23-Chem-B2 Environmental Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, December 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, 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), 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.
Inorganic contaminant — nitrate (NO₃⁻). Farmland runoff (fertilizer application, manure) downstream of a heavily farmed watershed is a classic source of elevated nitrate loading to a river intake, of concern for both eutrophication of the receiving water and, at the drinking-water tap, methemoglobinemia risk in infants; the standard treated-water measurement method is ion chromatography (IC) or the cadmium-reduction colorimetric method (Standard Methods 4500-NO₃⁻) run on the finished (post-treatment) water.
Microbiological contaminant — Cryptosporidium oocysts (or generically, faecal coliform/E. coli as an indicator). Agricultural runoff carrying livestock manure is a well-documented source of Cryptosporidium, which is chlorine-resistant and therefore not reliably removed by disinfection alone; the standard measurement method is USEPA Method 1623 (immunomagnetic separation and immunofluorescence microscopy) for direct oocyst enumeration, backed operationally by continuous filtered-water turbidity as a physical-removal surrogate.
Guaranteeing sufficient reduction. Because both IC/colorimetric nitrate analysis and Method 1623 are laboratory methods with hours-to-days turnaround, they cannot alone provide real-time assurance that a specific batch of finished water is safe; reliability is achieved by pairing the periodic laboratory confirmation with continuous, instantaneous online surrogates — a continuous online UV-absorbance nitrate analyzer (nitrate absorbs strongly near 220 nm) on the finished water, cross-checked against the laboratory ion-chromatography results, and continuous filtered-water turbidity (target <0.3 NTU) as the real-time proxy for oocyst removal via the coagulation-filtration barrier — with the periodic laboratory results used to re-validate that the online surrogate correlation still holds for the current source-water quality.
Given.
| Quantity | Symbol | Value |
|---|---|---|
| Tertiary effluent sample volume | $V_s$ | $300\ \text{mL}$ |
| Dilution water volume | $V_d$ | $150\ \text{mL}$ |
| Initial DO of mixture | $D_0$ | $8\ \text{mg/L}$ |
| DO after 5 days | $D_5$ | $2\ \text{mg/L}$ |
| DO after 20 days (stabilized) | $D_{20}$ | $0.2\ \text{mg/L}$ |
Find. (a) The 5-day CBOD of the undiluted tertiary effluent, CBOD₅. (b) The ultimate CBOD, CBOD∞.
Approach. The observed DO depletion occurs inside a diluted mixture, so it is scaled by the dilution fraction $P$ (sample volume/total mixture volume) to recover the neat-effluent BOD; with nitrification inhibited, the 20-day plateau is taken directly as the ultimate carbonaceous demand.
| Quantity | Result |
|---|---|
| Dilution factor | $P=0.6667$ (2/3) |
| 5-day CBOD | 9.0 mg/L |
| Ultimate CBOD | 11.7 mg/L |
A dilution-water blank DO depletion is normally subtracted in a standard BOD determination to correct for oxygen demand contributed by the dilution water itself; none is supplied here, so it is assumed negligible. Reading the 20-day plateau directly as CBOD∞ assumes the first-order decay constant $k_1$ is large enough for the reaction to be essentially complete by day 20 — a reasonable approximation for a well-treated tertiary effluent, but not universal.
Referring to the DAF schematic below (an open rectangular tank with a surface skimmer; a recycle stream is drawn from the clarified subnatant, pressurized through a recycle pump into an air-saturated contact tank, then released through a pressure/flow control valve into a sparge at the tank bottom, precipitating fine microbubbles), the three governing design parameters are as follows.
1 — Air-to-solids (A/S) ratio. The mass ratio of air released to solids fed governs how many microbubbles are available to attach to and lift each kilogram of floc; typical design values for waste-activated-sludge thickening are $A/S\approx0.02$–$0.04$ kg air/kg solids, established by bench or pilot flotation testing across the expected solids-loading range.
2 — Recycle ratio and saturator pressure. The fraction of flow recycled through the pressurized air-contact tank, together with the saturator operating pressure (typically 275–480 kPa, with recycle ratios of roughly 30–150% of the influent flow), sets how much dissolved air is actually available to release as microbubbles at the sparge — too low a pressure or recycle fraction starves the A/S ratio regardless of how it was nominally specified.
3 — Hydraulic and solids loading rate (tank sizing). The flotation tank surface area is sized as the larger of the solids-loading-rate requirement (typically 4–10 kg/m²·h for waste-activated sludge) and the hydraulic-loading-rate requirement (typically 5–15 m³/m²·h, i.e. m/h), because a dilute, low-solids feed can be hydraulically limited even while comfortably under its solids-loading capacity, and vice versa for a thick, concentrated feed.
