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Water Matters: NSF/ANSI 55 Class A Versus Class B UV Systems

By Rick Andrew

UV systems have been utilized successfully for decades to help control microorganisms or disinfect water for drinking purposes. The UV radiation emitted by these systems can inactivate the organisms and prevent them from reproducing, thus disinfecting the water. Because disinfection is such a critical element in treatment of drinking water, there has been great thought and a high degree of rigor incorporated into NSF/ANSI 55 Ultraviolet Microbiological Water Treatment Systems.

Class A and Class B

NSF/ANSI 55 establishes two classes of UV systems: A and B. Class B UV systems are for supplemental bactericidal treatment of disinfected public drinking water or other drinking water that has been tested and deemed acceptable for human consumption by the state or local health agency having jurisdiction. These systems are intended to reduce normally occurring non-pathogenic nuisance microorganisms only. Class B systems are not intended for disinfection of microbiologically unsafe water. Individual or general cyst claims may not be made on Class B systems, nor can microbiological health effects claims be made.

Class A systems are intended to inactivate microorganisms, including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts in contaminated water. These systems are not intended for the treatment of water that has an obvious contamination or intentional source, such as raw sewage. Class A systems are not intended to convert wastewater to drinking water. The systems are intended to be installed on visually clear water (not colored, cloudy or turbid). Claims of reduction of Cryptosporidium oocysts and Giardia cysts may be made on Class A systems not installed downstream of a device tested for cyst reduction/inactivation in conformance to the appropriate NSF/ANSI standard. General cyst claims may be made on Class A systems installed downstream of a device tested for cyst reduction/inactivation in conformance to the appropriate standard.


While the requirements for both Class A and Class B systems are specific and rigorous, Class A systems include UV dosage and fail-safe requirements that go above and beyond those of Class B systems. Figure 1 describes the requirements of each of these systems, allowing an overview as well as a comparison. Note that NSF/ANSI 55 addresses only low-pressure mercury systems. Alternate technologies such as LEDs and other types of UV emitters are not currently addressed by the standard.

Similarities and differences

Both Class A and Class B systems share strict requirements for safety of materials in contact with drinking water and for structural integrity of systems connected to a pressurized water supply. Both must include flow restrictors to assure that the UV dosage cannot drop below that which results from testing. Class A systems, however, require a higher UV dosage and they must also include a UV sensor and alarm to act as a fail-safe in case the system is not functioning in a manner to provide safe drinking water.

Dosage versus log reduction

The UV requirements for Class A and Class B systems are based on UV dosage. The idea here is that there have been scientific studies performed demonstrating that these UV dosages are sufficient to inactivate microorganisms. In the case of Class A systems, studies have demonstrated that all of the microorganisms identified for testing under the US EPA Guide Standard and Protocol for Testing Microbiological Water Purifiers from 1987 are effectively inactivated to the log reductions specified in the guide standard at the 40 mJ/cm2 dosage level (see Figure 2). By specifying the UV requirements for Class A and Class B systems based on dosage in the standard as opposed to specifying log reductions as in the guide standard, a single surrogate organism can be used for testing, instead of testing with actual pathogenic microorganisms. This approach eliminates safety concerns regarding testing in a laboratory with pathogenic organisms. Also, there may be cost savings for testing with only one surrogate organism as opposed to multiple pathogenic organisms to establish a log reduction for each organism.

There is a limitation created by this approach, which is that the surrogate organism dose response is clearly understood only for low-pressure mercury lamp UV systems. This is because there is a dose response curve generated for the test organism using a highly calibrated low-pressure mercury lamp source. This curve is used to correlate the log reduction (obtained when the sur- rogate organism is subjected to the test UV system) to the actual UV dosage obtained at that log reduction. This dose response relationship is not clear, however, if the test UV system uses a technology other than low-pressure mercury lamps to generate UV radiation. For example, if there is a 2.0-log inactivation of a specific organism by a UV system using a non-low-pressure mer- cury emission source, what does this mean if that same organism is known to be inactivated by 2.0 logs at a 40 mJ/cm2 dosage by a highly calibrated low-pressure mercury lamp? It could be that the emission spectrum of the UV system, which is probably different from the emission spectrum of a low-pressure mercury lamp, is particularly effective against the specific organism being tested. Testing a different organism in the same manner, and compar- ing the log reduction to the dosage at the same log reduction in a highly calibrated low-pressure mercury lamp, could result in a calculated UV dosage less than 40 mJ/cm2. This is one of the challenges faced as the NSF Joint Committee on Drinking Water Treatment Units considers expanding the scope of NSF/ANSI 55 to include UV-emission technologies other than low-pressure mercury lamps.

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Rigorous testing

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While this discussion has focused specifically on UV systems, it highlights a cornerstone of the philosophy supporting all of the NSF/ANSI Drinking Water Treatment Unit standards: contaminant reduction claims for health effects must be based on sound science and rigorous testing methodologies designed to challenge the effectiveness of the treatment. UV systems can play a very important role in the disinfection of water for drinking purposes. It is critical that the standard we rely on to establish the capabilities of this technology be one in which we can have complete confidence. With the science and requirements included in NSF/ANSI 55, we can indeed have that complete confidence.

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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