PRINCIPLE OF ACTION UVC
WHY UV-C DISINFECTION IN PARTICULAR?
Microorganisms such as bacteria, molds, yeasts and protozoa can be destroyed or removed by physical, biological and chemical methods. The type of radiation that causes inactivation of bacteria is now known as ultraviolet radiation; research into the behavior of microorganisms to this radiation has led to the development of new disinfection options, both of air and of surfaces of solid materials. Disinfection, without the use of chemical disinfectants or high temperatures, is possible where it was previously impossible or difficult to achieve.
Short-wave UVC radiation has a strong germicidal effect. UVC uses the photolytic effect for this, whereby the microorganisms are destroyed or inactivated by radiation, which prevents them from reproducing.
This means that thymine bases lying next to each other on the DNA strand form a chemical bond and thus form a bridge (dimer). If enough of these dimers are formed, the DNA can no longer replicate. However, some microorganisms can regenerate themselves by absorbing UVA. In other cases, UVC can also cause UVA or UVB to cleave a compound in a molecule, resulting in free radicals, which are often highly unstable and can react to form an inactive end product. When disinfecting, these effects occur at wavelengths below 320 nm, with the optimum at approx. 260 nm. The disfiguring or destructive effect on microorganisms is independent of the pH value, temperature or whether they are present in a liquid, solid or gaseous medium. What is important is that the radiation reaches the organism. This means that a bacterium that is covered by another bacterium or a particle escapes the influence. Unlike other techniques, UVC photolysis produces hardly any potentially dangerous by-products.
ULTRAVIOLET RADIATION
Ultraviolet radiation is one of the many types of radiation (X-rays, light, radio and television waves, alternating current, etc.) that all belong to the same category of electromagnetic wave radiation. The difference between the types of radiation is determined solely by the wavelengths. Cosmic rays and X-rays have the shortest wavelengths. Alternating currents are at the other extreme. Only a very small area is covered by optical radiation, which includes UV radiation. Visible radiation and infrared radiation continue the wave spectrum in the long-wave range.
Ultraviolet is a part of electromagnetic radiation, limited at the top by the visible spectrum and at the bottom by X-rays. By definition, the spectrum of ultraviolet radiation is between 100 nm and 400 nm (1 nm = 10-9 m) and is invisible to the human eye.
For practical applications, the UV spectrum has been divided into three areas:
- UVA (long-wave radiation) from 315 nm to 400 nm
- UVB (medium-wave radiation) from 280 nm to 315 nm
- UVC (short-wave radiation) from 100 nm to 280 nm
Radiation in the short-wave UVC band has a strong germicidal effect. The UV radiation emitted by a source is given in watts (W), the density of the irradiance in W/m². The dose, which multiplies the density of the irradiance by the time (t) in seconds, is important for the germicidal effect. It is expressed in J/m². 1 joule = 1 watt second.
The resistance of microorganisms to UV irradiation varies considerably. In addition, the microorganism's environment influences the dose required to destroy it. Water, for example, can absorb part of the effective radiation, depending on the concentration of foreign substances it contains. Iron salts in solution are known inhibitors. Iron ions absorb UV radiation.
WHY UV-C DISINFECTION IN PARTICULAR?
The most commonly used disinfection process today is chlorination. However, this process can produce haloforms such as chloroform, the carcinogenic effects of which are being discussed. Understandably, there is therefore a growing desire to replace this process or limit the use of chlorine. In addition, it is increasingly necessary to use near-surface water, which is more contaminated or polluted than groundwater (e.g. with faecal germs). One method in which no substances are added to the water to be treated
is disinfection using UVC radiation. This method does not leave any toxic substances in the water. In principle, nothing else happens here than in nature. This is because the oxidizing and germicidal effects of solar radiation make a significant contribution to keeping our environment clean. The amount of radiation intensity or more radiation in the effective range, and therefore the efficiency, is increased with artificial UV disinfection.