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Environmental Engineering Flashcards

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  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?

    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.

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

    Answer: Separate suspended solids from treated effluent by gravity settling

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

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

    Answer: Precipitate or dissolve calcium carbonate

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

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

    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.

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

    Answer: 292 lb/day

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

  6. Which air pollution control device is most effective for removing fine particulate matter (PM2.5) from industrial emissions?

    Answer: Fabric filter (baghouse)

    Fabric filters (baghouses) can achieve >99% removal efficiency for fine particulates including PM2.5.

  7. Under the Clean Water Act, a NPDES permit regulates:

    Answer: Point source discharges of pollutants into navigable waters

    NPDES (National Pollutant Discharge Elimination System) permits control point source discharges to surface waters.

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