Author: C. Reid Thornley B.Sc.

Article: Using ultraviolet (UV) light to purify drinking water is a concept that has existed for more than one hundred years. Despite its early beginnings, the science behind UV disinfection is complex. Understanding the fundamentals of how UV is able to purify drinking water requires a relatively deep understanding of physics, chemistry and biology.

The average consumer of this technology rarely has the science background to fully grasp how a "light in a pipe" will be able to protect a water supply from dangerous microorganisms. This often leads to consumer frustration which in many cases is exacerbated by faulty information provided by ill-equipped salespeople.

Often the outcome of this frustration is consumer inaction which is unfortunate because UV systems are an extremely effective and relatively inexpensive way to purify drinking water. The goal of this article is to explain, as simply as possible, the science behind the disinfection of drinking water using ultraviolet light.

UV light refers to wavelengths of light that exist between visible light and x-rays on the electromagnetic spectrum. UV can be broken into three subcategories: UV-A, UV-B, and UV-C. UV-A and UV-B are probably the best known of the UV wavelengths since they are responsible for giving us a suntan or sunburn. UV-C light is a much higher energy and much more damaging form of UV light. It is UV-C that is used in an ultraviolet light water purification system.

As mentioned above, UV-C is a very powerful and very damaging form of ultraviolet light. When a living cell is exposed to UV-C, specifically the 254nm (nm = "nanometer") wavelength, the light is able to penetrate through the cell and damage the DNA of the organism.

DNA, or deoxyribonucleic acid, is responsible for directing the activities within all living cells. All cells must have intact DNA in order to function properly. The structure of DNA is very similar to a ladder that has been twisted from both ends resulting in a spiral staircase appearance. It is sometimes referred to as a "double helix" since all DNA is actually comprised of two molecules. You can think of each side of the ladder as one molecule and the rungs of the ladder as the connection points between the two.

When cells reproduce, the end result is two identical cells each with a functional copy of DNA and all of the other necessary structures for the cell to function. This is true of the cells in humans, other animals, plants, and the bacteria and viruses that might be found in drinking water. So at some point in the cell replication cycle the DNA must be copied also.

When DNA replicates there is a special protein that travels the length of the DNA and splits it in half. Using the ladder analogy, this protein travels down the ladder and splits each rung. The result is two separate molecules that each resembles one side of a ladder. In normal DNA replication the protein travels down the ladder and as each rung is split each side is quickly rebuilt. The result is two identical strands of DNA - one for each of the resulting cells.

UV-C light has the ability to penetrate through the cell and attack the DNA. It actually fuses some of the rungs of the DNA ladder together. Recall above that during DNA replication a protein travels along the ladder and splits each rung. When the DNA of a cell has been exposed to enough UV-C light and some rungs have been fused, the protein responsible for splitting the replicating DNA can't do its job. When it encounters a fused rung it just stops, and the DNA replication stops with it. This prevents the cell from being able to reproduce. A bacterial or viral cell that cannot reproduce is not capable of causing infection. That statement bears repeating: if a bacterial or viral cell cannot reproduce, it cannot make us sick.

So, UV light does not "kill" microorganisms - it simply makes it impossible for them to reproduce, rendering them harmless to anybody that might ingest them in a drink of water. It is critical that the DNA of the organism is exposed to enough UV light in this process. When discussing UV water purification systems, the amount of UV light is usually called dose. UV dose is simply the amount of UV light an organism is exposed to as it travels through an ultraviolet water purification device. The good news about UV is that it takes relatively little exposure for a microorganism to be inactivated, and it's impossible to over dose UV light. Because of differing water conditions and in order to purify different water flow rates, UV manufacturers produce different sized models of UV water filter systems. For higher flow rates, a longer UV lamp is required. This is simply to ensure that an adequate UV dose is being applied. For slower flow rates a shorter UV lamp can be used since the water being treated resides in the UV system for a longer period of time.

About the author:
C. Reid Thornley is a B.Sc. Biology and a former research associate for a world class water purification manufacturer. He has been a presenter for the US Water Quality Association and he now owns and operates aQuatell - Water Purification Made Easy!