23-Chem-B2 Environmental Engineering · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B2, Environmental Engineering — December 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.
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) Min. particle size / range | (b) Typical efficiency (% mass) | (c) Advantage | (d) Challenge |
|---|---|---|---|---|
| (i) Wet (venturi) scrubber | ∼0.5–1 µm (sub-micron with a high-pressure-drop venturi throat) | 90–99% for PM <1 µm at high ΔP; also removes soluble/reactive gases simultaneously | Handles hot, wet, sticky or explosive gas streams that a fabric filter cannot (no blinding/fire risk; also achieves simultaneous gas absorption). | Generates a liquid waste (scrubbant/slurry) stream that itself needs treatment/disposal, and pressure drop (energy cost) scales sharply with the fine-particle target efficiency. |
| (ii) Cyclone collector | ∼5–10 µm (coarse/mechanical collector; poor below ∼5 µm) | 70–90% for PM >10 µm, but only 30–50% for PM2.5–PM10 | Low capital cost, no moving parts, low maintenance, and useful as a pre-cleaner ahead of a finer downstream device (ESP/baghouse) to reduce its dust loading. | Cannot economically meet modern fine-particulate (PM2.5) emission limits alone — must be paired with a polishing device for fine dust. |
| (iii) Thermal oxidizer | Not a particulate-capture device; applies to gas-phase VOC/organic-aerosol destruction (no particle-size metric) | 95–99.9%+ destruction and removal efficiency (DRE) for combustible organics, achieved via the “3 T's” (temperature, time, turbulence) | Destroys the pollutant (mineralizes to CO2/H2O) rather than merely transferring it to another phase, so there is no secondary liquid or solid waste stream to manage (barring acid-gas scrubbing for halogenated feeds). | High fuel/energy cost to maintain combustion temperature continuously, and halogenated feeds need a downstream acid-gas scrubber to avoid trading a VOC problem for an HCl/Cl2 emission. |
