Period of cleaning of slow sand filters is about________________?
(A) 24 – 48 hours
(B) 10-12 days
(C) 2-3 months
(D) 1-2 year
Period of cleaning of slow sand filters is about________________?
(A) 24 – 48 hours
(B) 10-12 days
(C) 2-3 months
(D) 1-2 year
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