NCEES Environmental Engineering 2

Question 1

A wastewater treatment plant uses activated sludge. The food-to-microorganism (F/M) ratio is 0.3 kg BOD/kg MLVSS·day and the MLVSS concentration is 2,500 mg/L. What is the hydraulic retention time (HRT) if the influent BOD is 250 mg/L?

Accepted Answer

3.3 hours

Answer

HRT = (BOD_in) / (F/M × MLVSS) = 250 / (0.3 × 2500) = 0.333 days = 8 hours — wait, HRT = S0 / (F/M × X) = 250/(0.3×2500) = 0.333 days ≈ 8 h; recalc: 250/(750) = 0.333 days = 8 h, so answer is 8.0 hours.

Question 2

Which of the following best describes the purpose of a clarifier in a wastewater treatment system?

Accepted Answer

Separate suspended solids from treated effluent by gravity settling

Answer

A clarifier uses gravity settling to separate suspended solids (sludge) from the liquid effluent.

Question 3

The Langelier Saturation Index (LSI) is used to assess water's tendency to:

Accepted Answer

Precipitate or dissolve calcium carbonate

Answer

LSI = pH − pH_s; a positive value indicates scaling (CaCO₃ precipitation), a negative value indicates corrosion.

Question 4

In a soil contamination remediation project, pump-and-treat is selected. Which contaminant characteristic makes this method LEAST effective?

Accepted Answer

High sorption to soil organic matter (high Koc)

Answer

Contaminants with high Koc sorb strongly to soil particles, causing tailing and rebound, which makes pump-and-treat less effective over time.

Question 5

A 10 MGD municipal water treatment plant adds 3.5 mg/L of chlorine. How many pounds of chlorine are added per day?

Accepted Answer

292 lb/day

Answer

lbs/day = flow (MGD) × dose (mg/L) × 8.34 = 10 × 3.5 × 8.34 = 291.9 ≈ 292 lb/day.

NCEES Practice Test

NCEES Practice Test

NCEES Environmental Engineering 2