260.  Velocity of flow of water in plain sedimentation water tank, is normally kept
A. 3 cm/minute
B. 10 cm/minute
C. 20 cm/minute
D. 30 cm/minute

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261.  Mostly used coagulant, is
A. chlorine
B. alum
C. lime
D. bleaching powder.

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262.  Pick up the incorrect statement from the following :
A. Iron salts produce heavy flocs and hence remove more suspended matter
B. Iron salts remove hydrogen sulphides
C. Iron salts can be used over a limited range of PH values
D. Iron salts impart corrosiveness to water.

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263.  To remove very fine suspended particles from water, the method adopted is
A. screening
B. sedimentation
C. boiling
D. filtration.

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264.  The best quality of filter material is obtained from quartzite if it does not loose weight when placed in hydrochloric acid for 24 hours, more than
A.5%
B.8%
C.10%
D.12%

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265.  A slow sand filter is cleaned if its filter head is higher than
A. 10 cm to 20 cm
B. 20 cm to 40 cm
C. 40 cm to 70 cm
D. 70 cm to 120 cm

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266.  After cleaning a slow sand filter, the filtered water is not used for
A. 6 hours to' 12 hours
B. 12 hours to 18 hours
C. 18 hours to 24 hours
D. 24 hours to 36 hours

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