Ultraviolet germicidal irradiation

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A low pressure mercury vapor discharge tube floods the inside of a biosafety cabinet with shortwave UV light when not in use, sterilizing microbiological contaminants from irradiated surfaces.

Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses ultraviolet (UV) light at sufficiently short wavelength to kill microorganisms.[1] It is used in a variety of applications, such as food, air, and water purification. UVGI utilises short-wavelength ultraviolet radiation (UV-C) that is harmful to microorganisms. It is effective in destroying the nucleic acids in these organisms so that their DNA is disrupted by the UV radiation, leaving them unable to perform vital cellular functions.

The wavelength of UV that causes this effect is rare on Earth as the atmosphere blocks it.[2] Using a UVGI device in certain environments like circulating air or water systems creates a deadly effect on micro-organisms such as pathogens, viruses and molds that are in these environments. Coupled with a filtration system, UVGI can remove harmful micro-organisms from these environments.

The application of UVGI to disinfection has been an accepted practice since the mid-20th century. It has been used primarily in medical sanitation and sterile work facilities. Increasingly it was employed to sterilize drinking and wastewater, as the holding facilities were enclosed and could be circulated to ensure a higher exposure to the UV. In recent years UVGI has found renewed application in air sanitation.

History[edit]

UV has been a known mutagen at the cellular level for more than one-hundred years. The 1903 Nobel Prize for Medicine was awarded to Niels Finsen for his use of UV against lupus vulgaris, tuberculosis of the skin.[3]

Using UV light for drinking water disinfection dates back to the year 1910 in Marseille, France.[4] Over the years, UV costs have declined as researchers develop and use new UV methods to disinfect water and wastewater. Currently, several countries have developed regulations that allow systems to disinfect their drinking water supplies with UV light.

Method of operation[edit]

UV light is electromagnetic radiation with wavelengths shorter than visible light. UV can be separated into various ranges, with short range UV (UVC) considered “germicidal UV”. At certain wavelengths UV is mutagenic to bacteria, viruses and other micro-organisms. At a wavelength of 2,537 Angstroms (254 nm)[5] UV will break the molecular bonds within micro-organismal DNA, producing thymine dimers in their DNA thereby destroying them, rendering them harmless or prohibiting growth and reproduction. It is a process similar to the UV effect of longer wavelengths (UVB) on humans, such as sunburn or sun glare. Micro-organisms have less protection from UV and cannot survive prolonged exposure to it.

A UVGI system is designed to expose environments such as water tanks, sealed rooms and forced air systems to germicidal UV. Exposure comes from germicidal lamps that emit germicidal UV electromagnetic radiation at the correct wavelength, thus irradiating the environment. The forced flow of air or water through this environment ensures the exposure.

Effectiveness[edit]

The effectiveness of germicidal UV in such an environment depends on a number of certain factors: the length of time a micro-organism is exposed to UV, power fluctuations of the UV source that impact the EM wavelength, the presence of particles that can protect the micro-organisms from UV, and a micro-organism’s ability to withstand UV during its exposure.

In many systems redundancy in exposing micro-organisms to UV is achieved by circulating the air or water repeatedly. This ensures multiple passes so that the UV is effective against the highest number of micro-organisms and will irradiate resistant micro-organisms more than once to break them down.

The effectiveness of this form of sterilization is also dependent on line-of-sight exposure of the micro-organisms to the UV light. Environments where design creates obstacles that block the UV light are not as effective. In such an environment the effectiveness is then reliant on the placement of the UVGI system so that line of sight is optimum for sterilization.

Sterilization is often misquoted as being achievable. While it is theoretically possible in a controlled environment, it is very difficult to prove and the term "disinfection" is used by companies offering this service as to avoid legal reprimand. Specialist companies will often advertise a certain log reduction e.g., 99.9999% effective, instead of sterilization. This takes into consideration a phenomenon known as light and dark repair (photoreactivation and base excision repair, respectively) in which the DNA in the bacterium will fix itself after being damaged by UV light.[6]

A separate problem that will affect UVGI is dust or other film coating the bulb, which can lower UV output. Therefore bulbs require annual replacement and scheduled cleaning to ensure effectiveness. The lifetime of germicidal UV bulbs varies depending on design. Also the material that the bulb is made of can absorb some of the germicidal rays.

Lamp cooling under airflow can also lower UV output, thus care should be taken to shield lamps from direct airflow via parabolic reflector. Or add additional lamps to compensate for the cooling effect.

Increases in effectiveness and UV intensity can be achieved by using reflection. Aluminum has the highest reflectivity rate versus other metals and is recommended when using UV.

Inactivation of microorganisms[edit]

The degree of inactivation by ultraviolet radiation is directly related to the UV dose applied to the water. The dosage, a product of UV light intensity and exposure time, is usually measured in microjoules per square centimeter, or alternatively as microwatt seconds per square centimeter (µW·s/cm2). Dosages for a 90% kill of most bacteria and virus range from 2,000 to 8,000 µW·s/cm2. Dosage for larger parasites such as Cryptosporidium require a lower dose for inactivation. As a result, the U.S. Environmental Protection Agency has accepted UV disinfection as a method for drinking water plants to obtain Cryptosporidium, Giardia or virus inactivation credits. For example, for one-decimal-logarithm reduction of Cryptosporidium, a minimum dose of 2,500 µW·s/cm2 is required based on the U.S. EPA UV Guidance Manual published in 2006.[7]:1–7

Weaknesses and strengths[edit]

Advantages[edit]

For more details on this topic, see Disinfectant.

UV water treatment devices can be used for well water and surface water disinfection. UV treatment compares favorably with other water disinfection systems in terms of cost, labor, and the need for technically trained personnel for operation: deep tube wells fitted with hand pumps, while perhaps the simplest to operate, require expensive drilling rigs, are immobile sources, and often produce hard water that is found distasteful. Chlorine disinfection treats larger organisms and offers residual disinfection, but these systems are expensive because they need special operator training and a steady supply of a potentially hazardous material. Finally, boiling water over a cook stove is the most reliable treatment method but it demands labor, and imposes a high economic cost. UV treatment is rapid and, in terms of primary energy use, approximately 20,000 times more efficient than boiling.[discuss]

Drawbacks[edit]

UV disinfection is most effective for treating a high-clarity, purified reverse osmosis distilled water. Suspended particles are a problem because microorganisms buried within particles are shielded from the UV light and pass through the unit unaffected. However, UV systems can be coupled with a pre-filter to remove those larger organisms that would otherwise pass through the UV system unaffected. The pre-filter also clarifies the water to improve light transmittance and therefore UV dose throughout the entire water column. Another key factor of UV water treatment is the flow rate—if the flow is too high, water will pass through without sufficient UV exposure. If the flow is too low, heat may build up and damage the UV lamp.[8]

A disadvantage of the technique is that water treated by chlorination is resistant to reinfection, where UVGI water must be transported and delivered in such a way as to avoid contamination.

Safety[edit]

In UVGI systems the lamps are shielded or are in environments that limit exposure, such as a closed water tank or closed air circulation system, often with interlocks that automatically shut off the UV lamps if the system is opened for access by human beings.

In human beings, skin exposure to germicidal wavelengths of UV light can produce sunburn and skin cancer. Exposure of the eyes to this UV radiation can produce extremely painful inflammation of the cornea and temporary or permanent vision impairment, up to and including blindness in some cases. UV can damage the retina of the eye.

Another potential danger is the UV production of ozone. Ozone can be harmful to health. The United States Environmental Protection Agency designated 0.05 parts per million (ppm) of ozone to be a safe level. Lamps designed to release UVC and higher frequencies are doped so that any UV light below 254 nm will not be released, thus ozone is not produced. A full-spectrum lamp will release all UV wavelengths and will produce ozone as well as UVC, UVB, and UVA. (The ozone is produced when UVC hits oxygen (O2) molecules, and so is only produced when oxygen is present.)

UV-C radiation is able to break down chemical bonds. This leads to rapid ageing of plastics (insulations, gasket) and other materials. Note that plastics sold to be "UV-resistant" are tested only for UV-B, as UV-C doesn't normally reach the surface of the Earth. When UV is used near plastic, rubber, or insulations care should be taken to shield said components; metal tape or aluminum foil will suffice.

Uses[edit]

Air disinfection[edit]

UVGI can be used to disinfect air with prolonged exposure. Disinfection is a function of UV concentration and time, CT. For this reason, it is not as effective on moving air, when the lamp is perpendicular to the flow, as exposure times are dramatically reduced. Air purification UVGI systems can be freestanding units with shielded UV lamps that use a fan to force air past the UV light. Other systems are installed in forced air systems so that the circulation for the premises moves micro-organisms past the lamps. Key to this form of sterilization is placement of the UV lamps and a good filtration system to remove the dead micro-organisms.[9] For example, forced air systems by design impede line-of-sight, thus creating areas of the environment that will be shaded from the UV light. However, a UV lamp placed at the coils and drainpans of cooling systems will keep micro-organisms from forming in these naturally damp places.

ASHRAE covers UVGI and its applications in IAQ and building maintenance in its 2008 Handbook, HVAC Systems and Equipment in Chapter 16 titled Ultraviolet Lamp Systems. ASHRAE's 2011 Handbook, HVAC Applications, covers ULTRAVIOLET AIR AND SURFACE TREATMENT in Chapter 60.

Water sterilization[edit]

A portable, battery powered low pressure mercury vapor discharge lamp for water sterilization.

Ultraviolet disinfection of water consists of a purely physical, chemical-free process. UV-C radiation attacks the vital DNA of the bacteria directly. The bacteria lose their reproductive capability and are destroyed. Even parasites such as Cryptosporidia or Giardia, which are extremely resistant to chemical disinfectants, are efficiently reduced.[10] UV can also be used to remove chlorine and chloramine species from water ; this process is called photolysis, and requires a higher dose than normal disinfection. The sterilized microorganisms are not removed from the water. UV disinfection does not remove dissolved organics, inorganic compounds or particles in the water.[11] However, UV-oxidation processes can be used to simultaneously destroy trace chemical contaminants and provide high-level disinfection, such as the world's largest indirect potable reuse plant in New York which opened the Catskill-Delaware Water Ultraviolet Disinfection Facility on the eighth of October 2013.[12] A total of 56 energy-efficient UV reactors were installed to treat 2.2 billion US gallons (8,300,000 m3) a day to serve New York City.[12]

It used to be thought that UV disinfection was more effective for bacteria and viruses, which have more exposed genetic material, than for larger pathogens that have outer coatings or that form cyst states (e.g., Giardia) that shield their DNA from the UV light. However, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the microorganism Cryptosporidium. The findings resulted in the use of UV radiation as a viable method to treat drinking water. Giardia in turn has been shown to be very susceptible to UV-C when the tests were based on infectivity rather than excystation.[13] It has been found that protists are able to survive high UV-C doses but are sterilized at low doses.

UV tube project[edit]

The UV Tube is a design concept for providing inexpensive water disinfection to people in poor countries. The concept is based on the ability of ultraviolet light to kill infectious agents by disrupting their DNA. It was initially developed under an open-source model at the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley. The form and composition of the UV Tube can vary depending on the resources available and the preferences of those building and using the device. However, certain geometric parameters must be maintained to ensure consistent performance. Several different versions of the UV Tube are currently being used in multiple locations in Mexico and Sri Lanka.


Wastewater treatment[edit]

Ultraviolet in wastewater treatment is replacing chlorination due to the chlorine's toxic by-products. Individual wastestreams to be treated by UVGI must be tested to ensure that the method will be effective due to potential interferences such as suspended solids, dyes, or other substances that may block or absorb the UV radiation. According to the World Health Organization, "UV units to treat small batches (1 to several liters) or low flows (1 to several liters per minute) of water at the community level are estimated to have costs of 0.02 US$ per 1,000 liters of water, including the cost of electricity and consumables and the annualized capital cost of the unit".[14]

Large-scale urban UV wastewater treatment is performed in cities such as Edmonton, Alberta, in Canada. The use of ultraviolet light has now become standard practice in most municipal wastewater treatment processes. Effluent is now starting to be recognised as a valuable resource, not a problem that needs to be dumped. Many wastewater facilities are being renamed as water reclamation facilities, and whether the waste water is being discharged into a river, being used to irrigate crops, or injected into an aquifer for later recovery. Ultraviolet light is now being used to ensure water is free from harmful organisms.

Aquarium and pond[edit]

Ultraviolet sterilizers are often used in aquaria and ponds to help control unwanted microorganisms in the water. Continuous sterilization of the water neutralizes single-cell algae and thereby increases water clarity. UV irradiation also ensures that exposed pathogens cannot reproduce, thus decreasing the likelihood of a disease outbreak in an aquarium. UV irradiation can also have a positive impact on an Aquariums Redox balance.

Aquarium and pond sterilizers are typically small, with fittings for tubing that allows the water to flow through the sterilizer on its way from a separate external filter or water pump. Within the sterilizer, water flows as close as possible to the ultraviolet light source. Water pre-filtration is critical so as to lower water turbidity which will lower UVC penetration. Many of the better UV Sterilizers have long dwell times and limit the space between the UVC source and the inside wall of the UV Sterilizer device.[15]

Laboratory hygiene[edit]

UVGI is often used to disinfect equipment such as safety goggles, instruments, pipettors, and other devices. Lab personnel also disinfects glassware and plasticware this way. Microbiology laboratories use UVGI to disinfect surfaces inside biological safety cabinets ("hoods") between uses.

Food and beverage protection[edit]

Since the U.S. Food and Drug Administration issued a rule in 2001 requiring that virtually all fruit and vegetable juice producers follow HACCP controls, and mandating a 5-log reduction in pathogens, UVGI has seen some use in sterilization of fresh juices such as fresh-pressed apple cider.

Technology[edit]

Lamps[edit]

Main article: Germicidal lamp

Germicidal UV is delivered by a mercury-vapor lamp that emits UV at the germicidal wavelength. Mercury vapour emits at 254 nm. Many germicidal UV bulbs use special ballasts to regulate electrical current flow to the bulbs, similar to those needed for fluorescent lights. In some cases, UVGI electrodeless lamps can be energised with microwaves, giving very long stable life and other advantages[clarification needed]. This is known as "Microwave UV".

Lamps are either amalgam or medium pressure lamps. Each type has specific strengths and weaknesses.

Low-pressure UV lamps
These offer high efficiencies (approx 35% UVC) but lower power, typically 1 W/cm power density (power per unit of arc length).
Amalgam UV lamps
A high-power version of low-pressure lamps. They operate at higher temperatures and have a lifetime of up to 16,000 hours. Their efficiency is slightly lower than that of traditional low-pressure lamps (approx 33% UVC output) and power density is approximately 2–3 W/cm.
Medium-pressure UV
These lamps have a broad and pronounced peak-line spectrum and a high radiation output but lower UVC efficiency of 10% or less. Typical power density is 30 W/cm³ or greater.

Depending on the quartz glass used for the lamp body, low-pressure and amalgam UV lamps emit light at 254 nm and 185 nm (for oxidation).

185 nm light is used to generate ozone.

The UV units for water treatment consist of a specialized low pressure mercury vapor lamp that produces ultraviolet radiation at 254 nm, or medium pressure UV lamps that produce a polychromatic output from 200 nm to visible and infrared energy. The optimal wavelengths for disinfection are close to 260 nm.[7]:2–6 Medium pressure lamps are approximately 12% efficient, whilst amalgam low-pressure lamps can be up to 40% efficient. The UV lamp never contacts the water, it is either housed in a quartz glass sleeve inside the water chamber or mounted external to the water which flows through the transparent UV tube. It is mounted so that water can pass through a flow chamber, and UV rays are admitted and absorbed into the stream.[16]

Water treatment systems[edit]

Sizing of a UV system is affected by three variables: flow rate, lamp power and UV transmittance in the water. UV manufacturers typically developed sophisticated Computational Fluid Dynamics (CFD) models validated with bioassay testing. This typically involves testing the UV reactor's disinfection performance with either MS2 or T1 bacteriophages at various flow rates, UV transmittance and power levels in order to develop a regression model for system sizing. For example, this is a requirement for all drinking water systems in the United States per the U.S. EPA UV Guidance Manual.[7]:5-2

The flow profile is produced from the chamber geometry, flow rate and particular turbulence model selected. The radiation profile is developed from inputs such as water quality, lamp type (power, germicidal efficiency, spectral output, arc length) and the transmittance and dimension of the quartz sleeve. Proprietary CFD software simulates both the flow and radiation profiles. Once the 3D model of the chamber is built, it's populated with a grid or mesh that comprises thousands of small cubes.

Points of interest—such as at a bend, on the quartz sleeve surface, or around the wiper mechanism—use a higher resolution mesh, whilst other areas within the reactor use a coarse mesh. Once the mesh is produced, hundreds of thousands of virtual particles are "fired" through the chamber. Each particle has several variables of interest associated with it, and the particles are "harvested" after the reactor. Discrete phase modeling produces delivered dose, headless and other chamber specific parameters.

When the modeling phase is complete, selected systems are validated using a professional third party to provide oversight and to determine how closely the model is able to predict the reality of system performance. System validation uses non-pathogenic surrogates to determine the Reduction Equivalent Dose (RED) ability of the reactors. Most systems are validated to deliver 40 mJ/cm2 within an envelope of flow and transmittance.

To validate effectiveness in drinking water systems, the methods described in the US EPA UV Guidance Manual is typically used by the U.S. Environmental Protection Agency, whilst Europe has adopted Germany's DVGW 294 standard. For wastewater systems, the NWRI/AwwaRF Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse protocols are typically used, especially in wastewater reuse applications.[17]

UV systems destined for drinking water applications are validated using a third-party test house to demonstrate system capability, and usually a non pathogenic surrogate such as MS 2 phage or Bacillus subtilis is used to verify actual system performance. UV manufacturers have verified the performance of a number of reactors, in each case iteratively improving the predictive models.


UV dosing[edit]

One method for gauging UV effectiveness is to compute UV dose. The U.S. EPA publishes UV dosage guidelines.[18]

Dosage involves the following parameters:

  • Flow rate (reflecting contact time)
  • Transmittance (reflects light reaching the target)
  • Turbidity ("cloudiness")
  • Lamp age (reflects reduction in UV intensity)
  • Lamp fouling
  • Percentage of active lamps (reflects lamp outages in each lamp bank)

See also[edit]

References[edit]

  1. ^ National Institute for Occupational Safety and Health. (2008, April). NIOSH eNews, 5(12). Retrieved September 10, 2008, from http://www.cdc.gov/niosh/enews/enewsV5N12.html
  2. ^ Lupu, Alexandra (2006-07-20). "UV Radiation – What UVA, UVB and UVC Rays Are and How They Affect Us". Seasonal Discomforts. Softpedia. Retrieved 2006-09-09. 
  3. ^ "The Nobel Prize in Physiology or Medicine 1903". Nobelprize.org. The Nobel Foundation. Retrieved 2006-09-09. 
  4. ^ "Ultraviolet Light Disinfection in the Use of Individual Water Purification Devices". U.S. Army Public Health Command. Retrieved 2014-01-08. 
  5. ^ Kowalski W.J.; Bahnfleth W.P.; Witham D.L.; Severin B.F.; Whittam T.S. (October 2000). "Mathematical Modeling of Ultraviolet Germicidal Irradiation for Air Disinfection". Quantitative Microbiology (Springer) 2 (3): 249–270. doi:10.1023/A:1013951313398. 
  6. ^ "How does UV disinfection work". R. Franklin. UVO3.co.uk. Retrieved 2011-06-22. 
  7. ^ a b c Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule (PDF). Washington, DC: United States Environmental Protection Agency. November 2006. Retrieved 30 January 2011 
  8. ^ GADGIL,A.,1997, Field-testing UV Disinfection of Drinking Water, Water Engineering Development Center, University of Loughborough,UK: LBNL 40360.
  9. ^ "Environmental Analysis of Indoor Air Pollution". CaluTech UV Air. Retrieved 2006-12-05. 
  10. ^ "Introduction to UV Disinfection". TrojanUV. 2012. Retrieved 24 May 2012. 
  11. ^ HARM, W., 1980, Biological Effects of Ultraviolet Radiation, International Union of Pure and Applied Biophysics, Biophysics series, Cambridge University Press.
  12. ^ a b Catskill-Delaware Water Ultraviolet Disinfection Facility
  13. ^ Ware, M. W. et al. Inactivation of Giardia muris by Low Pressure Ultraviolet Light (PDF). United States Environmental Protection Agency. Archived from the original on 27 February 2008. Retrieved 2008-12-28. 
  14. ^ WHO
  15. ^ "UV Sterilization; Aquarium and Pond". 
  16. ^ WOLFE, R.L., 1990, Ultraviolet Disinfection of Potable Water, Env. Sci. and Technology 24(6):768-773
  17. ^ Treatment Technology Report for Recycled Water (PDF). State Of California Division of Drinking Water and Environmental Management. January 2007. p. [page needed]. Retrieved 30 January 2011 
  18. ^ Chapter 7

External links[edit]