Airing it Out: Design Considerations for UV Disinfection Installations

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The use of ultraviolet light for disinfection in drinking water treatment has become increasingly prevalent since the findings of Clancy et al. were published in 1998. This increased use has been helped by the promulgation of the Long Term 2 Enhanced Surface Water Treatment Rule and the requirement for additional Cryptosporidium inactivation for certain source waters. UV disinfection use has also been aided by the increased awareness of endocrine disruptors and potential for advanced oxidation to deal with these compounds. As a result of this increased awareness many utilities are considering the use of ultraviolet light (UV) disinfection at their facilities. When considering how to implement UV disinfection at a facility, there are several design areas that should be considered to ensure a successful installation. One such area is the impact of air on the operation of UV disinfection equipment.

Whether it is a retrofit or a new facility, addressing the potential for air entrainment is a critical component in the successful design of a UV disinfection system. The purpose of this paper is to highlight some of the lessons learned from recent UV disinfection installations relative to air entrainment. Some of the highlights include:

• Review the piping configuration and think critically about where to install air release valves. Often times, even when you think you have enough air release valves in the system, it’s probably not enough. The vents on the reactors are not intended to be used as the primary method of getting air out of the reactor / piping network, and relying solely on these vents can lead to problems in reactor operation.

• Review the piping configuration and think critically about where to install air release valves. Often times, even when you think you have enough air release valves in the system, it’s probably not enough. The vents on the reactors are not intended to be used as the primary method of getting air out of the reactor / piping network, and relying solely on these vents can lead to problems in reactor operation.

• Avoid installing UV reactors at high points in the piping network. If you absolutely must install UV at the high point, be sure any reducing fittings (if used) are of the eccentric type such that air will be able to pass through the system and not get trapped in the reactor.

• Remember that medium-pressure reactors have temperature probes that are very sensitive to the presence of air. The temperature probes are typically installed on the top of the reactor, which is also where entrained air will often collect, which causes the temperature probe to issue a “high temperature” alarm and shut down the unit.

• Reactor tilt is a critical installation issue. While not always clear from the manufacturers’ literature, the reactors are meant to be installed at a slight angle such that the vents are at the highest point in the reactor. If the vents are not at the high point, they will not be able to help to vent air out of the system (which can lead to high temperature alarms and other problems). Working with installing contractors to ensure the reactors are installed correctly is of utmost importance.

By being cognizant of these and other issues associated with air entrainment in UV disinfection systems, utilities considering the use of this technology will have a more successful installation.

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