23-Chem-B2 Environmental Engineering · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B2, Environmental Engineering — December 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.
Three main causes. (1) Abundant NOₓ emissions (chiefly NO from vehicle and industrial combustion) that photolyze under UV sunlight to release atomic oxygen, initiating ozone formation. (2) Abundant reactive volatile organic compounds (VOCs, from vehicle exhaust, solvent evaporation and industrial sources) that react with the OH radical/NOₓ cycle to regenerate NO₂ without consuming ozone, driving net ozone accumulation. (3) Meteorological conditions that trap the precursors near ground level long enough to react — strong sunlight (photolysis energy), low wind speed, and a temperature inversion capping vertical mixing, all of which are common in urban basins during summer high-pressure systems.
| Control method | Advantage | Disadvantage / challenge |
|---|---|---|
| Vehicle catalytic converters + tighter tailpipe NOₓ/VOC standards (hard engineering) | Directly cuts both precursor classes at their largest urban source (mobile fleet), with a well-proven, mass-deployed technology. | Only affects new vehicles going forward — fleet turnover takes years to a decade, and does nothing for the existing on-road fleet or non-road engines. |
| Vapour-recovery / low-VOC product substitution at stationary sources (hard engineering) | Reduces the reactive-VOC precursor pool from fuel dispensing, industrial solvent use and consumer/coating products, a source category that is otherwise very diffuse and hard to regulate. | Effectiveness depends on widespread compliance across many small, dispersed sources, which is harder to inspect/enforce than a small number of large point sources. |
| Urban transportation planning / transit-oriented development (soft engineering) | Reduces total vehicle-kilometres travelled (and therefore total precursor mass), addressing the emission at its root cause rather than per-vehicle, with co-benefits for congestion and greenhouse-gas emissions. | Slow to implement (decades for land-use/transit infrastructure change) and politically difficult, so it cannot deliver near-term air-quality relief on its own. |
Three key causes of odorous emissions. (1) Anaerobic decomposition of septic wastewater and screenings generates hydrogen sulfide (H₂S) from sulfate reduction, the dominant "rotten egg" odour source at headworks. (2) Ammonia and volatile amines released from protein/urea decomposition in the raw wastewater and captured screenings. (3) Volatile organic sulfur compounds (mercaptans, dimethyl sulfide) from microbial breakdown of organic matter, released as the screenings are mechanically agitated and exposed to air during raking/compaction.
Two control technologies achieving >99% removal.
| Technology | How it achieves >99% removal |
|---|---|
| Chemical (wet) scrubber — caustic/hypochlorite stages | Captured headworks off-gas is contacted counter-currently with a caustic (NaOH) stage to strip H₂S as a soluble sulfide, followed by an oxidizing hypochlorite stage to destroy residual H₂S and mercaptans — a two-stage system is routinely designed and operated to exceed 99% H₂S removal. |
| Biotrickling filter / biofilter | Off-gas is passed through a media bed hosting an acclimated sulfur- and nitrogen-oxidizing microbial population that biologically oxidizes H₂S to sulfate and ammonia to nitrate/nitrite, achieving >99% H₂S removal at properly maintained empty-bed residence time and moisture. |
Operation and maintenance issues. The chemical scrubber requires continuous monitoring and feed control of caustic/hypochlorite dosing against the fluctuating H₂S loading (under-dosing lets odour breakthrough, over-dosing wastes chemical and can create a chlorine-odour nuisance of its own), plus periodic replacement of packing media as scale/biological growth accumulates. The biotrickling filter/biofilter requires ongoing media moisture and pH control (sulfuric-acid by-product from H₂S oxidation can acidify the bed and kill the microbial population if not periodically flushed/buffered) and media replacement as it compacts and channels over its multi-year service life.
