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UV Disinfection

Ultraviolet (UV) rays are part of the light that comes from the sun. The UV spectrum is higher in frequency than visible light and lower in frequency compared to  x-rays.  This also means that the UV spectrum has a longer wavelength than x-rays and a shorter wavelength than visible light; the order of energy, from low to high, is visible light, UV, than x-rays. As a water treatment technique, UV is known to be an effective disinfectant due to its strong germicidal (inactivating) ability; UV is energetic enough (ionizing radiation) that it can break chemical bonds, killing microbes. UV disinfects water containing bacteria and viruses and can be effective against protozoans like Giardia lamblia cysts or Cryptosporidium oocysts. UV has been used commercially for many years in the pharmaceutical, cosmetic, beverage, and electronics industries, especially in Europe. In the US, it was used for drinking water disinfection in the early 1900s but was abandoned due to high operating costs, unreliable equipment, and the expanding popularity of disinfection by chlorination.

Because of safety issues associated with chlorination and improvements in UV technology, UV disinfection has experienced an increased acceptance in both municipal and household systems. There are only a few large-scale UV water treatment plants in the United States although there are more than 2,000 such plants in Europe.  There are two classes of disinfection systems, certified and classified by the NSF under Standard 55 – Class A and Class B Units.

Class A — These ultraviolet water treatment systems must have an ‘intensity & saturation’ rating of at least 40,000 µsec/cm2.  Class A point-of-entry and point-of-use systems covered by this standard are designed to inactivate (kill) or remove from contaminated water microorganisms including bacteria, viruses, Cryptosporidium oocysts, and Giardia cysts..  Systems covered by this standard are not intended for the treatment of water that has obvious contamination or intentional sources such as raw sewage, nor are systems intended to convert wastewater to drinking water. These systems are intended to be installed on visually clear water.

Class B — These ultraviolet water treatment systems must have an ‘intensity & saturation’ rating of at least 16,000 µW-sec/cm2 and possess designs that will allow them to provide supplemental bactericidal treatment of water already deemed ‘safe’. i.e., no elevated levels of E. coli. or a standard plate count of less than 500 colonies per 1 ml.  NSF Standard 55 "Class B" UV systems are designed to operate at a minimum dosage and are intended to "reduce normally occurring non-pathogenic or nuisance microorganisms only." The "Class B" or similar non-rated UV systems are not intended for the disinfection of "microbiologically unsafe water."

Therefore, the type of unit depends on your situation, source of water, and your water quality. Transmitted UV light dosage is affected by water clarity. Water treatment devices are dependent on the quality of the raw water. When turbidity is 5 NTU or greater and/or total suspended solids are greater than 10 ppm, pre-filtration of the water is highly recommended.  Normally, it is advisable to install a 5 to 20 micron filter prior to a UV disinfection system.

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 (which, unlike glass, is transparent to UV). Traditionally, most mercury arc UV lamps have been the so-called "low pressure" type, because they operate at a relatively low partial pressure of mercury, a low overall vapor pressure (about 2 mbar), a low external temperature (50-100 °C), 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); the UV breaks bonds in the nucleic acids, killing the microorganism.

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, a  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 a typical UV system, approximately 95 percent of the radiation passes through a quartz sleeve and into the untreated water.  The water flows as a thin film over the lamp.  The quartz 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 (kills) 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" (µWatt•sec/cm²), and the U.S. Department of Health and Human Services has established a minimum exposure of 16,000 µWatt•sec/cm² for UV disinfection systems.  Most manufacturers provide a lamp intensity of 30,000-50,000 µWatt•sec/cm².  In general, coliform bacteria, for example, are destroyed at 7,000 µWatt•sec/cm².  Since lamp intensity decreases over time with use, lamp replacement and proper pretreatment are keys 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 a 30,000 µWatt•sec/cm² dose at a UV wavelength of 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. Unlike chlorination, there is no residual disinfection in the water to inactivate bacteria that may survive or may be introduced after the water passes by the UV source. The percentage of microorganisms destroyed depends on the intensity of the UV,  the contact time, raw water quality, and proper maintenance of the equipment.  If material builds up on the quartz sleeve or the particle load is high, the UV 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. Note: There has been a change of units in this paragraph, from µW•sec/cm2 to mJ/cm2. Both are units of irradiance (power/area); a watt (W) is a joule/sec; 1000 µ (micro) = 1 m (milli). 1000 µW•sec/cm2 = 1 mJ/cm2. 

However, dissolved organic matter, such as natural organic matter, certain inorganic contaminants, 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 overcoming or overwhelming DNA repair mechanisms.

Table 1 | Estimated Irradiation Time to Inactivate Microorganisms at a dosage of 30,000 µW•sec/cm² of UV at a wavelength of 254 nm

Name 100% lethal Dosage Name 100% lethal Dosage
  (Seconds)   (Seconds)
Dysentery bacilli 0.15 Micrococcus candidus 0.4 - 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 (E. 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
Bacteriophages 0.2 Echovirus 11 0.75
Influenza 0.23 Poliovirus 1 0.8
Rotavirus SA 11 0.52 Tobacco Mosaic 16
Mold Spores
Mucor mucedo 0.23 - 4.67 Penicillium roqueforti 0.87 - 2.93
Oospora lactis 0.33 Penicillium chrysogenum 2.0 - 3.33
Aspergillus amstelodami 0.73 - 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 - 40

Inactivation Doses for Giardia and Cryptosporidium

The UV dose is a product of UV light intensity (irradiance) and exposure time in seconds (IT), stated in units: mW•s/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. Similar results have been obtained using low-pressure, medium-pressure, and pulsed UV irradiation - Look for a Class A UV disinfection system. The UV dose required for a 4 log inactivation of selected waterborne pathogens is shown below:

Table 2 | UV Dose 4 log Inactivation

Pathogen UV dose mJ/cm/2
4log inactivation
Cryptosporidium parvum oocysts <10
Giardia lamblia cysts <10
Vibrio cholerae 2.9
Salmonella typhi 8.2
Shigella sonnei 8.2
Hepatitis A virus 30
Poliovirus Type 1 30
Rotavirus SA11 36

UV Irradiation Pretreatment

Either sediment filtration or activated-carbon filtration should take place before water passes through the UV unit. Particulate matter, color, and turbidity affect the transmission of UV to the microorganisms and so must be removed for successful disinfection.

Table 3 | Recommended maximum contaminant levels in water entering a UV treatment device.

Parameter Units/Range
Turbidity 5 FTU or 5 NTU
Suspended solids
(5 to 10 micron
prefiltration recommended)
< 10 mg/L
Color None
Iron < 0.3 mg/L
Manganese < 0.05 mg/L
pH 6.5-9.5

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

The typical UV treatment device consists of a cylindrical chamber housing the UV bulb along its central axis. A quartz 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 organisms 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 assure 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 UV source. An overnight cleaning with a solution of 0.15 percent sodium hydrosulfite or citric acid effectively 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 intensity 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 to the water.

2. | UV is effective against bacteria and viruses; and may be effective against Giardia lamblia or Cryptosporidium if the system is custom designed to meet these disinfection requirements.

3 | UV disinfection has no residual disinfection.

4 | There should be a minimum lamp intensity of 16,000 µwatt•sec / cm² .

5 | UV is often the last device in a treatment train of water treatment devices.

6 | The UV device should have an audible UV emission detector to notify the user when the 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.

Special Considerations

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.

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 [ FIX: Need a link to Tests Product Page ]

3 | Determine which problems are associated with infrastructure deficiencies, i.e., cracked casing, no well cap, an improper seal, poor surface drainage, etc.  Make the necessary repairs and improvements to the system.

4 | Install the necessary Water Treatment Systems.  I have provided some online links for water treatment systems, but I always recommend a preliminary water test.

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