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Engineering Chemistry Dr. Payal Joshi

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Water Technology

Hardness of Water: Hardness in water is that characteristic, which prevents lathering of soap. This is due to presence of large quantities of dissolved salts of Calcium (Ca) & Magnesium (Mg). Ca2+ & Mg2+ ions react with soaps which are salts of fatty acids (stearic or palmitic acid) to give insoluble precipitates of calcium or magnesium stearate or palmitate. Typical reactions of soap (sodium stearate) with calcium chloride and magnesium sulphate are depicted as follows: 2C17H35COONa + CaCl2 → (C17H35COO)2 Ca (scum) + 2NaCl 2C17H35COONa + MgSO4 → (C17H35COO)2 Mg(scum) + Na2SO4 This property or tendency is called hardness of water. The cause of hardness is the precipitation of soap and hence prevents lathering. Further, water which lathers easily on shaking with soap solution is called soft water which does not contain dissolved calcium and magnesium salts in it. Hardness of water is of two types: 1. Temporary/Alkaline/Carbonate hardness is due to dissolved bicarbonates of Ca2+ & Mg2+ in water. It is so called since it can be easily removed simply by boiling with bicarbonates readily getting precipitated. Ca(HCO3)2 Æ CaCO3 + H2O + CO2 Mg(HCO3)2 Æ MgCO3 + H2O + CO2 Ca(HCO3)2 /Mg(HCO3)2 + 2C17H35COONa Æ (C17H35COO)2 Ca/Mg + 2 NaHCO3 CaCl2/MgCl2 + 2C17H35COONa Æ (C17H35COO)2Ca/Mg + 2NaCl CaSO4/MgSO4 + 2C17H35COONa Æ (C17H35COO)2 Ca/Mg + Na2SO4 2. Permanent Hardness/Non-alkaline/Non-carbonate is due to the presence of dissolved sulfates & chlorides of Ca, Mg, Fe, etc in water. Permanent hardness cannot be removed easily on boiling, but requires special chemical treatment. Temporary & Permanent hardness added together gives Total hardness. Determination of Hardness of water by EDTA Titration: It is for determining total amount of Ca2+ & Mg2+ ions in water. The procedure involves titrating sample with standard EDTA solution using an organic dye indicator EBT (Eriochrome Black T). EDTA is a weak acid with the structure given below:

Contents: Hardness of water. Determination of hardness of water by EDTA titration. Numericals based on hardness of water and EDTA method. Softening Methods: Hot and Cold Lime-Soda Method, Zeolite Method and Ion-Exchange Method. Numerical based on lime-soda and zeolite method. Drinking water purification: Removal of microorganisms, by adding bleaching powder, chlorination (no breakpoint chlorination), Ozonization, Desalination by Reverse Osmosis, Ultrafiltration.


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(a) Structure of ethylenediaminetetraacetic acid, EDTA. (b) Structure of tetracarboxylate ion, [EDTA]4–, formed by dissociation of EDTA.

In an aqueous solution buffered at pH 10, EDTA dissociates to form a maximum amount of tetracarboxylate ion, [EDTA]4-

. This ion is electron-rich, having six bonding sites: four carboxylate groups and two nitrogen atoms. Each site has an electron pair available for bonding. [EDTA]4– anion wraps itself around Ca2+ or Mg2+ ions and all six electron pairs are shared with the metal ions. The reactions occurring in EDTA titration is shown as below: Mg2+ + EBT (blue) ------------> [Mg2+ ---EBT] [Mg2+ ---EBT] (wine red unstable complex) ---> [Mg2+ --- EDTA] (stable)+ EBT (blue)

Structure of [Mg–EDTA]2– chelate (1:1 complex) [Complexation Reaction]

Experimental Procedure: Preparation of Reagents: Standard Hard Water: 1 gm pure dry CaCO3 dissolved in minimum quantity of dilute HCl. Solution evaporated to dryness on water bath & residue left is dissolved in distilled water & solution is diluted to 1 litre. The hardness of solution is 1 mg of CaCO3 equivalent/ml. EDTA solution, EBT indicator => prepared Buffer solution : NH4Cl + conc. NH3. Standardization of EDTA solution: Pipette 50 ml of standard hard water in conical flask. Add 10-15 ml buffer solution (pH=10), 4-5 drops of EBT indicator. Solution is titrated against EDTA till colour changes from wine red to deep blue (V1 ml). Total hardness of water sample: Titrate known volume of the water with standard EDTA solution, using Eriochrome Black T indicator. From molarity & volume of EDTA solution used & volume of water titrated, one can calculate total Ca2+ and Mg2+ ion concentration in water sample. Report this total hardness in ppm of equivalent CaCO3.


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Permanent hardness & temporary hardness of water sample: Boil known volume of water, filter any precipitated CaCO3 & MgCO3. Analyze the filtrate for permanent hardness by titrating with EDTA solution. Further calculate temporary hardness in water sample. Finally, determine individual Ca2+ & Mg2+ ion concentration in water sample using titration data For total hardness : 50 ml hard water sample titrated against EDTA (V2 ml) Permanent hardness : 50 ml hard water sample is boiled for 10-15 min, filtered, diluted with D/W to make up to 50 ml & titrated against EDTA (V3 ml) Temporary Hardness = Total hardness – Permanent hardness Units of Hardness: In terms of dissolved Ca & Mg salts calculated as CaCO3 equivalent Reason of expressing hardness as CaCO3 equivalents is due to the fact that it’s Molecular mass (M) = 40+12+48=100 (Equivalent weight = 50) makes calculations simple & quick. Also, it is the most insoluble salt that can be precipitated in water treatment. Equivalent of CaCO3 = Mass of hardness producing material x chemical equivalent of CaCO3/Chemical equivalent of hardness producing substance Eq CaCO3 = Mass of hardness producing material x 50/ Chemical equivalent of hardness producing substance Molecular mass of Ca(HCO3)2 = 162; Molecular mass of CaCO3 = 100; Thus, CaCO3: Ca(HCO3)2 = 100:162. Chemical equivalent of Ca(HCO3)2 = 162/2 = 81 Thus, 162 parts by mass of Ca(HCO3)2 or 2 equivalents would react with same amount of soap as 100 parts by mass of CaCO3 (or 2 eq). Ca(HCO3)2 expressed as CaCO3 eq can be written as, 162 x 100/162 = 100 equivalents of CaCO3 Effects of Hard water: There are numerous disadvantages of using hard water in domestic and industrial purposes. In domestic use: Domestic use: i. Washing : no lather formation ,wastage of soap ii. Bathing: no lather formation. Also the resulting precipitates stick on body iii. Cooking: due to dissolved salts boiling point of water is elevated, causing unnecessarily wastage of time & fuel. iv: Drinking: bad effect on metabolic system. Calcium oxalate stones may develop in urinary tracts, if used regularly. Also it causes deposition of Ca in the bone joints. In industrial use: i: Textiles: loss of soaps during washing of yarn, & fabrics. Ppt sticks on fabric & when it is colored. Also Fe, Mn etc salts leave colored spots on fabrics ii. Sugar: crystallization of sugar is affected due to presence of sulfate ions. iii. Dyeing: Dissolved Ca, Mg, Fe salts react with dye forming undesirable ppts giving poor shades of color on the fabric. iv. Pharmaceutical industry: If hard water is used in drug, injections, ointments preparations, it results in undesirable products in them causing ill health. Softening of water: Softening techniques involve removal of Ca, Mg, Fe salts and other similar metallic ions that form insoluble metallic soaps. Three important industrial methods employed for softening of water are: 1. Cold and hot lime soda process, 2. Permutit or zeolite process, 3. Ion-exchange or demineralization process. Clark’s Process: Calculated amount of slaked lime [Ca(OH)2] added to hard water. Bicarbonates are converted into insoluble carbonates & removed by settling & filtration.


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Lime-Soda Process: In this method hard water is treated with calculated amounts of lime [Ca(OH)2] and soda ash [Na2CO3] in reaction tanks, so as to convert hardness producing chemicals into insoluble compounds which are then removed by settling & filtration. About 10% excess of chemicals are added in the reaction of tanks to complete the reaction quickly. The following reactions take place: Reactions with Lime:

i) To neutralize any free acid present 2HCl + Ca(OH)2 → CaCl2 + 2H2O H2SO4 + Ca(OH)2 → CaSO4 + 2H2O ii) To precipitate aluminum & iron salts as hydroxide Al2(SO4)3 + 3 Ca(OH)2 → 2Al(OH)3 + 3CaSO4 FeSO4 + Ca(OH)2 → Fe((OH)2 + CaSO4 iii) To precipitate dissolved CO2 CO2 + Ca(OH)2 → CaCO3 + H2O iv) To ppt Ca and Mg bicarbonates as carbonates Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O Mg(HCO3)2 + 2Ca(OH)2 → Mg(OH)2 + CaCO3 + 2H2O (v) To ppt all permanent magnesium hardness as hydroxide MgCl2 + Ca(OH)2 → Mg(OH)2 + CaCl2 (vi) To convert HCO3

- ions (like NaHCO3 and KHCO3) in to carbonates 2NaHCO3 + Ca(OH)2 → CaCO3 + 2H2O + Na2CO3 Reaction with Soda: Soda removes all the soluble permanent hardness due to calcium salts.(ie that which is present originally as well as that which is introduced during removal of Mg2+ , Fe2+ , Al3+ , HCl, H2SO4, CO2 etc by lime ) CaCl2 + Na2CO3 → CaCO3 + 2NaCl CaSO4 + Na2CO3 → CaCO3 + Na2SO4 Points to be remembered while solving numerical: (1) One equivalent of Mg(HCO3)2 consume two equivalents of lime (2) CaCO3 & MgCO3 impurities should be considered as bicarbonate hardness. i.e, amount of

MgCO3 first convert into CaCO3 equivalent, then twice of it should be added in lime, as in case of Mg(HCO3)2.

(3) NaCl, KCl, SiO2, Fe2O3 etc do not consume lime or soda. (4) If there are extra bicarbonate ions, they consume one equivalent of lime &

simultaneously produce one equivalent of CO32- . So add it in lime & subtract from soda.

2 HCO3- + Ca(OH)2 → CaCO3 + CO3

2- + 2H2O (5) If NaAlO2 is added as coagulant, it produces one equivalent of OH- ions which is equivalent to one equivalent of lime. Thus this amount must be subtracted from lime. NaAlO2 + 2H2O → NaOH + Al(OH)3 Amount of Lime required = 74/100 [Temp Ca2+ + (2 × Temp Mg2+ ) + perm Mg2+ + Fe2+ , Al3+ + H+ (HCl/H2SO4) + HCO3

- - NaAlO2] ×Vol of water x 100/%pure ………. In kg 106 Amount of Soda = 106/100 [perm Ca hardness + perm Mg hardness + Salts like Fe, Al + Acids - HCO3

-] × Vol of water x 100/%pure ………. In kg 106


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Chemical reactions taking place during lime soda treatment are slow & ppt of CaCO3 & Mg(OH)2 are fine & produce supersaturated solution. As a result, after deposition occurs in pipes, boiler tubes they clog the valves & lead to corrosion. To avoid this; i) Thorough mixing of chemicals & hard water. ii) Sufficient time allowed to complete reactions iii) Coagulants ie. Substances which help in the formation of coarse precipitates are

added eg, alum, sodium aluminate, etc. iv) Proper sedimentation chamber for precipitation to settle, before filtration being

carried out. Cold Lime-Soda Process: When the chemicals (lime & soda) are added for softening water at room temperature, it is called cold lime soda process. At this temperature the precipitates are finely divided & do not settle easily, nor they can be filtered. It is necessary therefore to add coagulants like alum, sodium aluminate etc.

NaAlO2 + 2H2O → NaOH + Al(OH)3 This process provides water containing a residual hardness of 50 to 60 ppm. Tank is fitted with a stirrer and vigorous stirring is carried out for thorough mixing process. After softening, soft water rises upwards and heavy sludge settles down. Softened water passes through filtering media ensuring complete removal of sludge and finally filtered water flows out through the top.

Hot lime soda method: This process is similar to cold lime soda process. Here, the chemicals along with the water are heated near about the boiling point of water by exhaust steam. As the reaction takes place at high temperature, there are the following advantages: 1. Precipitation reaction becomes almost complete and reaction takes place at faster rate. 2. Sludge settles rapidly & hence no coagulant is needed. 3. Dissolved gases (which may cause corrosion) are removed. 4. Viscosity of soft water is lower, hence filtered easily. 5. Residual hardness is lower (15-30 ppm) compared to the cold process.


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Hot lime-soda process consists of three parts: 1. ‘Reaction tank’ in which complete mixing of the ingredients take place 2. ‘Conical sedimentation vessel’ where sludge settles down and, 3. ‘Sand filter’ where sludge is completely removed. Soft water from this process is used for feeding boilers. Advantages: Lime-soda process is economical, process involves corrosion resistance of the water, mineral content of water is reduced, pH of the water rises, which reduces the content of pathogenic bacteria.

Disadvantages: i) Huge amount of sludge is formed and disposal is difficult, ii) Due to residual hardness, water is not suitable for high pressure boilers. Zeolite/Permutit Softener: Hydrated sodium alumino silicate, Na2O.Al2O3.x SiO2.yH2O Zeolites are capable of exchanging reversibly their sodium ions for hardness producing ions in water. They are of two types Natural Zeolites: are non-porous more durable & are derived from green sands by washing, heating & treating with NaOH. Synthetic Zeolites: porous & possess gel structure. Synthetic Zeolites have higher exchange capacity per unit weight. Theory: When hard water is passed over a bed of sodium zeolite, Ca2+, Mg2+ ions are taken up by the zeolite simultaneously releasing equivalent Na+ ions in exchange for them. CaCl2 + Na2Ze → CaZe + 2NaCl MgSO4 + Na2Ze → MgZe + Na2SO4 Regeneration: When Zeolite is completely converted into calcium & magnesium Zeolites, it ceases to soften water i.e, it gets exhausted. It is generated by treating with 10% brine solution. CaZe + 2NaCl → Na2Ze + CaCl2 MgZe + 2NaCl → Na2Ze + MgCl2 Process: Hard water enters from top at a specified rate & passes over a bed of sodium zeolite kept in a cylinder. Softened water is collected at the bottom of cylinder & is taken out from time to time.


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Limitations:

(i) Turbid water should not be employed as it will block the pores of zeolite & make them inactive.

(ii) Any colored Fe ions must be removed earlier since it is difficult to regenerate it from iron zeolite.

(iii) Mineral acid present in water must be neutralized earlier with soda otherwise that may destroy zeolite bed.

Advantages: i) Water of about 10 ppm hardness is produced. ii) Process automatically adjusts itself for different hardness of incoming water. iii) Requires less skill in maintenance as well as operation. iv) Compact equipment, occupy less space. v) No impurities are formed, so no formation of sludge and hence it is a cleaner process.

Disadvantages: i) Treated water contains more sodium salts ii) The method only replaces Ca2+ & Mg2+ ions by Na+ ions, but leaves all acidic ions

(HCO3-- & CO3

2-) in soft water. Such soft water containing NaHCO3, Na2CO3 etc when used in boilers, NaHCO3 decomposes to give CO2 which cause boiler corrosion & Na2CO3 hydrolyses to NaOH causing caustic embrittlement.

Ion-Exchange Process: It is reversible exchange of ions of same charge between mobile liquid phase & an insoluble solid (stationary phase). Ion-exchanger is an insoluble material liberating counter ions by electrolytic dissociation. Cation exchanger: High molecular weight, cross-linked polymer containing sulphonic (-SO3H), carboxylic (-COOH), or phenolic (-OH) as a part of resin and equivalent amount of cations.

H-R (resin) + Na+ Na-R (resin) + H+ 2NaR (resin) + Ca2+ CaR2 (resin) + 2Na+ (resin)

Anion exchanger : Is a polymer containing quaternary ammonium groups (-N+R2 ) containing equivalent amount of anions, Cl-, OH-, etc

2RCl + SO42- R2(SO4) + 2Cl-

(resin) (solution) (resin) (solution) Water from which all cations & anions are removed is called demineralized or deionized water.


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Demineralization of water

Water is first passed to cation exchanger in acid form. All cations will be exchanged for H+ ions. Next, water coming from cation exchanger is passed into anion exchanger in basic form and anions are exchanged for OH-. Let us consider water contains a major impurity as calcium sulphate. Hence the reactions are as follows: 2RCOOH (Na+) + Ca2+ Ù (RCOO)2Ca + 2H+ + 2Na+ (Cation exchange) 2R’OH + SO4

2- Ù R’2SO4 + 2OH- (Anion exchange) H+ + OH- Ù H2O (demineralized water) After some time, resin requires regeneration, as they become ineffective. Adding a strong acid regenerates cation resin and the anion resin is regenerated by adding strong base. (RCOO)2Ca + H2SO4 Æ 2RCOOH + Ca2+ R’2SO4 + NaOH Æ R’OH + SO4

2- Justify the statement: During demineralization using ion-exchange, water is first passed through cation exchanger and next into anion exchanger. Cation exchangers are easily attacked by alkalies, whereas all types of ion-exchangers are not attacked by acids. When water is passed through cation exchanger, salts present in water are converted into corresponding acids, which on passing anion exchanger do not harm it and finally get converted into pure water. If reverse sequence is used, then alkalies produced on passing water through anion exchanger will harm the cation exchanger in the subsequent step. Drinking Water or Municipal Water Removal of microorganisms: Water after passing through sedimentation, coagulation and filtration operations still contain a small percentage of pathogenic bacteria. The process of destroying or killing disease-producing bacteria, microorganisms, etc from water and making it safer for use is called disinfection. Chemicals or substances added to water for killing bacteria, etc are known as disinfectants. Chlorination: Chlorine (gas or in concentrated solution form) produces hypochlorous acid which is a powerful germicide. Cl2 + H2O Æ HOCl + HCl Bacteria + HOCl Æ Bacteria killed HOCl Ù H+ + OCl-


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Hypochlorite ion (OCl-) cause inactivation of enzymes of microorganisms that is necessary for metabolism thereby leading to death of microorganisms. Hence chlorine is found to be the most effective disinfectant.

Chlorine is the most widely used disinfectant. Liquid chlorine is more effective when applied to filtered water at such a point, where adequate mixing is done. The apparatus in which chlorination is carried out is known as chlorinator. It is a high tower with number of baffle plates. Water and proper quantity of concentrated chlorine solution (0.3-0.5 ppm) are introduced at its top. During their passage through the tower, they get thoroughly mixed. Treated water is taken out from the bottom. Advantages of chlorine: Effective and economic, requires little space for storage, stable & does not deteriorate on keeping, does not introduce salt impurities in water & hence is an ideal disinfectant. Disadvantages of chlorine: Excess of chlorine, if added, produces a characterisitic unpleasant taste & odour causing irritation to mucuos membrane. Quantity of free chlorine in treated water should not exceed 0.1-0.2 ppm. Bleaching powder treatment: It is used for the disinfection of drinking water or swimming pool water. 1kg of bleaching powder (Calcium hypochlorite, Ca(ClO)2 per 1000 kilolitres of water is mixed and water is allowed to stand undisturbed for several hours. Chemical action produces hypochlorous acid (powerful germicide). Disinfecting action of bleaching powder is due to chlorine made available by it. CaOCl2 + H2O Æ Ca(OH)2 + Cl2 Cl2 + H2O Æ HCl + HOCl (hypochlorous acid) Germs + HOCl Æ Germs killed Drawbacks: Bleaching powder introduces calcium in water, thereby making it harder. Bleaching powder gets deteriorated due to its continuous decomposition while storage. Only calculated quantity of bleaching powder should be used, since an excess of it gives bad taste and smell to treated water. Disinfection by Ozone: Ozone is an excellent disinfectant which is produced by passing electric discharge through oxygen.

3O2 Æ 2O3 Ozone is highly unstable and breaks down liberating nascent oxygen, O3 Æ O2 + [O] Nascent oxygen is very powerful oxidizing agent and kills all bacteria and also oxidizes the organic matter present in water. For carrying out disinfection by ozone, ozone is released or


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injected (2-3ppm) in water and the two are allowed to come in contact for 10-15 min in sterilizing tank (shown below). Disinfected water is removed from the top.

Advantages: Though the method is costlier than chlorination, it simultaneously removes colour, odour and taste without leaving any residue. Its excess is not harmful as it is unstable and decomposes in oxygen. Disadvantages: It is an expensive method and cannot be employed for disinfection of municipal water supply. Reverse Osmosis: Osmosis describes the flow of solvent from dilute to concentrated solution

through a semipermeable membrane. Reverse osmosis describes the flow of solvent in the opposite direction, i.e, from concentrated solution to dilute solution across a semipermeable membrane by applying hydrostatic pressure in excess of osmotic pressure. The diagram of a reverse osmosis cell is shown here, where desalination of brackish water is carried out. This method is largely used for purification of sea water for domestic use. Reverse osmosis has many advantages like; i) removal of ionic, non-ionic, colloidal and high molecular weight solutes from water; ii) regeneration of process involves easy replacement of semipermeable membrane; iii) easy maintenance and economical; iv) uninterrupted supply of large

volume of water for industrial or domestic purpose. Ultrafiltration: Ultrafiltration (UF) is a pressure-driven separation process, governed by a screening principle and dependent on particle size. UF membranes have a pore size between 1 nm and 100 nm, thus allowing retention of compounds with a molecular weight of 300 to 5,00,000 Dalton. It is suitable for retaining biomolecules, bacteria, viruses, polymers, colloidal particles and sugar molecules. UF membranes have the ability to purify, separate, and concentrate target macromolecules. UF does this by pressurizing the solution flow. Solvent and other dissolved components that pass through the membrane are known as permeate. Components that do not pass through are known as retentate.


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Ultrafiltration removes bacteria, protozoa and some viruses from the water. Applications: a) Pretreatment in sea water desalination plants in combination with reverse osmosis. b) Sterile filtration of drinking and beverage water. c) Wastewater treatment and re-use. d) Removal of metal hydroxides in wastewater treatment. Distinguish between:

Temporary Hardness Permanent Hardness It is caused due to the presence of bicarbonates and carbonates of Ca2+, Mg2+, Fe2+ etc. Salts like Ca(HCO3)2, CaCO3, Mg(HCO3)2, MgCO3, etc.

It is caused due to the presence of Cl-, SO42-,

nitrates of Ca2+, Mg2+, Fe2+, etc other than carbonates and bicarbonates. Salts like CaCl2, CaSO4, Ca(NO3)2, MgCl2, MgSO4, Mg(NO3)2.

It can be removed by boiling. It cannot be removed by boiling, but needs chemical treatment.

It is also known as carbonate or alkaline hardness.

It is also known as non-carbonate or non-alkaline hardness.

Temporary hardness leads to formation of loose deposits (sludge) of carbonates and hydroxides of Ca2+, Mg2+, if used in boilers

Permanent hardness leads to formation of hard deposits (scales) if used in the boilers

Cold Lime soda method Hot lime soda method

Process requires several hours to complete. Process requires 15 minutes. Coagulants are required, like sodium aluminate NaAlO2+H2OÆ NaOH + Al(OH)3

Coagulants are not required, precipitate settles rapidly.

Dissolved gases like oxygen, CO2 are not removed. Dissolved gases like oxygen, CO2 are removed.

Filtration takes place very slowly. Filtration takes place rapidly as viscosity of water decreases.

This process gives water with residual hardness of 50-60 ppm.

This process gives water with residual hardness of 15-30 ppm.

Zeolite Process Lime soda Method

Produces water with hardness of almost zero hardness.

This process produces water having hardness of 15-60 ppm depending on whether it is a cold or hot process.

Capital cost is higher Capital cost is lower. Exhausted zeolite bed can be regenerated with brine, hence operating cost is low.

Chemicals like lime, soda, coagulants are consumed making operation cost higher.

Salts causing temporary hardness are converted to NaHCO3 which is present in soft water creating problems to feed such waters in boilers.

Temporary hardness is completely removed.

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