How Does UV Light Kill Bacteria?

Q. – How Does UV Light Kill Bacteria?

A. – UV or ultraviolet light is a type of radiation. In simple terms, when bacteria or another type of microbe is directly exposed to certain types of UV light, the DNA (its fundamental building block) of the cell is damaged, preventing it from replicating. If a cell cannot reproduce, then the cell cannot cause infection, which is how UV light kills bacteria.

The UV process is a physical process as opposed to the addition of chlorine to the water to address microbiological issues which is a chemical process. This is important, because it allows UV to kill bacteria and other microorganisms without adding anything to the water or creating what is known as disinfection by-products (like trihalomethanes, called THMs for short, which are proven to be carcinogenic).

Q. – Does the UV process change the taste of my drinking water?

A. – No, disinfection by UV is a physical process and not a chemical process. UV does not alter the water chemistry and will not affect the taste or odor of your water. UV only addresses the microbiological concerns, killing bacteria.

Q. – Is a UV system expensive to operate and maintain?

A. – When compared to other disinfection systems, the answer is no. As long as proper pre-treatment is maintained, an annual system check-up and lamp replacement is all that is required. A UV light kills bacteria with just a lightbulb. As most systems are designed to be left on at all times, a typical household system will draw about the same energy requirements as a 40 watt light bulb!

Q. – I've never heard of UV as a treatment method before, is it new?

A. – Ultraviolet light has been used for water treatment for over 100 years. Scandinavian scientist Niels Finsen was awarded the Nobel Prize in Physiology in 1903 for his studies on phototherapy, and how UV light kills bacteria. UV water disinfection was first used by the French in 1906 and advancements in the technology have allowed it to become commonplace in the residential, commercial, and municipal markets over the past 20 years. Today, UV is likely the first form of treatment that many people consider when looking to address microbiological concerns in their drinking water.

Q. – The media says UV is bad for me, how can UV be good?

A. – Although it sounds conflicting, there is truth to this statement. The term "UV" is most commonly associated with the potentially harmful exposure to the sun, which can be harmful to your health. Direct exposure to the sun should be carefully monitored in order to minimize the exposure risk.

Ultraviolet energy is actually broken up into three distinct wavelengths; UV-A, UV-B, and UV-C. Sunlight is comprised of all three UV rays, but the most powerful UV-C rays are blocked out by the earth's ozone layer, which protects us on earth. It's actually the UV-C portion that is used for the disinfection of drinking water because it's deadly to microorganisms such as E coli. It's true that you don't want to expose your eyes or skin directly to the UV light in a treatment system; however, the system is designed to prevent this from happening. In a properly designed UV system, there is absolutely no risk to the homeowner.

Q. – Does UV work on all microorganisms?

A. – UV does have some effect on all microorganisms. Whether it is bacteria, virus, algae, protozoan cysts, spores, mold, or other organism, exposure to UV light will cause harm. Each individual organism requires a different level of exposure (known as UV dose) in order to prevent cell replication. Some organisms, usually viruses, require extremely high doses of UV light in order to achieve disinfection. The important issue here is that you should ensure that the UV system that you are purchasing delivers enough UV dose at the end of the lamp life to ensure adequate disinfection against a typical array of organisms found in drinking water.

Q. – I have been hearing about a lot about E. coli on the news. Is UV effective against this microorganism?

A.Escherichia coli, or E. coli for short, is a bacterium found in the lower intestine of warm blooded organisms. There are many strains of E. coli, some of which can be found in the water supply. Although this bacteria has been blamed for many deaths (i.e. Walkerton in 2000), when exposed to ultraviolet light, at relatively a relatively low dose, it is easily destroyed. Even the particularly virulent O157:H7 strain of E. coli has a 4-log (99.99%) reduction at a UV dose of 6 mJ/cm². It should be mentioned that all US Water UV systems deliver a UV dose in excess of 30 mJ/cm² at the end of the lamp life.

Q. – My area has had problems with Cryptosporium and Giarda lamblia, and I've read that UV is not effective against these organisms. Is this true?

A. – No. Although you may have read scientific articles from prominent researchers and from other UV companies, this information was based on old research studies. Original studies performed in the 1980s were based on excystation methods, which lead to the belief that UV was ineffective against these protozoan cysts. In the early 2000s, it was proven by a host of independent research that UV was in fact extremely effective against both Cryptosporium and Giarda lamblia at a UV dose of less than of 10 mJ/cm². The change was a result of testing methodology used in the earlier testing. This new information has opened the doors for UV to become a mainstream disinfection method.

Q. I've read about "log kills." What does this mean and how does it apply to UV?

A. – The term "log kill" or, more accurately, "log reduction," refers to the level of reduction that the ultraviolet energy has on a specific organisms in an logarithmic ratio. The term "sanitation" is commonly referred to as a 2-log reduction, which translates into a 50-99% reduction or a reduction of 1 out of 100. The term "disinfection" is commonly referred to as a 4-log reduction, which translates into a 99.99% reduction or a reduction of 1 out of 10,000. The term "sterilization" is sometimes incorrectly associated with UV technology and represents a 6-log reduction, which translates into a 99.9999% reduction or a reduction of 1 out of 1,000,000.

Q. – The word "dose" is mentioned in the UV literature. What is dose and why is it important?

A. – UV dose is the product of UV intensity (total UV energy per unit volume) and residence time (total time the water is in contact with the UV light), expressed by the equation D = I x T. Typically, UV dose is expressed in the units mJ/cm² but can also be in microWsec/cm². The conversion between the two is, 1 mJ/cm² equals 1,000 microWsec/cm².

Knowing the UV dose of your disinfection system is important because different microorganisms require a different UV dose to prevent them from replicating. Viruses, for example, need to be exposed to a high UV dose, while Cryptosporium can be treated with a lower dose.

Q. – What level of UV dose is right for my application?

A. – The right UV dose to kill bacteria isn't always obvious and is directly dependent on the application. Many industrial applications are pathogen specific, meaning that the equipment and UV dose is chosen based on what does will kill the specific microorganisms or group of microorganisms in the water. For the average home owner, the target pathogens typically require low UV dosages to be eradicated. As an example, E coli requires a UV dose of 6 mJ/cm² for a 4-log or 99.99% kill.

Currently, you can find equipment on the market with one of three different UV dose levels: 16, 30, or 40 mJ/cm². The 16 mJ/cm² dose is based on a US Public Health document produced in the 1960s. Typical waterborne pathogens will be eradicated at this UV dose, but with the emergence of some new viral contaminants and with the lack of a safety margin, this dose is typically used in instances to treat non-pathogenic or nuisance bacteria.

Industry standards have long suggested that a disinfection system should provide a UV dose of 30 mJ/cm² at the end of the lamp life. This dose allows for nearly double the original US Public Health dose and allows for a comfortable safety margin. The most recent UV dose suggested is that of NSF International and the US EPA drinking water regulations, who suggest a UV dose of 40 mJ/cm². This dose is the direct result of the introduction of certain emerging pathogens (specifically rotavirus) that was found to require a dose of 36 mJ/cm² for eradication. LUMINOR publishes system flow rates at all three dose levels in their literature and allows the customer to make their own educated decision on what is best for their requirements.

Q. – I draw my water from the lake and have received a clean "bill-of-health" for the water from my local Health Board. Why do I need to disinfect my water?

A. – Even though you may have received a "negative" test result for bacteriological contaminants, it is important to realize that this is a single test and a single point-in-time. If you are drawing water from a non-municipal source, or a source that is of unknown quality, it is imperative that you follow some form of disinfection prior to the consumption of any water from that source. The reason is that there can be seasonal fluctuations in the quality of the water, and one single test does not guarantee a safe result for 365 days of the year. If you live near a local beach that happens to be closed periodically throughout the summer months, you will easily understand the reasoning. The bottom line is that if the source is unprotected, you are the only one who can ensure the safety of that source, and it is your responsibility to do so.

Q. – Once I install my UV, do I need to continue the bacteriological testing?

A. – Yes. Although you have taken the right step to install a UV system to protect you and your family from bacteriological contaminants, it is important to continue to test your water to ensure everything is working as intended. Having your water tested every six months will give you additional peace of mind and takes very little time.

Q. – I live in the city. Do I need to install a UV system?

A. – The answer to this question depends on your level of confidence. Although most municipalities do provide bacteriological safe water to their clients, and this is closely monitored by most provincial, state, or other regulators, there have been many instances around the globe where water has been provided from the municipality that may not be safe. In these cases, the municipality will inform their clients that the water is in fact unsafe for consumption and will suggest alternate means. In these cases, the UV system installed in your home acts as "insurance" for your water supply. The need for this level of insurance is up to you, but considering the fact that water is the number one key to life, it seems to be a very inexpensive way to offer the security that you may need for you and your family.

Q. – What is a Boil Water Advisory and how does it affect me?

A. – A Boil Water Advisory (BWA) will be issued by your municipal water provider if the municipal water being provided to your home or business is deemed to be unfit for human consumption. This may be the result of a number of circumstances, but the municipality is informing their customers that at this point in time, the water is not safe for drinking unless treated by some other method, such as physically boiling the water for a minimum of 1 minute at a rapid boil. Depending on the exact nature of the BWA, other precautions may need to be taken, such as avoiding baths or showers (especially if water ingestion is possible). The municipality will advise when the BWA is over and how to ensure your home's water supply is made safe.

If you have a UV system installed, you may not need to boil your water. Because UV light kills bacteria and many other microorganisms, a correctly working system with a high enough dose should kill any potential pathogens in your water.

Q. – Do I need to get a plumber to install the UV or can I do it myself?

A. – If your local state or provincial regulations allow it, any competent handyman or even a homeowner can install a residential UV system. The typical installation involves cutting into the cold water supply and installing the UV equipment with unions and the appropriate fittings. Once installed, the unit is plugged into a separate GFCI plug. Please refer to one of the installation manuals found within this site for more detailed installation instructions.

Q. – I see both POU and POE units on your site, how do I know what system I need to install?

A. – In deciding whether a point-of-use (POU) or point-of-entry (POE) unit should be installed, you should determine the potential locations (taps or faucets) within the home where people could consume water. If there is only one faucet in a small cottage or apartment, then you might consider a smaller POU system to treat a single location. The same would be true if you were looking to add either pre or post treatment to a residential RO system.

Typically, if there is more than one tap (a kitchen and bathroom) in a home, it's best to install a POE system in a main cold water location before any cold water lines split to send water to different locations in the home. The slight increase in price for a larger system and the POE installation ensures that all locations within the home are treated.

Q. – I don't drink the water in my bathroom, so why should I install a POE system that also treats the bathroom?

A. – You may decide not to drink form the bathroom tap, but what if you have guests, or someone who visits your home and assumes the bathroom water is okay for consumption? Where do you brush your teeth? If it is in the bathroom, then treating potential contaminated water in the bathroom is imperative. You don't have to swallow the water to become infected with a potentially harmful organism. Consider the brushing action against sore or bleeding gums and you can easily see the consequences. It is because of this that it is highly recommended that homeowners install a whole house POE system whenever possible.

Q. – Does UV need any pre-treatment?

A. – Yes. UV works extremely well at addressing the microbiological problems, but can be greatly impeded by the presence of other water chemistry issues. The main concerns for UV are that of water hardness, tannins, turbidity, iron, manganese, and UV transmittance. Remember our first question: How does UV light kill bacteria? The light waves break down the DNA in the microorganisms. If the light can't easily penetrate the water, it can't effectively kill the pathogens.

Hard water can physically stain the quartz sleeve component of a UV, so it is recommended that your water hardness be less than 7 gpg. Tannins should be less than 0.1 ppm or 0.1 mg/l. Turbidity should be less than 1 NTU. Iron should be less than 0.3 ppm (0.3 mg/l), and manganese should be less than 0.05 ppm (0.05 mg/l). Although this type of disinfection system can be used on waters with a UV transmittance of less than 75%, for residential applications at the systems rated flow, it is recommended that UVT levels be greater than 75%.

All these issued can be addressed by other water treatment equipment, such as filters and water softeners (many of which can be found on this site). The minimum pre-treatment that is recommended is the installation of a 5 micron prefilter in front of the UV system. Again, these systems can be found in the filtration section of this site.

Q. – Tannins are mentioned in the pre-treatment process. What are tannins and why do I need to remove them from my water before using UV?

A. – Tannins, also known as tannic acid or humic acid, are a nuisance contaminant that affects the functionality of a UV system. Tannin in drinking water usually comes from composting organic matter (i.e. leaves, vegetable matter, etc.) and is most likely to appear in springs and wells bordering swampy areas or near coniferous tress. This contaminant can color the water with a yellow hue and can cause staining on fixtures and affect the aesthetic quality of drinks. In relation to UV, tannins interfere with the UV's ability to penetrate into the water due to their absorption capabilities. For a UV system to function properly, tannins must be removed from the water via a special anion exchange resin.

Q. – What is UVT and why is it important?

A. – UV transmittance, or simply UVT, describes the measurement of a fluid's ability to transmit UV light. Typically, municipal water supplies have UVT levels of greater than 95%, whereas deep water wells typically have UVT levels around 85%. The UVT of surface waters can vary greatly depending on the source and surrounding geography. UVT can be easily tested with a spectrophotometer which most laboratories or UV manufacturers have. Waters with lower UVTs will require more UV energy to deliver a similar UV dose than waters with a higher UVT.

US Water Systems UV systems are tested at a 95% UVT level and their rated flow is based on the water having a minimum UVT level of 95%. If your water has a lower UVT, the system will deliver a lower UVT dose, or the flow rate through the system will need to be decreased to achieve the same UV dose. Dosage curves are available for all US Water systems to aid in this selection. As a precaution, it is recommended that all water treated by a UV system have a minimum UVT level of 75%.

Q. – I understand the need for pre-treatment, but how can I tell if I have any of the problems that potentially require pre-treatment?

A. – Before the installation of a UV system, your water should be tested to determine the basic water chemistry. The minimum tests should include hardness, tannins, turbidity, iron, manganese, and UV transmittance. US Water offers a complete range of water testing kits.

Q. – I've made the decision to install a whole home POE UV system, but where exactly do I install the system?

A. – The UV system should be the last piece of treatment equipment. Water softeners, filters, and any other treatment systems should all be installed before to the UV system. All the treatment equipment should be installed on the main cold water line feeding the house and before any branch lines, including those feeding the hot water heater.

Q. – I've been told that UV does not have any residual disinfection like chlorine, so how do I get rid of the bacteria that may reside in my plumbing system?

A. – Before the water is consumed after the installation of a UV system, the entire distribution system (piping) must be disinfected with chlorine to ensure that the UV system delivers its water through a disinfected distribution system. This can be easily done by adding household bleach, or similar disinfectant, in the prefilter housing (that must be installed with each UV system).

First, remove he cartridge and then fill the filter sump with the bleach. Then go to each tap in the house, run the water until you smell the bleach, and then turn off the tap. Make sure you flush the toilets, run water into the dishwasher and washing machine, and turn on any showers. Finally, run water through any outside hose bibs and any other location that has water. Leave the entire system for a minimum of 30 minutes, and then flush the system to remove the bleach. Once this is complete, the water will be ready for consumption. Throughout this process, do not shut off the UV system. Please note that, if you have a UV system that contains a UV monitor, the alarm may sound during this process as the bleach interferes with the proper reading of the UV monitoring system.

Q. – I have a vacation property and I only use it occasionally. Can I turn off the system to save energy?

A. – You can turn the system off if you are away for extended periods of time. However, you must remember that if you turn the system off, you will need to disinfect the distribution system each time you restart the UV system as there is a possibility that the bacteria that may exist in the water can migrate and replicate when the system is off and could contaminate the distribution system after the UV system. As UV has no residual disinfection, there will be no way to clean up the down bound distribution system besides disinfecting the entire system with bleach. Considering this and the fact that a typical household UV system consumes about as much power as a 40 watt light bulb, many people elect to leave the UV system on, even when they are away for an extended period of time.

Q. – I have a vacation property that I don't use in the winter. Do I have to worry about the UV freezing?

A. – Yes. As the UV is part of your regular water system, you will need to properly drain the water from the UV reactor the same as you would with any other fixture in your vacation property.

Q. – I've read that some UV systems turn the lamp on and off every time water is drawn. Should the US Water UV system be turned on and off?

A. – There are some countertop POU systems that are specifically designed to be turned on and off every time water is drawn from the faucet. These systems are specifically engineering for this function. US Water System's POU and POE systems should not be turned off unless it is for an extended period of time. The systems are designed for continuous disinfection and not for on-off cycling, as this can severely reduce the overall life of the UV lamp.

Q. – What makes the US Water UV lamps so special?

A. – The UV lamp acts as the heart of the UV system and US Water uses the best available lamp technology. All of our lamps are manufactured with a proprietary internal coating that provides consistent UV output over the entire life of the lamp. Ceramic lamp bases are used for thermal efficiency and structural integrity. The lamps bases are color coded and base marked for easy identification and maximum output.

Q. – How long do the UV lamps last?

A. – UV lamps do age and do so in the same manner as a standard fluorescent lamp. US Water System's low pressure (LP) and low pressure, high-output (LP-HO) lamps have a rated service life of 9,000 hours or approximately one year of continuous use. US Water's amalgam lamps used in our commercial/industrial and municipal lines have a rated service life of 12,000 hours.

At US Water Systems, we guarantee all our lamps to have a minimum life of 1 year. It should be noted that illumination does not necessarily mean disinfection. The bluish glow given off by a low pressure mercury vapor lamp is actually the glow of the mercury vapor. The actual UV energy providing the disinfection is invisible to the naked eye. Just because a lamp is still emitting the bluish light does not mean it is providing enough, or any, UV energy. This is why the lamps should be changed on time as recommended by the manufacturer. On some US Water UV models, there is an integral lamp change reminder built into the system to warn the user when it is time to change the UV lamp.

Q. – Should I install a bypass to allow for water flow in case the UV is offline?

A. - Yes. The installation of a bypass valve is highly recommended in order to allow water flow in case the UV system has to be removed for some reason. Please note that notification should be provided, and posted, at each location where water is drawn to ensure that no one accidently consumes contaminated water. When the system is put back online, a full chemical disinfection must be made on the distribution system.

Q. – Will my water pressure be reduced with the installation of a UV system?

A. – No. All US Water UV systems are engineered to achieve a minimal pressure drop for the specific flow rates of each individual model. Typically, pressure drops are less than 3 psi.

Q. – I've heard that I may need to clean the quartz sleeve in the unit. How is this done?

A. – Depending on the influent water quality, the quartz sleeves may need to be removed and physically cleaned. On most US Water UV systems, there is no need to drain the reactors as is the need in competitive systems. The sleeve is easily removed by removing the top retaining nut and carefully sliding out the quartz sleeve. A commercially available scale cleaner, such as Lime-Away or CLR, can be used. When cleaning the sleeve, always wear gloves to ensure that no fingerprints are left on the quartz and ensure that the sleeves are thoroughly rinsed with clean water to remove any of the cleaning agents before reassembling.

Q. – How do I know my UV system is working?

A. – All US Water UV systems come with a lamp monitoring system. If the lamp is not illuminated, an audible alarm will sound. Please note that this alarm deals with a "lamp-on" condition only. Lamps that are not changed out on an annual basis, or systems that do not follow the pre-treatment protocols, may give you a false sense of security. The only way to ensure the system is working properly is to obtain a bacteriological test.

Q. – What is the typical space needed for the average household installation?

A. – As the UV lamps are removed through the axial length of the reactor, a space equal to twice the length of the reactor is needed. In the case of a typical 38 lpm (10 gpm) reactor, this would mean a space requirement of approximately 165 cm (65").

Q. – Are the UV systems designed to be installed vertically or horizontally?

A. – As the UV unit is a self-contained pressurized vessel, the orientation of the reactor chamber is not a concern how the UV light kills bacteria. However, for ease of installation and service, it's a good idea to install the system in a vertical orientation with the retaining lamp (and lamp removal) located at the top.

Q. – If the units are rated at a certain flow rate, how do I make sure I don't exceed that rated flow?

A. – To be absolutely sure that the UV system does not exceed the manufacturers rated flow, the installation of a flow restrictor is recommended. These flow restrictors are designed with a variable orifice that fluctuates with variations in water pressure. The maximum flow is controlled by the restrictor, which is ideally installed on the outlet port of the UV reactor. Flow restrictors can be purchased through your local plumbing supply distributor.

Q. – Can I install a UV system outside?

A. – No. These systems are designed to be installed indoors and away from the elements. The manufacturer's warranty will not apply if the units are installed in an outside environment, or if they are exposed to spraying or splashing water.

Q. – I've been told my system must be NSF approved. Is this true?

A. – This does depend on where you live and on what type of application the unit is being installed on. You should check with your local government and should follow their recommendation. It is important to remember that NSF International is a voluntary testing organization and is just one of the many testing organizations that exist.

Q. – What if I need a part for my system, where can I get them?

A. – To obtain replacement components for your UV system, you can contact US Water at 1-855-923-6913 or e-mail us at

Q. – The water coming out of my cold water tap is warm. What's up?

A. – The UV system is installed on the cold water line in your home. How UV light kills bacterial and other microorganisms is by exposing the contaminated water to the ultraviolet lamp contained inside the stainless steel reactor chamber. Although the UV energy itself gives off no heat, the low pressure mercury vapor lamp ("light bulb") does emit energy in the form of heat. It is this energy that can, in some cases, warm the water up.

Heat may be a problem when there is a long period of dormant (no flow) activities, such as at night. The problem may be compounded if the UV system is located close to a tap. To fix this situation, and recommend by most municipalities, run the water for a few seconds before filling a glass. This will not only rinse the lines of any stagnant water, but will also flush the warm water from the system. Most people view this feature as an additional way to know that your system is still on and operating, as in most cases the UV is installed somewhere out of the way and away from normal view.

Q. – How do I dispose of my old UV lamp?

A. – The UV lamp should be disposed in accordance with your local regulations. The lamps should be disposed in the same fashion as any other fluorescent light fixture that may be in your home or office. At US Water Systems, we strongly encourage recycling.