1. EG-CE 441 Environmental Engineering
Homework #5 (Due 11/14/2013)
1. Use the design information below to answer the questions regarding a sedimentation
tank:
flow rate to the tank = 1 MGD
hydraulic retention time = 2 hours
surface overflow rate (SOR) = 1,000 gpd/ft2
weir loading rate = 15,000 gpd/ft
The minimum required surface area of the sedimentation tank = 1000 ft2
The liquid depth in the sedimentation tank = 11.1 ft.
The minimum length of weir = 66.7 ft.
The following three problems are adapted from “Fundamentals of Engineering (FE)
Sample Questions” by National Council of Examiners for Engineering and Surveying
(NCEES), 1996).
2. Routine operation and maintenance processes in a water treatment plant include chemical
feed, mixing, coagulation, flocculation, sedimentation, filtration, chlorination,
fluoridation, pH adjustment, and pumping. You are asked to review the treatment
processes for a 3.0 MGD plant and provide answers for the following equations.
(a) The gas chlorination system is set to feed chlorine at a rate of 16 lb/day into a daily
water flow of 3,000,000 gallons for Spring operation. The chlorine residual is 0.4
mg/L. Estimate the chlorine demand (mg/L). Answer: 0.24
(b) The water treatment plant uses filters that are 10 ft in diameter. If the filters are
backwashed at 15 gpm/ft2 for 12 minutes, estimate the amount (gallons per wash) of
wash water used. Answer: 14,130 gal
3. A groundwater source is treated for removal of iron and hydrogen sulfide to supply
potable water for a community with a population of 4,000. If chlorine gas (Cl2) is added
at a dose of 2 mg/L, estimate the quantity (pounds) of chlorine that must be added to treat
15,000,000 gallons of water. Answer: 250 lbs
4. A lift station must be sized to serve a subdivision with a design population of 1,500. The
average per capita wastewater flow rate of the region is estimated at 90 gcpd (gallons per
capita per day). Assuming the peak flow factor is 4, the firm pumping capacity (gpm) for
this lift station is most nearly: 375.