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756 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters <br /> the septic tank by installing a small low-rate trickling filter above the septic tank <br /> (see Cha 13). Additional details on the removal of nitrogen in ISFs and RSFs may <br /> UjJY�q e found in Sandy et al. (1987) and Whitmyer et al. (1991). <br /> PROBLEMS AND DISCUSSION TOPICS <br /> 11-1. Demonstrate that the air at atmospheric pressure and 20°C contains about 275 mg/L <br /> of oxygen. <br /> 11-2. Verify that Eq. (11-6)is correct. <br /> 11-3. Detemnine the effective size and uniformity coefficient for the following sands(to be <br /> selected by the instructor). <br /> Percent retained <br /> Sieve size <br /> or number A B C <br /> s 0 0 0 <br /> 10 1 0.1 28 <br /> 16 15 3.9 22 <br /> 30 30 26 20 <br /> 50 29 48 18 <br /> 60 19 19 8 <br /> 100 2 2.9 3 <br /> Pan 4 0.1 1 <br /> 114. Washed sand is available in the following size ranges. Determine the effective size <br /> and uniformity coefficient if equal parts of each size of sand were mixed. Prepare a <br /> size distribution plot for the final sand. <br /> Sand retained <br /> Size between indicated <br /> designation sieve sizes <br /> 6 6xi0 <br /> 8 8x14 <br /> 12 12x25 <br /> 20 20 x 40 <br /> 30 30 x 60 <br /> 50 50 x 100 <br /> 11-5. Using the data from Prob. 11-4, determine what mixture of the size ranges should <br /> be used to produce a sand with: (1) an effective size of 0.30 and a uniformity coef- <br /> ficient of 2.50, (2) an effective size of 0.38 and a uniformity coefficient of 3.0, and <br /> (3)an effective size of 0.42 and a uniformity coefficient of 3.5 (to be selected by the <br /> instructor). <br /> 11-6. Determine the effective surface area in a cubic meter of sand composed of spherical <br /> particles 0.5 mm in diameter. Assume 1 percent of the area is taken up by particle- <br /> particle contact. <br />