In the primary treatment process of sewage, various suspended; floating and oily substances are removed from the sewage. The units of primary treatment are:
- Screens.
- Grit chamber.
- Detritus tank.
- Skimming tank.
- Primary sedimentation tank.
Flow diagram of Primary Treatment Process of Sewage
1) Screens
The screen is the first unit of the primary treatment plant.
The function of the screen is to remove all the floating matter of comparatively large size. Screens are located just before the grit chamber.
The screen may be constructed of M.S bars or rods, gratings, wire meshes or perforated plates. Screens are usually placed in an inclined position with an angle of 30° to 60° with the direction of flow.
Comminutors: Comminutors is the patented devices which break the larger sewage solids into small solids particles when the sewage is screened through them.
2) Grit Chamber
A grit chamber is an important segment in the primary treatment process of sewage. The grit chamber is an enlarged chamber or a long basin. They may be placed either before or after the screens.
The function of the grit chamber is to remove grit, sand and other inorganic matter from sewage before it enters the next treatment stage.
When sewage enters the grit chamber, it reduces the velocity of sewage movement through the chamber to allow heavy particles to settle down due to gravity.
3) Detritus Tank
A detritus tank is nothing but a grit chamber designed to flow with smaller flow velocity and a longer detention period ( 3 to 4 minutes).
The function of a detritus tank is to remove finer particles than those removed by a grit chamber.
4) Skimming Tank
In the primary treatment process of sewage skimming tanks are typically used for removing oil and grease from the sewage. They are placed before the primary sedimentation tanks.
The skimming tank consists of different compartments for completing different tasks. It works in several stages. The first stage is usually for the oil separation, in which oil and other floating pollutants are skimmed from the surface using special equipment such as oil skimmers or floating oil collectors. The second stage may include additional separation steps.
Overall we can say that the skimming tank is an effective and reliable process for the removal of oil and grease from sewage.
5) Primary Sedimentation Tank
It is also known as a primary clarifier or primary setting tank. The purpose of this unit is to remove the settleable solids present in the sewage. It is a rectangular tank constructed with brick masonry.
Baffle walls are provided in a zigzag way. Inlet and outlet pipes with valves are provided on opposite corners.
A sludge removal pipe is provided at the bottom of the tank.
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