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23-Chem-B2 Environmental Engineering · December 2015

Question 4 of 7: Water-quality parameter measurement, BOD dilution-test analysis, and dissolved air flotation

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

Notes on this paper

Paper format. EGBC 04-Chem-B2 Environmental Engineering, December 2015, 3 hours, closed-book with a candidate-prepared double-sided 8½×11-inch aid sheet. Seven problems, each worth 20 marks; candidates attempt any five, and only the first five answers in the workbook are marked. All seven problems are solved below as a complete study resource.

Reference texts: G. Tchobanoglous, F. L. Burton & H. D. Stensel (Metcalf & Eddy), Wastewater Engineering: Treatment and Reuse (4th ed., McGraw-Hill) — BOD kinetics, dissolved air flotation, activated-sludge design; M. L. Davis & D. A. Cornwell, Introduction to Environmental Engineering (5th ed., McGraw-Hill) — drinking-water treatment, air pollution control, ion exchange, reverse osmosis, soil remediation; C. D. Cooper & F. C. Alley, Air Pollution Control: A Design Approach — cyclones, scrubbers, fabric filtration, electrostatic precipitation, odour control; S. P. Turner, Workbook of Atmospheric Dispersion Estimates (2nd ed., CRC Press) — the Gaussian plume model and Pasquill–Gifford stability classes. Canadian context follows the Canadian Environmental Protection Act (CEPA 1999), the Canadian Council of Ministers of the Environment (CCME) Municipal Wastewater Effluent and Drinking Water Quality guidelines, and provincial air/water permitting practice (e.g. BC Environmental Management Act, Metro Vancouver air-quality bylaws).

Question 4: Water-quality parameter measurement, BOD dilution-test analysis, 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) Measurement method and significance of turbidity, conductivity and pH

ParameterHow it is measuredFundamental significance
(a) TurbidityNephelometric turbidimeter — a light beam is passed through the sample and the intensity of light scattered at 90° is compared against a formazin-standard calibration curve, reported in NTU.An indirect surrogate for suspended/colloidal particulate load; strongly correlated with filtration performance and, in drinking water, with pathogen (especially Cryptosporidium/Giardia) risk, since particles can shield organisms from disinfection.
(b) ConductivityA conductivity probe (two electrodes, or an inductive cell) measures the sample's electrical conductance, reported in µS/cm, calibrated against a standard KCl solution.A rapid surrogate for total dissolved ionic solids (TDS ≈ conductivity × an empirical factor, typically 0.55–0.7); used to flag saline intrusion, road-salt runoff, or process-water contamination in near-real time without a full ion analysis.
(c) pHA glass-membrane pH electrode measures the potential difference generated by the hydrogen-ion activity gradient across the membrane, referenced to a calibration buffer pair.Governs speciation and toxicity of many contaminants (e.g. un-ionized ammonia, metal solubility), corrosivity of the water toward distribution infrastructure, and the effectiveness of downstream chemical treatment (coagulation, disinfection) that is itself pH-dependent.

Given.

QuantitySymbolValue
Secondary effluent sample volume$V_s$$250\ \text{mL}$
Dilution water volume$V_d$$250\ \text{mL}$
Initial DO of mixture$D_0$$6\ \text{mg/L}$
DO after 5 days$D_5$$0.4\ \text{mg/L}$
DO after 20 days (stabilized)$D_{20}$$0.1\ \text{mg/L}$

Find. (a) The 5-day CBOD of the undiluted secondary effluent, CBOD₅. (b) The ultimate CBOD, CBOD∞.

Approach. The observed DO depletion occurs inside a diluted mixture, so it is scaled by the dilution fraction $P$ (sample volume/total mixture volume) to recover the neat-effluent BOD; with nitrification inhibited, the 20-day plateau is taken directly as the ultimate carbonaceous demand.

  1. Dilution fraction. $$P=\frac{V_s}{V_s+V_d}=\frac{250}{250+250}=0.5$$
  2. 5-day CBOD. $$\text{CBOD}_5=\frac{D_0-D_5}{P}=\frac{6-0.4}{0.5}=11.2\ \text{mg/L}$$ ==**CBOD₅ = 11.2 mg/L**==
  3. Ultimate CBOD. The 20-day DO has stabilized, so essentially all carbonaceous demand has been exerted: $$\text{CBOD}_\infty\approx\frac{D_0-D_{20}}{P}=\frac{6-0.1}{0.5}=11.8\ \text{mg/L}$$ ==**CBOD∞ = 11.8 mg/L**==
QuantityResult
Dilution factor$P=0.5$ (1/2)
5-day CBOD11.2 mg/L
Ultimate CBOD11.8 mg/L
Check — blank correction and rate-constant assumption

A dilution-water blank DO depletion is normally subtracted to correct for oxygen demand from the dilution water itself; none is supplied here, so it is assumed negligible. Reading the 20-day plateau directly as CBOD∞ assumes the first-order decay constant $k_1$ is large enough that the reaction is essentially complete by day 20 — reasonable for a well-treated secondary effluent.

(iii) Dissolved air flotation (DAF) — mechanism, schematic and key equations

A slipstream of clarified subnatant is pressurized and saturated with air in a contact tank, then released back to atmospheric pressure through a control valve at the flotation-tank sparge; the sudden pressure drop precipitates the dissolved air as a cloud of fine microbubbles (30–100 µm) that attach to the influent's suspended solids/oil droplets, lowering their effective density below that of water so they rise and are skimmed from the surface, rather than settling as in conventional sedimentation.

Flotation Tank(surface scraper)RecyclePumpAir Contact Tank(level control)InfluentTreatedeffluentrecycle drawpressurized recycleCompressor(air)P/F control valve -> recycle injection sparge (tank bottom)
Fig. 3: Dissolved air flotation (DAF) schematic. A slipstream of clarified subnatant is recycled through a pump to a pressurized air-contact tank; the pressure/flow control valve drops the saturated recycle back to atmospheric at the tank-bottom sparge, precipitating fine microbubbles that float the influent solids to the surface for scraper removal.

Key engineering equations. Henry's law governs how much air the recycle stream can hold at the saturator pressure: $$S_a=1.3\,s_a\,(fP-1)$$ where $S_a$ is the air released per unit recycle volume (mL air/L), $s_a$ is air solubility at 1 atm and process temperature, $f$ is the fraction of saturation achieved (typically 0.5–0.8), and $P$ is the saturator absolute pressure (atm). The resulting air-to-solids ratio, the primary DAF design parameter, is $$\frac{A}{S}=\frac{S_a\,R}{Q\,X}$$ with $R$ the recycle flow, $Q$ the influent flow, and $X$ the influent suspended-solids concentration; typical design values are $A/S\approx0.02$–$0.04$ for waste-activated-sludge thickening, achieved with a saturator pressure of 275–480 kPa and a recycle ratio $R/Q$ of roughly 0.3–1.5.