Calculate the amount of lime and soda required per litre for chemical treatment of water containing: CaCO3 =80 ppm; MgCO3=32 ppm; HCO3-=195 ppm; Fe SO4.7H2O added as coagulants=73.5 ppm. The purity of lime is 92% and soda is 84%.
Question
Calculate the amount of lime and soda required per litre for chemical treatment of water containing: CaCO3 =80 ppm; MgCO3=32 ppm; HCO3-=195 ppm; Fe SO4.7H2O added as coagulants=73.5 ppm. The purity of lime is 92% and soda is 84%.
Solution
To calculate the amount of lime and soda required per litre for chemical treatment of water, we need to consider the chemical reactions involved.
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Lime is used to remove the hardness caused by CaCO3, MgCO3 and half of the hardness caused by HCO3-.
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Soda is used to remove the remaining hardness caused by HCO3- and the hardness caused by FeSO4.7H2O.
The chemical reactions are as follows:
CaCO3 + Ca(OH)2 → 2CaCO3↓ + 2H2O
MgCO3 + 2Ca(OH)2 → 2CaCO3↓ + Mg(OH)2↓
HCO3- + Ca(OH)2 → CaCO3↓ + 2H2O
FeSO4.7H2O + Na2CO3 → Fe(OH)3↓ + 7H2O + Na2SO4
From these reactions, we can see that:
1 mole of CaCO3 requires 1 mole of Ca(OH)2 (74 g)
1 mole of MgCO3 requires 2 moles of Ca(OH)2 (148 g)
1 mole of HCO3- requires 1 mole of Ca(OH)2 (74 g)
1 mole of FeSO4.7H2O requires 1 mole of Na2CO3 (106 g)
Given that the molecular weights of CaCO3, MgCO3, HCO3-, and FeSO4.7H2O are approximately 100 g/mol, 84 g/mol, 61 g/mol, and 278 g/mol respectively, we can calculate the amount of lime and soda required as follows:
Lime required for CaCO3 = (80 ppm / 100 g/mol) * 74 g/mol = 59.2 mg/L Lime required for MgCO3 = (32 ppm / 84 g/mol) * 148 g/mol = 56.6 mg/L Lime required for HCO3- = (195 ppm / 2 / 61 g/mol) * 74 g/mol = 112.3 mg/L
Total lime required = 59.2 mg/L + 56.6 mg/L + 112.3 mg/L = 228.1 mg/L
Considering the purity of lime, the actual amount of lime required = 228.1 mg/L / 0.92 = 248 mg/L
Soda required for HCO3- = (195 ppm / 2 / 61 g/mol) * 106 g/mol = 169.8 mg/L Soda required for FeSO4.7H2O = (73.5 ppm / 278 g/mol) * 106 g/mol = 28.1 mg/L
Total soda required = 169.8 mg/L + 28.1 mg/L = 197.9 mg/L
Considering the purity of soda, the actual amount of soda required = 197.9 mg/L / 0.84 = 235.6 mg/L
So, the amount of lime and soda required per litre for chemical treatment of water are 248 mg/L and 235.6 mg/L respectively.
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