The objective of treatment in a Base Exchange Softner is to convert hardness forming salts of Calcium and Magnesium to soluble Sodium salts. Due to the low solubility of Calcium and Magnesium Salts, they tend to precipitate and form scales when the temperature of water us increased.
Sodium salts are highly soluble and hence do not form hard scales. The hard water, to be treated, flows through bed of bead type Polystyrene Cation Exchange resins in sodium form, which exchanges sodium ions with Calcium and Magnesium ions present in hard water.
There are two types of hardness present in water.
1. Temporary Hardness: This occurs due to presence of Carbonates and Bicarbonates of Calcium and Magnesium.
2. Permanent Hardness: This occurs due to presence of Sulfates, chlorides and nitrates of Calcium and Magnesium.
The output capasity of the water softner is inversely proportional to the hardness present in the raw water. The resign has to be regenerated periodically with Sodium Chloride solution of 10-13% concentration.
The ion exchange reaction for the service runcan be presented as follows:
Ca++ + 2 NaR = CaR2 + 2 Na+
Mg++ + 2 NaR = MgR2 + Na+
(hard water) (resin in Na form) (exhausted resin) (soft water)
where R is the resin.
From the above reaction, it will be seen that resign retains the Calcium and Magnesium and releases equivalent of sodium to the water leaving the Softner. After the Softner has produced specific quantity of soft water, the softner should be regenerated with Sodium Chloride. The reaction for the regeneration is given below:
CaR2 + 2NaCl = 2 NaR + CaCl2
MgR2 + 2NaCl = 2 NaR + MgCl2
(exhausted resin) (common salt) (regenerated Resin) (waste to drain)
Where R is the resin.
It should be noted that the same Softner could produce less amount of Soft Water between two regenrations, with increase in raw water hardness. The quality of soft water may also get affected due to this increase/change in composition of raw water.
