Hazard Knowledge

The waste carried in sewers may result in the atmosphere being:

• Toxic (e.g. presence of hydrogen sulphide H₂S)
• Explosive/flammable (e.g. presence of methane CH₄)
• Oxygen deficient (caused by certain micro-organisms)

If there has been a pumping failure, there may also be a considerable amount of organic material that will have been there for some time, which may lead to significant amounts of hydrogen sulphide being produced if disturbed.

Oxygen-deficient atmosphere

A reduced through-flow of fresh air due to a lack of natural ventilation or insufficient air currents from sewerage movement (blockage or pump failure) can lead to higher concentrations of other gases (asphyxiants), resulting in an oxygen-deficient atmosphere.

The operation of equipment, such as internal combustion engines, in or near the opening may not only use up oxygen from the air but also produce exhaust gases, such as carbon monoxide, which can accumulate in low areas