Correct Answer - Option 4 : Triangular tank with radial flow
Concept:
A sedimentation tank is structure in which wastewater is filled and stored for some time to remove the suspended particles present in the water by the action of gravitational forces. These particles may settle at the bottom of the tank and are removed by using scrapers.
Based on shape, a sedimentation tank has three types:
1. Circular tank: In this case influent is sent through central pipe of the tank and radial flow takes place. Mechanical sludge scrappers are provided to collect the sludge and collected sludge is carried through sludge pipe provided at the bottom.
2. Rectangular tank: Rectangular sedimentation tanks are mostly preferred sedimentation tanks and are used widely. The flow takes place in horizontal direction that is length wise in rectangular tanks
3. Hopper bottom tank: In case of hopper bottom tank, a deflector box is located at the top which deflects the influent coming from central pipe to downwards. Sludge is collected at the bottom and it is disposed through sludge pump.
The triangular tank with radial flow is not used for plain sedimentation tank.
A. Types of Sedimentation Tanks based on Location:
1. Primary Sedimentation Tank
Primary sedimentation tank is a normal sedimentation tank in which water is stored at rest for some time and sludge collected at bottom and oily matter collected at top are removed.
2. Secondary Sedimentation Tank
After activated sludge process the wastewater enters secondary sedimentation tank in which suspended particles contains microbes are removed .
B. Types of Sedimentation tanks based on Methods of Operation
1. Fill and Draw Type Sedimentation Tank
In case of fill and draw type sedimentation tank, water from inlet is stored for some time. The 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.
2. Continuous Flow Type Sedimentation Tank
In this case, water is not allowed to rest. Flow always takes place but with a very small velocity. During this flow, suspended particles are settled at the bottom of the tank. The flow may be either in horizontal direction or vertical direction.
