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Environment & Health

Question
6 out of 72
 

Which of the following is not feature of rapid sand filter (DNB June 09)



A Bacteriological quality of water is superior to that of slow sand filter

B Occupies less space

C Effective size of the partical is 0.4 - 0.7 mm

D Rate of filtration is 5 – 15 cu m / sq m / hour

Ans. A Bacteriological quality of water is superior to that of

slow sand filter Ref: Park; 22nd edi, pg – 660

Characteristics of a rapid sand filter are as follows:

Water+ alummixing chamberflocculation chambersedimentation tankfilterchlorine addedclear water storageconsumption.

Rate of filtration is 5-15 m3 / m2/ hour

Effective size of sand particle is 0.4-0.7mm

Back washing: cleaning of filter.

  1. Treatment by chemical coagulation & sedimentation
  2. Before the water comes to the filter it is subjected to a process of coagulation with alum.
  3. The filter bed is essentially similar to slow sand filter with two differences:
  4. The sand is coarser
  5. The biological membrane in slow filter is replaced by a layer of alum floc
  6. The rate of filtration in a rapid filter is 4000- 7500 liters per square meter as against 100 – 400 liters in a slow sand filter .
  7. Removes 98-99% bacteria
  8. Occupies very little space
  9. Advantages: can deal with raw water directly, filter beds occupy less space, filtration is rapid, washing of filters is easy, more flexibility in operation

Disadvantage: Bacteriological quality is inferior to that of slow sand filter.

Comparison of rapid and slow sand filters [Pg-663 (Table-3)]

Rapid sand filter

Slow sand filter

1. Space:

2. Rate of Filtration:

3. Effective size of sand:

4. Preliminary treatment:

5. Washing:

6. Operation:

7. Removal of bacteria:

Occupies very little space

200 m.g.a.d

0.4-0.7mm

Chemical coagulation & sedimentation

By back-washing

98-99 per cent

Occupies large area

2-3 m.g.a.d

0.2-0.3 mm

Plain sedimentation

By scraping sand bed

99.9-99.99 per cent

Environment & Health Flashcard List

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