In practice three types of sedimentation tanks are used, which are as follows: 1. Rectangular Tanks 2. Circular Tanks 3. Hopper Bottom Tanks.
Type # 1. Rectangular Tanks:
These are rectangular in plan and consist of large number of baffle walls. The function of baffle walls is to reduce the velocity of incoming water to increase the effective length of travel of the particle and prevent the short- circuiting.
These tanks are generally provided with channel type inlet and outlet extending on the full width. The floor between two baffles is made like a hopper sloping towards centre where sludge-pipe is provided. The sludge is taken out through sludge outlet under hydrostatic force by operating the gate-valve.
Type # 2. Circular Tanks:
These are generally not used in plain sedimentation, but are mostly used in sedimentation with coagulation.
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There are two types of circular sedimentation tanks classified on the basis of flow of water inside it:
(i) Radial Flow Circular Tank:
The section through a typical type of radial flow circular tank has been shown in Fig. 11.7. The water enters in this tank through the central inlet pipe placed in-side the deflector box.
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The deflector box deflects the water down wards and then it goes out through the holes provided in the bottom sides of the deflector box as shown in Fig 11.7. The water flows radially from the deflector box towards the circumference of the tank, where an outlet is provided on the full periphery as shown. All the suspended on the sloppy floor and clear water goes through the outlet.
The sludge is removed by scrapper (known as raking arm) which continuously moves around the floor at a very small velocity. The maximum velocity of raking arm does not exceed 4.5 metres/hour.
(ii) Circumferential Flow Circular Tank:
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Fig. 11.8 shows the plan of a circumferential flow circular tank. Water enters in the tank through two or three vertical slits. There is one rotating arm in the tank, which allows the water to move along the circumference of the tank.
Water while moving at very low velocity allows its suspended impurities to settle in the tank, which can be removed from sludge outlet. The clear water is drawn over a small weir type outlet shown in Fig. 11.8. The length of this outlet weir should not exceed 1/8th circumference of the tank.
Type # 3. Hopper Bottom Tanks:
These are vertical flow tanks, because water flows upward and downward in these tanks. The water enters in these tanks from the top into deflector box. After flowing downward inside the deflector box the water reverses its direction and starts flowing upward around the deflector box as shown by arrows in Fig. 11.9.
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The suspended particles having specific gravity more than one, cannot follow the water at the time of reversing its direction, and settle in the bottom, from where they are removed through sludge outlet pipe under hydrostatic pressure.
Rows of decanting channels are provided at the top to collect the clear water. The water after flowing in the channel is taken out from the outlet channel provided on one side of the tank. The sectional elevation of a typical type of hopper-bottom settling tank is shown in Fig. 11.9. These tanks are mostly used in sedimentation with co-angulation process.
