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a 11-3 SINGLE-PASS (INTERMITTENT) PACKED-BED FILTERS 723 <br /> 2. Determine the effective size and the uniformity coefficient. <br /> a. The dip and d6© sizes obtained from the plot are: <br /> djo = 0.30 mm <br /> d6o = 1.50 mm <br /> b. The UC value is: <br /> UC - dfl 0.3 = 5.0 <br /> The importance of the effective size can be appreciated by considering Hazen's <br /> formula for the velocity of flow of water through a porous medium under saturated <br /> flow conditions. <br /> 2 It <br /> vh = Qdto) L <br /> where vi, = superficial (approach) filtration velocity, m/d <br /> C = coefficient of compactness (varies from 700 to 1000 for new sand, and <br /> from 500 to 700 for sand that has been in use for a number of years) <br /> d]o = effective size of medium, mm <br /> 1 h = headloss in (ft) <br /> L = depth of filter bed or layer, m (ft) <br /> T = temperature, °F <br /> Two very important issues in the selection of a sand for use in ISFs are: (1) the <br /> use of a durable sand and(2) sand that is free of dust, fine organic particles, fine silt, <br /> and clay particles. Dirty sand has been the cause of poor operation and clogging of <br /> ISF systems on'numerous occasions. In time,,the fine material will migrate to the <br /> E bottom of the filter where it can form a restrictive layer. The impact of fine material <br /> is illustrated in Example 11-3. Ideally, the sand should be washed thoroughly and <br /> Un-dried; however, it is sometimes difficult, if not impossible, to find kiln-dried <br /> sand. Recommended sand sizes are given in Table 11-8. <br /> P <br /> TABLE 11-8 i <br /> Typical sand gradation for ISF' <br /> i <br /> r Cumulative i <br /> Size of percent passing <br /> Sieve size opening, <br /> or number mm Range Typical <br /> sr Bin 9.5 100 100 <br /> 4 4.75 40-100 99 <br /> 10 2.0 62--100. 84 <br /> 16 1.18 45-62 54 I <br /> 30 0.6 25-55 25 <br /> 50 0.3 5-20 6 <br /> 60 0.25 0-10 4 <br /> 100 0.15 0-4 Trace <br /> 'From Orenco Systems,Inc. <br />