The method of design of filter will be clear from the following solved examples:
Example 1:
Design five slow sand filter beds from the following data for the water works of a town of population 75,000; Per capita demand =135 litres/day/capita
Rate of filtration = 210 litres/hour/m2
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Assume maximum demand as 1.5 times the average demand. Out of five units, one is to be kept as stand by and used while repairing other units.
Solution:
Example 2:
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Design set of three rapid gravity filters for treating the water at a water works, which has to supply the water to a town of population 1,00,000. Per capita demand of the town is 270 litres/day. The rate of filtration of the rapid gravity filters may be taken as 4500 litres/hour/sq.m.
Solution:
Example 3:
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Design a rapid sand filter unit for treating 5 x 106 litres/day supply for a town. The filter are to work day and night. Take 4,500 litres/m2/hour as the rate of filtration.
Solution:
Let half an hour be lost in washing, draining and returning to the service after 24 hours. Therefore, total time will be 23.5 hrs. for 4,500litre/m2/hour as the rate of filtration.
Quantity of water to be treated per hour
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= 5.20 × 106/23.5 liters
... Filter area required = 5.2 × 106/23.5 × 4.5 × 103 = 49.19 m2 say 49.2m2
Providing two units, area of each = 24.6 m2.
Therefore provide three units of 7m × 3.50 m.
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Two units of filter will always work. Third is required during repair work of any one of the other, which can also be used during emergency.