14 Water Softening

Babita Khosla

15.1      Introduction

The term hardness was originally applied to waters that were hard for the purpose of washing means no foam formation with the soap. Hardness prevents soap from lathering by causing the development of an insoluble curdy precipitate in the water; hardness typically causes the buildup of hardness scale (such as seen in cooking pans). Dissolved calcium and magnesium salts are primarily responsible for most scaling in pipes and water heaters and cause numerous problems in laundry, kitchen, and bath. Hardness is usually expressed in grains per gallon (or ppm) as calcium carbonate equivalent. The hardness of water is caused mainly by the salts of calcium and magnesium, which are acquired through contact with rocks and sediments in the environment. The positive electrical charges of these ions are balanced by the presence of anions (negative ions), of which bicarbonate HCO3– and carbonate CO32– are most important. These ions have their origins in limestone sediments and also from carbon dioxide which is present in all waters exposed to the atmosphere and especially in groundwaters.

15.2     Origin of water “hardness”

Carbon dioxide reacts with water to form carbonic acid which exists mostly as bicarbonate ion in the environmental. Microscopic marine organisms take this up as carbonate to form calcium carbonate skeletons which, over millions of years, have built up extensive limestone deposits. Groundwaters, made slightly acidic by CO2 (both that absorbed from the air and from the respiration of soil bacteria) dissolve the limestone, thereby acquiring calcium and bicarbonate ions and becoming “hard”. If the HCO3– concentration is sufficiently great, it causes calcium carbonate (“lime scale”) to precipitate out on surfaces such as the insides of pipes.

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Generally, water containing more than 100 mg/l of hardness expressed as calcium carbonate (CaCO3) is considered to be hard. Excessive hardness is undesirable because it causes the formation of soap curds, increased use of soap, deposition of scale in boilers, pipelines and home appliances, damage in industrial processes and can cause objectionable tastes.

The degree of hardness standard as established by the American Society of Agricultural Engineers and the Water Quality Association (WQA) is as follows:

15.3      Types of water hardness

Carbonate/Temporary hardness: This refers to hardness whose effects can be removed by boiling the water in an open container. Such waters have usually percolated though limestone formations and contain bicarbonate (HCO3– ) of calcium and magnesium along with small amounts of carbonate (CO32– ) as the principal negative ions. Both calcium and magnesium bicarbonates decompose when water is boiled. Boiling the water promotes the reaction by driving off the carbon dioxide gas.

2 HCO3– → CO32– + CO2

The CO32– reacts with Ca2+ or Mg2+ ions, to form insoluble calcium and magnesium carbonates which precipitate out, thus removing the calcium and magnesium ions from the water, and so removes the hardness. Therefore, hardness due to bicarbonates is said to be temporary.

Non-carbonate/Permanent hardness Water containing other anions such as chloride or sulfate cannot be remediated by boiling, and are said to be “permanently” hard.

When measuring hardness, we typically consider total hardness which is the sum of all hardness compounds in water, expressed as a calcium carbonate equivalent. Total hardness includes both temporary and permanent hardness caused by calcium and magnesium compounds.

15.4 Water Softening is the removal of hardness from water. This is not a required part of the water treatment process since hard water does not have any health consequences. In fact, the National Research Council states that drinking hard water generally contributes to the total calcium and magnesium needs in humans. However, hard water is problematic for a variety of reasons. Hard water makes soap precipitate out of water and form a scum, such as the ring which forms around bathtubs. In addition to being unsightly, the reaction of hard water with soap results in excessive use of soaps and detergents. Hard water may also cause taste problems in drinking water and may shorten the life of fabrics washed in hard water. Finally, hard water harms many industrial processes, so industries often require much softer water than is usually required by the general public. Excessively hard water will nearly always have to be softened in order to protect the water treatment plant equipment and piping systems. At a hardness of greater than 300 mg/L as calcium carbonate, scale will form on pipes as calcium carbonate precipitates out of the water.

15.5 Processes for Water softening: In each of the treatment processes, the goal is the same that the softened water should have a hardness of about 80 to 90 mg/L as calcium carbonate. Some of the methods used are,

15.5.1 Chemical Lime-Soda Process: The process of water softening involves the addition of chemicals like calcium hydroxide, or Ca(OH)2 and soda ash (sodium carbonate, Na2CO3) to make hard water softer.

Cold Lime-Soda Process: In this method, raw water and calculated quantities of chemicals (Lime+Soda) are mixed with water at room temperature. At room temperature, the precipitates formed are finely divided, so they do not settle down easily and cannot be filtered easily. Ca(HCO3)2 + Ca(OH)2 → 2 CaCO3 + 2 H2O

Mg(HCO3)2 + 2 Ca(OH)2 → Mg(OH)2 + 2 CaCO3 + 2H2O

The non-carbonate calcium hardness can be further reduced by adding sodium carbonate.

CaCl2 + Na2CO3 → 2 NaCl + CaCO3

The optimum pH for this process is about 10- 11. This pH can be obtained by using additional lime. These precipitates are then removed by conventional processes of coagulation/flocculation, sedimentation, and filtration. Because precipitates are very slightly soluble, some hardness remains in the water–usually about 85 mg/l (as CaCO3).

In the process, it is essential to add small amounts of coagulants like Alum, aluminum sulphate, sodium  aluminate,  etc.  which  hydrolyze  to  flocculent,  gelatinous  precipitate  of  aluminum hydroxide, and entraps the fine precipitates. Use of sodium aluminate as coagulant, also helps the removal of silica as well as oil, if present in water. This process provides water, containing a residual hardness of 50 to 60 ppm.

NaAlO2 + 2H2 O → NaOH +Al(OH) 3

Al2(SO4)3+ 3 Ca(HCO3)2 → 2Al(OH)3 + 3CaSO4 + 6CO2

Hot Lime-Soda Process: The process involves the treatment of water with softening chemicals at a temperature of 80 to 1500 C. Since hot process is operated at a temperature close to the boiling point of the solution, so the reaction proceeds faster and the precipitate formed settle down rapidly and hence, no coagulants are needed. This process produces water of comparatively lower residual hardness of 15 to 30ppm.

After softening, the water will have high pH and contain the excess lime and the magnesium hydroxide and the calcium carbonate that did not precipitate. Re-carbonation (adding carbon dioxide) is used to stabilize the water. The excess lime and magnesium hydroxide are stabilized by adding carbon dioxide, which also reduces pH from 10.8 to 9.5 as the following:

CO2 + Ca(OH)2 → CaCO3 ↓ +H2O

CO2 + Mg(OH)2 → MgCO3 + H2O

Further recarbonation, will bring the pH to about 8.5 and stablize the calcium carbonate as the following:

CO2 + CaCO3 + H2O → Ca(HCO3)2

It is not possible to remove all of the hardness from water. In actual practice, about 50 to 80 mg/l will remain as a residual hardness.

Advantages of Lime Soda Process:

It is a very economical and if the process is combined with sedimentation/coagulation, lesser amounts of coagulants shall be needed The process increased the pH value of the treated water, thereby corrosion of the distribution pipes is reduced

Besides the removal of hardness, the quantity of minerals in the water are reduced To certain extent, iron and manganese are also removed from the water. Due to alkaline nature of treated- water, amount of pathogenic bacteria’s in water is considerably reduced

Disadvantages of Lime Soda Process:

Disposal of large amounts of sludge (insoluble precipitate) poses a problem. This can remove hardness only up to 15ppm, which is not good for boilers.For efficient and economical softening, careful operation and skilled supervision is required

15.5.2 Ion exchange Processes: Ion exchange processes fall into several categories as softening; the process is effective at removing both carbonate and non-carbonate hardness and is often used for waters high in non-carbonate hardness and with a total hardness less than 350 mg/L. In ion exchange softening processes, calcium and magnesium ions (hardness) are exchanged for sodium ions. However, ion exchange softening has its disadvantages as well. The  calcium and magnesium in the hard water are replaced by sodium ions, which may cause problems for people with health problems who are not supposed to eat any salt. Softeners have to be backwashed in a manner similar to a filter, and the recharge water, known as brine, can cause disposal problems.

Demineralization: In this process the hard water is passed through ion exchange resins, which are insoluble, cross linked long chain organic polymers with micro porous structure, and the functional groups attached to the chains are responsible for the Ion –exchanging properties. Ion exchange resins may be classified as

Cation exchange resins are mainly styrene-divinyl benzene copolymers, which on sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cations in water Anions exchange resins are styrene-divinyl benzene or amine-formaldehyde,copdymers,which contains amino or quaternary ammonium or quaternary phosphonium or tertiary sulphonium groups as an integral part of the resin matrix these after treatment with dilute NaOH solutions become capable to exchange their OH-anions with anions present in water.

The hard water in this process is passed first through cat ion exchange column, which removes all the cations like Ca+2 etc, from it and equivalent amount of H+ ions released from this column to water, thus

2RH+ + Ca 2+ → R2Ca 2+ + 2H+

2RH+ + Mg 2+ → R2Mg 2+ + 2H+

After cation exchange column, the hard water is passed through anion exchange column which removes all the anions like SO42- ,Cl- etc. present in the water and equivalent amount of OH- ions are released from this column to water thus:

R’OH- + Cl- → R’Cl- + OH-

2R’OH- + SO42- → R’2SO42- + 2OH-

2R’OH- + CO32- → R’2 CO32- +2OH-

H+and OH- ions get combined to produce water molecule and the water coming out from the exchanger is deionized or demeneralised water. When capacities of cation and anion exchangers to exchange H+ and OH- ions respectively are lost, they are then regenerated. The cation exchange column is regenerated by passing a solution of diluted HCl and the exhausted anions exchange column is regenerated by passing a solution of diluted NaOH.

The regeneration can be represented as
R2Ca 2+ + 2H+	→ 2RH+ + Ca 2+
R2SO42- + 2OH-	→  2ROH- + SO42-

15.5.3 Reverse Osmosis (Membrane Process): Membrane technology is widely accepted as a means of producing various qualities of water from surface water, well water, brackish water and seawater. In the treatment of water for drinking purposes first of all pressure-driven membrane techniques are used. The choice of the suitable membrane process depends on the size of the removed contaminants and admixtures from the water.

In the normal osmosis process, the water naturally moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). But, with the application of an external pressure, the natural flow of water can be reversed, which is reverse osmosis. The removal of minerals occurs at the osmotic membrane chamber that usually has tiny pores that can only allow water molecules to pass  through. The pressure of around 35- 40 psi is applied to push water molecules through. All other substances, including calcium and magnesium ions, are left behind, and softened water is produced.

This is an effective method of both water purification and softening. This is an effective method of both water purification and softening. Its disadvantages are the usage of electricity and wastage of a lot of water during the purification process.

Figure 4: The process of normal and reverse osmosis

Source: http://jjtlucero.blogspot.in/

References:

Skipton, S.O.; Dvorak, B.I.; Niemeyer, M.N. (2008): Drinking Water Treatment: Water Softening (Ion Exchange). In: University of Nebraska, Lincoln.