July 2002: Volume 44, Number 7
An Overview of the Standard for RO Treatment Technologies
by Rick Andrew
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Reverse osmosis (RO) systems continue to be a large segment of the point-of-use (POU) drinking water treatment market, both in point-of-purchase retail and dealer distribution, domestic and abroad. This popularity is due in large part to the broad range of claims that RO systems can achieve. To address the growing market demand for independent demonstration of product quality and reliability, many of these systems are tested and certified against ANSI/NSF Standard 58 - Reverse Osmosis Drinking Water Treatment Systems for performance capabilities, structural integrity and materials safety.
This standard, one of six ANSI/NSF standards for Drinking Water Treatment Units (DWTUs), is specific to RO technology. There are many unique aspects of this technology that result in a standard that’s very different from the other standards. The following is a brief discussion of the scope and test methods for evaluating performance of RO systems under the standard.
RO membrane elements, when used with proper pretreatment, can perform at acceptable levels for extended periods of time. Under ideal conditions, RO elements used in home treatment systems can last for five years or more. Conducting performance testing over the lifetime of an RO element is, therefore, impractical. As an alternative, all RO systems are performance tested for a fixed seven-day period.
This seven-day period allows for performance to be evaluated as the system comes to equilibrium following initial start-up and conditioning. It also allows for a large number of product water samples to be collected. Finally, it includes a 54-hour stagnation period to evaluate the performance of the system after a period of non-use.
Systems are performance tested in duplicate under Standard 58. The initial, inlet pressure is 50 pounds per square inch gauge (psig). Both systems must meet the performance requirements of the standard at all sample points throughout the seven-day test period for claims to be valid.
For most of the performance testing under Standard 58, the pre- and post-filters are removed from the RO system. The standard evaluates only performance of the RO element and not the pre- and post-filters. For mechanical reduction claims—including asbestos, cyst and turbidity reduction—pre-filters are left in place, but post-filters are removed. Volatile organic compounds (VOC) reduction claims under the standard are different from other performance claims, as they aren’t based on the RO element, but rather on the RO system’s activated carbon post-filter. As a result, protocols for testing VOC reduction claims under Standard 58 are more similar to test protocols in Standard 53 - Drinking Water Treatment Units - Health Effects that rely on capacity and flow rates as critical parameters.
Arsenic reduction claims apply only to water supplies chlorinated at the system inlet where residual free chlorine may be detected. Changes to the arsenic reduction claim will be forthcoming in the near future, allowing for a broader scope of claims based on new federal drinking water regulations.
Recovery rating, daily production rate and efficiency rating are calculated based on data collected during the first and seventh days of the test. These values are averaged to determine the overall ratings for the system.
RO technology has proven to be capable of reducing a significant number of health effects contaminants. All RO systems conforming to Standard 58 must have a total dissolved solids (TDS) reduction performance claim. Additional performance claims beyond TDS are optional, as selected by the manufacturer. Once selected, the standard specifies the same seven-day test regimen along with the required influent and maximum effluent criteria.
Table 1 describes the influent challenge levels and maximum allowable product water levels or reduction requirements specified for the various performance claims available under Standard 58. While procedures for measuring the performance of RO systems differ from other technologies and standards, these test conditions of influent and effluent are nearly identical across the various standards. Influent levels are derived generally from U.S. Geological Survey data, when available, and represent 95 percent of the highest concentration found in the survey. The effluent criteria are generally based upon the U.S. Environmental Protection Agency (USEPA) regulated maximum contaminant levels (MCLs) for drinking water.
Performance testing studies conducted by NSF International in conjunction with RO element manufacturers have demonstrated that TDS reduction can serve as a surrogate for certain other performance claims. Several membrane companies have certified their RO elements with NSF, including performance testing across a broad range of contaminants. As a result, system manufacturers that use these same elements in their RO systems can achieve significant cost and time benefits.
When an RO system manufacturer selects a certified membrane having established performance data, the system is evaluated at a minimum for TDS reduction. Depending on the requested claims and system configuration, other tests may not be necessary. Nitrate/nitrite reduction claims are one example where RO element test data cannot be included in a data transfer, as performance studies indicated that TDS reduction isn’t a valid surrogate for membrane performance. Most others, however, can be transferred using the single TDS surrogate test of the system.
NSF currently has performance test data on file for three RO element manufacturers, as indicated in online listings.
Today, there are a total of 574 systems and components certified by NSF to Standard 58, as listed by 68 individual companies. There are products that can reduce every available contaminant listed under Standard 58. There are several claims that many products are certified to reduce, as shown in Table 2.
RO technologies provide a unique set of conditions for determining contaminant reduction capabilities and related performance characteristics. This uniqueness has resulted in a rather complex set of test conditions, as captured and detailed in ANSI/NSF Standard 58. Understanding these test conditions and associated criteria are important for manufacturers and dealers when deciding on testing to Standard 58 when conveying to buyers the process used to demonstrate product performance.
About the author
Rick Andrew is the technical manager for the NSF DWTU Certification Program. He has been with the program since 1999. Previously, he was an environmental and analytical chemistry consultant. He has a bachelor’s degree in chemistry and a master’s degree in business from the University of Michigan. He can be reached at (800) 673-6275 or email: email@example.com