Civil Engineering PE Water and Wastewater Treatment 3 — Questions and Answers
Question 1: A wastewater treatment plant has a BOD₅ of 200 mg/L and a flow of 2 MGD. What is the BOD load entering the plant in lb/day?
- 3,338 lb/day (Correct answer)
- 400 lb/day
- 1,000 lb/day
- 2,000 lb/day
Correct answer: 3,338 lb/day
Load (lb/day) = concentration (mg/L) × flow (MGD) × 8.34 = 200 × 2 × 8.34 = 3,336 ≈ 3,338 lb/day.
Question 2: Which type of lagoon uses algae to supply oxygen to heterotrophic bacteria for BOD removal?
- Anaerobic lagoon
- Aerated lagoon
- Facultative lagoon (Correct answer)
- Polishing lagoon
Correct answer: Facultative lagoon
Facultative lagoons have an aerobic upper zone where algae supply oxygen via photosynthesis and an anaerobic lower zone for sludge digestion.
Question 3: In a water softening process using lime-soda ash, what does the addition of soda ash (Na₂CO₃) primarily remove?
- Carbonate hardness caused by Ca²⁺ and Mg²⁺
- Non-carbonate (permanent) hardness caused by calcium sulfate or chloride (Correct answer)
- Iron and manganese
- Total dissolved solids
Correct answer: Non-carbonate (permanent) hardness caused by calcium sulfate or chloride
Soda ash supplies CO₃²⁻ to precipitate non-carbonate (permanent) hardness cations like Ca²⁺ associated with sulfate or chloride.
Question 4: What is the mean cell residence time (MCRT or SRT) if the aeration basin volume is 500,000 gallons, MLSS = 2,500 mg/L, daily sludge wasting = 5,000 lb/day, and effluent TSS is negligible?
- ~2.1 days (Correct answer)
- ~4.2 days
- ~6.3 days
- ~8.4 days
Correct answer: ~2.1 days
Mass in basin = 500,000 gal × 2,500 mg/L × 8.34×10⁻⁶ lb·L/(mg·gal) ≈ 10,425 lb; SRT = 10,425/5,000 ≈ 2.1 days.
Question 5: Ultraviolet (UV) disinfection of wastewater is most effective at which UV wavelength range?
- 200–230 nm
- 254 nm (germicidal UV-C) (Correct answer)
- 300–320 nm (UV-B)
- 365–400 nm (UV-A)
Correct answer: 254 nm (germicidal UV-C)
UV-C at approximately 254 nm is most germicidal because it is absorbed by DNA, causing thymine dimer formation and inactivating microorganisms.
Question 6: In a trickling filter, the hydraulic application rate (HAR) is typically expressed in units of:
- lb BOD/1,000 ft³/day
- gallons per minute per square foot
- million gallons per day per acre (Correct answer)
- gpm/ft² of cross-section
Correct answer: million gallons per day per acre
Hydraulic application rate for trickling filters is expressed as MGD/acre (or sometimes gpm/ft²), while organic loading is in lb BOD/1,000 ft³/day.
Question 7: Which parameter is used to measure the oxygen-depleting strength of industrial wastewater when biological methods are not applicable?
- Total suspended solids (TSS)
- Chemical oxygen demand (COD) (Correct answer)
- Biochemical oxygen demand (BOD₅)
- Total organic carbon (TOC)
Correct answer: Chemical oxygen demand (COD)
COD uses a strong chemical oxidant (dichromate) to measure total oxidizable matter, including non-biodegradable compounds, making it suitable for toxic industrial wastewaters.
Civil Engineering PE Water and Wastewater Treatment 3
A wastewater treatment plant has a BOD₅ of 200 mg/L and a flow of 2 MGD.
What is the BOD load entering the plant in lb/day?