By Rick Andrew

Ultraviolet (UV) drinking water disinfection reactors are proven, effective technologies for helping to assure the microbiological safety of drinking water. Because of the acute health effects associated with drinking microbiologically contaminated water, such as diarrhea, cramps, and vomiting, it is essential to verify the disinfection performance of these reactors. There are numerous protocols, requirements, and methodologies for evaluation of UV drinking water disinfection reactors. Two of the most relevant in the US include:

  • US EPA Long Term 2 Enhanced Surface Water Treatment Rule (LT2 Rule)
  • NSF/ANSI 55 Class A

In the United States, the Safe Drinking Water Act defines public water systems as those serving more than 25 people. These systems fall under the LT2 Rule in terms of protecting public health from illness due to Cryptosporidium and other microbial pathogens in drinking water and to address risk-based trade-offs with the control of disinfection byproducts. The LT2 Rule requires public water systems to monitor frequently for Cryptosporidium if their water source is a lake, river or other “surface water”. Depending on the results of the monitoring, the public water system fits into one of four risk-related Bins. Depending on which Bin a system is placed, the LT2 Rule may require additional treatment to reduce the risk to Cryptosporidium. The treatment requirements for the Rule range from no additional treatment in the first Bin to treatment effective in reducing Cryptosporidium by 2.5 logs in the fourth.

Under the LT2 Rule, many different treatment technologies may be used to achieve reduction of Cryptosporidium. The Rule specifies criteria and requirements for these technologies, including validation;. UV is one such technology. The LT2 Rule requires that public water systems using UV to achieve compliance must use a UV reactor that has been independently validated. These validation requirements and test protocol are described in US EPA’s Ultraviolet Disinfection Guidance Manual (UVDGM). These public water systems vary in size, although they all have trained operators and plans for monitoring treatment effectiveness. NSF/ANSI 55, on the other hand, includes criteria for validating POU/POE UV reactors. They are intended to be installed in residences and other facilities in which there typically would not be a trained operator. Considering the application, NSF/ANSI 55 focuses on fail-safe requirements and approaches to the technology and the evaluation.

UVDGM versus NSF/ANSI 55
The UVDGM includes detailed guidelines regarding UV reactor validation testing. Taken into consideration is that public water systems using these UV reactors will have trained operators and treatment effectiveness monitoring plans. These guidelines specify, among other requirements, that validation testing of UV reactors must include the following:

  • Non-uniform lamp aging must be accounted for. There are several ways to accomplish this. One approach is to perform research on specific lamps to establish that aging is uniform. If this is the case, a new lamp with the power reduced to simulate an aged lamp can be used for validation testing. Another approach is to use an aged lamp for validation testing.
  • Validation testing must be conducted at the minimum and maximum design flowrates and at least one other flowrate.
  • The microorganism used for testing may be MS-2 coliphage or another organism with a UV sensitivity similar to the UV sensitivity of Cryptosporidium and Giardia.
  • Validation tests must be conducted with water adjusted to both the minimum and maximum levels of UV transmittance.
  • Duty sensors must be evaluated and compared to reference sensors.
  • Validation testing may be performed over a range of UV dosages.
  • Detailed validation reports must be include specifics regarding the test and quality control data for each test including bioassays, control and trip blanks, flowmeters and the calibration of measurement instruments.
  • The final dosage determination, the reduction equivalent dose (RED), is calculated using the validation test results and taking into account safety factors such as test organism dose-response, uncertainty with test organism dose-response and bias sensor uncertainty.

This approach is different from NSF/ANSI 55 Class A, which covers POU/POE systems installed in a facility with no trained operator and no treatment efficiency monitoring plan. Specifically, NSF/ANSI 55 Class A differs as follows:

  • The UV system must include a flow restrictor, UV sensor and alarm. The system testing is conducted at the highest achievable flowrate with parahydroxybenzoic acid (PHBA) added to reduce the UV transmittance to the alarm set point. Under this worst case condition (highest possible flow rate, system alarming), the system must still achieve the required 40 mJ/cm2 dosage. With this approach of testing under worse case conditions, testing at lower flowrates or higher UV transmittance conditions is not necessary.
  • The test organism is always MS-2 coliphage.
  • Typical certification reports are all that is required because of the detailed test method spelled out in the Standard.
  • Although system dosage can be calculated, the Standard requires simply that the system must achieve a minimum UV dosage of 40 mJ/cm2 under the worst case test conditions described above.

NSF/ANSI 55 and very small systems (VSS)
Despite these differences in approach due to the differences in end use, UVDGM references and recognizes NSF/ANSI 55. Specifically, UVDGM states that UV reactors evaluated to NSF/ANSI 55 should be evaluated on a case-by-case basis in terms of possible use in public water supplies and conformance to the requirements of the LT2 Rule. In light of this allowance, some regulatory agencies have been considering the use of NSF/ANSI 55 as a way to meet the Rule for small public water systems. Additionally, there are many very small systems (VSS) that are smaller than the SDWA definition of a public water system, which includes those serving more than 25 people. Depending on location, VSS serving fewer than 25 people are regulated either by state, local or provincial agencies. These agencies us a variety of approaches with respect to ensuring the performance of UV reactors used in these small public water systems. Many look to the UVDGM for their guidance.

Some of these agencies also see the value of UV reactors certified to NSF/ANSI 55, Class A. With some of these certified UV reactors providing flowrates exceeding 25 gpm (94.6 liters/minute), they can (in some situations) meet the needs of VSS. Reactors can also be arranged in parallel to provide necessary flowrates. Because these reactors include flow restrictors, UV sensors and alarms, they can be simpler to understand and operate than other types of reactors with fewer fail-safe mechanisms intended for use in public water supplies with trained operators.

End use dictates criteria
With any product or technology, the end use dictates the criteria for evaluation. For example, commercial kitchen equipment must meet different criteria than cooking utensils designed for home use. Coffeemakers intended to be used on aircraft are evaluated much differently from coffeemakers used in cafeterias. The same is true of UV systems. Those intended for use in public water supplies must meet the LT2 Rule through the UVDGM, whereas those for residential use are evaluated to NSF/ANSI 55. When it comes to VSS, however, the state, local or provincial agencies must use their best judgment when considering treatment options and appropriate validation criteria.

About the author
Rick Andrew is the General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols), and Biosafety Cabinetry Programs. He has previously served as the Operations Manager and prior to that, Technical Manager for the program. 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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