Cation Exchange, Part 4: Back to the Basics
By Gary Battenberg
In Part 3, we looked at the combined regeneration sequence of brine and slow rinse for down-flow regeneration of a water softener. We identified the method of brine draw, where brine is injected into the resin bed via a balanced throat and nozzle assembly sized for a specific media tank diameter. This injector assembly regulated the motive flow, which created the partial vacuum to atmosphere that in turn drew the brine solution from the brine tank. We included the math formula to determine the rinse water to brine draw volumes (2/3 to 1/3, respectively) to ensure the required brine strength and contact time to efficiently bathe the resin bed for a full hardness-capacity recovery. This was followed by the slow rinse (or displacement rinse), which displaced most of the brine to approximately one-percent salt in preparation for the fast-rinse cycle. And that is where we will resume in this installment.
The purpose of the fast rinse is to purge the resin bed of any remaining byproducts of regeneration still present after the brine and slow-rinse cycle and prepare the bed for service. This fast rinse is usually the same flowrate of the backwash that is typically sized for the five to six gpm (18.9 to 22.7 L/min) per square foot flowrate needed to loosen the resin bed for regeneration. While there is no specific length of time recommended for the fast-rinse cycle, it is important that the remaining salt in the resin bed is flushed away to prevent salty water to the service plumbing upon the return to service. The best way to determine how long the fast rinse should last is to test the hardness and salt at the termination point of the drain line to make sure neither are present. A TDS test of the raw water and the treated water using a TDS meter is a good way to tell if there is still salt in the water. The TDS of the raw water and the softened will be fairly equal, where the raw water TDS is 500 ppm (Mg/L) or less. Of high importance is ensuring the fast-rinse cycle is long enough to flush all traces of regeneration byproducts from the resin bed, especially where iron is also being removed by the softener. Failure to allow sufficient fast-rinse time may allow migration of oxidized iron into not only the service plumbing, but also in the brine tank during the brine tank (soft water) refill.
Most control valves are designed to provide for a soft-water refill of the brine tank. The reason for this feature is to prevent iron and/or manganese and sediment from being introduced into the brine tank. This feature has been available for many years and has made it possible for a water softener to remove ferrous (clear-water) iron and/or manganese where the pH value of the raw water is sufficient to allow the iron and manganese to come out of solution and be exchanged for sodium during the service run of the softener. Additionally, where an iron filter may have been required previously to remove the iron and/or manganese before softening, that cost has been eliminated because the water chemistry allows the softener to do all the work of cleaning up the water for service to the home. A good way to tell if the softener will remove the iron is to test the effluent water from a miniature demo softener typically used in a residential softener demonstration. If the hardness, iron and/or manganese are not detected when testing the effluent (soft) water, then you can be fairly confident that a softener will effectively exchange all three. If the iron and/or manganese are only partially removed but the water is soft, then pretreatment is required to remove the iron and manganese from the water before softening.
It is important to properly program the softener to compensate for the iron and/or manganese removal. Typically (some manufacturers use different formulae, so consult with your supplier to ensure compatibility) the iron and/or manganese content is calculated as hardness equivalent by multiplying the iron content by three and the manganese content by five, then adding the total(s) to the raw water hardness to obtain the total compensated hardness number. With this number, the softener can be programmed to ensure regeneration before resin-bed exhaustion.
Hardness: 25 grains per gallon (gpg)…..25
+ Iron: 2.0 mg/L x 3 = 6 gpg………………6
+ Manganese: 0.5 mg/L x 5 =2.5 gpg……2.5
= Total compensated hardness gpg……33.5
Note! In this example the compensated hardness number would be 34. Always round up to the next whole number when programming the controller.
A word of caution is advised at this point. Although a softener can easily remove iron and manganese, it is advisable to program the softener control valve for a three- or four-day regeneration override where a softener is programmed for demand initiated regeneration (DIR). Extended service runs beyond four days increase the probability of iron and/or manganese fouling of the resin bed. Fouling is evidenced when the water is soft but iron staining is present. This indicates that the exchange sites on the resin beads are coated with iron and/or manganese. Dosing the brine well of the brine tank with a food-grade acid (such as citric acid) will generally clean up the fouled resin. Highly fouled resin may require a second dose if iron remains in the soft water.
Salt or potassium chloride?
Whether using salt (sodium chloride) or potassium chloride, a word of caution is urged with regard to the available exchange capacity of each type. At a water temperature of 66 degrees Fo (20 Co), salt (NaCl) has a hardness recovery capacity of about 5,994 grains per pound. Potassium (KCl) has a hardness recovery capacity of about 4,699 grains per pound. That represents approximately 21 percent less capacity when using potassium chloride (KCl). Therefore, it would be necessary to adjust the total exchange capacity of the softener when using Kcl. Technical Tip: It is not advisable to use KCl to regenerate a water softener where iron and/or manganese is present. Potassium is a nutrient and may encourage the growth of iron and/or manganese bacteria. Also known as muriate of potash, it is used in farming as fertilizer. Use caution here!
Finally, use a very clean grade of salt that is guaranteed pure, such as solar or pellet salt or potassium chloride. When inspecting a brine tank, there are a couple of indicators that the salt being used is not a clean, pure salt. First is brown/black discoloration on the inside of the brine tank, brine well and salt grid. This condition indicates the salt being used could contain ‘miner’s carbon,’ which is typically found in rock salt. This condition causes not only odor problems in the brine tank, but the carbon will also plate out in the injector assembly, the air-check assembly and the resin bed. Dosing the brine tank with citric acid or an advanced resin cleaner will clean up the resin bed and can also be used to wash down the brine tank and internal components. It is not uncommon for this condition to also yield a thick sludge (mud-like) condition on the bottom of the tank, requiring a thorough cleaning.
Another problem indicator that the salt is not pure is the presence of silica. When you see what looks like beach sand in the bottom of the brine tank, it’s actually silica (sand). Here again, the salt is not pure and another source of salt is recommended. The easy way to determine which salt in your region is the best for your customers is to conduct a jar test. This involves a small glass jar with a sealed lid, a handful of salt and water. Obtain several different salt types for evaluation. Place the salt in the jar and fill it two thirds with water. Let it set for a couple of hours, shake vigorously for 15 seconds and then let set overnight. The next day, the samples with cloudy water or what looks like dust settling on the salt is not suitable salt and should be eliminated as a source for your customers.
A high grade of salt will be very white and without odor. The (brine) water in the brine tank should be clear when visible. If odor develops in the brine tank when there is no odor in the raw water, it’s an indication that an atmospheric intrusion has occurred. This could be dust, insects, rodents or even snakes. If this happens, steps should be taken to prevent this from recurring. Remember, the brine water is being drawn into the softener so if there is an intrusive problem of any kind, careful consideration should be given to protecting the brine tank from hostile environmental conditions. Some customers drink soft water; the health of the brine tank is actually a critical condition that must be attended to along with educating the customer on scrupulous maintenance.
The basics we have covered in this series should serve as a good foundation to build on your expertise in cation exchange. There is much more to learn, understand and know when working with softening, whether it be residential, commercial, industrial or farm and ranch applications. The fundamentals do not change, only the scope and size of the installation. Taking into consideration the environmental conditions leans more toward the technical side of the issue, but it is necessary in order to understand the basics of cation exchange. In the final installment of this series we will look at upflow (countercurrent) regeneration, including the benefits and efficiencies of full-strength brining and some of the early history of this regeneration method. 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,email@example.com