Lime and soda ash is used to soften the following water. The ion concentrations

Question

Lime and soda ash is used to soften the following water. The ion concentrations reported below are all given in units of mg/L as CaCO_3. The pH of the water is 7.7.Calculate the total hardness, carbonate hardness and noncarbonated hardness. Sketch mg/L as CaCO_3 bar graph, and list the hypothetical combinations of chemical compounds in solution. Calculate the softening chemicals required, expressing lime dosage as mg/L of CaO and soda ash as mg/L of Na_2CO_3. CO_2 = 15.0 Ca^2+ = 220.0 Mg^+2 = 60.0 Na^+ = 45.0 HCO_3- = 230.0 SO_4^2- = 20.0 Cl^- = 75

Explanation / Answer

a) calculation of total hardness-

total hardness= (temporary hardness + permanent hardness) mg/L

temporary hardness= hardness due to [Ca+2 + Mg+2 + HCO3-]

= 220+60+230= 510 mg/L

permanent hardness= hardness due to [Ca+2 + Mg+2 + SO4-2 + Cl-]

=220+60+20+75= 375 mg/L

total hardness= 510+375= 885 mg/L

b) calculation of lime requirement-

=74/100 [ Temp.hardness of Ca+2 + 2(temp.hardness of Mg+2) +Perm.hardness of Mg+2 + CO2] mg/L

=74/100 [450 + 2(290) + (80+135) + 15]

=74/100[450+580+ 215 +15]

=74/100 [1260]= 932.4 mg/L

Temp.hardness of Ca+2= hardness of Ca+2 + hardness of HCO3-= 220+230=450 mg/L

Temp.hardness of Mg+2= 60+230 =290 mg/L

Perm.hardness of Ca+2= CaSO4=240mg/L & CaCl2= 295mg/L

Perm.hardness of Mg+2= MgSO4= 80mg/L & MgCl2 = 135 mg/L

Soda requirement= 106/100 [perm.hardness of Ca+2 + perm.hardness of Mg+2]

   =106/100[(240+295) +(80+135)]

   =106/100[535+215]=795 mg/L

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