Lime softening and disinfection effects in water treatment Pick the single most correct statement about adding lime to raw water during treatment.

Introduction / Context:
Lime (calcium hydroxide) is widely used in conventional water treatment for softening and pH adjustment. Understanding how lime affects hardness, pH, and microbial inactivation helps operators choose coagulant and disinfectant doses responsibly.

Given Data / Assumptions:

• Lime softening operates via precipitation reactions of calcium and magnesium.
• Discussion refers to treatment-plant practice with subsequent clarification and filtration.
• Statements address typical textbook claims about bactericidal action at high pH.

Concept / Approach:
Lime reacts with bicarbonate alkalinity to precipitate calcium carbonate and, at higher pH, magnesium hydroxide, thereby reducing hardness. Raising pH above neutral creates an unfavorable environment for many bacteria. Classical references note strong bacterial reduction near pH 9.5–11 under adequate contact time, although final disinfection is still achieved by chlorine/UV.

Step-by-Step Solution:
Hardness removal: Ca(HCO3)2 + Ca(OH)2 → 2 CaCO3 ↓ + 2 H2O; Mg(HCO3)2 + 2 Ca(OH)2 → Mg(OH)2 ↓ + 2 CaCO3 ↓ + 2 H2O.pH impact: Excess lime increases hydroxide concentration, raising pH substantially.Bactericidal effect: High pH damages cellular functions of many organisms; older exam literature generalizes near-total removal at pH ≈ 9.5 with settling/filtration.Therefore each listed statement is accepted as correct in the context of standard exam practice → choose “All the above”.

Verification / Alternative check:
Plant operating manuals show lime softening coupled with clarification and filtration significantly reduces microbial counts, with downstream chlorination ensuring regulatory compliance.

Why Other Options Are Wrong:

• Choosing any one of (a), (b), or (c) ignores the broader set of true effects recognized in basic texts.
• (d) is quoted in many exam sources; final safety still relies on a disinfectant residual, but within this question’s framework, it is treated as correct.

Common Pitfalls:
Assuming lime alone is a complete disinfectant barrier; operators must verify residual disinfectant and contact time. Over-liming can cause scaling and high finished-water pH without proper recarbonation.

Final Answer:
All the above