23-Chem-B2 Environmental Engineering · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, December 2013, 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 — membrane/condensation/adsorption control technologies, thermal 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 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 — dissolved manganese (Mn) (or arsenic, depending on the specific watershed geology). A river receiving upstream mining, industrial or agricultural drainage commonly carries elevated dissolved Mn, which is aesthetically objectionable (staining, taste) and, chronically, a neurological health concern; the standard raw-water measurement method is atomic absorption spectroscopy (AAS) or ICP-MS on a filtered (0.45 µm) sample to distinguish the dissolved fraction (which drives coagulation/oxidation dose selection) from particulate-bound Mn.
Microbiological contaminant — Cryptosporidium oocysts. Cryptosporidium is chlorine-resistant and a common contaminant of surface water impacted by agricultural runoff or wastewater, making it the microbiological parameter of greatest concern for a surface-water-sourced plant (it drives the need for physical removal via filtration rather than relying on disinfection alone); the standard raw-water measurement method is USEPA Method 1623 (immunomagnetic separation, immunofluorescence microscopy) to enumerate oocysts per unit volume.
Reliability of the measurement methods after filtration/disinfection. Because both AAS/ICP-MS for Mn and Method 1623 for Cryptosporidium are laboratory methods with a turnaround of hours to days, they cannot be used as real-time process control on their own; reliability is ensured by pairing them with a continuously-monitored surrogate that responds instantly to a treatment upset — continuous filtered-water turbidity (as an oocyst-removal surrogate, since Cryptosporidium is removed primarily by physical straining/coagulation-filtration, and a well-run granular-media filter run should hold effluent turbidity below 0.3 NTU) and continuous online Mn or ORP monitoring downstream of the oxidation step. Periodic laboratory confirmation (AAS, Method 1623) is then used to validate that the turbidity/ORP surrogate correlation still holds for the current source-water quality, closing the loop between a method that is accurate but slow and one that is continuous but indirect.
Given.
| Quantity | Symbol | Value |
|---|---|---|
| Primary effluent sample volume | $V_s$ | $300\ \text{mL}$ |
| Dilution water volume | $V_d$ | $200\ \text{mL}$ |
| Initial DO of mixture | $D_0$ | $5\ \text{mg/L}$ |
| DO after 5 days | $D_5$ | $0.5\ \text{mg/L}$ |
| DO after 20 days (stabilized) | $D_{20}$ | $0.2\ \text{mg/L}$ |
Find. (a) The 5-day CBOD of the undiluted primary effluent, CBOD₅. (b) The ultimate CBOD, CBOD∞.
Approach. The observed DO drop occurs inside a diluted mix, so it must be scaled by the dilution fraction $P$ (sample volume / total mixture volume) to recover the BOD of the neat effluent; with nitrification inhibited, the 20-day plateau is taken directly as the ultimate carbonaceous demand.
| Quantity | Result |
|---|---|
| Dilution factor | $P=0.6$ |
| 5-day CBOD | 7.5 mg/L |
| Ultimate CBOD | 8.0 mg/L |
As in a standard 5-day BOD determination, a dilution-water blank DO depletion is normally subtracted to correct for oxygen demand contributed by the dilution water itself; none is supplied here, so it is taken as negligible (clean dilution water assumed). Reading the 20-day plateau directly as CBOD∞ assumes the first-order rate constant $k_1$ is high enough for the reaction to be effectively complete by day 20; no independent $k_1$ is given, so this is the best available estimate from the data provided.
Referring to the DAF schematic below (influent to a rectangular tank with surface scraper; a recycle loop draws clarified subnatant through a recycle pump to a pressurized air contact tank, then releases it through a pressure/flow control valve into a recycle-injection sparge at the tank bottom), the engineering design proceeds through the following key steps.
1 — Establish the solids/hydraulic loading basis. Convert the influent flow and feed-solids concentration into a mass loading rate (kg solids/h); this mass flux, not the raw volumetric flow, is what the flotation tank surface area and the air-to-solids (A/S) ratio must be sized against.
2 — Set the air-to-solids (A/S) ratio via bench/pilot testing. Run bench flotation tests (or use manufacturer pilot data) across a range of A/S ratios (typically 0.02–0.04 kg air/kg solids for waste-activated sludge thickening) to find the value that produces the target float-solids concentration and a clear subnatant; A/S, not recycle percentage alone, is the governing design variable because it fixes how many microbubbles are available to attach to and lift each kilogram of floc.
3 — Size the pressurized-recycle saturation system and the tank surface area. Compute the recycle flow and saturator pressure (typically 275–480 kPa for sludge-thickening duty) needed to deliver the target A/S ratio via $A/S=1.3\,s_a(fP-1)R/(Q_{ws}S_{ws})$, then size the flotation tank surface area as the larger of the solids-loading-rate requirement ($A=\dot m_{solids}/\text{SLR}$) and the hydraulic-loading-rate requirement ($A=(Q_{ws}+R)/\text{HLR}$), applying a peaking/safety factor to accommodate diurnal loading variability.
