Does UV Light Kill Germs in Water? Let’s Find Out!

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Drinking germ-infested water can lead to many unpleasant health effects, so it’s only natural to look for ways to protect yourself.

UV water purifiers are marketed as one of the best methods to do so.

But does UV light really kill germs in water? Let’s find out!

Key Takeaways

  • UV light kills germs in water. It eliminates bacteria, fungi, protozoans, and viruses.

What Is UV Light Water Purification?

UV light water purification is a water treatment method that uses UV light to kill harmful microorganisms in water. UV light purification is exceptional at eliminating waterborne bacteria, protozoa, fungi, and even viruses.

The device that carries out the water disinfection is called a UV water purifier.

How Does UV Light Kill Germs?

UV light is perfect for destroying microorganisms. Brief exposure to UV rays of 254 nm wavelength destroys the DNA of most germs, making them unable to reproduce which renders them harmless.

Although ultraviolet light kills at least 99.99% of most microorganisms in water, there’s a small catch: UV purifiers can only perform optimally when no solid or suspended particles are in the water. This is because said particles can shield germs from the UV rays and compromise the disinfection process.

Thus, usually the best way to use a UV water purifier is to install pre-filter stages to provide for ideal water conditions.

Is UV Light Effective Against All Types of Germs?

UV light is effective against all types of germs. It works against bacteria, fungi, protozoans, and even viruses. Depending on the exact process applied, UV light kills up to 99.9999% of all germs.

Note: Although UV purifiers kill the germs in water, they don’t remove their dead bodies. So even after the germs are killed, they will continue to float in your water supply – which is not a health issue at all!


Some of the most common bacteria present in tap water include E. coli, Vibrio, and Salmonella. These bacteria are responsible for many waterborne diseases and cause gastrointestinal disturbances like vomiting, diarrhea, and intestinal cramps.

Fortunately, you can count on UV light to eliminate them.

bacteria in water


Viruses are the tiniest form of microorganisms. They’re often present in water and typically enter our water supply through the fecal waste of humans or animals. Common viruses found in water are the Rotavirus, Adenovirus, and Hepatitis A virus.

UV light helps to deactivate these viruses. However, a higher UV dose is required.


Most protozoans in water do not pose health risks; only some are responsible for waterborne diseases. Two of these disease-causing protozoans include giardia and cryptosporidium.

UV light can eliminate these and other protozoans. But, they tend to have thick cell walls that make it harder for low-power UV light to penetrate them. So if you’re facing this type of germ, ensure that your UV light system is strong enough to kill it.


Fungi in water are especially dangerous to people who are immunocompromised. Immunocompromised people include people with chronic illnesses, the elderly, babies, and pregnant women.

Aspergillus fungi are the most common fungi found in drinking water responsible for 90% of contamination. The good news: UV light can take them out.

Do I Need to Pre-Filter My Water Before UV Disinfection?

Using a UV system without pre-filters can give you sub-optimal results. Without pre-filtering your water, microorganisms can hide behind dirt avoiding the UV light (this issue is called shadowing).

The most basic pre-filtration for UV purification is done using a sediment filter. This filter removes dirt, rust, sand particles, and more. Depending on your water conditions, you might also need water softening, iron removal, and more.

UV Water Filter Maintenance

UV water filters are not very high-maintenance, but you still need to do your part to ensure they work at full capacity. There are two parts of the UV filter we need to look at when it comes to maintenance: The UV light bulb and the quartz glass sleeve.

  • The UV light bulb typically requires replacement at least once a year. This is because most UV bulbs have lost 40% of their power after 9,000 hours of continuous use.
  • As for the quartz glass sleeve, you’d need to clean it and check it for permanent fogging. However, you don’t need to clean the quartz sleeve too often; an annual clean-up is usually enough to handle most of the build-up.
  • Remember that the fogging becomes permanent after a while, and you’d need to change the quartz glass sleeve. You can replace it once every two years.
  • Another part of a UV filter to watch is the sealing rings at each end of the reactor chamber. The sealing rings usually work for a long time without leaking, but you should always look out for issues. Unaddressed leaks can balloon into bigger problems.

Test Your Water After Treatment

Testing your water after treatment helps to know if the UV disinfection process is effective.

Here are 2 ways to test your water:

Testing At a Certified Lab

Testing your water at a lab gives you detailed and accurate results. However, you have to test your water at an EPA-certified lab. The EPA has standards that all labs must meet. You can find an EPA-certified lab on the EPA website. Most states have multiple approved labs, so finding one won’t be a problem.

Using a Test Kit

If you’d like to test your water yourself, then you can use a DIY water test kit. DIY kits are cheap and quite accurate. They’re also uncomplicated, so you don’t need any expert science knowledge. There are clear instructions on the pack for you to follow.

If you have any thoughts about the question, does UV light kill germs in water, please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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