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PS 3 Examples Even Year

# PS 3 Examples Even Year - UNIVERSITY OF CALIFORNIA LOS...

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UNIVERSITY OF CALIFORNIA, LOS ANGELES Civil and Environmental Engineering Department CEE 153 Introduction to Environmental Engineering Science Example Problems for Problem Set # 3 _____________________________________________________________________________ 1. Water “hardness” is defined as the concentration of calcium and magnesium ions (Ca ++ and Mg ++ ). One way to “soften” water is to add a chemical that will combine with these ions to form a solid precipitate that can then be settled out. For calcium hardness, soda ash (Na 2 CO 3 ) may be added to produce the following reaction: Ca ++ + Na 2 CO 3 CaCO 3 (s) + 2Na + where the calcium carbonate CaCO 3 is in solid form. a) A water treatment plant must soften a total flow Q = 1 m 3 /sec that has a calcium ion concentration of 100 mg/L. What mass of soda ash must be added each day to remove completely the calcium hardness? [22,848 kg] b) The softened water passes into a settling basin. If the solid CaCO 3 particles in the settling basin have a settling velocity w s = 100 m/day, what should the area A of the settling basin be if the objective is to remove 80% of the CaCO 3 solids? [3456 m 2 ] 2. In a wastewater treatment plant sludge from the primary clarifier and waste-activated sludge from the secondary system are mixed and thickened in a gravity thickener. The primary sludge contains 1250 kg of dry solids per day with a 4% solids content by volume (relative to the total volume including liquid) and the waste-activated sludge contains 525 kg of dry solids per day with a solids content by volume of 1.2%. After thickening the mixture has a solids content by volume of 3%. Assume that all dry solids have a density of 1000 kg/m 3 . a) Calculate the volume of thickened sludge produced each day. [59 m 3 /day] b) Calculate the percent volume reduction in the thickener. [79% = volume after thickening/volume before thickening]

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3. A water treatment plant with a flow Q = 1 m 3 /sec adds FeCl 3 to increase the concentration of suspended solids. The reaction of FeCl 3 with water that produces the solids (in the form of Fe(OH) 3 ) is: FeCl 3 + 3H 2 0 Fe(OH) 3 + 3HCl This reaction may be assumed to happen very rapidly and to result in the formation of small particles. After FeCl 3 addition and reaction, the flow passes through a flocculation
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