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Primary Treatment Process of Sewage
In the primary treatment process, 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 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
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. Grit chambers are that any can be stopped for the purpose of cleaning.
3) Detritus Tank
Detritus tank is nothing but grit chamber designed to flow with smaller flow velocity and longer detention period ( 3 to 4 minutes).
The function of detritus tank is to remove finer particles than those removed by a grit chamber.
4) Skimming Tank
Skimming tanks are employed for removing and grease from the sewage. They are placed before the primary sedimentation tanks.
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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