What is ultraviolet light?

Ultraviolet disinfection systems are mysterious to many people: How can “light” kill bacteria? To start with, ultraviolet technology has been around for 50 years, and its effectiveness has been well documented both scientifically and commercially. Nature’s own disinfection/purification method, UV light is a form of light that is invisible to the human eye. It occupies the portion of the electromagnetic spectrum between X-rays and visible light. The sun emits ultraviolet light, but much of it is absorbed by the earth’s ozone layer.

A unique characteristic of UV light is that a specific range of its wavelengths, those between 200 and 300 nanometers (billionths of a meter), are “germicidal” – meaning they are capable of inactivating microorganisms such as bacteria, viruses, and protozoa. That has led to widespread adoption of UV light as an environmentally friendly, chemical-free, and highly effective way to disinfect and safeguard water against harmful microorganisms.

How does UV work?

Unlike chemical approaches to water disinfection, UV light provides rapid, effective inactivation of microorganisms through a physical process. When bacteria, viruses, and protozoa are exposed to the germicidal wavelengths of UV light, they are rendered incapable of reproducing and infecting.

UV light inactivates micro-

organisms via damage to nucleic acids. The high energy associated with short-wavelength UV energy, primarily at 254 nanometers, is absorbed by cellular RNA and DNA.

That absorption of UV energy forms new bonds between adjacent nucleotides, creating double bonds or “dimers.” Dimerization of adjacent molecules, particularly thymine, is the most common photochemical damage. Formation of numerous thymine dimers in the DNA of bacteria and viruses prevents replication and destroys the ability to infect.

What are the benefits of using UV?

For many decades, chlorine has been used successfully in drinking water treatment. However, some chlorine-resistant organisms, such as Cryptosporidium and Giardia, are harmful to humans. For example, in 1993, more than 400 people died and some 400,000 became ill as a result of a Cryptosporidium outbreak in the drinking water supply of Milwaukee, WI. UV is highly effective at treating these chlorine-resistant organisms.

The advantages of using UV disinfection include:

• No known toxic or significant nontoxic byproducts

• No danger of overdosing

• Some organic contaminants removed

• No volatile organic compound emissions or toxic air emissions

• No on-site smell and no smell in the final water product

• Very little contact time (seconds versus minutes for chemical disinfection)

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