By Rick Andrew
In this day and age of potential bans on water softeners due to concerns about salinity, efficiency is a very hot topic. But one might wonder, exactly what does efficiency mean in terms of water softeners? Obviously it must relate to how efficiently these systems soften the water, but how does this translate to something measurable? Fortunately, this question has been answered by the NSF Joint Committee on Drinking Water Treatment Units through NSF/ANSI 44 – Residential cation exchange water softeners. The standard describes the requirements and test methodology used to determine efficiency of water softeners.
A note on scope
Before delving into efficiency, it is necessary to be clear on the scope of the standard. NSF/ANSI 44 covers residential cation exchange water softeners. Although other technologies that are sometimes used for the purpose of reduction of scale and/or reduction of hardness exist, these technologies are not addressed by this standard, which is limited to cation exchange technologies. Further, the standard defines residential water softeners as those having an inlet not exceeding 1.25 inches (31.75 mm) in nominal pipe size (NPS). Systems utilizing larger control valves fall outside the scope of what the standard defines as residential. Finally, only POE regenerating systems are covered by NSF/ANSI 44. Disposable cartridge-type POU systems sometimes used for softening for coffee brewing or other purposes are not POE and are not regenerating, so they fall outside the scope of the standard.
What is efficiency?
NSF/ANSI 44 defines efficiency in terms of minimum performance according to two different inputs: salt and water. Softeners must meet minimum performance criteria for both salt and water usage in order to be considered efficient. This usage is expressed in terms of the relationship of the amount of hardness removed from the water to the amounts of salt and water used to regenerate the softener. These relationships and the criteria for efficiency are described in Figure 1. The standard also disqualifies time-clock type softeners from being efficiency rated. Although these softeners may indeed regenerate quite efficiently, they will continue to regenerate on schedule regardless of the amount of actual water usage. A softener that regenerates and uses salt and water even when it still could have plenty of softening capacity because not much water has been used does not really meet the ‘eyeball test’ for an efficiency rating. With this in mind, the joint committee limited efficiency ratings only to demand-initiated regeneration (DIR) type softeners.
Testing for efficiency
The data used to determine whether DIR softeners are efficiency rated is generated during the softening capacity test, in which a softener is regenerated with a known amount of salt, according to a salt dosage specified by the manufacturer. The volume of water required for regeneration is measured. The softener is then operated at 50 percent of the manufacturer’s rated service flow, treating water with 20 grains per gallon of hardness. The test is stopped when the treated water hardness rises to one grain per gallon. At this point, the amount of hardness treated can be calculated because the hardness of the untreated and treated water is known and the volume of water treated has also been measured. Because the amount of salt and volume of water used for regeneration are also known, the calculations for efficiency can also be performed and the efficiency rating can be determined.
Relationship between efficiency and salt dosage
The efficiency of softeners varies with the amount of salt used for regeneration. A minimum amount of salt is required to effectively regenerate the resin. Beyond that minimum effective salt dosage, there are diminishing returns in terms of salt efficiency: the higher the salt dosage, the lower the salt efficiency. In fact, there is a point at which additional salt used in regeneration will not achieve any more softening capacity and will simply be rinsed out of the system. For this reason, many DIR water softeners meet the requirements for efficiency ratings at lower salt dosages, but not at higher dosages. This can be a challenge for softener manufacturers who wish to promote both high efficiency and high capacity. In these cases, the manufacturer must be sure to clearly associate these claims with a specific salt dosage. In fact, NSF/ANSI 44 specifies that efficiency-rated softeners shall not deliver more salt or be operated at a sustained maximum service flowrate greater than their listed rating.
Efficiency is optional
Efficiency is not required for certification to NSF/ANSI 44; however, it may be required by state or local regulations. This is true especially in western states that have water salinity problems, such as California. And states that currently do not have efficiency requirements for softeners may be moving in that direction. California actually requires an efficiency of 4,000 grains of capacity per pound of regenerant salt as opposed to the 3,350 grains of capacity per pound of regenerant salt required by NSF/ ANSI 44. This difference is sometimes called ‘California efficiency’ as opposed to efficiency in general.
Opportunities for improvement
Having criteria for efficiency tied to specific salt dosages makes sense for traditional softeners that operate based on a salt setting that is programmed or otherwise set by the dealer or end user. One can program the salt setting and know how efficiently the softener will operate. Recent advances in electronics, however, have led to a number of intelligent softener valve options that vary the regeneration parameters based on water usage. These systems are flexible in how they operate, so as to best serve the end users in making sure they don’t run out of soft water, while at the same time, operating as efficiently as possible per the algorithm used in their programming. Having an efficiency rating based on specific salt dosages, however, may not make as much sense for these systems that regenerate with varying amounts of salt. There has been a fair amount of discussion among manufacturers and certifiers regarding potential changes to the efficiency-rating approach used in NSF/ANSI 44. These discussions are likely to continue, possibly resulting in updated criteria and test methodologies to determine efficiency for advanced softener technologies.