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Reduce Your Backwash Flow or Lament “Why is my ion exchange resin in the drain?”

It is that time of year.  You must reduce your backwash flow in the Fall or you will find your ion exchange resin in the drain or waste treatment plant.

Why do I need to reduce my backwash flow rate?

If the backwash rate for an ion exchange system is set when the water is warm, the flow rate will be too high when the water temperature is cold.  Anion resins are more susceptible to this problem because their densities are lower than cation resins.  Your ion exchange supplier should have curves of each resin in their literature.

A chart that shows an example of backwash flow rates
Example of Backwash Flow Rates and Bed Expansion Chart
Purolite C-100

For example, if a water softening system and therefore the tank are designed for a 50% bed expansion at 77F, the backwash flow rate will be about 6.5 gpm/sq ft for Purolite C-100 cation exchange resin.  If the temperature of the water decreases to 41F, the bed will expand 120% at that same 6.5 gpm/sq ft sending most of the resin down the drain.

Please note, each resin will have a different backwash rate, so look in the manufacturer's bulletins for the correct value.

How do I prevent this from happening?

  • If you have a backwash valve with a position stop, decrease the flow rate to the correct value at the colder operation temperature. 
  •  If you have a backwash controller with rubber inserts, remove and plug enough holes to decrease the flow rate to the correct value at the colder operation temperature.


Some of the next-to-limitless flow controls and flow inserts that Res-Kem offers
Some of the Numerous Flow Controls and Flow Inserts Offered by Res-Kem


  • Install a resin trap in the backwash line of the system.  If there is any media carry-over, it will be trapped, hence the name.


A resin trap that Res-Kem offers to keep resin from harming the environment
Res-Kem Manufactures Resin Traps for 2", 3" and 4" Piping to Prevent Resin and
Other Media From Contaminating Downstream Equipment

Need Help?

Contact Res-Kem if you need help determining what needs to be done or to have us service you equipment.

Need Technical Information?

Posted on November 18, 2011 by Tom Dupnik | Tags: , , ,

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