23-Chem-B2 Environmental Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, May 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, nutrient removal, activated-sludge design, sedimentation design; 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, air quality modelling; C. D. Cooper & F. C. Alley, Air Pollution Control: A Design Approach — particulate/gas/vapour control, thermal/catalytic 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 Guidelines for Canadian Drinking Water Quality (Health Canada), the Canadian Council of Ministers of the Environment (CCME) Municipal Wastewater Effluent guidelines, and provincial air/water permitting practice (e.g. BC Environmental Management Act and 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.
Inorganic contaminant — arsenic (As). Arsenic is a common naturally-occurring groundwater contaminant in geologically susceptible aquifers, regulated in Canada under the Guidelines for Canadian Drinking Water Quality (maximum acceptable concentration 0.010 mg/L) because of its chronic carcinogenic and dermal toxicity. Measurement method: inductively coupled plasma mass spectrometry (ICP-MS) on a filtered, acid-preserved raw-water sample gives a low, defensible detection limit (well below the 0.010 mg/L standard) and distinguishes total arsenic reliably enough to size the pre-treatment (typically coagulation/filtration or adsorptive media) needed ahead of final disinfection.
Microbiological contaminant — E. coli / total coliform. A polluted aquifer with surface-water influence or septic-system impact can carry faecal-indicator organisms signalling pathogen risk. Measurement method: membrane filtration or a defined-substrate (Colilert-type) most-probable-number (MPN) test on the raw water quantifies indicator-organism density, which is used to size the disinfection (chlorine CT or UV dose) needed to achieve the required log-reduction before the water is deemed safe to distribute.
Ensuring measurement reliability. Both methods must be run by an accredited laboratory following a standard method (e.g. Standard Methods for the Examination of Water and Wastewater) with documented quality assurance/quality control: field/lab blanks and duplicates to characterize sampling and analytical variability, spiked recovery samples to confirm the method detection limit is well below the regulatory standard being demonstrated, and a fixed monitoring frequency appropriate to the raw-water variability (a groundwater source with a stable geology can be monitored less frequently than one with seasonal recharge-driven variability). Chain-of-custody documentation and regular instrument calibration/certification close the loop, so that a compliance decision (safe to serve) can be defended against a true, not an artifact, measurement.
Given.
| Quantity | Symbol | Value |
|---|---|---|
| Secondary effluent sample volume | $V_s$ | $400\ \text{mL}$ |
| Dilution water volume | $V_d$ | $100\ \text{mL}$ |
| Initial DO of mixture | $D_0$ | $6\ \text{mg/L}$ |
| DO after 5 days | $D_5$ | $1\ \text{mg/L}$ |
| DO after 20 days (stabilized) | $D_{20}$ | $0.1\ \text{mg/L}$ |
Find. (a) The 5-day CBOD of the secondary effluent, CBOD₅. (b) The ultimate CBOD, CBOD∞.
Approach. The 500 mL bottle contains a high proportion of neat effluent (400 of 500 mL), so the dilution fraction $P$ is close to 1; scaling the observed DO depletion by $1/P$ recovers the BOD of the neat effluent, and the 20-day plateau is read directly as the ultimate demand since nitrification has been inhibited and the reaction has essentially stopped by day 20.
| Quantity | Result |
|---|---|
| Dilution factor | $P=0.8$ |
| 5-day CBOD | 6.25 mg/L |
| Ultimate CBOD | 7.38 mg/L |
No dilution-water blank DO depletion is given, so it is taken as negligible (consistent with clean dilution water and an already nitrification-inhibited, secondary-treated effluent). Reading the 20-day plateau directly as CBOD∞ assumes the first-order rate constant is high enough that the reaction is essentially complete by day 20 (roughly $k_1\gtrsim0.15\ \text{d}^{-1}$); with no independent $k_1$ supplied this is the best available estimate from the data given.
Selecting the secondary (final) clarifier following an activated-sludge aeration tank as the treatment system, the design proceeds through three key steps.
1 — Size the surface area on overflow rate (governing the clarification function). The clarifier's primary job is to let floc settle faster than the upward (or, in a rectangular tank, horizontal-then-upward) escape velocity of the liquid; surface area is sized on a design surface overflow rate (SOR, m³/m²·d) selected from the sludge's settling-velocity distribution (from a settling-column test) at both average and peak-hour flow, since a clarifier that passes average-flow SOR but is under-sized for peak flow will lose solids in the effluent during wet-weather events.
2 — Size the depth and detention time on solids-loading rate and thickening function. Beyond clarification, the tank floor and hopper must also thicken the settled sludge to a pumpable underflow concentration; depth (and hence detention time) is checked against a solids loading rate (kg TSS/m²·h) limit, because an under-deep tank can be adequately sized for clarification (SOR) yet still fail on thickening (the sludge blanket rises and solids wash over the weir) — the governing criterion is whichever of clarification and thickening is more restrictive for the specific MLSS concentration and settleability (SVI) of the sludge.
3 — Design the inlet, weir, and sludge-removal mechanics to control short-circuiting. The inlet diffuser/baffle (shown in the figure) must spread the incoming flow evenly across the tank width and damp inlet momentum so it does not create a jet that short-circuits directly to the outlet weir; the outlet weir loading rate (m³/m·d of weir length) is limited to avoid pulling a localized upward velocity that re-entrains settled floc near the outlet, and a continuously moving scraper/plow mechanism (or the sloped hopper floor shown) must move settled sludge to the drain/hopper faster than it can accumulate and go septic, since septic sludge releases gas bubbles that float solids back into the clarified effluent.
