Methods of Removal of Hardness of Water | Engineering chemistry notes by Mohan Dangi (Gold medalist)

Methods of Removal of Hardness of Water (with Figures)

Below are the four principal methods used to remove hardness from water, each accompanied by an inline SVG schematic to aid conceptual understanding.

1. Boiling Method

Applicable for: Temporary hardness (bicarbonates of Ca & Mg).

Principle & reactions:

\(Ca(HCO_3)_2 \xrightarrow{\Delta} CaCO_3\downarrow + CO_2\uparrow + H_2O\)

\(Mg(HCO_3)_2 \xrightarrow{\Delta} Mg(OH)_2\downarrow + 2CO_2\uparrow\)

Simple boiling setup: precipitate (scale) forms and can be removed by sedimentation/filtration.

2. Lime–Soda Process

Applicable for: Both temporary and permanent hardness. Common for municipal water softening.

Reactions (summary):

• \(Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2CaCO_3\downarrow + 2H_2O\)
• \(CaSO_4 + Na_2CO_3 \rightarrow CaCO_3\downarrow + Na_2SO_4\)
• \(MgCl_2 + Ca(OH)_2 \rightarrow Mg(OH)_2\downarrow + CaCl_2\)

Lime–soda process schematic: chemical addition, flocculation to form carbonate/hydroxide precipitates, followed by clarification and sludge removal.

3. Zeolite (Permutit) Process

Applicable for: Permanent hardness removal by cation exchange (Ca²⁺/Mg²⁺ ↔ Na⁺).

Principle: Na-form zeolite exchanges Na⁺ for Ca²⁺ and Mg²⁺. The exhausted zeolite is regenerated by brine (NaCl) solution.

Exchange reactions:

\(Ze–Na_2 + Ca^{2+} \rightarrow Ze–Ca + 2Na^+\)

\(Ze–Na_2 + Mg^{2+} \rightarrow Ze–Mg + 2Na^+\)

Zeolite column: continuous softening via ion exchange; periodic regeneration required with concentrated NaCl solution.

4. Ion‑Exchange Method (Resin-based)

Applicable for: Both temporary & permanent hardness; also used for demineralization.

Principle: Organic polymeric cation-exchange resins (sodium or hydrogen form) exchange their counter-ions for Ca²⁺ and Mg²⁺. Anion resins remove anions if full demineralization is required.

Representative reactions:

\(2R–Na + Ca^{2+} \rightarrow R_2–Ca + 2Na^+\)

\(R'–OH + Cl^- \rightarrow R'–Cl + OH^-\)

Ion-exchange unit for softening/demineralization. Cation resins (Na⁺ form) remove Ca²⁺/Mg²⁺; anion resins remove anions for full deionization.

Comparison Table (Concise)

Method	Removes	Scale	Regeneration/Disposal
Boiling	Temporary	Small-scale/domestic	Precipitate disposal
Lime–Soda	Both	Municipal/large	Sludge disposal (CaCO₃ etc.)
Zeolite	Permanent (cations)	Industrial/continuous	Brine regeneration & spent brine disposal
Ion‑Exchange	Both / demineralization	Lab/industrial	Chemical regeneration (acid/base) & waste streams

Revision Box: Key Takeaways

• Boiling is simple but only removes temporary hardness.
• Lime–soda is economical for municipal use but generates sludge.
• Zeolite process is continuous and regenerable; effective for cation removal.
• Ion‑exchange gives highest purity but requires resin regeneration and skilled operation.