By Michael Sheffield
Utilizing ultraviolet technology for the treatment of public drinking water has been a successful method to disinfect and reduce the abundance of microorganisms in water. The foundational principle of UV technology is based upon the properties of the electromagnetic spectrum of light and the energy it transmits. Certain wavelengths have the ability to penetrate microorganisms that are present in water. Scientific research has shown that the ideal wavelength to penetrate microorganisms is 265 nm, which falls within the ultraviolet range of the electromagnetic spectrum.
How UV treatment works
Ultraviolet technology exposes water to UV light by running water through a UV reactor, which allows the necessary contact time between the water and UV light for efficient treatment. The contact time and concentration of energy produced by the UV light source can be described by the UV dose. When microorganisms present in water are exposed to the correct UV dose, it induces a genetic change from within. The UV light then works as a disabling energy inside the microorganisms. The result is that the microorganisms are unable to replicate, which essentially makes them harmless and in turn disinfects the water. Given that disinfection is imperative to the treatment of drinking water, it is important to have a way to monitor drinking water treatment units that claim to be able to treat water using UV technology. In response to this, NSF/ANSI Standard 55 was established to verify the effectiveness of ultraviolet microbiological water treatment systems.
NSF/ANSI Standard 55: Ultraviolet Microbiological Water Treatment Systems
This standard distinguishes between two classes of systems, denoted by A and B. Class A systems are designed to inactivate and/or remove microorganisms from contaminated water. Microorganisms of importance for inactivation include bacteria, viruses, Cryptosporidium oocysts and Giardia cysts. Water that has an obvious contamination or comes from an intentional source, such as raw sewage, is not within the framework of Class A systems. As a result, Class A systems are not intended to convert wastewater to drinking water. In order to get the optimal UV dose, water intended to be treated with Class A systems must be visually clear (low turbidity, not cloudy or colored).
Class B systems 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. These systems are only designed for the reduction of normally occurring, non-pathogenic nuisance microorganisms, not for disinfection of microbiologically unsafe water. Given that Class B systems are not designed for disinfection, these systems should not make microbiological health effects claims, neither individual nor general cyst claims.
It should be noted that NSF/ANSI 55 only addresses low-pressure mercury systems. These systems have the ability to emit a spectral output of 253.7 nm, a wavelength that is very close to the 265 nm, the optimal wavelength for microbiological inactivation. Class A and Class B systems have rigid requirements in order to gain certification under NSF/ANSI 55. Both systems need to be evaluated for structural integrity if they are connected to pressurized water supplies. Additionally, there are stringent safety requirements set in place to assess all materials that come in contact with drinking water. Another requirement for both systems is the necessity of flow restrictors to guarantee effective UV dosage throughout the water treatment process. One difference in the requirements for the two system types is that Class A systems require a UV sensor and alarm to notify when the system is not operating in an effective manner to provide clean drinking water. The sensor and alarm are needed due to the fact that Class A systems require a higher UV dosage than Class B systems. Other similarities and differences in the requirements for Class A and Class B UV systems can be seen in Figure 1.
Importance of third-party certification
Drinking water quality can vary depending upon where one lives, thus confidence in the standards put in place to protect the public from contaminated drinking water is of utmost importance. UV technology can play an important role in the disinfection of water to supply clean drinking water. As such, it is particularly important that monitoring the effectiveness of UV technology is derived from sound science and demanding testing methodologies. The scientific principals behind the development of NSF/ANSI 55 give the assurance that UV technology is being critically monitored to provide clean drinking water.
About the author
Michael Sheffield is a Senior Technical Reviewer at NSF International, serving in the Drinking Water Treatment Unit (POU/POE) program. He has an MS Degree in environmental science from American University, specializing in water toxicology.