Cation Exchange, Part 2: Back to the Basics
By Gary Battenberg
In Part 1, we looked at early history of cation exchange from 1766 to the early/mid-1900s with an acknowledgment of chemical and membrane softening methods. Next, we looked at the water softening basics relative to the different styles of softeners and the fundamental requirements of the softener control valve and the importance of each step of the regeneration process. Additionally, we covered the primary purpose of a softener and the basic rule of resin capacity, and finally an introduction to the two types of regeneration. In this installment, we will discuss the fundamental and technical aspects of softener regeneration and their importance to long-term performance and reliability of a water softener.
Basic softener components
To obtain a firm grasp of the fundamentals, one must know what comprises the basic components of a water softener. They are:
• Pressure vessel or media tank, typically a fiberglass or similar composite tank
• Lower collector and riser tube
• Under bed, typically 0.25 x 0.125-inch (6.35 x 3.17-mm) AWWA-certified gravel or garnet
• Cation resin, typically eight-percent crosslink
• Control valve with bypass, typically demand-initiated regeneration (DIR) with time-clock option
• Brine tank or cabinet for salt or potassium storage for brine makeup for regeneration
• Brine valve with safety float and connection tube to control valve
• Salt grid (optional)
Regeneration sequence, down flow
When regeneration is initiated, the first step in the sequence is the backwash cycle. In backwash sequence, the control valve internally redirects the inlet flow down the riser tube and out the lower collector, creating a counter-current flow upward through the resin bed. The purpose of this cycle is to thoroughly expand and loosen the resin bed, which is tightly compressed during the service run. Backwashing the resin will flush accumulated sediment (as well as any fractured resin beads or fines) from the top of the bed, as well as smaller particulates trapped within the bed to a suitable drain.
Expansion is important to sufficiently fluidize the bed, which in turn ensures efficient brine contact with all of the resin during the brine cycle. Insufficient expansion of the resin bed may result in partial regeneration, which is caused by ‘resin clumps’ that were not loosened during the backwash cycle. In this condition, the brine solution will not contact the entire resin bed, which in turn will yield less available total soft-water capacity to the service plumbing.
Another problem that may present itself is discolored water and/or salty water after regeneration. The discolored water may be the result of a condition known as ‘hideout,’ where iron, silt, dust or other particulates in large amounts create a filter-cake condition that effectively surface-blinds the resin (thereby creating these clumps) and is only slightly loosened during backwash. When the service cycle is restored, these particulates migrate to the service plumbing and the result is a plume of discolored water. This condition may also be accompanied by salty water, which when combined with color (or in some cases alone), is an indication of a hydraulic imbalance in the softener.
One of the most common causes of these conditions is what is technically known as the ‘transverse distributor effect.‘ This effect is typically associated with a conventional softener where there is a freeboard area between the top of the resin bed and the control valve. Transporting the conventional softener in an upright position is very important so that the gravel under-bed completely covers the lower collector and is level in the bottom of the tank. When a softener is laid on its side for transport, travel vibrations or sudden braking may cause the resin bed to shift, which in turn allows the gravel to shift to one side of the softener tank, thereby exposing the lower collector, which in turn creates the transverse distributor effect.
When the softener is up-righted, the resin (being lighter than the gravel) will replace the area the gravel previously occupied. This creates a hydraulic imbalance both for service and regeneration, where the water will take the path of least resistance. Gravel weighs 100 pounds (45.35 kilos) per cubic foot and resin weighs 52 pounds (23.5 kilos) per cubic foot. In service, this transverse distributor effect will allow channeling of the water within the resin bed to naturally migrate to the exposed collector covered only by the resin.
A level under-bed is very important for maintaining a uniform lift of the resin bed during backwash. The water will flow more easily through the exposed area of the collector instead of uniformly through the level gravel under-bed. The result is insufficient bed expansion, which creates the problems described above. Backwash volume is only sufficient to lift and loosen the resin in preparation for the brine cycle and is therefore insufficient to reclassify the heavier gravel and restore a level under-bed. Left in this condition, the performance of the softener will be diminished, the quality of treated water will be marginalized and eventual plugging of the resin bed with the sediment and particulates will render the softener inoperable.
Where the water being treated contains filterable solids, prefiltering the water to reduce the loading in the resin bed will be required. There are several methods available to provide filtered feed stock to the softener. The first (and often used) is a cartridge filter in a small housing. This option should be carefully evaluated before installing it upstream (before) of the softener. Too many times a simple 2.5 x 9.75-inch (63.5 x 247.65-mm) or 20-inch (508-mm) sediment filter is installed on a 3/4-inch (19.05-mm) water supply line. The problem here is that these filters typically are rated for 1-2 gpm (3.78 to 7.57 L/m) continuous flow.
The average domestic softener requires a backwash rate of between 1.5 and 3.0 gpm (5.67 and 11.35 L/m). If the sediment filter is not replaced as needed, the combined pressure drop and loss of flow will create a hydraulic deficit during backwash that may not allow for full bed expansion. Choking off the water flow and pressure to a softener is equivalent to trying to run uphill with your hand over your mouth. You won’t get too far before your progress is stalled for lack of oxygen.
The newer dual-gradient and multi-gradient sediment filters will perform somewhat better due to their inherent structure but are still too small to be a viable option for prefiltration for a typical residential softener. Most filter cartridge suppliers make larger 4.5 x 10-inch (114.3 x 254-mm) or 20-inch filters that offer a greater filter surface area, which in turn translates to longer service runs and lower pressure drops. The average household service flow is between 2.5 to 4 gpm (9.46 to 15.14 L/m) with some of the new laundry machines filling at 6 gpm (22.71 L/m).
The pressure drop through these small filters increases as the flowrate increases and greatly increases as the filter accumulates sediment. For moderate or intermittent sediment intrusion, these filters are an economical alternative to a manual-flushing screen filter or an automatic backwash filter. The best way to determine what type of filter is required for prefiltering water for a softener is to have it tested for turbidity and/or a silt density index to determine the filter-loading rate based on a specific volume of water.
For heavy sediment loading, an automatic backwash filter would be the preferred method to provide clarified feed water to the softener. This type of filter yields a consistent feed-water quality to the softener and eliminates the need to shut down the water supply to change a filter cartridge that always presents itself at the most inconvenient time. As stated in Part 1, the primary purpose of a softener is the removal of hardness, but in many instances, the softener is relied upon to provide filtration as well. A softener can do this somewhat effectively provided that sufficient flow and pressure are available for optimal regeneration of the water softener.
It is very apparent that a good regeneration starts with a good backwash flowrate that produces the required 50-percent bed expansion as recommended by resin manufacturers’ performance specifications. If backwash requirements are diminished, the subsequent regeneration sequences will suffer and treated water quality will be less than ideal for the customer. As we have seen, the all-important first step of an unimpeded backwash will establish the baseline health index of the softener. In Part 3, we will continue with the remaining regeneration sequences and their requirements to maintain proper operation and long-term reliability of a water softener. Stay tuned!
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