Q&A Ultraviolet Disinfection

Q: What is ultraviolet disinfection?
Ultraviolet light is radiation that lies between the X-ray region and visible region on the electromagnetic spectrum. UV ranges from 200 nanometers (nm) to 390 nm. Optimally, germicidal action occurs at 260 nm.

UV is produced through the conversion of electrical energy in a low-pressure mercury vapor quartz lamp. Electrons flow through the ionized mercury vapor between the electrodes of the lamp, which then create UV light.

UV disinfection can kill or render harmless microorganisms such as bacteria, viruses, algae and protozoa. The process consists of the UV light breaking through the cell wall and cytoplasmic membrane, rearranging the DNA and rendering the microorganism harmless.

Proper pretreatment must be used to effectively destroy microorganisms to a 99.9 % kill rate. UV does not leave any residuals, so it should be located as close as possible to the final distribution system. Any water treatment device installed downstream of the UV system could recontaminate the water. UV systems, alone, do nothing to reduce organic or inorganic contaminants in your water. Used alone, UV radiation does not improve the taste, odor, or clarity of water.

2. Q: What are the advantages of UV?
Some of the advantages of using ultraviolet include:

- It effectively destroys microorganisms such as bacteria, algae, protozoa and viruses;
- Does not require chemicals, so it does not produce byproducts;
- It's readily available;
- Equipment is relatively easy to operate and maintain;
- It has short contact times;
- Can eliminate chlorine / chloramines and associated byproducts;
- In air treatment, can destroy mold spores;
- Does not affect the taste or mineral content of the water;
- It does not waste water.

3. Q: What are the disadvantages?
Some limitations of using ultraviolet include
- Reduced performance in water with high levels of iron or color; and
- Requires pretreatment with a sediment filter (5-micron nominal).

4. Q: Which factors may affect UVs performance?
Installation of the required sediment prefiltration (5-micron nominal) will eliminate the possibility for certain water quality factors, such as turbidity and suspended solids, to affect the performance of a UV system.

There are other water quality factors not taken care of by the standard, required prefiltration. Specifically, iron and hardness should be considered and pretreatment for these parameters may be required if levels exceed those specified by the UV manufacturer.

In addition, the location of UV could affect system performance. The UV system should be the last water treatment device before point of use because other devices may cause recontamination.

Q: What is advanced oxidation?
One of several processes making use of two or more treatment processes used conjointly for the specific purpose of oxidizing and destroying chemical constituents in the water. A common form of advanced oxidation makes use of UV in combination with hydrogen peroxide. UV may also be applied in conjunction with ozone. This process accelerates the decomposition of ozone, yielding a sufficient quantity of hydroxyl radicals to affect water purification and lower disinfection byproducts. Advanced oxidation can be a combination of:

- Ultraviolet and hydrogen peroxide;- Ultraviolet and ozone;- Ozone and hydrogen peroxide; and/or
- Ozone and elevated pH.

Sources: 1. IUVA.org. 2003. 2. National Drinking Water Clearinghouse. Tech Brief One, June 1996. 3. Triangular Waves Technology, Inc. 4. McGowen, Wes. All About Water, Scranton Gillette Communications, 2000. 5. NSF International. Drinking Water Treatment Units: A Certification Guide. 6. www.wqa.org. The Water Quality Association, 2003.