Sequencing of the Sodium Softeners and RO

Traditionally, sodium softeners have been used as pretreatment to RO. Sodium softeners remove hardness and metals, such as iron and manganese, that scale, foul, or degrade RO membranes. Anecdotally, softeners are also used to help reduce suspended solids and SDI from surface or other highly fouling feed waters prior to RO. The sodium softener, in essence, acts as another barrier in front of the membrane. 

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  

The softener system must be relatively large, as the service flow rate through a softener vessel should be about 6-8 gpm/ft2.1 A 500-gpm RO operating at 75% recovery (see Chapter 3.1) would require two 120-inch diameter vessel to soften the feed water and maintain the desired service flow rate while one unit is in regeneration. 

Chloride discharge may become a concern. One 120-inch diameter vessel will generate about 3,400 gallons of 10% brine waste just from the brining step alone. 

A 10% brine solution contains about 6,000-ppm chloride. Current Environmental Protection Agency (EPA) guidelines call for a chronic (or continuous) chlorine discharge limit of 230 ppm to a controlled watercourse. 

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. 

Ihe sodium softener is only treating the RO permeate, typically about 75% of the feed flow rate. 

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