Environmental Engineering

Simple 8 questions .It will be done in no time.The answer keys are there just need the solution.

EG-CE 441 Environmental Engineering

Homework #4
(Due 10/29/2013)

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

Ca+2 = 120; Mg+2 = 30, HCO3- = 70, CO2 = 10.
Ca+2 = 220; Mg+2 = 75, HCO3- = 265, CO2 = 17.