By Lorna Badman

ANSI/NSF Standard 55 Ultraviolet Microbiological Water Treatment Systems was initially adopted in 1991. Standard 55 covers two classes of devices: Class A and Class B.

Class A point-of-use/point-of-entry (POU/POE) devices are designed to disinfect and/or remove microorganisms, including bacteria and viruses, from contaminated water to a safe level. They aren’t intended for treatment of water that has an obvious contamination source, such as raw sewage; nor are systems intended to convert wastewater to microbiologically safe drinking water. Class A POU or POE disinfection treatment devices are required to deliver a minimum UV dose of 38 milliJoules per square centimeter (mJ/cm2), or 38,000 milliWatts per second per square centimeter (mW-sec/cm2), at the failsafe point as determined by inactivation of Bacillus subtilis spores and using a sensitivity calibration curve.

Meanwhile, Class B POU systems are designed for supplemental bactericidal treatment of treated and disinfected public drinking water or other drinking water tested and deemed acceptable for human consumption by the state or local health agency with jurisdiction. Class B systems aren’t intended for disinfection of microbiologically unsafe water, but are designed to reduce only normally occurring nonpathogenic or nuisance microorganisms. Class B microbial reduction devices are required to deliver a minimum UV dose of 16 mJ/cm2 or 16,000 mW-sec/cm2 at 70 percent of the UV lamp’s normal output or at the fail-safe point, as determined by inactivation of Saccharomyces cerevisiae cells and using a sensitivity calibration curve.

Revision of standard
With advances in technology, however, Standard 55 is in the process of undergoing a comprehensive revision.

Structural performance
Structural performance tests have been harmonized over NSF’s family of drinking water treatment unit (DWTU) standards to allow for more consistency. Standard 55 is included in this harmonization and a table has been added to more clearly define testing requirements.

Use of Coliphage MS-2
Coliphage MS2 is currently being validated as an additional test surrogate to determine bacterial and viral pathogen disinfection efficiency of UV water treatment systems. The validation testing is being completed by NSF and an independent laboratory to determine reproducibility of the coliphage as a surrogate organism. The existing Standard 55 requires a Class B system to deliver a UV dose of 16 mJ/cm2 employing S. cerevisiae as the test organism. Class A POE or POU UV treatment devices are required to deliver a minimum UV dose of 38 mJ/cm2 for certification. Class A devices are challenged with B. subtilis because of its relatively high resistance to UV light.

A study conducted by Bart Wilson1 shows a 99.5 percent reduction of coliphage MS2 after UV treatment is equivalent or greater than a 99.999 percent reduction or a 5-log reduction of bacterial pathogens and a 99.99 percent reduction or a 5-log reduction of viral pathogens. The UV inactivation rate of coliphage MS2 was compared to common microbial contaminants and pathogens (S. cerevisiae, B. subtilis, Vibrio cholerae, Salmonella typhi, Escherichia coli O157:H7, Shigella dysenteriae, Yersinia enterocolitica, Campylobacter jejuni, Aeromonas hydrophila, Legionella pnuemophila, Hepatitis A, Rotavirus SA-11, and Polivirus type 1). Of all these organisms, coliphage MS2 was found to be the most resistant to UV radiation.

Additional benefits of using coliphage MS2 include: 1) linear response over a wide range of UV dose levels, 2) highly reproducible UV inactivation results, 3) low cost, 4) it’s easily grown and propagated to high titers or culture concentration, and 5) it’s non-pathogenic to humans.

Cryptosporidium claim
Currently, a manufacturer wanting to make a cyst reduction claim on a Class A device is required to a have a prefilter that complies with ANSI/NSF Standard 53 for cyst reduction upstream of the UV device. With data now showing UV light does inactivate Cryptosporidium oocysts, an additional technology for Cryptosporidium oocysts reduction/inactivation won’t be needed when treating municipal chlorinated waters. Class A systems without a general cyst reduction device used for the treatment of untreated surface waters must have a device found to be in conformance for cyst reduction under Standard 53 installed upstream of the system.

ANSI formatting
In addition to the technical revisions, Standard 55 is being editorially updated to the American National Standards Institute or ANSI-recommended format for American national standards.

Future revisions and PIDs
Currently, Class A qualification is only available to devices equipped with UV sensors. A sub-task group on sensors is looking at further defining the performance qualifications of UV sensors and ways for a device without a Performance Indication Device (PID) to obtain Class A qualification.

The sub-task group is currently focusing on defining the performance qualifications for sensors. The group is systematically looking at this issue by first addressing the consumer’s specifications, performance specifications and qualification specifications.

Consumer specifications
The sub-task group first discussed consumer expectations on a UV device. The sub-task group determined the following were important characteristics of a UV device to the consumer:

  • An audio or visual alarm when the water is unsafe to drink. POE devices would need both a visual and audio device with minimum and maximum limits. For POU devices, either an audio or visual device would be required but, whichever was chosen, the alarm would need to be easily discernable to the consumer,
  • A provision for a remote readout,
  • Ability by the consumer to monitor the disinfection level for the lifetime of the unit,
  • Ability of the unit to notify the consumer when the sensor system fails, and
  • No field calibration required by the consumer.

Performance specifications
Once the sub-task group had covered consumer specifications, it turned the focus to performance specifications of the sensor. Following are the performance specifications:

  • Accuracy and precision are two different requirements but both will be addressed. Accuracy will be based on intensity at the trip point—the point at which the unit is no longer delivering the prescribed dose—± 10 percent and precision will be determined at the trip point ± 5 percent.
  • Life expectancy of the UV bulb will be expressed in operational lives. The lamp-life must exceed the design life or service contract.
  • The sensor must fail in the failsafe mode and shouldn’t exceed a 2 percent unsafe failure mode. The sensor must notify the user when UV intensity has dropped below 70 percent of the initial reading.
  • Thermal stability, maximum percent signal degradation over time, linearity of the sensor’s response and sensitivity of the sensor will all be considered together.
  • The manufacturer will be responsible for maintaining calibration of the sensor, which will be evaluated against a known calibrating reference at the final point of manufacturing with records being maintained.

NSF is performing a comprehensive revision of ANSI/NSF 55—Ultraviolet Microbiological Water Treatment Systems. The proposed revisions will include ANSI formatting, structural integrity harmonization, the addition of a coliphage MS-2 as a surrogate test organism, and a Cryptosporidium claim. NSF hopes to ballot the above revisions to Standard 55 by early this summer. Future revisions to the standard will include the performance qualifications of sensors and the Class A device performance specifications without sensors.


  1. Wilson, Barth R., et al., “Coliphage MS2 as a UV water disinfection efficacy test surrogate for bacterial and viral pathogens,” Poster Session, 1992 Water Quality Technology Conference, November 1992.

About the author
Lorna Badman is project manager for NSF International DWTU standards. Badman has been with NSF since 1999. She has a bachelor’s degree in biology from Eastern Michigan University. She can be reached at (734) 827-6806, (734) 827-6831 (fax) or email:

Sub-task group’s list of qualifications considered

  • Test methods need to be designed and validated.
  • A failsafe point needs to be determined.
  • Bioassay testing needs to be completed.
  • A method for monitoring UV transmittance needs to be determined.
  • A UV light source reference is needed.
  • Duty cycles need to be addressed.
  • A specification for a standard optical configuration for the sensor testing needs to be determined.

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