Aeration is an important unit operation in which the principle of gas transfer is used. The process of exposing large surface of water to the atmospheric air is called aeration. Its main purpose is to absorb more oxygen and let out unpleasant odours and gases and oxidise Fe and Mn in water.

Objectives of Aeration

• To increase oxygen contents in water for imparting freshness.
• To expel volatile substances and gases like $H_2$S, C$O_2$ causing bad taste and odour.
• To oxidise iron and manganese so that these can be precipitated and removed.
• To destroy bacteria to some extent, by agitation of water during aeration.

Methods of Aerations

• By using Fountains, Spray Nozzles Water is discharged under the effect of fountain action, in the form of fine spray. Spraying results in the formation of extremely minute droplets of water. This increases its surface area of contact with air. Spray nozzles are located in a pool of water. They involve considerable loss of head. The pressure required to operate spray nozzles is about 0.7 to 1.4 kg/c$m^2$. This removes C$O_2$ and $H_2$S to the extent of 90%. Spray nozzles may be of fixed type or movable type as shown in the Fig.

Gravity or free fall aerators

(a) Cascade Aerators : They are the simplest free fall aerators. A simplest cascade consists of a series of 3 or 4 steps, either circular or straight type as shown in Fig.. Water is allowed to fall through a height of 1 to 3 metres. These may be in the open air or under shelter.

Limitations of Aeration

• It is inefficient to remove or reduce tastes and odours caused by

(a) Non-volatile substances like oils of algae.

(b) Chemicals discharged in industrial wastes.

• Due to over oxygenation, water becomes corrosive and de-aeration may be required.

• Aeration is economical only in warmer climate months.

• Possibility of air-borne contamination in water.

• Iron and Manganese can be precipitated by aeration only when organic matter is absent