Civil Engineering PE Water and Wastewater Treatment 2 — Questions and Answers
Question 1: A water treatment plant uses rapid sand filtration. The design filtration rate is 4 gpm/ft². If the filter box is 20 ft × 30 ft, what is the design flow through this single filter?
- 1,200 gpm
- 2,400 gpm (Correct answer)
- 600 gpm
- 4,800 gpm
Correct answer: 2,400 gpm
Flow = rate × area = 4 gpm/ft² × (20 × 30) ft² = 4 × 600 = 2,400 gpm.
Question 2: Which disinfection byproduct (DBP) is primarily formed when chlorine reacts with natural organic matter (NOM) in drinking water?
- Chloramines
- Trihalomethanes (THMs) (Correct answer)
- Perchlorates
- Nitrosamines
Correct answer: Trihalomethanes (THMs)
Trihalomethanes (THMs) such as chloroform are the primary DBPs formed when free chlorine reacts with NOM.
Question 3: In a conventional activated sludge system, the sludge volume index (SVI) is 120 mL/g and the MLSS is 3,000 mg/L. What is the settled sludge volume after 30 minutes?
- 250 mL/L
- 360 mL/L (Correct answer)
- 480 mL/L
- 120 mL/L
Correct answer: 360 mL/L
SVI = (settled volume mL/L × 1,000) / MLSS mg/L, so settled volume = SVI × MLSS / 1,000 = 120 × 3,000 / 1,000 = 360 mL/L.
Question 4: A clarifier has a surface overflow rate of 800 gpd/ft². If the flow is 1.5 MGD, what is the required surface area?
- 1,875 ft² (Correct answer)
- 1,500 ft²
- 1,200 ft²
- 2,500 ft²
Correct answer: 1,875 ft²
Area = Q / SOR = 1,500,000 gpd / 800 gpd/ft² = 1,875 ft².
Question 5: Which of the following best describes the purpose of alkalinity in coagulation during water treatment?
- It increases turbidity to improve floc formation
- It provides bicarbonate/carbonate ions that react with coagulant to form floc (Correct answer)
- It reduces pH below 5 to precipitate metals
- It provides nutrients for biological growth
Correct answer: It provides bicarbonate/carbonate ions that react with coagulant to form floc
Alkalinity (bicarbonate/carbonate) reacts with alum or ferric coagulants to form hydroxide floc and release CO₂.
Question 6: In wastewater treatment, what does a Return Activated Sludge (RAS) ratio of 0.5 mean?
- 50% of the influent flow is returned from the clarifier (Correct answer)
- The sludge age is 0.5 days
- The F/M ratio is 0.5
- Half the sludge is wasted daily
Correct answer: 50% of the influent flow is returned from the clarifier
RAS ratio = RAS flow / influent flow, so 0.5 means the return flow is 50% of the incoming flow.
Question 7: The CT concept in disinfection represents:
- Total chlorine residual times turbidity
- Disinfectant concentration times contact time (Correct answer)
- Carbon treatment efficiency times time
- Coagulant dose times treatment volume
Correct answer: Disinfectant concentration times contact time
CT = disinfectant concentration (mg/L) × contact time (min), used to ensure adequate inactivation of pathogens per the Surface Water Treatment Rule.
Civil Engineering PE Water and Wastewater Treatment 2
A water treatment plant uses rapid sand filtration.
The design filtration rate is 4 gpm/ft².
If the filter box is 20 ft × 30 ft, what is the design flow through this single filter?