---
title: "Sand, grease and oil trap"
subject: "Sanitary Engineering"
chapter: 4
key: "semester_6/sanitary_engineering:sand-grease-oil-trap"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sand-grease-oil-trap
site: BCT Engineering
---

# Sand, grease and oil trap

Sanitary Engineering · Chapter 4: Sewer Appurtenances

- Asked in: 2075 Bhadra, 1+3 marks; 2073 Magh, 4 marks (asked 2 times)
- Video: [Grease And Oil Traps & Flushing Traps](https://www.youtube.com/watch?v=hiZ9vcv-9Bc) — Ahmed Shaban (Sanitary&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/sand-grease-oil-trap
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4

## Question

Why should grease and oil be excluded from wastewater? Explain the principle of working of a sand, grease and oil trap with neat sketch.

## Short answer

Grease and oil clog sewers, interfere with treatment, and cause foul smells. A sand, grease, and oil trap uses gravity separation at low velocity: heavy sand settles, lighter grease floats and is trapped by a baffle, while clear water flows out.

## Answer

#### Why grease and oil must be excluded
1. They are lighter than water, float, and **stick to the walls of the sewer**, reducing its capacity and causing blockages.
2. They **clog pumps, screens and treatment units** and interfere with the oxygen transfer, so the biological treatment fails.
3. They form scum layers in the treatment tank and **cause foul smell** as they decompose.
4. They **pollute rivers**, forming a film which stops oxygen entering the water.
5. They can ignite and cause explosions if petroleum oils enter the sewer.

#### Sand, grease and oil trap
It is a small tank built in the line of the sewer from hotels, restaurants, garages, workshops and petrol stations to remove sand (grit), grease and oil from wastewater before it enters the sewer.

#### Principle of working
It works on **gravity separation due to difference in density and a low velocity**.
- The tank reduces the velocity of flow (to about 0.1-0.3 m/s), giving a detention time of a few minutes.
- Heavy particles like sand and grit (specific gravity about 2.65) **settle** to the bottom.
- Grease and oil, which are lighter than water (specific gravity about 0.9), **float** to the surface and are trapped by a baffle (a T pipe or a submerged outlet).
- The clear water from the middle layer flows out.

```text
   Inlet               Baffle         Outlet
     |                   |              |
  -->|___________________|___           |-->
     |  grease/oil  ~~~~~|~~~~~          |
     |                   |  clear water  |
     |   . . . sand . . .|_______________|
     |___sand pocket_____|
        removable cover
```

#### Construction and operation
- A brick or RCC tank, with a cover and an inlet pipe well below the water level, an outlet with a dip pipe (baffle) that leaves the floating layer behind, and a sump at the bottom for sand.
- The trap is cleaned regularly: floating grease is skimmed off and the sand removed manually or by pump, otherwise it will be re-suspended and carried away.

## Check yourself

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

1. Why is grease harmful in sewers?
   A. It floats and sticks to sewer walls, reducing capacity and causing blockages
   B. It makes sewage heavier so it sinks
   C. It increases the oxygen in the water
   D. It speeds up biological treatment
   Answer: A. Grease sticks to walls, blocks sewers and interferes with oxygen transfer. (Key point: Why excluded)

2. On what principle does a sand, grease and oil trap work?
   A. Chemical coagulation
   B. Gravity separation by density difference at low velocity
   C. Filtration through fine sand
   D. Centrifugal force
   Answer: B. Low velocity lets sand settle and oil float. (Key point: Principle)

3. What happens to sand and oil inside the trap?
   A. Both settle to the bottom
   B. Both float to the surface
   C. Sand settles; grease and oil float and are held by a baffle
   D. Both pass out with the effluent
   Answer: C. Sand has specific gravity about 2.65 and settles; oil about 0.9 floats. (Key point: Behaviour)

4. Why must the trap be cleaned regularly?
   A. To raise the flow velocity
   B. To make the water acidic
   C. To remove the baffle
   D. Otherwise deposits are re-suspended and carried away
   Answer: D. Uncleaned deposits get carried downstream. (Key point: Maintenance)

## Related in Sanitary Engineering

- [Sand, grease, oil traps: need, O&M](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asand-grease-oil-trap-om) — Sanitary Engineering, chapter 4

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