Water Treatment, Purification & Hardness of Water
Sources of water in Ghana, stages of municipal water treatment, physical and chemical tests for pure water, causes and effects of water hardness (temporary vs permanent), and softening methods.
The Weija Water Works and Kpong Water Treatment Plants supply millions of residents in the Greater Accra Metropolitan Area by purifying water abstracted from the Densu River and Volta River through multi-stage industrial clarification and chlorination.
Key Syllabus Learning Objectives
- 1Describe the 6 major stages of municipal water purification used by Ghana Water Company Limited
- 2State the physical and chemical tests used to identify water and verify purity
- 3Distinguish between temporary and permanent hardness of water by their chemical causes and removal techniques
- 4Evaluate the industrial, economic, and health consequences of hard water vs soft water
Municipal Water Purification by GWCL
- Coagulation Chemistry: Alum [Al₂(SO₄)₃] hydrolyzes in water to form gelatinous aluminum hydroxide [Al(OH)₃] which binds negatively charged colloidal clay particles into heavy settleable flocs.
- Disinfection: Chlorine forms hypochlorous acid (HOCl), a powerful oxidizing agent that penetrates bacterial cell walls and inactivates metabolic enzymes.
- Residual Chlorine: GWCL maintains a small residual concentration of free chlorine (~0.2 - 0.5 mg/L) in distribution mains to prevent bacterial re-growth during piping to homes.
During heavy rainfall in the Birim and Pra river basins polluted by galamsey mining, GWCL must use triple the normal dose of alum to clarify high-turbidity raw water.
Coagulation aggregates colloidal particles for sedimentation; chlorination destroys pathogenic microorganisms.
Chemistry of Water Hardness: Scum & Scale Formation
- Scum Formation: Soap (sodium stearate, C₁₇H₃₅COONa) reacts with Ca²⁺ to precipitate insoluble calcium stearate: 2C₁₇H₃₅COO⁻ + Ca²⁺ → (C₁₇H₃₅COO)₂Ca(s).
- Kettle Scale (Fur): Boiling temporary hard water decomposes calcium hydrogen carbonate into insoluble calcium carbonate (CaCO₃ scale), which coats heating elements and reduces thermal efficiency.
- Ion-Exchange Resin (Permutit): Hard water passes through a column containing zeolite (sodium aluminosilicate, Na₂Z). Ca²⁺ and Mg²⁺ ions displace Na⁺ ions into solution, completely softening both temporary and permanent hard water.
Electric kettles in Tema and Cape Coast develop thick crusts of white limescale after months of boiling borehole water.
Boiling removes only temporary hardness by precipitating CaCO₃; permanent hardness requires chemical precipitants or ion-exchange resins.
- Distinguish between a test for water (turns anhydrous CuSO₄ from white to blue) and a test for PURE water (boils sharply at 100°C at 1 atmosphere).
- When asked to write the equation for removing temporary hardness by boiling, write: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g).
- State clearly that synthetic detergents do NOT form scum with hard water because their calcium and magnesium salts are water-soluble.
- Claiming that boiling removes permanent hardness (boiling has zero effect on CaSO₄ or MgCl₂).
- Stating that anhydrous copper sulfate turns pink (it turns blue; cobalt chloride turns pink).
- Confusing distilled water (pure H₂O) with potable water (safe to drink, but contains dissolved beneficial minerals).
- Can I list the six stages of municipal water purification in chronological order?
- Can I write balanced equations for the thermal decomposition of Ca(HCO₃)₂?
- Can I explain how an ion-exchange resin softens water?
- Can I list two industrial disadvantages and two health benefits of hard water?
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