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Water Softening : Lime and Soda Ash Softening Is a Widely Used Method

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Water Softening : Lime and Soda Ash Softening Is a Widely Used Method :Water Softening : Lime and Soda Ash softening is a widely used method of removing hardness from water using lime and soda ash as flocculating agents. The process utilizes the low solubility of CaCO3 and Ca(OH)2 to precipitate out hardness causing cations like calcium and magnesium. Theoretically hardness is defined as sum of the soluble concentrations of polyvalent cations expressed as equivalent concentration of calcium carbonate. Carbonate hardness is defined as the concentrations of Ca+2 and Mg+2 and other polyvalent cations that are associated with the anions that comprise alkalinity. Non Carbonate hardness is defined as the concentration of Ca+2 and Mg+2 and other polyvalent cations associated with non alkaline anions(e.g. SO4-, Cl-). Hardness is derived in the water due to dissolution of minerals from geologic formation that contains calcium and magnesium. Most common forms are CaCO3(calcite) and dolomite[CaMg(OH)2]. The removal of these hardness causing cations depend upon the relative insolubilities of calcium carbonate and magnesium hydroxide. Within the pH range and concentration of cations found in water , it is almost imperative to increase the pH of water for the precipitation of calcium and magnesium. Lime and Soda Ash are the most commonly used chemicals. When carbonate hardness is adequate both calcium carbonate and magnesium hydroxide can be precipitated out using pH correction alone. When the carbonate hardness is too low, carbonate content must be supplemented using sodium carbonate (soda ash) and the minimum hardness achieved would be dependent upon the solubility of Ca , Mg and pH. Aside from aesthetic benefits lime softening can effectively remove heavy metals, iron and manganese, turbidity and come organic compounds including a substantial amount of NOM, kill algae, bacteria and virus and provide around 40 to 80% TOC reduction. Principle reaction for lime softening for carbonate hardness: CO2 + Ca(OH)2 CaCO3(s) + H2O 3- Ca+2 + 2HCO + Ca(OH)2 2CaCO3(s) + 2H2O +2 3- Mg + 2HCO + 2Ca(OH)2 2CaCO3(s) + Mg(OH)2(s) + 2H2O +2 -2 +2 -2 Mg + SO4 + Ca(OH)2 Mg(OH)2(s) + Ca + SO4 When lime is added to water, it first reacts with any free CO2 present in the water as CO2 is stronger than HCO3-. It is for this reason that with waters having high free CO2 aeration tank is added prior to softening to remove free CO2 and thus to minimize the consumption of lime during softening process. Theoretically for complete conversion of HCO3- to CO3-2 , a pH of greater than 12 is required, however in practice it is found that calcium carbonate precipitation may occur at pH as low as 9.3 because a significant amount of carbonate is in equilibrium with bicarbonate and more carbonate is formed during precipitation. For, the precipitation of magnesium carbonate hardness a pH of at least 10.5 or greater is required for effective precipitation of Mg(OH)2. The range of excess lime dosage to raise pH is reported in the range of 30 to 70 mg/L as CaCO3. When the amount of carbonate alkalinity present in the water is not sufficient to react with lime, additional alkalinity is added in the form of soda ash. Moreover for the removal of non carbonate hardness, soda ash is added with lime. In case of non-carbonate hardness, MgSO4 will first react with lime and produce Mg(OH)2 precipitate and CaSO4. This caSO4 will further react with soda ash and will produce calcium carbonate precipitate. MgSO4 + Ca(OH)2 Mg(OH)2(S) + CaSO4 CaSO4 + Na2CO3 CaCO3(S) + Na2SO4 Stabilization of Softened Water: After the water is softened, stabilization process is required to minimize the corrosion and scaling in the distribution system. Two major factors contribute to corrosion and scaling. As during the softening process a significant amount of lime is added to water, it leaves enough amount of alkalinity in the effluent water to cause corrosion of distribution mains. Second, theoretically the precipitation of calcium carbonate during softening process does not reach its equilibrium state at a rapid rate. Though most of the calcium carbonate precipitates out from the solution, even after sufficient reaction time, there would be a residual of calcium carbonate particles in the effluent water and it continues to react and remain in suspension. Even in a continuous stirred system, after a three hours of reaction time, the residual concentration of CaCO3 in water would not reach the equilibrium concentration of 15 mg/L as CaCO3. Thus this continual reaction of calcium carbonate within the solution would cause scaling of the downstream filtration distribution system. The following methods are used to aid stabilization in softening system. 1) Long Reaction Time 2) Recarbonation 3) Addition of raw water to softened or settled water ahead of filtration 4) Addition of sodium bicarbonate, soda ash or lime 5) Filtration through sand filters 6) Sludge blanket Filtration 7) Treatment with phosphate such as sodium hexametaphosphate 1) Long reaction time: When the supersaturated softened water is allowed to stand, the excess calcium carbonate will precipitate out. However, for this longer standing time is required along with large reservoirs and space to build it. 2) Re carbonation : Re carbonation to stabilize the water can be performed by addition of CO2 in liquid or gaseous form. The addition of CO2 will precipitate out additional Ca present in the water. When excess alkalinity is present in the water, CO2 added will react with hydroxide ion and will form carbonate ion. This carbonate ion will react with any calcium present above saturation limit and would produce calcium carbonate precipitate. Addition of any additional CO2 will lower the pH of water to saturation limit. In the case of selective calcium removal, as the water is treated with excess lime and the pH will be between 10.0 to 10.6 , addition of CO2 will convert the carbonate into bicarbonate and will dissolve some of the calcium carbonate which was precipitated out as calcium bicarbonate. Ca+2 + CO3-2 + CO2 + H2O Ca+2 + 2HCO3- In cases where calcium and magnesium removal is required, excess amount of lime is added and the operation is carried out at pH higher than 11.0 to precipitate magnesium hydroxide. Thus, enough amount of CO2 is required to convert the hydroxide ions to carbonate ions and then to bicarbonate ions. This conversion will bring the pH to 10.0 to 10.5 and calcium hydroxide will precipitate as CaCO3 and magnesium will be converted to magnesium carbonate which is soluble in the water. Ca+2 + 2OH- + CO2 CaCO3(s) + H2O Mg+2 + 2OH- + CO2 Mg+2 + CO3-2 + H2O As further CO2 is added to bring pH to 8.4 to 8.6, some of the calcium carbonate which was precipitated previously would dissolve as calcium bicarbonate and magnesium carbonate will convert to magnesium bicarbonate. CaCO3(s) + CO2 + H2O Ca+2 + 2HCO3- Mg+2 + CO3-2 + CO2 + H2O Mg+2 + 2HCO3-2 Types of Softening Processes: 1) Single Stage Process : Single stage process is used when removal of non carbonate hardness is not required. Lime is added upstream of the reactor/clarifier using flash mixer or added directly in the clarifier. The pH of the mixer leaving flash mixer is 10.2 to 10.5. Soda ash can be added if non carbonate hardness removal is required. Following sedimentation the treated water will pass through the recarbonation and filtration stage to adjust the pH, removal of excess lime and particles formed during re carbonation stage. 2) Two Stage Softening : It is known as split re carbonation as well and used when magnesium removal is required along with non carbonate hardness. Excess lime is added during first stage to raise pH to 11.0 for magnesium removal. After sedimentation pH is reduced to 10.0 to 10.6 and soda ash is added for excess lime precipitation. After second stage precipitation and sedimentation, second stage re carbonation is followed to reduce the pH to 8.3 to 8.5. Finally filtration is used remove any excess particles formed during second stage. Split Treatment Softening: It consists of treating of a part of the source water to varying degrees of treatment and then blending it to get desired results. There are principally three methods available: 1) parallel softening, and coagulation 2) parallel lime softening and RO/IX 3) split treatment with excess lime. Split treatment with excess lime is most commonly used method. Parallel softening along with coagulation is used in the case of water containing high turbidity, color and hardness while parallel softening along with RO/IX is used when removal of dissolved solids is required or a high non carbonate hardness removal is sought after. Split Treatment with Excess Lime : It is used when magnesium must be removed and a very little non carbonate hardness is available in the source water. Initially a part of the raw water is treated with excess lime for the removal of calcium and magnesium and the level of magnesium is brought down to 10 mg/lit. The remaining part of the water is blended with treated water prior to sedimentation and the alkalinity of raw water is used to neutralize the excess caustic alkalinity that is used to remove magnesium. Normally re carbonation step is not required.The ratio of bypass flow is derived from the mass balance equation of magnesium as follows: X = [Mg+2]e - [Mg+2]t / [Mg+2]o - [Mg+2]t Where e = finished water , t = treated water at the outlet of first stage, o = source water. Chemical dosage estimation: The chemical dosage can be estimated using one of the following methods.
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