NCEES Environmental Engineering 2 — Questions and Answers
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?
- 4.0 hours
- 8.0 hours
- 3.3 hours (Correct answer)
- 6.6 hours
Correct answer: 3.3 hours
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?
- Remove dissolved nutrients by biological uptake
- Separate suspended solids from treated effluent by gravity settling (Correct answer)
- Oxidize organic compounds using ozone
- Disinfect effluent using ultraviolet radiation
Correct answer: Separate suspended solids from treated effluent by gravity settling
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:
- Form trihalomethanes during chlorination
- Precipitate or dissolve calcium carbonate (Correct answer)
- Promote biological growth in distribution systems
- Remove heavy metals by ion exchange
Correct answer: Precipitate or dissolve calcium carbonate
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?
- High water solubility
- Low Henry's Law constant
- High sorption to soil organic matter (high Koc) (Correct answer)
- Low vapor pressure
Correct answer: High sorption to soil organic matter (high Koc)
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?
- 175 lb/day
- 292 lb/day (Correct answer)
- 350 lb/day
- 438 lb/day
Correct answer: 292 lb/day
lbs/day = flow (MGD) × dose (mg/L) × 8.34 = 10 × 3.5 × 8.34 = 291.9 ≈ 292 lb/day.
Question 6: Which air pollution control device is most effective for removing fine particulate matter (PM2.5) from industrial emissions?
- Cyclone separator
- Wet scrubber
- Fabric filter (baghouse) (Correct answer)
- Gravity settling chamber
Correct answer: Fabric filter (baghouse)
Fabric filters (baghouses) can achieve >99% removal efficiency for fine particulates including PM2.5.
Question 7: Under the Clean Water Act, a NPDES permit regulates:
- Groundwater withdrawals from aquifers
- Point source discharges of pollutants into navigable waters (Correct answer)
- Non-point source runoff from agricultural fields
- Underground injection of industrial wastes
Correct answer: Point source discharges of pollutants into navigable waters
NPDES (National Pollutant Discharge Elimination System) permits control point source discharges to surface waters.
NCEES Environmental Engineering 2
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?