Homework
1. (Problem 6-13) Determine the lime and soda ash dose, in mg/L as CaCO3, to soften the following
water to a final hardness of 80 mg/L as CaCO3. The ion concentrations reported below are all mg/L as CaCO3: Ca+2 = 120; Mg+2 = 30, HCO3- = 70, CO2 = 10.
Answer: 100 and 40mg/L respectively 2. (Problem 6-16) Determine the lime and soda ash dose, in mg/L as CaCO3, to soften the following
water to a final hardness of 70 mg/L as CaCO3. The ion concentrations reported below are all mg/L as CaCO3: Ca+2 = 220; Mg+2 = 75, HCO3- = 265, CO2 = 17.
Answer: 352 and 0, respectively 3. (Problem 6-30) What is the volume required for a rapid-mix basin that is to be used to treat 0.05
m3/s of water if the detention time is 10 seconds? Answer: 0.5 m3 4. (Problem 6-31) Two parallel flocculation basins are to be used to treat a water flow of 0.150 m3/s.
If the design detention time is 20 minutes, what is the volume of each tank? Answer: 90 m3 5. (Problem 6-45) If the settling velocity of a particle is 2.80 mm/s and the overflow rate of a
“horizontal” clarifier is 0.56 cm/s, what percent of this kind of particles are retained in the clarifier?
Answer: 50% 6. (Problem 6-47) If the flow rate in Problem 5 is doubled, what percent removal of this kind of
particles would be expected? Answer: 25% 7. (Problem 6-54) For a flow of 0.8 m3/s, how many rapid sand filter boxes of dimensions 10m × 20
m are needed for a loading rate of 110 m3/d•m2? Answer: 4 filters (rounding to the nearest even number) 8. A water plant superintendent tells you the four dual media filters (each 5m × 10m) are loaded at a
velocity of 280 m/d. What is the flow rate through the filters in m3/s? Answer: 0.65
9. The average flow rate of a wastewater treatment plant is 20 MGD and the peak factor is 2.0. Design the sedimentation tanks by using the following information:
• Hydraulic retention time = 90 minutes (based on peak flow) • Hydraulic loading rate = 800 gpd/ft2 (based on peak flow) • Weir loading rate = 18,000 gpd/ft • Number of tanks = 10 (in parallel) • Length : width = 7:1
Answer: L:W:H = 187’ : 26.7’ : 6.7’, Weir length 222’. 10. A 5-MGD surface water supply is treated with conventional filtration followed by chlorine
disinfection. The chlorine dose is controlled to be less than 2 mg/L to avoid total trihalomethane (a disinfection by-product, or DBP) problems. The chlorine demand is 60% of the dosage. Estimate the maximum free available chlorine concentration (in mg/L).
Answer: 0.8