Preliminary Treatment of Sewage 

The sewage treatment process consists of four steps

1.Preliminary treatment 
2.Primary treatment 
3.Secondary treatment 
4.Final or advanced treatment 

From all the above steps preliminary treatment is discussed here. Preliminary treatment is mainly used to separate the floating materials such as dead animals, woods, rags ( organic matter), and also heavy inorganic materials. This process involves Screenings, Grit chamber, and Skimming tanks. Preliminary treatment is sometimes considered as apart of primary treatment. 


This is the first step of a sewage treatment plant and it uses different screens to remove the floating matters from the sewage. The floating matters can be paper, wood, kitchen wastes, or fecal wastes. The purpose of screening is to remove large size material so that they will not choke the pipes. The position of the screen is in front of the grit chamber. 


They are also known as shredders. They are used to break the large solids present in the sewage and make them as small as 6mm size. The basic comminutor includes a screen and cutting teeth.

Grit chamber-

These are the sedimentation tank placed after the fine screen but before the primary settling tank. The grit chamber removes sand or gravel if present in the sewage. Detritus tank is a type of grit chamber. It is rectangular in shape and has a longer retention period of 3-4 minutes.
Grit Chamber

Skimming tank- 

This tank helps in removing oil and grease. They are placed before the primary sedimentation tank. If oil and grease are not removed from the sewage it can form a scum on the surface of the tank.  

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