---
title: "Slow sand filter construction and design"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:ssf-construction-design"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ssf-construction-design
site: BCT Engineering
---

# Slow sand filter construction and design

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2073 Chaitra, 8 marks
- Video: [Slow Sand Filter- Filtration - Environmental Engineering 1](https://www.youtube.com/watch?v=BUrnYITrQQ4) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ssf-construction-design
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Describe the construction of a slow sand filter with neat sketches and its design consideration.

## Short answer

A slow sand filter consists of a rectangular tank with a supernatant water layer 1.0–1.5 m deep over a fine sand bed (0.9–1.2 m, effective size 0.2–0.3 mm) and graded gravel under-drainage. Design rate is 0.1–0.4 m/h; area is determined from Q/rate with one extra unit; cleaning is by scraping the top 1–3 cm when head loss reaches 0.6–1.2 m.

## Answer

A **slow sand filter (SSF)** is a gravity filter in which water passes slowly (0.1 to 0.4 m/h) through a fine sand bed. It works by straining, sedimentation, adsorption and especially biological action, and it needs no chemicals.

#### Construction
```text
   Inlet          supernatant water 1.0-1.5 m
     |      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     v      | schmutzdecke (biological film)
  ______    |:::::::::::::::::::::::::::::|
 |      |   | fine sand 0.9-1.2 m         |
 |      |   |-----------------------------|
 |      |   | gravel 0.3-0.45 m (graded)  |
 |______|   |==under-drain pipes (laterals)|
 Free board 0.5 m       |             |
                   main drain -> clear water
```
- **Tank**: rectangular RCC or masonry, with watertight floor, L : B about 2:1 to 3:1; area up to about 2000 m² per bed, with a minimum of two units.
- **Supernatant water**: 1.0 to 1.5 m depth above the sand, plus 0.5 m free board.
- **Filter sand**: depth 0.9 to 1.2 m, effective size 0.2 to 0.3 mm, uniformity coefficient 2 to 3 (not more than 5), free of clay and organic matter.
- **Base material**: gravel 0.3 to 0.45 m in 3 or 4 layers from 3 mm (top) to 40 mm (bottom).
- **Under-drainage**: central main drain with lateral perforated pipes or open-jointed tiles covered with gravel, bed slope 1 in 100.
- **Inlet and outlet chambers** with valves, weirs, a filter-to-waste valve, and a rate (flow) controller.

#### Design considerations
- **Rate of filtration**: 0.1 to 0.4 m/h (100 to 400 l/h/m²).
- **Area**: $A=Q/\text{rate}$, with one extra unit as standby for cleaning.
- **Sand cleaning**: when the head loss reaches 0.6 to 1.2 m, remove the top 1 to 3 cm by scraping, and re-sand when the bed depth is below 0.4 to 0.5 m (cleaning every 1 to 3 months).
- Influent turbidity not above 30 NTU (use a roughing filter or sedimentation for turbid water).
- After scraping, the filtrate of the first 1 to 2 days is run to waste until the schmutzdecke is formed again.
- Efficiency: removes 98 to 99 % of bacteria and the turbidity to below 1 NTU; chlorination is still required.

## Check yourself

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

1. What is the usual filtration rate of a slow sand filter?
   A. 3 to 6 m/h
   B. 0.1 to 0.4 m/h
   C. 1 to 2 m/h
   D. 10 to 20 m/h
   Answer: B. SSFs work slowly at 0.1 to 0.4 m/h (100 to 400 l/h/m2), which allows biological action. (Key point: SSF filtration rate)

2. Which is the correct order of layers from top to bottom?
   A. Supernatant water, graded gravel, fine sand, under-drains
   B. Fine sand, supernatant water, gravel, schmutzdecke
   C. Supernatant water, schmutzdecke on fine sand, graded gravel, under-drains
   D. Schmutzdecke, gravel, fine sand, under-drains
   Answer: C. Water stands over the sand; the biological film forms on its top; gravel supports the sand above the drains. (Key point: Layers of the bed)

3. Which sand specification suits a slow sand filter?
   A. Effective size 0.45 to 0.7 mm, uniformity coefficient 1.3 to 1.7
   B. Effective size 0.2 to 0.3 mm, uniformity coefficient 10, depth 0.3 m
   C. Effective size 2 to 3 mm, depth 3 m
   D. Effective size 0.2 to 0.3 mm, uniformity coefficient 2 to 3, depth about 0.9 to 1.2 m
   Answer: D. Fine sand with a modest uniformity coefficient gives slow, thorough straining. (Key point: Sand specification)

4. How is a slow sand filter cleaned when head loss reaches 0.6 to 1.2 m?
   A. By scraping off the top 1 to 3 cm of sand, then running filtrate to waste for 1 to 2 days
   B. By backwashing with upward flow of clean water
   C. By adding more coagulant
   D. By chlorinating the bed overnight
   Answer: A. The clogged top layer is scraped, and the schmutzdecke needs time to form again. (Key point: Cleaning of SSF)

## Related in Water Supply Engineering

- [Slow sand filter, 15000 present population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-c76) — Water Supply Engineering, chapter 6
- [Slow sand filter, 35,000 in 2019 for 2039](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-c75) — Water Supply Engineering, chapter 6
- [Slow sand filter, 15,000 population, 150 L/h/m2](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-a75) — Water Supply Engineering, chapter 6
- [Slow sand filter, 10000 pop, 1.5% growth](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-a74) — Water Supply Engineering, chapter 6
- [Slow sand filter versus rapid sand filter](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-vs-rsf) — Water Supply Engineering, chapter 6
- [Slow sand filter, 10000 population, base year 2068](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-bk68) — Water Supply Engineering, chapter 6

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