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Water Softening/Conditioning

The ABCs of ion-exchange resin cleaning

October 13, 2010
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Ion-exchange water softening is an inexpensive method for improving a home’s water quality and preventing costly plumbing repairs.

Water softeners are low-maintenance, and the cation resin has a working life of seven to 10 years before replacement is required.

There are only two primary conditions that cause water softeners to have problems and lose efficiency:

  • They become fouled with iron and manganese; or
  • They become fouled by organics such as tannin or humic acids.

This is a general information article on how to clean and prevent these two problems.

Cation resin fouling
Iron and manganese are positively charged cations that are attracted to the cation resin like calcium and magnesium. However, unlike calcium and magnesium, iron and manganese are not effectively removed during the brine regeneration.

Iron and manganese can be left on the resin after regeneration and will continue to accumulate cycle after cycle.

Cation resins are sold in either black or amber color. It is very difficult to visually inspect the resin for iron and manganese fouling.

The best method to determine fouling is to soak a small sample in a 1 percent hydrochloric acid solution for five to 10 minutes. If the acid solution has more than 5 ppm of iron, the resin is fouled and requires cleaning.

The traditional cleaning reagents are sulfamic acid, hydrochloric acid, citric acid, acetic acid and sodium hydrosulfite. The product of choice for resin cleaning should be based on the material of construction of the softener’s shell and internal distributors.

Iron cleaning products work on one or more similar principles:

  • They drop the pH of the resin column, making the iron more soluble; and/or
  • They change the ferric form of iron to the more soluble ferrous form of iron; and/or
  • They react with the iron to form complex (chelated) compounds that are not attractive to cation resin.

Cleaning with sodium hydrosulfite
Below is a typical resin cleaning procedure using sodium hydrosulfite.

Most resin manufacturers recommend a dose rate of sodium hydrosulfite of 1.25 pounds per cubic foot of resin.

  1. Backwash the resin.
  2. Regenerate the resin with brine.
  3. Slow rinse the resin to a slight salty taste.
  4. Isolate the softener by closing valves from main outlet.
  5. Depressurize and remove the upper man-way cover or vessel head.
  6. Lower the water level until the amount of water above the resin bed is equal to half of the volume of the resin bed.
  7. Prepare a concentrated sodium hydrosulfite solution so that it makes about a 4 percent solution when added to the water volume above the resin.
  8. Mix the solution with the water.
  9. Drain the solution from the bottom of the softener until just above the resin bed level.
  10. Mix the solution and resin. Do not use air sparging. This will oxidize the sodium hydrosulfite making it ineffective.
  11. Soak resin for four to12 hours.
  12. Replace the upper man-way cover or vessel head.
  13. Drain, downflow rinse and backwash for 30 minutes.
  14. Regenerate the resin, doubling the regenerant concentration.
  15. Return to service after the effluent pH level is normal.

Resin cleaners
To prevent iron and manganese accumulation in the softener, cleaners can be added to the brine solution. The typical dose rate of cleaner is 1 pint to 50 gallons of brine.

There are many powdered or liquid resin cleaners. Some of the products available are ResUp, Super Iron Out and Industrial Cleaner 3105.

The easiest — but more expensive — method is using a salt with resin cleaners. These salt products inhibit the build-up of iron in the water softener and they remove any accumulated iron on the resin during each regeneration cycle.

The most popular salt products available are Morton Rust Remover and Diamond Crystal Red-Out.

In some areas, iron can be found in the form of an organic compound. This iron-based organic compound carries a negative or anionic charge. These organo-ferric compounds can be removed by an anion-ion exchange resin.

However, these compounds in high concentrations can accumulate and foul cation exchange resins. Resins fouled with the organic form of iron require a brine/caustic soak.

The resin is typically soaked overnight, thoroughly rinsed, regeneraed with brine and put back in service.

Cleaning organically fouled cation resins
Cation exchange resins can accumulate organic compounds. This is a rare problem in homes on municipal water supplies. It occurs in homes with shallow wells.

The organic molecules most common in fouling water softeners are tannin and humic acids.

Tannin and humic acids are chemical by-products from decomposed plant materials. These organic molecules cause water supplies to have color, odor and a bad taste.

Tannin and humic acids do not exchange on cation resin. Tannin and humic acids form precipitates as a result of pH changes and become trapped in the resin bed.

This reduces exchange capacity, and the precipitate accumulates because normal regenerations with brine are not effective in removing the precipitant.

Below is a typical procedure for cleaning organically fouled cation resins:

  1. Thoroughly backwash the resin. Regenerate the resin in the normal manner using twice the normal brine dose.
  2. Isolate the softener by closing valves from main outlet.
  3. Depressurize and remove the upper man-way cover or vessel head.
  4. Lower the water level until the amount of water is 3 inches above the resin bed.
  5. Add the caustic soda solution.
  6. Drain the solution from the bottom of the softener until just above the resin bed level.
  7. Soak in the caustic solution for three to four hours.
  8. Drain the caustic solution.
  9. Pass the spent caustic through a neutralizer material before putting into drain.
  10. Replace the man-way cover or vessel head.
  11. Rinse the softener in downflow mode until the water is clear.
  12. Regenerate the resin at twice the normal brine dose.
  13. Regenerate again at normal operating conditions.

Tannin and humic acids can be easily identified in water sources because of the water’s tea-like color. To prevent organic fouling of the water softener, frequent regeneration cycles are required.

A maximum interval of three days between regenerations is a reasonable compromise between the degree of fouling of the resin and overall operating convenience and efficiency.

Failure to regenerate at least every three days may lead to tannin and humic fouling for waters that are known to have high fouling potential.


Phil Adams is southeast technical representative for resin manufacturer ResinTech, Inc., West Berlin, NJ.
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