23-Chem-B2 Environmental Engineering · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B2, Environmental Engineering — May 2016. 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.
PM2.5 (fine, largely secondary/combustion-derived particulate) is best captured by an electrostatic precipitator (ESP), which can reach sub-micron collection efficiencies that a depth filter struggles to match economically at high gas volumes. PM10 (coarser, mechanically generated particulate) is well suited to a fabric filter (baghouse), which physically sieves the gas stream through a dust cake built up on woven or felted media.
| Method | Design principle 1 | Design principle 2 | Operational consideration 1 | Operational consideration 2 |
|---|---|---|---|---|
| ESP — PM2.5 | Corona discharge wires ionise the gas and negatively charge particles; migration to grounded collection plates follows the Deutsch–Anderson relation η = 1−exp(−wA/Q), so plate area A and drift velocity w are sized for the target efficiency at gas flow Q. | Multiple electrical fields in series (sectionalisation) let each field be energised independently, improving reliability and allowing higher overall efficiency than one large field. | Rapping cycle (mechanical or electric) timed to dislodge the collected cake without re-entraining fines back into the gas stream. | Maintain flue-gas resistivity in the 104–1010 Ω·cm window (condition with SO3 or moisture injection if resistivity drifts too high) to avoid back-corona sparking. |
| Baghouse — PM10 | Size the air-to-cloth (face-velocity) ratio to the dust/media pair (typically 2–4 ft/min for a pulse-jet unit) so the dust cake that forms is the true filtering medium, not the bag fabric itself. | Select bag media (woven vs. felted; fibre type) for gas temperature, moisture and chemical compatibility so the cake releases cleanly on each cleaning cycle. | Trigger cleaning (pulse-jet or reverse-air) from a differential-pressure setpoint, not a fixed timer, to balance cake retention (needed for fine-particle capture) against blinding. | Continuous opacity or particulate bypass monitoring on individual compartments to catch bag leaks/tears before they become a stack-emission exceedance. |
Three independent process routes reach the <5 mg/L TAN limit; each is shown as a labelled block diagram. Method (a) removes nitrogen biologically, (b) strips it as a gas, and (c) oxidises it chemically — giving the plant three genuinely different technology options to compare on cost, footprint and byproduct handling.
(a) Biological nitrification–denitrification. Autotrophic nitrifiers (Nitrosomonas, Nitrobacter) oxidise NH4+ to NO2− then NO3− in an aerobic reactor; an anoxic zone (with an added carbon source or internal recycle) then reduces NO3− to N2 gas, which strips out of solution. This is the lowest-cost route at municipal scale because it uses conventional activated-sludge infrastructure, but it needs enough SRT and alkalinity for the slow-growing nitrifiers.
(b) Air stripping. Raising the pH to ∼11 with lime or NaOH converts NH4+ to volatile free NH3, which is then stripped in a counter-current packed tower against a large air flow; the off-gas must be scrubbed (acid absorption) before release, and the stripped liquid is re-neutralised. This route is fast and insensitive to toxic shock loads but is capital- and reagent-intensive and can freeze/scale in cold climates.
(c) Breakpoint chlorination. Chlorine dosed past the breakpoint (∼10:1 mass ratio Cl2:N) oxidises NH4+ directly to N2 gas; residual chlorine is then removed by dechlorination (SO2 or bisulfite) before discharge. It is compact and fast-acting but the most expensive route per kg N removed and produces chlorinated by-products (e.g., trihalomethanes) that must be managed.
