How Does UV Water Purification Work? A Simple Explanation

The sun is constantly emitting the same UV rays utilized to treat water

Ultraviolet radiation or more commonly, UV radiation is something most of us have heard of, particularly those who live in sunnier environments. Growing up in sunny climates we are bombarded with information on how to protect ourselves from the sun’s UV rays which can cause damage to our cells and even cancer. It is this same radiation that can cause sunburn that is utilized against microorganisms like bacteria and pathogens to purify water, the exact nature of the interaction between UV radiation and pathogens pushed me to write this article.

Ultraviolet radiation purifies water by disrupting the DNA of pathogens and bacteria, preventing their reproduction. UV energy is able to pass through cell membranes and protective layers, directly interacting with the pathogen’s DNA allowing its destruction without changing the composition of the water.

What is ultraviolet radiation?

Ultraviolet (UV) is a type of electromagnetic radiation with a wavelength shorter than that of visible light, but a longer wavelength than X-rays. UV radiation exists naturally as a part of sunlight and the corresponding radiation that it puts out, it can also be produced artificially by lamps and LEDs, like those used in UV water treatment. Although ultraviolet radiation is not considered ionizing radiation as its photons lack the energy to ionize atoms, it can cause chemical reactions. These chemical reactions that UV radiation can cause lead to many of its practical applications, in particular, its interactions with organic molecules like tanning beds and more relevantly water purification.

Tanning beds are another common application of UV radiation

History of UV water treatment

The idea of using UV radiation to disinfect water was first discovered in the mid-19th century in the form of utilizing direct sunlight. Two British scientists looked into the effect of sunlight on microorganism growth exposing brown sugar solutions to sunlight and shade, bacteria grew in the shaded samples however it was inhibited in the samples exposed to sunlight. This test was the first recorded scientific proof of sunlight/UV’s effect on bacterial growth in water.

Prior to the end of the 19th century, scientists started to understand the different wavelengths of light and used prisms to separate light into distinct wavelengths similar to a rainbow. Upon separating the light they realized that it was the ultraviolet portion of the light spectrum that inhibited the growth of microorganisms rather than all sunlight as previously thought.

The first large-scale UV water treatment facility was commissioned in Marseille, France in 1910, however, chlorine disinfection was the more popular method of the day so UV treatment facilities were far less numerous. If you happen to be curious about chlorination, I wrote the article What Is Chlorination: A Simple Explanation to succinctly explain the key points of the process.

Light is separated into different wavelengths in the form of a rainbow, UV is at the bottom

How does UV radiation purify water?

Arguably the most important question is, how does UV radiation actually purify water, how does it damage microorganisms? It had become clear to researchers that UV radiation did something to the cells which caused the microorganisms to cease to reproduce and survive.

The part of the cells that are rendered inactive is the DNA, the part containing genetic information required for reproduction, growth and development. UV photons penetrate through the cell walls and break the original chemical bonds holding the DNA together, with new bonds replacing them however the damage is done and DNA can no longer be replicated. As the UV-affected microorganisms have lost their ability to reproduce, they can no longer infect a host (Human or animal) that consumes them, rendering the water they are contained in safe to drink. It must be noted that UV water treatment does not kill nor remove microorganisms from the water, it merely renders them inactive and unable to cause harm.


Does UV treatment work for all pathogens?

Ultraviolet radiation has an effect against all pathogens, although, some pathogens have considerably more resistance than others to its impact. This question is something which is the topic of a lot of research, due to UV disinfection’s multiple practical applications in the healthcare world. A journal article published in NPJ clean water studied the effects of UV treatment in water-borne bacterial communities before and after UV treatment and then again after post-treatment storage. The findings were interesting in that UV treatment had a disinfection effect against all bacteria however post-treatment, certain bacterial families started to increase in number again. This increase highlights that there was some form of resistance to UV’s disabling effect on DNA and reproduction which allowed the bacteria to thrive when given time. From a practical sense, a key finding here is that UV treatment is best used as a purification method in line, right before consumption when it has been disinfected to the required levels. Due to some pathogens having a slight resistance to UV radiation, the treated water’s potable status can will often not last long-term if put in post-treatment storage. For water that is destined for storage post-treatment chlorination is often the most effective treatment technique, which explains its popularity worldwide.

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