---
title: "Working of two sewer appurtenances"
subject: "Sanitary Engineering"
chapter: 4
key: "semester_6/sanitary_engineering:two-sewer-appurtenances-sketch"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/two-sewer-appurtenances-sketch
site: BCT Engineering
---

# Working of two sewer appurtenances

Sanitary Engineering · Chapter 4: Sewer Appurtenances

- Asked in: 2079 Chaitra, 4 marks
- Video: [Sewer Appurtenances](https://www.youtube.com/watch?v=mSTvKWct5E0) — Environmental Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/two-sewer-appurtenances-sketch
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4

## Question

Describe with neat sketches the working of any two sewer appurtenances that are usually provided on any sewer line.

## Short answer

Manholes allow worker access for inspection and cleaning; sewage flows through a half-round channel, and benching prevents solids settling. Catch basins collect silt and grit from storm water by reducing velocity; heavy particles settle in the sump, and a hooded outlet traps floating matter and provides a water seal.

## Answer

Two commonly provided appurtenances are the **manhole** and the **catch basin** (or street inlet). Their working is explained below.

#### 1. Manhole
A manhole is a vertical chamber built over a sewer to let a worker go down for inspection, cleaning and repair.

```text
      ___ cover
     |   |
     | S |   shaft with steps
     | H |
    /  A  \
   |Inlet  |
   |--->===|---> Outlet
   |_______|
   Half-round channel
```

**Working:** the sewage from the inlet sewer flows through a half-round channel built in the base to the outlet sewer without splashing; side benching sloped to the channel stops solids from settling. The cast iron cover also ventilates the sewer. When a blockage occurs, workers enter and remove it or insert rods from the manhole. Manholes are provided at changes of direction, gradient and size, at junctions and every 30-90 m in straight length.

#### 2. Catch basin
A catch basin is a small chamber with a silt pit below the level of outlet, provided at the inlet of a storm or combined sewer.

```text
   Street inlet
        |
     ___v___
    |       |  <- cover
    |       |_ outlet to sewer (hooded)
    |_______|
    silt/grit settles here
```

**Working:** storm water enters from the street inlet and falls in the chamber. Its velocity is reduced, so heavy silt, sand, grit and rubbish settle to the bottom pit, and only the clear water flows out through the outlet pipe which is higher than the bottom and often has a hood or elbow dipping under water to hold back floating matter and foul gas (water seal). The deposit is removed periodically by hand or by a bucket. This protects the sewer from silting and blockage.

## Check yourself

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

1. What do the half-round channel and benching in a manhole do?
   A. Store silt
   B. Carry flow smoothly without splashing and stop solids settling
   C. Hold back oil
   D. Reduce gas
   Answer: B. The channel guides flow; sloped benching prevents deposits. (Key point: Manhole working)

2. Why is the catch basin's silt pit below the outlet level?
   A. So water can flow backwards
   B. To store drinking water
   C. So heavy silt and grit settle below the outlet and do not enter the sewer
   D. To increase velocity
   Answer: C. Velocity falls and heavy matter settles in the pit below the outlet. (Key point: Catch basin working)

3. What does the hood or elbow dipping under water in a catch basin do?
   A. Increases flow speed
   B. Lets air enter the sewer
   C. Collects silt
   D. Holds back floating matter and gives a water seal against foul gas
   Answer: D. The hooded outlet traps floating matter and acts as a water seal. (Key point: Hood)

4. Where is a catch basin provided?
   A. At the inlet of a storm or combined sewer
   B. At the outfall into the river
   C. At the top of a ventilating shaft
   D. In the middle of a treatment tank
   Answer: A. It lies between the street inlet and the sewer. (Key point: Catch basin position)

## Related in Sanitary Engineering

- [Sewer appurtenances and their necessity](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-appurtenances-necessity) — Sanitary Engineering, chapter 4, asked 2×
- [Separate sewer, 0.6 full, 250000 persons](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-0p6-full-250000) — Sanitary Engineering, chapter 3, asked 2×
- [Separate sewer, 0.70 full, 100000 persons](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-0p7-full-100000) — Sanitary Engineering, chapter 3, asked 2×
- [Steps of sewer construction](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-construction-steps) — Sanitary Engineering, chapter 3
- [DWF and WWF in sewer design](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-importance) — Sanitary Engineering, chapter 2
- [Sewer pipe materials: merits, demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asewer-pipe-materials) — Sanitary Engineering, chapter 3

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