Cation Exchange, Part 5: Back to the Basics
By Gary Battenberg
In the final installment of this series, we will look at up-flow (counter-current) regeneration, the benefits and efficiencies of full-strength brining, the precautions required for effective regeneration and some early history of this regeneration method.
The most widely used softener design is the co-current or down-flow mode of operation for domestic, commercial and industrial softeners, where the service and brine draw both flow in the same direction (downward) through the resin bed. The typical down-flow design includes a fresh-water zone that is referred to as the free-board area, above the resin bed. This area is typically one-third of the side-shell height of the media tank. For example, an eight-inch diameter x 44-inch tall tank would have a free-board area of 14 inches. This area is necessary to provide sufficient lift of the resin bed without flushing the resin to drain during backwash.
The brine solution is introduced at the top of the tank where it passes through the fresh water zone. The brine strength is diluted at the beginning of the this regeneration cycle, which results in the bottom layer of the resin bed, typically three to five inches (7.62 to 12.7 centimeters) being only partially regenerated at the lower salt settings of five pounds or less per cubic foot. Insufficient brining is manifest when hardness leakage occurs before the calculated soft-water volume has been reached. High hardness and high TDS waters should always be regenerated at a higher salt setting, typically between six and nine pounds of salt per cubic foot of resin capacity.
In counter-current regeneration, the service flow and brine draw flow are opposite each other and referred to as up-flow regeneration. Two immediate benefits are realized with up-flow brining. First, the brine solution is introduced immediately at the bottom of the resin bed so there is very little dilution. The benefit of this full-strength brine is virtually no hardness leakage because the bottom of the resin bed is the most fully regenerated. Therefore, when the softener control valve returns to service, water entering the service plumbing passes through the most highly regenerated portion of the resin bed last, thus yielding maximum softening. Secondly, lower salt settings mean lower operating cost.
Early full-strength brining
Historically speaking, full-strength brining was inherent in early water softener designs (1920s-1940s) because of the way they were regenerated. These early softeners were referred to as ‘salt-in-head’ designs and were manually regenerated. Brine tanks were not introduced until the late 1940s. The process required the owner or operator to first backwash the resin bed by manipulating a manual valve nest or by moving a lever on a four-cycle solo valve (or similar manual control valve) to backwash the softener to drain. Next, the water was turned off to depressurize the softener. A cap on the top of the tank was then removed and water was allowed to continue to drain to displace the water of the fresh-water zone in the top of the tank. The owner or operator would then pour about 15 pounds of fine salt into the top of the tank directly on top of the resin bed. After replacing the cap, the valve(s) would be repositioned to allow a very slow flow to drain, where the inlet water would dissolve the salt, flow down through the resin and out to drain. Finally, the system was fast-rinsed to purge the resin bed of remaining brine traces and the softener was placed back in service.
As control-valve technology became more advanced, softener regeneration eventually became semi-automatic. This required the owner or operator to set a timer to start the regeneration. Early timers were either a twist timer or electric. The twist timer required the operator to simply turn the timer indicator for the recommended time and the valve would automatically regenerate and shut off when the timer turned off. The electric semi-automatic was similar; a simple switch was activated for regeneration. Next came fully automatic time-clock controls, followed by demand initiated regeneration (DIR). Several manufacturers developed a way to introduce full-strength brine in the concurrent (down-flow) softeners by using a drop tube on the brine port of the control valve. This drop tube would allow the brine to be introduced at the resin bed level, effectively eliminating brine dilution by passing the brine through the fresh-water zone, as previously mentioned.
Advancements in resin technology have also kept pace, making it feasible for some softener manufacturers to offer systems that regenerate with as little as one pound of salt and nine gallons of water, using fixed-bed resin tank designs and fine-mesh resin. The call for higher salt efficiencies and reduced water per regeneration in the West Coast, Southwest and Central plains areas of the US prompted equipment manufacturers and resin suppliers to work together in developing highly efficient softening systems. The results were capacities approaching 5,800 grains recovery per pound of salt and up to 80-percent less water used per regeneration.
Reduction in water usage was achieved by creating media tank designs that prevented the resin from fluidizing during regeneration. The first designs with conventional tanks were to fill the fresh-water zone with a lightweight, inert media to hold the softening resin in a static or fixed state. Next came the bifurcated vane collector for conventional tanks that replaced the cone-shaped lower collector. These were followed by advanced tank designs in which directional flow screens and others with distributor plates were integrated into the tank, effectively creating a multi-compartment tank where a combination of media could be configured to treat multiple water problems in one system. An inherent benefit to these designs was the elimination of a gravel under-bed, which reduced the shipping weight and the frustration of resetting a standard riser and lower collector when re-bedding a system.
Fluidizing of the resin during regeneration had to be eliminated in order to obtain the very high efficiencies claimed by equipment manufacturers. This was accomplished by eliminating the first backwash typical of a concurrent water softener.
It was vital that the resin bed of a conventional tank with a free-board area set up for counter-current regeneration was not expanded any more than 20 percent and preferably not at all. The tightly packed resin from the down-flow service in a conventional tank was essential for full-contact, up-flow brining of the resin. With a tightly packed resin bed, the next step was to create a uniform flow through the bed to ensure that all of the resin benefited from full-strength brining. (This is why advanced designs of lower collectors and upper distributors were developed.) For conventional tanks with a standard cone-shaped lower collector, it is still important to maintain the gravel underdrain, to ensure a more equal distribution of the brine upward through the full cross-section of the resin tank. The captured resin-bed type of softeners (utilizing the advanced screen-collector and distributor designs) inherently created a more even, uniform brine distribution through the resin bed.
After the brine and rinse cycles had been completed, an up-flow or down-flow rinse cycle could be used for packed-bed systems. This fundamental step is still necessary to flush away any remaining brine traces from the resin bed. Finally, the soft-water brine refill concludes the regeneration process, followed by the return of soft water to the service plumbing.
More recently, softener design has advanced even further with the development of a softener that uses ultrafine-mesh resin. This resin is so fine that standard distributor and collectors cannot prevent loss of the resin to the service lines or to drain during regeneration. Advanced screen technologies have proven effective at capturing resin between the screen plates. The kinetics of ultrafine-mesh resin has allowed manufacturers to design a very small water softener capable of providing service with tolerable pressure drop for the average three-bedroom/two-bath home. Counter-current regeneration has the advantage of exposing the resin to full-strength brining, which in turn creates a higher-force energy to drive off all of the cations exchanged during the service run. Additionally, iron and manganese are more easily pushed off by this same force of the full-strength brine (as well as other metals such as lead, cadmium, copper, zinc and barium) if they are present in the water supply.
Build your knowledge base
Cation exchange water softening is much more complex than the basics we have covered in this series. Regional issues and conditions must be evaluated at the local level with careful scrutiny of the water supplies in your market area. A thorough water analysis and accurate interpretation of that analysis is essential in order to effectively specify the proper equipment to treat any water source. Be diligent in applying the fundamentals of water treatment in your work and build your career on solid information available through our industry associations. Use caution when reviewing information available on the Internet. Always check the information from non-industry sources against established industry standards so you can avoid serious mistakes that could compromise your integrity and credibility.
About the author
Gary Battenberg is a Technical Support and Systems Design Specialist with the Fluid System Connectors Division of Parker Hannifin Corporation in Otsego, MI. He has 34 years of experience in the fields of domestic, commercial, industrial, high-purity and sterile water treatment processes. Battenberg has worked in the areas of sales, service, design and manufacturing of water treatment systems and processes utilizing filtration, ion exchange, UV sterilization, reverse osmosis and ozone technologies. He may be reached by phone at (269) 692-6632 or by email, gary,firstname.lastname@example.org