Short note: Testing of sewers

Testing of sewers is done after laying and jointing, before backfilling, to check that the sewer is watertight, straight, true to gradient and free from obstruction. A defective sewer causes exfiltration (pollution of groundwater) or infiltration (extra flow).

Tests

1. Water (hydrostatic) test: the length between two manholes is plugged at the lower end and filled with water to give a head of about 1.5 m above the crown at the upper end. After the pipe has absorbed water, the fall of level in a standpipe over a set time (often 30 min) is noted. Leakage must be within the specified limit.
2. Air test: the sealed pipe is pressurised with air to about 100 mm of water gauge; the drop of pressure over 5 minutes should not exceed about 25 mm. It is quick and does not require water.
3. Smoke test: smoke is forced into the sewer; escape at any point shows a leak (mostly for house drains and old sewers).
4. Mirror and lamp test (alignment test): a mirror at one manhole reflects light from a lamp at the next manhole. A full bright circle shows that the sewer is straight and clear.
5. Ball (obstruction) test: a ball slightly smaller than the pipe is rolled along; obstructions or projecting joint material stop it.
6. Infiltration test: in a sewer below the water table, the flow entering is measured at the lower manhole.

Defects found are repaired and the test is repeated until the line is accepted.