23-Chem-B2 Environmental Engineering · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, December 2013, 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, nutrient removal; 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 — membrane/condensation/adsorption control technologies, thermal oxidation, 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), which govern effluent/emission limits and treatment-technology selection referenced throughout.
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.
Physical atmospheric conditions. Smog forms and accumulates when emissions are trapped near ground level under a stagnant, temperature-inverted air mass (warm air aloft over cooler surface air suppresses vertical mixing), typically within a topographically or thermally bounded airshed (a valley or coastal basin), and requires abundant sunlight to drive the photochemistry described below — conditions most common on clear, calm, sunny days with light or no wind.
Chemical reactions necessary. The primary photochemical driver is the NO₂ photolysis cycle: sunlight (UV) splits NO₂ into NO and atomic oxygen ($\text{NO}_2+h\nu\rightarrow\text{NO}+\text{O}$), the atomic oxygen combines with O₂ to form ozone ($\text{O}+\text{O}_2\rightarrow\text{O}_3$), and in a clean NOx-only atmosphere ozone would simply react back with NO to regenerate NO₂ (a null cycle). In the presence of reactive VOCs, however, hydroxyl-radical-initiated VOC oxidation produces peroxy radicals (RO₂•) that oxidize NO to NO₂ without consuming ozone, breaking the null cycle and allowing ozone (and secondary products such as peroxyacetyl nitrate, PAN, and fine secondary particulate) to accumulate through the day — this VOC + NOx + sunlight combination is what distinguishes true photochemical smog from simple primary-pollutant haze.
A 99.99% destruction/removal efficiency (DRE, “four nines”) is the benchmark performance standard applied to hazardous-waste incinerators burning principal organic hazardous constituents (99.9999% for PCB/dioxin-bearing wastes). It leaves a residual 50× smaller than the roughly 99.5% a basic thermal oxidizer is often specified for, and is achieved by thermal oxidation with acid-gas scrubbing designed and operated with a wide margin above the minimum conditions typically quoted for chlorinated-compound destruction.
Stated assumptions. (1) The emission is a dilute, combustible vapour stream (below its lower flammable limit), not a liquid/solid waste requiring separate feed handling; (2) sufficient auxiliary natural-gas fuel is available to sustain the required combustion temperature independent of the pollutant's own heat of combustion; (3) chlorine content is low enough that HCl, not the more difficult Cl₂/dioxin-precursor chemistry of high-chlorine waste incineration, is the principal combustion by-product; (4) the compound's destruction kinetics at the design temperature are known (or a trial burn is performed), since a 99.99% DRE target for a specific compound cannot be assumed — it must be demonstrated.
1 — Oversize the thermal oxidizer's temperature and residence time margin. Rather than the minimum ~980 °C / 1 s conditions often quoted for oxidizing chlorinated VOCs, the chamber is designed for ≥1,100 °C with a longer residence time (≥1.5–2 s) and high turbulence to push destruction kinetics further toward completion — each additional “nine” of DRE requires disproportionately more severe conditions because the remaining undestroyed fraction shrinks geometrically.
2 — Two-stage combustion with excess-air control. A two-stage (primary/secondary) combustion chamber design, with tightly controlled excess air in each stage, ensures thorough mixing and complete oxidation even of the last trace fraction of the compound, which a single-stage chamber can miss due to local under-mixed pockets.
3 — Acid-gas quench and multi-stage scrubbing. The hot, HCl-laden combustion gas is quenched and passed through a caustic (NaOH) wet scrubber — potentially staged (venturi quench + packed-tower polishing) — to both meet acid-gas emission limits and protect downstream equipment from corrosion.
4 — Continuous monitoring with automatic feed interlock. A continuous temperature (and surrogate destruction, e.g. CO or THC) monitor is interlocked to automatically divert or shut off feed the instant chamber conditions drop below the validated 99.99%-DRE setpoint, since destruction efficiency collapses rapidly below the design temperature — this closed-loop control is what actually guarantees the 99.99% figure is met continuously, not just during a compliance test.
5 — Stack testing with a sensitive analytical method. Verifying 99.99% DRE requires an analytical method with a detection limit low enough to reliably measure the tiny residual outlet concentration (0.01% of inlet); a standard method adequate for confirming 99.5% DRE may not have sufficient sensitivity, so method selection itself becomes a design consideration at this stringency.
Technology: biofiltration. Foul air from the odour-generating enclosure (headworks, digester, biosolids-handling building) is drawn through a moist, biologically active filter bed (compost, wood chips, or engineered media colonized by sulfur-oxidizing bacteria such as Thiobacillus); H₂S and reduced-sulfur odorants transfer from the gas phase into the biofilm on the media surface, where they are biologically oxidized to sulfate, destroying the odorant rather than merely transferring or masking it. Design principle: empty-bed residence time and media surface area are sized to give the pollutant enough contact time with the active biofilm for oxidation to go to completion at the design odour loading, and the bed is kept continuously moist (humidification of the inlet air, periodic irrigation) since the biological activity stops if the media dries out. Operational requirements: maintaining bed moisture content in the target range (typically 40–60% by weight) and monitoring backpressure across the bed to detect channeling or compaction; balancing airflow to avoid both under-loading (media dry-out) and over-loading (breakthrough of un-oxidized odorant). Maintenance requirements: periodic media replacement or turning as the organic media biodegrades and compacts over time (reducing porosity and increasing pressure drop); nutrient (N, P) supplementation if the sulfur-oxidizing population becomes nutrient-limited; monitoring effluent pH at the media surface, since biological sulfate production can acidify the bed over time and eventually inhibit the same bacteria that are doing the treatment.
