23-Chem-B2 Environmental Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC 04-Chem-B2 Environmental Engineering, May 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, phosphorus removal; M. L. Davis & D. A. Cornwell, Introduction to Environmental Engineering (5th ed., McGraw-Hill) — air pollution control, ion exchange, reverse osmosis, soil remediation; L. Theodore & A. J. Buonicore / C. D. Cooper & F. C. Alley, Air Pollution Control: A Design Approach — fabric filtration, absorption, 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 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), which govern effluent/emission limits, monitoring frequency, and buffer-strip / best-management-practice programs 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.
Total phosphorus in secondary effluent is dominated by soluble orthophosphate plus a small particulate fraction, so meeting a 1 mg/L monthly-average limit almost always requires a dedicated polishing step beyond conventional secondary treatment.
Physical/chemical method — metal-salt coagulation with tertiary filtration. Alum (Al₂(SO₄)₃) or ferric chloride (FeCl₃) is dosed ahead of a rapid-mix/flocculation basin, precipitating AlPO₄ or FePO₄; the floc is removed by a downstream granular-media (sand/anthracite) or cloth-disk tertiary filter, which is what drives TP consistently below 1 mg/L (chemical clarification alone typically levels off near 0.3–0.5 mg/L soluble P but the residual particulate P needs the filter). Advantages: (1) fast, robust process that responds within minutes to load changes, so it copes well with wet-weather TP spikes; (2) very low, predictable effluent TP (<0.1 mg/L achievable) independent of biology, useful when the discharge is to a sensitive lake. Disadvantages: (1) continuous chemical cost and a metal-hydroxide sludge that increases sludge production 20–30% and complicates biosolids land application (aluminum/iron loading); (2) requires close pH and dose control — overdosing wastes chemical and can carry residual metal into the effluent, underdosing lets TP through, so it needs either jar-testing or a P-based feedback dosing controller.
Biological method — enhanced biological phosphorus removal (EBPR). An anaerobic zone (no oxygen or nitrate) is placed ahead of the aerobic zone; phosphorus-accumulating organisms (PAOs) ferment volatile fatty acids there and release stored polyphosphate for energy, then in the following aerobic zone they take up phosphorus in excess of normal metabolic need (“luxury uptake”) to rebuild polyphosphate reserves, so the wasted PAO biomass carries the phosphorus out of the system. Advantages: (1) no ongoing chemical purchase, lower operating cost and a smaller, more land-applicable biosolids stream; (2) can reach <1 mg/L TP with a well-designed anaerobic contact time and can be combined with biological nitrogen removal (A²/O configuration) in one process train. Disadvantages: (1) fragile — nitrate or dissolved oxygen leaking into the anaerobic zone (e.g. from RAS) suppresses fermentation and collapses P release/uptake, so performance is sensitive to plant hydraulics and to cold-weather kinetics; (2) cannot reliably hit <1 mg/L on its own during wet-weather or high-influent-P events, so most plants still need a small chemical polishing dose (“chemically-assisted EBPR”) as a backstop, which partly erodes the cost advantage.
Given.
| Quantity | Symbol | Value |
|---|---|---|
| Effluent sample volume | $V_s$ | $100\ \text{mL}$ |
| Dilution water volume | $V_d$ | $200\ \text{mL}$ |
| Initial DO of mixture | $D_0$ | $6\ \text{mg/L}$ |
| 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. Because the sample is diluted inside the 300 mL BOD bottle, the observed DO drop must be scaled up by the dilution factor $P$ (fraction of the bottle that is actual sample) to recover the BOD of the neat effluent; the 20-day plateau is taken as a direct estimate of the ultimate (asymptotic) oxygen demand since the reaction has effectively stopped.
| Quantity | Result |
|---|---|
| Dilution factor | $P=0.333$ |
| 5-day CBOD | 12.0 mg/L |
| Ultimate CBOD | 17.4 mg/L |
Standard Methods normally subtracts a dilution-water blank DO depletion $(B_1-B_2)\cdot f$ to correct for oxygen demand contributed by the seed/dilution water itself; none is given here, so it is taken as negligible (consistent with using clean dilution water and an already-nitrification-inhibited, low-strength tertiary effluent). Reading the 20-day plateau directly as CBOD∞ also implicitly assumes a first-order rate constant high enough that the reaction is >95% complete by day 20 (roughly $k_1\gtrsim0.15\ \text{d}^{-1}$); with no independent $k_1$ given this is the best available estimate.
Selecting option (1): a DAF thickener for 10 ML/d of waste activated sludge (WAS), concentrating it from 0.5% to 2% total solids. The design proceeds through the following steps.
1 — Establish the solids and hydraulic loads. Convert the 10 ML/d flow at 0.5% (5,000 mg/L) solids to a mass loading: $10{,}000\ \text{m}^3/\text{d}\times5{,}000\ \text{mg/L}=50{,}000\ \text{kg/d}$ of dry solids. This mass, not the volumetric flow, is the design basis for the flotation unit (loaded as a solids flux in kg/m²·h).
2 — Bench-test the air-to-solids (A/S) ratio. Run a bench flotation cell (or use manufacturer pilot data) at several A/S ratios to find the value — typically 0.02–0.04 kg air/kg solids for WAS — that gives the target 2% float-solids concentration and a clear subnatant; the A/S ratio, not just recycle percentage, is the controlling 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. Compute the recycle flow and saturator pressure needed to deliver that A/S ratio: $A/S = \dfrac{1.3\,s_a(f\,P-1)R}{Q_{ws}\,S_{ws}}$, where $s_a$ is air solubility (mL/L) at the saturator pressure $P$ (atm), $f$ is the fraction of saturation achieved (0.5–0.8 typical), $R$ is recycle flow and $Q_{ws}S_{ws}$ is the WAS mass loading from Step 1. Saturator pressures of 275–480 kPa (40–70 psig) are typical for sludge thickening (higher than for clarification duty because of the higher solids loading).
4 — Size the flotation tank surface area. Apply an allowable solids loading rate (SLR, kg/m²·h, typically 4–10 for WAS thickening) and hydraulic loading rate (HLR, m/h) from the bench/pilot data or design tables; the tank area is the larger of $A=\dot m_{solids}/\text{SLR}$ and $A=(Q_{ws}+R)/\text{HLR}$, so both constraints are checked and the governing one sized.
5 — Detail the mechanical elements. Specify the skimmer/scraper mechanism and float-removal rate to carry the 2% float layer off continuously without re-entraining it, the subnatant (underflow) withdrawal to recycle or discharge to the head of the plant, chemical-conditioning (polymer) dosing ahead of the DAF if the bench tests show it improves float quality, and freeboard/baffling to prevent short-circuiting of the incoming feed to the outlet.
6 — Confirm with a mass balance and safety factor. Close the solids balance (feed = float + subnatant solids) using the bench-determined float concentration (2%) and an assumed capture efficiency (typically ≥90–95% for WAS), then apply a peaking/safety factor (commonly 1.5–2× average solids loading) to the sized area to accommodate diurnal WAS wasting variability.
