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1.7: Lime Softening

  • Page ID
    5729
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    Learning Objectives

    • Describe hard and soft water
    • Explain hardness of water
    • Explain soft water
    • Describe lime softening

    Water hardness is technically caused by divalent metallic cations that are capable of reacting with soap to form precipitates and with certain anions present in water to form scale. Calcium and magnesium are usually the only cations that are present in significant concentrations. Hardness is generally considered to be an expression of the total concentration of calcium and magnesium ions that are present in the water. If any metallic ions are present in significant amounts, they should be included in the hardness determination.

    Hard water is water having a high concentration of calcium and magnesium ions. Water is considered hard if it has a hardness greater than the typical hardness of water from the region. Hard water is defined as water with a hardness of more than 100 mg/L as calcium carbonate.

    Hardness is a characteristic of water caused by the salts of calcium and magnesium, like bicarbonate, carbonate, sulfate, chloride, and nitrate. Excessive hardness in water is undesirable because it causes the formation of soap curds, increased use of soap, deposition of scale in boilers, damage in some industrial processes, and it causes objectionable tastes in drinking water.

    Calcium hardness is caused by calcium ions (Ca+2). Magnesium hardness is caused by magnesium ions (Mg +2). Total hardness is the sum of the hardness caused by calcium and magnesium ions. Carbonate hardness is caused by the alkalinity present in water up to the total hardness. This value is usually less than the total hardness. Non-carbonate hardness is the portion of the total hardness in excess of the alkalinity.

    Alkalinity is the capacity of water or wastewater to neutralize acids. This capacity is caused by the water’s content of carbonate, bicarbonate, hydroxide, and sometimes borate, silicate, and phosphate. Alkalinity is expressed in mg/L of equivalent calcium carbonate. Alkalinity is not the same as pH because water does not have to be strongly basic to have high alkalinity. Alkalinity is a measure of how much acid must be added to a liquid to lower the pH to 4.5.

    Calcium carbonate is an expression of the concentration of specified constituents in water in terms of their equivalent value to calcium carbonate.

    The dissolved minerals in water cause difficulties in doing the laundry and in dishwashing in the household. These ions cause a coating to form inside the hot water heater similar to that in a tea kettle after repeated use.

    Hardness tends to shorten the life of fabrics that are washed in hard water. The scum or curds may become lodged in the fibers of the fabric and cause them to lose their softness and elasticity.

    In industry, hardness can cause greater problems. Many processes are affected by the hardness content of the water used. Industrial plants using boilers for processing steam or heat must remove the hardness from their makeup water, beyond what the water treatment plants would do. The reason is that the minerals will plate out on the boiler tubes and form a scale. This scale forms an insulation barrier that prevents proper heat transfer, causing excessive energy requirements for the boiler.

    In addition to removing hardness from water, other benefits of softening include:

    • Removal of iron and manganese
    • Control of corrosion when proper stabilization of water is achieved
    • Disinfection due to high pH values when using lime
    • A reduction in taste and odors
    • Reduction of total solids content by the lime treatment process
    • Removal of radioactivity

    Some of the possible limitations of softening include the following:

    • Free chlorine residual is predominantly hypochlorite at pH levels above 7.5 and it is less powerful
    • Cost and benefits must be carefully weighed to justify softening
    • Ultimate disposal of process wastes
    • The pH levels associated with softening chemical precipitation of the trihalomethane fraction in the treated water can increase
    • Production of aggressive water that would tend to corrode metal ions from the distribution system piping. Hard water does not corrode the pipe. Excessively hard water can cause scaling on the inside of the pipes and restrict the flow

    The decision to soften water is up to each community because softening is done mostly as a customer service. Hard water does not have an adverse effect on health but can create several unwanted side effects:

    • Over a period of time, the detergent consuming power of hard water can be costly
    • Scale problems on fixtures will be more noticeable
    • The lifetime of several types of fabrics will be reduced with repeated washing in hard water. Also, a residue can be left in clothing, creating a dirty appearance

    Two common methods are used to soften water. They are chemical precipitation (lime-soda ash) and ion exchange. Ion exchange softening is applied to water high in noncarbonated hardness and the total hardness does not exceed 350 mg/L. This method of softening can produce water of zero hardness, as opposed to lime softening where zero hardness cannot be reached.

    Ion exchange softening will also remove noncarbonated hardness without the addition of soda ash, which is required with lime softening. Ion exchange is a nonselective method of softening. This method will remove total hardness which is the sum total of carbonate and non-carbonate hardness.

    The limitation of ion exchange softening processes includes an increase in the sodium content of the softened water if the ion exchange is regenerated with sodium chloride. The sodium level should not exceed 20 mg/L in treated water because of the potentially harmful effect on persons susceptible to hypertension. The ultimate disposal of spent brine and rinse water from softeners can be a major problem for many installations.

    Hardness is due to the presence of divalent metallic cations in water. Hardness is a factor commonly measured by titration. Individual divalent cations can be measured in the laboratory using an atomic absorption spectrophotometer for accurate work. Hardness is usually reported as calcium carbonate equivalent. This procedure allows operators to combine or add up the hardness caused by calcium and magnesium and reported as total hardness.

    • Calcium Hardness, mg/L s CaCO3 = (Calcium, mg/L) (Equivalent Weight of CaCO3)/(Equivalent Weight of Calcium)
    • Equivalent Weight of Calcium = Atomic Weight/Valence

    To express the magnesium hardness of water as calcium carbonate equivalent:

    • Magnesium Hardness, mg/L as CaCO3 = (Magnesium, mg/L) (Equivalent Weight of CaCO3)/(Equivalent Weight of Magnesium)

    When treating water the pH is important. The pH of water can be increased or decreased by the addition of certain chemicals used to treat water. In many instances, the effect on the pH of adding one chemical is neutralized by the addition of another chemical. When softening water by chemical precipitation processes, like lime-soda softening, the pH must be raised to 11 for the desired chemical reaction to occur. The levels of carbon dioxide, bicarbonate ion, and carbonate ion in water are very sensitive to pH.

    The stability of treated water is determined by measuring the pH and calculating the Langelier Index. This index reflects the equilibrium pH of water with respect to calcium and alkalinity.

    • Langelier Index (LI) = pH – pHs where...
      • pH = actual pH of the water
      • pHs = pH at which water having the same alkalinity and calcium content is just saturated with calcium carbonate

    A negative Langelier Index indicates that the water is corrosive and a positive index indicates that the water is scale forming. After the water has been softened, the treated water distributed to consumers must be stable which means that the water can be neither corrosive nor scale forming.

    Alkalinity

    Alkalinity is the capacity of water to neutralize acids. This capacity is caused by the water content of carbonate, bicarbonate, hydroxide, borate, silicate, and phosphate. Alkalinity is expressed in mg/L of equivalent calcium carbonate. Alkalinity is not the same as pH.

    Alkalinity is measured in the laboratory by the addition of color indicator solutions. The alkalinity is then determined by the amount of acid required to reach a titration endpoint for a specific color change. The P (phenolphthalein) endpoint is pH 8.3. When the pH is below 8.3, no P alkalinity is present. When the pH is above 8.3, P alkalinity is present. No carbon dioxide is present when the pH is above 8.3, so no carbon dioxide is in the water when P alkalinity is present. Also, hydroxide and carbonate alkalinity are not present when the pH is below 8.3.

    The relationship between the various alkalinity constituents, like bicarbonate, carbonate, and hydroxide, can be based on the P (phenolphthalein) and T (total or methyl orange) alkalinity.

    When the pH is less than 8.3, all alkalinity is in the bicarbonate form and is commonly referred to as natural alkalinity. When the pH is above 8.3, the alkalinity can consist of bicarbonate, carbonate, and hydroxide. As the pH increase, the alkalinity progressively shifts to carbonate and hydroxide forms.

    Total alkalinity is the sum of the bicarbonate, carbonate, and hydroxide. Each of these values can be determined by measuring the P and T alkalinity in the laboratory. Alkalinity is expressed in mg/L as calcium carbonate equivalents.

    Softening

    Hardness is not completely removed by the chemical precipitation methods used in water treatment plants. Hardness cannot be reduced to zero using the chemical precipitation method of softening water. Water having a hardness of 150 mg/L as calcium carbonate or more is usually treated to reduce the hardness to 80 to 90 mg/L when softening is used as a water treatment option.

    The minimum hardness that can be achieved by lime softening-soda ash processes is around 30 to 40 mg/L as calcium carbonate. Regardless of the method used to soften water, the consumer usually receives blended water with a hardness of 80 to 90 mg/L as calcium carbonate when softening is utilized in water treatment facilities.

    Lime-soda softening produces benefits in addition to softening water. The advantages include:

    • Removal of iron and manganese
    • Reduction of solids
    • Removal and inactivation of bacteria and viruses from the high pH involved in the treatment
    • Control of corrosion and scale formation with proper stabilization of treated water
    • Removal of excess fluoride

    Limitations of the lime-soda softening process include:

    • An inability to remove all hardness
    • A high degree of operator control must be exercised for maximum efficiency in cost, hardness removal, and water stability
    • Color removal may be complicated by the softening process because of high pH levels
    • Large quantities of sludge are created that must be handled and disposed of in an acceptable manner
    Lime softening
    Figure \(\PageIndex{1}\): Lime Softening - Image by COC OER is licensed under CC BY

    Chemical Reaction

    In chemical precipitation, the hardness causing ions are converted to insoluble forms. Calcium and magnesium become less soluble as the pH increases. Calcium and magnesium can be removed from the water as insoluble precipitates at high pH levels.

    The addition of lime to water increases the hydroxide concentrations so that the pH increases. The addition of lime to water also converts alkalinity from the bicarbonate form to the carbonate form which causes the calcium to be precipitated as calcium carbonate. As additional lime is added to the water, the phenolphthalein alkalinity increases to a level where hydroxide becomes present, allowing the magnesium to precipitate as magnesium hydroxide.

    Following the chemical softening process, the pH is high and the water is supersaturated with excess caustic alkalinity in the hydroxide or carbonate form. Carbon dioxide can be used to decrease causticity and scale forming tendencies of the water prior to filtration.

    The chemical reactions that take place in water during the chemical precipitation process depend on whether the hardness to be removed is carbonate or non-carbonate hardness. Carbonate hardness, temporary hardness, can be removed with the use of lime only. Removal of non-carbonate hardness, permanent hardness, requires lime and soda ash.

    Chemicals

    The lime used in the chemical precipitation softening process can be hydrated lime, (Ca(OH)2,, calcium hydroxide, or calcium oxide, CaO. Hydrated lime can be used directly. The calcium oxide or quicklime must first be slaked. This process involves adding the calcium oxide to water and heating it to cause slaking, which is the formation of calcium hydroxide (Ca(OH)2) before use. Small facilities commonly use hydrated lime. Large facilities find it more economical to use quicklime (CaO) and slake it on site.

    The application of lime for the removal of carbonate hardness also removes carbon dioxide. Carbon dioxide does not contribute to hardness and does not need to be removed. However, carbon dioxide will consume a portion of the lime that is used and must be considered in the dosing process.

    When lime is added to water, carbon dioxide present in the water is converted to calcium carbonate if enough lime is added. Adding more lime, the calcium bicarbonate will be precipitated as calcium carbonate. To remove calcium and magnesium bicarbonate, an excess of lime must be used.

    Magnesium carbonate hardness requires the addition of lime and soda ash, Na2CO3.

    The primary chemical reactions products from the lime-soda softening process are calcium carbonate and magnesium hydroxide. The water treated has been chemically changed and is no longer stable because of pH and alkalinity changes. Lime-soda ash softened water is usually supersaturated with calcium carbonate. The degree of instability and excess calcium carbonate depends on the degree to which the water is softened. Calcium carbonate hardness is removed at a lower pH than magnesium carbonate hardness. If maximum carbonate hardness removal is practiced so that a high pH is required to remove magnesium carbonate hardness, the water will be supersaturated with calcium carbonate and magnesium hydroxide. Under these conditions, deposition of precipitates will occur in filters and pipelines.

    Excess lime addition to remove magnesium carbonate hardness results in supersaturated conditions and a residual of lime, which will produce a pH of about 10.9. The excess lime is called caustic alkalinity since it raises the pH. If the pH is then lowered, better precipitation of calcium carbonate and magnesium hydroxide will occur. Alkalinity will be lowered also. This process is usually accomplished by pumping carbon dioxide gas into the water. This addition to the treated water is called re-carbonation.

    Re-carbonation can be carried out in two steps. The first addition of carbon dioxide would follow excess lime addition to lower the pH to about 10.4 and encourage the precipitation of calcium carbonate and magnesium hydroxide. The second addition of carbon dioxide after treatment removes non-carbonate hardness. The pH is lowered to about 9.8 and it encourages precipitation. By carrying out re-carbonation prior to filtration, the buildup of excess lime and calcium carbonate and magnesium hydroxide precipitates in the filters will be prevented or minimized.

    Care must be exercised when using re-carbonation. Feeding excess carbon dioxide can result in no lowering of the hardness by causing calcium carbonate precipitates to go back into solution and cause carbonate hardness.

    An alternative method to the lime-soda ash softening process is the use of sodium hydroxide (NaOH), which is called caustic soda. The chemical reactions using caustic soda demonstrate that in removing carbon dioxide and carbonate hardness, sodium carbonate (soda ash) is formed, which will react with to remove non-carbonate hardness. Sodium hydroxide substitutes for soda ash and part of the lime to remove carbonate hardness. The use of caustic soda may have several advantages including stability in storage, less sludge formation, and ease of handling.

    Lime softening process
    Figure \(\PageIndex{2}\): Lime Softening – Image by COC OER is licensed under CC BY

    Ion Exchange Softening

    Electrically charged atoms or molecules are known as ions. Ion exchange treatment processes use special resins to remove charged, inorganic contaminants like arsenic, chromium, nitrate, calcium, radium, uranium, and excess fluoride from water. When source water is passed through a series of resin beads, it exchanges its charged contaminants for the harmless charged ions stored on the resin surface. The contaminants accumulate on the resins and must be periodically cleaned with a solution that recharges the interchangeable ions.

    Ion exchange resin comes in two forms: cation resins, which exchange cations like calcium, magnesium, and radium, and anion resins, which are used to remove anions like nitrate, arsenate, arsenite, or chromate. Each resin type is usually regenerated with a salt solution (sodium chloride). In the case of cation resins, the sodium ion displaces the cation from the exchange site, and in the case of anion resins, the chloride ion displaces the anion from the exchange site. As a rule, cation resins are more resistant to fouling than are anion resins. Resins can be designed to show a preference for specific ions so that the process can be easily adapted to a wide range of different contaminants. This treatment process works best with particle-free water, because particulates can accumulate on the resin and limit its effectiveness.

    Ion exchange is a common water treatment system that can be scaled to fit any size treatment facility. It may also be adapted to treat water at the point-of-use and point-of-entry levels.

    Activated Alumina

    Activated alumina treatment is used to attract and remove contaminants, like arsenic and fluoride, which have negatively charged ions. Activated alumina (aluminum oxide) is typically housed in canisters through which source water is passed for treatment. A series of such canisters can be linked together to match the water volume requirements of any particular system.

    As alumina absorbs contaminants, it loses its capacity to treat water. Therefore, treated water quality must be carefully monitored to ensure that cartridges are replaced before they lose their treatment effectiveness. Also, the capacity of the alumina is strongly influenced by the pH of the water. Lower pHs function better. Many systems use acid pretreatment to address this need.

    Source water quality is an important consideration for activated alumina systems. The treatment agent will attract contaminants, as well as other negatively charged ions found in the source water. This characteristic can limit the alumina’s ability to attract and remove the targeted contaminants.

    Activated alumina technology can be expensive, and its costs are associated with the disposal of the contaminated water that is created when alumina is purged of contaminants and recharged for future use. Large-scale activated alumina systems also require a high level of operational and maintenance expertise and, consequently, are relatively rare. Small-scale systems are more common and can be tailored to accommodate specific water volume requirements.

    Blending

    Ion exchange softeners will produce water with zero hardness. Water with zero hardness must not be sent into the distribution system. Water with zeros hardness is corrosive and over a period of time will attack steel pipes in the system and cause red water problems.

    At most softening plants, the zero hardness effluent from the softeners is mixed with filtered water having a known hardness concentration. A certain amount of water that the plant produces bypasses the softening process. This water has a known hardness concentration and is mixed in various proportions with the softener effluent to arrive at the desired level of hardness in the finished water.

    The blending of water is simple and is usually controlled by a valve and meter. The operator adjusts the exact gallons per minute bypassing the softener to produce the desired hardness.

    Review Questions

    1. Describe hard and soft water.
    2. Explain the hardness of water.
    3. Explain soft water.
    4. Describe lime softening

    Test Questions

    1. ________ is generally considered to be an expression of the total concentration of calcium and magnesium ions that are present in the water.
      1. Softness
      2. Hardness
      3. Stabilization
      4. Alkalinity
    2. ________ is the capacity of water or wastewater to neutralize acids. This capacity is caused by the water’s content of carbonate, bicarbonate, hydroxide, and sometimes borate, silicate, and phosphate.
      1. Softness
      2. Hardness
      3. Stabilization
      4. Alkalinity
    3. ______is expressed in mg/L of equivalent calcium carbonate.
      1. Softness
      2. Hardness
      3. Stabilization
      4. Alkalinity
    4. In addition to removing hardness from water, which of the following is not a benefit of softening?
      1. Removal of iron and manganese
      2. Control of corrosion when proper stabilization of water is achieved
      3. Disinfection due to high pH values when using lime
      4. Production of aggressive water that would tend to corrode metal ions from the distribution system piping
    5. Which of the following is not a typical limitation of softening?
      1. Free chlorine residual is predominantly hypochlorite at pH levels above 7.5
      2. Removal of radioactivity
      3. Disposal of process wastes
      4. At the pH levels associated with softening chemical precipitation of the trihalomethane
      5. The fraction in the treated water can increase
    6. _________ is used to soften water that is high in noncarbonated hardness and where the total hardness does not exceed 350 mg/L.
      1. Lime
      2. Lime-soda ash
      3. Calcium hydroxide
      4. Ion exchange softening
    7. Limitation of ________ processes include an increase in the sodium content of the softened water if the ion exchange is regenerated with sodium chloride. The sodium level should not exceed 20 mg/L in treated water because of the potentially harmful effect on persons susceptible to hypertension.
      1. Lime
      2. Lime-soda ash
      3. Calcium hydroxide
      4. Ion exchange softening
    8. The primary chemical reactions products from the lime-soda softening process are _______ and __________.
      1. Calcium hydroxide, magnesium hydroxide
      2. Calcium carbonate, magnesium carbonate
      3. Calcium carbonate, magnesium hydroxide
      4. Calcium hydroxide, magnesium carbonate
    9. Electrically charged atoms or molecules are known as ______.
      1. Cations
      2. Anions
      3. Electrons
      4. Ions
    10. Resins are usually regenerated with _______.
      1. Sodium chloride
      2. Potassium chloride
      3. Ferric chloride
      4. Hydrochloric acid
    11. Activated alumina treatment is used to attract and remove ______, which have negatively charged ions. Activated alumina (aluminum oxide) is typically housed in canisters through which source water is passed for treatment. A series of such canisters can be linked together to match the water volume requirements of any particular system.
      1. Iron and magnesium
      2. Magnesium and calcium
      3. Nitrate and nitrite
      4. Arsenic and fluoride
    12. Ion exchange softeners will produce water with zero hardness. Water with zero hardness must not be sent into the distribution system. Water with zeros hardness is ___________.
      1. Hard water
      2. Corrosive water
      3. Scaling forming water
      4. Black water
    13. At most softening plants, the zero hardness effluent from the softeners is mixed with filtered water having a known hardness concentration in a process called ______.
      1. Blending
      2. Conservation
      3. Oxidation
      4. None of the above
    14. Calcium hardness is caused by calcium ions (Ca+2). Magnesium hardness is caused by magnesium ions (Mg +2). Total hardness is the sum of the hardness caused by calcium and magnesium ions. Carbonate hardness is caused by the _______ present in water up to the total hardness.
      1. Softness
      2. Hardness
      3. Stability
      4. Alkalinity

    1.7: Lime Softening is shared under a CC BY license and was authored, remixed, and/or curated by John Rowe.

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