Ultraviolet germicidal irradiation (UVGI) is a sterilization method that uses short wavelengths of ? ultraviolet (short-wave UV or UV-C) light to kill or inactivate microorganisms by destroying nucleic acids and damaging their DNA so that they are unable to perform important cellular functions.UVGI is used in a variety of applications such as food, air and water purification.
UV-C light is weak at the Earth's surface because the ozone layer of the atmosphere blocks it. UVGI devices can generate UV-C light in circulating air or water systems strong enough to make them inhospitable environments for microorganisms such as bacteria, viruses, molds, and other pathogens.UVGI can be used in conjunction with a filtration system to sanitize air and water.
The use of UVGI in disinfection has been an accepted practice since the mid-20th century. It is primarily used in healthcare and sterile work facilities. It is increasingly being used to disinfect drinking water and wastewater because storage facilities are enclosed and can be cycled to ensure greater exposure to UV. UVGI is being reapplied to air purifiers.
The effectiveness of UV sterilization depends on the duration of exposure of microorganisms to UV light, the intensity and wavelength of the UV radiation, the presence of particles that can protect the microorganisms from UV light, and the ability of the microorganisms to resist the UV light during exposure.
In many systems, redundancy of microbial exposure to UV is achieved by repeatedly circulating air or water. This ensures multiple passes so that the UV light is effective against the greatest number of microorganisms and will irradiate resistant microorganisms multiple times to break them down.
"Sterilization" is often mistakenly thought to be achievable. While it is theoretically possible in a controlled environment, it is difficult to prove, and companies offering the service often use the term "sterilization" to avoid legal condemnation. Instead of sterilization, professional companies often advertise a certain log reduction, such as 6-log reduction or 99.9999% efficiency.
This takes into account a phenomenon known as photorepair and dark repair (photoactivation and base excision repair, respectively), in which cells can repair DNA damaged by UV.
The effectiveness of this form of sterilization depends on the line-of-sight exposure of the microorganisms to UV light. Environments designed to create barriers that block UV light are not as effective. In such environments, effectiveness depends on the placement of the UVGI system so that the line of sight is best suited for disinfection.
Dust and film on the bulb can reduce UV output. Therefore, bulbs need to be cleaned and replaced regularly to ensure effectiveness. The life expectancy of germicidal UV bulbs varies by design. In addition, the material from which the bulb is made can absorb some of the germicidal rays.
Cooling the lamp under airflow can also reduce UV output. The use of reflection can increase efficiency and UV intensity. Aluminum has the highest reflectivity compared to other metals and is recommended when using UV.
Disadvantages
Ultraviolet disinfection is most effective for treating high-purity, purified reverse osmosis distilled water. Suspended particles are a problem because microorganisms buried in the particles are shielded by UV light and can pass through the unit unaffected. However, the UV system can be used in conjunction with a prefilter to remove those larger organisms that would otherwise pass through the UV system unaffected.
The pre-filter can also purify the water to increase light transmission, thereby increasing the UV dose throughout the water column. Another key factor in UV water treatment is the flow rate - if the flow rate is too high, water will pass through without sufficient UV exposure. If the flow rate is too low, heat can build up and damage the UV lamps.
One of the drawbacks of UVGI is that while chlorinated water resists reinfection (until the chlorine is out), UVGI water does not. UVGI water must be transported or delivered in a way that avoids reinfection.