Permanent hardness in water treatment In municipal water-supply engineering, which of the following processes is specifically used to remove permanent hardness (caused by calcium and magnesium salts of sulphates, chlorides, etc.)?

Introduction / Context:
Hardness of water is of two types: temporary (carbonate hardness) and permanent (non-carbonate hardness). Permanent hardness arises mainly from calcium and magnesium sulphates and chlorides. It does not respond to simple boiling and needs specific softening techniques used in water-treatment practice.

Given Data / Assumptions:

• Permanent hardness is due to Ca^2+ and Mg^2+ salts of sulphates/chlorides/nitrates.
• Municipal-scale treatment options are being compared.
• Goal is hardness removal, not turbidity removal or disinfection.

Concept / Approach:
Temporary hardness can be removed by boiling or by lime softening as bicarbonates decompose. Permanent hardness requires ion exchange or advanced chemical softening. The zeolite (sodium ion-exchange) process exchanges Ca^2+/Mg^2+ with Na^+ on the resin/zeolite, effectively removing non-carbonate hardness without forming scale.

Step-by-Step Solution:

Verification / Alternative check:
Plant design manuals list sodium zeolite softeners and lime-soda ash processes for permanent hardness. Boiling or alum/chlorine addition do not remove non-carbonate hardness.

Why Other Options Are Wrong:

• Adding alum: coagulant for turbidity/colour, not hardness.
• Adding lime only: primarily addresses temporary hardness unless combined with soda ash.
• Chlorination: for disinfection, no hardness reduction.
• Boiling: precipitates bicarbonates only (temporary hardness).

Common Pitfalls:
Confusing turbidity removal with hardness removal, and assuming boiling treats all hardness types.

Final Answer:
Zeolite (sodium ion-exchange) process