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.
Method 1 — Electrostatic precipitator (ESP). Design principles: (1) particles passing between a high-voltage discharge electrode and a grounded collection plate are field/diffusion-charged and migrate across the interelectrode gap under the applied electric field, so the plate spacing, applied voltage and gas residence time must be sized (via the Deutsch–Anderson relation) to give the migration velocity time to cross the full gap before the gas exits; (2) particle/gas electrical resistivity must be characterized and, if too high or too low, conditioned (e.g. SO₃ or moisture conditioning), because resistivity outside the workable 10⁴–10¹² Ω·cm range causes back-corona or poor charge retention and collapses collection efficiency. O&M considerations: (1) rapping/cleaning cycle timing must be tuned — too frequent re-entrains dust, too infrequent lets the cake grow thick enough to insulate the plate and reduce the effective field; (2) periodic inspection and cleaning of discharge electrodes and insulators is required, since electrode fouling or insulator tracking is the dominant cause of ESP spark-over and efficiency loss over time.
Method 2 — Fabric filter (baghouse). Design principles: (1) air-to-cloth ratio (gas volumetric flow per unit fabric area) must be sized to the specific dust/fabric combination — too high a ratio drives excessive pressure drop and rapid cake blinding; (2) fabric material and cleaning method (pulse-jet, reverse-air, shaker) must be matched to the gas temperature and moisture content, since exceeding a fabric's temperature rating or condensing moisture within the bags causes irreversible damage or cake blinding. O&M considerations: (1) pressure-drop monitoring across the bag bank to trigger cleaning cycles and detect bag failure (a sudden drop in ΔP indicates a torn bag bypassing collection); (2) periodic bag replacement on a scheduled interval (bags degrade with thermal/chemical cycling) and hopper dust removal to prevent bridging and re-entrainment.
NH₃ toxicity — add a nitrification stage (e.g. nitrifying trickling filter or MBBR). A primary-only plant provides no biological ammonia removal, so essentially all influent NH₃–N passes through to the receiving lake, where the un-ionized fraction is directly toxic to fish gill tissue. Retrofitting a secondary biological stage designed with sufficient aerobic solids retention time (SRT) for the slow-growing nitrifiers (Nitrosomonas/Nitrobacter) — either a nitrifying trickling filter or a moving-bed biofilm reactor (MBBR) downstream of the existing primary clarifiers — oxidizes NH₃ to NO₃⁻, converting the acutely toxic species to a far less toxic one. Sizing the aerobic SRT to exceed the nitrifier washout SRT at the plant's coldest design temperature is the controlling design step, since nitrification kinetics are strongly temperature-dependent and a plant sized only for the summer condition will lose nitrification (and the toxicity protection it provides) every winter.
P eutrophication — chemical precipitation with alum or ferric chloride. Dosing alum (Al₂(SO₄)₃) or ferric chloride (FeCl₃) ahead of (or into) the existing primary clarifiers precipitates AlPO₄ or FePO₄, which settles out with the primary sludge and is removed from the liquid stream before discharge; this can be implemented as a retrofit to the existing primary process (no new biological reactor required) and responds within minutes to changes in influent P loading, unlike a biological nutrient-removal upgrade. Because chemical precipitation is a physicochemical, not biological, process it is comparatively insensitive to the cold-temperature kinetic penalty that limits the nitrification retrofit, making it a robust year-round solution for the P/eutrophication half of the problem, at the cost of an ongoing chemical purchase and an increased (metal-hydroxide) sludge production that the plant's solids-handling capacity must be checked against.
