By Marianne Metzger
Water treatment can be very complex, whether you’re dealing with a homeowner looking for a safer supply of drinking water or a business that requires a particular level of quality for water being used in a process. Testing is critical to treating the water; it shows us what contaminants are present and at what levels so we can determine which contaminants need to be reduced and what treatment technologies are best suited for treating those contaminants.
Having a complete water analysis is vital to provide the correct answers on water-treatment options. Uncovering what is present in water is important to all treatment applications but can be extra helpful when dealing with ion exchange. Managing competing ions and different water chemistry can have a dramatic impact on how effective ion exchange will be. Applying ion exchange correctly is a cost-effective way to reduce contaminants, so testing is crucial for proper application.
Cation Exchange: Simple Softening
Cation resin, a negatively charged media, attracts positively charged ions called cations. In a basic water softener, an exchange of ions takes place: Sodium or potassium is exchanged for calcium and magnesium. Additionally, resin beads have different affinities toward different ions. In the case of cations, the following is the typical order of affinity:
Hg 2+ < K + ≈ NH4 + < Cd 2+ < Cs + < Ag + < Mn 2+ < Mg 2+ < Zn 2+ < Cu 2+ < Ni 2+ < Co 2+ < Ca 2+ < Sr 2+ < Pb 2+ < Al 3+ < Fe 3+
Affinity refers to the level of attractiveness to each cation and in what order. Affinity generally is not problematic, as the cations with higher affinities are typically not present in high enough levels to cause problems with a typical residential softening application. However, in wastewater applications, it could cause problems, so analyzing for all cations present and at what levels is important.
For residential softening, at a minimum, test for conductivity, pH, hardness, iron, manganese, sodium, potassium, and chloride. If the conductivity/total dissolved solids (TDS) are high, then consider sending the sample to a laboratory to determine what else is making up the TDS.
Anion Exchange: A Bit More Complicated
Anion resin, a positively charged media, exchanges for negatively charged ions, also referred to as anions. Anion-exchange media is typically regenerated using chlorides for residential applications, and, depending on the application and the type of media, you may not even need to exchange. This treatment demands a more comprehensive water test, as it typically involves treatment of a health-based contaminant, such as nitrate, arsenic, or uranium. It can also be used to treat tannins, a non-health-related anion. However, when a health-based contaminant is found, it is especially important to address it, so be sure to do a complete analysis to consider all contaminants present.
As mentioned before, cation resin has an affinity order, and anion resin also has an affinity order. The relative order of affinity of these strong-base anion resins for some common ions in drinking water is as follows:
Uranium/Perchlorate > Sulfate/Chromium > Selenium/ Arsenate > Nitrate > Chloride > Bicarbonate > Fluoride
Note that the highest affinity is for uranium and perchlorate; this means the resin will remove these contaminants before removing contaminants like arsenic, nitrate, and even sulfate. Unfortunately, this could become a problem, depending on the levels of uranium present and if not properly sized and maintained. Thus, knowing the levels of uranium can help you determine how often the resin will need to be replaced before it becomes a concern for disposal.
What’s That Fishy Smell?
If you work with anion resins, you likely have experienced a service call for which the complaint is a fishy odor. This smell is usually the result of using anion resin when there is an issue with the pH of the water. The smell comes from amines, which are in a gaseous form at higher pH levels, creating an odor. At lower pH levels, they are ionized, and no odor is produced. In general, as pH levels approach 8.2, a fishy odor may be noticeable in certain areas, but once it is above 8.5 the odor can persist throughout the system. Consequently, the only way to correct this fishy odor is to lower the pH level on the influent or effluent water; carbon will not correct this issue.
Testing pH levels is critical for this type of application. It’s important to test for pH on-site, which yields the most accurate results. Labs can run pH, but the results can be skewed based on the water chemistry and how long it takes to begin the analysis. Therefore, an on-site pH test is the best option.
Anion Resin and Alkalinity
Using anion resin with chloride regeneration will reduce bicarbonate and carbonate alkalinity of the water and show a drop in pH initially after start-up and regeneration. If the pH levels are already low or if they dip too low, this may lead to corrosion problems. We typically see a pH drop of 1.0 to 1.5 levels for about 500 gallons per cubic foot to 1,500 gallons per cubic foot. It is important to keep this in mind when using anion resins.
What Do I Include in a Test?
Water from a private well that hasn’t been tested for some time should have a comprehensive analysis to get a baseline of the water quality. When testing, it’s helpful to start with your basic wet chemistry, including pH, alkalinity, and TDS/conductivity, which are commonly run on-site. Additionally, it is imperative to test for heavy metals, minerals, and inorganic compounds, including nitrate, chloride, fluoride, sulfates, and phosphates—essentially all the cations and anions.
Specific applications may require additional analysis. For example, arsenic would require an arsenic speciation test to determine if additional pretreatment may be needed. If working on a job for PFAS reduction, it is imperative that PFAS levels be known in order to predict the estimated life of the resin and protect customers by recommending replacement at an appropriate interval. Obtaining accurate test results allows for the proper application of treatment to alleviate problem water. Ion exchange is a crucial water-treatment technology that depends on testing to accurately apply.
About the author
Marianne R. Metzger has spent a majority of her career working at laboratories in a variety of capacities, from technical support representative to vice president of sales. She currently heads up the Laboratory Services Division at ResinTech Inc. Metzger has presented at several national and regional water-quality conferences about water-testing topics and has contributed several articles to pertinent trade publications within the water-treatment industry. In addition to her work at ResinTech, she also serves as the executive director of the Eastern Water Quality Association, where she assists the board of directors in developing membership through education.