Where
[Ca(OH)2] = amout of Ca(OH)2 required for softening in me/L
[CO2] = concentration of CO2 in me/L
[HCO3-] = concentration of HCO3- in me/L
[Mg2+] = concentration of Mg2+ in me/L
[excess lime] = additional amount of lime needed to maintain an elevated pH value, usually 1 me/L
The dosage of soda ash required is expressed by Eq. (8.15).1,27
[Na2co3] = [Ca2+] + [Mg2+] – [Alkalinity]