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.
Reaction 1 — NO₂ photolysis. Sunlight (UV) splits nitrogen dioxide into nitric oxide and a highly reactive ground-state atomic oxygen: $$\text{NO}_2+h\nu\rightarrow\text{NO}+\text{O}$$
Reaction 2 — Ozone formation. The atomic oxygen produced in Reaction 1 combines with atmospheric molecular oxygen (with a third body $M$ to absorb excess energy) to form ozone: $$\text{O}+\text{O}_2+M\rightarrow\text{O}_3+M$$
Reaction 3 — VOC-driven radical propagation, breaking the null cycle. In a clean NOx-only atmosphere, ozone would simply react back with NO ($\text{O}_3+\text{NO}\rightarrow\text{NO}_2+\text{O}_2$), regenerating NO₂ with no net ozone accumulation. In the presence of reactive VOCs, hydroxyl-radical-initiated VOC oxidation instead produces peroxy radicals (RO₂•) that oxidize NO to NO₂ without consuming ozone: $$\text{RO}_2^{\bullet}+\text{NO}\rightarrow\text{RO}^{\bullet}+\text{NO}_2$$ This breaks the null cycle, allowing ozone (and secondary products such as PAN and fine secondary particulate) to accumulate through the day — the simultaneous presence of VOCs, NOx and sunlight is what distinguishes true photochemical smog from simple primary-pollutant haze.
Technology: regenerative thermal oxidizer (RTO), or activated-carbon adsorption for a lower-concentration/intermittent source. For a continuous stationary source (e.g. a benzene-handling process vent), an RTO heats the vapour stream to 800–1,000 °C for ≥0.5–1 s residence time, oxidizing benzene to CO₂+H₂O with ceramic heat-recovery beds recouping ≥90% of the combustion energy from the hot outlet gas — destroying rather than merely transferring the pollutant, appropriate since benzene is a known human carcinogen with no acceptable disposal route other than destruction. For a mobile source (vehicle evaporative/refuelling emissions), the equivalent engineering control is a carbon-canister vapour-recovery system that adsorbs benzene-laden fuel vapour during refuelling/parking and purges it back into the engine intake for combustion during normal operation — the same underlying principle (capture and destroy) implemented at vehicle scale rather than stack scale.
Technology: biofilter (or biotrickling filter). Foul air from rendering operations (dominated by reduced-sulfur and amine/organic-nitrogen odorants) is drawn through a bed of biologically active organic media (compost, bark, or an engineered synthetic packing colonized by an acclimated microbial population); odorants transfer from the gas phase into the moist biofilm on the media surface, where resident bacteria biologically oxidize them to non-odorous end products (CO₂, H₂O, sulfate) — a passive, low-chemical-input alternative to a chemical scrubber, well suited to the large, dilute, continuous airflows typical of a rendering plant.
O&M requirement 1 — Bed moisture content control. The microbial population and the gas-liquid mass-transfer step both require the media to be maintained within a defined moisture range (typically 40–60% by weight); a bed that dries out kills the biofilm and collapses removal efficiency, while an over-wet bed increases pressure drop and can create anaerobic (odour-generating) zones — requiring a scheduled humidification/irrigation system with moisture monitoring, not a "set and forget" installation.
O&M requirement 2 — Media replacement and bed-channeling prevention. Organic media compacts and biodegrades over time (unlike an inert chemical scrubber packing), progressively reducing porosity, increasing pressure drop, and creating preferential flow channels that let untreated air bypass the active biofilm; periodic media replacement (typically every 2–5 years, rendering-plant duty being on the more frequent end given high odour loading) and bed raking/turning to break up channeling are required to sustain long-term removal performance.
