---
title: "Short note: Catch basin"
subject: "Sanitary Engineering"
chapter: 4
key: "semester_6/sanitary_engineering:catch-basin-note"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/catch-basin-note
site: BCT Engineering
---

# Short note: Catch basin

Sanitary Engineering · Chapter 4: Sewer Appurtenances

- Asked in: 2068 Bhadra (old course), 4 marks
- Video: [CEW 254.  Infrastructure 🦜 part8. Storm Catch basin inlet / outlet المدخل والمخرج](https://www.youtube.com/watch?v=WFuGGusLEo8) — C.E.W عالم الهندسة المدنية
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/catch-basin-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4

## Question

Write a short note on catch basin.

## Short answer

A catch basin is a chamber between a street inlet and sewer that traps silt, sand, and floating refuse. Velocity reduction allows heavy solids to settle in the sump, while a hooded outlet retains floating matter and provides a water seal. It requires regular cleaning.

## Answer

A **catch basin** is a small chamber built in the line between a street inlet and a storm or combined sewer, to collect and hold silt, sand, grit and floating refuse so that they do not enter and block the sewer.

#### Construction
```text
         Cast iron cover
       __________________
      |                  |
      |                  |  hooded outlet
      |            ______|--->  to sewer
      |   water   |
      |   level   |  Outlet 0.3-0.6 m
      |___________|  above floor
      |  silt pit |
      |___________|
```
- Circular or rectangular, 0.6-1.0 m in diameter, depth 0.9-1.5 m, built of brickwork or concrete, plastered inside.
- The floor of concrete is below the outlet invert to give a **sump (silt pit)**.
- A cast iron frame and cover or the street inlet grating at the top.
- The outlet is connected to the sewer through a trapped or hooded elbow dipping below the water level, which retains floating matter and gives a **water seal** against foul gas.

#### Working
Storm water from the inlet falls into the basin, velocity drops, and heavy particles settle in the silt pit while clear water overflows through the outlet. Floating matter stays behind the hood.

#### Maintenance
Periodic cleaning is needed (silt removed by buckets or a suction machine, especially before the monsoon), otherwise the basin fills up, loses its function and breeds mosquitoes.

**Merits:** keeps silt out of the sewer, prevents blockages, reduces sewer size by avoiding silting.
**Demerits:** requires frequent cleaning, can give foul smell and breed mosquitoes; not needed if the sewer is self-cleansing.

## Check yourself

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

1. What does a catch basin do?
   A. Flushes the sewer
   B. Holds silt, sand, grit and refuse so they do not block the sewer
   C. Joins two sewers
   D. Releases gas
   Answer: B. It traps solids between the street inlet and the sewer. (Key point: Purpose)

2. What gives the catch basin a water seal against foul gas?
   A. The cast iron cover
   B. The silt pit
   C. Hooded or trapped elbow outlet dipping below water level
   D. The street grating
   Answer: C. The dipping hood seals and retains floating matter. (Key point: Water seal)

3. Where does the silt collect?
   A. Above the outlet
   B. In the outlet pipe
   C. On the cover
   D. In the sump below the outlet invert
   Answer: D. The floor lies below the outlet invert to form a silt pit. (Key point: Silt pit)

4. Which is a demerit of a catch basin?
   A. Needs frequent cleaning and may breed mosquitoes
   B. Allows silt into the sewer
   C. Cannot be built of brick
   D. Needs no maintenance
   Answer: A. Basins must be cleaned often; otherwise they fill and breed mosquitoes. (Key point: Demerits)