By Rick Andrew
Most people know that replacement elements in water treatment systems with replacement elements must be…replaced. As more and more water is treated, these elements may become clogged and suffer from a reduced flowrate or they may diminish in treatment effectiveness, or both. In fact, the NSF/ANSI Drinking Water Treatment Unit (DWTU) standards were developed to take this reality into account. This applies to both mechanical filtration tests and also testing of chemical reduction performance of active media technology. Let’s set aside the issues of mechanical filtration and clogging, and examine more closely this issue of diminishing treatment effectiveness and treatment capacity for chemical reduction functions of active media replacement elements, and how this issue is addressed by the NSF/ANSI DWTU standards.
Adsorptive capacity
Active media filtration systems rely on adsorptive or absorptive mechanisms to treat the water, with the more typical case being adsorption. A generalized, oversimplified explanation of this technology is that the media includes a finite number of adsorptive sites. As more and more water is treated, these adsorptive sites become occupied by contaminants that have been removed from the water, thus reducing the number of available adsorptive sites to treat additional contaminants. Eventually, treatment becomes less and less effective due to fewer sites being available, so that the contaminant begins to break through the media at higher and higher concentrations in the treated water. This deterioration in treatment performance continues until the treatment is completely ineffective or until the media is replaced.
Standard requirements
As mentioned, this concept of adsorptive capacity is reflected in the NSF/ANSI DWTU standards. The structure of methods for testing of chemical reduction performance of active media filtration systems with replacement elements under both NSF/ANSI 42 Drinking Water Treatment Units – Aesthetic Effects and NSF/ANSI 53 Drinking Water Treatment Units – Health Effects is based on the manufacturer’s recommended treatment capacity. Under NSF/ANSI 42, these systems are tested to 100 percent of the manufacturer’s recommended treatment capacity, with samples of the influent and treated water taken at specific intervals throughout the test to assure continuing performance. NSF/ANSI 53 is structured similarly, with the end point for systems with performance indication devices (PIDs) at 120 percent of manufacturer’s recommended treatment capacity, and the end point for systems without PIDs at 200 percent of capacity. The rationale for testing beyond capacity is based on providing a buffer for greater assurance of protection of public health.
Limitations
All technologies used to treat drinking water have limitations associated with them and active media technologies are no exception. Water characteristics affect the performance of active media treatment systems in many different ways. For example, pH can have an effect on the form of metals present in water, which in turn can affect adsorption characteristics of these metals with various media. The presence of total organic carbon in the form of natural organic matter in water can cause confounding effects for activated carbon treatment of organic contaminants in the water. Temperature can have an effect on many different forms of treatment, with variations in effectiveness occurring at high and low temperatures. The level of TDS can impact some forms of treatment of certain contaminants. And of course the concentration of the contaminant being treated is very important to performance of an active media treatment system—the higher the concentration of contaminant, the sooner the adsorptive capacity of the media will be reached, among other effects.
The NSF/ANSI DWTU standards are designed with these limitations in mind. The design of the various test waters specified for contaminant reduction tests takes into account these factors (pH, temperature, total organic carbon, TDS and others), resulting in test conditions that are protective of public health and help assure that products tested according to these standards will perform under most water conditions encountered in real-world situations. For aesthetic effects contaminant reduction testing under NSF/ANSI 42, this means setting contaminant concentrations in the influent challenge water at reasonable levels of occurrence and setting maximum allowable contaminant concentrations in treated water at levels consistent with treatment for aesthetic effects, such as treatment below the taste or odor threshold. When it comes to testing for health effects contaminants, this means setting influent concentrations typically at the 95th percentile of occurrence, meaning that of those water supplies containing that contaminant, 95 percent have a concentration equal to or lower than the concentration required for testing by the standard. In those cases where the 95th percentile of occurrence is low with respect to the regulated levels, the influent concentration is set at three times the regulated level to allow for meaningful testing conditions. The maximum allowable treated water concentrations are set at regulated levels stipulated by US EPA or Health Canada for regulated contaminants.
Metals tested under NSF/ANSI 53 are also tested at both low and high pH, with a requirement that both tests must pass in order for product claims to be made. By taking this approach, the effectiveness of treatment when the metals take different forms because of the pH variation can be assessed. Figure 1 includes a summary of some of these key considerations and how they are addressed in the NSF/ANSI DWTU standards. Even considering the thoughtfulness and protectiveness built in to testing according to these standards, there are still limitations to consider. Some water supplies may have contaminant concentrations or other water characteristics at the extremes, in which case conforming products may not perform as well under these conditions as they do when tested in the laboratory.
Statements in product literature
Because laboratory testing and evaluation cannot possibly account for every extreme situation, the NSF/ANSI DWTU standards require statements in the product literature to help alert end users to this reality. These standards require a statement in the product’s performance data sheet that while testing was performed under standard laboratory conditions, actual performance may vary.
This is quite similar to the disclaimer associated with US EPA fuel economy rating statements for automobiles, both in terms of the reasons for making the statement, and the language used.
Truth in advertising and changing filters
One of the origins of the NSF/ANSI DWTU standards, and one of the driving forces behind these standards to this day, is the concept of honest portrayal of product performance, measured on a level playing field. This consideration of limitations and dealing with limitations in a comprehensive, truthful manner that protects public health and informs the public rings true to this origin and driving force.
And underlying all of this discussion is a fundamental concept that is sometimes (unfortunately) ignored by end users: active media treatment systems with replacement elements will at some point cease to be effective if those elements are not replaced. One of the greatest services the water treatment industry can offer to end users is to provide advice and reminders about the importance of system maintenance according to the manufacturer’s recommendations. Without maintenance, most products will eventually fail to perform. This is certainly applicable to water treatment products with replacement elements.
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]