Sodium water softening issues

The disadvantage with using sodium softening as RO pretreatment is that the softener must treat not only the permeate volume but also the volume of water that will become the reject. In other words, the softener must be large enough to treat the entire feed volume to the RO. This brings up two issues ... 

These two issues have prompted several users to move or consider moving the sodium softener from in front of the RO to after the RO, to polish the RO permeate. Post treatment of RO permeate is often necessary because the RO does not reject 100% of the hardness in the feed water. And, since the feed water is not softened, the concentration of hardness in the RO effluent will be higher than if the feed water were softened prior to the RO. Depending on the application of the permeate, polishing with a softener to remove hardness may be required. 

During the softening of an edible oil, solid sodium methoxide was being added when one of the operators raised the inspection hatch. Flames issued from it. The investigation revealed that water leaking from a cooling pipe reacted with the methoxide to hydrolyse it to methanol. 

These two issues have prompted several users to move or consider moving the sodium softener from in front of the RO to after the RO, to polish the RO permeate. Post treatment of RO permeate is often necessary because the RO does not reject 100% of the hardness in the feed water. 

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