By Rick Andrew
Ion exchange has long been one of the important tools in the water treatment toolbox. Recognizing its significance in the industry, a number of NSF/ANSI standards address ion exchange in various ways, depending on the end use and the particular standard involved. In some cases, these standards address only the safety of the ion exchange resin for contact with drinking water, utilizing an extraction (or leaching) test and a toxicological evaluation of any residuals from the manufacturing process or any other contaminants that may extract out during the test. In other cases, the standards go further. In addition to addressing safety of the ion exchange resin, they include test protocols for evaluating the contaminant reduction capability of specific system designs incorporating ion exchange resin. Figure 1 provides an overview of the standards and their applications to ion exchange resins.
Applications of ion exchange resins
Different applications in water treatment utilize various ion exchange resins in different ways. For example, typical POE domestic water softeners use cation exchange resin to exchange sodium or potassium for hardness, primarily calcium and magnesium. Most of these systems are designed and manufactured to be regenerated on site, using pellets of high-purity sodium chloride or potassium chloride. Alternatively, POU perchlorate reduction systems might use a highly selective anion exchange resin in a single-use application, where there is no regeneration of the media and new resin is periodically installed according to a replacement schedule.
Ion exchange treatment also includes important considerations related to the characterization of the water, especially the concentrations of various ions. Ion exchange resins have various selectivity for ions and are impacted by the concentrations of other ions in the water relative to the ion that is intended to be exchanged. For example, although a given ion exchange resin may be more selective to the ion being targeted, other ions (such as nitrate or sulfate) in high concentrations can become problematic by reducing the exchange capacity for the target ion. pH can also be a consideration related to the effectiveness of certain ion exchange resins for exchanging specific ions.
Contaminant reduction protocols
When developing contaminant reduction protocols, it is important to take into account the operation of the treatment technology being tested. For example, is the technology intended for single use and replacement of treatment media or is it intended for on-site regeneration? Another consideration is whether the flow through the system is continuous or near continuous, or is it more intermittent with frequent starting and stopping of flow and periods of stagnation. Other considerations include the impact of water characteristics on the treatment technology. Does the pH impact the effectiveness of treatment? Does the concentration of other ions such as nitrate and/or sulfate impact the effectiveness of treatment?
The goal is to develop a test protocol that is repeatable, reproducible, protective of human health and beneficial to consumers and manufacturers. Each part of this goal has considerations:
- Repeatable. The test protocol must be specific enough such that when the same technology is tested in the same laboratory multiple times, the results are sufficiently similar.
- Reproducible. The test protocol must be described clearly enough such that different laboratories interpret it similarly and achieve results that are sufficiently similar when testing the same technology.
- Protective of human health. The test protocol must be rigorous enough so as to ensure treatment effectiveness under most conditions that could be encountered in real-world applications. When it comes to health effects contaminants, the NSF Joint Committee on Drinking Water Treatment Units has tended toward the 95th percentile, such that 95 percent of end-use cases would be covered by the criteria outlined in the standards.
- Beneficial to consumers and manufacturers. Cost is important. The standards could be developed to attempt to be protective in 100 percent of end-use applications for health effects contaminants (instead of 95 percent), regardless of how extreme the conditions might be. Covering the most extreme five percent of cases, however, would likely lead to over-designing solutions for the other 95 percent of cases, leading to potentially much more expensive technology that would be out of reach for some consumers who could be helped effectively by much less expensive solutions.
Bearing in mind all of these factors, the various standards have specific test protocols designed, specifying the operational cycle, the composition of the test water, the end point of the test, the sample points and the pass/fail criteria.
For applications like water softeners that often include on-site regeneration, the test protocol also involves regeneration. It requires multiple exhaustion cycles with continuous flow, with the regeneration cycle itself also being evaluated for efficiency when demand-initiated regeneration systems make claims of salt and water efficiency.
For applications like POU, single-use (non-regenerable) perchlorate reduction systems, the test protocol includes testing through and beyond the manufacturer’s recommended treatment capacity with samples taken at intervals and with on- and off- cycling of intermittent flow. It also specifies the composition of the test water to include specific concentrations of other ions that could be competing ions or influence the system performance in some way.
Separate test protocols exist for Arsenic III and Arsenic V, with the composition of the test water including specific concentrations of other ions. Reduction of Arsenic III and Arsenic V must each be verified at both high pH (8.5) and low pH (6.5). And as with perchlorate reduction, testing through and beyond a manufacturer’s recommended treatment capacity with interval sampling is required.
A highly scientific approach
Water treatment is complicated. Understanding the complications, the challenges presented by treating certain contaminants with various technologies in water of various compositions and developing solutions that perform well and are cost effective is not simple. The water treatment industry’s role in providing solutions to these complex problems is crucial. Likewise, the standards for verifying the effectiveness of treatment solutions must leverage the knowledge developed by the industry and other stakeholders to develop test protocols that are repeatable, reproducible, protective of public health and beneficial to consumers and manufacturers. Reviewing the standards and their applications to ion exchange technology provides an excellent example of how their development has taken these factors into account to achieve these goals.
About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: [email protected]