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Type of settling tank - Coggle Diagram
Type of settling tank
Location
Primary
Water is stored at rest for some time and sludge collected at bottom and oily matter collected at top are removed.
After primary sedimentation process, the wastewater is discharged into aerobic filter where activated sludge process take place
Secondary
Suspended particles containing microbes are removed and are reflected towards aerobic filter to maintain high microbe concentration in aerobic filter
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Shape
Circular
Mechanical sludge scrappers are provided to collect the sludge and collected sludge is carried through sludge pipe provided at the bottom.
Preferred for continuous vertical flow type sedimentation tanks. In this case influent is sent through central pipe of the tank and radial flow takes place.
But circular tanks are uneconomical as compared to rectangular tanks but they have high clarification efficiency.
Rectangular
Maintenance costs are low in case of rectangular sedimentation tanks. They are also suitable for large capacity plants.
Flow takes place in horizontal direction that is length wise in rectangular tanks. Sometimes baffle walls are provided for rectangular tank to prevent short circuiting.
Hopper Bottom
Deflector box is located at the top which deflects the influent coming from central pipe to downwards.
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Methods of Operation
Fill & draw type tank
Water from inlet is stored for some time, may be 24 hours. In that time, the suspended particles are settled at the bottom of the tank.
After 24 hours, the water is discharged through outlet. Then settled particle are removed. This removal action requires 6-12 hours.
So, one complete action of sedimentation requires 30-40 hours in case of fill and draw type sedimentation tank.
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