Water Conditioning & Purification Magazine

Testing Softening Capacity of Residential Water Softeners

By Rick Andrew

Most professionals in the water treatment industry are familiar with the concept of softening capacity for water softeners. Many have seen or heard terms such as 30,000-grain or 40,000-grain softener and those who are familiar with these terms generally know (or surmise) that a 40,000-grain softener is bigger than a 30,000-grain softener. But many probably don’t know the details behind how a softener is established to be rated at these capacities. Softening capacity is actually established through a standardized laboratory test that is described in great detail in NSF/ANSI 44 Residential Cation Exchange Water Softeners.

Capacity versus salt setting
As many of us already know, salt is used to regenerate cation exchange resin. It would make sense, then, that if only a little salt is used for regeneration, then the softener won’t have as much resulting softening capacity compared to if a lot of salt were being used for regeneration. It would also make sense, however, that there are limits on how much capacity can be gained by adding more and more salt. At some point, when adding more salt, the amount of additional salt isn’t useful for regeneration and simply ends up flushed out of the softener during rinsing (or maybe even ends up in the softened drinking water).
There is actually a theoretical maximum amount of salt and amount of resulting capacity that can be used for regenerating cation exchange resin. It is based on the chemical structure of the cation exchange resin, as well as the atomic and molecular weights of the salt and the hardness. Beyond the theoretical maximum, more salt is useless. In practice, various factors cause softeners to operate less efficiently than at the theoretical maximum. This is why measurements of softening capacity must be performed to find out how efficiently the specific softener operates at the conditions being tested. In general, the more salt that is added, the more softening capacity is achieved, although the additional capacity per the additional amount of salt decreases and eventually becomes zero, as more and more salt is added. So, softener capacities are specific to associated salt settings.

The test
To make sure the test results are repeatable and reproducible, the hard water used for testing softener capacity is defined quite precisely, per Figure 1. Changing the characteristics of the water could affect the softening capacity and could make the test less repeatable and/or reproducible. For example, if the test water contained more iron, it could foul the cation exchange resin and affect the test results. Also, the cation exchange resin must be conditioned to a specific state of regeneration prior to testing softening capacity. So, the test requires that the softener be operated using the test water per Figure 1 at a flowrate of four gpm per cubic foot of cation exchange resin until the softened water has 10 grains per gallon of hardness in it. The softener is then regenerated at the salt setting being tested for capacity and is ready for testing.
In order to make the test as accurate as possible, a measured amount of salt (as brine) is introduced for regeneration, as opposed to simply programming the softener to regenerate at the tested salt setting. Any impact due to inaccuracy of the softener’s brine system is eliminated by taking this approach. Once the softener is regenerated, it is operated at 50 percent of the softener’s maximum published flowrate until the softened water has breakthrough of one grain per gallon of hardness. The test is repeated until three successive repetitions result in capacities that are within 10 percent of the average of the three. In practice, this usually means that the first run is discarded because during the first run the softener is regenerating from a more depleted cation exchange resin bed—the softener is exhausted until the softened water has 10 grains per gallon hardness during conditioning, but the softener is exhausted only to the point where the softened water has one grain per gallon hardness during each test run.

Calculating softening capacity
As the softening capacity test is conducted, a drip sample of hard challenge water is collected throughout the test and analyzed to determine the hardness of the water. This hardness is multiplied by the number of gallons of water softened to determine the total hardness introduced into the softener during the test.
The calculation of hardness removed (softening capacity) is not quite finished yet. During the test, some hardness is passing through the softener into the softened water and this amount is increasing throughout the test until the point where it reaches one grain per gallon and the test is stopped. A calculation is performed to determine how much hardness passed through the softener (this amount is sometimes referred to as leakage) and this amount is subtracted from the total hardness introduced to the softener during the test to obtain the resulting softening capacity for the salt setting being tested.
For example, if the softener reached 1,600 gallons with test water that contained 20 grains per gallon hardness, the total amount of hardness introduced to the softener during the test would be:
• 1,600 gallons x 20 grains per gallon = 32,000 grains total hardness introduced to the softener during the test
If the calculation to determine how much hardness passed through the softener over the course of the 1,600-gallon test indicated that it was 650 grains, then the softening capacity of the softener at the tested salt setting would be:
• 32,000 grains total introduced – 650 grains that passed through (leakage) = 31,350 grains softening capacity
And once again, this softening capacity is associated with the specific amount of salt used for regeneration. If less salt is used for regeneration, the resulting softening capacity would be lower than 31,350 grains.

Knowing the details
Professionals in the water treatment industry generally understand that softening capacity ratings are relative to the size of the water softener. Understanding that there is a detailed test procedure that establishes the softening capacity rating helps professionals better explain to consumers the significance of the rating and to have more confidence when they talk with consumers about what the softening capacity rating means. The fact that these ratings are based on highly repeatable and reproducible, detailed test methods increases confidence that the ratings are meaningful and reliable and can be used to help better understand water softeners and how they function.

About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: Andrew@nsf.org

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