The U.S. Environmental Protection Agency (EPA) promulgated in 2006 the Long Term 2 Enhanced Surface Water Treatment Rule (LT2 rule) under its authority in the Safe Drinking Water Act (SDWA) as amended in 1996. The new LT2 rule was developed “to protect public health from illness due to Cryptosporidium and other microbial pathogens in drinking water and to address risk-risk trade-offs with the control of disinfection byproducts.”
The SDWA defines public water systems as those serving more than 25 people and the LT2 rule affects systems of all sizes, small to large. The LT2 rule requires public water systems to monitor frequently for Cryptosporidium if their source water is from a lake, river or other surface water. The results of the monitoring will then place the system into one of four “bins,” representing increasing risk to exposure from Cryptosporidium and other microbial pathogens. Depending on which bin a system is placed into, the LT2 rule may require additional treatment to reduce the risk of Cryptosporidium. The treatment requirements for the LT2 rule range from no additional treatment (Bin 1) to treatment effective in reducing Cryptosporidium by 2.5 log10 (Bin 4).
UV Reactor Validation
The LT2 rule addresses the use of many treatment technologies that a system may use to achieve reduction of Cryptosporidium. One frequently mentioned treatment technology in the LT2 rule is ultraviolet (UV) radiation. If systems use UV to achieve compliance with the LT2 rule, then the UV reactor must be independently validated. In November 2006, the EPA provided the final version of the Ultraviolet Disinfection Guidance Manual (UVDGM), which summarizes the LT2 rule requirements for validation testing and presents the EPA’s recommended validation protocol.
The UVDGM provides a detailed description for how to perform an UV reactor validation test. It also discusses alternative procedures and standards for the validation of UV reactors, which include:
- German DVGW Standard W294
- Austrian ONORM
- National Water Research Institute (NWRI) guidelines
- NSF/ANSI Standard 55
The UVDGM states that UV reactors certified by DVGW and ONORM should be granted a 3 log10 Cryptosporidium and Giardia credit. It cautions that the use of the NWRI guidelines and NSF/ANSI Standard 55 should be evaluated on a case-by-case basis. Some regulatory agencies have been considering the use of NSF/ANSI Standard 55 as a way to meet the new LT2 rule for small systems; however, there are significant differences in the requirements and application of the UVDGM and NSF/ANSI Standard 55.
The LT2 rule and UVDGM will not apply to most UV point-of-use or point-of-entry (POE) reactors, especially those that meet the Class B requirements of NSF/ANSI Standard 55. The concern about the differences between the documents is with UV POE Class A devices that could be applied to small systems.
The significant differences between the requirements of UVDGM and NSF/ANSI Standard 55 are the scope and application of the respective documents.
First, NSF/ANSI Standard 55 defines its scope to be exclusive to private residences based on its definition of POE, while the UVDGM is designed for validation of UV reactors used for larger community systems. Another critical difference between NSF 55 and the UVDGM is that under the LT2 rule, each UV reactor’s make and mode must be validated. The LT2 rule does not allow engineering scaling in situations where the use of a single UV reactor validation is applied to a “family” of products. A separate UV reactor validation and report must be performed on each model. Other prominent differences in requirements include the following:
- The UVDGM requires validation testing to account for non-uniform lamp aging. There are several ways to to do this. One approach to confirm uniform lamp aging is by looking at data or performing research on specific lamps. Once it is confirmed, a new lamp’s power can be turned down to simulate an aged lamp. Another approach is to use an aged (near end life) lamp for the validation test.
- Each UV reactor model must be tested at the minimum and maximum design flow rates and at least one other flow rate in between.
- The challenge tests may be performed with microorganisms other than MS2 coliphage, whose UV sensitivity is similar to that of Cryptosporidium and Giardia.
- UV reactors undergo bioassay tests with water at the minimum and maximum levels of UV transmittance.
- Duty sensors are also evaluated under the UVDGM and compared to the reference sensors.
- There is no established set point, and validation may be performed over a range of UV doses.
- A detailed validation report must be issued that defines the parameters of the test and quality control data for each test including bioassays, control and trip blanks, flowmeters, and the calibration of UV absorbance and other measurement instruments.
- A reduction equivalent dose is calculated by taking into account safety factors such as microorganism UV sensitivity, bias sensor uncertainty and collimated beam test uncertainties.
Very Small Systems
The differences between the NSF/ANSI Standard 55 for Class A devices and the UVDGM suggest that NSF/ANSI Standard 55 would not meet the LT2 rule’s UVDGM. There are many very small systems (VSS), however, that are not covered under the SDWA definition of 25 people or greater. VSS are regulated either by state, local or provincial agencies. Many of these agencies are uncertain as to what standard or protocol to use in validating UV reactors. Many look to the UVDGM for guidance.
Because the SDWA does not apply to VSS and the NSF/ANSI Standard 55 is restricted to single residences, there is a need for a standardized protocol to evaluate the performance of UV devices designed for VSS application. There are many different paths to address the need for a UV validation standard or protocol for VSS application. This may require some changes to existing standards or protocols to address VSS and drinking water regulators’ concerns. It also may involve the application of other standards such as the German DVGW Standard W294. The creation of a new standard or protocol could also be considered. NSF looks forward to hearing from stakeholders regarding this issue and their suggestions to address this public health concern.