---
title: "Short note: Testing of sewers"
subject: "Sanitary Engineering"
chapter: 3
key: "semester_6/sanitary_engineering:sewer-testing-note"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sewer-testing-note
site: BCT Engineering
---

# Short note: Testing of sewers

Sanitary Engineering · Chapter 3: Design and Construction of Sewers

- Asked in: 2068 Magh (old course), 4 marks
- Video: [Testing of Sewers | Wastewater Engineering | Explained Concepts of Civil Engineering](https://www.youtube.com/watch?v=w3tdPqBympo) — Simplified Learning Civil Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sewer-testing-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-3

## Question

Write a short note on testing of sewers.

## Short answer

Sewers are tested after laying to ensure watertightness, alignment, and freedom from obstruction. Key tests are water (hydrostatic) test with 1.5 m head, air test (100 mm water gauge, 5 min), smoke test, mirror-lamp test, ball test, and infiltration test for groundwater entry. Leakage must meet specifications.

## Answer

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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Sewers are tested mainly to check they are
   A. Cheap and light
   B. Deeper than 1 m
   C. Watertight, straight, true to gradient and free from obstruction
   D. Free from all sewage
   Answer: C. Defects cause exfiltration (groundwater pollution) or infiltration (extra flow). (Key point: Purpose of testing)

2. In the water test the head is about
   A. 1.5 m above the crown at the upper end
   B. 1.5 m below the invert
   C. 15 m above the crown
   D. 0.1 m above the invert
   Answer: A. The lower end is plugged and the upper manhole filled to give about 1.5 m head. (Key point: Water test head)

3. The ball test detects
   A. Leakage through joints
   B. Obstructions or projecting joint material, using a ball slightly smaller than the pipe
   C. Pipe misalignment sideways
   D. Acid attack
   Answer: B. If the ball is stopped there is an obstruction. (Key point: Ball test)

4. Where is smoke testing mostly used and how does it reveal faults?
   A. Large new trunk sewers; smoke stays inside
   B. Water supply mains; smoke shows pressure
   C. Manholes only; smoke shows depth
   D. House drains and old sewers; smoke escaping shows a leak
   Answer: D. Smoke is forced into the sewer and emerges at defects. (Key point: Smoke test)

## Related in Sanitary Engineering

- [Sewer construction and testing](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-construction-testing) — Sanitary Engineering, chapter 3