Soft Water and Water Softening Processes
Soft Water and Water Softening Processes
Water is the most wonderful, abundant, and useful compound in nature. It is used in industry for conducting of several processes. In industrial uses, hard water causes the problem of deposition of insoluble salts which interferes with the process. Hard water creates several problems such as (i) scale and sludge formation, (ii) corrosion, (iii) priming and foaming, and (iv) caustic embrittlement. Hence, hard water needs softening for which it is necessary to know the analysis of water and the compounds and their quantities present in water for deciding the methods to be used for water softening.
In industrial water treatment, softening process refers to the removal of those dissolved salts which makes the water hard. Soft water is used in steel plants in closed loop cooling circuits. Soft water is that water which is free from dissolved salts of such metals as calcium (Ca), magnesium (Mg), or / and iron (Fe) which form insoluble deposits that appear as scale in the pipe lines and the water-cooling elements.
Water is termed as hard when it has a higher mineral content than needed. Hardness of water is the characteristic of preventing lather formation of water with soap. Normally salts like chlorides, bicarbonates and sulphates of Ca2+, Mg2+ and Fe2+ make water hard. This hard water on treatment with soap which is stearic or palmitic acid salts of sodium or potassium causes white precipitate formation of calcium or magnesium stearate or palmitate.
Water used for closed cooling circuits is to be pure, i.e., it is to be free from hardness, scale forming substances, and corrosive agents like dissolved oxygen (O2) etc. The process of removing hardness producing salts from water is known as softening of water. Soft water prevents scale build-up in the water-cooling elements and in the pipe lines which are circulating water for equipment cooling.
Water is a universal solvent, which is capable of dissolving several gases, liquids, and solids. Water is classified into two types based on the property of it with soap solution as (i) soft water, and (ii) hard water. Soft water is one which gives good lather readily with soap solution. Hard water is the water which does not produce lather with soap readily but forms an insoluble precipitate like white scum.
Soft water is described as the water produced by the process of softening water. It can have increased levels of bi-carbonate and sodium ions. Since soft water has reduced calcium ions, soft water does not produce any calcium deposits in the cooling elements and in pipe lines circulating water for equipment cooling.
Soft water has 0 parts per million (ppm) to 17 ppm of hardness imparting elements. Because of the comparative absence of these minerals, soft water has increased sodium content. Typically, soft water tends to be slippery. It can have a salty taste because of dominant sodium content. Soft water does not leave any mineral deposits. Extremely softened water tends to leach metals like copper and lead from pipes, and pipe fittings. The leaching effect causes metallic odour and taste, and increased content of the metal in the water.
Soft water has several benefits, several of which are sourced from reducing the hardness minerals levels which include elimination of build-up of scales and increased efficiency of the cooling water systems which in turn improve the performance of the equipments to which the cooling system is catering.
The impurities present in water are categorized into three types namely (i) physical impurities, (ii) chemical impurities, and (iii) biological impurities. Physical impurities are colour, turbidity, taste, and odour. It includes suspended and colloidal impurities. Suspended impurities are clay, sand, decayed vegetable and, animal matter. These impurities make the water turbid. Colloidal impurities consist of finely divided clay and silica, colouring matter, waste products, complex proteins, amines etc. These impurities impart colour, odour, and taste to the water.
Chemical impurities are inorganic and organic chemicals which are present in water in the form of dissolved salts and dissolved gases. Dissolved salts are chlorides, sulphates, bi-carbonates, carbonates of calcium, magnesium, and sodium (Na). Calcium chloride (CaCl2), calcium sulphate (CaSO4), calcium bi-carbonate [Ca(HCO3)2], magnesium chloride (MgCl2), magnesium sulphate (MgSO4), and magnesium bi-carbonate [Mg(HCO3)2] make the water hard. Mg(HCO3)2, Ca(HCO3)2, Na2CO3 (sodium carbonate), and NaHCO3 (sodium bi-carbonate) make the water alkaline. Dissolved gases are oxygen, carbon di-oxide (CO2), hydrogen sulphide (H2S). Oxygen accelerates the rate of corrosion while CO2 and H2S makes the water acidic and corrosive.
Biological impurities are algae, pathogenic bacteria, fungi, viruses, pathogens, and parasite-worms. Biological impurities are cause of different disease.
Common hardness producing salt present in water are chlorides, sulphates and bi-carbonates of calcium and magnesium. i.e., CaCl2, CaSO4, MgCl2, MgSO4, Ca(HCO3)2 and Mg(HCO3)2 .The quantity these salts present in water decides the extent of hardness of water.
The water quality parameters are roughly categorized into three categories namely (i) physical, (ii) chemical, and (iii) biological. Physical parameters are colour, turbidity, taste, odour, and total dissolved solids (TDS). Chemical parameters are hardness, acidity, alkalinity, dissolved oxygen, pH, biological oxygen demand (BOD), and chemical oxygen demand (COD). Biological parameter includes pathogenic micro-organisms.
Alkalinity of water is a measure of its acid neutralizing ability or it is the tendency of water to accept hydrogen (H+) ions in order to neutralize it with the supply of hydro-oxide (OH-) ions. In water analysis, it is frequently desirable to know the kinds and quantities of the different forms of alkalinity present in water. Hardness is measured in ppm of CaCO3 (calcium carbonate).
Calcium carbonate is chosen as a standard since (i) its molecular weight is 100 and equivalent weight is 50 which is a whole number, hence the calculations in water analysis can be simplified. (ii) it is the most insoluble salt which can be precipitated during the water treatment. Hardness is expressed in terms of CaCO3 or its equivalent. If water contains CaCO3 alone the hardness is a measure of number of parts of CaCO3 in water. Normally, water contains some other salts. The quantity of these salts is converted into their CaCO3 equivalent. The conversion of the hardness causing salts into CaCO3 equivalents can be achieved by using the formula ‘degree of hardness = (weight of hardness causing salts / molecular weight of hardness causing salts) × 100 (molecular weight of CaCO3)’.
Hardness is the characteristic of water which is because of the presence of bi-carbonates, chlorides and sulphates of calcium, magnesium and other salts in water. It leads to adverse and harmful effects when hard water is used. Water with high concentration of calcium carbonate has detrimental effects. Hardness salts build up lime-scale.
Hardness of water can be classified into two categories namely (i) temporary hardness or carbonate hardness, and (ii) permanent hardness or non-carbonate hardness. Total hardness includes the content of all calcium and magnesium salts in water. Total hardness of water = carbonate hardness + non-carbonate hardness or = temporary hardness + permanent hardness. Temporary water hardness is the type, which is eliminated by boiling, while permanent hardness remains after boiling. The reduction or removal of hardness from water is known as the water softening process.
Temporary or carbonate hardness is caused by the presence of dissolved bi-carbonates of calcium and magnesium [Ca(HCO3)2 and Mg(HCO3)2] or carbonate of iron (FeCO3). Temporary hardness is mostly destroyed by mere boiling of water. During boiling, the bi-carbonates are decomposed and form insoluble precipitates. Carbonates and hydroxides can be removed by filtration, while carbon di-oxide (CO2) escapes out. The chemical equations are Ca(HCO3)2 = CaCO3 (insoluble) + H2O + CO2, and Mg(HCO3)2 = Mg(OH)2 (insoluble) + 2CO2. Maintenance is always a concern with equipment when hard water containing temporary hardness is present because of its scale forming potential, especially when water is heated as in the case of water-cooling system.
Permanent or non-carbonate hardness is because of the presence of dissolved chlorides, sulphates, and nitrates of calcium and magnesium. Unlike temporary hardness, permanent hardness is not destroyed on boiling. It is removed by special methods. it can be removed by the use of chemical agents.
There are several units in which the hardness of the water is measured. The first unit for measuring hardness of water is parts per million (ppm). It is the number of parts of calcium carbonate equivalent hardness present in one million parts of water i.e., 1 ppm is 1 part of CaCO3 equivalent hardness in one million parts of water. The second unit for measuring hardness of water is milligram per litre (mg/l). It is the number of milligrams of calcium carbonate equivalent hardness present in one litre of water. Hence, 1 mg/l is 1 mg of CaCO3 equivalent hardness in 1 litre of water. The third unit for measuring hardness of water is degree Clarke (deg Cl). It is the number of parts of CaCO3 equivalent hardness present in 70,000 parts of water. The fourth unit for measuring hardness of water is degree French (deg Fr). It is the number of parts of CaCO3 equivalent hardness present in 100,000 parts of water. These four units are correlated as 1 ppm = 1 mg/l = 0.07 deg Cl = 0.1 deg Fr.
The determination of hardness is carried out by titrating water sample with sodium salt of ethylene diamine tetra acetic acid (EDTA) using Eriochrome Black-T (EBT) as an indicator and keeping the pH of the water at 9 to 10. The end point is the change in colour from wine-red to blue, when the EDTA solution complexes the calcium and magnesium salt completely. The reactions are (i) Ca2+ or Mg2+ (hardness salts) + EBT (indicator) = (Ca-EBT) or (Mg-EBT) [unstable complex (wine-red)], and (Ca-EBT), and (ii) (Mg-EBT) + EDTA [unstable complex (wine-,red)], = (Ca-EDTA) or (Mg-EDTA) [stable complex (colourless)] + EBT (blue).
The quality of water determines the working efficiency of the cooling system of an equipment, or a furnace. When hard water is used, then because of continuous evaporation, the salts present in the hard water gets saturated and are finally deposited in the areas where the flow is slow. When the precipitates formed are loose and slimy in nature, it is called sludge, whereas when the precipitates formed are hard and adhere strongly on the inner walls of the pipes or cooling elements, they are called scale.
Sludge can be easily removed by scrapping with a brush. Sludge is formed by the presence of MgCO3, MgSO4, MgCl2, and CaCl2. These salts are more soluble in hot water. Disadvantages of sludge formation are (i) poor heat conduction because of the presence of sludge on the surface, (ii) difficulty in the operation of the cooling system, (iii) if sludge is formed along with the scale and is trapped within the scale formed then it is difficult to remove, and (iv) clogging of the pipe lines and other fittings in the cooling system especially in those places where water circulation rate is slow. Sludge formation can be prevented by (i) using soft water in the cooling system, and (ii) removing the concentrated salty water from time to time so that deposition of sludge is prevented.
Scales are the hard deposits, which stick very firmly to the inner surface of the cooling elements, pipes and fittings of a cooling system. Scales are difficult to remove, even with the help of hammer. Scales are the main sources of trouble in the water-cooling systems. Formation of scales is because of decomposition of calcium bi-carbonate into CaCO3, which is an insoluble salt. Decomposition take place because of high water temperature. CaCO3 forms the scale. Deposition of calcium sulphate (CaSO4) can also form the hard scale. This is since the solubility of CaSO4 decreases with increase in temperature. Hard scale can also be formed when silica (SiO2) is present in the hard water. It deposits as calcium silicate (CaSiO3) or magnesium silicate (MgSiO3). These calcium or magnesium silicate scales are very difficult to remove. Dissolved magnesium salts can also precipitate as magnesium hydroxide [Mg(OH)2] which forms soft type of scale.
Disadvantages of scale formation are similar to the sludge formation but the severity is more, since its removal is more difficult. Disadvantages include (i) poor heat transfer leading to deterioration in cooling, (ii) because of ineffective cooling there is adverse effect on the process which needed efficient removal of heat from the cooling system, and (iii) pipes, valves, and pipe fittings get choked because of scale formation leading to efficiency decrease of the cooling system.
Chemistry of hardness removal process – During precipitation softening, calcium is removed from water in the form of CaCO3 precipitate and magnesium is removed as Mg(OH)2 precipitate. The carbonic acid concentration present and the pH play an important role in the precipitation of these two solids. Carbonate hardness can be removed by the addition of hydroxide ions and raising the pH by which the bi-carbonate ions are converted to carbonate form having a pH value above 10. Because of the increase in carbonate concentration, precipitates of CaCO3 (calcium carbonate) are formed. The remaining calcium, i.e., non-carbonate hardness, cannot be removed by simple adjustment of pH. Hence, soda ash (sodium carbonate) is to be externally added to precipitate this remaining calcium. Magnesium is removed because of the precipitation of magnesium hydroxide.
Water softening is an ion exchange process which exchanges hard water ions like Ca2+ or Mg2+ for single-charged ions like sodium (Na+) or potassium (K+). Water softening resins do not remove negative ions like bi-carbonate and chlorides. When in solution with bi-carbonate, sodium ions do not show inverse solubility. Even when the bi-carbonate ion breaks down upon heating to form carbonate, it is still over 30,000 times more soluble than calcium carbonate. Hence, instead of forming calcium carbonate scale, the sodium and carbonate ions stay in solution to much higher concentrations. The reaction which takes place is 2Na+ + 2(HCO3)- = 2Na+ + (CO3)2- + H2O (liquid) + CO2 (gas).
Softening of water is one approach to prevent scale build up in cooling systems. Water softening chemicals remove hard water cations and exchange them for softened water cations like sodium ions. Also, softening water does not appreciably change the conductivity of the water, since for every calcium ion (containing two positive charges) which is removed, it is replaced with two sodium ions, each of which have one positive charge.
Water softening processes
Water used for cooling systems is required to be sufficiently pure. It is to be free from hardness producing salts before put to use. The process of removing hardness producing salts from water, is known as softening of water. The choice of water softening method is based on physical and chemical analysis of water, including determination of turbidity, hardness, and temperature of water. In case of high turbidity levels in water, it is necessary to remove suspended solids in advance for a better result during the softening process of water. The methods usede for softening of water are of two types namely (i) internal treatment, and (ii) external treatment.
Internal treatment – The internal treatment for softening of water is also known as conditioning of water. During the internal treatment, the softening of water is carried out in the water-cooling circuit. In this method, some types of chemicals are added to hard water to remove the negative effect of calcium and magnesium. Chemical treatment for softening results into low levels of hardness of water. For purifying hard water from a single source, it is economically feasible method. Selection of the proper chemical is determined by the raw water composition and the desired quality after softening. In case several chemicals are applicable, aspects of operational management also become important.
During the internal treatment for softening of water, the hardness causing salts are removed (i) by complexing the hardness causing salts to soluble salts by adding suitable reagents, (ii) by precipitating the scale forming impurities in the form of sludge which can be removed by blow down operation, and (iii) by converting the scale forming salts into other compounds which stay in ‘dissolved form’ and do not cause any trouble to the cooling elements, pipelines, and fittings of the cooling system.
The important internal conditioning methods are (i) colloidal conditioning, (ii) phosphate conditioning, (iii) carbonate conditioning, (iv) Calgon conditioning, and (v) conditioning with sodium aluminate. The water softening method is to be chosen to reduce the requirements of maintenance.
In the colloidal conditioning, the scale formation in cooling system can be prevented by the addition of kerosene, tannin (a class of astringent, polyphenolic bio-molecules), and agar-agar (a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ‘ogonori’ and ‘tengusa’) etc. which gets coated over the scale forming precipitates. These forms loose, non-sticky deposits (sludge) which can be removed by blow down.
In the phosphate conditioning process, the permanent hardness causing salts in the cooling systems are removed by reacting with sodium phosphate. The complex formed is soft, non-adherent and easily removable. The typical reaction is CaCl2 + 2 Na3PO4 = Ca3(PO4)2 + 6 NaCl. The three phosphates used in this process are (i) tri-sodium phosphate (Na3PO4, alkaline), (ii) disodium hydrogen phosphate (Na2HPO4, weakly alkaline), and (iii) sodium di-hydrogen phosphate (NaH2PO4, acidic).
In the carbonate conditioning, the hard and strong adherent scales formed because of CaSO4 are avoided by the addition of sodium carbonate to the cooling water. The reaction which takes place during carbonate conditioning is CaSO4 + Na2CO3 = CaCO3 + Na2SO4. The CaSO4 is converted to CaCO3, which is loose sludge and it can be removed by blow down.
In the Calgon conditioning, sodium hexa-meta phosphate Na2[Na4(PO3)6] or (NaPO3)6 is used. The commercial name of sodium hexa-meta phosphate is Calgon. Calgon forms soluble complex compounds with CaSO4. The reactions which take place during treatment of cooling water with sodium hexa-meta phosphate are (i) Na2[Na4(PO3)6] = 2Na+ + [Na4P6O18]2-, and (ii) 2CaSO4 + [Na4P6O18]2- = [Ca2P6O18]2- + 2Na2SO4.
In the method consisting of treatment with sodium aluminate (NaAlO2), sodium aluminate is hydrolyzed in the cooling system to give NaOH. The formed NaOH immediately precipitates by the reaction with some of the magnesium salts as Mg(OH)2. The reactions in this method are (i) NaAlO2 + H2O = Al(OH)3 + NaOH, (ii) MgCl2 + 2 NaOH = Mg(OH)2 + NaCl.
External treatment – It is the treatment which is given to the hard water for the removal of hardness causing salts before the water is taken into the cooling system. Three main processes for the external treatment are (i) lime soda process, (ii) Zeolite process or Permutit process, and (iii) ion exchange process.
Lime soda process uses slaked lime [Ca(OH)2] and soda ash (Na2CO3) which are added to the water. Lime and soda ash reacts with the calcium and magnesium salts to form insoluble precipitate of CaCO3 and Mg(OH)2. The chemical reactions which take place for carbonate hardness are (i) Ca(HCO3)2 + Ca(OH)2 = CaCO3 + 2H2O, (ii) Mg(HCO3)2 + Ca(OH)2 = Ca(HCO3)2 + Mg(OH)2, (iii) Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 + 2H₂O, (iv) MgCO3 + Ca(OH)2 = CaCO3 + Mg(OH)2, (v) MgCl2 + Ca(OH)2 = Mg(OH)2 + CaCl2. The chemical reactions which take place for non-carbonate hardness are (i) MgSO4 + Ca(OH)2 = Mg(OH)2 + CaSO4, (ii) CaCl2 + Na2CO3 = CaCO3 + 2NaCl, (iii) CaSO4 + Na2CO3 = CaCO3 + Na2SO4, and (iv) CO2 +Ca(OH)2 = CaCO3 + H2O.
Lime helps in removing the entire carbonate hardness and it reacts with non-carbonate hardness of magnesium to convert it to non-carbonate hardness of calcium. Soda ash then removes the non-carbonate hardness of calcium. Similarly, lime removes the CO2, from the water.
Re-carbonation takes place when CO2 pass through the water containing finely divided particles of CaCO3. It combines with CaCO3 to form soluble bi-carbonate. The chemical reactions which take place are (i) CaCO3 (insoluble) + CO2 +H2O = Ca(HCO3)2 (soluble), (ii) Mg(OH)2 + CO2 = MgCO3 + H₂O, and (iii) MgCO3 + CO2 + H2O = Mg(HCO3)2.
Advantages of lime soda process are (i) it is economical, (ii) it easily combines with other water treatments without any problem, (iii) when used along with coagulant, quantity of coagulants can be reduced, (iv) increased pH reduce corrosion of distribution pipes, and increased causticity cause killing of pathogenic bacteria, especially when calcium and magnesium hydroxide alkalinity is between 20 mg/l to 50 mg/l. (v) helps to reduce the total mineral content of the water, and (vi) removes iron and manganese, to some extent.
Disadvantages of lime soda process are (i) formation of a large quantity of sludge, (ii) needs careful operation and skilled supervision, (iii) there is incrustation of pipes and trouble in filter bed, if re-carbonation is not done properly, and (iv) there is no zero hardness since CaCO3 is slightly soluble in water, and hence remaining hardness after the treatment is up to around 50 mg/l.
Zeolite process or Permutit process is a process of removing the permanent as well as temporary hardness of the water. It involves the precipitation of calcium and magnesium ions present in water. The exchange of and ions occurs with the help of zeolite and hence, it is known as Zeolite softening process.
Zeolite (Ze) is hydrated sodium alumina silicate, which is capable of exchanging reversibly its sodium ions for hardness-producing ions in water. Chemical formula of sodium zeolite can be represented as; Na2O.Al2O3.x SiO2.y H2O where x is 2 to 10 and y is 2 to 6.
Zeolites are of two types namely (i) natural zeolites, and (ii) synthetic zeolites. Natural zeolites are non-porous, e.g., Natrolite (Na2O.Al2O3.4SiO2.2H2O). Synthetic zeolites are porous and prepared by heating together china-clay, feldspar (AlNaO8Si3) and soda ash. Synthetic zeolites possess higher exchange capacity per unit weight than natural zeolites.
For softening of water by zeolite process, hard water is percolated at a specified rate through a bed of zeolite, kept in a cylindrical vessel. The hardness causing ions (Ca2+, Mg2+) are retained by the zeolite as CaZe and MgZe, while the outgoing water contains sodium salts. Reactions taking place during the softening process are (i) Na2Ze + Ca(HCO3)2 = CaZe + 2NaHCO3, (ii) Na2Ze + Mg(HCO3)2 = MgZe + 2NaHCO3, (iii) Na2Ze + CaCl2 = CaZe + 2NaCl, and (iv) Na2Ze + MgCl2 = MgZe + 2NaCl.
After some time, the zeolite is completely converted into calcium and magnesium zeolites and it ceases to soften water, i.e., it gets exhausted. At this stage, the supply of water is stopped and the exhausted zeolite is reclaimed by treating the bed with brine solution (10 % NaCl solution). The reaction which takes place during regeneration is given by the equation CaZe (or MgZe) + 2NaCl = Na2Ze + CaCl2 (or MgCl2). The washings (waste liquor) containing CaCl2 and MgCl2 are sent to the drain and the regenerated zeolite bed hence obtained is used again for softening purpose. Fig 1 shows the softening of hard water by the Zeolite process.
Fig 1 Softening of hard water by Zeolite process
Limitations of the Zeolite process include (i) If the supplied water is turbid, the suspended matter is required to be removed, before the water is admitted to the zeolite bed, otherwise the turbidity clogs the pores of zeolite bed, thereby making it inactive, (ii) If water contains large quantities of coloured ions such as Mn2+ and Fe2+, they are to be removed first, since these ions produce manganese and iron zeolites, which cannot be easily regenerated, and (iii) if mineral acids are present in water, they destroy the zeolite bed and, hence, they are to be neutralized with soda, before admitting the water to the zeolite softening plant.
Advantages of zeolite process are (i) It removes the hardness almost completely, and water of around 10 ppm hardness is produced, (ii) The equipment used is compact, occupying a small space, (iii) no impurities are precipitated, so there is no danger of sludge formation in the treated water in a large scale, (iv) the process adjusts automatically itself for variation in hardness of incoming water, and (v) the time needed for softening is very low.
Disadvantages of zeolite process include (i) the treated water contains more sodium salts than in lime-soda process, (ii) The method only replaces Ca2+ ions and Mg2+ ions by Na+ ions, but leaves all the acidic Ions in the soft water and when this soft water containing NaHCO3 and Na2CO3 is used in cooling system, NaHCO3 gets decomposed producing CO2, which causes corrosion, and Na2CO3 undergoes hydrolysis to sodium hydroxide (NaOH) causing caustic embrittlement, and (iii) high turbidity water cannot be treated efficiently by this method, since fine impurities get deposited on the zeolite bed, thereby creating problem for its working.
Ion exchange process uses resin beads with sodium (sometimes potassium) attached to them. An ion exchange occurs when hard water containing ions of calcium and / or magnesium flows through the resin beads. The resin beads release sodium ions to the water while capturing the calcium and / or magnesium ions. Once the resin is depleted, it is to be regenerated to flush out the captured hard water ions and replace them with a new source of sodium ions. This is done by using a salt solution (sodium chloride) from a brine tank. Once the regeneration process is complete, the system is rinsed to remove residual hard water ions and chloride ions from the salt, and the rinse water is sent to the drain. After being put back on-line, the newly soft water contains all the original substances which are in the incoming supply water minus the hard water ions which have been replaced with sodium ions. The process continues until the resin is again depleted of sodium, and regeneration of the resin beads is to be done. process occurs. Fig 2 shows the process of ion exchange for water softening.
Fig 2 Process of ion exchange for water softening
Ion exchange resins are insoluble, cross-linked, long chain organic polymers with a micro porous structure, and the ‘functional groups’ attached to the chains are responsible for the ion exchange properties. Resins containing acidic functional groups (-COOH, -SO3H) are capable of exchanging their H+ ions with other cations, which comes in their contact. Resins containing basic functional groups (amino groups) are capable of exchanging their anions with other anions, which comes in their contact. Ion-exchange resins can be classified as (i) cation exchange resins (R.H+), and anion exchange resins (R.OH-)
Cation exchange resins (R.H+) are mainly styrene-divinyl benzene copolymers, which on sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cations in their water. The cations exchange sulphonic acid-formaldehyde resin exchange their H+ ions with the cations present in the water i.e., Ca2+ and Mg2+.
Anion exchange resins (R.OH-) are styrene-divinyl benzene or amine formaldehyde copolymers, which contain amino or quarternary ammonium or quarternary phosphonium or tertiary sulphonium groups as an integral part of resin matrix. These, after treatment dilute NaOH solution, become capable to exchange their OH- anions with anions in water. These resins on treatment with hard water are capable of exchanging the OH- anions with different anions of water i.e., Cl-, and (SO4)2-.
In the process of the ion exchange process, the hard water is passed first through cation exchange column, which removes the cations like Ca2+ and Mg2+ etc. from it, and equivalent quantity of H+ ions are released from the column to water. The reactions which take place are (i) 2RH+ + Ca2+ = R2Ca2+ + 2H+, and (ii) 2RH+ + Mg2+ = R2Mg2+ + 2H+. After cation exchange column, the hard water is passed through anion exchange column, which removes all the anions like (SO4)2-, Cl-, and (CO3)2- etc. present in the water and equivalent quantity of OH- ions are replaced from this column to water. The reactions which take place are (i) R.OH- + Cl- = R.Cl- + OH-, (ii) 2R.OH + (SO4)2- = R2.(SO4)2 + 2OH-, and (iii) 2R.OH- + (CO3)3- = R2.(CO3)2 + 2OH-. H+ and OH- ions released from cation exchange and anion exchange columns respectively get combined to produce water molecule. The reaction is H+ + OH- = H2O. Hence, the water coming out from the exchanger is free from cations as well as anions. Ion-free water is known as deionized or soft water.
When capacities of cation and anion exchangers to exchange H+ and OH- ions respectively are lost, they are then said to be exhausted. The exhausted cation exchange column is regenerated by passing a solution of dilute HCl or dilute H2SO4. The regeneration can be represented as (i) R2Ca2+ + 2H+ = 2RH+ + Ca2+. The exhausted anion exchange column is regenerated by passing a solution of dil. NaOH. The regeneration can be represented by the equation R2.(SO4)2 + 2OH- = 2R.OH- + (SO4)2. The regenerated ion exchange resins are then used again. Fig 3 shows the softening of hard water by ion exchange process.
Fig 3 Softening of hard water by ion exchange process
Advantages of ion exchange process are (i) the process can be used to soften highly acidic or alkaline waters, (ii) it produces water of very low hardness (around 2 ppm), and hence it is very good for treating water used in cooling systems.
Disadvantages of ion exchange process are (i) the equipment is costly, (ii) expensive chemicals are needed for treatment of water, and (iii) output of the process is reduced in case water contains turbidity. The turbidity is to be less than 10 ppm. In case of higher turbidity, it is to be removed by coagulation and filtration before the ion exchange process.
There are another four methods which are being used for water softening. These are (i) reagent addition, (ii) reverse osmosis, (iii) electro-dialysis, and electro-dialysis reversal, and (iv) electro-magnetic softening.
In the reagent addition method of water softening, specially formulated mixture of chemicals is added to the water for reducing its hardness to the level which is needed by the cooling system. The choice and the quantities of chemicals in the mixture is based on physical and chemical analysis of the available water, including determination of turbidity, hardness and temperature. In case of high turbidity levels, it is necessary to remove suspended solids in advance for a better result of water softening. This method is the internal treatment of water.
Reverse osmosis is based on the use of partially permeable membranes, through which water is passed under pressure. The process retains 90 % to 99 % of hydrated salt ions, as well as other organic and mineral dissolved compounds, the smallest colloidal impurities and bacteria. In the process the membranes get clogged and need periodic replacement. Reverse osmosis process can be with nano-filtration. The process is used when there is a need for higher level of water softening. The process has a high capital cost.
Electro-dialysis, and electro-dialysis reversal methods are driven by direct current (DC) in which ions (as opposed to water in pressure driven methods) flow through ion selective membranes to electrodes of opposite charge.
Electro-magnetic water conditioning devices have been marketed based on a variety of claims regarding their effect on water hardness and related scale formation. These devices are reported to have inconsistent performance to date. There are some reports that there is no change in the physical and chemical properties or the calcium ion concentration of water treated with these devices.
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