Unit 1: Water and its Treatment

Water is the most widely used industrial and domestic solvent, and its dissolved impurities decide its fitness for boilers, municipal supply and manufacturing. This unit hangs on one governing idea: the salts dissolved as water passes over rock (mainly Ca²⁺ and Mg²⁺) make it "hard", and every treatment method targets these ions.

• Hardness: the soap-consuming capacity of water caused by dissolved Ca²⁺ and Mg²⁺ salts (chlorides, sulphates, bicarbonates).
• Equivalence convention: all hardness-causing salts are expressed as an equivalent amount of CaCO₃ (molar mass 100, equivalent weight 50) so that unlike salts can be added together.
• Soft vs hard water: soft water lathers readily with soap; hard water first precipitates soap as scum before lathering.
• Reaction basis of scum: 2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl, where sodium stearate (soap) is wasted as insoluble calcium stearate.

II. Types of Hardness

Hardness is classified by whether boiling removes it.

A. Temporary (carbonate) hardness

• Cause: dissolved bicarbonates of calcium and magnesium, Ca(HCO₃)₂ and Mg(HCO₃)₂.
• Removal by boiling: decomposes bicarbonate to insoluble carbonate:
  
  TEXT

  Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑
  Mg(HCO₃)₂ → Mg(OH)₂↓ + 2CO₂↑

B. Permanent (non-carbonate) hardness

• Cause: chlorides and sulphates of Ca and Mg (CaCl₂, MgCl₂, CaSO₄, MgSO₄).
• Not removed by boiling: requires chemical softening (lime-soda, zeolite, ion exchange).
• Total hardness: temporary + permanent, all reported as CaCO₃ equivalent.

III. Units of Hardness of Water

Hardness is a small quantity, so it is scaled to a common CaCO₃ basis.

A. Units of hardness of water

• ppm (parts per million): parts of CaCO₃ equivalent per 10⁶ parts of water.
• mg/L: milligrams of CaCO₃ equivalent per litre; numerically 1 mg/L = 1 ppm.
• Clarke's degree (°Cl): parts of CaCO₃ per 70,000 parts of water; 1 °Cl = 1 grain per gallon.
• Degree French (°Fr): parts of CaCO₃ per 10⁵ parts of water.
• Interconversion: 1 ppm = 0.1 °Fr = 0.07 °Cl = 0.001 meq/L.
• CaCO₃ equivalent formula:
  
  TEXT

  CaCO₃ equivalent = mass of salt × (50 / equivalent weight of salt)

IV. Determination of Hardness by EDTA Method

A complexometric titration that measures total, and then permanent, hardness.

A. Determination of hardness by EDTA method

• Reagent: disodium salt of ethylenediaminetetraacetic acid (EDTA), a hexadentate ligand forming 1:1 metal complexes.
• Indicator: Eriochrome Black-T (EBT), used at buffered pH 9–10 (NH₄Cl–NH₄OH buffer).
• Colour change principle: EBT first binds Ca²⁺/Mg²⁺ giving a wine-red M–EBT complex; EDTA is added and pulls the metal off EBT because M–EDTA is more stable, freeing the indicator to its blue end point.
  
  TEXT

  M²⁺ + EBT → [M–EBT] (wine red)
  [M–EBT] + EDTA → [M–EDTA] (stable) + EBT (blue)

• Total hardness titration: titrate the raw water sample against standard EDTA; volume V₁.
• Permanent hardness titration: boil a fresh sample to remove bicarbonates, filter off the precipitate, then titrate; volume V₂.
• Temporary hardness: obtained by difference, Total − Permanent.
• Calculation (with 1 mL EDTA ≡ 1 mg CaCO₃):
  
  TEXT

  Total hardness (ppm)     = (V₁ / V_sample) × 1000
  Permanent hardness (ppm) = (V₂ / V_sample) × 1000
  Temporary hardness       = Total − Permanent

V. Alkalinity of Water and its Significance

Alkalinity is the acid-neutralising capacity due to hydroxide, carbonate and bicarbonate ions.

A. Alkalinity of water

• Ions responsible: OH⁻, CO₃²⁻ and HCO₃⁻; the combinations OH⁻+CO₃²⁻ or CO₃²⁻+HCO₃⁻ can coexist, but OH⁻ and HCO₃⁻ cannot.
• Two indicators: titrate against standard acid using phenolphthalein (end point pH 8.3, reading P) then methyl orange (end point pH 4.4, total reading M).
• Reactions: OH⁻ + H⁺ → H₂O; CO₃²⁻ + H⁺ → HCO₃⁻ (to P end); HCO₃⁻ + H⁺ → H₂O + CO₂ (to M end).
• Standard interpretation:

B. Its significance

• Boiler control: excess OH⁻/CO₃²⁻ causes caustic embrittlement and interferes with coagulation.
• Corrosion balance: correct alkalinity buffers pH and limits pipe corrosion.
• Dosing guide: the P and M values decide the lime–soda quantities needed for softening.

VI. Boiler Feed Water and its Treatment

Feed water must be nearly free of hardness and dissolved gases to protect boilers.

A. Boiler feed water and its treatment

• Scale and sludge: hardness salts deposit as hard scale (CaSO₄, Mg(OH)₂) that insulates and causes overheating, or loose sludge (MgCO₃) removed by blow-down.
• Priming and foaming: carry-over of water droplets with steam, caused by high dissolved solids and alkalinity.
• Caustic embrittlement: inter-crystalline cracking of boiler metal from concentrated NaOH in crevices; controlled by adding Na₂SO₄, tannin or lignin.
• Boiler corrosion: from dissolved O₂, CO₂ and acids; O₂ removed chemically by hydrazine N₂H₄ + O₂ → N₂ + 2H₂O or sodium sulphite.
• Internal treatments: phosphate conditioning (Na₃PO₄ gives soft, non-adherent Ca₃(PO₄)₂), carbonate and calgon (sodium hexametaphosphate) conditioning.

VII. Softening Methods and Numerical Problems Based on These Methods

Softening removes Ca²⁺ and Mg²⁺ so water suits boilers and processes.

A. Softening methods

1. Lime–soda process: lime Ca(OH)₂ removes temporary hardness and Mg; soda Na₂CO₃ removes permanent Ca hardness.
   
   TEXT

   Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O
   CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄

2. Zeolite (permutit) process: sodium aluminosilite Na₂Ze exchanges Na⁺ for hardness ions, then regenerates with brine.
   
   TEXT

   CaCl₂ + Na₂Ze → CaZe + 2NaCl   (softening)
   CaZe + 2NaCl → Na₂Ze + CaCl₂   (regeneration)

B. Numerical problems based on these methods

• Lime requirement (as CaO, eq. wt 28):
  
  TEXT

  Lime (mg/L) = (74/100) × [temp Ca + 2×temp Mg + perm Mg + CO₂ + HCO₃⁻ ... as CaCO₃]
  Soda (mg/L) = (106/100) × [perm Ca + perm Mg as CaCO₃]

• Worked example: water contains Ca(HCO₃)₂ = 16.2 mg/L. Convert to CaCO₃: 16.2 × (100/162) = 10 ppm. Lime needed for this component = (74/100) × 10 = 7.4 mg/L (as pure Ca(OH)₂). Each hardness salt is converted to CaCO₃ equivalent first, then summed before applying the lime and soda factors.

VIII. Membrane Based Treatment Processes

Pressure-driven membranes separate dissolved and suspended matter by size.

A. Membrane based treatment processes

• Reverse osmosis (RO): pressure (15–40 atm) above osmotic pressure forces water through a semipermeable cellulose-acetate/polyamide membrane, rejecting dissolved salts and giving desalinated water.
• Osmosis direction reversed: normally solvent flows to concentrated side; applied pressure reverses it, leaving ions behind.
• Ultrafiltration: lower pressure membrane removing colloids, macromolecules and bacteria but not small ions.
• Electrodialysis: ions migrate through ion-selective membranes under a DC field, concentrating salts in alternate compartments.
• Advantages: removes ionic, colloidal and microbial impurities in one step; long membrane life; continuous operation.

IX. Water Treatment by Chlorination

Chlorination disinfects water by destroying pathogenic microorganisms.

A. Water treatment by chlorination

• Chemistry: chlorine forms hypochlorous acid, the active germicide:
  
  TEXT

  Cl₂ + H₂O → HOCl + HCl
  HOCl → H⁺ + OCl⁻

• Forms used: chlorine gas, bleaching powder CaOCl₂, and chloramines (from Cl₂ + NH₃) for lasting residual action.
• Break-point chlorination: chlorine added until the demand from organic matter and ammonia is satisfied; beyond the break point free residual chlorine appears, ensuring disinfection.
• Residual chlorine: 0.1–0.2 ppm free chlorine kept to guard against recontamination.

X. Specifications for Drinking Water

Potable water must meet chemical, physical and microbiological limits.

A. Specifications for drinking water

• Turbidity: ≤ 5–10 NTU; free from suspended matter.
• pH: 6.5–8.5 for taste and corrosion control.
• Total hardness: desirably ≤ 300 ppm as CaCO₃.
• Total dissolved solids (TDS): ≤ 500–2000 mg/L.
• Chlorides / sulphates: ≤ 250 mg/L each.
• Fluoride: 1.0–1.5 mg/L limit; excess causes fluorosis.
• Bacteriological: free from coliform (E. coli) organisms; residual chlorine maintained.
• Colour, odour, taste: should be nil or negligible.