---
title: "Slow sand filter versus rapid sand filter"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:ssf-vs-rsf"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ssf-vs-rsf
site: BCT Engineering
---

# Slow sand filter versus rapid sand filter

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2070 Chaitra, 8 marks
- Video: [Rapid Sand Filter V/S Slow Sand Filter | All you need to know about these filters | Water Treatment](https://www.youtube.com/watch?v=bvb-njVSmwE) — Simplified Learning Civil Engineering
- 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-vs-rsf
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Differentiate between slow sand filter and rapid sand filter.

## Short answer

Slow sand filters use low rate (0.1–0.4 m/h), fine sand (effective size 0.2–0.3 mm), biological schmutzdecke action, scraping cleaning, and no pre-coagulation. Rapid sand filters use higher rate (3–6 m/h), coarser sand (0.45–0.7 mm), adsorption, require coagulation-flocculation, and are backwashed.

## Answer

| Point | Slow sand filter | Rapid sand filter |
|---|---|---|
| Rate of filtration | 0.1 to 0.4 m/h (100 to 400 l/h/m²) | 3 to 6 m/h (3000 to 6000 l/h/m²) |
| Effective size of sand | 0.2 to 0.3 mm | 0.45 to 0.7 mm |
| Uniformity coefficient | 2 to 3 | 1.3 to 1.7 |
| Depth of sand | 0.9 to 1.2 m | 0.6 to 0.75 m |
| Area required | Large | Small |
| Pre-treatment | Plain sedimentation only (no coagulant) | Coagulation, flocculation and sedimentation needed |
| Principal action | Straining and biological (schmutzdecke) | Straining and adsorption within the bed |
| Cleaning | Scraping the top 1 to 3 cm every 1 to 3 months | Backwashing by upward flow every 24 to 72 h |
| Loss of head | Up to 1.0 to 1.2 m | Up to 2.5 to 3.0 m |
| Quality of effluent | Bacteria removal 98 to 99 % | 80 to 90 % bacteria removal |
| Operation | Simple, unskilled labour | Skilled operation, more complex |
| Construction cost | Higher (large area) | Lower per unit, but equipment cost |
| Running cost | Low | High (chemicals, power for washing) |
| Suitable for | Small towns and rural areas, low turbidity | Large towns and cities, turbid water |
| Wash water | Not needed | 2 to 4 % of filtered water |

## Check yourself

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

1. How do filtration rates compare?
   A. SSF 0.1 to 0.4 m/h; RSF 3 to 6 m/h
   B. SSF 3 to 6 m/h; RSF 0.1 to 0.4 m/h
   C. Both about 1 m/h
   D. SSF 0.4 to 1 m/h; RSF 100 m/h
   Answer: A. RSF is about 10 to 30 times faster, so it needs much less area. (Key point: Filtration rate)

2. How is each filter cleaned?
   A. SSF by backwashing; RSF by scraping
   B. SSF by scraping the top layer; RSF by upward-flow backwashing
   C. Both by scraping every week
   D. Both by chemical dosing
   Answer: B. SSF's biological layer is scraped off; RSF is backwashed every 24 to 72 h. (Key point: Cleaning methods)

3. What pre-treatment does each need?
   A. SSF needs coagulation; RSF needs none
   B. Both need only chlorination
   C. SSF only plain sedimentation; RSF coagulation, flocculation and sedimentation
   D. Neither needs any pre-treatment
   Answer: C. RSF cannot handle turbid water without coagulated, settled feed. (Key point: Pre-treatment)

4. Which statement on effluent quality is correct?
   A. SSF removes 80 to 90 %; RSF removes 98 to 99 %
   B. Both remove 100 %
   C. SSF removes none, RSF removes 99 %
   D. SSF removes 98 to 99 % bacteria; RSF removes about 80 to 90 %
   Answer: D. SSF's schmutzdecke gives superior bacterial removal. (Key point: Bacterial removal)

## Related in Water Supply Engineering

- [Rapid sand filter construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-construction-working) — Water Supply Engineering, chapter 6, asked 5×
- [Rapid sand filter design for 25 MLD](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-b81) — Water Supply Engineering, chapter 6
- [Rapid sand filter for 5000 persons](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-k78) — Water Supply Engineering, chapter 6
- [Rapid sand filter, 45,000 population in 2021](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-a76) — Water Supply Engineering, chapter 6
- [Rapid sand filter design, 50,000 population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-c72) — Water Supply Engineering, chapter 6
- [Rapid sand filter for 60000 population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-c69) — Water Supply Engineering, chapter 6