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

# Sand, grease, oil traps: need, O&M

Sanitary Engineering · Chapter 4: Sewer Appurtenances

- Asked in: 2076 Bhadra, 8 marks
- 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-om
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4

## Question

Why are sand, grease and oil traps necessary in sewer lines? Describe them with neat sketches. Briefly discuss their operation and maintenance frequency and locations adopted in road network.

## Short answer

Sand, grease, and oil traps prevent blockages, pump wear, and treatment interference. Sand traps reduce flow velocity to 0.2-0.3 m/s for grit settling; grease/oil traps use baffles to separate floating substances. Clean grease traps weekly or at 25% capacity, sand traps monthly and pre/post-monsoon. Locate at hotels, garages, petrol stations, road inlets, and pumping station entries.

## Answer

#### Why sand, grease and oil traps are necessary
- **Sand and grit** settle in sewers, reducing the capacity and causing blockages, wear pumps and fill treatment tanks.
- **Grease and oil** float and stick on the sewer wall and on screens, blocking sewers, choking pumps, forming scum in tanks, interfering with biological treatment and causing odour. Petroleum oils may cause fire or explosion in the sewer.
So the traps are fitted on the wastewater from kitchens, hotels, garages, petrol pumps, workshops and slaughter houses.

#### Description (with sketch)
**Sand trap (grit chamber):** a chamber which reduces the velocity of flow to about 0.2-0.3 m/s so that sand and grit settle in a sump, and the lighter organic matter remains in the flow.

**Grease and oil trap:** a rectangular tank with an inlet deep below the water level, a baffle and an outlet. Flow velocity is reduced, grease and oil rise to the top and are held back by the baffle; the clean water goes out from lower level.

```text
  inlet            baffle        outlet
   -->|_____________|___          |-->
      | oil/grease ~~|~~~~~~~     |
      |              |  water ----|
      |  sand . . . .|__________  |
      |__sand pocket_|
```

A combined sand, grease and oil trap is used where all are present.

#### Operation and maintenance
- **Frequency:** grease traps of restaurants and hotels should be cleaned **weekly or when 25% of the volume is filled** (daily in heavy kitchens); sand traps of garages and road traps at least **monthly**, and before and after the monsoon. Larger traps may be cleaned every 3-6 months.
- Skimmed grease is removed with a ladle and disposed to a landfill or used for soap making. Sand is shovelled out or removed by suction.
- Never use hot water or detergents to wash out the grease, as this only moves the problem downstream.

#### Locations in a road network
- At **petrol stations, workshops, garages** and car washes (oil traps).
- At **hotels, restaurants and canteens** (grease traps, inside the premises).
- Along **roads with heavy silt** (sand traps) at the inlets and at the entry to a pumping station or the treatment plant (grit chamber).
- At the junctions where storm drains from unpaved areas join the sewer.

## Check yourself

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

1. Why are traps necessary on wastewater from garages and kitchens?
   A. They make sewage clearer
   B. They raise pH
   C. Grease and oil block sewers, choke pumps, upset biological treatment and may cause fire
   D. They add oxygen
   Answer: C. Grease/oil clog and petroleum may explode. (Key point: Necessity)

2. At what velocity does a sand trap operate so that sand settles?
   A. About 3 m/s
   B. About 10 m/s
   C. Zero
   D. About 0.2-0.3 m/s
   Answer: D. Velocity is reduced to 0.2-0.3 m/s so grit settles and organics stay in flow. (Key point: Sand trap)

3. What is the recommended cleaning frequency for a restaurant grease trap?
   A. Weekly or when 25% full
   B. Once in 10 years
   C. Yearly
   D. Never
   Answer: A. Restaurant traps need frequent cleaning. (Key point: Maintenance)

4. Why should hot water or detergents not be used to wash out grease?
   A. It makes grease sink
   B. It only moves the problem downstream
   C. It destroys the trap
   D. It is too costly
   Answer: B. Grease re-solidifies downstream. (Key point: Cleaning)

## Related in Sanitary Engineering

- [Sand, grease and oil trap](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asand-grease-oil-trap) — Sanitary Engineering, chapter 4, asked 2×
- [Unit operation, process; need of trickling filter](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aunit-operation-process-trickling-need) — Sanitary Engineering, chapter 7
- [Sludge treatment need; sand drying bed](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-treatment-necessity-drying-bed) — Sanitary Engineering, chapter 8
- [Wet weather discharge, combined sewer](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Awet-weather-discharge-small-town) — Sanitary Engineering, chapter 2

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- [Road maintenance; gully control measures](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aroad-maintenance-gully-control) — Transportation Engineering II, chapter 4