The continuous flow type of sedimentation tanks are widely adopted at present and hence, it becomes essential to study in detail some of the design aspects of these tanks. Following are the aspects of continuous flow sedimentation tanks: 1. Velocity of Flow 2. Capacity of Tank 3. Miscellaneous Considerations.
1. Velocity of Flow:
The velocity of flow of water in sedimentation tanks should be sufficient enough to cause hydraulic subsidence of suspended impurities. It should remain uniform throughout the tank and it is generally not allowed to exceed 15 cm/min to 30 cm/min.
2. Capacity of Tank:
Following are the two methods by which capacity of a sedimentation tank is determined:
(i) Detention Period, and
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(ii) Overflow Rate.
(i) Detention Period:
As stated earlier, the theoretical time taken by a settling tank is known as “detention period”, the relation between capacity and the detention period can be established as follows:
T is the detention period in hours.
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The value of detention period depends on the quality of suspended impurities present in water. For plain sedimentation tanks, t varies from 4 to 8 hrs and when coagulants are used, it may vary from 3 hrs to 4 hrs.
In this method, the depth of settling tank is to be taken into consideration the depths usually provided for settling tanks vary from 3.0 m to 6.0 m.
(ii) Overflow Rate:
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In this method, it is assumed that the settlement of a particle at the bottom of tank does not depend on the depth of tank. But it depends on the surface area of the tank.
From the eqn. (iii) it can be stated that the quantity of water passing per hour per unit area of settling tank is known as “surface overflow rate” (S.O.R.). For plain rectangular sedimentation tanks, the overflow rate is about 500-750 lit/hr/m2 and when coagulants are used, it is about 1000 to 1250 lits/hr/m2 of plan area.
3. Miscellaneous Considerations:
In addition to the main design aspects, some miscellaneous considerations should also be properly paid attention to:
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(i) Construction of Tank:
The tanks should be watertight and should be provided with roofs, if water from them is to be supplied directly for consumption.
(ii) Free board:
The vertical distance between top of wall and water level in the tank is known as free board and a depth of about 30 cm to 60 cm should be kept as free board.
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(iii) Sludge Capacity:
For the collection of sludge, it is necessary to provide enough space at the bottom of tank. A depth of about 50 cm to 60 cm is provided for this purpose.
(iv) Sludge Removal:
In order to remove sludge, the working of tank is stopped and water from the tank is drained out. The sludge is removed by gravity or pumping the compressed air and manual labour may be applied, if necessary. The frequency of sludge removal will depend upon the amount of suspended matter retained from water in the settling tank. Generally, the settling tanks are cleaned at an interval of 1 to 3 months.
Following solved examples will clearly illustrate the design of simple sedimentation tanks:
Example 1:
A water has to purify the water for a town whose daily demand is 9 × 106 litres/day. Design the suitable sedimentation tank of the water works fitted with mechanical sludge remover. Assume the velocity of flow in the sedimentation tank as 22 cm/minute and the detention period as 8 hours.
Solution:
Example 2:
Design a sedimentation for a water works, which supplies 1.4 x 106 litre/day water to the town. The sedimentation period is 5 hours, the velocity of flow is 12 cm/minute, depth of water in the tank is 4.0 m. Assuming an allowance for sludge is to be made as 80 cm.
Solution:
Example 3:
Design a circular sedimentation tank Jilted with mechanical sludge remover for a water work which has to supply daily 4.2 million litres of water to the town. The detention period in the tank for water is 4.5 hours, and the depth of the water in the tank may be assumed us 3.3 m.
Solution:
Example 4:
If a rectangular sedimentation tank is treating 2.5 × 106 litres/day.
The size of the tank is 17.5 × 5.5 × 3.5 m. If 80 p.p.m. suspended solids are present in the water, assuming the 75% removal in the basin and the average specific gravity as 2.0, determine the following:
(i) Average flow of water through tank.
(ii) Detention time.
(iii) Deposition of the solids in the tank.
(iv) Overflow rate.
Solution:
