By Rick Andrew
Last month this column focused on accuracy of the brine system, which is an important attribute of a water softener system that many of us probably either take for granted or don’t think about as a key performance parameter. As we discussed last month, softening capacity, salt efficiency and pressure drop are all very important performance attributes to consider when thinking about operation costs and impact on the plumbing system. We should also think about testing for safety of materials in contact with drinking water and structural integrity. No one wants a water softener that unintentionally contaminates the drinking water through leaching or one that fails due to leakage.
It makes sense, then, that these testing requirements are all included in the scope of NSF/ANSI 44 Residential cation exchange water softeners. Considering that is the American National Standard for residential cation exchange water softeners, it seems natural that all of these criteria important to water softener design and construction are addressed. There is indeed yet another requirement included in the standard, in addition to accuracy of the brine system, that may not be an obvious one that immediately comes to mind. That is testing for rinse effectiveness.
Cation exchange water softeners use salt to regenerate the resin. Exposing the resin to high concentrations of salt during the regeneration cycle causes the calcium and magnesium ions on the resin to be displaced by sodium ions. These sodium ions are then displaced by new calcium and magnesium ions once the softener is returned to service. Once this regeneration is completed, it is necessary to rinse the excess salt out of the cation exchange resin. If this is not done, the water coming from the softener when it is first returned to service will be quite salty and basically not acceptable to users. For this reason, softeners have rinse cycles as part of the overall regeneration process. The effectiveness of this rinsing is evaluated during softening capacity testing conducted under NSF/ANSI 44.
NSF/ANSI 44 requires that after regeneration, the chloride content of the product water shall not exceed 100 mg/L above the influent challenge chloride level. The requirement is written based on chloride level because salt is sodium chloride; by measuring chloride, we are actually measuring salt. The requirement is based on a comparison to the influent challenge chloride level because the hard water used for the softening capacity test has chloride in it and the exact concentration of chloride can vary, depending on the source water used for this test.
Example: The hard water used for softening capacity testing has 29 mg/L of chloride. The softened water has 110 mg/L of chloride. This is an increase of 110 – 29 = 82 mg/L. 82 ≤ 100; therefore this softener meets the requirement for rinse effectiveness under NSF/ANSI 44.
Rinse effectiveness versus softener efficiency
When first thinking about this requirement, it might seem that manufacturers would want to simply include a relatively long rinse cycle as part of their softener system operation. This approach would make it quite simple to meet this requirement of the standard.There is a trade-off involved, however, that pushes manufacturers to limit the length of the rinse cycle. The efficiency requirement of NSF/ANSI 44 is based both on salt usage and water usage. Although many people tend to focus on the use of salt in regeneration, it is just as important to limit the use of water in the regeneration process. A softener system that is efficient in its use of salt, but not efficient in its use of water, is not considered to be efficient under NSF/ANSI 44.
The requirement for efficiency-rated softeners is that they may use no more than five gallons (18.9 liters) of water in the regeneration cycle per 1,000 grains of hardness removed during the service cycle. So, a softener with a capacity of 18,000 grains can use no more than 18,000/1,000 * 5 = 90 gallons (340.6 liters) of water during regeneration, which includes all cycles during regeneration: backwash, brine, fast rinse, slow rinse, etc. Ultimately there exists a trade-off between rinse effectiveness, which pushes manufacturers to want to have their softener systems use more water during regeneration, and efficiency levels that push manufacturers to want to have their softener systems use less water during regeneration.
A complete standard
When looking at NSF/ANSI 44 in its entirety, we can see that it is quite comprehensive in terms of covering the various safety and performance aspects of a water softener. These aspects include the ones that come immediately to mind, such as the softener’s capability to soften the water, amount of soft water it can deliver, efficiency of its use of salt, pressure drop resulting from the softener being plumbed in to the building, etc. But beyond these performance aspects that immediately occur to us, NSF/ANSI 44 includes the rinse effectiveness requirement. This requirement helps assure that the softened water is acceptable to end users in that it doesn’t have too much residual salt left in it after regeneration, despite the push to abbreviate the post-brine rinse cycle or cycles for the sake of efficiency ratings.
This issue of rinse effectiveness is one more level of assurance provided for softeners that have been evaluated for conformance to NSF/ANSI 44: that these softeners will not deliver salty tasting water to end users after a regeneration cycle has been completed. This requirement may not be the most important one in NSF/ANSI 44, but nonetheless, it is good to understand it in case questions arise about salty water.
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: [email protected]