Filter Sand [with Discussion]

formity coefficient" which are in use today. As Baylis has pointed out, however, Hazen's work was primarily applicable to "slow" rather than to "rapid" sand filters. Sand employed in the former was often unwashed and ungraded and hence contained a relatively high per cent of "fines" and the sizes extended over a wide range. It was not uncommon to have an effective size as low as 0.2 mm. with a uniformity coefficient of 3.0 or higher. With these conditions, Hazen found that the fine sand (0 to 10 per cent) had about as much effect upon filtration efficiency as practically all the other sizes combined. This is natural. Today, the "fines" (retained on 150, 100 and 65 mesh), as well as the coarse sand (retained on 12 and 14 mesh screens) are eliminated by the producer through washing and screening. Under these circumstances, effective size and uniformity coefficient lose much of their original significance. There are two screen scales available. The clear opening between meshes of the "square root of 2 series" increases in the ratio of 1.414. This is entirely adequate for filter sand analysis. The greater refinement enabled by the "fourth root of 2 series" (1.189) is not warranted. According to the theory of hydraulic subsidence, backwashing should grade sands within a layer, leaving the coarsest material at the bottom and the finest at the top.