--- title: "Removal mechanism of permanent hardness" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:permanent-hardness-removal" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/permanent-hardness-removal site: BCT Engineering --- # Removal mechanism of permanent hardness Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2081 Baisakh, 5 marks; 2079 Bhadra, 2 marks (asked 2 times) - Video: [IonExchange](https://www.youtube.com/watch?v=7he0Z1snrHg) — Clayton Spencer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/permanent-hardness-removal - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question Discuss the removal mechanism of permanent hardness in water. ## Short answer Permanent hardness from Ca/Mg sulfates and chlorides is removed by lime‑soda softening (soda ash precipitates CaCO₃, lime precipitates Mg(OH)₂), sodium zeolite base‑exchange (replacing Ca/Mg with Na), or demineralization with cation and anion exchange resins. ## Answer **Permanent (non-carbonate) hardness** is caused by the chlorides, sulphates and nitrates of calcium and magnesium (CaSO$_4$, MgSO$_4$, CaCl$_2$, MgCl$_2$). It is not removed by boiling, because these salts remain dissolved. It is removed by chemical precipitation or ion exchange. #### 1. Lime-soda process Soda ash (Na$_2$CO$_3$) is added; it converts the soluble calcium salts into insoluble calcium carbonate: $$ CaSO_4 + Na_2CO_3 \rightarrow CaCO_3\downarrow + Na_2SO_4 $$ $$ CaCl_2 + Na_2CO_3 \rightarrow CaCO_3\downarrow + 2NaCl $$ Magnesium carbonate is fairly soluble, so magnesium salts are first precipitated with lime as Mg(OH)$_2$: $$ MgSO_4 + Ca(OH)_2 \rightarrow Mg(OH)_2\downarrow + CaSO_4 $$ and then the CaSO$_4$ formed is removed by soda ash as above. The precipitates are removed by sedimentation and filtration. The remaining hardness is about 30 to 50 mg/l. #### 2. Zeolite (base-exchange) process Water passes through a bed of sodium zeolite (Na$_2$Z). Ca and Mg exchange with sodium: $$ CaSO_4 + Na_2Z \rightarrow CaZ + Na_2SO_4 $$ $$ MgCl_2 + Na_2Z \rightarrow MgZ + 2NaCl $$ When the zeolite is exhausted, it is regenerated with strong brine (NaCl): $CaZ + 2NaCl \rightarrow Na_2Z + CaCl_2$. It gives almost zero hardness water. #### 3. Demineralisation (ion exchange) Water passes through a cation exchange resin (which replaces Ca$^{2+}$ and Mg$^{2+}$ with H$^+$) and then an anion exchange resin (which replaces SO$_4^{2-}$ and Cl$^-$ with OH$^-$), removing practically all dissolved ions. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is permanent hardness not removed by boiling? A. It is caused by bicarbonates B. It is caused by chlorides, sulphates and nitrates of Ca and Mg that stay dissolved C. It is caused by gases D. It is caused by iron Answer: B. Non-carbonate salts do not precipitate on boiling, so chemical precipitation or ion exchange is required. (Key point: Nature of permanent hardness) 2. What does soda ash do in the lime-soda process for CaSO4? A. Converts it to insoluble CaCO3 B. Converts it to soluble salts only C. Adds more hardness D. Lowers pH to zero Answer: A. CaSO4 + Na2CO3 gives CaCO3 precipitate and Na2SO4. (Key point: Soda ash) 3. In the zeolite process, how is exhausted zeolite regenerated? A. With lime B. With acid C. With strong brine (NaCl) D. With alum Answer: C. Brine restores sodium zeolite: CaZ + 2NaCl gives Na2Z + CaCl2. (Key point: Zeolite process) 4. How are magnesium salts handled in the lime-soda process? A. Soda ash alone removes them directly B. Lime precipitates Mg(OH)2, then the CaSO4 formed is removed with soda ash C. They are not removed D. They are boiled off Answer: B. Mg needs lime and soda; the CaSO4 produced is removed by soda ash. 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