water softening ce3
TRANSCRIPT
WATER
SOFTENING
ENVIRONMENTAL ENGINEERING.
Courtesy: dupont.com
RGU IIIT NUZVID 2
Contents• Introduction• Why softening?• Removal of temporary hardness
• Boiling• Addition of lime
• Removal of permanent hardness• Chemical precipitation• Ion exchange
• Demineralization• Stages of softening• Split treatment• Softening operations• Problems
Introduction
• Softening is reduction or removal of hardness.• Hardness is due to divalent metallic ions like −𝐶𝑎+2, 𝑀𝑔+2, 𝐹𝑒+2, 𝑀𝑛+2, 𝑆𝑟+2 etc.• Two types of hardness.
• Temporary or carbonate hardness• Permanent or non-carbonate hardness
• Permissible hardness: 75 – 115 mg/l• Softening may be done by the water utility at the treatment plant or by the
consumer at the point of use.
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Why softening?• Reduction of soap consumption• Lowered cost in the maintenance of plumbing• Improved taste in foods prepared• A must for industrial supplies
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Courtesy: lenntech.com
Courtesy: ndsu.edu
Lead contaminated site water softening basin
Removal of temporary hardness
• Temporary hardness is due to carbonates and bicarbonates of calcium and magnesium.
• Can be removed either by boiling or by adding lime to the water.
Boiling
• 𝐶𝑎𝐶𝑂3 is slightly soluble in water.• So it usually exists in water as a bicarbonate.• Boiling will lead to the precipitation of 𝐶𝑎𝐶𝑂3 and release of 𝐶𝑂2.
• 𝐶𝑎(𝐻𝐶𝑂3)2 + Heat 𝐶𝑎𝐶𝑂3 + 𝐶𝑂2 + 𝐻2O
• 𝐶𝑎𝐶𝑂3 is precipitated.• Magnesium carbonates and bicarbonates are not satisfactorily removed.• 𝑀𝑔𝐶𝑂3 is fairly soluble.
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Addition of lime
• Hydrated lime is added to the water.• Efficient in removal of both calcium and magnesium carbonates.
• 𝑀𝑔𝐶𝑂3 + 𝐶𝑎(𝑂𝐻)2 𝑀𝑔(𝑂𝐻)2 + 𝐶𝑎𝐶𝑂3
• 𝑀𝑔(𝐻𝐶𝑂3)2 + 𝐶𝑎(𝑂𝐻)2 𝐶𝑎(𝐻𝐶𝑂3)2 + 𝑀𝑔(𝑂𝐻)2
• 𝐶𝑎(𝐻𝐶𝑂3)2 + 𝐶𝑎(𝑂𝐻)2 2𝐶𝑎𝐶𝑂3 + 2𝐻2O
• The precipitated compounds can be removed in the sedimentation tank.
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Removal of permanent hardness Lime soda ash – Chemical precipitation
• Both the carbonate and non carbonate hardness can be removed.• Lime also helps in removal of free dissolved 𝐶𝑂2.
• 𝐶𝑎(𝐻𝐶𝑂3)2 + 𝐶𝑎(𝑂𝐻)2 2𝐶𝑎𝐶𝑂3 + 2𝐻2O
• 𝑀𝑔(𝐻𝐶𝑂3)2 + 𝐶𝑎(𝑂𝐻)2 𝐶𝑎(𝐻𝐶𝑂3)2 + 𝑀𝑔(𝑂𝐻)2
• 𝑀𝑔𝐶𝑂3 + 𝐶𝑎(𝑂𝐻)2 𝑀𝑔(𝑂𝐻)2 + 𝐶𝑎𝐶𝑂3
• 𝑀𝑔𝐶𝑙2 + 𝐶𝑎(𝑂𝐻)2 𝑀𝑔(𝑂𝐻)2 + 𝐶𝑎𝐶𝑙2
• Large quantity of sludges are encountered.8
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Lime soda ash – Chemical precipitation (Contd.)
𝑀𝑔𝑆𝑂4 + 𝐶𝑎(𝑂𝐻)2 𝑀𝑔(𝑂𝐻)2 + 𝐶𝑎𝑆𝑂4
𝐶𝑂2 + 𝐶𝑎(𝑂𝐻)2 𝐶𝑎𝐶𝑂3 + 𝐻2O
𝐶𝑎𝐶𝑙2 + 𝑁𝑎2𝐶𝑂3 𝐶𝑎𝐶𝑂3 + 2NaCl
𝐶𝑎𝑆𝑂4 + 𝑁𝑎2𝐶𝑂3 𝐶𝑎𝐶𝑂3 + 𝑁𝑎2𝑆𝑂4
• Incrustation of filter media due to the precipitates. • Recarbonation by passing 𝐶𝑂2 to again form the soluble bicarbonates.(Stabilization)
• 𝐶𝑎𝐶𝑂3 + 𝐶𝑂2 + 𝐻2O 𝐶𝑎(𝐻𝐶𝑂3)2(Soluble)(Insoluble) 9
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Mg
Equilibrium concentration of calcium and magnesium ions as a function of pH
pH value
Co
nce
ntr
atio
n o
f C
alci
um
an
d
Mag
nes
ium
ion
s, p
pm
Ca
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Caustic Soda– Chemical precipitation All forms of hardness can be converted to the precipitates.
𝐶𝑂2 + 2NaOH 𝑁𝑎2𝐶𝑂3 + 𝐻2𝑂
𝐶𝑎+2 + (H𝐶𝑂3)2− + 2NaOH 𝐶𝑎𝐶𝑂3 + 𝑁𝑎2𝐶𝑂3 + 2𝐻2𝑂
𝑀𝑔+2 + (H𝐶𝑂3)2− + 4NaOH 𝑀𝑔(𝑂𝐻)2 + 2 𝑁𝑎2𝐶𝑂3 + 2𝐻2𝑂
𝑀𝑔𝑆𝑂4 + 2NaOH 𝑀𝑔(𝑂𝐻)2 + 𝑁𝑎2𝑆𝑂4
Ion exchange to remove hardness
• Cation exchange• Calcium and Magnesium can be replaced by a nonhardness cation usually
Sodium.• Use of zeolite(a naturally occurring sodium - alumino silicate) – Green sand.• Synthetic resin(polymer) coated with exchange material in the modern
applications.
• 𝑁𝑎2𝑍 + 𝐶𝑎(𝐻𝐶𝑂3)2 𝑁𝑎2(𝐻𝐶𝑂3)2 + CaZ
• 𝑁𝑎2(𝐻𝐶𝑂3)2 don’t cause hardness.
• Home-use softeners are almost ion-exchange units.• They may be either gravity or pressure filters.
(Active zeolite) (Exchanged zeolite)
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Resin with𝑁𝑎+ + 𝑅adsorbed
Resin with𝐶𝑎2+ + 𝑅adsorbed
Active exchange zone
𝑁𝑎++ Anion 𝑁𝑎++ Anion 𝑁𝑎++ Anion 𝑁𝑎++ Anion
𝐶𝑎2++ Anion 𝐶𝑎2++ Anion 𝐶𝑎2++ Anion 𝐶𝑎2++ Anion
Ion exchange processRGU IIIT NUZVID
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Scheme of Ion exchange
Courtesy: Venu
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Advantages of Ion exchange
No problem of incrustation of pipes as there in lime soda process. 𝐹𝑒+2, 𝑀𝑛+2 can also be removed. The RMO cost is relatively less. Water of varying quality can also be treated. Very useful in textile industries, boilers etc. as zero hardness can be
achieved.
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Ion exchange to remove hardness (Contd.)
Courtesy: healthgoods.com
• Capacity and efficiency of ion-exchange softeners• Type of solid medium• Type of exchange material• Contact time
• Limitations• Water must be free from turbidity.• Softening of groundwater should be done immediately
before aeration takes place.• Water should not be chlorinated prior to this process.• Costlier for treating waters containing Fe and Mn
because regeneration of zeolite is not possible.
Demineralization to remove hardness
• Removal of minerals from water.• Even to obtain mineral free water which is as pure as distilled water.• Achieved by passing water through a bed of cation exchange resins, and
then through a bed of anion exchange resins.
• 𝐶𝑎(𝐻𝐶𝑂3)2 + 𝐻2𝑅 CaR + 2𝐻2𝑂 + 2 𝐶𝑂2
• ROH + HCl RCl + HOH
• 2ROH + 𝐻2𝑆𝑂4 𝑅2𝑆𝑂4 + 2HOH
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Schematic Diagram of Demineralization Process
Courtesy: Envicare Technologies Pvt ltd.
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Single – stage softening• Water is to be softened to the minimum calcium hardness by the lime-
soda ash process.• Magnesium removal is not conveyed necessary.
Two – stage softening• Water is softened to the minimum possible hardness by lime-soda-ash-
excess-lime process.
Split treatment
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Split treatment (Contd.)• To treat water with high magnesium hardness.• This process bypasses the first-stage softening unit with a part of the incoming
water.• No magnesium is removed in the bypassed water.
• 𝑄𝑥 = 𝑀𝑔𝑓−𝑀𝑔1
𝑀𝑔𝑟−𝑀𝑔1where 𝑄𝑥 = fraction of the total flow bypassed.
𝑀𝑔𝑓 = Mg concentration in the finished water
(40-50 mg/L as 𝐶𝑎𝐶𝑂3)𝑀𝑔𝑟 = Mg concentration in the raw water, mg/L𝑀𝑔1 = Mg concentration remaining in the fraction of the
water receiving first stage treatment.
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Split treatment(Contd.)
𝐶𝑂2
𝐵𝑦 𝑝𝑎𝑠𝑠𝑒𝑑 𝑓𝑙𝑜𝑤 𝑄𝑥
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Schematic diagram of a split system softening water treatment plant
Courtesy: Dr. Fahid Rabah Ph.D., P.E
slide7
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Advantages of split treatment
1) Reduces chemical dosage of lime & CO2
2) No need for first stage Recarbonation
3) Reduce capital cost
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Softening operations
• Mixing of chemicals with water.• Flocculation to help the growth of precipitate.• Settling of precipitate.• Stabilization.
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Problems
Free 𝐶𝑂2 = 3mg/l; Alkalinity= 68mg/l; Non-carbonate hardness=92mg/lTotal Mg = 15 mg/l35mg/l of carbonate hardness can’t be removed. Treated water is to have 80mg/l. Find the amount of hydrated lime and soda required to treat 10lakh lit. of raw water.Non carbonate hardness to be left = 80 – 35 = 45mg/lNon carbonate hardness of raw water = 92mg/l To be removed = 92-45 = 47 mg/li. Lime required for free 𝐶𝑂2= 44mg/l requires 56 mg/l of CaO
3 mg/l of 𝐶𝑂2 requires 56
44* 3 = 3.82kg for 10 lakh lit.
ii. Lime for carbonate hardness = alkalinity ; 𝐶𝑎𝐶𝑂3 = 40+12+48= 100
68mg/l of 𝐶𝑎𝐶𝑂3 requires56
100*68 = 38.08 kg for 10lakh lit.
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Problems(Contd.)
iii. 24mg/l of magnesium reacts with 56mg/l of CaO
15 mg/l of Mg requires 56
24*15 = 35 kg for 10 lakh lit.
Total pure lime required = 3.82+38.08+35 = 77.9 kg56 kg of pure lime = 74 kg of hydrated lime
Per 10 lakh lit. of raw water, hydrated lime = 74
56*77.9 = 1029.94kg
Soda – to remove non – carbonate hardnessNon – carbonate hardness = 47mg/l100 mg/l of 𝐶𝑎𝐶𝑂3 requires 106 mg/l of 𝑁𝑎2𝐶𝑂3
47mg/l requires 106
100*47 = 49.8 kg for 10lakh lit.
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Design of a Zeolite – Softener – Industry works 2 shifts of 8hrs. eachSoftwater = 25000lit./hr; Hardness in raw water = 400ppm as 𝐶𝑎𝐶𝑂3Hardness in treated supplies = 50 ppm; Ion exchange capacity of zeolite = 10kg/𝑚3
Brine required for regeneration = 50kg/𝑚3
Qty. per shift = 25000*8 = 2 lakh lit.350 ppm has to be removed 350 is 87.5% of raw water, 12.5% is to obtain 50ppm hardnessQty. per shift = 2,00,00*0.875 = 1.75 lakh lit1hr per regeneration; 7hrs. of service.(Assumption)Hardness to be removed per shift = 1.75 lakh * 400 ppm = 70 kg.Qty. of resin = 70/10 = 7 𝑚3 ; Provide 6 units of 1.4 and one 1.4 𝑚3 is stand by; 1.4m depth10% of brineSalt required for regeneration = 50 * 7 = 350 kg.
Volume of brine = 350
10%= 3500lit. = 3.5 𝑚3
2 tanks of 1.75𝑚3 each. Assume dia.= 1.2m ; so height required = 1.75
3.14
4∗1.2∗1.2
= 1.55m
Overall tank size will be 1.2m dia. * 1.7 m height; free board = 0.15m
