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.
Treatment system — Modified Ludzack–Ettinger (MLE) biological nutrient removal. Total nitrogen exists in wastewater predominantly as ammonia/organic nitrogen, so removing it below 10 mg/L requires two sequential biological reactions in series — nitrification (ammonia→nitrate) followed by denitrification (nitrate→N₂ gas) — rather than a single-stage aerobic process, which nitrifies but cannot itself remove the resulting nitrate. The MLE configuration places an anoxic (unaerated, mixed) zone ahead of the aerobic zone and recycles nitrified mixed liquor back to the anoxic zone, so a single carbon source (the raw influent BOD) drives both the growth of the nitrifiers downstream and the denitrification reaction upstream.
Component description and key design principle.
| Component | Description | Key design principle |
|---|---|---|
| Anoxic zone | Mixed (not aerated) reactor receiving raw influent plus the internal mixed-liquor recycle (IMLR) and return activated sludge (RAS); facultative heterotrophs use the influent BOD as electron donor and the recycled nitrate as electron acceptor. | Anoxic hydraulic retention time (typically 1–3 h) and the influent BOD₅/TKN ratio (generally ≥4–5 for adequate carbon) must be sufficient to complete denitrification of the recycled nitrate load before the flow proceeds to the aerobic zone. |
| Aerobic zone | Conventional aerated activated-sludge zone where autotrophic nitrifiers (Nitrosomonas, Nitrobacter) oxidize ammonia through nitrite to nitrate, and heterotrophs complete BOD removal. | Aerobic solids retention time (SRT) must exceed the nitrifier minimum growth-rate SRT with an adequate safety factor (commonly a design SRT of 10–20 d, temperature-dependent, since nitrifiers grow far slower than heterotrophs and are the rate-limiting population) and dissolved oxygen must be maintained above ≈2 mg/L to avoid inhibiting nitrification. |
| Internal mixed-liquor recycle (IMLR) | A dedicated recycle stream (typically 200–400% of influent flow) pumped from the end of the aerobic zone back to the head of the anoxic zone, carrying nitrate produced by nitrification back to where carbon is available for denitrification. | The IMLR ratio sets the theoretical maximum TN removal (removal fraction ≈ IMLR/(1+IMLR+RAS ratio)); it must be sized so the anoxic zone actually receives enough nitrate load to hit the <10 mg/L target, not just enough carbon. |
| Secondary clarifier + RAS/WAS | Gravity settling separates the biomass from the treated flow; RAS returns settled biomass to the anoxic zone to sustain the mixed-liquor concentration, and WAS controls the overall SRT. | Surface overflow rate and solids loading rate sized conventionally (as in Question 4); RAS is routed to the anoxic zone (not directly to the aerobic zone) so its nitrate content, too, is available for denitrification. |
A 10 mg/L monthly-average TN limit (rather than a tighter daily-maximum limit) gives the MLE process useful operating margin against day-to-day load variability; if the receiving aquifer were unusually sensitive, a tertiary step (e.g. a supplemental post-anoxic carbon-fed denitrification filter) could be added to push TN below 3 mg/L, but the two-stage MLE train is normally sufficient for a 10 mg/L target.
| Contaminant type | Control method | Main engineering design principle | Important O&M issue |
|---|---|---|---|
| VOCs (benzene, methylene chloride, hexane) | Regenerative thermal oxidizer (RTO) | Vapour stream is heated to 800–1,000 °C for ≥0.5–1 s residence time to oxidize the VOC to CO₂+H₂O (+HCl for the chlorinated methylene chloride, requiring downstream acid-gas control); ceramic heat-recovery beds recoup ≥90% of combustion energy from the hot outlet gas. | Continuous chamber-temperature monitoring with a feed-diversion interlock, since destruction efficiency collapses quickly below the design temperature; periodic inspection of the ceramic media and combustion-chamber refractory for fouling/degradation. |
| Particulates (PM10, aerosols) | Fabric filter (baghouse) | Dust-laden gas is drawn through woven/felted bags; the growing dust cake itself becomes the primary filtering medium (depth filtration transitioning to surface/cake filtration), achieving >99% removal across the sub-micron to coarse range that a cyclone alone cannot capture. | Pressure-drop monitoring to trigger a defined pulse-jet/reverse-air cleaning cycle before the cake chokes flow, while not cleaning so aggressively/frequently that dust re-entrains; protecting bag material from condensation ("blinding") and from exceeding its temperature rating. |
| Air toxics (CO, NOx, SOx) | Selective catalytic reduction (SCR) for NOx, paired with a wet limestone flue-gas desulfurization (FGD) scrubber for SOx; CO is addressed by ensuring complete combustion (excess-air/burner tuning) upstream rather than end-of-pipe capture. | SCR injects ammonia/urea upstream of a catalyst bed operated in its effective temperature window (≈300–400 °C) to reduce NOx to N₂+H₂O; FGD contacts flue gas with a limestone slurry so SO₂ is absorbed and converted to gypsum; CO control relies on maintaining adequate excess O₂ and mixing at the burner to complete combustion to CO₂. |
