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.
| Technology | (a) Min. particle size / range | (b) Typical efficiency (% mass) | (c) Advantage | (d) Challenge |
|---|---|---|---|---|
| (i) Gravimetric settlers | Effective mainly >50–75 µm (coarse particulate only) | Low, ≈50–70% for coarse PM; negligible for PM10/PM2.5 | Very low capital and operating cost, virtually no moving parts, negligible pressure drop. | Large footprint required for adequate settling residence time; ineffective as a stand-alone control for fine/respirable particulate, so almost always used only as a coarse pre-cleaner ahead of a higher-efficiency device. |
| (ii) Electrostatic precipitators | Effective down to <1 µm (sub-micron range) | High, typically 95–99.5%+ (multi-field units can exceed 99.9%) | Very low pressure drop for the efficiency achieved (low ongoing fan-energy cost), can handle very high gas volumes and elevated temperatures. | Efficiency is strongly sensitive to particle/dust electrical resistivity — high-resistivity dust causes "back corona" that collapses collection efficiency, requiring conditioning (e.g. SO₃ injection) or a different technology. |
| (iii) Fabric filters | Effective down to sub-micron range, including PM2.5 | Very high, typically >99% across essentially the full particle-size range | Consistently very high efficiency largely independent of particle electrical properties (unlike ESP), robust across load variability. | High pressure drop (higher fan-energy cost than ESP) and bag material limits on temperature/chemical compatibility; bags are consumable and require periodic cleaning-cycle management and replacement. |
Synthesis. Read across the rows, the three devices form a ladder of increasing capture of fine particulate: a gravimetric settler relies only on gravity and so is limited to coarse dust, an ESP adds an electrical force that reaches sub-micron sizes at low pressure drop, and a fabric filter physically sieves the gas through a dust cake for the highest and most size-independent efficiency. In practice the choice follows the dust and the gas: a settler is used as a cheap pre-cleaner ahead of either of the other two, an ESP suits very large, hot gas volumes with moderate-resistivity dust (e.g. utility boilers), and a fabric filter suits fine or high-resistivity dust where the gas temperature and chemistry are within the bag material's limits. Under Canadian practice, the fine-particulate (PM2.5) limits set by provincial permits and the Canadian Ambient Air Quality Standards usually rule out a settler as the final control device.
