specific permit with additional safety precautions.
Personal gas monitors must be worn at all times by everyone working on a wastewater treatment site.
If you smell gas or your detector alarms, evacuate upwind immediately and report to the site controller.
Applicable Legislation: Dangerous Substances and Explosive Atmospheres Regulations 2002 · COSHH Regulations 2002 · Confined Spaces Regulations 1997 · Health and Safety at Work etc. Act 1974