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23-Chem-B2 Environmental Engineering · May 2018

Question 4 of 7: Water Quality Measurement, BOD Test and Dissolved Air Flotation

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exam 16-Chem-B2, Environmental Engineering — May 2018. 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.

Problem 4: Water Quality Measurement, BOD Test and Dissolved Air Flotation (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) pH, conductivity and toxicity measurement

CharacteristicMeasurement technique
(a) pHDirect potentiometric measurement with a glass combination electrode/pH meter, calibrated against certified buffer standards (e.g. pH 4, 7, 10) immediately before use; continuous online probes are used for process control.
(b) ConductivityDirect measurement with a conductivity (electrical resistivity) cell/meter, calibrated against a certified KCl conductivity standard; conductivity is used as an indirect surrogate for total dissolved solids (TDS ≈ conductivity × a site-specific empirical factor, typically 0.55–0.9).
(c) ToxicityMeasured indirectly via standardized whole-effluent toxicity (WET) bioassays (e.g. Ceriodaphnia or fathead-minnow acute/chronic tests, or the Microtox® bioluminescence assay) that report an LC50/EC50 or percent-effect endpoint, since no single chemical measurement captures the combined/synergistic toxic effect of a real effluent's contaminant mixture.

(ii) BOD5 dilution test

Given.

QuantityValue
Tertiary effluent sample volume300 mL
Dilution water volume200 mL
Total mixture volume500 mL
Initial DO, DOi7 mg/L
DO after 5 days, DO52.0 mg/L
DO after 20 days (stabilized), DO200.05 mg/L

Find. The 5-day CBOD (CBOD5) and the ultimate CBOD (L0) of the undiluted tertiary effluent.

Approach. Apply the standard BOD dilution formula, scaling the bottle's DO depletion by the reciprocal of the sample fraction P = Vsample/Vtotal (no seed correction given, so none is applied); the stabilized 20-day reading is taken as the ultimate carbonaceous demand.

  1. Dilution fraction. $$P = \frac{V_{sample}}{V_{total}} = \frac{300}{500} = 0.60$$
  2. (a) 5-day CBOD of the undiluted effluent. $$CBOD_5 = \frac{DO_i - DO_5}{P} = \frac{7 - 2.0}{0.60} = \boxed{8.33\ \text{mg/L}}$$
  3. (b) Ultimate CBOD (L0), from the stabilized 20-day DO. Since the DO has stopped falling by day 20, essentially the full carbonaceous demand has been exerted, so DOi−DO20 scaled the same way is the ultimate CBOD of the undiluted sample. $$L_0 \approx \frac{DO_i - DO_{20}}{P} = \frac{7 - 0.05}{0.60} = \boxed{11.58\ \text{mg/L}}$$
  4. Consistency check (not asked, verification only). Back-calculating the first-order rate constant from CBOD5/L0 via $CBOD_5=L_0(1-e^{-k\cdot5})$ gives k ≈ 0.254 d-1, inside the typical municipal-effluent range (0.1–0.4 d-1), confirming the two readings are mutually consistent with first-order BOD kinetics.
QuantityValue
Dilution fraction, P0.60
(a) 5-day CBOD, CBOD58.33 mg/L
(b) Ultimate CBOD, L011.58 mg/L

(iii) Dissolved air flotation (DAF): main functions

The DAF unit sits between flocculation and filtration in the treatment train (rapid mix → floc tank → DAF tank → filtration), with a side-stream of clarified/filtered water pressurized through a saturator and recycled back into the DAF tank as the air source:

RapidMixingFlocTankDAFTankFiltrationSaturatorInfluent, QCoagulant +pH controlEffluent, QRecycle, Qr
Fig. 2a — Water treatment train: rapid mix → floc tank → DAF tank → filtration, with recycle taken after DAF/filters, pressurized through a saturator, and re-injected into the DAF tank.
Contact ZoneSeparation ZoneInfluent, QRecycle (Qr)+ dissolved airFroth (float) layerSkimmed floatClarified effluent, Q
Fig. 2b — DAF reactor, side-view cross-section: influent and recycle (with dissolved air) enter the contact zone; bubble-floc agglomerates float to the surface and are skimmed as froth in the separation zone; clarified effluent exits at the bottom.
Air distribution systemBubbles rising uniformly across width, attaching to flocs
Fig. 2c — DAF reactor, front-view cross-section: the air-distribution system releases micro-bubbles uniformly across the full tank width so bubble–floc attachment is not confined to one side of the reactor.
  1. Clarification (solids/floc removal). The DAF tank replaces (or precedes) a gravity-settling clarifier for light, poorly-settling floc — attaching micro-bubbles to the flocculated solids lowers their effective density below water, so they float and are skimmed rather than needing to settle.
  2. Thickening of the removed solids. The skimmed froth is a concentrated (thickened) sludge (typically several percent solids), reducing the downstream sludge-handling volume compared to the more dilute underflow of a conventional gravity clarifier.
  3. Pre-conditioning ahead of filtration. By removing the bulk of the floc load upstream, the DAF stage protects the downstream granular-media filter from premature headloss build-up/breakthrough, extending filter run length between backwashes.