Volume 44 Number 7
An Overview of the Standard for RO Treatment Technologies
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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.
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.
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.
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.
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