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Dealkalizer Performance Calculations

We recently had questions about a perceived problem with a dealkalizer. This customer believed the capacity of the dealkalizer was lower than originally specified and was unhappy with its performance. The customer was regenerating this new system more often than they regenerated the older system. We reviewed the system design and operation and found the problems.

Determining the Dealkalizer Capacity:
In order for us to know how long a dealkalizer system will produce dealkalized water between regenerations, we need a complete water analysis. There are a number of factors in determining the capacity of Dealkalizer Systems. The two most important things used to determine resin capacity are: Influent TDS and Alkalinity (as a percentage of the TDS). Once these are known, the resin capacity can be determined by using established resin manufacturers' charts, or by doing calculations based on total exchangeable anions and percentages of alkalinity and chlorides.

Purolite A-300 Strong Base Anion Resin Dealkalizer Capacity Curves

Based on using the charts and info from Purolite and Rohm & Haas the capacity they should be getting is approximately 2,900 grains/cu ft x 15 cu ft = 43,500 grains removal. With 50 ppm (2.93 grains/gallon) alkalinity, this equates to 14,846 gallons between regenerations.

Minimum Flow Rate:
This capacity is further based on the flow rate of the system. Ideally, the unit should run @ 2 gpm/cu ft of resin or 30 gpm. Further, the minimum flow rate to insure proper kinetics and to prevent channeling is 2 gpm/sq ft of tank surface area. In their case, with a 30" diameter fiberglass tank, which has a surface area of 4.6 sq ft, the minimum flow rate should be 9.2 gpm.

We advised the customer that their average real time flow rate is 271 gph, which is 4.5 gpm. This is well below the recommended flow rate of 9.2 gpm. At this rate, the water is definitely channeling resulting in premature alkalinity breakthrough.

Premature alkalinity breakthrough would necessitate regenerating the system more often. If the dealkalizer was being run at the design rate of 9.2 gpm it would regenerate less often.

Resin Regeneration Frequency and Resin Life:
Before this customer understood the cause of the dealkalizer problem he was concerned the dealkalizer resin had lost its original capacity. Furthermore he was concerned that regenerating the dealkalizer more often would affect the life of the resin. The answer in both cases was no. The resin was in good shape and regenerating the system more often would not be detrimental to it.

Their very old dealkalizer did not appear to regenerate as often as the new system. There could have been any number of reasons it didn't.

  • Was it actually producing dealkalized water in the 5 ppm range?
  • Was the resin broken down to the point where it could operate at low flows and still work?
  • Is the water analysis from 2008 the same as now?

We don't know the answers to the first two questions but it's very doubtful that the analysis is the same, as water in most areas can change dramatically from summer to winter and from drought to rainy times. We asked them to check the water in the winter when there is a lot of snow and/or ice. The salt on the roads increases the TDS of the water, not to mention the alkalinity and chlorides; all of which will have a significant affect on the performance of a dealkalizer.

Solutions and Recomendations:

Adding Caustic to Salt During Regeneration Increases Resin Capacity
They may be able to increase the capacity of the dealkalizer somewhat by increasing the amount of the salt and caustic. However, it may not be worth it given the low flow rate of operation.

We don't feel making changes to the dealkalizer will enable it to produce more treated water between regenerations. If the real time flow will continue at 4.5 gpm (or less), we suggested they installation of a recirculation pump on the softener/dealkalizer system. This will insure there is enough water going through the units and prevent channeling.


Posted on December 2, 2009 by Tom Dupnik | Tags: , ,

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