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724 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters <br /> f <br /> FIGURE 11-10 <br /> .lar test used-to assess <br /> cleanliness of filtering <br /> material. If the layer of solids <br /> that forms on the top of the <br /> filter medium after shaking <br /> and settling for 30 minutes <br /> exceeds ;s in,the filter <br /> material should be rejected. <br /> The cleanliness of the sand or gravel can be assessed qualitatively in the field <br /> with a simple jar test as illustrated in Fig. 11-10. A quart jar is filled half full of the <br /> material to be tested and water is added to fill the jar_ The material is then shaken <br /> vigorously and then allowed to settle. If, after settling for 30 minutes, a perceptible <br /> layer of material [e.g., greater than t6 in (1.5 mm)] has accumulated on top of the <br /> filter material, the sand is not clean enough to be used as filtering material. <br /> EXAMPLE 11-3. EFFECT OF SAND SIZE ON FILTRATION RATE. An unwashed filter sand with <br /> an effective size of 0.3 mm is to be used in an intermittent sand filter. 1f the size of <br /> silt present in the sand is 0.03 mm, estimate the difference in acceptance rates, under <br /> saturated flow conditions, if the silt were to migrate and form a layer on the bottom of <br /> the filter. <br /> Solution <br /> 1. Determine the velocity of flow through a layer of each material using Eq. (11-3), <br /> assuming saturated flow and a unit value for the term h1L. Use a C value of 800 and <br /> assume the temperature of the wastewater is 50°F. <br /> vh = C(dlo)2hT + 10° <br /> L � <br /> a. The velocity for the 0.3-mm sand is: <br /> vh = 8Q0(0.3)2(l) 50 fi+030° = 72 m/d <br />