Sanitary installations, drainage, sewerage and traps
Sanitary installations and drainage
This domain tests whether you can lay out a drainage system that flows, vents and seals properly, and choose the right trap and chamber.
Systems
- Two-pipe system: soil (WC) and waste (sink, basin) run separately; most reliable, more pipe.
- One-pipe system: soil and waste share one stack with a common vent; each fixture has its own trap.
- Single stack: one pipe with a ventilating pipe; cheapest, needs careful venting.
Traps and seals
A trap holds a water seal that stops sewer gas; the seal should be not less than 50 mm. P-trap for basins and sinks, S-trap (avoid - it siphons), Q-trap for WC pans, gully trap at the boundary of the house drain, floor trap with grating in wet areas. Each trap needs ventilation or an anti-siphon valve to survive siphonage.
Drains and chambers
House drain gradient commonly 1:60 for 100 mm, steeper for smaller sizes; too steep and solids separate from water. Self-cleansing velocity is about 0.6 to 0.75 m/s for house drains. Inspection chamber at every change of direction or junction, manhole on the public sewer, septic tank plus soak pit where there is no sewer, and rainwater led to a separate system, never into the foul drain.
Worked example
A 100 mm house drain runs 12 m at 1:60. Reasoning: fall = 12000 / 60 = 200 mm. If the upstream invert is 0.450 m, the downstream invert is 0.450 − 0.200 = 0.250 m, and this is the level written on the drawing.
Traps that cost marks
An unvented S-trap that siphons; a gully trap without a seal; rainwater connected to the foul sewer; too steep a slope; forgetting invert levels on the section.