UV Water Treatment

Spartan Environmental Technologies supplies UV Systems for water disinfection among other applications from NeoTech Aqua Solutions. This page and the associated links provide information on UV basics, applications and systems.

Ultraviolet light (UV) includes the wavelengths between visible light and X-rays in the electromagnetic spectrum. UV can be divided into the four regions: Vacuum UV (VUV), Short Wave UV (UV-C), Middle Wave UV (UV-B), and Long Wave UV (UV-A).

UV Lamp types

UV lamps are typically tubular with an outer casing, or “envelope” which is made from quartz. Inside the quartz envelope, electrodes emit electrons which cause an inert gas to ionize. A plasma is created and the inert gas becomes heated. This causes mercury atoms to vaporize and collide with the high-energy plasma electrons. The mercury atoms then fluctuate between many excited states and a ground state. During this fluctuation the mercury atoms release energy in the form of UV light.

The most commonly used UV lamps for water purification are Low Pressure (LP) and Medium Pressure (MP). MP lamps produce more UV energy across a broader range of wavelengths, however they require significantly more energy to operate. Low Pressure UV lamps are considered monochromatic with approximately 82% of the ultraviolet light emitted at 253.7 nm and approximately 7% emitted at 184.9 nm. These peak wavelengths are often referred to as 254 nm and 185 nm respectively.

In general, low pressure lamps require less energy to create a given level of UV output and therefore are more efficient than medium pressure lamps. In addition, LP lamps last longer, generate much less heat, and are less expensive to own and operate than MP lamps.

Medium pressure lamps, with broad spectrum UV and greater UV intensity, are often used for large volume applications (> 300 gpm), such as municipal water treatment systems where heat is less of an issue.

Disinfecting Water

UV radiation can be an effective for disinfecting water. UV works by destroying an organisms ability to reproduce and infect its host. Disinfection using UV radiation is more commonly used in wastewater treatment applications but is finding increased usage in drinking water treatment. It used to be thought that UV disinfection was more effective for bacteria and viruses, which have more exposed genetic material, than for larger pathogens which have outer coatings or that form cyst states (e.g., Giardia) that shield their DNA from the UV light. However, it was recently discovered that ultraviolet radiation is effective for treating the microorganism Cryptosporidium. Giardia in turn has been shown to be very susceptible to UV-C when the tests were based on infectivity rather than excystation.

UV for Ozone Creation

Ultraviolet light can be used to create ozone in air, which may be desirable for air purification applications. The creation of ozone requires the use of a lamp which is designed to allow 185 nm to transmit through the lamp envelope. 185 nm lamps will simultaneously create and destroy ozone. However, the ozone creation occurs at a faster rate and the net result is a propagation of ozone in air. UV lamps are also used for creation of ozone in water, but at very low rates of production.

UV for Ozone Destruction

Germicidal ultraviolet light destroys Ozone in water very quickly. Therefore, the 254 nm wavelength produced by low- pressure germicidal UV lamps is quite effective for ozone removal from water. The mechanism for removing ozone is dissociation, which occurs when 254 nm UV energy “breaks” one of the oxygen bonds in an ozone molecule. As a result of this reaction, each ozone molecule is converted into one oxygen atom and one oxygen molecule. Free oxygen atoms will combine with each other to form oxygen molecules.

Although ozone is readily destroyed by UV, it requires more UV energy than inactivation of microorganisms (approx 90 mJ/cm² versus 30 mJ/cm²). Therefore, in order to ensure effective ozone destruction, UV systems are often sized using a “flow-rate adjustment” of 40% the flow-rate of a disinfection system. For example, a system designed for 100 gpm disinfection flow-rate would be required to achieve 40 gpm for ozone destruction.

UV for Dechlorination

An increasingly popular dechlorination technology, with limited drawbacks, is ultraviolet (UV) treatment. Successful UV dechlorination applications range from pharmaceutical, food and beverage processing to semiconductor fabrication and power generation. In all these industries, dissatisfaction with conventional dechlorination methods has encouraged alternative methods to be found. The following link on when to apply UV dechlorination technology discusses the economic consideration.