The aims of the current study is to investigate the constraint of using caustic soda in water treatment and evaluating
its performance in water softening, compared to the other chemical group, including lime and sodium carbonate.
Based on mass balance of reactants in the caustic softening process, a mathematical relation for expressing the
constraint of using caustic soda in water softening was derived. To evaluate caustic soda performance in water
softening and prove the derived relation as well, some experimental works on some water sources including well
water and clarifi er’s inlet water in two oil refi neries were performed. The results showed that compared to limesodium
carbonate, the caustic soda is the best choice for water softening; however, using caustic soda in water
softening, while restrictive mathematical relation does not verify the chemical characteristics of water, could lead
to an extreme increase in alkalinity.