Water softner before ro/di

Florida Sunshine · Dec 6, 2021

Howdy all.
I have a RO/DI unit in the garage which has worked great for my tank for years. Now we are considering doing something about the water for humans. This leads me to getting a water softner for the house. I've had one before and loved it , but do not have any idea how the water softner will work with my RO/DI.
Will it lower my TDS going into the RO/DI or should I keep the water for the tank away from the RO/DI?

AJsReef · Dec 6, 2021

Water softeners generally increase TDS slightly but the salt added to the water to soften it is easier on your membrane than hard water.

In other words, RO post water softener is best

Worth noting too that softening water doesn’t do anything for humans beyond change the “feel” of the water. It will allow soap to sud better and reduce build up with pipes/appliances. If you’re looking to actually treat your drinking water it’s technically worse given the increased salt content.

JGT · Dec 6, 2021

Water softeners do not add salt to the water. The salt is used to backwash the resin that catches all the minerals that make the water hard to begin with. Essentially, the salt ions cause the resin to release the minerals and regenerate the resin.
I have a whole house softener and my RODI tied into it. Year later, haven't changed any filters or membranes and my water is still 0 TDS.

AJsReef · Dec 6, 2021

JGT said:
Water softeners do not add salt to the water. The salt is used to backwash the resin that catches all the minerals that make the water hard to begin with. Essentially, the salt ions cause the resin to release the minerals and regenerate the resin.
I have a whole house softener and my RODI tied into it. Year later, haven't changed any filters or membranes and my water is still 0 TDS.

They don’t add significant levels of salt, but the ion transfer does leave more residual salt than the source water. The low levels can have adverse impacts for some which is exactly why there are potassium chloride alternatives.

JGT · Dec 6, 2021

True, but i have yet to see any negative impact on my RODI/membrane, in fact the opposite has been true.
it also depends on your water. If your source is really bad, then the softened water will be better. If your source isn't too bad then maybe not.

Rmckoy · Dec 6, 2021

Or .. plumb the rodi for the tank in the line before the softener ?

AJsReef · Dec 6, 2021

JGT said:
True, but i have yet to see any negative impact on my RODI/membrane, in fact the opposite has been true.
it also depends on your water. If your source is really bad, then the softened water will be better. If your source isn't too bad then maybe not.

Softened water is easier on membranes. The ion exchange in the softener strips the “hard” minerals before they hit the RO membrane thus the membrane works “less hard” and is easier flushed.

Malcontent · Dec 6, 2021

Randy Holmes-Farley · Dec 7, 2021

A water softener before the RO/DI is generally considered desirable because it greatly reduces one of the reasons for membrane failure: precipitation of calcium and magnesium carbonate onto it.

BUT, there is a small price to pay, and that is a little bit faster depletion of the RDI resin.

The reason is a little more complicated than stated above.

Even with 100% perfect ion exchange (meaning no more sodium added than twice the calcium and magnesium content that is removed), the TDS may drop a little because two sodium ions are less conductive than one calcium or one magnesium ion.

Still, that is no concern or benefit because no one really cares about conductivity.

The small problem is the next step, in the ro membrane. It turns out that sodium is a little bit more poorly rejected than is either calcium or magnesium. Thus, a few more ions (total charges) get through the RO membrane with sodium present than with calcium or magnesium present.

These ions are all readily removed by the DI, but since there are more charges getting through, the DI is depleted a little faster when sodium is present.

Thus, whether it is desirable to put a softener before an RO/DI will depend on how hard the water is and how likely membrane fouling is. But if it is hard enough for your home to benefit from a softener, the membrane likely will too.

Saab95conv · May 21, 2022

JGT said:
Water softeners do not add salt to the water. The salt is used to backwash the resin that catches all the minerals that make the water hard to begin with. Essentially, the salt ions cause the resin to release the minerals and regenerate the resin.
I have a whole house softener and my RODI tied into it. Year later, haven't changed any filters or membranes and my water is still 0 TDS.

Thsnks you answered my question I will have better wster for tank with softner and to. Need opinion of to buddy system I'm using in garage? I've had for several years for tank.

Saab95conv · May 21, 2022

Bad spelling. Sorry it's a ro buddy system I found online for my tank water. Interested in people's opinion. Getting a undersink ro installed in kit hen this week.