23-Chem-B2 Environmental Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, May 2014, 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, nutrient removal, activated-sludge design, sedimentation 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, air quality modelling; C. D. Cooper & F. C. Alley, Air Pollution Control: A Design Approach — particulate/gas/vapour control, thermal/catalytic 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 Guidelines for Canadian Drinking Water Quality (Health Canada), the Canadian Council of Ministers of the Environment (CCME) Municipal Wastewater Effluent guidelines, and provincial air/water permitting practice (e.g. BC Environmental Management Act and 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.
Definition. Photochemical smog is a secondary air-pollution haze formed when sunlight drives photochemical reactions among primary pollutants — principally nitrogen oxides (NOx) and volatile organic compounds (VOCs) — producing ground-level ozone (O₃), peroxyacetyl nitrate (PAN), and fine secondary particulate, none of which is emitted directly but which together cause the characteristic brown haze and respiratory irritation.
Atmospheric conditions. Smog forms preferentially under (1) strong sunlight/UV, which supplies the photon energy to initiate the NO₂ photolysis cycle; (2) a temperature inversion or otherwise stagnant, low-mixing-height air mass that traps emissions near the ground instead of dispersing them vertically, common in topographically bounded urban basins (e.g. the Lower Fraser Valley, Los Angeles basin); and (3) a sufficient co-located density of both NOx and VOC precursor emissions, typically from vehicle traffic and industrial/solvent sources.
Key chemical reactions. The core photolytic cycle is $\text{NO}_2+h\nu\rightarrow\text{NO}+\text{O}$, $\text{O}+\text{O}_2\rightarrow\text{O}_3$, followed by $\text{O}_3+\text{NO}\rightarrow\text{NO}_2+\text{O}_2$, which alone would be a null cycle (no net ozone build-up); the smog-forming step is that VOCs are oxidized by hydroxyl radicals to form peroxy radicals (RO₂•), which oxidize NO to NO₂ without consuming ozone, breaking the null cycle and allowing O₃ to accumulate through the day as VOC and NOx continue reacting.
A 99.99% destruction/removal efficiency (four-nines DRE) is a demanding target that, for halogenated hydrocarbons, is achieved with high-temperature thermal oxidation coupled to acid-gas scrubbing — the four-nines target sets tighter temperature, residence-time and control requirements than a conventional 99–99.5% VOC oxidizer.
Stated assumptions. (1) The stream is a dilute, combustible vapour (below its lower flammable limit), not a liquid/solid waste needing separate incineration feed handling; (2) auxiliary fuel is available to sustain combustion temperature independent of the pollutant's own heating value; (3) chlorine (not bromine/fluorine) is the dominant halogen, so HCl is the principal acid-gas by-product; (4) the required 99.99% DRE is demonstrated for the specific compound(s) present, since destruction efficiency is compound-specific, not universal.
1 — High-temperature, extended-residence-time thermal oxidizer. Reaching four-nines DRE on halogenated compounds (harder to fully oxidize than plain hydrocarbons because of strong C–Cl bond energy) typically needs a higher chamber temperature (1,100–1,200 °C) and longer residence time (≥2 s) than a conventional 99–99.5% VOC oxidizer, verified against compound-specific trial-burn data.
2 — Two-stage acid-gas quench and scrubbing. Combustion converts essentially all chlorine to HCl; a quench followed by a two-stage caustic (NaOH) scrubber (rather than a single stage) is specified to reach the very low residual HCl carryover consistent with an overall four-nines removal claim on the halogenated-compound side, and to protect downstream equipment from corrosion.
3 — Continuous emissions monitoring with a redundant interlock. A continuous temperature monitor, backed by a second independent sensor (redundancy, given the tighter DRE margin for error), automatically diverts/shuts off feed if chamber temperature drops below the validated four-nines setpoint, since even a brief excursion can consume most of the destruction “budget” available at a 99.99% target.
4 — Heat recovery and periodic compliance stack testing. Recuperative/regenerative heat exchange reduces auxiliary fuel demand for sustaining the higher chamber temperature; a compliance stack test measures inlet/outlet concentration to confirm $\text{DRE}=(1-C_{out}/C_{in})\times100\%\ge99.99\%$ under representative operating conditions.
Technology: biofiltration. Foul air containing methane and mercaptans (odour thresholds in the low parts-per-billion range) is passed through a bed of moist organic media (compost, wood chips, or a peat/bark mix) that hosts an acclimated microbial population. Design principle: odorous compounds partition from the gas into the moist biofilm coating the media particles, where resident bacteria biologically oxidize them (mercaptans to sulfate, co-emitted organics to CO₂/H₂O), so the odour is destroyed rather than merely transferred; note that methane itself is odourless and sparingly water-soluble, so a biofilter removes little of it — the mercaptans are the odour drivers and are well removed, and any significant methane fraction should be flared or thermally oxidized separately; sizing is based on an empty-bed residence time (typically 30–60 s) and a maximum media loading rate that keeps the bed from going anaerobic. Operational requirements: maintaining bed moisture content in the target range (40–60% by weight) via an irrigation/humidification system, since a bed that dries out loses microbial activity and one that is waterlogged goes anaerobic and can itself begin producing odour; monitoring bed backpressure as an indicator of media compaction/channelling. Maintenance requirements: periodic media replacement (typically every 2–5 years) as the organic media decomposes and loses porosity/void space; occasional bed turning or reworking to prevent channelling, and nutrient supplementation if the media's native nutrient content is depleted over extended service.
