Effect of Backwash on Filter Effluent Quality

by Robert L. Johnson,
John L. Cleasby,


Serial Information: Journal of the Sanitary Engineering Division, 1966, Vol. 92, Issue 1, Pg. 215-228


Document Type: Journal Paper

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Abstract: The effect of backwash rate and backwash duration on the quality of water produced by sand filters has been investigated on a plant scale. The best water quality was produced following backwash rates of 16.7 gpm per sq ft in filters with sand of effective size of 0.64 mm, even though the average sand expansion was only 17%. This observation is supported by a theoretical evaluation of the forces acting on the sand during backwash. Therefore, the early idea that 50% expansion is required for good backwashing is not supported by this research. With coarser sands being more widely used today (1966), this will mean savings in cost of backwash piping, equipment, and in filter underdrain systems. It was observed that the turbidity of the backwash water alone is not a valid indicator of the cleanliness of the sand.

Subject Headings: Backwashing | Sand filters | Water quality | Soil water | Soil analysis | Coarse-grained soils | Sandy soils

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