23-Chem-B2 Environmental Engineering · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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.
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.
| Technology | (a) Key design parameter | (b) Typical efficiency (% mass) | (c) Maintenance issue |
|---|---|---|---|
| (i) Electrostatic precipitator | Specific collection area and migration velocity, via the Deutsch–Anderson relation $\eta = 1-e^{-wA/Q}$ (collection area A and migration velocity w sized to gas flow Q for the target efficiency). | 95–99.9% for fine PM at properly sized collection area (drops sharply if flue-gas resistivity drifts out of the 104–1010 Ω·cm window). | Rapping cycle (mechanical/electric) must be tuned to dislodge the collected cake into the hopper on schedule — too infrequent causes re-entrainment of built-up dust, too frequent re-entrains freshly collected fines. |
| (ii) Wet scrubber | Pressure drop across the contacting zone (e.g. venturi throat ΔP), which sets the gas-to-liquid relative velocity and hence droplet size/inertial-impaction efficiency for the target particle size. | 70–99%, strongly particle-size dependent (low-ΔP spray towers ~70–90% for coarse PM; high-ΔP venturi 95–99% down to sub-micron). | Scaling/plugging of nozzles and demister mist-eliminator from dissolved solids in the recirculated liquor — requires blowdown/makeup water management and periodic nozzle inspection. |
| (iii) Fabric filter (baghouse) | Air-to-cloth (face velocity) ratio, sized to the dust/media pair (typically 0.6–1.2 m/min, about 2–4 ft/min, for pulse-jet) so the dust cake itself becomes the fine-particle filtering medium. | 99–99.9%+ for PM of essentially any size once a cake has formed (highest of the three technologies). | Bag replacement/leak monitoring — a single torn bag can dominate total emissions, so continuous opacity or bag-leak detection per compartment is needed to catch failures between scheduled bag changes. |
