Short note: Ventilating shaft

A ventilating shaft (vent stack) is a vertical pipe or shaft connected to the sewer or to a manhole, which lets the sewer gases escape to the atmosphere at a safe height and lets fresh air in.

Need

Sewage decomposes and produces gases such as hydrogen sulphide, methane, carbon dioxide, ammonia and carbon monoxide. These are

• poisonous (H₂S, CO) and dangerous to sewer workers,
• explosive or inflammable (methane), and
• corrosive; H₂S changes into sulphuric acid and attacks the concrete crown of the sewer. Ventilation also reduces sewer odour and lets air in for the aerobic condition.

Arrangement

        ___ Cowl
       |   |
       | V |  vertical pipe, 100-150 mm CI,
       | e |  fixed on a building wall or a pole
       | n |  at least 3 m above ground (above roofs)
       | t |
   ____|   |____
  Manhole / sewer

• A 100-150 mm cast iron or PVC pipe is fixed along a building wall and connected to the sewer; the top has a cowl to keep rain out. Its height is above the roofs (at least 2-3 m above the highest window).
• Ventilation is also obtained through the perforated manhole covers (which let air in), and the house vent pipes (soil pipes carried up above the roof), and ventilating columns at the end of the sewers.
• Ventilating shafts are provided at the ends of branch sewers, at the highest point of the network, and spaced at regular distances along trunk sewers; if the sewer is long they may use forced draught or fans.

Precautions

Workers must still test for gases and ventilate by blowers before entering a sewer.