Water Contaminant Characterization, BOD and Sedimentation: 23-Chem-B2 Environmental Engineering, December 2019, Q6

Problem 6: Water Contaminant Characterization, BOD and Sedimentation (20 marks)

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.

(i) Groundwater-under-surface-water-influence contaminants in a farming community

Contaminant	Type	Treatment method
Nitrate (NO₃⁻), from fertilizer/manure runoff infiltrating the aquifer	Chemical	Anion-exchange (nitrate-selective resin) or biological denitrification, both sized on the influent nitrate concentration and required removal to meet the drinking-water MAC.
Cryptosporidium/Giardia (oo)cysts, from livestock manure reaching the surface-water-influenced aquifer	Microbiological	Granular media filtration (physical removal) followed by UV disinfection (chemically resistant (oo)cysts are inactivated by UV dose even though they resist chlorine).

Measurement confirmation combines a process-performance indicator (continuous turbidity monitoring on the filter effluent, since (oo)cyst removal correlates strongly with turbidity/particle removal) with periodic direct analyte testing (laboratory nitrate analysis by ion chromatography against the maximum acceptable concentration, and UV-dose validation/UV-transmittance monitoring for the disinfection step) — the continuous surrogate catches short-term treatment upsets in near-real time, while the periodic direct analysis confirms the specific health-based limit is actually being met, since neither measurement alone can verify both operational stability and regulatory compliance.

(ii) CBOD₅ test

Given.

Quantity	Value
Sample volume (secondary effluent), V_s	250 mL
Dilution water added	250 mL
Total mixture volume, V_t	500 mL
Initial DO, DO_i	10.0 mg/L
DO after 5 days, DO₅	3.0 mg/L
DO after 20 days (stabilized), DO₂₀	0.5 mg/L

Find. (a) 5-day CBOD (CBOD₅), mg/L. (b) Ultimate CBOD (L₀), mg/L.

Approach. With nitrification inhibited, all bottle oxygen depletion is carbonaceous demand; dividing by the dilution factor P scales the diluted-bottle depletion back to the demand of the undiluted effluent, and the stabilized 20-day reading is taken directly as the ultimate carbonaceous demand.

Dilution factor. $P = \frac{V_s}{V_t} = \frac{250}{500} = 0.500$
5-day CBOD. $\text{CBOD}_5 = \frac{DO_i - DO_5}{P} = \frac{10.0 - 3.0}{0.500} = \boxed{14.00\ \text{mg/L}}$
Ultimate CBOD. $L_0 = \frac{DO_i - DO_{20}}{P} = \frac{10.0 - 0.5}{0.500} = \boxed{19.00\ \text{mg/L}}$

Quantity	Value
Dilution factor, P	0.500
5-day CBOD, CBOD₅	14.00 mg/L
Ultimate CBOD, L₀	19.00 mg/L

The ratio CBOD₅/L₀≈0.737 implies a first-order rate constant k≈0.27 d⁻¹ (from L₀(1−e^−5k)=CBOD₅), squarely within the typical municipal 0.1–0.3 d⁻¹ range — reported as informational context only and does not change either boxed result.

(iii) Primary sedimentation design parameters

Parameter	How it influences performance	Typical design value
Surface overflow rate (SOR)	The upward flow velocity a particle must out-settle to be captured; SOR (not detention time) is the fundamental sizing parameter for an ideal settling basin because capture depends only on the particle's settling velocity relative to the basin's overflow rate, not tank depth.	≈ 32–48 m³/m²·d (average flow) for primary clarifiers.
Weir loading rate	Flow per unit length of effluent weir; an excessive weir loading rate creates high approach velocities near the weir that can re-entrain and carry over settled/settling floc before it reaches the sludge hopper.	≈ 125–500 m³/m·d.
Detention time	Average hydraulic residence time in the basin; must be long enough for the design particle to actually traverse the basin depth at its settling velocity, and also provides time for flocculation of fine particles into larger, faster-settling aggregates.	≈ 1.5–2.5 hours.

Topic: Groundwater-under-surface-water-influence treatment; CBOD dilution-test calculation; primary-clarifier design parameters.

Key relations: CBOD=(DO_i−DO_f)/P; ideal settling-basin theory (capture depends on SOR, not depth); weir loading rate vs. approach-velocity carryover.

Why this works: Groundwater under the direct influence of surface water is regulated like surface water precisely because it can carry the same pathogen/agricultural-contaminant risk, which is why both a physical (filtration) and disinfection (UV) barrier are required rather than relying on the aquifer's natural filtration alone; ideal settling theory shows tank capture efficiency is independent of depth given a fixed SOR, which is why SOR (not detention time alone) is the primary sizing parameter for primary clarifiers.

Common pitfall: Forgetting to divide by the dilution factor P (using the raw bottle DO depletion as "the" BOD); assuming a deeper tank alone improves removal without also checking SOR and weir loading, when in ideal settling theory basin depth does not change removal efficiency at a fixed SOR.

Source: Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 4 (groundwater under the influence) and Ch. 5 (sedimentation theory); Metcalf & Eddy, Wastewater Engineering, 4th ed., Ch. 2 (BOD test) and Ch. 5 (primary clarifier design).

Question 6 of 7: Water Contaminant Characterization, BOD and Sedimentation

Problem 6: Water Contaminant Characterization, BOD and Sedimentation (20 marks)

(i) Groundwater-under-surface-water-influence contaminants in a farming community

(ii) CBOD₅ test

(iii) Primary sedimentation design parameters