By Rick Andrew
There are a number of important requirements for water softeners under NSF/ANSI 44 Residential Cation Exchange Water Softeners. The materials in contact with drinking water must not excessively leach contaminants into the drinking water. The structural integrity of the softener must be tested. Softening capacity at given salt dosages must be determined and there are requirements included for determining salt and water efficiency of demand initiated regeneration (DIR) water softeners. The brine system must be tested for accuracy in delivering specific amounts of salt. Pressure drop must be tested to assure that it is not excessive.
These are all definitely the types of key aspects and functions of a water softener that one would reasonably expect to be addressed by the standard. There is one more criterion for water softeners, however, that must be met under NSF/ANSI 44 and it is one that probably would not immediately come to mind: the softener performance test.
The test itself is quite simple. First, the softener is conditioned according to manufacturer’s instructions and then regenerated at the manufacturer’s minimum salt dosage. Water containing 20 ± 2 gpg (342 ± 34.2 mg/L) hardness is introduced to the softener. The full specifications for this water can be seen in Figure 1. The softener is put into service at the manufacturer’s maximum published flowrate. Samples of the softened water are collected every minute for 10 minutes and all samples are analyzed for hardness. The softened water must contain less than 1 gpg (1 gpg = 17.1 mg/L) hardness at each sample point during the test.
One’s first thought upon learning about this test might be to wonder why the standard actually includes a test to see if a softener will perform for 10 straight minutes. It might seem as if every softener would do this and therefore, unusual to try to consider scenarios in which a softener might fail this test. But let’s explore some scenarios and see where the exercise leads us.
Let’s consider a softener with a maximum published flowrate of 10 gpm and 0.4 ft3 of cation exchange resin. Further, the manufacturer specifies a minimum salt dose of five pounds (2.2 kilograms) of salt per cubic foot of cation exchange resin. For this softener, that would be a salt setting of 5 * 0.4 = 2 lbs of salt. The softener, being operated for 10 minutes at 10 gpm (37.8 L/m), would result in 100 gallons (378.5 liters) of water being softened. At 20 gpg, that is 2,000 grains of hardness that must be removed during the softener performance test. Given that it was regenerated with two pounds (0.9 kilograms) of salt, it should not be difficult for the softener to pass this test because it will require a salt efficiency of only 2,000 / 2 = 1,000 grains of hardness per pound of salt, which should be easily achievable even at the 10 gpm flowrate.
Now consider a softener that, once again, has a maximum published flowrate of 10 gpm, but this time it has 0.25 ft3 of cation exchange resin. And this time, the manufacturer specifies a minimum salt dose of three pounds (1.3 kilograms) of salt per cubic foot of cation exchange resin, which equates to a salt setting of 3 * 0.25 = 0.75 lbs of salt. Because our maximum flowrate is still 10 gpm, we still have the 2,000 grains of hardness that must be removed during the softener performance test. With a regeneration of only 0.75 lbs of salt, this means the softener must have a salt efficiency of at least 2,000 / 0.75 = 2,667 grains of hardness per pound of salt. It may not under these conditions, given the 10-gpm flowrate and the shallow cation exchange resin bed (having only 0.25 ft3 of cation exchange resin). This softener could potentially fail the softener performance test.
Over the years, NSF has tested a few softeners that have failed this requirement and the failure occurs due to three main causes, working in combination with each other:
1. The softener contains a relatively small amount of cation exchange resin.
2. The manufacturer is recommending a relatively low salt dosage as the minimum.
3. The manufacturer has a relatively high maximum published flowrate.
Under these conditions, it is possible to end up with a softener that will not deliver soft water for 10 minutes when put into service at the maximum published flowrate. Some of these softeners, although in the minority compared to typical softeners sold in North America, might actually perform acceptably under certain conditions where household water usage is minimal, especially if the softener is typically being regenerated at higher than the minimum salt dosage. For the typical North American household, however, it is likely that a softener that fails the softener performance test will underperform for them.
The complete picture
We often like to focus on some of the universal requirements for the NSF/ANSI Drinking Water Treatment Units standards, including material safety through extraction testing, structural integrity testing for pressure-bearing products, contaminant reduction testing for complete systems, and use information conveyed through the product literature. But sometimes, it’s nice to step back a bit and examine a test like the softener performance test, which may not immediately come to mind when considering these standards. This test serves a very distinct purpose and adds overall value to the standard by addressing an unusual situation that does not come up very often, but is important to overall buyer acceptance of the product.
By including test requirements, such as the softener performance test, the NSF/ANSI Drinking Water Treatment Unit standards can take a step beyond the basics and go further into the realm of fitness for purpose of products that conform to the standards. Requirements like this one can further limit the risk of consumer dissatisfaction with POU/POE equipment, thereby helping to support the industry and add value to the entire supply chain, from component suppliers to OEMs to dealers to consumers.
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