23-Chem-B2 Environmental Engineering · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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 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.
| Step | Primary engineering design principle |
|---|---|
| (a) Sedimentation | Discrete/flocculent particles settle out of the flow under gravity; the controlling design parameter is the surface overflow rate (SOR), $\text{SOR}=Q/A_s$ (flow divided by tank surface area, not depth), so any particle whose settling velocity exceeds the SOR is captured regardless of tank depth — depth only affects detention time and sludge storage, not the ideal removal efficiency. |
| (b) Aeration | Dissolved oxygen must be supplied at a rate matching the biological oxygen demand of the mixed-liquor microbial population (typically maintained ≥2 mg/L DO) while providing enough mixing energy to keep the biomass in suspension; the design parameter is the oxygen transfer rate of the diffuser/mechanical aerator system under field (non-standard) conditions, corrected from the manufacturer's clean-water standard rating via alpha/beta/temperature/altitude factors. |
| (c) Final effluent disinfection | Pathogen inactivation is governed by the CT concept (disinfectant residual concentration × contact time) for chlorination, or delivered UV dose (mJ/cm²) for UV systems, sized against the design pathogen's inactivation kinetics; a hydraulically baffled contact chamber (minimizing short-circuiting) is essential so the design contact time is actually achieved by the bulk of the flow, not just its fastest streamline. |
The two control devices shown target physically different pollutant classes — the cyclone removes large particulate matter by inertial separation, while the scrubber removes corrosive gases by gas-liquid mass transfer — so their governing design considerations are correspondingly different.
| Equipment | Design consideration 1 | Design consideration 2 |
|---|---|---|
| Cyclone separator | Inlet velocity / cyclone diameter. Collection efficiency (via the cut diameter, $d_{50}\propto\sqrt{D_c/v_i}$) improves as the tangential inlet velocity increases and the barrel diameter decreases, since a smaller, faster vortex generates higher centrifugal acceleration on the particles — but velocity is capped by an economic pressure-drop limit and by particle re-entrainment at excessive velocity. | Dust hopper seal and vortex-finder length/diameter. An unsealed dust outlet lets the internal vortex short-circuit air back up through the collected dust, re-entraining fines and collapsing efficiency; the vortex-finder (gas outlet tube) must extend far enough into the body to prevent the incoming dusty gas from bypassing directly to the clean-gas outlet. |
| Wet scrubber | Gas-liquid contact area and liquid-to-gas (L/G) ratio. Absorption of a corrosive gas is a mass-transfer-limited process, so the packing/spray contact area and the L/G ratio must be sized against the target removal efficiency using the gas's absorption/reaction equilibrium (e.g. an alkaline scrubbing liquor for an acid gas) — under-sizing either leaves the gas under-absorbed regardless of overall flow capacity. | Materials of construction and mist-eliminator performance. The corrosive gas and its absorbed liquid product (often a strong acid, e.g. HCl) demand corrosion-resistant wetted-surface materials (FRP, PVC-lined steel); a properly sized mist eliminator at the outlet prevents liquid droplet carryover, which would otherwise re-introduce the removed pollutant downstream and corrode downstream ductwork. |
