By Bruce Laing
History—UV is not a new technology UV disinfection is an established technology supported by decades of use in applications from drug manufacturing to wastewater treatment. The germicidal properties of sunlight were discovered by Downes and Blunt (1877). Once it was understood that UV light was the wavelength responsible for this germicidal activity, the development of mercury lamps as artificial UV light sources in 1901, and the use of quartz as a UV transmitting material in 1906, paved the way for the technology to be developed and used in a controlled and meaningful way. The first drinking water disinfection application took place in Marseilles, France in 1910, and considerable research on the mechanisms of UV disinfection and the inactivation of microorganisms has since been completed.
Why use UV?
UV is effective at inactivating bacteria, viruses and protozoa, such as Cryptosporidium and Giardia, which may be present in water supplies from potentially all sources. Many people believe that well water is pristine, glacier water is pure and municipal water is treated to safety standards specified by regulatory bodies. While all of this is generally true, even these ’good’ sources of water may be contaminated. Groundwater quality can be degraded by failing septic systems, animal farms and many other sources. Groundwater in aquifers is continuously moving, which results in variable quality. It can test good today but fail tomorrow. People who fall sick tend to blame it on the food they ate or some other explanation because they believe their water is safe. Areas where microorganism contaminants are highest are at the base of mountains where pure water from mountain streams is collected. Yet the water picks up contaminants on its journey down the mountain that can create the need for disinfection.
Cryptosporidium and Giardia protozoa are more evident in drinking water supplies. The occurrence of Cryptosporidium parvum in drinking water sources is recognized as a significant threat to private and public water supplies throughout the world (Rose et al., 1991; Lisle and Rose, 1995; Messner and Wolpert, 2000). Water treatment plants usually cannot guarantee the removal of all Cryptosporidium because oocysts are very small (four to five micrometers in diameter), and are resistant to chlorine and most other disinfectants.1 It is for this reason that many municipal treatment plants are installing UV systems.
A report published by the US EPA2 indicates that,“… Cryptosporidium is not only a surface water problem.” In Canada and the US, 60.2 percent of surface water samples contained oocysts in a study done by LeChevallier and Norton in 1995. The same report also cites a study done by Hancock et al. (1998), reporting a study of 199 ground water samples tested for Cryptosporidium. They found that five percent of vertical wells, 20 percent of springs, 50 percent of infiltration galleries, and 45 percent of horizontal wells contained Cryptosporidium oocysts. The significance of this is that normal water testing does not test groundwater for oocysts.
The most challenging water source is the dug well, where runoff is an issue. As runoff enters the well, it can carry with it such contaminants as surface animal waste and septic drainage from the aquifer. Municipal water can be deemed perfectly safe when it leaves the treatment plant. Nonetheless, Boil Water Alerts (BWA) happen frequently as a result of the unexpected. Many times the BWA is issued 24 to 48 hours after the contamination is detected. Residential POE UV can be a primary barrier to protect people from contamination in a well that has failed a water test, and it can be inexpensive insurance to others who think water is safe all the time, but want to be sure that their family is fully protected.
How does it work?
Ultraviolet light, in the 200 to 300-nm (UV-C) range, is most effective at destroying bacteria and viruses by altering their DNA. This natural, non-chemical method of treatment penetrates and permanently alters the DNA of the microorganisms in a process called thymine dimerization. The microorganisms are inactivated and rendered unable to reproduce or infect. Typically, UV light is generated by applying a voltage through a gas mixture, resulting in a discharge of photons. Nearly all UV lamps currently designed for water treatment use a gas mixture containing mercury vapor. Mercury gas is advantageous for UV disinfection applications because it emits light in the germicidal wavelength range. The UV light output from mercury-based lamps depends on the concentration of mercury atoms, which is directly related to the mercury vapor pressure. Low-pressure (LP) UV lamps contain mercury at low vapor pressure, which produces primarily UV light at 253.7 nm. Variations of LP lamps are low-pressure/high-output (LPHO) and amalgam UV lamps, which operate at higher current and use different gas mixtures to increase the amount of germicidal UV that is emitted. An easy way to think of the relative lamp power is that LPHO produces approximately twice as much UV light as an LP lamp, and an amalgam lamp produces about four times as much UV light as an LP lamp. Higher-output UV lamps allow reactors to be smaller and higher flowrates to be treated with one lamp.
Factors affecting UV performance
Once UV light is generated, it begins its journey toward the microorganisms that are its intended target. Along the way, a certain amount of UV energy is lost due to absorption and scattering of the light.
Absorption happens in two significant ways. First, a certain amount of the UV light is absorbed by the quartz lamp envelope and sleeve. It is important to be sure UV manufacturers are using high-quality quartz, and that lamps and sleeves are replaced with OEM parts to ensure consistent performance of the system. Second, minerals like iron and manganese, and organic compounds (like tannins from decayed organic material in the water) will absorb UV light, reducing the transmitted energy and its ability to alter DNA in microorganisms.
Scattering is primarily caused by particles in the water, which can cause shadowing. Some microorganisms may pass through the UV reactor without receiving enough UV exposure to inactivate them.
Water quality (hardness and iron)
Depending on the concentrations of the minerals, the water’s hardness can also affect the performance of the UV system, as water hardness can cause scale to form on lamp sleeves. This can reduce UV light transmission and the inactivation of pathogens.
The combination of the amount of UV energy and water flowrate determine contact time (CT) or UV dose. Each microorganism has a different susceptibility to UV light and therefore, requires a different dose to be inactivated. To achieve successful disinfection, the equipment capacity must match the target microorganism’s UV dose requirement. Microorganism susceptibility to UV Numerous studies have been done to determine the UV dose needed to inactivate a specific microorganism. Table 1 provides a sample of this valuable information. Many universities and research companies continue to add to the literature as new information is discovered. (Log reductions are based on UV equipment flowrate specs.)
Summary of water quality parameters
Listed below are common water quality parameters that ensure the best performance from a UV system in a residential or commercial application. If the water is outside these parameters,
a water treatment dealer should be contacted for equipment recommendation.
- < 0.3 ppm iron
- < 7 grains per gallon (120 ppm) hardness
- > 75-percent UVT (UV transmission through water sample) UV light advantages
- Low lifetime ownership cost
- No moving parts
- Minimal maintenance requirements
- No DBPs, which can occur using chlorine
- No risk to children from chlorine storage at home
- No change to the taste or odor of the water
- No handling or mixing of chemicals
Carefully review the specifications and be sure the system is capable of operating in a UVT range that your application may encounter. Be sure the system will provide a minimum dose of 30 mJ/cm2 at the end of the lamp life, and pay attention to the UVT at which this dose is quoted. A dose quoted at 85-percent UVT is safer to use than the same dose at 95 percent, because there is a wider safety factor built in for lower UVT water. NSF Standard 55 Class A certified systems are tested at 70-percent UVT or lower, and must provide 40 mJ/cm2 to meet the requirements for certification. If you compare UV systems, keep in mind this is a science; similar lamp power and water layers with similar chamber designs should receive similar UV dose. Refer to industry specs to determine if what you are reading is too good to be true.
How to sell UV
As a water treatment dealer or contractor selling a system, it is important to follow manufacturer’s recommendations when selecting the correct size of the UV system for your application. Consider factors like flowrate and UVT of the water. Since many people do not know this parameter, set yourself apart and purchase a UVT meter to help design the right UV system for the client. For those who do not want to go to that expense, be sure to select a system that will be capable of delivering a minimum of 30 mJ/cm2 at the end of lamp life on 85-percent UVT water. It is important to test the water weekly for three or four weeks after installation to ensure that the installation was done properly. If the installation is a commercial or institutional application, it is always a good idea to recommend a spare lamp and sleeve be kept onsite. This will come in handy when there is a lamp failure during a time when you cannot source another one immediately. The sleeve is good to have in the event it is accidentally broken during cleaning.
The future of UV in residential applications
Over the next three to five years, technological improvements to UV systems will change the way UV light is created and delivered to the water. Improvements to reduce maintenance requirements even further will make this technology the most attractive option for anyone wanting to ensure their family has safe, reliable drinking water.
- Khan, Omar A. Water Health Advisory: A Review of Cryptosporidiosis. Johns Hopkins University School of Public Health.
- US EPA report. Cryptosporidium: Drinking Water Health Advisory. EPA-822-R-01-009, March 2001.
- US EPA report. Ultraviolet Disinfection Guidance Manual for the Final
Long Term 2 Enhanced Surface Water Treatment Rule. EPA 815-R-06-007, November 2006.
About the author
Bruce Laing is Sales Strategy Manager at VIQUA– a Trojan Technologies Company. During his 12 years with VIQUA, he has been involved in many facets of product and standards development. Laing has served as the Chairperson of the NSF Standard 55 UV Task Group for the past nine years, and works closely with many industry leaders on the NSF Joint Committee. He is also a member of the CSA B483.1 Committee. Over the past 11 years, Laing has presented eight times at WQA conventions on various topics, and has written many articles for industry publications. Please contact him at [email protected].