DEPARTMENT OF CIVIL & ENVIRONMENTAL ENGINEERING; LAFAYETTE COLLEGE
Homework Assignment 7, Due Oct 30, 2008
CE 321, Fall 2008
1. At the Easton Water Treatment Plant on the day of our visit, four parallel rectangular
clarifiers, each 140 ft long, 17 ft wide, and 18 ft deep were being used to settle solids from an
approximately 8 MGD flow. Calculate the HRT and surface loading rate per tank.
2. A 1.5 million gallon clearwell operating as a plug-flow reactor (PFR) is used to disinfect a
flow of 8 MGD, resulting in a 99.9% bacteria removal. What would the % removal be if short-
circuiting of flow in the tank reduced the HRT by 2 hrs?
3. Hypochlorous acid (HOCl) dissociates into hypochlorite ions (OCl-) and hydrogen ions (H+)
during chlorination. As discussed in class, the equilibrium constant for this reaction is 10-7.5.
Plot the ratio of [OCl-] to [HOCl] as a function of pH ranging from 6 to 9. Given that HOCl is a
much more effective disinfectant than OCl-, which is more advantageous for disinfection:
slightly acidic or slightly basic water?
4. A major natural disaster has occurred and you need clean drinking water. There is no
sunshine and your stove isn’t working, but you find a container of liquid bleach. How much
bleach (in teaspoons) should you add to a 5-gal bucket of water for a OCl- concentration of 2
mg/L in the water? (Notes: the bleach is 6% by weight NaOCl, the density of the bleach is ~1.1
g/mL, 1 teaspoon = 5 mL)
5. A groundwater supply of 3 MGD has 120 mg/L of Ca+2 and 50 mg/L of Mg+2. Using the
following two softening reactions, how much lime (Ca(OH)2) in kg must be added per day to
precipitate the Ca+2 and Mg+2? (you may assume that HCO3- is plentiful and will not limit the
precipitation reaction)
OHCaCOOHCaHCOCa s2)(323
222)(2 +→++ −
+
OHOHMgCaCOOHCaHCOMg s
s2
)(
2)(323
22)(2)(22 ++→++ −
+
6. Problem 10-10 [Design problem]
7. Problem 10-11 [Design problem]
8. Problem 10-16 (use the data in Table 10.4) [Design problem]