Increased pollution of the environment by inorganic and organic substances has lead to UV disinfection gaining increased acceptance and use for treatment of surfaces, air and fluids across a wide spectrum of applications.
The process is simple, reliable, economical and is employed either as a stand alone solution or in combination with other methods such as filters, ozone and chemicals.
UV exposure initiates a photochemical reaction which effectively damages the DNA molecule to such an extent that cell division, and thus multiplication, can no longer occur. The relative effectiveness of UV light wavelengths for this process is known as the germicidal action spectrum, which peaks at a maximum wavelength 265nm, as illustrated in figure 1.
There are a wide variety of micro-organisms (germs) which cause contamination and include bacteria, viruses, moulds, yeasts, algae and protozoans. All vary in their structure and sensitivity to UV-C radiation. This is illustrated for bacteria, yeast and fungi in figure 2.
Variations in ultraviolet light sensitivity can by due to cell size, structure of cell wall or membrane, pigmentation or the existance or capacity of repair systems.
Selection of UV light equipment for a particular purpose depends primarily on the UV light exposure necessary to achieve the required reduction in the number of specified micro-organism types. Figures 3 and 4 illustrate the different lethal exposures required for different micro-organisms to achieve a kill rate of 99.9%. Other factors include illumination area or volume, environment, process integration and temperature considerations. The necessary exposure for many applications ranges between 10mJ/cm2 and 100mJ/cm2.
The destruction of micro-organisms by ultraviolet is an exponential process. The higher the given exposure, the higher will be the proportion of micro-organisms destroyed. Consequently, the exposure necessary to destroy 99% is double the value to destroy 90%. It follows therefore that the exposure required to kill 99.9% is three times the value to destroy 90% and the exposure required to kill 99.9% is four times the value to destroy 90%. This is illustrated in figure 5.
Types of UV light sources employed for UV-disinfection processes can be conveniently split into two categories. These are known as low pressure mercury discharge tubes and medium pressure mercury discharge lamps. The low pressure UV tube category can be further subdivided into convention and amalgam types.
Conventional low pressure UV tubes are the most energy sufficient source with approximately 40% of the electrical input power converted into UV-C light output at wavelength 254nm, which is close to the maximum for germicidal effect, see figure 1. Their spectral power distribution is illustrated in figure 6.
Amalgam UV tubes offer up to three times the UV-C light output compared with conventional types, with approximately 30% of the electrical input power converted into UV-C light output at wavelength 254nm. In addition this type of tube is less sensitive to ambient temperatures. Their spectral power distribution is illustrated in figure 7.
Medium pressure UV lamps (often refered to as high pressure UV lamps due to their higher gas pressure), produce more spectral lines and a continuum due to recombination radiation. Their spectral power distribution is illustrated in figure 8. This UV lamp type is less electrically efficient than the above types but produces much greater UV-C light intensity and thus will achieve a given UV light exposure in a much shorter irradiation time.
A summary comparison of the three different UV sources is shown in fugure 9.
Figure 9.
|
Conventional low presssure tubes |
Amalgan low pressure tubes |
Medium pressure lamps |
| UV emission spectrum |
Narrow band |
Narrow band |
Broad band |
| UV-C wavelength |
254nm |
254nm |
200nm - 280nm |
| % of electrical power input converted into UV-C light |
40% |
30% |
15% |
| Power |
0.5W/cm |
2W/cm |
100W/cm |
| UV-C radiation flux |
0.2W/cm |
0.6W/cm |
15W/cm |
| Surface temperature |
40deg C |
100deg C |
600 - 900 deg C |
| Influence of ambient temperature |
Large |
Lower |
Negligible |
| Electrical input power range |
5 - 50W |
50 - 300W |
1 - 30KW |
