A conventional water treatment plant currently treats of river water for a drink

Question

A conventional water treatment plant currently treats of river water for a drinking water supply. The plant contains filters that are 6 m × 10 m in dimension. The filters operate on a 48 h cycle with 12 min for backflushing, 10 min for draining and air injection, and 18 min for rinsing. If water is applied at 4.80 L·m-2·s-1 during operation, and this value is doubled during backflushing:

b. If the plant utilizes 10 filters, what is the maximum flow it can produce?

c. If the plant needs to expand its capacity from this maximum flow by 25%, how many new filters need to be added?

d. Due to space constraints, the plant managers want to know if it would be possible to increase the filter loading rate by investing in new filtration media and deepening the filtration beds. If no new filter beds are added, what filter loading rate would be required to meet the new demand?

e. Since you know that rapid sand filtration loading rates are confined, for practical reasons, to 5 – 25 m3·m-2·hr-1, do you think it is reasonable to use the manager’s idea in part (d)?

Explanation / Answer

Area of one filter: 60 m2 The flow rate is 4.80 L. m-2.s

b) We have 10 filters, i.e 600 m2 ares for that we can produce maximum flow of, 4.80 * 600 = 2880 Liter per second flow.

c) If plant needs to expand by 25% then it needs to increase the maximum flow generation to 2880*1.25 = 3600 liter per second.

Every m2 can generate 4.80 liter per second so for 3600 liter per second we will need, 3600/4.80 = 750 m2 area.

So increase in area is, 750-600 = 150 m2 for 150 m2 we will need approximately 3 more filters.

d) to have the 3600 liter per second filter rate from 10 filter we need 3600/600 = 6 L. m-2s-1 filter loading rate.

e) The filter loading rate is limited to 25 m3 m-2 hr-1 i.e. 25000/3600 = 6.94 L. m-2 s-1

The filter rate we will change to is  6 L. m-2s-1 which is below the permissible limits for practical application.

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