16-Civ-A3 Elementary Environmental Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams 98-Civ-A3 Environmental Engineering, May 2014 — 3 hours, closed book with one candidate-prepared double-sided aid sheet, approved Casio or Sharp calculator only. Seven questions are offered; any five constitute a complete paper (20 marks each, 100 marks maximum), and only the first five answers in the work book are marked. All seven are solved here, because the set is intended as a study resource rather than an examination script. Section marks are shown in brackets at the left margin of each part, and the marking scheme on page 6 confirms the split.
Reference texts. Davis & Cornwell, Introduction to Environmental Engineering (5th ed.); Mihelcic & Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design (3rd ed.); Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.); Crittenden et al., MWH’s Water Treatment: Principles and Design (3rd ed.). Canadian regulatory frame: the federal Impact Assessment Act (2019) and the Impact Assessment Agency of Canada, the Canadian Environmental Protection Act (CEPA 1999), CCME Canadian Environmental Quality Guidelines, and Health Canada’s Guidelines for Canadian Drinking Water Quality (GCDWQ).
Check: Henry’s law constant units in Question 1(i). The paper writes the constant as “0.30 (mol/atm)”, which is dimensionally incomplete — a Henry’s constant in the concentration/pressure form must carry a volume in the denominator. It is taken here as 0.30 mol/(L·atm), i.e. the aqueous-concentration form $C_{aq}=K_H\,p$. That reading is confirmed by the published value for ethyl acetate, $H \approx 1.3\times10^{-3}\ \text{atm}\cdot\text{m}^3/\text{mol}$, whose reciprocal is $\approx 0.77\ \text{mol}/(\text{L}\cdot\text{atm})$ — the same order of magnitude. Per NOTE 1 on page 1, this assumption is stated with the answer.
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.
Principle 1: transport of particles to the collector surface. A granular filter does not work as a sieve. The pore openings between 0.5–1.0 mm sand grains are on the order of 100 µm, whereas the particles that matter — clay colloids, precipitated floc fragments, bacteria and Cryptosporidium oocysts — are one to fifty micrometres or smaller and would pass straight through an opening of that size. Instead, removal begins with transport mechanisms that carry a particle out of the streamline and onto a grain: interception, where a particle following a streamline that passes within one particle radius of a grain touches it; sedimentation, where a particle denser than water settles across streamlines onto the upper surface of a grain; and Brownian diffusion, which dominates for submicron particles undergoing random thermal motion. Hydrodynamic action and inertial impaction contribute at higher velocities. These mechanisms are why filter performance is a strong function of grain size, filtration rate and particle size, and why there is a characteristic “most difficult” particle size near 1–2 µm where transport by both sedimentation and diffusion is weak.
Principle 2: attachment through destabilisation of surface charge. Transport only brings a particle close; it must then stick. Natural particles and sand grains both carry a negative surface charge at typical water pH, so the electrical double layers repel and a colliding particle is simply carried away again. Effective filtration therefore depends on upstream coagulation — adding alum or ferric salts, or a cationic polymer — to compress the double layer, neutralise charge and permit the short-range van der Waals attraction to hold the particle to the grain. The efficiency of this step is captured by the collision (attachment) efficiency $\alpha$, and it is the single most common reason a filter underperforms: a filter with excellent hydraulics but poorly coagulated influent will pass particles. Once attachment begins, previously deposited particles act as additional collectors, so removal improves through the ripening period before head loss and eventual breakthrough force a backwash.
Filtration is depth-based rather than surface-based, which is why dual-media beds place coarse anthracite above fine sand: the coarse layer removes larger floc without clogging the surface, and the fine layer polishes, so the whole bed depth participates and the run length is set by head loss rather than by a surface mat.
Why turbidity indicates effective filtration. The single most important reason is that turbidity is a real-time, continuously measurable surrogate for the particles that carry pathogen risk, particularly chlorine-resistant protozoan cysts, which cannot themselves be monitored on any useful timescale. A Cryptosporidium analysis takes days and is neither continuous nor sensitive at the concentrations of concern, whereas a nephelometer on each filter effluent reports every few seconds and will register the particle breakthrough that would accompany an oocyst passage. Low filtered-water turbidity also protects downstream disinfection, since particles shield organisms from oxidants and UV and exert disinfectant demand. Canadian practice under the Guidelines for Canadian Drinking Water Quality therefore sets a treatment-based target for chemically assisted filtration of < 0.3 NTU in at least 95 % of measurements per month, and never above 1.0 NTU, monitored on individual filter effluent rather than only on the combined stream so that one failing filter cannot hide behind the others.
Given. A lake water near a rock quarry is analysed for its divalent cations, and hardness must be expressed on the conventional calcium-carbonate equivalent basis.
| Cation | Concentration | Atomic weight | Charge | Equivalent weight |
|---|---|---|---|---|
| $\text{Ca}^{2+}$ | 70 mg/L | 40 (given) | 2+ | 20 g/eq |
| $\text{Mg}^{2+}$ | 50 mg/L | 24.3 | 2+ | 12.15 g/eq |
| $\text{Fe}^{2+}$ | 10 mg/L | 55.85 | 2+ | 27.93 g/eq |
| $\text{CaCO}_3$ reference | — | 40 + 12 + 3(16) = 100 | 2 | 50 g/eq |
Find. Total hardness in mg/L as $\text{CaCO}_3$, and the classification of the water on the standard soft / moderately hard / hard scale.
Check: atomic weights not supplied by the paper. The question lists atomic weights for Ca, H, C and O only — enough to build the $\text{CaCO}_3$ reference and convert calcium — but gives none for Mg or Fe, which are nonetheless required. Standard values Mg = 24.3 and Fe = 55.85 are adopted and stated here per NOTE 1. Rounding these to 24 and 56 changes the total hardness by less than 1 % (401 rather than 399 mg/L as $\text{CaCO}_3$) and does not affect the classification.
Approach. Convert each divalent cation to its calcium-carbonate equivalent by the ratio of equivalent weights, then sum. Hardness is an equivalence concept, so the conversion must go through charge equivalents, not molar mass alone.
| Contribution | mg/L as $\text{CaCO}_3$ |
|---|---|
| Calcium hardness | 175.0 |
| Magnesium hardness | 205.8 |
| Ferrous iron hardness | 17.9 |
| Total hardness (Ca + Mg) | 380.8 |
| Total hardness (Ca + Mg + Fe) | 398.7 |
| Classification | Hard (very hard on the four-band scale) |
The engineering consequence is direct. At close to 400 mg/L as $\text{CaCO}_3$ this water would scale hot-water systems and heat exchangers, consume excessive soap, and justify lime–soda ash or ion-exchange softening; a practical target of 80–120 mg/L as $\text{CaCO}_3$ is usual, since fully softened water is corrosive and must be stabilised. The high magnesium fraction matters operationally, because magnesium removal requires excess lime to raise the pH above about 11 and is considerably more costly than removing calcium alone. The ferrous iron would also be oxidised and filtered in any case, since 10 mg/L is far above the 0.3 mg/L aesthetic objective for iron in Canadian drinking water.
Carbon monoxide — health impact. CO is an odourless, colourless product of incomplete combustion whose toxicity arises from its affinity for haemoglobin, roughly 200 to 250 times that of oxygen. It binds to form carboxyhaemoglobin, which both removes haemoglobin from oxygen transport and shifts the oxyhaemoglobin dissociation curve leftward so that the remaining oxygen is released less readily to tissue. The result is chemical asphyxiation: headache and impaired judgement at COHb levels of a few percent, and unconsciousness or death above roughly 40 %. Because the gas gives no warning and the early symptoms mimic influenza, CO is the leading cause of fatal poisoning in Canadian homes, and cardiac and foetal tissue are the most vulnerable.
Carbon monoxide — engineering control. The primary control is combustion management: ensuring adequate excess air and good air–fuel mixing so oxidation proceeds fully to $\text{CO}_2$, with staged or low-NOx burners tuned so that the measures taken to suppress nitrogen oxides do not quench CO oxidation. For mobile sources the equivalent control is the three-way catalytic converter, which oxidises CO to $\text{CO}_2$ over platinum and palladium under closed-loop oxygen-sensor control — the measure responsible for the collapse of urban CO concentrations in Canadian cities since the 1970s. In enclosed spaces such as parking garages, the control is ventilation interlocked to CO sensors, backed by CSA-certified CO alarms, which most provincial building codes now require in dwellings with fuel-fired appliances or attached garages.
Hydrogen sulphide — environmental and health impact. $\text{H}_2\text{S}$ is generated when sulphate-reducing bacteria in the anaerobic slime layer of a sewer reduce sulphate to sulphide, which then partitions into the sewer atmosphere. Its most costly environmental impact is crown corrosion of concrete sewers: Thiobacillus bacteria on the moist pipe crown oxidise the gas to sulphuric acid, which attacks the cement matrix and can destroy the upper wall of a large trunk sewer within a decade or two, an infrastructure loss running to hundreds of dollars per metre to replace. Its health impact is more acute — the gas is detectable by odour at a few parts per billion, but it paralyses the olfactory nerve above roughly 100 ppm, so the warning disappears exactly when the danger becomes severe, and concentrations of several hundred ppm cause rapid respiratory arrest. It is a leading cause of confined-space fatalities in municipal work, frequently killing would-be rescuers as well.
Hydrogen sulphide — engineering control. The most effective measures act on the sewer chemistry to prevent generation: chemical dosing of the upstream force main with nitrate, so that bacteria respire nitrate in preference to sulphate; with iron salts, which precipitate sulphide as insoluble iron sulphide; or with magnesium hydroxide to raise the pH so that sulphide stays in the non-volatile $\text{HS}^-$ form. These are combined with hydraulic measures — shortening force-main detention time, eliminating turbulent drops that strip the gas, and providing forced ventilation at drop structures — and with materials measures such as PVC-lined or high-alumina-cement pipe where crown corrosion is unavoidable. Wet wells and headworks are ventilated to a foul-air scrubber or biofilter, and all entries follow a confined-space entry programme with continuous gas monitoring, mandated by provincial occupational health and safety regulation and WorkSafeBC in British Columbia.
