Volume 44 Number 6
Ultraviolet Light: An Alternative Disinfectant
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According to the Centers for Disease Control and Prevention (CDC), disinfection of public water supplies is recognized as one of the greatest achievements in public health during the 20th century. The most commonly used disinfectant in the United States is free chlorine. Unfortunately, chlorine efficacy is highly dependent on water quality parameters. In addition, chlorine combines with natural organics and manmade contaminants in some source waters to form potentially harmful disinfection by-products (DBPs). Perhaps most daunting is the known resistance of certain human pathogenic microbes to conventional chlorine treatment, especially protozoa, i.e., Cryptosporidium and Giardia (see “On Tap,” WC&P, April 2002).
Over the last decade, the emergence of Cryptosporidium as a major waterborne pathogen has refocused attention on the need for alternative disinfectants. Cryptosporidia have an environmentally resistant oocyst stage and are highly robust. Conventional methods of water treatment frequently fail to remove or inactivate oocysts from source waters. Their small size and low infectious dose have added to the challenge that these organisms pose to water utilities and consumers.
Issues of emerging waterborne pathogens are disconcerting to consumers and the water treatment industry alike. Advances in treatment technologies, however, continue to develop that minimize the impact of newly identified pathogens. Although ultraviolet (UV) light irradiation isn’t a new technology, it’s being considered for increased application in the water treatment industry. The advantage of UV over other disinfectants is it can inactivate hard to control organisms without chemical additions to the final product that can alter taste and without producing harmful DBPs.
How does it work?
UV irradiation is typically produced by electrically powered quartz lamps following electron flow through mercury gas in the lamps. UV light is distributed by low-pressure lamps emitting at wavelengths of approximately 254 nm; medium pressure lamps emitting at wavelengths from 180 nm to 1,370 nm; or lamps that emit at other wavelengths in a high intensity pulsed manner. To increase dosage -- usually defined as UV intensity × time over a specified area -- additional lamps are used or exposure times increased. This dosage measure translates to milliWatt seconds per square centimeter (mW-sec/cm2), equal to milliJoules per square centimeter (mJ/cm2). You may also see microWatt-seconds per square centimeter (µW-sec/cm2) -- 1 mJ/cm2 equals 1,000 µW-sec/cm2. In a pulsed UV reactor, capacitors build up and deliver electricity in pulses to xenon flash tubes in the center of a 2-inch diameter flash chamber through which the water flows. High intensity UV irradiation (75 mW/cm2) is emitted and can be adjusted by altering the frequency of pulsing.3 Multiple UV reactor designs, including flow-through systems, appear to be effective for protozoa disinfection and applicable to drinking water treatment operations.5
The germicidal effect of UV light is due to the photochemical damage to the DNA or RNA of the organism. Following UV irradiation, thymine dimers occur, hooking nucleic acid strands together and preventing reproduction of the organism. It’s the nature of UV irradiation to inactivate a constant fraction of the microbial population during each progressive increment of time. Therefore, there’s a direct and proportional relationship between the UV dose and microbial response. As such, high intensity UV energy applied for shorter periods of time is equally as effective as low intensity UV energy applied for longer periods of time.
Viability vs. infectivity
In the evaluation of previous studies and for future studies, it’s important to distinguish between viability and infectivity and realize the public health implications of each. More research is needed to look at rapid methods of protozoa infectivity detection for increased monitoring of water quality and disinfection efficacy. For example, cell culture assays are reported as being equivalent to the animal assays, and an appropriate alternative, for determining the infectivity of Cryptosporidium parvum6 and molecular methods, used in conjunction with cultural methods, promise to enhance detection speed.
Advantages & disadvantages
There are some potential disadvantages, though, as well. UV irradiation isn’t only absorbed by DNA but also by proteins. This characteristic can cause variable results with UV disinfection efficacy among different organism. For example, single-strand RNA viruses (i.e., poliovirus) are much more susceptible to UV irradiation than double-strand DNA viruses (i.e., enteric adenoviruses). In addition, certain microbial agents, especially bacteria, and possibly adenoviruses, are capable of directly or indirectly repairing the damage caused by irradiation and reverting back to a viable state.1,8 This phenomenon, known as photoreactivation, usually occurs following exposure to sunlight. The extent of reactivation varies among microbes but appears to be minimized by shielding the treated water from sunlight until a time that the DNA damage is irreversible. Although photoreactivation has been shown not to affect infectivity of UV irradiated Cryptosporidium, more studies are needed to evaluate a wide range of pathogens and the effect that varying UV dose has on a variety of microbes and time limits required after which photoreactivation can no longer occur.
A number of factors can alter the rate of UV absorption and decrease the efficacy of the disinfectant. These factors are often site specific and thus calculations of time and intensity required for treatment aren’t applicable to sites with different water quality parameters. The presence of dissolved or suspended matter, including chemical and dissolved organics or inorganics, can protect microbes from UV irradiation. Likewise, turbidity, color, or clumping of microbes can all affect UV disinfection efficiency. In addition, poor flow conditions in UV reactors can create dead spaces where inadequate disinfection occurs. Finally, operational considerations for optimum UV efficacy must include monitoring of diminished output over time due to either aging of the lamp -- average life of low pressure UV lamps is 8,800 hours -- or fouling of the lamp surface due to scaling or biofilm. Proper maintenance and lamp cleaning can easily remedy these potential problems.
With the absence of residual disinfectant in UV treated water, the water is subject to recontamination via the distribution system, prior to consumption. In addition, biofilm formation and coliform regrowth may occur. To address this problem, a secondary chemical disinfectant could be used to maintain a residual during distribution. Although recent epidemiological studies suggest a significant level of endemic waterborne gastrointestinal illness could be due to intrusions in the distribution system, residual chlorine was determined to have little impact on protozoan and viral pathogens reintroduced in the distribution system.9
A promising future
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