Question 3 of 7: Particle Characteristics, Solution Chemistry and Gaseous Emissions

Question 3: Particle Characteristics, Solution Chemistry and Gaseous Emissions (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

Most turbidity in surface water is carried by colloidal particles (clays, natural organic matter, microorganisms) that are too small to settle and that repel one another because they carry like (usually negative) surface charges. Coagulation, flocculation and filtration act as a sequence that overcomes this stability. Coagulation is the rapid-mix addition of a metal-salt coagulant (alum or ferric chloride) or polymer, which neutralises the colloids’ surface charge and compresses their electrical double layer so they can approach one another. Flocculation is gentle, prolonged stirring that promotes particle collisions, so the destabilised colloids aggregate into larger, denser, settleable flocs; too much shear breaks the flocs, too little fails to grow them. Sedimentation then removes the bulk of the floc by gravity, and filtration through a rapid granular (sand/anthracite) bed captures the residual fine floc and any carried-over particles, polishing the water to low turbidity before disinfection. The processes are complementary: filtration alone would blind almost immediately on raw colloids, so coagulation–flocculation is what makes the particles large enough for sedimentation and filtration to remove economically.

Part (ii) — Hardness of the lake water as CaCO₃

Given. A divalent-cation analysis to be expressed as calcium carbonate, using the atomic weights supplied.

Find. The total hardness in mg/L as CaCO₃ and the classification of the water.

Approach. Convert each multivalent metal cation to a common CaCO₃ basis through equivalent weights — CaCO₃ has molar mass 100 and equivalent weight 50 — then sum.

1. Conversion factor. Hardness of a species as CaCO₃ is $$H_i=C_i\left(\frac{50}{\text{EW}_i}\right),\qquad \text{EW}_i=\frac{\text{AW}_i}{2}.$$
2. Calcium. $H_{Ca}=100\left(\dfrac{50}{20}\right)=250\ \text{mg/L as CaCO}_3.$
3. Magnesium. $H_{Mg}=500\left(\dfrac{50}{12}\right)=2083\ \text{mg/L as CaCO}_3.$ Although Mg is reported at 5× the calcium load, its smaller equivalent weight (12 vs. 20) makes it dominate even more strongly — each mg of Mg carries 1.67× the charge of a mg of Ca.
4. Copper. $H_{Cu}=80\left(\dfrac{50}{32}\right)=125\ \text{mg/L as CaCO}_3.$ Cu²⁺ is a divalent metal cation and strictly contributes to hardness, though "hardness" conventionally means the calcium–magnesium sum.
5. Totals. $$\boxed{\text{Ca}+\text{Mg}=250+2083=2333\ \text{mg/L as CaCO}_3}$$ Including copper, the strict total is $2333+125=2458\ \text{mg/L as CaCO}_3.$
6. Classify. On the three-band scale offered (soft < 75, moderately hard 75–150, hard > 150 mg/L as CaCO₃), any of these totals is far above 150, so the water is hard — in fact extremely (very) hard. On the Health Canada four-band scale (very hard > 180) it is likewise very hard.

Part (iii) — Chlorine gas for drinking-water disinfection

Taking chlorine gas [Cl₂(g)] as the selected disinfectant: gaseous chlorine is dosed into the water where it hydrolyses to hypochlorous acid, $\text{Cl}_2+\text{H}_2\text{O}\rightleftharpoons \text{HOCl}+\text{H}^{+}+\text{Cl}^{-}$, the active biocide that oxidises microbial enzymes and nucleic acids and inactivates bacteria and viruses. Its principal advantages are low cost, a strong and well-understood oxidising action, and a persistent free-chlorine residual that continues to protect water throughout the distribution system — the reason it remains the most widely used municipal disinfectant. The chief drawbacks are the formation of disinfection by-products (trihalomethanes and haloacetic acids) when chlorine reacts with natural organic matter, taste and odour, reduced effectiveness at high pH, and above all the hazard of the gas itself.

Special precautions centre on the acute toxicity of chlorine gas, which is heavier than air, corrosive and lethal at low concentration. Cylinders and ton containers must be stored in a separate, cool, well-ventilated chlorine room with low-level exhaust ventilation and gas detection/alarms; operators must have self-contained breathing apparatus and emergency scrubber or neutralisation systems; leak repair kits and buddy-system entry procedures are required; and the room must be isolated from occupied and public areas. Dosing is controlled to leave an adequate but not excessive free residual, and pH is managed to keep HOCl dominant. These handling requirements are why many utilities switch to sodium hypochlorite or on-site generation despite chlorine gas’s cost advantage.