Factors that affect use of direct filtration in treating surface waters

This paper evaluates direct filtration as a treatment method for water from five sources in Virginia. The most effective filtration scheme consisted of a three-minute rapid mix with alum and a cationic polymer used iri combination as primary coagulants. The rapid mix was followed by filtration at 3.5 mm/s (5 gpm/sq ft) through 51 cm (2Q in.) of 1.3-mm effective size (e.s.) anthracite coal and 25 cm (10 in.) of 0.45-mm e.s. silica sand. This configuration produced filtered water that met the AWWA goal for turbidity with filter run times of at least eight hours when color and turbidity in the raw water were less than 15-20 units.

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