23-Chem-B2 Environmental Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, May 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, nutrient removal; 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 — fabric filtration, thermal oxidation, adsorption, 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 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.
| Contaminant | Treatment component | Key design principle |
|---|---|---|
| Settleable particulates | Coagulation/flocculation/sedimentation (clarifier) | A metal-salt coagulant (alum/FeCl₃) destabilizes colloidal turbidity, flocculation basins (sized by the velocity-gradient product $G\cdot t$) grow settleable floc, and the clarifier is sized on surface overflow rate (SOR, typically 15–30 m/d) so particles have time to settle before carryover to the next unit. |
| Dissolved solids | Granular media / membrane filtration — or reverse osmosis for a genuinely dissolved (ionic) load | Dissolved species are not removed by settling at all; RO membrane flux and required feed pressure are sized against the feed stream's total dissolved solids (osmotic pressure) and target permeate quality, with recovery limited by scaling of sparingly-soluble salts concentrating in the reject. |
| Microbial contaminants | Disinfection (chlorination or UV) following filtration | The CT concept (disinfectant concentration × contact time) or, for UV, delivered dose (mJ/cm²) is sized against the design pathogen's known inactivation kinetics, with a hydraulically baffled contact chamber ensuring the design contact time is actually achieved (avoiding short-circuiting). |
Given.
| Quantity | Symbol | Value |
|---|---|---|
| Secondary effluent sample volume | $V_s$ | $300\ \text{mL}$ |
| Dilution water volume | $V_d$ | $200\ \text{mL}$ |
| Initial DO of mixture | $D_0$ | $7\ \text{mg/L}$ |
| DO after 5 days | $D_5$ | $1.5\ \text{mg/L}$ |
| DO after 20 days (stabilized) | $D_{20}$ | $0.2\ \text{mg/L}$ |
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.
| Quantity | Result |
|---|---|
| Dilution factor | $P=0.6$ (3/5) |
| 5-day CBOD | 9.17 mg/L |
| Ultimate CBOD | 11.33 mg/L |
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.
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.
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.
