23-Chem-B2 Environmental Engineering · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B2, Environmental Engineering — May 2019. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.
Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
A river intake downstream of heavily farmed land typically carries an inorganic contaminant such as nitrate (NO₃⁻) from fertilizer application, and a microbiological contaminant such as fecal coliform bacteria (or the more specific indicator, E. coli) from livestock manure and septic runoff.
| Contaminant | Standard measurement method |
|---|---|
| Nitrate (inorganic) | Ion chromatography or the cadmium-reduction colorimetric method (Standard Methods 4500-NO₃⁻), both giving a quantitative concentration traceable to a calibration curve. |
| Fecal coliform / E. coli (microbiological) | Membrane filtration (Standard Methods 9222) with selective agar incubation, reported as colony-forming units (CFU) per 100 mL. |
Both methods are applied to the treated water leaving the plant (and at the raw-water intake, to track removal across the plant), and results are compared with the Health Canada Guidelines for Canadian Drinking Water Quality: nitrate MAC 45 mg/L as NO₃⁻ (10 mg/L as nitrate-nitrogen), and no detectable E. coli per 100 mL. For the methods to show that the contaminants really are sufficiently reduced, each method's detection limit must sit well below the guideline value, sampling must follow an approved frequency and location plan, and analyses must be done by an accredited laboratory (ISO/IEC 17025). To ensure the measurement results are reliable, the laboratory program runs field/lab duplicate samples and method (or trip) blanks alongside every sampling event, and periodically analyzes certified reference standards (for nitrate) or positive/negative biological controls (for coliform culturing) to confirm the method's accuracy and detection limit remain within the certified range; results outside statistical control limits trigger re-sampling before a treatment or compliance decision is made on that data.
Given.
| Quantity | Value |
|---|---|
| Sample (tertiary effluent) volume, Vs | 300 mL |
| Dilution water added | 150 mL |
| Total bottle volume, Vt | 450 mL |
| Initial DO, DOi | 8 mg/L |
| DO after 5 days, DO₅ | 2 mg/L |
| Stabilized DO after 20 days, DO₂₀ | 0.2 mg/L |
Find. (a) 5-day CBOD (CBOD₅) of the effluent, mg/L; (b) ultimate CBOD (L₀), mg/L.
Approach. The standard BOD dilution-bottle equation scales the measured DO depletion by the inverse of the seed/sample fraction P=Vs/Vt to back out the concentration in the undiluted effluent.
| Quantity | Value |
|---|---|
| Dilution fraction, P | 0.667 |
| 5-day CBOD, CBOD₅ | 9.00 mg/L |
Approach. The question states that the DO has stabilized at 0.2 mg/L after 20 days, so the carbonaceous demand is fully exerted by day 20. The total DO drop over 20 days, divided by the dilution fraction, is therefore the ultimate CBOD directly; no rate constant has to be assumed.
CBOD₅/L₀ = 0.77 and k ≈ 0.29 d⁻¹ (base e) are within the usual range for treated municipal effluent, so the three DO readings are self-consistent.
| Quantity | Value |
|---|---|
| Ultimate CBOD, L₀ | 11.70 mg/L |
| Implied rate constant, k | 0.29 d⁻¹ (check only) |
| Design parameter | Influence on performance | Typical value (secondary, activated sludge) |
|---|---|---|
| Surface overflow rate (SOR = Q/Asurface) | Sets the maximum particle/floc settling velocity that can still be captured before the flow reaches the outlet weir; too high an SOR causes solids (and effluent TSS) carryover, especially during peak wet-weather flow. | 16–28 m³/m²·d at average flow (up to about 40–48 m³/m²·d at peak flow). |
| Weir loading rate (Q/Lweir) | Governs the approach velocity near the outlet launder; an excessive weir loading rate creates local upflow currents that re-entrain settled or settling floc just before it exits, degrading effluent quality even if the SOR itself is acceptable. | 125–250 m³/m·d. |
| Solids (mass) loading rate (kg MLSS/m²·d) | Controls whether the clarifier is solids- (rather than hydraulically-) limited: too high a mass loading causes the sludge blanket to rise and eventually carry solids over the weir, independent of the hydraulic SOR. | 4–6 kg/m²·h (roughly 100–150 kg/m²·d) for a conventional activated-sludge secondary clarifier. |
The three parameters must be checked together, not in isolation: a clarifier can pass an SOR check yet still fail on weir loading (short-circuiting near the launder) or on solids loading (blanket rise from a high MLSS/recycle rate), which is why secondary clarifier design in practice is normally governed by whichever of the three constraints is most restrictive for the plant's actual MLSS concentration and peak/average flow ratio, not by SOR alone.
