Question 3 of 7: Particles, Gaseous Emissions and Solution Chemistry

Question 3: Particles, Gaseous Emissions and Solution Chemistry (20 marks)

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.

Part (i) — Coagulation, flocculation and filtration

Colloidal and fine suspended particles in surface water (clay, silica, organic matter, some pathogens) are too small to settle in any practical time and carry a negative surface charge that keeps them mutually repelled and stable. The three units act in sequence to overcome this. Coagulation is rapid, high-intensity mixing of a metal-salt coagulant (alum or ferric chloride) that neutralises the particle charge and generates gelatinous metal-hydroxide precipitate, destabilising the colloids so they can approach one another. Flocculation is slow, gentle stirring that promotes particle–particle collisions so the destabilised fines aggregate into larger, denser, settleable flocs — too much energy would shear the flocs apart. Sedimentation then removes the bulk of these flocs by gravity. Filtration (granular or dual-media beds) is the polishing barrier that captures the residual small flocs and fine particles that did not settle, by straining, interception and adsorption within the media, producing low-turbidity water suitable for disinfection. The units are complementary: coagulation makes particles stickable, flocculation makes them settleable, and filtration removes what remains.

Part (ii) — Gaseous emissions at pumping stations

Anaerobic conditions in collection systems generate hydrogen sulfide (H₂S), methane (CH₄) and other reduced-sulfur odorants that accumulate at pumping stations. Impacts (health & safety / environmental): (1) H₂S is acutely toxic — it paralyses the sense of smell at high concentration and can be fatal in confined spaces (a confined-space asphyxiation and poisoning hazard for workers); (2) methane is flammable and forms an explosion hazard, while H₂S oxidises to sulfuric acid that causes “crown corrosion” of concrete and steel, and both contribute nuisance odours to the surrounding community. Engineering controls: (1) source control and ventilation — force-ventilate wet wells and confined spaces, and add oxygen/nitrate or chemical dosing (e.g., iron salts, hydrogen peroxide) to the sewer to suppress sulfide formation; (2) capture and treatment — enclose and extract the foul air to a treatment unit such as a biofilter, activated-carbon adsorber or chemical scrubber before discharge. Confined-space entry procedures, gas detection and adequate ventilation are mandatory under Canadian OH&S regulations.

Part (iii) — Hardness as CaCO₃

Given. Groundwater near a limestone quarry with the divalent-cation analysis below (printed atomic weights adopted).

Find. Total hardness expressed as mg/L CaCO₃, and the classification of the water.

Approach. Convert each multivalent cation to its calcium-carbonate equivalent through equivalent weights (CaCO₃ equivalent weight = 50), sum the conventional hardness cations, and classify.

1. Convert each ion to CaCO₃ equivalents. Using C_{as CaCO3} = C × (50 / EW_ion): $$\text{Ca: } 40\times\frac{50}{20.0}=100.0\ \tfrac{\text{mg}}{\text{L}},\qquad \text{Mg: } 20\times\frac{50}{12.0}=83.3\ \tfrac{\text{mg}}{\text{L}}$$ $$\text{Cu: } 40\times\frac{50}{32.0}=62.5\ \tfrac{\text{mg}}{\text{L}}$$
2. Conventional (Ca + Mg) hardness. Total hardness is, by definition, the sum of the calcium and magnesium contributions: $$\boxed{\ TH=100.0+83.3=183.3\ \text{mg/L as CaCO}_3\ }$$

Classification. On the common three-band scale (soft < 75, moderately hard 75–150, hard > 150 mg/L as CaCO₃) the water at 183 mg/L is hard; on the four-band GCDWQ scale (soft < 60, moderate 60–120, hard 120–180, very hard > 180) it sits just into the very hard band. Either way it warrants softening for domestic use, consistent with a limestone-quarry setting rich in calcium.