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The use of ultraviolet (UV) light has become standard practice in most industrial processes. If the water is used as a product or as part of the process, spoilage organisms cause nuisance and potential harm to people or stock, and a product recall can lead to lasting damage to market reputation.
UV light was first discovered in 1801. It took more than 100 years to develop lamps and power supplies to allow the application of the technology on an industrial scale when a drinking water plant treating 0.2 mgd of potable water briefly used UV light in Marseilles, France, in 1910.
The general application of chlorine to drinking water after World War I led to UV only being used in a municipal application where chlorine could not be used.
UV light was routinely used to address vitamin deficiencies caused by poor diets during the war years in Europe, and in 1937 the directors of Sherwood Colliery, Mansfield, England, installed a “Light Corridor” that miners walked down after showering and before dressing, as a means of providing artificial sunlight to those working underground to overcome vitamin deficiencies.
UV light was successfully used for medical and therapeutic applications from the 1940s until the mid 1980s, when clinical drugs became widely available and were able to address root causes rather than merely dealing with a symptom.
Industrially, the use of UV light developed as a number of process users of water, such as breweries and pharmaceutical plants, adopted UV as a means of extending submicron filter runs or preventing spoilage.
In many industries, UV was the only available means of ensuring the water was free from harmful organisms, such as fish farms, crab grow-outs or shellfish depuration. Early UV systems lacked meaningful UV monitors, optical fouling was not easy to remove and high prices meant the technology did not gain widespread acceptance until the 1970s.
By 2000, a number of key drivers had led to the technology being readily incorporated into most process users’ water treatment facilities. The main drivers were the flight from chemical-based regimes, pressure to extend shelf life, increasing immunity to conventional disinfectants by emerging pathogens and increased chlorine intolerance of many membranes.
The use of UV today has been extended to disinfect brines, marinades and fruit juices, and increasingly expensive water has led many users to reuse and reclaim their discharges or to partner with a local municipal water provider and accept municipal reuse for industrial and other nonpotable applications.
The use of UV is now extended to the dechlorination and dechloramination of water, as well as selectively removing a wide variety of endocrine disrupting chemical compounds, pesticides and other contaminants such as N-Nitrosodimethylamine.
How does it Work?
UV light is a nonintrusive method of ensuring that organisms in water, wastewater and other process fluids are rendered inert. Light at 260 nm has the ability to cause permanent damage to the DNA contained in all living species. The damage to the DNA occurs in a few picoseconds and is permanent at a UV dose of 40 mJcm-2, which is a usual UV dose.
No organisms can demonstrate any immunity to UV; however, a number of organisms such as Listeria Monocytogenes, Cryptosporidium Parvuum and Giardia are now chlorine immune, with others demonstrating increased tolerance. The incidence of organisms such as methicillin-resistant staphylococcus aureus becoming tolerant to antibiotics is well documented.
UV does not alter the pH, color, taste, odor or the physical property of water. While this is one of its main benefits, it can also be a crucial drawback. UV offers no residual protection, and the fluid can be subsequently reinfected if the plant design or housekeeping is poor.
Pipe dead legs, biofilm and other process barriers such as GAC contactors or submicron filters can all be microbial breeding grounds. The technology should be part of a multibarrier approach, and plant operators should periodically sanitize with steam or a chemical clean-in-place regime.
The UV light is created by a low-pressure, high-output lamp, or by a medium-pressure lamp. The low-pressure, high-output lamps use an amalgam of mercury to produce a single (monochromatic) line output at 254 nm. These lamps operate independently of fluid temperature, and their main advantage is the electrical efficiency, which can be up to 40%. Medium-pressure lamps are used where footprint is more important than electrical efficiency. These types of lamps are 12% to 15% efficient, and the large amount of heat (infrared) energy emitted causes significant fouling issues.
The large amount of visible light also emitted does cause algal growth, which can be an issue when using UV to dechlorinate reuse water. The medium-pressure lamps are polychromatic, and produce a continuous spectral output from 190 nm to the long-wavelength visible light and infrared parts of the spectrum.
Electronic ballasts are used to continuously vary the lamp output, compensate for lamp aging and ensure that a uniform UV dose is delivered to the fluid.
Wipers are used to keep the optical path free from fouling. Iron and calcium will foul the quartz sleeves, and often these mechanical wiping systems need chemical assistance to clean the quartz. A UV monitor camera is used to continuously measure the amount of UV light being emitted from the lamp and through the quartz sleeve.
One of the newer applications for UV light is the dechlorination of potable or reuse water that is increasingly being supplied as a lower cost alternative to potable water. The local municipal water provider will usually add chlorine or chloramines to ensure the reuse water remains free from pathogens. Chemical dechlorination regimes often lead to fouling of RO membranes and have fallen out of favor.
UV light is able to render organisms nonviable by breaking bonds in their DNA so selective wavelengths are able to cleave the bonds in the chlorine and chloramine species. This type of photolysis also delivers a high disinfection dose, which is a secondary benefit.
The use of UV in swimming pools is well documented and standard practice, with hundreds of pools in the U.S., Europe and Asia now using UV systems.
A dose of 90 mJcm-2 is normally required to sufficiently reduce the combined chlorine (chloramine) load, and it also ensures high removal of any waterborne organisms. As with chloramines, the mechanisms for the removal of chlorine depend on a number of factors such as water pH, water chemistry and the level of removal required.
UV has been successfully used to remove ozone from ultrapure water systems for many years, and as new contaminants such as Urea emerge, the technology will play an important role, often when used in conjunction with a catalyst or accelerant.
When used to dechlorinate poor transmittance reuse water, UV light can cause algal growth inside pipelines. This is caused by the system geometry permitting long-wavelength light to travel extended distances. As the penetration depth increases, all of the germicidal light will be absorbed by the fluid, leaving visible light that stimulates algal growth. This problem can be overcome by modifying the chamber geometry to prevent the passage of long wavelength visible light or by treating the immediate connecting pipe walls.
The algae growth can become so profound that large clumps break off and can actually break the quartz sleeves. This problem can be overcome, but illustrates the selection of a qualified vendor and a degree of application understanding.
Water will continue to be used in a very wide variety of industrial processes, and as it becomes more expensive, industrial users will be encouraged to reuse their waste. The flight from chemical regimes will continue—driven by costs and risks (both terrorist attacks and disinfection byproducts)—so UV will continue to gain popularity. Energy will become a far more urgent issue, and energy-efficient designs and lamps will gain favor over less efficient technologies.