Engineering Chemistry S Datta

WATER TREATMENT 371
∴ Lime required =
74
100 [2 × MgCO 3 + CaCO 3 + MgCl 2
] × volume of water
= 74 [2 × 171.4 + 2500 + 100.0] × 50,000 mg
100
= 17,309,000 mg = 17.31 kg.
Problem 6. A water sample on analysis gives the following data : Ca 2+ = 20 ppm, Mg 2+
= 25 ppm, CO 2
= 30 ppm, HCO 3
–
= 150 ppm, K + = 10 ppm. Calculate the lime (87% pure) and
soda (91% pure) required to soften 1 million litres of water sample.
Sol. Ca 2+ = 20 ppm =
Mg 2+ = 25 ppm =
CO 2
= 30 ppm =
HCO 3
–
= 150 ppm =
20 × 100
40
25 × 100
24
30 × 100
44
150 × 100
122
= 50 ppm CaCO 3
= 104.65 ppm CaCO 3
= 68.18 ppm CaCO 3
= 122.95 ppm CaCO 3
∴ Lime requirement = 74
100 [Mg2+ + CO 2
+ HCO 3
–
] × vol. of water
Soda requirement = 106
100
= 74 [104.65 + 68.18 + 122.95] × 106
100
= 218.9 × 10 6 mg = 218.9 kg.
2+ 2+ −
3
Ca + Mg − HCO × vol. of water.
= 106 [ 50 + 104. 65 − 122. 95]
× 106
100
= 33.6 × 10 6 mg = 33.6 kg.
Problem 7. An exhausted zeolite softener was regenerated by passing 150 litres of NaCl
sol. having a strength of 150 g/l of NaCl. If the hardness of water is 600 ppm, calculate the total
volume of water that has been softened in the softener.
Sol. 150 litres of NaCl solution contain = 150 × 150 = 22,500 gm NaCl
= 22,500 × 100
117 CaCO 3
= 1.932 × 10 4 g CaCO 3
If the hardness of water is 600 ppm
4
1932 . × 10 × 1000
∴ Vol. of water = litres
600
= 3.129 × 10 4 litres
F M I
Problem 8. 10 ml of 0.85
HG
100 K J Zn (OAC) 2
required mol of an EDTA solution for
standardization. 50 ml of a water sample required 20 ml of the above EDTA solution. Calculate
the total hardness of water in ppm.