Ultraviolet (UV) Light

What is UV?

Along the electromagnetic spectrum, there are several types of waves. Visible light only takes up a small section of that scale. Even though the human eye can’t see UV waves, they can cause biological changes to living organisms. There are several different types of UV waves: UV-A and B waves are used in tanning beds and are present in sunlight, which allows them to cause sunburn. UV-C has a strong germicidal effect. It can destroy bacteria, mold, viruses, and spores. It is this type of ray that is used in water disinfection. Some exposure to UV, usually through sunlight, is necessary for good health. Sometimes, UV lamps can be used for medicinal purposes as well, to treat psoriasis or jaundice.

How is UV Used in Water Treatment?

UV treatment has been used for decades to provide safe, disinfected water to many homes and businesses. This type of UV radiation penetrates the microbes, permanently changing their DNA, inactivating or disabling them, preventing them from reproducing or infecting. UV treatment does not change the chemistry or taste of your water, since nothing is added to the water. Unlike chemical treatment, UV treatment does not cause harmful residual effects for humans or aquatic life.

UV is used in municipal water treatment plants, wastewater plants, and can also be used in homes and businesses. This treatment is especially important if the source water comes from a well or a surface water source like a pond or a lake. Wells, lakes, and ponds are highly susceptible to bacteria, viruses, and protozoa, particularly those from animal and human waste, like cyclospora, e.coli, and giardia lambia. Some estimates state that over 20% of all residential wells in America are contaminated with these dangerous microorganisms. Cryptosporidium and giardia are resistant to chemical disinfection and are too small for typical water filtration. UV irradiation is the only option for fighting against these dangerous bugs.

To be effective, the UV treatment system should always include effective pre-filtration. Pre-filtration options might include sediment filters, carbon filters, iron removal systems, and water softeners depending upon your water quality. The minimum requirement for pre-filtration is a 5 micron sediment filter.

Whether at point of entry or point of use, the UV system should be plumbed as the last stage of the water filtration or treatment system. Before entering the UV chamber, iron, sediment, tannins, or other particulates should be removed from the water. These contaminants can coat the UV lamp, blocking the UV radiation. Further, the viruses and bacteria are much smaller than most of these floating particulates. By "hiding" behind these larger pieces, the microorganisms may not be “zapped” by the UV radiation. The UV light has to penetrate the microorganism to disable it.

The effectiveness of a UV system is measured in the percentage of microorganisms that are rendered inactive. This is described in terms of the “log reduction scale.” Because different microorganisms require different levels of UV dosage, it is advisable to choose a higher log reduction level, allowing for a higher percentage of reduction. Many water professionals will suggest a system that will provide 4-6 log reduction.

Log Reduction Microorganism Elimination
1 Log Reduction 90% Greater Assurance of Microorganism Elimination
2 Log Reduction 99%
3 Log Reduction 99.9%
4 Log Reduction 99.99%
5 Log Reduction 99.9999%

Factors to Consider When
Choosing the Right UV System

Source Water

The short answer is: “it depends on your water.” Your source water will be a large factor in deciding which UV system is the right fit for your home or business. Before purchasing a UV treatment system, consider where your water comes from. Different UV systems should be used for different source water.

Lakes, ponds, and even mountain streams can carry dangerous contaminants, particularly parasites and diseases from animal waste. Sewage treatment overflow or flooding can also carry contaminants for miles into a lake, pond, or a well, contaminating the water going to the tap. Even bacterial tests may not always be accurate as the source water may test acceptable one week, but fail the following week.

Even municipal water may not always be safe to drink. Water may be of acceptable quality when it leaves the plant, but as it travels through miles and miles of pipes, it can pick up bacteria, viruses, and other microorganisms. This is why "boil alerts" are issued so frequently. Implementing a UV treatment system can allow a protective final barrier, keeping out the bugs that might make people sick.

The NSF has broken UV systems down into two rating systems based on source water and the likely dosage required for that water: Class A and Class B. Class A systems can be used with surface water, including wells and lakes. Class B systems should only be used with municipal drinking water that has already been filtered and treated for human consumption.


To inactivate the microorganisms, the UV lamp must fully penetrate the water and the bug. In addition to considering source water, two other factors should be considered to make sure that the water receives the correct UV dosage: the intensity of the lamp and the water's contact time. Measuring the lamp’s intensity and the water’s exposure time is expressed in mJ/cm².

Here are some examples of typical doses:

Dosage Level Type of Source Water Example Applications
16 mJ/cm² Municipal water or water treated for human consumption Post reverse osmosis, water coolers, other point of use applications
30 mJ/cm² Some wells and households Many commercial and residential applications
40 mJ/cm² Required for potentially unsafe water sources including wells, lakes, and rivers. The EPA and NSF standard for these sources. Food service, restaurants, hotels, pharmaceutical manufacturing, and municipality regulations.

Flow Rate

The water’s flow rate through the UV treatment system is perhaps the most important factor to achieving the correct dosage. The faster the water flows through the UV chamber, the lower the UV dose will be. The slower the flow, the higher the dosage. All of the UV systems that Fresh Water Systems sells are clearly marked with the required gallon per minute flow rate.

Click here to calculate the correct flow rate for your home or business.

Advantages to UV Water Treatment

  1. UV is immediate. There is no storage tank or waiting needed.

  2. UV radiation is more effective on viruses than chlorine.

  3. Unlike other chemical treatments, UV systems are economical. Once installed they can treat hundreds of gallons for just a few pennies of operating costs.

  4. UV systems can be used in conjunction with other products like water conditioning or reverse osmosis.

  5. Unlike chemical treatments, UV has no residual effects on humans or marine life.