23-Chem-B2 Environmental Engineering · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B2, Environmental Engineering — December 2018. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.
Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.
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.
Monitoring of point and non-point source air pollution. Continuous emission monitoring systems (CEMS) on stacks and ambient air-quality monitoring networks around diffuse (non-point) sources supply the real-time concentration and mass-emission-rate data that drives the rest of the program: without a measured baseline, neither compliance nor the effectiveness of any control retrofit can be verified. Monitoring closes the feedback loop — it tells regulators and operators when an emission source is drifting out of its permitted envelope so a correction can be made before ambient air-toxic exposure accumulates.
Adopting best management practices (BMPs). BMPs (e.g. covering open storage piles, using low-VOC solvents and coatings, leak-detection-and-repair (LDAR) programs on fugitive valves/flanges, housekeeping to limit dust re-entrainment) reduce emissions at the source, before any capital-intensive end-of-pipe control is even needed. Because many air-toxic sources are diffuse (fugitive dust, evaporative losses) rather than a single stack, BMPs are frequently the only cost-effective lever available, and they work synergistically with monitoring: monitoring identifies which BMP gaps are actually driving excess emissions, so the practices adopted are targeted rather than generic.
A widely used advanced treatment method is the Modified Ludzack–Ettinger (MLE) biological nutrient removal (BNR) process, an activated-sludge configuration with an anoxic zone upstream of the aerobic zone plus a large internal mixed-liquor recycle, typically followed by chemical polishing for phosphorus.
Nitrogen reduction — nitrification/denitrification. In the aerobic zone, autotrophic nitrifiers (Nitrosomonas, Nitrobacter) oxidize ammonia to nitrite then nitrate (NH₄⁺ → NO₂⁻ → NO₃⁻); the nitrate-rich mixed liquor is then recycled to the upstream anoxic zone where heterotrophic bacteria use the readily biodegradable influent carbon (BOD) as electron donor and nitrate as electron acceptor in the absence of dissolved oxygen, reducing it to nitrogen gas (NO₃⁻ → N₂↑), which strips harmlessly to atmosphere — this is the key engineering process for nitrogen removal because it is the only step that actually removes nitrogen from the liquid phase rather than merely converting its oxidation state.
Phosphorus reduction — enhanced biological phosphorus removal (EBPR) or chemical precipitation. Adding an anaerobic zone ahead of the anoxic/aerobic train lets phosphate-accumulating organisms (PAOs) take up volatile fatty acids and store them as PHA while releasing stored phosphate; in the subsequent aerobic zone the PAOs oxidize the PHA and take up phosphate in excess of normal growth requirements ("luxury uptake"), which is then removed with the wasted sludge. Where EBPR alone cannot meet a low effluent phosphorus limit, chemical precipitation (alum or ferric salt dosed to the aeration basin or a tertiary stage) is added as a polishing step, forming insoluble AlPO₄/FePO₄ that settles out with the sludge.
| Component | Primary engineering principle | Main purpose |
|---|---|---|
| Flash mix (chemical addition) | A very high velocity gradient (G ≈ 700–1000 s−1) applied for only a few seconds so the coagulant hydrolyzes and disperses uniformly through the water column before it precipitates — the destabilization reaction (charge neutralization of the negatively-charged colloids) must occur while the coagulant is still molecularly dispersed, which is a very short-lived state. | Destabilizes colloidal and suspended particles (turbidity-causing clays, organics) so they can subsequently aggregate into settleable floc; this is the step that actually converts non-settling colloids into a form the rest of the train can remove. |
| Granular filtration | Depth filtration through a graded sand/anthracite bed removes particles by a combination of straining, interception, sedimentation and adsorption onto the grain surfaces within the bed depth — the hydraulic loading rate (typically 5–15 m/h for rapid filters) is sized below the rate that would push floc through the bed, and periodic backwashing (with air scour) fluidizes and expands the bed to scour captured floc from the grains. | Polishes the sedimentation-basin effluent to a low, consistent turbidity ahead of disinfection — low turbidity is essential because particulates shield pathogens from disinfectant contact (chlorine demand/UV shadowing), so filtration performance directly controls the achievable log-inactivation downstream. |
