How Does Ultraviolet Water Treatment Work?
What is UV
UV is Ultraviolet radiation, an energy band within the electromagnetic energy spectrum. Its wavelength is between that of visible light and x-rays and it has been found to be an effective method for destroying germs in a water supply. At a 254nm wavelength, UV reaches its peak germicidal effectiveness against most microorganisms.
UV destroys germs causing a molecular change in their DNA makeup. This prevents germs from multiplying and destroys the ability to spread disease. When germs cannot multiply, they are considered dead.
Different germs can tolerate different amounts of UV light and therefore require varying amounts of UV energy to be destroyed. This energy level is called the dosage. By definition, dosage is the intensity of UV light multiplied by time. The intensity is the amount of UV energy that the UV lamp produces at a certain distance from its surface per square centimeter of the lamp’s area. The time is the period it actually takes the water to travel inside the UV chamber.
How is UV created?
UV is created when an electrical current flows between two electrodes inside a UV lamp-shell containing mercury vapor. UV lamps are similar in design and construction to fluorescent lamps. The main differences are that fluorescent lamps use a coat of phosphorous on the inside surface to filter out UV light, where as UV lamps use none. Fluorescent lamps do not normally use hard glass which is commonly used in UV lamps to enhance UV light transmission.
Factors affecting UV Effectiveness
UV light can only be effective if it hits its target. If for example, UV light is prevented from reaching its mark due to suspended solids or other impurities in water, then UV will not be as effective as it can be. The following are the main obstacles that will reduce the effectiveness of UV disinfection:
- Suspended solids. These will act as an umbrella, ‘protecting’ germs from UV light. Suspended solids should be physically removed from water by pre-filtration before it goes to an UV unit
- Iron/manganese will cause discoloration on the quartz sleeve that surrounds the UV lamp and reduces the transmission of UV light, they will also absorb UV energy. They should be removed from water by pretreatment or at least reduced to no more than .3 ppm for iron and no more than .05 ppm for manganese.
- When water is hard, scale will eventually build up on the quartz sleeve around the UV lamp and will stop the UV light from passing through. A water softener should be used before an UV unit if the water is hard.
- Other organic and inorganic dissolved substances can also reduce the germicidal effectiveness of UV light and should be pre-treated before an UV unit.
Limitations OF UV Water Treatment
Excessive bacteria counts in water may require additional UV dosages or chemical treatment. Other factors, such as water temperature, should also be considered. UV units are normally designed to operate best between two and forty degree Celsius. Freezing will cause damage to the UV unit and water temperature higher than forty degree C will cause a reduction in UV energy and therefore, a reduced germ kill ratio.
Advantages of UV Water Treatment Systems
UV has many advantages over other disinfection processes:
- UV is effective and quick. No need for holding tanks and reaction times. No need for storing chemicals.
- UV does not alter the taste of water, which makes it ideal for use in bottling plants and food processing applications.
- UV is safe. NO need to add or handle hazardous chemicals or risk polluting the environment.
- UV is compatible with all other water treatment processes. No need for de-chlorination if using RO systems. In fact, UV enhances the use of other water treatment by keeping them free from germs.
- UV is economical. Almost always, the cost of UV disinfection units is much less than the cost of chemical treatment systems. The cost of service and maintenance of UV units is very low. The electrical running cost of an UV unit in a house is about that of a regular light bulb.
- UV is more effective against viruses than chlorine.
- Easy installation. UV units are very easy to install and require very little space.
The use of UV systems is found in residential, commercial and industrial applications, as well as in sewage treatment plants. UV can be used in: homes, cottages, hotels, motels, buildings, factories, rinse processes, bottling plants, food processing, cooling towers, breweries, hospitals, fisheries, farms, wineries, and the list goes on.
UV units should be installed after any water pre-treatment process, just before the cold and hot branch lines. If in doubt about the size of an UV unit that is needed, always use a larger unit rather than a smaller unit. Normally, an 8gpm UV unit will be installed as close as possible to the dispensing point. All the pipes should be treated with chemical shock treatment and flushed thoroughly before an UV unit is commissioned and prior to its use.
UV dis-infection is very quickly gaining ground on other conventional disinfection processes because it is effective, environmentally friendly and economical. UV does not change the taste of water and does not add anything to it. For many applications, it is the ideal choice for disinfecting water.