18-Env-A1 Principles of Environmental Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2014 — 04-Env-A1 / Principles of Environmental Engineering. 3 hours duration; closed book with an 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five questions constitute a complete paper (first five answers marked); all seven are solved below for completeness. Each question is worth 20 marks.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); MWH’s Water Treatment: Principles and Design (3rd ed.); Guidelines for Canadian Drinking Water Quality (Health Canada); Canadian Council of Ministers of the Environment (CCME) water-quality guidelines; Canadian Environmental Protection Act, 1999 (CEPA); Andrews, Canadian Professional Engineering and Geoscience (professional ethics).
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.
Given. A well-mixed lake with first-order TP decay:
| Quantity | Symbol | Value |
|---|---|---|
| Lake volume | $V$ | $1\times10^{7}$ m³ |
| Upstream river inflow | $Q_u$ | $9\times10^{4}$ m³/d |
| Evaporation loss | $Q_e$ | $2\times10^{3}$ m³/d |
| Lake outflow | $Q_o$ | $6\times10^{4}$ m³/d |
| Upstream TP concentration | $C_u$ | 20 mg/L |
| TP decay rate in lake | $k$ | 0.01 /d |
Find. The steady-state TP concentration $C$ in the lake (and, since the lake is modelled as completely mixed, in the outflow stream as well).
Approach. Model the lake as a single well-mixed (CSTR) reactor and write a steady-state mass balance on TP, with first-order decay removing mass within the lake volume; evaporation carries no TP (it removes pure water) and so appears in the lake's water budget but not in the TP mass balance.
| Quantity | Value |
|---|---|
| Effective removal capacity, $Q_o + kV$ | $1.6\times10^{5}$ m³/d |
| Steady-state TP concentration in the lake | ≈ 11.25 mg/L |
| Steady-state TP concentration in the outflow | ≈ 11.25 mg/L (same, completely-mixed lake) |
Two partition coefficients dominate how a dissolved organic contaminant redistributes between water and the other phases it contacts. The first is the octanol–water partition coefficient, $K_{OW} = C_{octanol}/C_{water}$, normally reported as $\log K_{OW}$, which uses n-octanol as a laboratory surrogate for biological lipid tissue. Compounds with high $\log K_{OW}$ (PCBs, organochlorine pesticides, dioxins typically fall in the 5–7 range) are strongly lipophilic: once such a molecule crosses a fish's gill or gut membrane, it partitions preferentially into the fish's fat rather than re-dissolving into the surrounding water, so tissue concentrations climb far above the ambient water concentration. This is the physical basis of bioaccumulation within one organism and biomagnification up the food chain, and engineers use $K_{OW}$ to screen contaminants for tissue-residue and human food-chain exposure risk even when the dissolved-phase concentration looks benign.
The second is the organic-carbon partition coefficient, $K_{OC} = C_{sorbed}/C_{water}$ normalized to the fraction of organic carbon in the solid phase, which governs how strongly a hydrophobic organic compound sorbs onto soil, sediment or suspended solids rather than remaining dissolved. A high $K_{OC}$ compound travels with sediment (settling out in quiescent zones, resuspending during storm events or dredging) rather than with the water column, which is why sediment quality guidelines, not just water-column criteria, are used to assess and remediate contaminated sites, and why removing suspended solids (Q3(i)) is itself an effective contaminant-control strategy for these compounds.
Regardless of the disinfectant chosen (chlorine, UV, ozone), three design parameters govern whether a disinfection system reliably achieves its target pathogen inactivation. The first is disinfectant dose/concentration ($C$) — the applied residual must be high enough, after satisfying any immediate demand from organic matter or reduced species in the water, to sustain an effective driving concentration through the full contact time; under-dosing (e.g., from unaccounted chlorine demand in a turbid or high-organic source) is the single most common cause of disinfection failure. The second is effective contact time ($T$), governed jointly by tank volume/flow and by hydraulic efficiency: because short-circuiting lets some water reach the outlet far faster than the nominal (volume/flow) detention time, contact tanks are baffled (serpentine, over-and-under) to push the residence-time distribution toward plug flow, and regulators require the design to use the conservative $T_{10}$ (time within which only 10% of tracer has broken through) rather than the nominal detention time. Together $C$ and $T$ combine in the Chick–Watson $C\cdot T$ criterion, $C\cdot T \geq (C\cdot T)_{required}$ for the target pathogen and log-removal credit. The third is water quality entering the disinfection step, principally turbidity and pH: particles can shield pathogens from disinfectant contact (embedded micro-organisms are not exposed to the bulk residual), which is why disinfection is placed after clarification/filtration (Q3(i)) rather than on raw water, and pH controls chlorine speciation ($\text{HOCl}$, the far more effective species, dominates below roughly pH 7.5; disinfection efficiency drops sharply as pH rises and the equilibrium shifts toward the weaker $\text{OCl}^-$ form).
