By Rick Andrew

POU RO systems are widely used to help treat residential water. Part of the reason for their broad use is their considerable contaminant reduction capability. This capability is derived from the RO membrane, as well as from the system design, potentially including other water treatment technologies being included in the system. Let’s explore the requirements for claims of contaminant reduction by POU RO systems as stated in NSF/ANSI 58 Reverse Osmosis Drinking Water Treatment Systems.

Inorganic chemical reduction claims
Inorganic chemical reduction claims for POU RO systems under NSF/ANSI 58 are based on the performance of the RO membrane in the system, independent of any treatment that could be provided by prefilters, postfilters or any other type of treatment incorporated into the system. The test method for these claims is designed to address treatment only by the RO membrane. It involves testing for seven days under a variety of usage scenarios involving partial and complete draws from the storage tank, with a 50-psi inlet pressure. This testing does not include any assessment of treatment capacity, because the inorganic chemical reduction claims are based on ionic rejection by the membrane as opposed to adsorption on any media. For these reasons, inorganic chemical reduction claims are tested on systems that have the pre- and postfilters removed (see Figure 1 for specific claims and reduction requirements).

A few notes regarding these claims:

  • TDS reduction is required for all POU RO systems for conformance to NSF/ANSI 58. This is the bare minimum performance that all conforming RO systems must demonstrate.
  • There are two challenge levels for pentavalent arsenic reduction because the US EPA MCL changed from 0.050 mg/L to 0.010 mg/L in 2006. So, some water supplies that previously were below 0.050 mg/L but above 0.010 mg/L needed treatment, whereas previously they hadn’t. The lower challenge level was included for this reason. Note also that there is no claim for trivalent arsenic reduction. For systems making pentavalent arsenic reductions, there is an Arsenic Fact Sheet required to be included with the user information, to help them understand the nuances of trivalent versus pentavalent arsenic reduction, how to oxidize trivalent arsenic to pentavalent arsenic and the difference in the challenge levels.
  • Claims of trivalent chromium reduction, hexavalent chromium reduction, or both, can be made under NSF/ANSI 58.
  • Nitrate and nitrite reduction claims are tied together under NSF/ANSI 58 because the two exist in equilibrium when present in drinking water. The maximum permissible product water concentration is a combination of the two, based on the US EPA MCL for nitrate and nitrite.
  • The claim of radium 226/228 reduction is based on the test for barium reduction. There is no actual testing with radium 226/228 specified in NSF/ANSI 58.
  • There are two different test water requirements specified in this standard. One is TDS reduction water, which is basically deoinized water with NaCl added to make up 750 mg/L and adjusted to 25°C. This water is used for TDS reduction testing, plus testing of reduction of pentavalent arsenic, barium, chromium and perchlorate. There is a second test water, inorganic chemical reduction test water, that is basically tap water adjusted if necessary to meet specifications for pH, turbidity, TDS and temperature. The remaining inorganic chemical reduction contaminants may be tested either in TDS water or in inorganic chemical reduction test water.

Mechanical filtration claims
Mechanical filtration claims are based on the performance of the complete RO system. The test methods are appropriate for evaluating whatever combination of prefilters, postfilters and RO membranes that may be present. For this reason, mechanical filtration claims are tested on complete systems with all pre- and postfilters included. Testing these claims involves using the same seven-day testing protocol used for evaluation of inorganic chemical reduction claims (see Figure 2 for specific claims and reduction requirements).

VOC reduction claims
NSF/ANSI 58 includes requirements for VOC reduction claims. However, unlike the other contaminant reduction claims included in NSF/ANSI 58, the VOC reduction claim is based on a carbon postfilter. Only systems that have activated carbon postfilters may be tested for VOC reduction under NSF/ANSI 58. As with NSF/ANSI 53, a large number of organic contaminant reductions may be claimed for systems that meet the requirements for VOC reduction, based on the test using chloroform as the surrogate compound. Further, the standard includes two separate test methods and approaches for conducting the test, depending on the location of the activated carbon postfilter in the RO system. There is one method for testing postfilters located downstream of the RO element but upstream of the storage tank, and there is a second method for testing postfilters located downstream of the storage tank.

The first method involves testing with very low flowrates typical of those seen by postfilters downstream of the RO element and upstream of the storage tank. Also, the test method involves a continuous flow of water, typically seen for extended periods when the storage tank is refilling after being emptied. There is an option to test at an accelerated flowrate if desired, to speed up the test. The test is conducted to 120 percent of rated capacity if there is a performance indication device (PID) present; otherwise it is conducted to 200 percent of rated capacity, just as required by NSF/ANSI 53. The second method used for systems with postfilter located downstream of the storage tank, is to test according to NSF/ANSI 53 for typical POU systems. For both test methods, the flowrate through the filter in the RO system is measured. But to facilitate testing in the laboratory, the actual test is conducted on activated carbon filters that have been removed from the RO system itself (see Figure 3 for specific claims and reduction requirements).

Versatile technology and appropriate test methods
Typical POU RO systems offer combinations of technologies that can be quite versatile, considering the RO membrane element itself, plus the various additional filter that can be included. NSF/ANSI 58 has been thoughtfully designed to provide a basis for verification of contaminant reduction capabilities based on appropriate and varying test methods for these technologies, taking into account exactly how they are utilized in POU RO systems. All of this adds up to a winning combination for all stakeholders—manufacturers, dealers, retailers, regulators and end users—because they can all rely on robust evaluations of this complex technology when considering claims of contaminant reduction.

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:


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