Water Hardness Determination and Industrial Treatment Methods

Posted on Nov 9, 2025 in Chemistry

Estimation of Water Hardness using EDTA Titration

B.Tech. Unit VI: Water Chemistry

Principle of EDTA Titration

  • Hardness in water is primarily due to Ca²⁺ and Mg²⁺ ions.
  • It is determined by complexometric titration using EDTA (Ethylenediaminetetraacetic acid).
  • EDTA forms a stable, soluble complex with Ca²⁺ and Mg²⁺ ions.
  • Indicator used: Eriochrome Black-T (EBT).
  • At pH ≈ 10 (buffered with NH₄OH–NH₄Cl buffer), EBT forms a wine-red complex with Ca²⁺/Mg²⁺.
  • During titration, EDTA replaces EBT, forming a colorless Ca/Mg–EDTA complex. The endpoint is indicated by a color change from wine-red to pure blue.

Procedure

  1. Take a measured volume (50–100 mL) of the hard water sample.
  2. Add buffer solution (NH₄OH–NH₄Cl) to maintain pH ≈ 10.
  3. Add a few drops of EBT indicator; the solution turns wine-red.
  4. Titrate against standard EDTA solution taken in a burette.
  5. At the endpoint, the color changes from wine-red to clear blue.

Calculations

  • Equivalence: 1 mL of 0.01 M EDTA ≡ 1 mg of CaCO₃ equivalent hardness.

Hardness (mg/L or ppm) is calculated using the formula:

Hardness (mg/L or ppm) = (V × M × 1000) / Volume of sample in mL

  • V = Volume of EDTA used (mL)
  • M = Molarity of EDTA

Advantages of the EDTA Method

  • Quick and accurate.
  • Can estimate total, temporary, and permanent hardness (by suitable pre-treatment like boiling).

Soda Lime Softening Methods

Hot Soda Lime Method

  • In this method, lime [Ca(OH)₂] and soda [Na₂CO₃] are added to water at 80–150°C.
  • Hardness-causing salts are precipitated as CaCO₃ and Mg(OH)₂.
  • Advantages:
    • Reactions are fast and complete.
    • Sludge settles quickly; no coagulant is required.
    • Removes dissolved gases (O₂, CO₂) which prevents corrosion.
    • Produces soft water with very low residual hardness.
  • Use: Primarily for boiler feed water treatment.

Cold Soda Lime Method

  • Lime and soda are added to water at room temperature.
  • Removes both temporary and permanent hardness by forming insoluble precipitates (CaCO₃, Mg(OH)₂).
  • Disadvantage: The sludge formed is very fine and does not settle easily, requiring coagulants like alum or sodium aluminate.
  • Produces water with moderate residual hardness (less soft than the hot process).
  • Use: Suitable for small industries and domestic supply.

Industrial Water Treatment Processes

Industrial water must be treated rigorously to avoid scaling, corrosion, and contamination.

Key Treatment Steps

  1. Screening & Sedimentation: Removal of large suspended solids.
  2. Coagulation & Filtration: Removal of colloids and fine particles.
  3. Softening: Methods include Lime-soda, Zeolite, or Ion-exchange.
  4. Demineralization/Desalination: Methods include Ion-exchange, Electrodialysis, or Reverse Osmosis (RO).
  5. Deaeration: Removal of dissolved O₂ and CO₂.
  6. Disinfection: Methods include chlorination, ozonation, or UV treatment.

This ensures water is suitable for critical applications such as boilers, cooling towers, textiles, food, and pharmaceutical industries.

Municipal Wastewater Treatment Processes

The treatment process involves several stages to purify wastewater before discharge.

Stages of Wastewater Treatment

  1. Preliminary Treatment
    • Screening: Removes large floating matter.
    • Grit Chamber: Removes heavy inorganic solids like sand and stones.
  2. Primary Treatment
    • Sedimentation: Allows suspended solids to settle.
    • Coagulation: Uses chemicals (e.g., alum, FeSO₄, NaAlO₂) to remove colloids.
  3. Secondary Treatment (Biological)
    • Aeration Tank: Utilizes activated sludge or trickling filters.
    • Microorganisms decompose organic matter, significantly reducing Biological Oxygen Demand (BOD).
  4. Tertiary Treatment
    • Advanced processes (Filtration, Ion Exchange, Reverse Osmosis) remove dissolved solids, nutrients (Nitrogen, Phosphorus), and heavy metals.
  5. Disinfection
    • Methods like chlorination, ozonation, or UV treatment are used to kill pathogens.
  6. Sludge Treatment
    • Involves digestion, drying, and safe disposal of the residual sludge.

Wastewater Treatment Flow Diagram

Screening → Grit Chamber → Sedimentation Tank → Aeration → Secondary Clarifier → Filtration → Disinfection → Treated Water.