Activated sludge :

Activated sludge process is a type of waste water treatment process for treating sewage or industrial waste water using aeration and a biological floc composed of bacteria and protozoa.

The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items: An aeration tank where air (or oxygen) is injected in the mixed liquor. This is followed by a settling tank (usually referred to as "final clarifier" or "secondary settling tank") to allow the biological flocs (the sludge blanket) to settle, thus separating the biological sludge from the clear treated water.

Types of aeration system :

• Diffused air aeration
• Mechanical aeration
• Combine aerator

Diffused air aeration:

A compressed air is blown through the sewage and air is diffused in the sewage by diffuser.

Mechanical aeration:

In this method the surface of sewage is agitated violently with the help of some mechanical equipment to encourage absorption of oxygen from atmosphere. There are two well known form of mechanical aerator vertical surface aerator and horizontal surface aerator.

Combined aerator:

In this system, diffused air aeration and mechanical aeration is combining in a single unit. The well known type of such combination is dorroco aerator. The aeration of sewage is done by air diffuser as well as mechanical aerator.