---
title: "Aeration: purpose and methods"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:aeration-purpose-methods"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/aeration-purpose-methods
site: BCT Engineering
---

# Aeration: purpose and methods

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2073 Chaitra, 8 marks; 2071 Chaitra, 8 marks; 2071 Shrawan, 8 marks; 2068 Chaitra, 8 marks; 2076 Asoj, 2 marks; 2070 Chaitra (old course); 2067 Asar (old course) (asked 7 times)
- Video: [61. Aeration || Water supply Engineering](https://www.youtube.com/watch?v=FodohwWS_7Y) — NITIANS ACADEMY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/aeration-purpose-methods
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

What do you mean by aeration of water? Why is it required (purposes)? Describe the various methods of aeration with sketches.

## Short answer

Aeration brings water and air into close contact to dissolve oxygen and release dissolved gases. It oxidises iron and manganese, removes carbon dioxide, hydrogen sulphide, tastes and odours, raises dissolved oxygen, and reduces corrosion. Methods include gravity or cascade aerators, spray fountains, air diffusion, mechanical aerators, and packed towers.

## Answer

#### Aeration
**Aeration** is the process of bringing water and air into close contact in order to dissolve oxygen from the air into water and release dissolved gases and volatile substances from the water into the air.

#### Purposes of aeration
1. To **oxidise iron and manganese** (Fe$^{2+}$ and Mn$^{2+}$) into insoluble hydroxides or oxides so they can be settled and filtered.
2. To **remove dissolved gases** such as carbon dioxide, hydrogen sulphide and methane, which cause corrosion and bad taste and smell.
3. To **remove taste and odour** caused by volatile organic matter, algae products and gases.
4. To **raise the dissolved oxygen** of deep well or reservoir water and improve its freshness.
5. To **reduce free CO$_2$** and so raise the pH, which reduces corrosion of pipes.
6. To **precipitate** some hardness-causing and other dissolved substances by oxidation, and to help with the removal of volatile chemicals.

#### Methods of aeration
**1. Gravity (waterfall) aerators**: water falls in thin layers or drops over steps, cascades or inclined planes through the air.
```text
   water in
     \__
        \__   cascade steps
           \__
              \__ -> collecting tank
```
- **Cascade / step aerator**: water flows down a series of steps, each about 0.2 to 0.3 m high, with a total fall of about 1 to 3 m. Good for CO$_2$ and iron removal.
- **Inclined plane aerator**: water flows in a thin sheet down an inclined plane fitted with riffle plates or studs that break it up.
- **Tray (coke tray) aerator**: water trickles down through 3 to 5 trays of coke, stones or slats, 0.3 to 0.5 m apart. The coke surface also helps iron oxidise.
- **Multiple-tray aerator**: 3 to 9 perforated trays, 30 to 75 cm apart, with the water distributed over the top tray.

**2. Spray aerators (fountain)**: water is pumped through nozzles 2.5 to 4 cm diameter at 70 kPa and sprayed in the air as fine drops (spray height 1 to 3 m). Gives a large contact area and good gas exchange; needs more space and head.
```text
        \ | /     spray
      .  \|/  .   droplets
        ~~~~~~~~~   <- basin (area ~ 0.03 m2 per m3/h)
```

**3. Air diffusion (bubble) aerators**: compressed air is bubbled through diffusers (perforated pipes or porous plates) at the bottom of a tank, with 0.3 to 0.75 m³ of air per m³ of water and contact time 10 to 30 min.
```text
   water in -> |~~~~~~~~~~~~~~~| -> out
               |  o  o  o  o   |
               |==diffuser====|<- air
```

**4. Mechanical aerators**: rotors, paddles or turbines stir the water surface to draw air in.

**5. Packed (forced-draft) towers**: air and water are brought into counter-current contact in a tower filled with packing, with the air blown in by a fan. This gives the highest efficiency for gas removal.

## Check yourself

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

1. Aeration is best defined as:
   A. Adding chlorine to water
   B. Bringing water and air into close contact to dissolve oxygen and release unwanted gases
   C. Filtering water through sand
   D. Removing hardness with lime
   Answer: B. Aeration exchanges gases: oxygen dissolves in water while CO2, H2S and volatile substances escape. (Key point: Definition of aeration)

2. Why is aeration used in treating water containing iron and manganese?
   A. It makes them more soluble
   B. It converts them to chlorides
   C. It oxidises Fe2+ and Mn2+ to insoluble compounds that can be settled and filtered
   D. It removes them directly as gas
   Answer: C. Oxygen oxidises soluble ferrous and manganous ions into insoluble hydroxides or oxides. (Key point: Iron and manganese)

3. What characterises a spray (fountain) aerator?
   A. Water is sprayed through nozzles as fine drops, giving large contact area but needing head and space
   B. Compressed air is bubbled from the tank bottom
   C. Water flows over trays of coke
   D. Rotors stir the surface
   Answer: A. Spray aerators pump water through nozzles into the air; good gas exchange but they need more space and head. (Key point: Spray aerator)

4. In an air diffusion aerator, how is oxygen transferred?
   A. Water falls over steps
   B. Rotating paddles beat the surface
   C. Compressed air is bubbled through diffusers at the bottom of the tank
   D. Water is passed through a sand bed
   Answer: C. Diffusers release fine air bubbles through the water during a contact time of 10 to 30 min. (Key point: Air diffusion aerator)

## Related in Water Supply Engineering

- [Aeration purposes; Fe/Mn removal; residual chlorine](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aaeration-iron-residual-chlorine) — Water Supply Engineering, chapter 6
- [Iron and manganese: removal methods, aeration](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Airon-manganese-objectionable-aeration) — Water Supply Engineering, chapter 6
- [Service reservoirs: purpose and construction](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aservice-reservoir-purpose-construction) — Water Supply Engineering, chapter 7, asked 2×
- [Purpose of valves; reflux valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Avalves-purpose-reflux) — Water Supply Engineering, chapter 9
- [Purpose of valves; cutoff valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Avalves-purpose-cutoff) — Water Supply Engineering, chapter 9

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- [Simplex method of aeration](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asimplex-aeration-method) — Sanitary Engineering, chapter 7
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