---
title: "Treatment processes, impurities removal, settlement"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:treatment-impurities-sedimentation-factors"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/treatment-impurities-sedimentation-factors
site: BCT Engineering
---

# Treatment processes, impurities removal, settlement

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2080 Baisakh, 8 marks; 2079 Bhadra, 4+4 marks (asked 2 times)
- Video: [Unit:6 | Water Treatment | Stages of Water Treatment | Methods of Water Treatment | Prashant YT |](https://www.youtube.com/watch?v=3n7_nR0zHe0) — Prashant YT
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/treatment-impurities-sedimentation-factors
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Discuss the treatment processes (unit operations) and the impurities they remove. Explain the factors affecting settlement (hydraulic subsidence; effect of temperature and diameter) in a sedimentation tank.

## Short answer

Treatment unit operations: screening removes debris; aeration removes gases, Fe/Mn; plain sedimentation settles coarse solids; coagulation and filtration remove colloids and bacteria; disinfection kills pathogens. Settling depends on particle diameter (v ∝ d²), specific gravity, viscosity (inversely proportional, so warmer water settles faster), overflow rate, detention time, and tank design.

## Answer

#### Treatment processes and the impurities they remove
| Treatment unit | Impurity removed |
|---|---|
| Screening | Floating matter, leaves, big debris |
| Aeration | Dissolved gases (CO$_2$, H$_2$S), taste and odour, Fe and Mn |
| Plain sedimentation | Heavy suspended solids, silt, sand (above 0.02 mm) |
| Coagulation, flocculation and sedimentation | Colloids, fine turbidity, colour, some bacteria |
| Filtration (slow or rapid) | Fine suspended solids, flocs, bacteria, cysts |
| Disinfection (chlorination) | Pathogenic bacteria, viruses |
| Softening (lime-soda, zeolite) | Hardness (Ca and Mg salts) |
| Activated carbon, special | Taste, odour, organic chemicals |

```text
 Raw -> Screen -> Aerator -> Sedimentation -> Coag/Floc
 water                                          |
 Supply <- Disinfection <- Filter <- Settling <-+
```

#### Factors affecting settlement in a sedimentation tank
1. **Size of particle**: $v_s \propto d^2$; bigger particles settle quicker.
2. **Specific gravity of particle**: heavier particles settle faster ($v_s \propto S_s-1$).
3. **Temperature (viscosity)**: warmer water has lower viscosity, so settling is faster; $v_s \propto 1/\nu$.
4. **Surface overflow rate / tank area**: larger surface area for the flow reduces the overflow rate and improves removal.
5. **Detention time**: the longer, the better the removal.
6. **Velocity of flow**: should be low (below about 0.3 m/min) so that the settled sludge is not scoured.
7. **Tank shape and depth**, inlet and outlet arrangement (short-circuiting, density currents, wind).
8. **Flocculation**: coagulated particles are bigger and settle faster.
9. **Hindered settling** at high concentration and tank turbulence.

**Hydraulic subsidence (settling) value**: the overflow rate $v_0=Q/A$ is the minimum settling velocity that gives 100 % removal. A particle of velocity $v_s\ge v_0$ is fully removed; if $v_s<v_0$ only the fraction $v_s/v_0$ is removed.

**Effect of temperature and diameter**: from Stokes' law, $v_s = \dfrac{g(S_s-1)d^2}{18\nu}$. Settling velocity varies as the **square of diameter** (doubling the diameter gives four times the speed) and is **inversely proportional to the kinematic viscosity**, which falls as temperature rises (the viscosity at 10 °C is about 1.3 times that at 20 °C, so settling at 10 °C is about 23 % slower). Tanks are therefore designed for the lowest water temperature.

## Check yourself

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

1. Which treatment unit is matched with the impurity it mainly removes?
   A. Screening with hardness
   B. Aeration with floating debris
   C. Filtration with dissolved hardness
   D. Coagulation and sedimentation with colloids, fine turbidity and colour
   Answer: D. Screening removes floating matter, aeration gases/Fe/Mn, softening hardness; coagulation handles colloids and colour. (Key point: Unit and impurity)

2. If the particle diameter is doubled, the Stokes settling velocity becomes:
   A. Twice
   B. Four times
   C. Eight times
   D. Half
   Answer: B. vs is proportional to d^2, so doubling d gives four times the velocity. (Key point: Stokes' dependence)

3. Why are sedimentation tanks designed for the lowest water temperature?
   A. Cold water has lower viscosity
   B. Cold water settles faster
   C. Viscosity is higher in cold water so settling is slower; a tank designed for it also works in warm weather
   D. Temperature has no effect
   Answer: C. vs is inversely proportional to kinematic viscosity, which rises as temperature falls. (Key point: Temperature effect)

4. For an ideal tank with overflow rate v0 = Q/A, what fraction of a particle with vs less than v0 is removed?
   A. 100 %
   B. 0 %
   C. v0/vs
   D. vs/v0
   Answer: D. Particles with vs >= v0 are fully removed; slower particles are removed in proportion vs/v0. (Key point: Overflow rate)

## Related in Water Supply Engineering

- [Treatment process; sedimentation factors; coagulants](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-coagulants) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank, 9 million L peak demand](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-c75) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank, 20000 population, 0.4 mm/s](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-s73) — Water Supply Engineering, chapter 6
- [Settling tank 15 m x 7 m x 3 m, 95% removal](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-b81) — Water Supply Engineering, chapter 6
- [Rectangular sedimentation tank, 0.2 mm/s, 3 hours](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-c71) — Water Supply Engineering, chapter 6, asked 2×
- [Rectangular sedimentation tank, 0.2 mm/s, 25000 population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ased-bk79) — Water Supply Engineering, chapter 6, asked 2×

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- [Surge tank objectives and economic penstock diameter factors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-objectives-economic-diameter) — Hydro Power, chapter 5
- [Biological treatment principles; grit vs sedimentation](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-vs-sedimentation-differences) — Sanitary Engineering, chapter 7
- [Circular sedimentation tank, population 5500](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Ased-tank-industrial-5500) — Sanitary Engineering, chapter 7, asked 2×