In 1976, a mysterious disease broke out in Philadelphia, USA. 221 people fell in ill, in which 34 people died later. In 1977, the disease had been known as legionnaires’ disease because it affected veterans. Later, research revealed that the bacterium that causes the disease was found to be a previously unknown type of bacteria. As a result, the disease was subsequently named Legionellosis. This is a type of atypical pneumonia caused by legionella. Humans get the disease mainly by drinking bacteria-infected water. The main physical symptoms are cough, fever, dyspnea, headache, muscle soreness, fatigue, anorexia and ataxia. Sometimes vomiting and diarrhea can occur. Among neurological symptoms, common symptoms include cognitive dysfunction, confusion, bradycardia, and low heart rate with fever. Legionella is estimated to cause 2-9% of out-of-hospital pneumonia cases in the United States and Europe. Legionellosis occurs mainly in the summer and fall, with an average of 8,000 to 18,000 people requiring hospital treatment each year in the United States.
The Environment in Which Legionella Exist
At least 70 serum groups of 50 species of Legionella have been identified and naturally occur in fresh water. Legionella are rarely found in moving water. But if water sits in large tanks or pipes for too long, there’s a good chance the bacteria will grow. They mainly pollute water heaters, bathtubs and the cooling towers of large air conditioners that are not properly maintained. These are relatively closed water storage environments. In addition, they have been found in swimming pools, spas and even dental equipment. Speaking of closed water storage environments, the UV Ballast Water Treatment in our previous Blog also falls into this category.
How to Deal with Legionella
There are a variety of ways to deal with legionella, including heating, chemical methods, membrane technology and UV technology. The heating method involves heating the water to over 70℃ and keeping it there for more than three minutes. The chemical sterilization method is similar to the traditional chlorination method for tap water. This method mainly uses chlorine dioxide, sodium chloride, hypochlorite water and calcium hypochlorite and other chemical solvents containing chlorine components for treatment. In the use of membrane technology, the micropores of the membrane are generally used to filter legionella. In UV sterilization, UV light with a wavelength of 254 nm is capable of inactivating legionella.
UV Water Treatment
UV radiation can be divided into UVA (long wave), UVB (medium wave) and UVC (short wave), respectively corresponding to the wavelength of 315-400 nm, 280-315nm, 100-280 nm. UV water treatment mainly adopts the UVC band, which is the band used for sterilization. UVC kills bacteria, viruses and other microorganisms based on the absorption of UVC by these microorganisms. The absorption of UVC by microorganisms leads to the destruction of microbial DNA and RNA. Microorganisms reproduce by replicating themselves. Since DNA and RNA contain the basic genetic information of microorganisms, the destruction of DNA and RNA by UVC can fundamentally inhibit the reproduction of microorganisms such as bacteria and viruses.
The use of UV water treatment to inactivate Legionella has been scientifically proven in recent years. Effective inactivation of bacteria can be achieved by irradiating water containing Legionella with a certain dose of UV light. Moreover, UV water treatment offers significant advantages over the more complex processes and difficult dose control that arise with the use of other methods.
Advantages of UV Water Treatment
UV sterilization is quick, safe, easy to use and to maintain. For Legionella, an instant elimination can be easily achieved by the use of UV sterilization. UV treatment is a physical method, and there will be no risk of overuse. During the UV water treatment process, no chemical ingredients will be added to the water body. So, compared with other methods in which chemicals will be added, there are no concern regarding the affecting of the color, smell or taste of the water to be treated.
The chlorination of water makes it difficult to avoid the formation of carcinogenic byproducts such as trihalomethane in the water, while no sterilization byproducts will be produced during UV water treatment. This is of vital importance in fields such as aquariums, swimming pools and even beverage industry that are sensitive to byproduct formation in water treatment.
When water is chlorinated, chlorine-resistant protozoans such as Cryptosporidium and giardia will develop resistance to chlorine-containing disinfectants. They are therefore difficult to kill in the chlorination process. This is not the case with the UV method, and chlorine-resistant microorganisms have no defense against UV radiation. For vibrio cholera, poliovirus, salmonella typhi, hepatitis viruses and other pathogenic microorganisms, UV also has a very good inactivation effect. In addition, the use of UV method is more economical since no chemical products are used. Typical UV lamps have a long life cycle, so the whole process only includes power consumption. This is in line with the current concept of energy conservation and environmental protection.
Disadvantages of UV Water Treatment
Although UV water treatment has many advantages which are mentioned above, but it is not always efficient in any case. It also has several limitations in some areas. First of all, UV water treatment requires a certain level of purity in the water to be treated. Muddy water will affect microorganisms’ exposure to UV radiation and thus creating shaded areas. Second, in contrast to the chemical methods, the effect of UV is only present at the time of exposure. There will be no subsequent sterilization capabilities when UV lamps go out. In some cases, insufficient exposure time may lead to the resurrection of some microorganisms. Finally, similar to those chlorine- resistant microorganisms, some microorganisms may develop a certain level resistance to UV radiation. Therefore, in the process, sometimes it is necessary to increase the UV dose and prolong the treating time according to the presence of UV-resistant microorganisms. Also, other methods can also be chosen to combine with the UV treatment. For instance, hydrogen peroxide, titanium dioxide and ozone can be used as auxiliary treatment methods.
For UV water treatment, it needs to be emphasized that its advantages are far greater than the disadvantages. UV water treatment can surpass many traditional methods with its benefits such as high efficiency, safety, environmental friendliness, convenience and low economic cost. We have detailed the advantages and disadvantages of UV water treatment compared with other methods in a previous Blog. If you are interested, please check it out.
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