Summary: With water softener efficiency gaining special emphasi, water treatment dealers are encouraged to consider some design features that can improve the unit’s capacity. Some of the more important factors are discussed here.
Softener efficiencies have received a large amount of press over the past several years, especially in California. Recently, the state implemented an efficiency standard of 4,000 grains per pound of salt. This efficiency rating is approximately 67 percent of the theoretical capacity of 1 pound of salt (5,950 grains). Potassium chloride has a theoretical capacity of 4,690 grains per pound. These theoretical capacities mean that every bit of sodium or potassium, depending on the regenerant, is being used in the regeneration. No unit can be made to produce an efficiency of 100 percent; however, there are factors that can be addressed to make a system more efficient.
Standard, 8 percent, crosslinked cation, softening resins have a typical theoretical capacity of 2 milliequivalents per milliliter (meq/ml). To convert this value to a more recognizable number, multiply by 21.8. This will convert meq/ml to grains per gallon (gpg); in this case, the theoretical capacity is 43,600 grains. Once again, it’s not possible to attain this capacity. One can only aspire to reach as close to it as possible.
From a resin standpoint, there are a few ways to increase efficiencies. Fine mesh resin, perhaps shallow shell technologies, and uniform particle size resin can improve softener efficiencies. These resins utilize better rates of reaction (also known as kinetics) and/or enhanced flow characteristics to produce a more efficient unit.
Area equals efficiency
Fine mesh resins that have a bead size between 40 mesh (0.4 mm) to 50 mesh (0.30 mm) will achieve a capacity approximately 10 percent higher than standard mesh resin that run 16 mesh (1.2 mm) to 50 mesh (0.3 mm) (see Figure 1). This increase is due to better kinetics and overall utilization of the resin in a given unit. The bottom line is—When there’s more surface area, the unit becomes more efficient. The downside to this type of resin is the greater amount of pressure drop, which is two to three times greater than standard softening resin. In a household system, this is generally not a significant issue, but it can be in an industrial application. Please note that the backwash flow rate for fine mesh resin is approximately 50 percent lower than standard mesh resin. Many of the ultra-efficient home water softeners on the market today use a fine mesh resin.
Uniform particle size (UPS) resins are also manufactured to make a system more efficient. Because the beads are all one size, the flow characteristic of the unit is good and a greater portion of the resin capacity is utilized. UPS resins promote better use of the overall resin bed and reduce rinse requirements, especially in industrial systems. Raw water with hardness is generally used to rinse softeners, which will decrease the operating capacity during the service cycle. There may not be much of a benefit in small systems like household water softeners. See Table 1 for capacities.
Shallow shell technology employs the use of a standard mesh size resin that has an inert core. The theory behind this technology is that the core of standard resin isn’t used efficiently so it doesn’t need to be functionalized, or capable of removing contaminants. Containing only a functionalized outer shell, this resin purportedly can produce a more efficient unit due to better kinetics from only having to regenerate the outer portion of the resin bead. Shallow shell resins may have an impact in industrial softening applications where low leakage is required. The hardness leakage from this type of resin can be lower than that of a conventional resin at a given salt dosage. This resin is generally not recommended for residential use since it’s difficult to post-treat for taste, odor and color throw.
As with any media, pilot testing is required to establish operating characteristics to a given product. When a new ion exchange resin is used, it’s likely to produce an operating capacity better than the existing used resin. Metal fouling and oxidation are common factors that will reduce operating capacity. It’s important to research how the old resin performed when it was new to establish a real efficiency change. It should also be mentioned that manufacturers’ capacity graphs are generated under the best conditions. Therefore, the pilot study will generate actual unit operation in the field.
Beyond these three types of resins, there are no other known ion exchange resins on the market that promote an increase in efficiency. The next step in producing a softener with greater efficiencies is to modify the unit itself.
Probably the greatest impact on efficiency can be made with the salt dosage. The lower the salt dosage, the greater the efficiency. A typical softener using standard bead size softening resin that’s fully salted/regenerated at 15 pounds of salt per cubic foot (lbssalt/ft3, lbs/ft3 or lbs/cf) will typically produce a capacity of 30,000-32,000 grains. This capacity produces an efficiency of 2,000 to 2,133 grains per pound of salt. The same unit regenerated at 5 lbssalt/ft3 can produce a capacity of 20,000-22,000 grains or 4,000-4,400 grains per pound of salt (see Figure 1).
The salt concentration will also impact capacity of the unit. The optimum brine concentration seems to be 10 to 12 percent in a dynamic regeneration (see Figure 2). Concentrated brine can reduce capacity by as much as 10-15 percent. This is probably due to the small quantity of brine that actually passes through the unit over a short period of time. Concentrated brine has 2.6 lbssalt/gallon (gal) while 10 percent brine has approximately 0.89 lbssalt/gal. If a system is regenerating at 5 lbssalt/ft3, only 1.9 gallons of saturated brine are used while 5.6 gallons of 10 percent brine will be used. If the saturated brine is maintained at the same flow rate as a 10 percent brine, the salt solution will pass through the resin bed very quickly and cause a reduction in capacity. The goal is to contact the resin bed with the salt for 20-30 minutes to achieve the best efficiency. If the saturated brine flow rate is reduced to a flow rate that achieves the 20-minute contact time, the salt may channel thereby reducing the capacity and efficiency. Some equipment manufacturers have tried to overcome this by pulsing the brine through the resin, which requires a specially designed valve. Lower salt concentrations don’t have the power to displace the hardness, causing a reduction in capacity as high as 10 percent.
As temperature decreases, the ion exchange reaction rate will also decrease. This is apparent in the service cycle as well as the regeneration cycle. Temperature generally doesn’t affect capacity until it dips to 40oF or below. This is generally not a factor except in cold weather climates where the water is coming from a surface water source.
Demand-initiated regeneration (DIR) valves monitor the amount of water that passes through a unit. It will regenerate at a pre-determined number of gallons calculated from the amount of salt used for regeneration and the hardness concentration found in the water. A time clock only regenerates after a certain amount of time. The unit may not be totally exhausted so it will regenerate before it’s really necessary thus wasting salt. On the other hand, if more water passes through the unit than expected over a calculated time, hard water will end up in the distribution system.
Under-bedding, which covers the bottom of the tank as well as the basket/distributor, will increase the unit’s capacity. Resin that’s found below or around the distributor may not be used efficiently since it may be found in a dead spot. Resin in a blind area of the tank will obviously not be used and will reduce efficiency. Under-bedding can increase capacity by as much as 6 percent. Under-bedding will also enhance the flow characteristic of the unit, so dead spots that cause channeling are reduced or eliminated throughout the tank. Channeling occurs when the water takes a path of least resistance and doesn’t contact the whole resin bed, which will obviously reduce capacity and therefore efficiency.
Resin bed depth should be typically set at a minimum of 24 inches. Shorter bed depths can lower capacity since the contact time of the water with the resin is reduced (see Figure 3). Flow rate is critical in shallow beds so as not to exceed kinetics of the resin. Properly designed packed beds and counter-currently regenerated systems can help to offset the lower capacity.
Typical flow rates should be maintained at five to 12 gallons per minute per square foot (gpm/ft2 or gpm/sq ft). Unfortunately, most systems run two to three times higher. An excessive flow rate will reduce the capacity of the system and increase hardness leakage(see Figure 4). In many household applications, the loss in capacity or increase of leakage isn’t a large concern as long as the water remains zero hard and the operating capacity isn’t greatly affected. Zero hard water is defined as hardness leakage of less than 1 grain (17.1 ppm). This goal is fairly easy to meet in most instances. Special care must be taken in applications where the hardness leakages are required to be less than 1 ppm. This leakage is difficult to meet even in the best of circumstances. Properly designed counter-current regeneration will help reduce hardness leakages significantly.
Most household systems in the United States are co-currently regenerated. In this type of unit, the service flow and the regeneration flow go in the same direction. Counter-current regeneration involves the regeneration flow running in one direction while the service flow runs in the opposite direction. Correctly built counter-current units will produce considerably lower hardness leakages than their co-current partners, which tend to rinse up more quickly—usually within a bed volume or two—vs. a co-current system that can be five to 10 bed volumes. In most systems, the rinse water is hard and exhausts the resin bed with each gallon used. One bed volume in a one cubic unit is equivalent to approximately eight gallons.
A duplex system—where one unit is operating and the other is either regenerating or in standby mode—can be an effective way to increase efficiency. The unit in operation can provide soft water for the complete regeneration so the system will be in its most highly regenerated state. There’s no need for a reserve water capacity to get the unit through the day so it can regenerate at night. When the unit is completely exhausted, the other tank will take over and supply water. The downfall to this type of system is the added cost for additional equipment. Costs can be reduced if only one valve can be used to operate both tanks.
Backwashing the unit will also help to keep the systems running efficiently. Any build-up of solids within a tank must be backwashed from the unit to prevent channeling and pressure drop. Most systems should plan to have a free board of 50 percent. Free board is the empty space in a tank above the resin bed. When the resin is backwashed, the water comes up through the bottom of the unit and causes the bed to fluidize—expand—and turn over. This allows trapped dirt and debris to be removed from the bed. Systems that use packed beds must be careful in applications where solids can be found in the water and pre-filtration is recommended.
A system can be the most efficient system in the world, but chemistry of the water can cause non-compliance if an efficiency standard has been established. High total dissolved solids (TDS) and high hardness will negatively impact efficiency. As the ionic load of the water is changed to sodium, due to the removal of hardness (calcium and magnesium) and replacement of sodium in the softening process, the treated water becomes a mild regenerant. An exhaustion band will form in all units during the service cycle. When a high TDS is present in the raw water, this exhaustion band expands, which reduces the amount of usable resin. In other words, the capacity and efficiency are lower.
There are many ways to produce a more efficient system. If some of these suggestions are incorporated into the engineering design, less salt will be used to operate the system. On the other hand, when a service call is made, a simple tune-up to the valve and new water analysis can go a long way to increase capacity and efficiency.
About the author
Mike Keller is production manager of household ion exchange at Sybron Chemicals, of Birmingham, N.J. He is also a member of WC&P’s Technical Review Committee. Keller can be reached at (800) 678-0020, (609) 894-8641 (fax), email: firstname.lastname@example.org or website: www.h2otreat.com