Achieving Brine Efficiency In Softening
Source: Pentair Water Solutions
What is Regeneration Efficiency?
There are two measures of regeneration efficiency - brine efficiency and water efficiency. Brine efficiency is a measure of how much salt a softener system uses to remove hardness from the water. The brine efficiency of a system is calculated as the grains of hardness removal capacity per pound of salt used to regenerate the system (grain/lb). Water efficiency is a measure of how much water the system uses to regenerate, and is usually calculated as gallons/regeneration, or gallons of regeneration water/1000 grains hardness removed.
Softeners that are tested to the NSF/ANSI standard 44 for Residential Cation Exchange Water Softeners can be efficiency rated if they use demand initiated regeneration (i.e. meter or sensor initiated, not time clock). Per the standard, efficiency rated softeners must have a rated salt efficiency of at least 3350 grains per pound of salt used for regeneration. Additionally, the salt efficiency for softeners installed in California must be at least 4000 grains per pound. Efficiency rated softeners must also meet a water efficiency of 5 gallons of regeneration water (or less) per 1000 grains of hardness removed.
In an ion exchange water softener, hardness ions in the water - primarily calcium and magnesium - are exchanged on the ion exchange resin for sodium. When all available exchange sites are converted from sodium to hardness, the softener is exhausted and needs to be regenerated. Regeneration is achieved by exposing the ion exchange resin to a brine solution. Sodium chloride is typically used, although potassium chloride may also be used. (We will focus on sodium chloride solutions although the same process takes place when a potassium chloride brine solution is used.) The brine concentration is high enough that the resin replaces the hardness ions on the resin with sodium ions. The hardness ions removed from the resin, along with some excess sodium and chloride ions, are sent to the drain.
To make the sodium exchange back onto the resin, excess sodium is required. To optimize brine efficiency, the system is designed so that as little excess sodium chloride as possible is discharged to the drain. Optimizing brine efficiency lowers system operating costs and reduces the level of brine discharged into the environment.
There are many ways to optimize the brine and water efficiency of a softener system through system design and operation. Factors affecting brine efficiency include salt dose, brine flow rate, brine flow direction, reserve setting, and others as discussed in this paper.
Section 1: Brining Considerations
Salt Dose and Concentration
Salt dose is the primary variable in achieving brine efficiency. A typical capacity curve and efficiency curve for an ion exchange resin is shown below:
As shown above, the capacity of a softener will increase as the salt does increases. However, it is also shown that brine efficiency decreases as the salt dose is increased because less capacity is gained for each additional pound of salt used. For example, 4 pounds of salt gives a capacity of 17,900 grains. Increasing the salt dose to 9 pounds of salt increases the salt used by a factor of 2.25, while capacity only increases to 26,600 grains, an increase of 1.5 times.
As shown, brine efficiency is higher at a lower salt dose - more hardness is removed per pound of salt used to regenerate the system. Therefore, the system can be run more efficiently by lowering the salt dose. There are practical limits to the minimum salt dosage depending on water characteristics and treated water quality requirements. Theoretically, the maximum efficiency for a softener is around 6000 grains/lb salt. Practically, systems are rarely set up with a salt dose of much less than 3 lb/ft3, and peak efficiencies rarely exceed 5100 gains/lb.
Typically these limits in salt efficiency are determined by the water's tendency to foul the resin and injectors with iron and sediment, and treated water hardness requirements. Low salt doses increase treated water hardness concentration. Removing iron from a softener bed requires higher concentrations of salt. Finally, smaller brine injectors are required for low salt doses as these small injectors are more susceptible to plugging with iron and sediment. Therefore, in applications with iron or where low hardness leakage is required, softeners are seldom run at maximum salt efficiency.
Lowering the softener's salt dose to increase brine efficiency will also decrease water efficiency. At a lower salt dose, there is less capacity per regeneration cycle. Because the system is regenerating more frequently and with minimal change in the regeneration water used in the draw/slow rinse cycle, more regeneration water is used per 1000 grains of hardness treated. So, while significant increases in brine efficiency can be achieved by decreasing the salt dose, its effect on water efficiency should be considered.
Brine concentration also has an impact on softener efficiency. Brine concentration should be between 8-15% NaCl (or 30-57% salometer) upon introduction to the softener for a downflow brining system. Testing indicates that a lower concentration (around 6%) provides the optimum efficiency for an upflow brining system. The brine in an upflow system sees less dilution before it is introduced to the resin bed. In upflow regeneration it is introduced directly to the bottom of the bed, while in downflow regeneration, it passes through the free board area necessary for backwashing and is thus diluted.
Fleck® Control Valve injectors typically produce brine in a concentration range of 6-13% (23-49% salometer), depending on injector size, style, feed pressure, and draw height.
Brining Rate and Contact Time
Just as salt dose and brine concentration has an impact on a system efficiency, the rate at which brine is introduced will also have an impact. For a given salt dose and concentration, the brining rate will determine brine contact time with the resin. Increased contact time will increase the capacity and efficiency of the softener system; therefore, the slower brine can be introduced the better. There are practical limitations to how low the flow rate can go. Too low a flow rate will increase the chances of channeling, where the brine isn't flowing evenly through the resin bed but is instead following several lower-resistance paths, resulting in uneven regeneration. In an upflow brining system, a low flow rate is also required to prevent the resin bed from lifting and becoming fluidized, which actually decreases resin-to-brine contact time, thus reducing system efficiency.
Testing has shown that 0.5-0.63 gpm/ft2 is the optimum flow rate for brining and slow rinse of an upflow regenerating softener with standard mesh, 8% crosslinked resin. The table below shows the recommended flows and injector sizes for a variety of tank sizes required to achieve the maximum system efficiency.
There are drawbacks to designing a system at low regeneration flow rates. The system uses smaller injectors which may be more prone to plugging with debris, as previously discussed. The system is more sensitive to changes in inlet pressure which can have an impact on the flow achieved by a small injector. (Injector caps with integrated pressure regulators are available for many Fleck valves to provide a constant feed pressure, therefore keeping the brine and slow rinse rates at the desired rate.) Lower brining and slow rinse rates also mean longer overall regeneration time, which can be an issue if there are multiple water treatment units to be regenerated in a limited period of time.
Decreasing brine flow rate to improve brine efficiency has little impact on softener water efficiency. By decreasing flow rate, the time to draw brine increases but the motive flow-to-brine draw ration remains the same, so approximately the same volume of motive water is required.
Section 2: System Design Considerations
Downflow Brining Versus Upflow Brining
In upflow (counter-current) regeneration, the hardness that is exchanged off of the resin during brining is pushed back up the bed, exiting the system at the top of the bed. The freshest brine is being used to regenerated the least depleted portion of the bed. This highly regenerated portion of resin acts as a polisher which decreases hardness leakage. This allows an upflow regenerated softener to use a lower salt dose while achieving the same quality water (i.e. low hardness leakage).
Care must be taken when choosing upflow versus downflow regeneration as upflow regeneration is not suitable for all installations. In particular, brine and slow rinse rates must be maintainable at a level low enough to prevent fluidizing of the resin bed. It is most suitable in cases where feed water has low concentrations of iron and particulates.
Most metered, single tank softeners delay regeneration until the night after the meter reaches its set capacity. To ensure that soft water is available throughout the day, a portion of the capacity equal to a days' usage is reserved.
Variable brining is a control feature available on some upflow softeners. With variable brining, the controller determines how much reserve capacity has been used when the regeneration time is reached. Based on that remaining capacity, the system adjusts the salt dose used for that regeneration.
salt dose adjustment avoids using salt for resin that is still regenerated.
Variable brining systems have brine fill as the first step of regeneration,
instead of the last step of regeneration. Fill time is varied to allow the
salt dose to be matched to the actual amount of resin that is exhausted.
Variable reserve is another means to minimize capacity
wasted by the reserve setting. With a variable reserve system, the
controller determines what the appropriate reserve for a system should be
based on recent water usage patterns. This system increases and decreases
the reserve capacity as required, helping to avoid both wasting salt and
running out of soft water by optimizing reserve capacity.
Twin Tank Systems
A softening system is designed to continuously supply soft
water. This is achieved in one of two ways. One option is to have a single
softener resin tank, regenerating it at times when no one is using water –
typically 2 a.m. In order to do this, the softener must have a reserve
capacity as discussed previously.
Using a twin tank system increases water efficiency as well as brine efficiency. Twin tank systems have more capacity between regenerations, with similar water usage per regeneration as a single tank system, thus utilize less regeneration water per 1000 grains of hardness treated.
Softener regeneration can be initiated in several ways,
including by a time clock or meter. With a time clock, the softener is set
to regenerate after a set period of time regardless of the water volume
treated (or amount of capacity used). A metered system is set to regenerate
after a specified volume of water has been treated. A metered system offers
a better estimate of the resin capacity that has been used. With a time
clock system, as demand varies, the softener may regenerate too soon wasting
salt by regenerating with capacity remaining. Or the softener may regenerate
too late, allowing hard water to pass through the system.
We have discussed that one upflow regenerated softener
benefit is that the most highly regenerated resin is at the bottom (outlet)
of the tank, whereas a downflow regenerated softener has the most exhausted
resin at the bottom of the tank.
Double backwash will have a negative impact on water efficiency as the extra backwash cycle adds to the water required per regeneration. The second backwash on an electronically controlled unit can be shorter than the first, as it is only necessary to mix the resin rather than clean it.
Section 3: Brine Recovery
Brine recovery is another method that claims to improve
brine efficiency. In residential applications, brine recovery is typically
applied with a simple reclaim and reuse method, where waste brine is
redirected to the brine tank in place of a portion of the fill water and
then reused. Brine recovery can only be used on high salt dose, and
therefore inefficient, systems. This is because there needs to be sufficient
salt remaining after the brine has passed through the resin bed to recover,
and that salt is not available in an inherently brine-efficient system.
Brine reclaim systems should have a slightly better water efficiency than a
system without reclaim. There is a decreased regeneration water volume because
the fill cycle is reduced, however, that volume decrease is partially offset by
the increase in volume required to draw larger brine volume at the same apparent
salt dose. In our example system at left, water savings from brine reclaim is
approximately 200 gallons/year.
Section 4: Water Efficiency
The simplest way to gain water efficiency is to optimize backwash, slow rinse, and rapid rinse cycles to be no longer than necessary. In clean water applications, a backwash of 7 to 8 minutes is usually sufficient. Slow rinse cycles only need to be long enough to rinse the excess salt off the resin. This is especially true with larger injectors as slow rinse rates are higher and more water is used for each extra minute of rinse. Additionally, rapid rinse cycles can sometimes be shortened by a couple of minutes. This adds up to signifi cant water savings as these are the highest flow rate regeneration cycles.
There are several ways to optimize water softener efficiency by adjusting regeneration flow rates and cycle durations. However, it should be noted that adjustments to improve brine efficiency or water efficiency often have an impact on one another, and should be both considered and balanced to meet the specific needs of the installation.
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