Split Stream Softening and Dealkalization

Split Stream Softening and Dealkalization

The cation split stream softening and dealkalization system consists of a brine regenerated sodium cycle softeners and a hydrogen cycle cation exchangers followed by a degasifier. The same strong cation resin is used in both sodium and hydrogen units. Water passes through the sodium softener and hydrogen cation units in parallel. When the two effluents are blended together in the optimum ratio, the bicarbonates and carbonates (commonly known as alkalinity) are converted to carbon dioxide, which is removed by the degasifier. The resultant treated water contains little or no hardness, greatly reduced alkalinity, and somewhat lower total dissolved solids content.

The sodium cycle softener removes divalent cations such as calcium and magnesium (commonly known as hardness) by exchanging them for sodium. The hydrogen cycle cation unit removes all cations (including sodium) by exchanging them for hydrogen. The exchange for hydrogen increases the acidity of the water and lowers the pH. The pH of the final treated water is still slightly acidic (pH less than 7.0), so it is common practice to add sodium hydroxide to the treated water in order to neutralize the acidity and raise the pH.

The approximate percentages of softened and decationized water required for a blend can be predicted from these factors:

1. The alkalinity of the influent (Ai)

2. The free mineral acidity (FMA) of the hydrogen cation effluent, which depends on the sulfate, nitrate, and chloride concentrations in the influent

3. The desired alkalinity in the mixed effluent (Am)

With the above information, the proper proportion can be calculated: 

Percent hydrogen zeolite = Ai - Am x 100 
                                            Ai + FMA

For example, given a water with: ppm as CaCO3 

Calcium 24 

Magnesium 12 

Alkalinity 34 

Sulfate 6.3 

Nitrate 2 

Chloride 6 

Carbon Dioxide 15

Assume an alkalinity of 15 ppm as calcium carbonate is required in the blended effluent, given Ai of 34 ppm as CaCO3, and the desired Am of 15 ppm as CaCO3.

The FMA is the only additional value needed to apply the formula and calculate the percentage of water treated by the hydrogen zeolite unit. The FMA value is equal to the sum of the acids of the sulfates, nitrates, and chlorides present in the water.