Civil Engineering PE Water and Wastewater Treatment 3

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?

Accepted Answer

3,338 lb/day

Answer

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?

Accepted Answer

Facultative lagoon

Answer

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?

Accepted Answer

Non-carbonate (permanent) hardness caused by calcium sulfate or chloride

Answer

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?

Accepted Answer

~2.1 days

Answer

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?

Accepted Answer

254 nm (germicidal UV-C)

Answer

UV-C at approximately 254 nm is most germicidal because it is absorbed by DNA, causing thymine dimer formation and inactivating microorganisms.

Civil Engineering PE Practice Test

Civil Engineering PE Practice Test

Civil Engineering PE Water and Wastewater Treatment 3