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When the U.S. Environmental Protection Agency (EPA) embarked on the long process of developing the Long Term 2 Enhanced Surface Water Treatment Rule (LT2), which was published in draft form in June 2003 and in final form in January 2006, it was determined at an early stage that guidance manuals would be produced for the various ‘approved’ treatment technologies. Because ultraviolet (UV) disinfection is one of the principal approved treatment technologies in the LT2, a major effort was undertaken to write the Ultraviolet Disinfection Guidance Manual (UVDGM) as a guide for utilities, UV equipment manufacturers and engineering consultants in the application of the LT2 rules. The UVDGM was issued in draft form in June 2003 and in final form in November 2006. This review will follow the chapter outline in the UVDGM.
The LT2 Rule targets drinking water utilities that rely on surface water as their water source. This rule specifically is aimed at protecting drinking water from the threat of protozoan pathogens such as Cryptosporidium parvum and Giardia lamblia. The approach taken in this rule is unique in that the rule is allied on a site-specific risk basis. Starting in late 2006, all public water systems (PWSs) serving more than 100,000 people were required to test their source water for Cryptosporidium. Based on the results of this testing, PWSs that have filtered systems will be placed in one of four “bins,” depending on the concentration of Cryptosporidium oocysts found in their source water.
If the concentration is very low (less than 7.5 oocysts per 100 L), the PWS would be placed in Bin 1, which requires no additional treatment, since the EPA has determined that the Cryptosporidium oocysts concentration found represents a very low risk for someone being infected after drinking the water. For PWSs that have conventional filtration equipment and find Cryptosporidium oocysts concentrations greater than 300 oocysts per 100 L (Bin 4), the utility is required to provide additional treatment such as to assure 2.5 logs additional credit for treatment of Cryptosporidium. Bins 2 and 3 require implementation of treatment that will assure up to 1 and 2 logs of additional credit, respectively. For PWSs that use unfiltered water, if the concentration of Cryptosporidium oocysts is less than 1 oocyst per 100 L, they still must provide additional treatment such as assuring 2 logs of additional credit for treatment of Cryptosporidium. If the concentration is higher than 1.0 oocysts per 100 L, the additional treatment credit must be 3 logs. For PWSs serving fewer than 10,000 people, Cryptosporidium monitoring does not have to start until 2010.
Prior to 1998, it was widely perceived that UV treatment was ineffective in treating Cryptosporidium or Giardia. However, in June 1998, I presented a paper on Cryptosporidium at the American Water Works Association’s annual conference in Dallas, Texas. This paper demonstrated for the first time that, rather than being among the most UV-insensitive, Cryptosporidium oocysts are among the most UV-sensitive of all microorganisms. Other researchers corroborated this finding as well. In addition, it was found that Giardia is also very UV-sensitive. The EPA evaluated all of the UV research on Cryptosporidium, Giardia and viruses, and has recommended UV dose targets for various log inactivation credits (See Table 1). The UV dose targets for viruses are based on research that shows that adenovirus is very insensitive to UV inactivation. All other viruses that have been studied have UV sensitivities such that a UV dose of 40 mJ/cm2 will assure more than 4 logs inactivation.
The UVDGM has been through an extensive process of writing and vetting by experts and receiving comments from stakeholders. This process paid off because the document is comprehensive and relatively easy to read and understand. Fortunately, it has been considerably simplified from the draft version that was issued in 2003. The UVDGM is organized into six chapters and seven appendices. The practical “guidance” has been largely put into the chapters with technical background relegated to the appendices. The rest of this review will follow the chapter outline in the UVDGM.
This chapter contains an excellent summary of the LT2 requirements, monitoring and reporting requirements and a timetable for implementation. It is interesting that although the UVDGM has been written in terms of “conventional” UV disinfection using mercury discharge lamps, the EPA does acknowledge that other light sources, such as pulse lamps and excimer lamps, may be of importance in the future.
After a brief overview of the history of UV disinfection and a summary of the nature, generation and transmission of UV light, this chapter lays out an excellent discussion of the microbial response to UV light and the mechanism of UV disinfection, including microbial repair (both dark and light induced). The importance of the UV dose distribution in a UV reactor is examined, along with a discussion of the influence of the microbial action spectrum on the effective UV dose delivered by a UV reactor. The chapter continues with a discussion of various types of UV reactors, UV lamps and their characteristics, ballasts, sleeves, cleaning systems, UV sensors and various other sensors (e.g., UVT analyzers, temperature sensors). It considers the effects of water quality, including the transmittance of the water, particle counts and upstream water treatment processes on UV disinfection efficiency. Finally, this chapter discusses possible byproducts of UV disinfection, including nitrite formation and findings that show UV disinfection has a negligible effect on the formation of disinfection byproducts such as trihalomethanes and haloacetic acids.
This chapter is devoted to outlining the essential site and facility information necessary to lay out the general design for a UV installation. This includes evaluating where the UV facility should be located in the treatment train, defining key design parameters, evaluating UV reactors and UV dose monitoring strategies, assessing how UV equipment should be validated, and finally considering other issues, such as head loss constraints, the UV facility footprint, estimating costs and reporting to the state. Most of the discussion relates to PWSs that use filtered water, but there is also some discussion of the issues associated with the use of unfiltered source water.
This chapter considers the factors necessary to put together a specific detailed design specification for a UV facility. These factors include hydraulics, selection of the operating approach, instrumentation and control, electrical power and back-up power and UV facility layout. The chapter gives examples of specific UV equipment specifications and final UV facility designs. Finally, it discusses the process of reporting to the state during the design stage.
This chapter is probably the most important in the UVDGM because it provides guidance as to how UV reactors should be validated. The guidance here is very detailed and considers many sources of error and inefficiency in the validation process. It is important to realize that the UV doses given in Table 1 are only applicable to a “perfect” UV reactor. Such reactors do not exist. To account for the fact that UV reactors are imperfect and that there are errors associated with the validation process, the EPA has introduced a term called the validation factor (VF). The UV dose obtained from the validation testing must be divided by the VF and then compared with the UV doses in Table 1 to ascertain whether or not the UV reactor meets the UV dose criteria. The VF replaces the complicated set of “bias factors” that were contained in the 2003 draft UVDGM.
In my opinion, this replacement is a great improvement. Only two factors enter into the VF: the RED bias factor that accounts for the influence of non-germicidal light on UV sensor readings and an uncertainty factor that accounts for the various errors (both random and systematic) associated with the validation process. Detailed descriptions and examples are given as to how to calculate the VF. This chapter ends with a useful section on documentation and checklists.
This chapter is the last in the main section of the UVDGM and deals with the practical issues of start-up, operation and maintenance of UV facilities, including what should go into an operation and maintenance manual, the monitoring and recording of UV facility operations, reporting to the state, operational challenges, and finally staffing, training and safety issues. Of particular importance is the section on UV sensors and how they must be calibrated.
The appendices contain a wealth of technical information and protocols, such as preparing and assaying challenge microorganisms (Appendix A), examples of UV reactor testing (Appendix B), protocols for collimated beam testing (Appendix C), background to the UV reactor validation protocols (Appendix D) and UV lamp breakage issues (Appendix E). Appendix F contains a very useful summary of specific case studies, and Appendix G gives reduction equivalent dose bias tables.
It was difficult to find much to criticize in the final version of UVDGM. In some places, the UVDGM does not do a good job explaining terms and units. This is particularly true in explaining the concepts of absorbance (symbol A and unitless) and absorption coefficient (symbol a units of cm–1). These two terms are confused and often interchanged. UV intensity is incorrectly defined as “the power passing through a unit area perpendicular to the direction of propagation,” when in fact “UV intensity” is used in the UVDGM as a replacement term for “irradiance” or “fluence rate,” which both have very different definitions than those given in the UVDGM. The EPA made a conscious decision to use the term “UV intensity” instead of the internationally recognized terms of “irradiance” and “fluence rate.” I have no qualms with that choice, but they should have provided a clear definition and shown how it related to the internationally accepted terms. However, these are minor criticisms and should not detract from the excellent job done. It is certain that the UVDGM will have a major impact on the implementation, use and maintenance of UV facilities, not only in the U.S. but also around the world.
This is an extended version of the article, which appeared in the 2007 Water Disinfection supplement of Water Quality Products.