Photo of a UV light water sterilizer - ugh Choices of UV Light Systems for Water Treatment Method of Contaminated Drinking Water

  • UV ULTRAVIOLET LIGHT WATER TREATMENT - CONTENTS: Water treatment for bacteria using UV Lights - ultraviolet light sterilizers on water systems. How does an ultra violet light fixture correct bacterial contamination in well water? How do UV lights work? Details of proper UV light installation and maintenance for well water treatment. Water treatment methods for contamination, bacteria, lead, minerals, etc. Water treatment choices for odors, smells, sediment, cloudiness. Choices of types of water treatment equipment
  • POST a QUESTION or READ FAQs about choosing, installing, & maintaining or repairing UV light water treatment systems or "water purifiers"

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UV Water Purification Systems guide: Here we explain procedures & reasons for using permanantly-installed UV light (ultra violet light) treatment systems to address bacterial contamination in drinking water or well water - one of the options for correcting unsanitary or unsatisfactory drinking water.

This series of articles explains many common water contamination tests for bacteria and other contaminants in water samples. We describe what to do about contaminated water, listing common corrective measures when water test results are unsatisfactory. We include water testing and water correction measures warnings for home owners and especially for home buyers when certain conditions are encountered, with advice about what to do when these circumstances are encountered.

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UV - Ultraviolet light as a Water Purifier - Installation, Inspection & Maintenance Guidelines for UV Water Treatment Systems

UV Disinfection system (C) Daniel FriedmanUV water treatment systems combine a

  • water disinfection chamber (a quartz tube or quartz sleeve that contains an ultraviolet light and keeps the bulb physically separated from the water),
  • quartz UV light bulb that kills bacteria in water as water passes past the light source
  • electrical and plumbing connections that provide electrical power to the bulb, allow water to pass through the unit, and include shutoff and drain valves for the equipment. The shutoff valves make unit cleaning and maintenance easier.
  • optional controls to sound an alarm or stop water flow if the UV system stops working

For the UV treatment system illustrated in most of the photographs in this article, we installed a Siemens Ultraviolet Water Disinfection System (formerly produced under the name Sunlight systems) shown in the photograph at left.

The UV disinfection system is installed either in a kitchen or point of use, or when protecting a whole building it is installed where water leaves the well pump and pressure tank and enters the building, so that water flowing through the disinfection system is exposed to a UV lamp to carry out the water disinfection process.

The "UV" light can indeed kill bacteria in water, provided that the flow rate of water through the device is not too fast (needs exposure time) and provided that the water is not too obstructed with sediment and provided that the light source is cleaned - the bulb needs to be changed periodically.

Proper UV Disinfection System Installation: How do we know the UV light is properly installed and effective in treating drinking water?

If you are relying on a UV light to treat bacterial contamination in drinking water these considerations apply:

  • Proper UV light selection for water treatment: The UV light must have been properly selected to handle the volume and flow rate of the water supply where it is installed. If ultra violet light is not of sufficient capacity to treat water flowing past its bulb it will not be effective.
  • The UV light water treatment system must be properly installed - at the right location in the water system;

Disinfecting building piping (C) Daniel Friedman Disinfecting building piping (C) Daniel Friedman

  • Sanitize the building water tank & piping: 13 Steps to Disinfect building piping: the UV disinfection system manufacturer may also recommend that the building's water piping be disinfected after the light has been installed. We accomplished this process for the installation shown here as follows:
    • Turn off electrical power to the UV light disinfection system.
    • Turn off water at the point of entry into the whole house water filter; leave the water filter bypass loop closed as well.
    • Drain building piping: connect a garden hose to the UV disinfection system drain or another convenient piping drain point, and drain water out of the building piping.
    • Remove the whole house filter canister and set the filter aside on a clean surface (paper towels are fine).
    • Fill the water filter canister with bleach solution;
    • Replace the bleach-filled canister onto its mount
    • Turn on water: Slowly open the water supply into the bleach-filled canister and into the building, first making sure that building faucets have been turned OFF. This will send disinfecting bleach-water mix into the building piping. (An accurate measure length of time that bleach-solution needs to be in contact with a surface to disinfect it is a function of the strength of the solution.)
    Chlorine test kit (C) Daniel Friedman
    • Fill building piping with bleach solution: By opening building faucets (photo above, right), re-fill the UV disinfection device and house piping with the bleach solution.

      We used chlorine test strips (photo at left) to confirm that we had adequate bleach solution in the piping by testing at each faucet. The right-hand square shows the chlorine concentration between 4.0 and 10.0 ppm in this sample. The other color squares report on water pH, alkalinity, hardness, etc.

      A low-tech method often used is to run water at each fixture until you smell bleach, then turn the fixture off.
    • Replace water filter cartridge: Turn off water at the point of entry to the whole house water filter, remove the water filter canister and replace the filter cartridge. We add a few ounces of bleach again into the canister to disinfect the filter cartridge after touching it, then we flush this water out of the system as described below.
    • Disinfection time: Allow the bleach solution to remain in the piping for 24-hours.
    • Turn on electrical power to the UV light disinfection system
    • Thoroughly flush out bleach from the system before using the water supply. Run water at least until there is no bleach odor at any faucet.
    • SAFETY WARNING: Failure to flush out all disinfecting bleach from building water piping is dangerous - drinking bleach is poisonous and can be fatal; leaving even a small amount of bleach in the building piping can result in accidental bleaching of the next laundry load, and it could accidentally subvert a follow-up bacteria test.
  • The water flow rate must match the UV light specifications - the manufacturer of the UV light for water treatment will specify the maximum permitted water flow rate through the light fixture. If water flow rate exceeds that number (7 gpm, for example), the light might not be fully effective in killing all bacteria in the water. (This risk is higher if the starting bacteria count was high.)

    The UV light kit will probably include a flow control device (or one is purchased at a local plumbing supplier) that is installed at the inlet side of the fixture - a small disc that is inserted into the plumbing connection on the light fixture.
  • The UV light water treatment system must be properly maintained - follow the manufacturer's instructions for periodic inspection and maintenance. If the bulb becomes dirty or is in fact not operating (burned out) it may not be obvious without inspecting the unit.

    Do not touch the UV light bulb glass surface when installing a new bulb. Oils from your fingers may cause damage to the bulb. We discuss UV disinfection system maintenance and detail below
  • A whole house filter may be necessary or recommended by the installing plumber. When a UV-light water treatment system is installed on a water supply that contains high levels of sediment or iron, those particles can settle on the UV bulb surface, ultimately blocking light and interfering with the UV water treatment system's effectiveness.

    A whole house filter can protect the Ultra Violet Light bulb from debris clouding. For the UV treatment system illustrated in most of the photographs in this article, we installed an Aqua-Pure whole house filter. Also see Advice for Water Testing after UV Installation
  • Automatic shutoff valve on UV disinfection systems: some UV light disinfection systems include a "normally closed" solenoid switch. This switch will "open" or turn off the water flow in the building if the UV lamp burns out or if the UV intensity fails.

    This switch will also stop water flow during a power failure. (At a home with a private well and well pump, a power failure will also stop the well pump from running anyway.)

    Some UV solenoid switches include a manual override (look for a white toggle switch on Siemens units) that can be used in emergency to deliver water even though the disinfection system has stopped working. If the UV solenoid switch turns off the water supply (other than during a power failure) you will need to determine and correct the problem:
    • There may have been a power failure
    • The UV lamp may have burned out
    • The UV lamp intensity may have fallen too low
    • The solenoid switch itself may have been damaged
    • See No Water Pressure for a detailed procedure for diagnosing loss of water pressure, since there are other possible causes of that problem.

Maintenance Instructions for UV Light Disinfection Systems

UV disinfection light operation (C) Daniel Friedman UV disinfection light operation (C) Daniel Friedman
  • How to tell if the UV light is on: even with the cover installed, on some UV disinfection systems you may see a small amount of purple or violet light shining on a nearby wall or leaking out of the cover (photo above-left). Removing the cover on a UV unit will show the light more easily.
    • UV Bulb light: As we show in our photo at above-left with the UV disinfection system cover in place, look for visible UV light from the UV bulb itself. It may be necessary to loosen or remove the cover to see light from the bulb (photo above-right) but be sure to see our safety warning just below - do not look directly at the bulb and do not pull the UV bulb out of its quartz tube when power is on to the unit.
    • UV Shutoff valve: If an automatic UV disinfection system shutoff solenoid valve is installed, review our description of that device at Automatic shutoff valve above.
    • UV Indicator light: Some UV disinfection systems include a status indicator lamp or bulb: a green LED indicates that the UV lamp is on and the UV disinfection system is operating.
    • UV Audible alarm: Some UV disinfection systems include an audible alarm that sounds if the unit stops working (and provided that there has not been a power failure)
    • UV Monitoring: UV disinfection systems that include an indicator light or alarm also include circuitry that monitors the effectiveness of the bulb disinfection system. The monitor is watching the bulb output, not water purity.
  • Watch out: Avoid exposure to the UV light when the system is on. UV light may be difficult or impossible to see when the UV disinfection system is operating - by design. Exposure to UV light is harmful to skin and eyes. If a cover has been removed from a UV disinfection system that is operating, do not look directly into the light. Unplug the unit before installing or removing the UV lamp.
  • How to tell if the UV disinfection system is working effectively: periodic water testing for bacteria is recommended when relying on a UV light to handle bacteria in the water supply.

    OPINION: It is uncertain how one knows that a UV light water sterilizer system is working without periodic testing and when necessary, UV bulb replacement. Furthermore, UV light treatment systems do not remove other contaminants (if any are present in the water supply, such as chemical contaminants, odors, or other problems that may be present.
  • Periodic changeout of the whole house water filter cartridge will be necessary, depending on the level of sediment and debris in the water supply. If your filter becomes visibly dirty, if the building water pressure drops, or if the water pump begins rapid short-cycling on and off, a clogged water filter could be the cause.
    See WATER FILTERS, HOME USE for details.
  • Periodic changeout of the UV light bulb: the manufacturer recommends that the bulb be changed annually and that the quartz sleeve is cleaned regularly. Some UV light disinfection systems include a quartz tube cleaning system that permits the owner to clean the quartz sleeve without having to disassemble the disinfection chamber.

    Otherwise, if the quartz sleeve becomes soiled with sediment you will need to turn off power to the unit, remove the cover, carefully remove and set aside the UV quartz bulb, shut off water to the disinfection unit, drain the unit, and remove the quartz sleeve for cleaning.

    Watch out: Be careful not to drop the quartz sleeve - it breaks easily. If you do break the UV light disinfection system quartz sleeve, don't panic: order a new sleeve from the manufacturer. The disinfection system model and serial number are on the unit, or on its cover.

What Contaminants are Removed by a UV Light Water Treatment System

Photo of a UV light water sterilizer - ughNothing is "removed" from the drinking water, but if the system is working properly, the UV light will kill bacteria in the water supply. The dead bacteria, along with other water contaminants (if there are other contaminants in the drinking water) are not removed by the light.

OPINION: Property sellers often install this quick and least-expensive "solution" in the course of a real estate transaction in order to meet the minimum requirements of a buyer's lender to provide "potable water."

Since the lending bank usually requires only a bacteria test as a measure of water potability, the new owner/residents may not know whether or not there are chemical or other contaminants in the water supply. When a UV light has been installed on a water supply, our recommendations for further diagnostic testing are outlined just below.

Look For the Source of Bacterial Contamination in a Well

Since one of the most common ways that bacteria enters a well is through a defect at the pitless adapter (joining the water pipe to the well casing) or at a buried well cap that is leaky or open. In such cases the presence of bacteria in water is really an indicator of ground water leakage into the well.

Surprisingly we have found significant non-coliform bacteria levels in well water traced to

Spiders living in the top of a well casing: the spiders eat insects, dropping flies or other insects into the well casing, a possible source of un-wanted bacteria.

Dead animals in the well? On occasion a mouse or other small rodent falls into a well casing, causing temporary contamination.

Well contamination sources such as these are easily corrected by sterilizing the well and its casing interior.


Should You Test for Other Contaminants in Well Water?

If ground water or surface runoff have been leaking into a well, this means that anything that is on the ground or in the soil around the well is likely to be entering the water supply. So treating for bacteria may be failing to address other contaminants. Further testing for other contaminants would be appropriate if a well fails a bacteria test.

In particular, if the well is located at a property where chemicals are likely to have been applied nearby, such as near an orchard, farm property, animal barns or pens, or where pesticides have been in use, or close to a gas station, or where other contaminants are known to have been in local ground water, tests for those specific contaminants are a good idea.

If a property is in a residential neighborhood that has not included farms, orchards, barns, gas stations, or nearby industrial processes, the chance of finding these other contaminants in a well is certainly much less.

See WATER TESTING ADVICE for an outline of when to test for what contaminants in well water.

See WATER TESTS for CONTAMINANTS for a list of articles on water contaminants and water tests.

Advice for Testing Water Systems after UV Light Installation

Often conditions cause for follow-up water testing after a UV water treatment light has been installed. For example a property owner may discover bacterial contamination in the water supply when a home is being sold. If the initial water test shows that the starting bacteria count is low - just a few CFU's/L, and if there is no reason to have concerns for other water contaminants, testing for bacteria again after the UV light has been installed is proper procedure, and should produce acceptable results.

Here are some post-UV-light installation water testing considerations:

  • If the water flow rate through the UV treatment device is higher than the device manufacturer specifies, then that condition can also interfere with and reduce the effectiveness of the UV light in killing bacteria in the incoming water.
  • If the installer did not install a whole house filter upstream (before) the UV light, if there is debris and sediment in the water, that material can obscure the UV bulb and interfere with its effectiveness.

What if the UV Light is Installed Without a Water Filter?

A reader recently asked for advice for a case in which the UV installer forgot to install the water filter. The water filter was to be re-installed the next day, but the homeowner had already observed a lot of debris coming out of faucets in the building after the new UV light was installed. That high level of debris may have been due to disturbances of piping and water tank during the plumbing work, or the incoming water supply might simply be high in debris and sediment.

Actually, flushing out the plumbing system before re-installing the cartridge in the water filter is not a bad idea because often during plumbing work lots of extra debris is stirred up or even introduced into the system. But the filter needs to be in place to protect the UV light from becoming obstructed.

We advised the following for this case:

If the person taking the sample fully flushes the system and piping before collecting the water test sample it is likely to "pass" with acceptable results (in most U.S. communities that's a count of less than 1 CFU/L or in a "presence/absence" test, with a result of "absent").

When we recently installed a UV light water treatment system, preceded by a whole house filter (which is important for UV effectiveness as it keeps debris off of the bulb), we also sterilized all of the house piping using the well shock procedure at WELL CHLORINATION & SHOCKING. The reasoning is that having been running a building plumbing system with bacteria in the water supply, there might be pockets of bacteria in debris trapped in the system.

In the particular case under discussion, the water test for a home buyer was planned for the day after the water filter was to be installed. Thus the home owner would not have time to go through the well shocking procedure in time for the water test.

Therefore the homeowner needs to either

  • delay the test for at least a week or better two (thus making the later test most reliable and avoiding any charges of tampering after the well shock), perform the well shock procedure, and flush out and wait period (4 days minimum, longer is more reliable), then perform the water test.


  • perform the water test on schedule with a thorough flush-out of the building water supply piping at the test point first. As the starting bacteria level was in this case reported to be low, in this case it's likely the test will produce acceptable results.

If it does not, then go through the well shock procedure as described and things will either be just fine, or the UV system is not properly installed or not properly working.

Limitations of UV Disinfection Systems for Drinking Water

Watch out: Factors that affect the effectiveness and operating time of any UV light used to treat water in an effort to make it potable, or at least improve its potability include at least the following:

  • What contaminants are present in the water. UV treatment does not address chemical contaminants, nor do some water filters.
  • The water temperature. (The SteriPen includes a temperature sensor)
  • The water clarity - the level of particles in the water and particle size have important effects on the UV disinfection process

    Qualls (2008), in a study discussing the advantages of UV light as an alternative to chlorination, point out that filtration is often necessary to reduce the level of suspended particles in water for UV treatment to be effective. Abstract excerpts:

    It is well known that aggregation of bacteria and viruses provides some degree of protection from halogen and ozone disinfection. Adsorption of microbes to inorganic surfaces such as clay provides little protection. However, organic particles can protect organisms from disinfectants and can become a major limiting factor in disinfection. Particulate materials also effect [sic] UV disinfection.

    Clays do little to inhibit UV disinfection because they tend to scatter UV light rather than absorb it. Oilver and Cosgrove attributed the difference between microbial survival in irradiated raw wastewter and secondary effluent to differences in particle sizes. They believed that bacteria inside aggregates of particulate matter were at least partially protected from UV light. They found that a sample dispersed by ultrasonication was more sensitive to UV disinfection.

Using UV light in a wide range of applications inculding the control of bacteria, mold, and algae or moss growth is also discussed at UV LIGHT BLACK LIGHT USES


Continue reading at WATER TREATMENT EQUIPMENT DISINFECTION or select a topic from the More Reading links shown below.

Or see UV WATER DISINFECTION, PORTABLE for emergency or camping use


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UV ULTRAVIOLET LIGHT WATER TREATMENT at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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