Sanitary Drainage, Ventilation and Sewerage Systems
Sanitary Drainage, Ventilation and Sewerage Systems tests slope, trapping, venting and sewer connection. The exam focuses on why waste flows, why traps lose their seal, and how gases leave the system safely.
Core rules
- Separate soil (WC) and waste (basin, sink) where required; connect through a gully or inspection chamber.
- Gradient: use the specified fall; a common guide is 1:60 for 100 mm soil pipe and 1:100 for 150 mm, but follow IS 1742 or the local bye-law.
- Traps: 50 mm seal for fixtures; provide access for cleaning; avoid double trapping.
- Ventilation: a vent stack prevents siphonage; a re-vent protects a fixture far from the stack; an air admittance valve is used only where permitted.
- Manholes/inspection chambers: at changes of direction, junctions and intervals; provide covers and channel flow.
- Sewer connection: use a gully trap before the sewer to keep foul air out.
Worked example
A 100 mm drain runs 12 m at 1:60. Fall = 12 ÷ 60 = 0.20 m = 200 mm. If the invert at the start is 0.80 m, the downstream invert is 1.00 m. Check that minimum cover is maintained and that the pipe does not fall below the sewer connection level.
Traps
Laying a drain too flat, adding an extra trap, omitting a vent, or discharging a WC directly into a gully causes marks to be lost. Also, a bend without an access fitting is a maintenance trap.