Hardness Removal

HARDNESS REMOVAL TECHNOLOGIES OF WATER Hardness is due to the presence of multivalent cations in water. Hardness in water is principally caused by the solution in water of carbonate, bicarbonate, and sulfate of calcium, and magnesium. Sometimes iron and aluminum cause hardness to a lesser degree. In other words, it is a property of water, which prevents the lathering of the soap. Hardness is usually expressed in mg/litre or p.p.m. of calcium carbonate in water. Hardness is of two types. 1. Temporary hardness: It is caused due to the presence of carbonates and sulfates of calcium and magnesium. It is removed by boiling. 2. Permanent hardness: It is caused due to the presence of chlorides and nitrates of calcium and magnesium. It is removed by zeolite method. Generally a hardness of 100 to 150 mg/l is desirable. Excess of hardness leads to the following effects. 1. Large soap consumption in washing and bathing 2. Fabrics when washed become rough and strained with precipitates. 3. Hard water is not fit for industrial use like textiles, paper making, dye and ice cream manufactures. 4. The precipitates clog the pores on the skin and make the skin rough 5. Precipitates can choke pipe lines and valves 6. It forms scales in the boiler tubes and reduces their efficiency and cause in encrustations 7. Very hard water is not palatable Softening is practices when hardness exceeds 300mg/l. Hardness > 600 mg/l rejected for drinking purpose. METHODS OF REMOVAL OF HARDNESS There are several general methods used for water softening 1. Boiling (Heating) 2. Lime addition (Lime process) 3. Lime soda process (lime and soda ash process) 4. Excess Lime treatment 5. Caustic soda process 6. Zeolite process 7. Demineralization or exchange process (base-exchange process). Methods 1 and 2 are suitable for removal of temporary hardness and 3 to 7 for both temporary and permanent hardness. 1

Removal of temporary Hardness: 1.

Boiling: Boiling or heating is effective only for the removal of bicarbonate hardness. Heating of water breaks down bicarbonate into carbonate, water and carbon dioxide. i)

heat Ca(HCO3)2 ------> CaCO3↓+ CO2↑+ H2O

heat ii) Mg(HCO3)2 ------> MgCO3↓+ CO2↑+ H2O

CaCO3 is insoluble and settles down and excess CO 2 escapes from water. Similarly hardness by Mg(HCO3)2 can also be removed by simple heating. 2.

Addition of lime: The lime process, also known as the Clark Process, reduces only carbonate hardness. The principle involved is to neutralize with milk of lime, i.e Ca(OH)2, forming normal carbonates which precipitate out when present in excess amount and removed by sedimentation and filtration. Ca (HCO3)2 + Ca(OH)2 -----------> 2CaCO3↓ + 2H2O Mg(HCO3)2 + Ca(OH)2 -----------> CaCO3 + MgCO3 + 2H2O

Removal of permanent Hardness: 1. Lime soda process: In this method, the lime and sodium carbonate or soda is used to remove permanent hardness from water. Lime has no effect on sulfate of calcium and magnesium, which are responsible for causing most non-carbonate hardness found in natural waters. The non-carbonate hardness is removed by the addition of soda ash (Na2CO3). The chemical reactions involved in this process are as follows. MgSO4 + Ca(OH)2 ----------> Mg(OH)2 + CaSO4 {conversion of MgSO4 to CaSO4} CaSO4 + Na2CO3 ----------> CaCO3 + Na2SO4 {removal of sulfates} MgCl2 + Ca(OH)2 ----------> Mg(OH)2 + CaCl2 {removal of chlorides} CaCl2 + Na2CO3 ----------> CaCO3 + 2NaCl MgCl2 + Na2CO3 ----------> MgCO3 + 2NaCl {removal of chlorides} Some of the insoluble materials formed in the process may remain in the water in a finely divided state and may be found deposited on the filter sand or in the distribution pipelines at a later time. This difficulty may be overcome by treating the water with carbon dioxide, a process called recarbonation. Advantages of lime soda process 1. The pH value of water treated by this process bring down to 9 and which results in decrease in corrosion of the distribution system. 2. Less quantity of coagulant will be required, if this process is adopted 3. Removal of iron and manganese to some extent 4. Reduction of total mineral content of water 5. Hardness of water is reduced to 40mg/l (of CaCO3) and magnesium upto 10mg/l 6. The process is economical 2

Disadvantages 1. Large quantity of sludge formed during this process to be disposed off by some suitable method 2. This process requires skilled supervision for its successful working 3. If recarbonation is omitted, a thick layer of calcium carbonate will be deposited in the filtering media, distribution pipes etc. 2. Zeolite process: This is also known as the base-exchange or Ion exchange process. The hardness may be completely removed by this process. Principle Zeolites are compounds (silicates of aluminum and sodium) which replace sodium ions with calcium and magnesium ions when hard water is passes through a bed of zeolites. The zeolite can be regenerated by passing a concentrated solution of sodium chloride through the bed. The chemical reactions involved are + Na2Al(SiO3)2 (Zeolite)

CaSO4 MgSO4 CaCO3 MgCO3 CaCl2

Na2CO3 Na2SO4 2NaCl etc.

Ca Al(SiO3)2

+

Mg

Regeneration: Ca

+ Al(SiO3)2

Mg

2NaCl

Na2Al(SiO3)2 (Regenerated Zeolite)

CaCl2 + MgCl2

Advantages 1. In this process, the sludge is not formed hence problem of sludge disposal does not arise 2. It can be operated easily and no skilled supervision required 3. The hardness of water reduces to zero and hence used for boiler and texile industries 4. The process is economical where salt is cheaply available 5. The load on Zeolite can be reduced by combining it with lime or aeration process Disadvantages 1. The Zeolite process cannot be used for turbid or acidic water 2. The Zeolite process is unsuitable for water containing Iron and Manganese 3. The Zeolite should be operated carefully to avoid injury or damage to the equipment 3

Exercise:

Calculate the quantity of zeolite required to soften 2.27ML of water of 572 ppm to 286 ppm hardness. The interval between successive regeneration is 3 hours and the capacity of exchanger is 27.46 x 106 mg/m3.

Soln.: L

Quantity of hard water to be softened in 3 hours = 2.27 x 10 6 x 3/24 = 283750 Reduction in hardness = (572 – 286) ppm = 286 mg/L Total hardness to be removed = 283750 x 286 = 81152500 mg So, Total quantity of zeolite required = 81152500/27.46 x 106 = 2.955 ≈ 2.96 m3.

Demineralization Both cations and anions are removed by resins similar to zeolites in two columns by ion exchange method; however, the exchanged ions must not contribute dissolved solids to the effluent. This is accomplished by exchanging hydrogen for the dissolved cations and hydroxide for the dissolved anions. The two then combine in equal amounts to form water (H2O), leaving no residual and not affecting the pH. The resins may be regenerated with acids and bases, respectively. This process is used in industries to get distilled water or quality water. Processes of demineralization Hydrogen zeolite (available under the name Zeo-karb catex and organolite) exchange all cations for hydrogen. (HCO ) CO Ca 3 2 3 Mg Cl2 NO Cl22 Fe SO4 SO4

+ H2 X

H2

+

Ca Mg NO2 Fe

X

Cation Exchange Process

CO3 O HH2Cl 2 2

CO3 +

NaOH

+ Na

Cl2 SO4

SO4 Anion Exchange Process mg/L equiv/ L =

4

Equivalence weight molecular weight

Equivalence weight =

valence (Mass +ve or –ve charge that results from dissolution) Ex Conuert 116 mg/L of NaCl to equiv/liter unit. [NaCl]=116mg/L molecular weights : NaCl = 23 + 35 =58 gr/ mole

Na+ + Cl -

NaCl

Na Cl Equivalence weight :

58 gr / mole = 58 gr/equiv 1 116 mg/L x 10-3 x g/mg

[NaCl] equiv/liter =

= 2 10-3 eq / liters = 2 meq/liters

58 g/equiv Ex convert 200 mg/L of CaCO3 to equivl / liter units [CaCO3]. molecular weight = CaCO3 = 40 + 12 + 3 (16) = 100 gr/mole ca equivalence weight :

carbon 100 gr/mole

oxygen

CaCO3

Ca2+ + CO2-3

= 50 gr/eq

2 200 mg/liter 10-3 = 4 10-3 eq/ liter = 4 meq/liter

[CaCO3] equiv/liters = 50 gr/eq

5