Purification: lime-soda, zeolite & ion-exchange softening; measurement of BOD and COD

Water Softening: Lime-Soda, Zeolite, Ion-Exchange; BOD & COD

1. Lime-soda process

Lime $[Ca(OH)_2]$ removes temporary hardness; soda $[Na_2CO_3]$ removes permanent hardness as $CaCO_3$/$Mg(OH)_2$ precipitates.

Lime requirement $= \frac{74}{100}$(temp. Ca hardness) $+ \frac{74}{100} \times \frac{1}{2}$(temp. Mg hardness, extra) $+ \frac{74}{100}$(perm. Mg hardness) $+$ excess/coagulant corrections; soda $= \frac{106}{100}$(perm. Ca + perm. Mg hardness) — all as CaCO₃ equivalents, × purity factor $100/\%$purity.

• Cold vs hot process (hot is faster, residual hardness lower ~15–30 ppm vs ~60 ppm).
• Pros: cheap for large volumes, removes silica/iron too. Cons: sludge disposal, can't reach zero hardness, needs supervision.

2. Zeolite (permutit) process

Sodium zeolite $Na_2Z$ exchanges $Na^+$ for $Ca^{2+}/Mg^{2+}$:

• Exhausted bed regenerated with 10% brine: $CaZ + 2NaCl \to Na_2Z + CaCl_2$.
• Output ~0–15 ppm hardness — good for boilers/laundries; fails on acidic, turbid, or $Fe^{3+}/Mn^{2+}$ water (clogs bed); adds $Na^+$ (bad for high-pressure boilers → caustic embrittlement).

3. Ion-exchange (demineralization)

Two beds in series: cation exchanger ($RH_2 + Ca^{2+} \to CaR + 2H^+$) then anion exchanger ($R'(OH)_2 + SO_4^{2-} \to R'SO_4 + 2OH^-$); $H^+ + OH^- \to H_2O$. Regenerated with HCl and NaOH respectively.

• Produces deionized/DM water (~pure); needed for printing, pharma, high-pressure boilers, labs. Costliest; resins fouled by organics/turbidity.

Process	Residual hardness	Removes	Limitation
Lime-soda	15–60 ppm	temp + perm, silica	sludge, not zero
Zeolite	~0–15 ppm	Ca/Mg only	Na⁺ added, Fe-sensitive
Ion-exchange	~0 (DM water)	all ions	costly, resin fouling

4. BOD and COD

• BOD (biochemical oxygen demand): $O_2$ consumed by microbes oxidizing organics in 5 days at 20 °C ($BOD_5$). High BOD = high biodegradable pollution. Drinking water < 1–2 ppm; sewage ~200–400 ppm.
• COD (chemical oxygen demand): $O_2$ equivalent of $K_2Cr_2O_7$ consumed oxidizing ALL oxidizable matter (biodegradable + non-biodegradable), in hours.

($B, S$ = blank/sample FAS titre). Always $COD > BOD$; ratio tells biodegradability ($BOD/COD > 0.5$ = easily treatable biologically).

Exam Points

• Lime and soda formulae with 74/100 and 106/100 factors + one full numerical with purity correction (5 marks favourite).
• Zeolite exchange + brine regeneration; ion-exchange two-bed scheme (3 marks).
• BOD vs COD definitions, conditions ($BOD_5$ at 20 °C), COD formula, why COD > BOD (3–5 marks).