23-Chem-B2 Environmental Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, December 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, 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), 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.
Given.
| Quantity | Symbol | Value |
|---|---|---|
| Ozone mixing ratio | $C_{ppm}$ | $0.15\ \text{ppm}$ |
| Molar mass, O₃ | $M$ | $48\ \text{g/mol}$ |
| Molar volume at reference conditions | $V_m$ | $24.45\ \text{L/mol}$ ($25^{\circ}\text{C}$, 1 atm) |
Find. The equivalent ozone concentration in µg/m³.
Approach. The ppm(v)→mass-concentration conversion for an ideal gas is $C\,[\mu g/m^3]=C_{ppm}\times(M/V_m)\times1000$; the "STP" reference condition must be stated explicitly since it changes the answer.
| Quantity | Result |
|---|---|
| Ozone concentration at 0.15 ppm | ≈294 µg/m³ (at 25 °C, 1 atm) |
"STP" is ambiguous between the chemistry convention (0 °C, 1 atm, $V_m=22.41$ L/mol ⇒ 321 µg/m³) and the 25 °C reference condition environmental-engineering ambient air-quality standards are normally tabulated against ($V_m=24.45$ L/mol ⇒ 294 µg/m³). The 25 °C convention is adopted here as it is the one used when comparing a calculated concentration against a published ambient air-quality objective.
Air monitoring/sampling to confirm abatement. Continuous ambient ozone monitoring uses UV-absorption photometric analyzers sited at representative downwind/population-exposure locations, logging data against the applicable averaging period (1-hour and 8-hour ozone objectives) so that the measured ambient concentration — not just the calculated stack-based estimate — can be directly compared to the established air quality standard. Because ozone is a secondary pollutant formed downwind of NOx/VOC sources rather than emitted directly, monitoring for source-abatement adequacy instead focuses on the precursors: continuous source or fenceline monitoring of NOx and VOC emission rates from the abated source, combined with periodic ambient ozone monitoring across the plume's typical photochemical-formation distance/time-of-day window, closes the loop between "the source is abated" (precursor emission rate reduced) and "the air quality standard is met" (ambient ozone concentration reduced).
Ambient air quality standard (350 µg/m³, 24-hour). This is a receptor-based, health-protective limit on the SO₂ concentration actually present in the air that people breathe, integrated over a 24-hour averaging period; it is used to judge whether the airshed as a whole — the combined effect of all sources plus dispersion — is safe, and is verified by continuous ambient monitoring at representative stations.
Source performance standard (90 g SO₂/10⁶ kJ). This is a source-based emission-intensity limit expressed per unit of fuel energy input, applied directly to an individual combustion source (e.g. this power plant) at the point of emission; it is used to set and enforce the permitted emission rate of a specific facility, verified by stack testing or continuous emission monitoring (CEMS) referenced to measured heat input.
How they relate. The two standards operate at different points in the same cause-and-effect chain: the source performance standard controls the emission rate $Q$ at the stack, and that $Q$, once diluted through atmospheric dispersion (the Gaussian plume relationship used in Problem 5), determines the resulting ground-level ambient concentration that the 24-hour standard judges. A source that just meets its performance standard is not automatically guaranteed to keep every downwind receptor under the ambient standard — that additionally depends on stack height, local meteorology, and the number/proximity of other sources contributing to the same airshed — which is why regulators use both: the source standard as the enforceable permit condition on the facility, and the ambient standard as the independent check on whether the airshed target is actually being achieved.
Treatment method — tertiary chemical phosphorus removal (metal-salt coagulation, flocculation and clarification) ahead of discharge. Ferric chloride (FeCl₃) or alum (Al₂(SO₄)₃) is dosed into the secondary effluent, precipitating FePO₄/AlPO₄ that is then removed by flocculation and sedimentation before the flow reaches the receiving river/lake, cutting the soluble phosphorus load responsible for eutrophication (algal-bloom-driven oxygen depletion) at the source rather than downstream in the receiving water.
Key design/operating parameter per unit. Rapid mix — velocity gradient $G$ (typically 700–1000 s⁻¹) and mixing time (≈30–60 s), which governs how quickly and completely the metal salt disperses and hydrolyses before it can precipitate uncontrolled floc; flocculation — the $G\cdot t$ product (typically $2\times10^4$–$10^5$), balancing enough agitation to promote particle collision/floc growth against too much, which shears floc back apart; sedimentation — the surface overflow rate (SOR, typically 15–30 m/d for a chemical floc, generally lower than a purely biological floc because metal-hydroxide floc is denser and settles somewhat faster but is also more shear-sensitive), which fixes the clarifier surface area needed to capture the settleable P-bearing floc before it carries over to the effluent.
