ULTRAVIOLET (UV) DISINFECTION FOR WATER TREATMENT
ULTRAVIOLET (UV) DISINFECTION FOR WATER TREATMENT
WATER TREATMENT AND ULTRAVIOLET (UV-C) SANITIZER
WHAT IS THE WATER DISINFECTION WITH ULTRAVIOLETS?
The sun emits invisible light, the ultraviolet light, having sterilizing effect. This natural phenomenon is reproduced inside reactor (irradiation chamber) in which water goes in and will be disinfected by UV radiation (learn more about how does the UV disinfection works).
WHICH KIND OF WATER CAN BE TREATED WITH THE UV TECHNOLOGY?
There is a wide range of applications for the water disinfection using the UV technology, BIO-UV Group offers solutions for both professionals and individuals. For instance, you can treat surface water, residual water, pool water, industrial process water, drinking water, etc…
Globally, as soon as you need to disinfect the water, the UV technology may be used.
HOW ULTRAVIOLET UV-C WATER TREATMENT WORKS?
THE DIFFERENT TYPES OF UV-RAYS
There are several categories of ultraviolet rays depending on their wavelengths measured in Nanometer (nm). The UV rays’ wavelengths start from 10nm (under it’s X-rays) to 400nm (beyond it’s visible radiation).
There are 3 main types of UV radiation and each one has specific phytochemical effects on the irradiated bodies:
• UV-A (wavelength from 315 to 400nm): they are causing skin pigmentation
• UV-B (from 285 to 315nm): allow the vitamin D synthesis
• UV-C (from 200 to 280nm): penetrate the microorganisms’ DNA and destroy it
HOW CAN “LIGHT” KILL BACTERIA?
Ultraviolet disinfection systems are mysterious to many people – how can “light” kill bacteria? But the truth is it can. Ultraviolet (UV) technology has been around for more than 60 years, and its effectiveness has been well documented both scientifically and commercially. It is nature’s own disinfection/purification method. With consumers becoming more concerned about chlorine and other chemical contamination of drinking water, more dealers are prescribing the ultraviolet solution suitable for both small flow residential applications as well as large flow commercial projects.
Ultraviolet is a means of killing or rendering harmless microorganisms in a dedicated environment. These microorganisms can range from bacteria and viruses to algae and protozoa. UV disinfection is used in air and water purification, sewage treatment, protection of food and beverages, and many other disinfection and sterilization applications. A major advantage of UV treatment is that it is considered safer and more reliable for disinfection of water than chemical alternatives, while the level of disinfection is much higher. UV treatment systems are also extremely cost efficient and require less space than alternative disinfection systems.
THE UV-C RAYS’ GERMICIDAL EFFECT
So, the UV disinfection technology deals with the UV-C rays use for their sterilizing effect. Indeed, UV-C radiation modify the chemical frame of the irradiated living microorganism.
If we observe the trending line of DNA’s absorption we can notice a significant peak at 254nm, UV-C rays penetrate the heart of the microorganism’s DNA and disturb the metabolism of cells until they are totally destroyed.
The ultraviolet UV-C disinfection process is able to kill 99,9% of the microorganisms like viruses, bacteria, algae, yeasts, mould…including Legionella and Cryptosporidium.
Furthermore, apart from the wavelength, the sterilizing effect will be also determined by the UV energy amount absorbed by the DNA:
• Weak to average UV-energy amount absorbed: bacteriostatic effect, the microorganism will still live but cannot reproduce
• High UV-energy amount absorbed: bactericidal effect, the microorganism will be destroyed
So that, the more UV-energy the DNA absorb, the more efficient the germicidal effect is.
THE EFFECTIVE UV DOSE
The effective UV dose, expressed in mJ/cm² (millijoules per square centimeter), refers to the necessary UV energy that the micro-organisms (bacteria, viruses, algae in suspension…) must absorb before being destroyed. When this UV dose is reached, the UV-C rays penetrate the micro-organisms’ DNA and eradicate 99,9% of them.
The UV dose is a result of an equation between:
• the flow rate
• the UV exposure time (depending on the device’s design)
• the UV-C power emitted by the lamp
Other parameters must to be taking into account while calculating the UV dose and notably, the UV transmittance measured as a percentage. It is the UV rays ease to go through water; the easier they are going through, the higher the percentage is.
Thanks to our expertise, we are able to offer optimum UV disinfection solutions even with low UV transmittance. (Learn more about BIO-UV Group’s expertise)
UV Types Comparison
Principles of UV Disinfection
UV radiation has three wavelength zones: UV-A, UV-B, and UV-C, and it is this last region, the shortwave UV-C, that has germicidal properties for disinfection. A low-pressure mercury arc lamp resembling a fluorescent lamp produces the UV light in the range of 254 manometers (nm). A nm is one billionth of a meter (10^-9 meter). These lamps contain elemental mercury and an inert gas, such as argon, in a UV-transmitting tube, usually quartz. Traditionally, most mercury arc UV lamps have been the so-called “low pressure” type, because they operate at relatively low partial pressure of mercury, low overall vapor pressure (about 2 mbar), low external temperature (50-100oC) and low power. These lamps emit nearly monochromatic UV radiation at a wavelength of 254 nm, which is in the optimum range for UV energy absorption by nucleic acids (about 240-280 nm).
In recent years medium pressure UV lamps that operate at much higher pressures, temperatures and power levels and emit a broad spectrum of higher UV energy between 200 and 320 nm have become commercially available. However, for UV disinfection of drinking water at the household level, the low-pressure lamps and systems are entirely adequate and even preferred to medium pressure lamps and systems. This is because they operate at lower power, lower temperature, and lower cost while being highly effective in disinfecting more than enough water for daily household use. An essential requirement for UV disinfection with lamp systems is an available and reliable source of electricity. While the power requirements of low-pressure mercury UV lamp disinfection systems are modest, they are essential for lamp operation to disinfect water. Since most microorganisms are affected by radiation around 260 nm, UV radiation is in the appropriate range for germicidal activity. There are UV lamps that produce radiation in the range of 185 nm that are effective on microorganisms and will also reduce the total organic carbon (TOC) content of the water. For typical UV system, approximately 95 percent of the radiation passes through a quartz glass sleeve and into the untreated water. The water is flowing as a thin film over the lamp. The glass sleeve is designed to keep the lamp at an ideal temperature of approximately 104 °F.
UV Radiation (How it Works)
UV radiation affects microorganisms by altering the DNA in the cells and impeding reproduction. UV treatment does not remove organisms from the water, it merely inactivates them. The effectiveness of this process is related to exposure time and lamp intensity as well as general water quality parameters. The exposure time is reported as “microwatt-seconds per square centimeter” (uwatt-sec/cm^2), and the U.S. Department of Health and Human Services has established a minimum exposure of 16,000 µwatt-sec/cm^2 for UV disinfection systems. Most manufacturers provide a lamp intensity of 30,000-50,000µwatt-sec/cm^2. In general, coliform bacteria, for example, are destroyed at 7,000 µwatt-sec/cm^2. Since lamp intensity decreases over time with use, lamp replacement and proper pretreatment are key to the success of UV disinfection. In addition, UV systems should be equipped with a warning device to alert the owner when lamp intensity falls below the germicidal range. The following gives the irradiation time required to inactivate completely various microorganisms under 30,000 µwatt-sec/cm^2 dose of UV 254 nm Used alone, UV radiation does not improve the taste, odor, or clarity of water. UV light is a very effective disinfectant, although the disinfection can only occur inside the unit. There is no residual disinfection in the water to inactivate bacteria that may survive or may be introduced after the water passes by the light source. The percentage of microorganisms destroyed depends on the intensity of the UV light, the contact time, raw water quality, and proper maintenance of the equipment. If material builds up on the glass sleeve or the particle load is high, the light intensity and the effectiveness of treatment are reduced. At sufficiently high doses, all waterborne enteric pathogens are inactivated by UV radiation. The general order of microbial resistance (from least to most) and corresponding UV doses for extensive (>99.9%) inactivation are: vegetative bacteria and the protozoan parasites Cryptosporidium parvum and Giardia lamblia at low doses (1-10 mJ/cm2) and enteric viruses and bacterial spores at high doses (30-150 mJ/cm2).
Most low-pressure mercury lamp UV disinfection systems can readily achieve UV radiation doses of 50-150 mJ/cm2 in high quality water, and therefore efficiently disinfect essentially all waterborne pathogens. However, dissolved organic matter, such as natural organic matter, certain inorganic solutes, such as iron, sulfites and nitrites, and suspended matter (particulates or turbidity) will absorb UV radiation or shield microbes from UV radiation, resulting in lower delivered UV doses and reduced microbial disinfection. Another concern about disinfecting microbes with lower doses of UV radiation is the ability of bacteria and other cellular microbes to repair UV-induced damage and restore infectivity, a phenomenon known as reactivation. UV inactivates microbes primarily by chemically altering nucleic acids. However, the UV-induced chemical lesions can be repaired by cellular enzymatic mechanisms, some of which are independent of light (dark repair) and others of which require visible light (photorepair or photoreactivation). Therefore, achieving optimum UV disinfection of water requires delivering a sufficient UV dose to induce greater levels of nucleic acid damage and thereby overcome or overwhelm DNA repair mechanisms.
Table. Estimated Irradiation Time to Inactivate Microorganisms at a Dosage of 30,000 µwatt-sec/cm^2 of UV 254 nm
Name 100% lethal Dosage
(Second) Name 100% lethal Dosage
Dysentery bacilli 0.15 Micrococcus Candidus 0.4 ¨C 1.53
Leptospira SPP 0.2 Salmonella Paratyphi 0.41
Legionella Pneumophila 0.2 Mycobacterium Tuberculosis 0.41
Corynebacterium Diphtheriae 0.25 Streptococcus Haemolyticus 0.45
Shigella Dysenteriae 0.28 Salmonella Enteritidis 0.51
Bacillus Anthracis 0.3 Salmonella Typhimurium 0.53
Clostridium Tetani 0.33 Vibrio Cholerae 0.64
Escherichia coli 0.36 Clostridium Tetani 0.8
Pseudomonas Aeruginosa 0.37 Staphylococcus Albus 1.23
Coxsackie Virus A9 0.08 Echovirus 1 0.73
Adenovirus 3 0.1 Hepatitis B Virus 0.73
Bacteiophage 0.2 Echovirus 11 0.75
Influenza 0.23 Poliovirus 1 0.8
Rotavirus SA 11 0.52 Tobacco Mosaic 16
Mucor Mucedo 0.23 ¨C 4.67 Penicillium Roqueforti 0.87 – 2.93
Oospara Lactis 0.33 Penicillium Chrysogenum 2.0 ¨C 3.33
Aspergillus Amstelodami 0.73 ¨C 8.80 Aspergillus Niger 6.67
Penicillium Digitatum 0.87 Manure Fungi 8
Chlorella Vulgaris 0.93 Protozoa 4 – 6.70
Green Algae 1.22 Paramecium 7.3
Nematode Eggs 3.4 Blue-Green Algae 10 ¨C 40
Inactivation Doses for Giardia and Cryptosporidium
UV dose is a product of UV light intensity and exposure time in seconds (IT), stated in units; mWs/cm2 or mJ/cm2. IT is analogous to the chemical dose or CT (concentration x time). Microbes show a range of sensitivities to UV as shown by the UV data. Cryptosporidium and Giardia are more sensitive to UV than bacteria and viruses are more resistant than bacteria. Similar results have been obtained using low-pressure, medium-pressure and pulsed UV irradiation- Look for a Class A UV disinfection system. UV dose required for a 4log inactivation of selected waterborne pathogens.
Table .UV Dose 4 log Inactivation
UV Irradiation Pretreatment
Either sediment filtration or activated carbon filtration should take place before water passes through the unit. Particulate matter, color, and turbidity affect the transmission of light to the microorganisms and must be removed for successful disinfection.
Table. Recommended maximum contaminant levels in water entering a UV treatment device.
Turbidity 5 FTU or 5 NTU
(5 to 10 micron prefiltration recommended) < 10 mg/L
Iron < 0.3 mg/L
Manganese < 0.05 mg/L
UV is often the last device in a treatment train (a series of treatment devices), following reverse osmosis, water softening, or filtration. The UV unit should be located as close as possible to the point-of-use since any part of the plumbing system could be contaminated with bacteria. It is recommended that the entire plumbing system be disinfected with chlorine prior to initial use of a UV system.
Types of UV Disinfection Devices A Final Barrier
The typical UV treatment device consists, of a cylindrical chamber housing the UV bulb along its central axis. A quartz glass sleeve encases the bulb; water flow is parallel to the bulb, which requires electrical power. A flow control device prevents the water from passing too quickly past the bulb, assuring appropriate radiation contact time with the flowing water. It has been reported that turbulent (agitated) water flow provides more complete exposure of the organism to UV radiation.
A UV system housing should be of stainless steel to protect any electronic parts from corrosion. To assure they will be contaminant-free, all welds in the system should be plasma-fused and purged with argon gas. The major differences in UV treatment units are in capacity and optional features.
Some are equipped with UV emission detectors that warn the user when the unit needs cleaning or when the light source is failing. This feature is extremely important to assurance of a safe water supply.
A detector that emits a sound or shuts off the water flow is preferable to a warning light, especially if the system might be located where a warning light would not be noticed immediately.
Maintenance of a UV System
Since UV radiation must reach the bacteria to inactivate them, the housing for the light source must be kept clean. Commercial products are available for rinsing the unit to remove any film on the light source. An overnight cleaning with a solution of 0.15 percent sodium hydrosulfite or citric acid effec tively removes such films. Some units have wipers to aid the cleaning process.
UV systems are designed for continuous operation and should be shut down only if treatment is not needed for several days. A few minutes for lamp warm-up is needed before the system is used again following shut-down. In addition, the plumbing system of the house should be thoroughly flushed following a period of no use. Whenever the system is serviced, the entire plumbing system should be disinfected with a chemical such as chlorine prior to relying on the UV system for disinfection.
UV lights gradually lose effectiveness with use, the lamp should be cleaned on a regular basis and replaced at least once a year. It is not uncommon for a new lamp to lose 20 percent of its intensity within the first 100 hours of operation, although that level is maintained for the next several thousand hours. As stated previously, units equipped with properly calibrated UV emission detectors alert the owner when the light intensity falls below a certain level.
The treated water should be monitored for coliform and heterotrophic bacteria on a monthly basis for at least the first 6 months of the device’s use. If these organisms are present in the treated water, the lamp intensity should be checked, and the entire plumbing system should be disinfected with a chemical such as chlorine.
Quick Facts about UV Water Treatment
1. UV disinfection does not add chemicals tothe water.
2. UV is effective against bacteria and viruses; and may be effective against Giardia lamblia or Cryptosporidium if the system custom designed to meet these disinfection requirements.
3. UV disinfection has no residual disinfection.
4. Minimum lamp exposure of 16,000 µwatt-sec / cm^2 .
5. UV often last device in a treatment train of water treatment devices.
6. UV device should have audible UV emission detector to notify user when lamp intensity is inadequate.
7. Regular maintenance and lamp replacement is essential.
Capacity of UV Disinfection Systems
UV is an in-line, point-of-entry system that treats all the water used in the house. The capacities range from 0.5 gallons per minute (gpm) to several hundred gpm. Since bacteria may be shielded by particles in the water, pretreatment to remove turbidity may be required. There is also a limit to the number of bacteria that can be treated. An upper limit for UV disinfection is 1,000 total coliform/100 mL water or 100 fecal coliform/100 mL.
Prefiltration is required to remove color, turbidity, and particles that shield microorganisms from the UV source. Water that contains high mineral levels can coat the lamp sleeve and reduce the treatment effectiveness. Therefore, pretreatment with a water softener or phosphate injection system may be necessary to prevent build-up of minerals on the lamp. Table 3 lists the maximum levels of certain contaminants that are allowable for effective UV treatment.
UV System Overall Recommendations
Installing an UV treatment system, or any other water disinfection system is not a substitute for proper well design and construction. If you have a dug well as a supply source, replacing the well is probably a more satisfactory long-term option. If a dug well or spring is your only supply option then look at all the treatment options before you decide what to do. Make sure you get advice from an expert! Recommended treatment process selection:
1. Obtain information about your water source.
2. Get your water tested – At least Annually
3. Determine which problems are associated with infrastructure deficiencies, i.e.,
cracked casing, no cap, improper seal, poor surface drainage, etc. Make the necessary repairs and improvements to the system.
4. Install the necessary drinking water treatment systems. I have provided some online links for water treatment systems, but I always recommend a preliminary water test.
UV dosage is the most critical function of UV disinfection, because the extent of inactivation is proportional to the dose applied to the water. As individual UV lamps emit a set amount of ultraviolet energy, it is important that a system be sized correctly. Flow rates are the determining factor and must not be overstated. Contact time, which is the time the water is within the sterilization chamber, is directly proportional to dosage, which is the amount of energy per unit area (calculated by dividing the output in watts by the surface area of the lamp), and thus the overall effectiveness of microbial destruction in the system. This product of intensity and time is known as the Dose and is expressed in microwatt seconds per centimeter squared (μWsec/cm2). Divide by 1000 to express the dose in the preferred notation mJ/cm2 (milli joule per centimeter squared).
DOSE = time (sec) x output (watts) over area (cm2)
For maximum UV transmission a “hard glass” quartz sleeve is recommended for two main reasons. It isolates the lamp from the water to offer more uniform operating temperatures and allows for higher UV output into the water.
The UV dose is the product of UV intensity (expressed as energy per unit surface area) and residence time.
Therefore: DOSE = I x T
This is commonly expressed as 1mJ/cm2=1000 micro Watt second/cm2
The minimum dose expressed by Willand gives the user the guaranteed assurance of success. Average and cumulative doses offered by others depend on turbulent flow characteristics which can disappear when flow is variable.
Willand recommend the appropriate UV dose for each application taking into account water quality, arc tube ageing, industry specifications, as well as microbiological standards.
DOSE / DESTRUCTION RELATIONSHIP
The relationship between the dose and the destruction achieved of a target micro-organism can be summarized as follows:
N/No = e-KD
N = Initial Number of target organism
No = Number of target organisms after treatment
K = Constant associated with target organisms
D = Dose
From the above relationship doubling of the dose applied will increase the destruction by a factor of 10. Therefore doubling the dose required for 90% destruction will produce 99% destruction of the target organism. tripling the dose will produce a 99.9% destruction of the target organism and so on.
WHATE ARE THE BENEFITS OF WATER UV STERILIZER?
There are plenty of benefits using the UV technology to purify water. Easy to use and install, BIO-UV devices do not require complicated maintenance. You only need to replace the lamp every two years as the BIO-UV lamps have a very long lifetime (16 000 hours) but this operation is fast and really easy to do (check out our video tutorial). Therefore, this is a very cost-effective solution for long and mid-term.
BENEFITS OF WATER UV STERILIZER
Furthermore, the UV disinfection does not alter the physicochemical composition of the water. Which means that water going through the UV reactor goes out with the same physical and chemical composition than it has before going in. The process only disinfects by removing viruses, bacteria, microbes, etc.
While all other water disinfection technologies like chlorine, bleach, ozone, chlorine dioxide and other will alter the water. They may even create harmful by-products and residuals that threaten the health.
So, this is not the case for UV treatment but also, it does not change the smell neither the taste which results in extreme comfort while consuming the water or swimming.
Besides, the UV technology prevents from corrosive effects and harmful product use while warrantying an efficient and safe treatment.
Finally, the main advantages of disinfecting the water with UV are the following:
• Easy to install, use and maintain
• Do not alter the physicochemical composition of the water
• Do not create byproducts neither residuals
• Do not change the taste neither the smell of the water
• No corrosive effects
• No needs for carrying or manipulate harmful products
• Cost efficient at medium and long term
COMPARISON OF DISINFECTANTS AND STERILIZATION METHODS
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