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23-Chem-B2 Environmental Engineering · May 2016

Question 3 of 7: Water Contaminants, BOD and Flotation

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

Notes on this paper

National Exam 04-Chem-B2, Environmental Engineering — May 2016. 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 3: Water Contaminants, BOD and 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) Surface-water contaminants and measurement

A common inorganic contaminant needing treatment from surface water is suspended clay/silt mineral particulate (measured as turbidity, which is also the standard surrogate for coagulation/filtration performance); a common microbiological contaminant is the protozoan Cryptosporidium (chlorine-resistant, so filtration performance is the real barrier). Turbidity is measured continuously with an in-line nephelometric turbidimeter (NTU), which feeds forward to coagulant/filter-aid dosing and back to filter-to-waste decisions. Cryptosporidium/pathogen risk is assessed indirectly via particle counting (size-resolved counts in the 3–7 µm oocyst range as a real-time surrogate) and confirmed on periodic large-volume raw-water samples by USEPA Method 1623 (cartridge filtration, immunomagnetic separation and immunofluorescence microscopy, reported as oocysts per litre); the oocyst concentration sets the log-removal credit the plant must achieve. (Coliform indicators alone are not adequate here, because oocysts survive chlorination that kills coliforms.)

Accuracy and precision are maintained by calibrating the turbidimeter against certified formazin/StablCal standards on a defined schedule, running duplicate samples and method (equipment) blanks with every batch of microbiological analyses, and participating in an external inter-laboratory proficiency-testing program — the standard QA/QC triad (calibration, replicate/blank checks, external audit) required under Standard Methods for both instrumental and culture-based measurements.

(ii) BOD test

The BOD-bottle DO drop measures the diluted mixture's oxygen depletion; scaling by the dilution fraction recovers the demand of the undiluted primary effluent. The 20-day plateau (no further DO drop) is taken as the point at which essentially all of the carbonaceous demand has been exerted, i.e. DO depleted at day 20 ≈ ultimate CBOD (L0).

Given.

QuantityValue
Primary effluent sample volume150 mL
Dilution water volume100 mL
Total mixture volume250 mL
Initial DO, DOi7 mg/L
DO after 5 days, DO52 mg/L
DO after 20 days (stabilized), DO200.2 mg/L

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

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).

  1. Dilution (seeding) fraction. $$P = \frac{V_{sample}}{V_{total}} = \frac{150}{250} = 0.60$$
  2. 5-day CBOD of the undiluted effluent. $$CBOD_5 = \frac{DO_i - DO_5}{P} = \frac{7 - 2}{0.60} = \boxed{8.33\ \text{mg/L}}$$
  3. 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. $$L_0 \approx \frac{DO_i - DO_{20}}{P} = \frac{7 - 0.2}{0.60} = \boxed{11.33\ \text{mg/L}}$$
  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 ≈ 0.266 d-1, well inside the typical municipal-effluent range (0.1–0.3 d-1), confirming the two readings are mutually consistent with first-order BOD kinetics.
QuantityValue
Dilution fraction, P0.60
5-day CBOD, CBOD58.33 mg/L
Ultimate CBOD, L011.33 mg/L

(iii) Flotation (dissolved-air flotation) design

Selecting dissolved-air flotation (DAF) as the treatment system in the figure (feed enters a contact zone, air-saturated recycle is injected, a froth "concentrate" is skimmed from the surface, and clarified "pulp" leaves as underflow to the next cell/stage):

FeedPressurized recycle+ dissolved air(entrained or forced)Froth / Concentrate layerConcentrate(skimmed)Pulp(to nextflotation cell)Contact zoneSeparation zone
Fig. 3 — DAF flotation cell, section view (contact zone with recycle/air injection, separation zone, skimmed froth concentrate, clarified underflow).
  1. Air-to-solids (A/S) ratio. Bench/pilot testing sets the A/S ratio (typically 0.02–0.06 kg air/kg solids) that gives adequate bubble attachment to the floc; this sizes the recycle-pressurization system (saturator pressure and recycle flow fraction).
  2. Hydraulic loading and retention time. The surface overflow rate (typically 4–10 gpm/ft²) and contact/separation retention time (roughly 20–60 minutes total) are sized so bubble–floc agglomerates have time to rise to the surface before the clarified underflow exits.
  3. Float (concentrate) removal. A skimmer/scraper mechanism and its cycle frequency are designed to remove the accumulating froth layer without re-entraining solids into the clarified stream, and the recycle pump/saturator are sized with maintenance access for fouling/air-binding.