Sanitary Drainage and Sewerage Systems
What the exam tests
This domain covers soil, waste and vent pipes, traps, gradients, inspection chambers, manholes, septic tanks and house sewer connections. The exam tests whether drainage flows by gravity without blockage or gas entry.
Core rules
- Drainage relies on gravity; use the specified gradient. A common guide is 100 mm pipe at 1:50 and 150 mm at 1:100, but follow the project specification.
- Separate sanitary sewage from storm water unless a combined system is approved.
- Every fixture needs a trap; the stack needs venting to protect seals.
- Provide inspection chambers at changes of direction, junctions and intervals; keep covers accessible.
- A septic tank needs retention, inlet/outlet tees and periodic desludging; the outlet goes to a soak pit or treatment system.
- Test drains for leaks and alignment before backfilling.
Worked example
A 100 mm drain runs 20 m at 1:50. Fall = length ÷ gradient denominator = 20 ÷ 50 = 0.4 m. Set the invert 0.4 m lower at the downstream end, check the chamber levels and confirm the pipe is laid straight between chambers. Reason: too little fall causes solids to settle; too much fall leaves liquid behind and encourages blockage.
Traps
Laying drain pipes level; wrong direction of fall; missing vent; connecting storm water to the sanitary sewer; no access chamber; ignoring septic tank sludge removal.