By Rick Andrew

“Softening capacity” is a widely discussed term related to residential water softeners, especially in the realm of marketing and advertising. Many of us are familiar with the advertising of softeners that include descriptions such as “30,000-grain softener” or “40,000-grain softener.” People with some understanding of these terms generally know, or at least assume, that a 40,000-grain softener is bigger than a 30,000-grain softener.

But how do you establish a softener to be a 30,000-grain or a 40,000-grain residential water softener? This question is answered by referring to NSF/ANSI 44 Residential Cation Exchange Water Softeners. This American National Standard includes specifi­cations for a standardized laboratory test to determine this.

Softener Capacity Is a Function of Salt Setting
Salt is used to regenerate cation exchange resin. It follows log­ically that if only a little salt is used for regeneration, then the softener won’t have as much resulting softening capacity as when a lot of salt is used to regenerate.

It makes sense that there are limits on how much capacity can be gained by adding more and more salt. At some point, when adding more and more salt, the additional salt isn’t useful for regeneration. Eventually, any more salt added simply ends up flushed out of the softener during rinsing or maybe even in the softened drinking water.

There is a theoretical maximum amount of salt and resulting capacity that can be used for regenerating cation exchange resin. This theoretical maximum is determined by 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, adding any more salt is purely wasteful.

In the real world, various factors cause softeners to operate less efficiently than at the theoretical maximum. Different water softeners have different designs that cause these factors to have more or less influence on the actual softening capacity compared to 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. The result is that water softener capacities are specific to associated salt settings.

Testing Softening Capacity
The hard water used for testing softener capacity is defined precisely by the standard to make sure the test results are repeatable and reproducible. The characteristics of this hard test water are described in Figure 1. Variation in the characteristics of the water could cause variation in the softening capacity resulting from the test, and therefore could make the test less repeatable and/or reproducible. For example, if the test water contained a higher concentration of iron, the iron could foul the cation exchange resin and adversely affect the test results.Cation exchange resin must be conditioned to a specific state of regeneration prior to testing softening capacity. The standard addresses this potential cause of variation in test results by requiring that before testing, the softener must be operated using the test water, per Figure 1, at a flow rate of 4 gallons per minute per cubic foot of cation exchange resin until the softened water has 10 grains per gallon of hardness in it. Once this step is completed, the softener is regenerated at the salt setting being tested for capacity and is ready for testing.

The standard includes another requirement 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.

The test requires that once the softener is regenerated, it is operated at 50 percent of the softener’s maximum published flow rate until the softened water has breakthrough of 1 grain per gallon of hardness. The test is repeated until three successive runs of the test 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 as, prior to testing, the softener is exhausted until the softened water has 10 grains per gallon. However, during actual testing, the softener is exhausted only when the softened water has 1 grain per gallon hardness during each test run.

Determining Softening Capacity
A drip sample of the hard challenge water is collected through­out the softening capacity test. This drip sample is analyzed to determine the hardness of the challenge water. This hardness is multiplied by the number of gallons of water softened to determine the total mass of hardness introduced into the softener during the test.

There is one more step in the calculation of hardness removed, or softening capacity. During the test, some hardness passes through the softener into the softened water. This amount increases throughout the test until it reaches 1 grain per gallon, and the test is stopped. Taking into account the hardness passing through the softener, a calculation is performed to determine what is sometimes referred to as hardness leakage. To obtain the resulting softening capacity, this hardness leakage amount is then subtracted from the total hardness introduced to the softener during the test.

For example, if the softener reached 1,700 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 determined as follows:

  • 1,700 gallons x 20 grains per gallon = 34,000 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 1,700 gallon test indicated that it was 750 grains, then the softening capacity of the softener at the tested salt setting would be determined as follows:

  • 34,000 grains total hardness introduced – 750 grains hardness that passed through (leakage) = 33,250 grains softening capacity

It is important to keep in mind that this softening capacity is associated with the specific amount of salt used for regeneration. If less salt were used for regeneration, the resulting softening capacity would be lower than 33,250 grains.

Understanding the Specifics
Water treatment professionals generally know that softening capacity ratings for residential water softeners are relative to the size of the water softener. They can confidently and clearly explain to consumers what the softening capacity rating means when they understand that there is a highly specific test that considers the many factors that could contribute to variation, and the test controls for them. The fact that this rating is based on 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.

Andrew_Rick_mugAbout 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: [email protected]


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