Ultraviolet germicidal irradiation (UVGI ) is a disinfection method that uses short-wavelength ultraviolet (UV-C) light to kill or deactivate microorganisms by destroying nucleic acids and disrupting their DNA, making they can not perform important cellular functions. UVGI is used in many applications, such as food, air, and water purification.
UV-C light is weak on Earth's surface because the ozone layer in the atmosphere is blocking. UVGI devices can produce strong UV-C light in air circulation or water systems to make them unfriendly to microorganisms such as bacteria, viruses, molds and other pathogens. UVGI can be combined with a filtering system to clean air and water.
The UVGI application for disinfection has been a accepted practice since the mid-20th century. It has been used primarily in medical sanitation and sterile work facilities. More is used to sterilize drinking and wastewater, as storage facilities are closed and can be circulated to ensure higher exposure to UV. In recent years, UVGI has found new applications in air purifiers.
Video Ultraviolet germicidal irradiation
History
In 1878, Arthur Downes and Thomas P. Blunt published a paper describing the sterilization of bacteria exposed to short-wavelength light. UV has been known as mutagen at cellular level for more than 100 years. The 1903 Nobel Prize in Medicine was awarded to Niels Finsen for his use of UV against lupus vulgaris, tuberculosis of the skin.
Using UV light for disinfection of drinking water dates back to 1910 in Marseille, France. The prototype factory was closed after a short time due to poor reliability. In 1955, the UV water treatment system was implemented in Austria and Switzerland; in 1985 about 1,500 plants were employed in Europe. In 1998 it was found that protozoa such as cryptosporidium and giardia were more susceptible to UV light than previously thought; this opens the way to the widespread use of UV water treatment in North America. In 2001, more than 6,000 UV water treatment plants operate in Europe.
Over time, UV costs have decreased as researchers develop and use new UV methods to disinfect water and wastewater. Currently, some countries have developed regulations that allow the system to disinfect their drinking water supply with UV light.
Maps Ultraviolet germicidal irradiation
Operation method
UV light is electromagnetic radiation with a wavelength shorter than visible light. UV can be separated into various ranges, with short-wavelength UV (UVC) considered "UV germ". At certain wavelengths, UV is mutagenic to bacteria, viruses and other microorganisms. Particularly at a wavelength of about 260 nm-270 nm, the UV breaks the molecular bond in the DNA of the microorganism, producing a thymine dimer that can kill or deactivate the organism.
- The mercury-based lamp emits UV light on line 253,7Ã, nm.
- Ultraviolet light emitting diodes (UV-C LEDs) emit UV light at selectable wavelengths between 255 and 280 nm.
- Pulsed-xenon lamps emit UV light across the UV spectrum with peak emissions approaching 230nm.
This process is similar to a longer wavelength effect (UVB) that produces burning skin in humans. Microorganisms have less UV protection, and can not survive long on exposure.
The UVGI system is designed to expose environments such as water tanks, enclosed spaces and forced air systems to UV germs. The exposure comes from a germicidal light that emits UV germs at the right wavelength, thus illuminating the environment. Airflow or water imposed through this environment ensures exposure.
Effectiveness
The effectiveness of UV germs depends on the length of time the microorganisms are exposed to UV, the intensity and wavelength of UV radiation, the presence of particles that can protect microorganisms from UV, and the ability of microorganisms to withstand UV during exposure.
In many systems, redundancy in exposing microorganisms to UV is achieved by circulating air or water repeatedly. This ensures some graduations so that UV is effective against the highest number of microorganisms and will irradiate resistant microorganisms more than once to break them down.
"Sterilization" is often misquoted as achievable. While theoretically possible in a controlled environment, it is very difficult to prove and the term "disinfection" is commonly used by companies that offer this service to avoid legal strikes. The specialist company will often advertise a certain log deduction eg, 99.9999% effective, not sterilization. It considers a phenomenon known as light and dark repair (photoreactivation and improvement of base removal, respectively), in which cells can repair DNA that has been damaged by UV light.
The effectiveness of this form of disinfection depends on the exposure of X-rays from microorganisms to UV light. The environment in which the design creates barriers that block UV rays is ineffective. In such an environment, the effectiveness then depends on the placement of the UVGI system so that the line of sight is optimal for disinfection.
Dust and film coat the lower UV output bulb. Therefore, tubers require regular cleaning and replacement to ensure their effectiveness. Age of UV bulb varies depending on the design. Also, the material that the bulb is made of can absorb some germicidal rays.
Cooling the lamp under air flow can also decrease the UV output; thus, care should be taken to protect the lamp from direct air flow, or to add additional lights to offset the cooling effect.
Increased effectiveness and intensity of UV can be achieved by using reflection. Aluminum has the highest reflectivity level compared to other metals and is recommended when using UV.
One method for measuring UV effectiveness in water disinfection applications is to calculate UV doses. The US EPA issued UV dosage guidelines for water treatment applications. UV doses can not be measured directly but can be inferred based on known or estimated inputs to the process:
- Flow rate (contact time)
- Transmission (light reaches target)
- Turbidity (turbidity)
- Age of light or fouling or off (UV intensity reduction)
In air disinfection applications and surface UV effectiveness is estimated by calculating the UV dose to be delivered to the microbial population. UV dose is calculated as follows:
UV Dosage Ã,ÂμWs/cmÃ,² = UV Intensity Ã,ÂμW/cmÃ,² x Exposure time (sec)
UV intensity is determined for each lamp at a distance of 1 meter. The intensity of the UV is inversely proportional to the square of the distance thus decreasing at a greater distance. Alternatively, it quickly increases at a distance shorter than 1m. In the above formula the UV intensity should always be adjusted for distance unless the UV dose is calculated exactly 1m from the lamp. Also, to ensure effectiveness, UV doses should be calculated at the end of the lamp life (EOL is determined in the number of hours when the lamp is expected to reach 80% of the initial UV output) and at the furthest distance from the lamp at the edge of the target area. Some anti-breakable lamps are coated with a fluorized ethylene polymer to contain glass and mercury fragments in case of damage; this layer reduces the UV output by 20%.
To accurately predict which UV dose will be sent to the UV intensity target, adjusted for distance, coating and end of life of the lamp, will be multiplied by the time of exposure. In a static application, the exposure time may be as long as necessary for the effective UV dose to be achieved. In the case of fast-moving air, in air-conditioning ducts for example, the exposure time is short so the UV intensity should be increased by introducing some UV lamps or even light banks. In addition, UV mounting should be placed in the long straight channel section with the lights perpendicular to the airflow to maximize the exposure time.
This calculation actually predicts flu fluence and it is assumed that UV fluency will be equal to UV dose. UV doses are the amount of UV germ energy absorbed by the microbial population over a period of time. If the microorganism is planktonic (free-floating), the UV flu will be the same as the UV dose. However, if the microorganisms are protected by mechanical particles, such as dust and dirt, or have formed biofilms, a much higher UV flu would be required for effective UV doses to be introduced into the microbial population.
Inactivation of microorganisms
The rate of inactivation by ultraviolet radiation is directly related to the UV dose applied to water. Dosages, UV light intensity products and exposure times, are usually measured in microjoules per square centimeter, or equivalent as microwatt seconds per square centimeter (Ã,ÂμWÃ, Â · s/cm 2 ). The dose to kill 90% of most bacteria and viruses ranges from 2,000 to 8,000 Ã,ÂμWÃ, s/cm 2 . Larger parasites such as cryptosporidium require a lower dose for inactivation. As a result, the US Environmental Protection Agency has received UV disinfection as a method for drinking water plants to get cryptosporidium, giardia or viral inactivation credits. For example, for the reduction of one decimal-logarithm of cryptosporidium, a minimum dose of 2,500 ÂμWÃ, s/cm 2 is required under the US UPA Guide EPA Guidelines published in 2006.
Strengths and weaknesses
Benefits
UV water treatment equipment can be used for disinfection of well water and surface water. UV treatment is better than other water disinfection systems in terms of cost, labor, and the need for technically trained personnel for surgery. Water chlorization treats larger organisms and offers residual disinfection, but the system is expensive because they require specialized operator training and supply of potentially harmful materials. Finally, boiling water is the most reliable but labor-intensive treatment method, and it imposes a high economic cost. Rapid UV treatment and, in terms of primary energy use, is approximately 20,000 times more efficient than boiling.
Losses
UV disinfection is most effective for treating reverse osmosis distilled water with high clarity and purified. Suspended particles are a problem because the microorganisms buried inside the particles are protected from UV light and pass through unaffected units. However, UV systems can be paired with pre-filters to remove larger organisms that otherwise would pass through the unaffected UV system. Pre-filters also explain water to increase light transmission and hence UV doses throughout the water column. Another key factor of UV water treatment is the flow rate - if the flow is too high, water will pass without sufficient UV exposure. If the flow is too low, the heat can accumulate and damage the UV lamp.
The disadvantage of UVGI is that while treated water with chlorination is resistant to reinfection (up to chlorine gases), UVGI water is not resistant to reinfection. UVGI water must be transported or shipped in such a way as to avoid re-infection.
Security
In UVGI systems, lamps are protected or are in an environment that limits exposure, such as closed water tanks or closed air circulation systems, often with interlocks that automatically turn off UV lights if the system is opened for human access.
For humans, exposure of skin to the wavelength of germs from UV rays can result in sunburn and skin cancer. Eye exposure to UV radiation can produce very painful inflammation of the cornea and temporary or permanent vision disturbance, up to and including blindness in some cases. UV can damage the retina of the eye.
Another potential danger is the production of UV ozone, which can be harmful to health. The US Environmental Protection Agency assigned 0.05 parts per million (ppm) of ozone to a safe level. Lights are designed to release UVC and higher frequencies doped so that any UV light below 254 nm wavelength will not be released, to minimize ozone production. The full spectrum light will release all the UV wavelengths, and will produce ozone when the UVC molecule hit oxygen (O 2 ).
UV-C radiation is capable of breaking chemical bonds. This causes rapid plastic aging, insulation, gaskets, and other materials. Note that the plastic sold into "UV resistant" is only tested for UV-B, because UV-C usually does not reach Earth's surface. When UV is used near plastics, rubber, or insulation, care must be taken to protect these items; metal band or aluminum foil will suffice.
The American Conference of Governmental Industrial Hygienists (ACGIH) Committee on Physical Agents has established TLV for UV-C exposure to avoid skin and eye injuries among the most vulnerable. For 254Ã, nm UV, this TLV is 6 mJ/cmÃ,² over a period of eight hours. TLV function is different according to wavelength due to variable energy and cell damage potential. The TLV is supported by the International Commission on the Protection of Non-Ionized Radiation and is used in setting lighting standards by the Illuminating Engineering Society of North America. When TUSS was planned, and up until recently, the TLV was interpreted as if the eye exposure in the room was continuous for eight hours and at the highest level of eye radiation found in the room. Under extremely unlikely conditions, a dose of 6.0 mJ/cmÃ,² is achieved under ACGIH TLV after just eight hours of continuous exposure to radiation of 0.2? W/cmÃ,². So, 0.2? W/cmÃ,² is widely defined as the upper limit of permissible irradiation at eye level.
Usage
Air disinfection
UVGI can be used to disinfect air with long exposure. Disinfection is a function of intensity and UV time. For this reason, it is not effective for moving the air, or when the light is perpendicular to the flow, since the exposure time decreases dramatically. The UVGI air purification system can be a freestanding unit with shielded UV lamps that use a fan to force air through UV light. Another system is installed in the forced air system so that the circulation to where the microorganisms move through the lamp. The key to this form of sterilization is the placement of UV lamps and a good filtration system to remove dead microorganisms. For example, a forced air system with a design obstructs the line of sight, thus creating an environmental area to be shaded from UV rays. However, UV lamps placed in cooling system coils and cooling pans will keep the microorganisms formed in these natural moist areas.
ASHRAE includes UVGI and its applications in indoor air quality and building maintenance in "Ultraviolet Lamp Systems", Chapter 16 of the 2008 Handbook, HVAC Systems and Equipment . 2011 Handbook, HVAC Application , includes "Ultraviolet water and surface treatment" in Chapter 60.
Water disinfection
Ultraviolet water disinfection is a purely chemical-free physical process. Even parasites such as cryptosporidia or giardia , which are highly resistant to chemical disinfectants, are reduced efficiently. UV can also be used to remove chlorine and chloramine from water; this process is called photolysis, and requires a higher dose than normal disinfection. Sterilized microorganisms are not excreted from water. UV disinfection does not remove dissolved organic, inorganic compounds or particles in water. However, the UV-oxidation process can be used to simultaneously destroy traces of chemical contaminants and provide high-level disinfection, such as the largest indirect reuse plant in New York that opens the Catskill-Delaware Water Ultraviolet Disinfection Facility on October 8, 2013. A total of 56 UV-saving reactors energy installed to treat US $ 2.2 billion gallons (8.300,000 m 3 ) a day to serve New York City.
It used to be considered that UV disinfection is more effective for bacteria and viruses, which have more open genetic material, than larger pathogens that have an outer layer or that form a cyst state (eg, Giardia) that protects their DNA from UV rays. However, it has recently been found that ultraviolet radiation can be somewhat effective in treating Cryptosporidium microorganisms. These findings result in the use of UV radiation as a viable method for treating drinking water. Giardia in turn have proven to be highly susceptible to UV-C when tests are based on infectivity rather than excystation. It has been found that protists are able to survive high UV-C doses but are sterilized at low doses.
Developing countries
A 2006 project at the University of California, Berkeley produced a design for inexpensive water disinfection in resource-poor settings. The project is designed to produce open source designs that can be adapted to meet local conditions. In a somewhat similar proposal in 2014, Australian students designed the system using a chip foil package to reflect solar UV radiation into glass tubes that had to disinfect water without electricity.
Wastewater treatment
Ultraviolet in waste treatment usually replaces chlorination. This is largely due to concerns that the chlorine reaction with organic compounds in the wastewater stream can synthesize potentially toxic and long-term chlorinated organic and also because of the environmental risks of storing chlorine or chlorine gas containing chemicals. Individual waste that must be treated by UVGI should be tested to ensure that this method will be effective due to potential interference such as suspended solids, dyestuffs, or other substances that may block or absorb UV radiation. According to the World Health Organization, "UV units to treat small batches (1 to several liters) or low flow (1 to liters per minute) of water at community level are estimated to cost US $ 20 per megaliter, including electricity and consumables annual unit cost of capital. "
Large-scale urban UV sewage treatment is carried out in cities such as Edmonton, Alberta. The use of ultraviolet light has now become standard practice in most municipal wastewater treatment processes. Waste is now beginning to be recognized as a valuable resource, not a problem that needs to be discarded. Many wastewater facilities are renamed water reclamation facilities, whether wastewater discharged into rivers, used to irrigate crops, or injected into aquifers for later recovery. Ultraviolet light is now used to ensure free water from harmful organisms.
Aquarium and pond
Ultraviolet sterilization is often used to help control undesirable microorganisms in aquariums and ponds. UV radiation ensures that the pathogen can not reproduce, thus reducing the possibility of an outbreak of disease in the aquarium.
Aquarium sterilization and ponds are usually small, with tube fittings that allow water to flow through the sterilizer on the way from separate external filters or water pumps. Inside the sterilizer, water flows as close to the source of ultraviolet light as possible. Pre-filtration water is essential because water turbidity decreases UVC penetration. Much better UV sterilization has long stalled and limited the space between the UVC source and the inner wall of the UV sterilizing device.
Laboratory cleanliness
UVGI is often used to disinfect equipment such as safety glasses, instruments, pipettors, and other devices. Laboratory personnel also sterilize glass and plastic equipment in this way. The microbiology laboratory uses UVGI to disinfect surfaces inside biological safety cabinets ("hoods") between usages.
Food and beverage protection
Since the US Food and Drug Administration issued a ruling in 2001 that requires almost all producers of fruit and vegetable juices to follow HACCP controls, and require a 5-log reduction of pathogens, UVGI has seen some use in juice sterilization such as Apple Apple Vinegar apples.
Technology
Lights
UV germs for disinfection are usually produced by mercury vapor lamps. Low pressure mercury vapor has a strong emission line at 254Ã,nm, which is within the wavelength range indicating a strong disinfection effect. The optimal wavelength for disinfection is close to 270 nm.
The lamp is an amalgam lamp or medium-pressure lamp. Low-pressure UV lamps offer high efficiency (about 35% UVC) but lower power, usually power density of 1 W/cm (power per unit of arc length). Amalgam UV Lamp is a higher version of low pressure lamp. They operate at higher temperatures and have a lifetime of up to 16,000 hours. Their efficiency is slightly lower than traditional low-pressure lamps (about 33% of UVC output) and power density of about 2-3 W/cm. Medium pressure UV lamps have broad and clear peak-line spectra and high radiation output but low UVC efficiency of 10% or less. The typical power density is 30 W/cmÃ,³ or greater.
Depending on the quartz glass used for the lamp body, low pressure and radiation emission of amalgam emitted at 254m and also at 185nm, which has a chemical effect. UV radiation at 185nm is used to produce ozone.
UV lamps for water treatment consist of special low-pressure mercury vapor lamps that produce ultraviolet radiation at 254m, or medium-pressure UV lamps that produce polychromatic output of 200 nm to visible and infrared energy. UV light never touches water; it is either housed in a quartz glass sleeve inside a water chamber or mounted externally to water that flows through a transparent UV tube. Water passing through the flow chamber exposed to UV light is absorbed by suspended solids, such as microorganisms and impurities, in the river.
Light emitting diodes (LED)
Recent developments in LED technology have produced commercially available UV-C LEDs. UV-C LEDs use semiconductors to emit light between 255Ã, nm-280Ã, nm. Wavelength emissions can be adjusted by adjusting the semiconductor material. The reduced LED size opens the option for a small reactor system that allows application and integration of point use to medical devices. Low power consumption of semiconductors introduces UV disinfection systems that utilize small solar cells in remote or Third World applications.
Water treatment system
The size of the UV system is affected by three variables: flow rate, lamp strength, and UV transmittance in water. Manufacturers typically develop sophisticated Computational Fluid Dynamics (CFD) models that are validated by bioasai testing. This involves testing the performance of UV reactor disinfection with bacteriophage MS2 or T1 at various flow rates, UV transmissions, and power levels to develop regression models for system size. For example, this is a requirement for all drinking water systems in the United States per EPA UV Guideline Manual.
The flow profile is generated from the space geometry, flow rate, and selected turbulence models selected. The radiation profile is developed from inputs such as water quality, lamp type (power, germ efficiency, spectral output, arc length), and the transmittance and dimensions of quartz arms. Proprietary CFD software simulates flow and radiation profiles. Once a 3D model of space is built, it is filled with a grid or mesh consisting of thousands of small cubes.
Points of interest - such as round the corner, on the surface of the quartz arm, or around the eraser mechanism - use high resolution nets, while other areas in the reactor use a coarse mesh. After the mesh is produced, hundreds of thousands of virtual particles are "fired" through the room. Each particle has several variables associated with it, and particles are "harvested" after the reactor. Discrete phase modeling generates submitted doses, head loss, and other space-specific parameters.
When the modeling phase is completed, the selected system is validated using a professional third party to provide surveillance and to determine how close the model is able to predict the system performance realities. The system validation uses non-pathogenic replacements such as MS 2 phage or Bacillus subtilis to determine the ability of Reduction Equivalent Dose (RED) from the reactor. Most systems are validated to provide 40 mJ/cm 2 in the flow envelope and transmittance.
To validate the effectiveness of drinking water systems, the methods described in EPA's UV Guidelines are normally used by the United States, while Europe has adopted the German DVGW 294 standard. For wastewater systems, NWRI/AwwaRF Ultraviolet Ultraviolet Disinfection Guidelines for Water and Water Reuse protocols are commonly used, especially in waste water reuse.
See also
- Portable water purification
- Sanitation
- Standard Operating Procedures Sanitation
- Solar water disinfection
References
External links
- [UVGI Country Publication by W. J. Kowalski]
- A residential and commercial UVGI system with detailed UV-C indoor air conditioning information
- ASHRAE 2008 Handbook - Table of contents
- WHO, Managing water at home
- Wastewater technology fact sheet: Ultraviolet disinfection
- Lawrence Berkeley National Laboratory, UV Water Disinfection Testers
- International Ultraviolet Association
- Cantaro Azul, Mexican Nonprofit Organization
- Prevent Sick Building Syndrome with Ultraviolet Sterilization
- Are UV Air Purifiers Really Working?
Source of the article : Wikipedia
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