Water and Its Treatment 3,4
Water and Its Treatment 3,4
Water and Its Treatment 3,4
Water Softening
Processes involved in removal of temporary and permanent hardness is called as “softening of
water”. It is very essential process since hard water is unsuitable for domestic and industrial use.
Use of hard water for generating steam in boilers so as to generate electricity results in problems
like sludge formation, priming, foaming, boiler corrosion and caustic embrittlement.
Boiler Hard water can be removed by two ways:
1. External Treatment: Treatment of hard water is done before its entry into the boiler.
This involves lime-soda, zeolite and ion exchange process. All are preventive methods.
2. Internal treatment: Treatment of raw water is done inside the boiler by two processes.
a. Appropriate chemicals are added to the boiler water to precipitate the scale forming
impurities in the form of sludges which can be later removed.
b. Cations causing hardness are converted into compounds which will stay in dissolved
form in water and thus do not cause any harm.
The processes involved are Colloidal conditioning, carbonate conditioning, phosphate
conditioning and calgon conditioning.
Difference b/w Internal and External treatment
Following are the reactions that takes place in this process when the following
substituent is present in hard water:
𝐌𝐖 𝐨𝐟 𝐬𝐨𝐝𝐚
Soda requirement = [Hardness in ppm] x Volume of water (L)
𝐌𝐖 𝐨𝐟 𝐂𝐚𝐂𝐎𝟑
Q2. Calculate the amount of lime required for softening 50,000L of hard water
containing Mg(HCO3)2 = 144ppm, Ca(HCO3)2 = 25ppm, MgCl2 = 95 ppm, CaCl2
= 111ppm, Fe2O3 = 25 ppm, Na2SO4 = 15 ppm
Ans:
Constituent Reactions Lime/Soda
required
Ca(HCO3)2 Ca(OH)2 + Ca(HCO3)2 → 2CaCO3 ↓ + 2H2O 1Lime
Fe2O3 and Na2SO4 do not cause hardness and hence do not require L or S
Lime requirement = 74/100 (2x98.6 +15.4 +100) mg/L x50, 0000L = 1156200mg =
11.57kg
Q4. Calculate the amount of lime and soda needed for softening a water sample containing
36ppm Mg2+, 20ppm Ca2+ and 183ppm HCO3-
Ans:
Lime required = 74/100[Mg2+ + HCO3- in CaCO3 equiv.] mg/L =
74/100[36x100/24 + 183x100/122] mg/L = 222ppm
Ans:
Lime required = 74/100[Mg2+ + HCO3- in CaCO3 equiv.] mg/L =
74/100[18x100/24 + 183x100/122] mg/L = 166.5ppm