Ultraviolet (UV) technology using a low-pressure mercury source has become increasingly popular for point-of-use and/or point-of-entry drinking water treatment. With enough energy, UV radiation at the 254-nm wavelength has the ability to disrupt DNA in pathogenic microorganisms so they cannot reproduce, which prevents them from causing disease in people drinking the water.

As the technology becomes more and more popular, more people are becoming aware of NSF/ANSI 55 for UV systems.

Should I Buy Class A or Class B? What's the Difference?

NSF/ANSI 55 separates UV systems into two distinct classes. Class A systems should be used with well or surface water (streams, lakes, etc.) and are designed to inactivate and/or remove microorganisms, including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts from contaminated water. Class A UV systems are not intended for the treatment of water that has an obvious contamination or intentional source, such as raw sewage, nor are they intended to convert wastewater to drinking water. They are intended to be installed on visually clear water (not colored, cloudy or turbid).

Class B systems, on the other hand, should be used with municipal water or treated water, and are designed 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. Class B systems are designed to reduce normally occurring, nonpathogenic nuisance microorganisms only. These systems are not intended for the disinfection of microbiologically unsafe water, and may not make individual or general cyst claims. Microbiological health effects claims may not be made on Class B systems.

Our Most Popular Class A SystemsOur Most Popular Class B Systems
Pura ABUV-6 UV Disinfection System NSF Class BSterilight SV5Q-PA Silver SeriesTrojan UVMax D4-V NSF Class B

Trojan UVMAX Pro20, 20 gpm,
NSF Class A

Trojan UVMAX Pro30, 30 gpm,
NSF Class A

Sterilight Platinum SPV-200, 2.6 gpm,
NSF Class A

Pura ABUV-6
UV System,
NSF Class B

Sterilight SV8Q-PA
Silver Series, 7 gpm,
NSF Class B

Trojan UVMax D4-V,
8.9 gpm,
NSF Class B

UV Dosage

UV dosage is the measurement of the energy delivered by UV systems, typically measured in mJ/cm². The more dosage a system provides, the more energy it can deliver to any microorganisms present in the water being treated. At a certain threshold, this energy becomes sufficient to inactivate most of the microorganisms present.

NSF/ANSI Standard 55 requires that Class A UV systems must deliver a high enough UV dose at 254 nm (40 mJ/cm²) to inactivate the pathogenic microorganisms that could be responsible for causing disease through contamination of our drinking water. Class B systems are required to deliver a lesser dose of 16 mJ/cm² that is sufficient to inactivate nonpathogenic organisms.

This dosage is verified through a test procedure detailed in the Standard. The test must be conducted at the highest achievable flow rate through the system, with UVT reduced to 70 percent, or to the alarm set point, whichever is lower.

Alarm or Fail Safe

NSF/ANSI 55 requires that Class A systems have alarm and/or fail-safe design elements incorporated into them. Specifically, Class A systems must include a UV sensor. A visible sensor is not sufficient to meet the Standard — it must be a UV sensor. The UV sensor must be connected to an alarm, which provides a visual and/or audible indication that the system is not performing, and/or terminates the discharge of treated water. The Standard includes a test to ensure that the UV sensor and alarm perform properly in low-dosage conditions. Although Class B systems are not required to have a UV sensor, if they are so equipped, the sensor must meet the test requirements.

Flow Control

Because UV dosage is inversely proportional to flow rate through the system, NSF/ANSI 55 requires that UV systems must include automatic, fixed flow-rate controls to prevent excessive flow over the manufacturer's recommended operating pressure range. The Standard requires that the flow rate of the system be evaluated over the manufacturer's operating pressure range and up to at least 100 psi (690 kPa), and that the Standard testing for UV dosage must be conducted at the highest flow rate achieved by the system.

Basic Requirements of NSF/ANSI 55
for Point-of-Use or Point-of-Entry UV Systems

USEWell or surface water (stream, lake, etc.)Municipal water or treated water
UV Dosage40 mJ/cm² at highest achievable flow rate with flow control in place, with UVT reduced to alarm set point or 70%, whichever is lower16 mJ/cm² at highest achievable flow rate with flow control in place, with UVT reduced to 70%
UV SensorRequiredNot required
Alarm (visual, audible, termination of treated water)RequiredNot required
Flow ControlRequiredRequired

Source: Rick Andrews, http://www.wcponline.com/pdf/WaterMatters.pdf