23-Chem-B2 Environmental Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, May 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, 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 — fabric filtration, thermal oxidation, adsorption, 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.
Atmospheric conditions. Smog accumulates when pollutants are trapped near ground level beneath a stagnant, temperature-inverted air mass (warm air aloft over cooler surface air, suppressing vertical mixing), typically within a topographically or thermally bounded airshed (a valley or coastal basin), and requires strong, sustained sunlight to drive the photochemistry — conditions most common on clear, calm, sunny days with light wind.
Key chemical reactions. Sunlight (UV) photolyses NO₂ ($\text{NO}_2+h\nu\rightarrow\text{NO}+\text{O}$); the resulting atomic oxygen combines with O₂ to form ozone ($\text{O}+\text{O}_2\rightarrow\text{O}_3$), which in a clean NOx-only atmosphere would simply react back with NO to regenerate NO₂ (a null cycle, no net ozone build-up). In the presence of reactive VOCs, hydroxyl-radical-initiated VOC oxidation instead produces peroxy radicals (RO₂•) that oxidize NO to NO₂ without consuming ozone, breaking the null cycle and allowing ozone (and secondary products such as PAN and fine secondary particulate) to accumulate through the day — the VOC + NOx + sunlight combination is what distinguishes true photochemical smog from simple primary-pollutant haze.
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) the compound's destruction kinetics at the chosen design temperature/residence time are adequate to reach the higher 99.9% destruction/removal efficiency (DRE) target — a full order of magnitude tighter than the routine 99.5% benchmark, requiring more margin on temperature, residence time, and continuous verification than a standard installation.
1 — Thermal oxidizer sized with extra margin above the standard 99.5%-DRE envelope. A combustion chamber operated at the upper end of the standard range (1,000–1,100 °C, vs. the 980 °C floor used for a 99.5% target) with a longer minimum residence time (≥1.5 s rather than the bare 1 s minimum) and strong turbulence/mixing (the "three T's") provides the additional margin needed to reliably reach 99.9% DRE, since destruction efficiency does not scale linearly — closing the last order of magnitude of destruction is disproportionately harder than the first two orders.
2 — Two-stage oxidation (primary chamber + afterburner/secondary chamber). A second combustion stage downstream of the primary chamber gives the gas a second, independent pass at the design temperature/residence time, so any parcel that escaped complete destruction in the primary chamber (e.g. due to a local under-mixed pocket) gets a second opportunity — a design approach specifically used to push DRE from the 99.5% single-stage benchmark up toward 99.9%+.
3 — Acid-gas quench and caustic scrubbing. The hot, halogen-laden combustion gas (producing HCl, HBr, etc. depending on the specific halogenated compound) is quenched and passed through a caustic (NaOH) wet scrubber to meet acid-gas emission limits and prevent downstream corrosion.
4 — Continuous temperature/CEMS monitoring with feed interlock. A continuous chamber-temperature monitor, backed by a continuous emissions monitoring system (CEMS) tracking a surrogate combustion-efficiency parameter (e.g. CO), is interlocked to automatically divert or shut off feed if conditions drop below the validated 99.9%-DRE setpoint — a tighter, more instrumented control loop than the single-point interlock adequate for a 99.5% target, since the smaller allowable residual concentration leaves much less margin for an undetected excursion.
5 — Trial-burn stack testing to formally verify the 99.9% figure. Because 99.9% DRE corresponds to a very small residual outlet concentration, verification requires an analytical method with correspondingly higher sensitivity than needed for a 99.5% target, run during a dedicated trial burn at worst-case feed conditions before the design is accepted as meeting the regulatory target.
Technology: wet chemical (oxidative) scrubbing, e.g. sodium hypochlorite or hydrogen peroxide solution. Foul air is drawn through a packed-tower scrubber where it contacts a recirculating oxidant solution counter-currently; reduced-sulfur odorants (H₂S, mercaptans, dimethyl sulfide) transfer from the gas phase into the liquid and are chemically oxidized (destroyed, not merely absorbed) by the hypochlorite/peroxide, converting them to non-odorous sulfate/sulfonate species.
Design principle 1 — gas-liquid contact area and residence time. The packing surface area and empty-bed residence time are sized against the design odour (mass) loading and the target removal efficiency, since the oxidation reaction can only proceed as fast as the odorant is transferred across the gas-liquid interface — under-sizing the packing bed leaves odorant untreated regardless of how much oxidant is present in the liquor.
Design principle 2 — oxidant residual control via ORP set point. The oxidant feed rate/concentration is controlled to maintain a minimum residual oxidant concentration in the recirculating liquor, monitored via oxidation-reduction potential (ORP), since insufficient residual oxidant lets odorant pass through partially treated while excess oxidant wastes reagent and can create its own by-product/corrosion issues — this closed-loop control is what actually sustains destruction performance as the odour loading varies through the day.
