Laserfiche WebLink
21-106 WATER ENGINEERING <br />INn nW OuTF:.Ow <br />Fig. 21-79. Longitudinal section through an ideal settling basin. <br />Some design criteria for sedimentation tanks are: <br />Period of detention -2 to 8 h <br />Length -to -width ratio of flow-through channel -3:1 to 5:1 <br />Depth of basin -10 to 25 ft (l5 ft average) <br />Width of flow-through channel—not over 40 ft (30 ft most common) <br />Diameter of circular tank -35 to 200 ft (most common, 100 ft) <br />Flow-through velocity—not to exceed 1.5 ft/min (most common velocity, 1.0 ft/min) <br />Surface loading or overflow velocity, gal per day per ft2 of surface area—between 500 and 2000 <br />for most settling basins <br />(American Water Works Association, "Water Quality and Treatment," McGraw-Hill Book <br />Company, New York; G. M. Fair, J. C. Geyer, and D. A. Okun, "Water and Wastewater <br />Engineering," John Wiley & Sons, Inc., New York.) <br />21-65. Coagulation -Sedimentation <br />Plain sedimentation is discussed in Art. 21-64. To increase the settling rate and remove finely <br />divided particles in suspension, coagulants are added to the water. Without coagulants, finely <br />divided particles do not settle out because of their high ratio of surface area to mass and the <br />presence of negative charges on them. The velocity at which drag and gravitational forces are <br />equal is very low, and the negative charges on the particles produce electrostatic forces of repul- <br />sion that tend to keep the particles separated and prevent agglomeration. When coagulating <br />chemicals are mixed with water, however, they introduce highly charged positive nuclei that <br />attract and neutralize the negatively charged suspended matter. <br />