By David J. Swiderski
Through my seven years traveling in Latin America I have discovered that weak acid cation is under-utilized and presents one of the best solutions to very common drinking water problems.
Weak acid cation (WAC) resin is a cation resin that is regenerated with very low concentrations of acid, most commonly hydrochloric acid (HCL). At first glance, the need for this resin to be regenerated with acid can make it appear like an incomplete solution. However, there are many reasons why careful study is worthwhile with this resin. Its major benefits include:
- 60,000 grains of capacity vs. a typical maximum 30,000 for softener resins.
- Both alkalinity and hardness are removed in one column.
- Total Dissolved Solids (TDS) level corresponding to the level of alkalinity actually drops.
What this means is you are using a column of resin that is half the size of normal softener resin and instead of an ion-exchange with salt, which leads to unpleasant taste, you end up producing CO2 which can be stripped with a degasifier.
Also, by removing both alkalinity and hardness in one column, after degasification you actually drop the TDS level by the level of alkalinity. There is one very important factor that needs to be understood with regard to WAC. The resin removes hardness only to the level of alkalinity. At first glance, this can make many shy away from using this resin, but if the alkalinity and hardness are close, an extremely small softener can be used to “polish” off the excess hardness.
In the case of beverage preparation, the problem of passing some of this hardness actually can be a blessing. As the bottled water market has matured, many are finding that maintaining some level of hardness in the water actually produces a better tasting product. There are many brands of bottled water that use reverse osmosis (RO) to completely remove all hardness in Latin America, as there are many areas that do not allow for high recuperation rates with RO which may flush 25-50 percent of the water down the drain.
The use of WAC resin actually allows for very efficient use of water and minimal loss. Another factor that can make this extremely attractive is that a resin bed system is a low-pressure system and can easily be assembled with a ball valve nest or automated with diaphragm or butterfly valves. Given that many on-site operators for companies in Latin America are often the lowest paid, sometimes the best solution is simplest. As water treatment professionals, we should focus on providing the best solution and this means understanding that what we provide should take into account the potential for operators to maintain and operate the systems.
WAC resin provides alternative solutions to some very common challenges and is also a highly effective pretreatment for membrane systems. In the case of membrane systems, not having to lower the pH too much and avoiding high sodium passage where a softener has been used avoid the problem of nano or loose RO. By using the WAC resin as pretreatment, significant levels of TDS can be eliminated outright. This means that with one pass in an RO system, you will be able to assure water quality that others will be challenged to produce and can help minimize injection of acid or membrane cleanings. In deionization (DI) systems, the load on both the cation and anion is significantly reduced and you can actually use the regenerate from the strong acid cation to regenerate the WAC resins. A good rule of thumb is that if about 25 percent of the anions are alkaline, the return on investment of a DI system is extremely quick.
Below are some design considerations which can be critical to the implementation of a WAC system.
- WAC resin swells by 100 percent, so always double your tank size. For example if you have six feet3 you need a tank that can hold 12. As the resin reaches capacity it swells and in the regeneration process it returns to its original size. Installing sight glasses on the tank makes it easy for an operator to determine if he needs to regenerate simply by observing the swell.
- The resin will only remove hardness to the level of alkalinity, therefore you will need to polish the excess hardness with a very small column if zero hardness is required.
- Degasifying the outlet water is critical because it will contain a very high level of CO2. Note that if you are testing the water, do not sample the water until it has been degasified. The water will be extremely acidic and most reagents in the test kits only work in limited pH ranges.
Conclusion
The use of weak acid cation resin has enormous possibilities not only for new equipment, but also for adding pretreatment to existing equipment in the field. As water treatment professionals, we tend to get involved in just one technology such as membrane equipment. But we need to be well versed in many to provide the best solutions or combinations of technology to truly satisfy a customer’s needs. Where Picasso used paints, we should look to filtration, membrane technology, resins and electro-deionization as our tools to create our works of art.
About the author
David J. Swiderski is manager of international relations with Bucyrus International Inc., 1100 Milwaukee Avenue, South Milwaukee, WI 53172-0500. He has extensive experience in the water treatment marketplace in Central and South America, including previous work with Water Tec International and Osmonics Inc. Swiderski can be reached at (414) 768-4405 or via email at [email protected]