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CE 321 Exam 2: Water and Wastewater Treatment, Exams of Civil Engineering
Exam questions from a fall semester 2003 civil engineering course focusing on water and wastewater treatment. Topics include water quality assessment using do and bod, conductivity, and coliform tests. Treatment system design covers clarifiers, batch reactors, and plug flow reactors. Drinking water treatment processes such as surface water filtration, coagulation, flocculation, and backwashing are also discussed.
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DEPARTMENT OF CIVIL & ENVIRONMENTAL ENGINEERING, LAFAYETTE COLLEGE
Exam 2, CE 321 Name:
Fall Semester 2003
(show your work; use backs of pages if necessary – ask if you need clarification about a problem)
Water Quality Assessment
- (5 pts) DO is used as a general indicator of water quality. List four factors that affect the measured DO of a stream or river.
- (10 pts) A BOD test is run on 5 mLs of wastewater in a standard 300-mL BOD bottle. The DO drops from 9 mg/L to 5 mg/L over the 5-day test. The degradation rate constant is determined to be 0.22 day-1.
What is the BOD 5 (mg/L)? (ans = 240 mg/L)
What is the BOD and DO after 10 days (mg/L)? (ans = 360 mg/L, 3.67 mg/L)
How long would the BOD test have to run to degrade 95% of original organic matter of the sample (days)?
(ans =14 days)
- (5 pts) What does the conductivity (or specific conductance) value tell you about a water sample? Why is it important to understand the geology of the area when using conductivity data to infer water quality/pollution?
[Note: Qs on water quality other than DO/BOD/DO sag and bacteria will not be on the 2008 test]
- (5 pts) The total and fecal coliform tests are used as indicators of pathogens. Explain what is meant by “indicators” in this context.
Treatment System Design
- (5 pts) Match each of the following with batch, PFR, or CSTR
No mixing: Stage 1 sludge digestion tank at Easton WWTP:
No flow: Clarifier at Easton WTP:
Clearwell at Easton WTP:
- (10 pts) Two parallel clarifiers for a 10 MGD water treatment plant are to be designed with surface loading (overflow) rates of 1000 gal/day per ft^2 , weir loading rates of 20,000 gal/day per ft of weir, and hydraulic retention times (HRT) of 3 hours. What is the required surface area (ft^2 ), length of weir (ft), and depth (ft) of the two clarifiers? What is the average vertical velocity of the water in the clarifiers (ft/sec)?
Ans: As = 5000 ft^2 each, Lw = 250 ft each, d = 16.7 ft, v = 0.00155 ft/s
- (10 pts) A plug flow reactor (PFR) is used to treat a steady waste stream of 500 L/min containing 100 mg/L of contaminant, which undergoes a second-order reaction with a rate constant of 0.9 L/mg-hr. If the PFR has a volume of 10 m^3 , what is the effluent concentration (mg/L)?
(Ans = 3.23 mg/L)
Drinking Water Treatment
- (5 pts) What is the “Surface Water Filtration Rule”?
- (10 pts) Explain how the processes of coagulation and flocculation aid in the settling of suspended particles.
- (5 pts) What is the purpose of backwashing, and how is it accomplished?
- (5 pts) If EPA increases the discharge standard for water treatment plants from 99% removal to 99.9% removal of bacteria, how will this affect the required contact time for treatment in a plug-flow reactor? Use a reaction rate constant of k = 2 hr -^.
(Ans = Increase by 1.15 hrs or 50%)
The plant superintendent suggests that the new regulations can be met by reducing short-circuiting in the existing reactor. What does he mean by this?
Wastewater Treatment
- (5 pts) Define the following terms and briefly state why they are important in wastewater treatment.
F/M ratio:
Industrial Pretreatment Program:
Grit chamber:
Part 503:
Return activated sludge:
- (5 pts) List and briefly describe three methods of secondary wastewater treatment
- (5 pts) Why is it necessary to a) digest and b) dewater wastewater sludges?
- (10 pts) A wastewater plant generates 250,000 gal per day of sludge with 50,000 mg/L solids and 5 mg/L of cadmium , which it dewaters to a concentration of 200,000 mg/L solids. There is no cadmium in the water removed from the sludge.
How much water (tons/year) has been removed from the sludge? ( Ans = 285,500 tons/yr )
What is the concentration of cadmium in the dewatered sludge (mg Cd/kg of solids)? ( Ans = 100 mg/kg )
Extra Credit (2 pts): What team has lost 2 of the last 3 World Series?
What does the acronym WET represent in the context of a wastewater treatment plant NPDES permit?