---
title: "Theory of filtration and its limitations"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:theory-of-filtration"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/theory-of-filtration
site: BCT Engineering
---

# Theory of filtration and its limitations

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2079 Baisakh, 8 marks; 2066 Bhadra (old course) (asked 2 times)
- Video: [Theories of filtration](https://www.youtube.com/watch?v=ySzfnypveGQ) — Surabhi Patil
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/theory-of-filtration
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

What is filtration? Explain the theory of filtration as used in the purification of water and its limitations.

## Short answer

Filtration passes water through granular material to remove fine suspended, colloidal and microbial impurities via straining, sedimentation in pores, adsorption, and biological action in slow sand filters. Limitations: cannot remove dissolved substances, experience head loss and clogging, require cleaning; viruses/cysts partially removed; slow filters need low turbidity and large area; rapid filters require coagulation and skilled operation.

## Answer

#### Filtration
**Filtration** is the process of passing water through a bed of granular material (usually sand) to remove suspended, colloidal and microbial impurities that sedimentation could not remove.

#### Theory (mechanisms) of filtration
1. **Mechanical straining**: particles larger than the pore spaces between sand grains (0.1 to 0.2 mm) are stopped at the top; this effect builds up the **schmutzdecke** in a slow sand filter, which then strains finer particles.
2. **Sedimentation**: the pores act like tiny settling basins; fine particles settle on the sand grains' upper surfaces.
3. **Adsorption / surface attraction**: the sand grain and the sticky film on it attract the oppositely charged colloids and bacteria (electrostatic, van der Waals forces).
4. **Biological action**: in a slow sand filter the zoogleal film feeds on organic matter and bacteria, oxidising them to harmless compounds, and converting ammonia to nitrates.
5. **Electrolytic / chemical action**: the ion exchange between sand grains and impurities, and the chemical bonding with coagulant flocs (important for rapid filters).
6. **Flocculation in the bed**: fine particles collide in the pore passages and combine into bigger ones which are then trapped.

```text
  water in
  ~~~~~~~~~~~~~~~~~
  ..  schmutzdecke     <- straining, biological film
  ::::::::::::::::::
  :::: sand grains ::: <- adsorption, sedimentation
  ::::::::::::::::::
  ooooo gravel ooooooo
  clean water out
```

#### Limitations
- Filtration cannot remove dissolved salts, hardness, dissolved gases or most tastes and odours.
- Filters need a clean influent: highly turbid water clogs the bed quickly (a slow sand filter needs turbidity below about 30 NTU; a rapid filter needs coagulation first).
- Head loss builds up with time, so the bed needs regular cleaning (scraping or backwashing), which adds to the maintenance work.
- Viruses and cysts are only partly removed; disinfection is still required.
- Efficiency depends on filtration rate, grain size and temperature; at high rates there is break-through of floc.
- The slow filter needs a large land area, and the rapid filter needs skilled operation, with possible mud balls, cracking and air binding.

## Check yourself

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

1. What is filtration?
   A. Dissolving impurities with chemicals
   B. Passing water through a granular bed to remove suspended, colloidal and microbial impurities
   C. Letting water stand to settle
   D. Removing hardness by boiling
   Answer: B. Filtration removes what sedimentation could not, by passing water through sand. (Key point: Definition)

2. Which is a mechanism of filtration?
   A. Evaporation
   B. Distillation
   C. Mechanical straining, sedimentation in pores, adsorption and biological action
   D. Crystallisation
   Answer: C. Straining, pore sedimentation, adsorption, biological, chemical and flocculation effects operate in the bed. (Key point: Mechanisms)

3. Which is a limitation of filtration?
   A. It removes dissolved salts and hardness completely
   B. It removes all viruses and cysts
   C. It needs no cleaning
   D. It cannot remove dissolved salts, hardness or gases, and highly turbid water clogs the bed quickly
   Answer: D. Filters remove particles only; dissolved matter stays, viruses are only partly removed and disinfection is still needed. (Key point: Limitations)

4. In a slow sand filter, what does the biological film do?
   A. Oxidises organic matter and bacteria to harmless compounds and converts ammonia to nitrates
   B. Adds chlorine
   C. Removes hardness
   D. Increases the filtration rate
   Answer: A. The zoogleal film feeds on organic matter and bacteria. (Key point: Biological action)

## Related in Water Supply Engineering

- [Theory of settlement; Stoke's law](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Astokes-theory-settlement) — Water Supply Engineering, chapter 6, asked 2×
- [Water sources: quality and quantity](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Asources-quality-quantity) — Water Supply Engineering, chapter 2, asked 3×
- [Membrane filter technique for coliforms](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Amembrane-filter-technique) — Water Supply Engineering, chapter 4, asked 2×
- [Number and size of RSF, 1,20,000 population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-b78) — 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

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- [Limitations of Mohr-Coulomb theory](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amohr-coulomb-limitations) — Soil Mechanics, chapter 10
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