Ion exchange is a proven technology that has been commercialized for many decades. Applications for ion exchange include simple processes, such as softening, as well as the more complex systems, such as deionization for water recycling. Recently, the technology has expanded to include more selective processes that target certain contaminants, such as nitrate and arsenic removal. With an expanding interest in ion exchange applications, it is appropriate to review one of its fundamental principles: the regeneration process. Regeneration is the backbone of any ion exchange application, since without the ability to reverse the ion exchange process (or regenerate), this technology would prove inefficient and uneconomical for most applications.
Upon further examination of the regeneration process, we find that for most applications, a countercurrent regeneration method (regenerant flow in the opposite – or counter – direction of the service flow) can provide substantial returns to the end-user in terms of product quality and operating cost reductions. Growing concerns regarding total life cost management and environmental impact will encourage the industry to re-learn the art of countercurrent regeneration.
Long viewed as a complicated process, in reality, the sequence used in countercurrent regeneration is no more complex than its co-current counterpart. The perception of complexity is likely rooted in the fact that it is different.
Understanding the practice of countercurrent regeneration starts with an understanding of the process by which an ion exchange media is exhausted. Following the service flow through the media bed, one will find the greatest concentration of exchanged ions at the entry point of the media bed. As the flow path is followed through the depth of the media bed, a natural concentration gradient is formed. Understanding this phenomenon is the key to unlocking the benefits of the countercurrent process. By reversing the flow through the exhausted bed, the concentration gradient works in favor of countercurrent regeneration. In principle, this allows regenerant chemicals to be introduced first to the media that is least exhausted and then, gradually, to the completely exhausted media at the service entry to the tank.
Benefits of this regeneration orientation include enhanced efficiencies as well as improved product quality. These benefits are further explained as the comparison between the two regeneration processes is discussed.
Technique comparison: co-current, the common
In examining co-current regeneration, we find the sequence typically consists of four stages: backwash, brine, slow rinse and fast rinse.
During the backwash stage, any debris trapped within the bed is backwashed out and the bed is reclassified and settled. Reclassification optimizes the regenerant flow through the bed and improves the effectiveness. Let’s illustrate this principle:
The standard backwash flow rate for adequate (30-50 percent) bed expansion is five to six gallons per square foot. After backwash, there is a distinct gradient of exhaustion through the bed. Regeneration chemical is required at a strength great enough to push the exhaustion completely through the resin bed; in typical co-current designs, the regeneration concentration needed is 6-15 lbs. salt/cu. ft., with a recovered capacity of about 20,000-30,000 grains per cubic foot.
Our first case shows co-current regeneration with an inadequate backwash sequence. For this discussion, we will use a factor of <10 percent bed expansion to identify a poor backwash sequence. For typical standard mesh, high capacity cation resin, this would be equivalent to less than 3.5 gallons per minute per square foot of resin (gpm/ft2.)
After backwash, there is a distinct gradient of exhaustion through the resin bed. In the following discussions, we will make comparisons using an industrial softening application requiring extremely low leakage, below one ppm. For this particular case, we will state the resin at the top of the bed is exhausted at 30,000 grains of hardness removed per cubic foot of media (grains/ft3), while the resin at the bottom of the bed has an exhaustion concentration of 15,000 grains/ft3.
The regeneration chemical is required at a strength great enough to push the exhaustion wave completely through the resin bed. In this case, the regeneration concentration needed is 10 pounds of salt per cubic foot of resin (lbs./ft3). (If the resin bed is one ft3, then 10 pounds of salt is needed to regenerate the media.)
To contrast the bed dynamics of the first case, here we target a backwash with a bed expansion of greater than 20 percent. At this targeted expansion flow rate, a standard mesh, high capacity resin, will exhibit a 25 percent increase in expansion for Na+ form resin compared to its Ca++ form counterpart (25 percent expansion versus 20 percent expansion.) This expansion difference is due to the shrinkage of the resin bead as it changes from the Na+ to the Ca++ state.
With these higher backwash flow velocities (> 5gpm/ft2), and the inherent resin expansion difference, the overall concentration gradient will further diffuse throughout the bed.
After backwash, the next stage in a co-current regeneration sequence is the introduction of brine. In this co-current process, the regenerant is processed in the same direction of the service flow. Residential systems that can tolerate modest leakage of hardness brine in the 6-10 lb. range per cubic foot; however, industrial systems requiring very low leakage (less than one ppm) typically regenerate at 10-15 lbs. per cubic foot. For our example application requiring minimal hardness leakage, a regeneration dosage of 15 lbs./ft3 would be recommended.
After the brining stage, water continues to flush the resin bed. The slow rinse allows the regenerant chemical to remain in contact with the resin before displacing it from the resin bed. Typically, the slow rinse time will equal the brining time.
The final stage of the regeneration process is the fast rinse. This stage is important as it rinses the residual brine from the tank. Depending on the regeneration concentration, one-to-two bed volumes of fast rinse water are recommended to complete the regeneration process. A quick measurement of the effectiveness of the rinse is to measure the conductivity of the regeneration drain. As brine is removed from the resin bed, this measurement will approach the inlet conductivity. More accurate measurements for rinse-up are typically conducted by analyzing the chloride content of the regeneration stream.
Here, designers must take a different approach to the regeneration sequence. The process for countercurrent regeneration generally consists of only three stages: brine, slow rinse and backwash.
In a countercurrent regeneration sequence, the brine is able to work with the exhaustion gradient, thus the required brine dosage is a fraction of that required in co-current applications. Even at these lower dosages – typically 5 to 7.5 lbs./ft3 – the quality of the regeneration is superior to the co-current version. This improvement is created by the pristine resin quality at the bottom of the tank. In a co-current system, the exhaustion front is pushed through the entire resin bed, requiring the entire bed to be completely exhausted and then regenerated. In the countercurrent system, the regenerant direction keeps the bottom of the bed clean. This virgin region of resin maximizes the effluent quality produced by the system.
The key design attribute to a countercurrent regeneration brine cycle is the ability to uniformly distribute the brine through the resin vessel while not mixing the resin. As indicated in the co-current backwash description of this paper, resin, when fluidized will begin to reclassify based on its form (Na+ or Ca++.) This inherent physical property is further magnified during the brine cycle. The specific gravity of the brine compared to water will cause media fluidization at low flow rates. To circumvent this challenge, a variety of techniques can be applied.
• Brine direction
One simple approach is to regenerate the resin bed in a down-flow direction, while providing service in the up-flow direction. Using this approach, gravity helps keep the bed in place during the regeneration sequence; however, the problem of keeping the bed from mixing is now shifted to the service, or up-flow cycle. In most commercial applications, this flow configuration is not easily adopted (to do so would require recycling pumps and other equipment) so it is not recommended.
• Packed bed
A common solution to the issue of bed movement is to lock the bed. This lock can be accomplished by filling the tank completely with resin, or by filling the normally void space with inert material, such as polypropylene. In either situation, the bed has little opportunity to move, thus keeping the exhaustion gradient fixed in the resin bed. This approach yields the most favorable results, based on the resulting system efficiency.
On the disadvantage side, a packed bed method will cause additional flow restrictions for the system due to the increased bed depth or increased compression of the media. This can be illustrated by comparing two operating resin beds, one with free-board (unpacked) and one that is packed.
For this comparison, we will use 24” x 72” resin tanks. Under typical design parameters, the tank would be loaded with 10 ft3 of media and use 2 ft3 of under bedding. The resulting configuration would yield a 10 psi pressure loss at 56 gpm.
• Packed tank
With an additional four ft3 of media, the 24” x 72” tank will be completely filled. However, under these conditions the tank will only be able to produce 37 gpm at the same pressure loss. For applications that have minimal operating pressures, this 33 percent reduction in flow capacity based on a fixed pressure loss may not be acceptable. For applications that have adequate pressure, the pack design will merely operate with a greater differential pressure.
In addition to the increased pressure loss of a packed, bed system, the lack of a backwash sequence that expands the resin may make the design vulnerable to solids fouling in applications with an elevated or inconsistent total suspended solids load. Due to this constraint, a pack bed system is recommended on streams that are either pre-filtered or have a consistent, low-level suspended solids load.
• Brine flow rate
The least-practiced solution to holding the media in place during a countercurrent brine sequence is to adjust the brine flow rate to a velocity that will not mix the bed. Finding the appropriate brine flow rate is a balance between various factors: distribution system design, brine strength concentration, volume of media, water temperature and media tank proportions. The simplest approach to brine flow rate optimization is to track system capacity while the brine flow rate is varied through repetitive exhaustion and regeneration sequences.
By starting at the lowest flow rate achievable by the regeneration process, then increasing flow rate, the system efficiency can be mapped, and the all site specific attributes can be simultaneously resolved. While this efficiency study may take considerable time during initial evaluations, data can then be applied quickly to designs using similar brine concentrations, distributors and resins.
While this method may not produce the regeneration results found in a packed design, this approach does have significant advantages. First, it allows for existing media tanks to be converted with little-to-no modification to the media or distribution system. Next, it allows for a combination fast rinse/backwash sequence (at the end of the regeneration cycle) allowing for application of the countercurrent sequence in streams with suspended solids concerns. Finally, a properly balanced brine flow rate can provide regeneration efficiency improvements of up to 40 percent, compared to co-current alternatives.
The slow rinse operation for the countercurrent regeneration provides the same function as its co-current counterpart. The recommendation of one additional bed volume is also beneficial to countercurrent regeneration, as this allows for extended contact time of the brine that enters the resin bed at the end of the brining cycle. Also, in a countercurrent regeneration, this additional contact time further improves the regenerated quality of the resin at the bottom of the bed, resulting in improved effluent quality during a service run.
The last stage in a counter-current system can be designed to completely rinse the bed of the regenerant and remove any debris from the bed. This dual action is achieved by countercurrent designs that use a down-flow service and up-flow regeneration, and do not use a packed bed technology. This design configuration requires the same volume of total rinse water, one-to-two bed volumes, yet satisfies two requirements as it rinses the resin and backwashes debris from the bed. This advantage allows countercurrent regeneration to be more efficient in the use of water, as the high flow of the fast rinse and backwash are combined into one stage.
As this fluidization stage occurs after the bed has been regenerated, there is no concern for media mixing, as it will all be in the same, Na+ form. For this countercurrent method, soft water is advised for this stage, otherwise, the pristine quality at the bottom of the bed will be contaminated by the water being used for this stage.
For units using a packed bed design, there is no fluidization and therefore no backwash. Under these conditions, rinse-up will occur quickly, requiring a mere one to one and a half bed volumes of rinse water.
Countercurrent regeneration requirements
Countercurrent systems require higher quality water to be used for the regeneration process. Since the goal of this method is to provide both efficiency and quality improvements, it is important to use only treated, soft water during the entire regeneration sequence. Also, as most softening systems make the brine regenerant solution by diluting solid salt, this dilution water should also be soft. These steps will help insure that any solution introduced to the bottom will not be contaminating or exhausting this region of the resin bed.
Soft water for the regeneration can be provided through two design alternatives: the use an external supply, or an additional resin tank. Designs incorporating an additional tank can also be configured to provide for a continuous production of soft water. With the benefit of continuous flow, a multi-tank configuration will be the preferred method for supplying the additional soft water needed for a countercurrent regeneration.
In addition to the use of soft water for the regeneration process, the distribution system used in a countercurrent regeneration can provide efficiency benefits. Most commercial applications use distribution systems that are designed to maximize flow performance and are not tuned to the needs of a countercurrent regeneration. For down-flow service systems (up-flow regeneration), the use of gravel underbedding is highly recommended as an inexpensive means to help provide flow diffusion though the bottom of the bed. Ideally, this underbedding should at least cover the lower distribution system (1”.) Hub-and-lateral distributors will provide the best flow distribution and when coupled with the gravel underbed, they will typically provide a design meeting the distribution needs for a countercurrent regeneration.
Hub designs are less efficient than their hub-and-lateral counterparts and require a deeper underbed to optimize flow diffusion. Three to six extra inches of gravel above the top of a hub distributor will provide the diffusion needed to operate a countercurrent regeneration without changing distributors. For existing systems looking to upgrade their distribution system, additional gravel can be added to the top of the resin bed. Due to its significant density compared to resin, the gravel will travel quickly to the bottom of the tank and provide improved distribution characteristics. The one concern in this method is to make sure there is enough space in the tank for the additional gravel added. If there is no space, then some resin may need to be removed.
The countercurrent effect
As described, the countercurrent regeneration process is inherently simple and requires only minimal additional equipment to properly operate – namely, a source of treated water for the regenerations and regenerant make-up. In many applications, the investment in these additional components is already included due to other requirements of the job, such as a continuous supply of treated water through the use of a multi-tank system. Here, the multi-tank configuration is a well-suited source for the required treated water for the regeneration sequence.
The benefits realized by the simple change from co-current to countercurrent will have significant impact on the industrial softening industry, especially in terms of the overall value of products offered. When evaluating designs, we find that implementing a countercurrent strategy will cover the additional associated capital costs within the first year. Of course, with higher volumes of water processed per day, or higher levels of inlet hardness, the payback becomes even more significant.
The cost savings are only part of the overall countercurrent effect. In addition to the system performing more efficiently and saving money over each year of operation, the following benefits are also realized simply by design:
• Improved effluent hardness quality based on the bleed concentration of hardness from the softener. This is due to the bottom of the resin tank being cleaner in the countercurrent method than with the co-current process.
• Improved effluent iron quality for systems operating on high levels of ferrous iron. The same benefits of improved hardness removal also enhance the system’s ability to remove elevated levels of iron.
• Reduced salt consumption since units operate more efficiently. The overall salt consumption of a countercurrent system can be 40-50 percent less than a similar co-current system.
• Reduced water consumption per regeneration cycle.
• Environmental benefits are also addressed, as countercurrent systems are inherently more efficient and less wasteful than co-current designs. Counter-current technology can be deemed the greenest softener design.
While ion exchange technology has experienced recent growth through new applications, our industry should recognize that the next new thing is really something old. Quality, product efficiency and an elevated awareness of environmental issues can be accomplished by implementing the concept of countercurrent regeneration. It is tried and true and more importantly, the design is simply a better approach than the co-current regeneration sequence used today. Organizations that invest the time to learn this regeneration art will create an advantage for their company. This advantage is marketable to any industry looking for a solution beyond the common practices of the past.
About the Author
Jerome Kovach is the Director of Product Management with Kinetico Incorporated, a supplier of softening, filtration and reverse osmosis systems based in Newbury, OH. He has 18 years of experience in the water treatment industry. He may be reached at (440) 564.9111 or email: [email protected].