UV light is the component of solar radiation occurs naturally. UV light falls in the range of between X-rays and visible lights in the electromagnetic spectrum. Generally, this region falls in the range of between 100nm-400nm wavelength. UV region is also divided into sub-bands.
Scientist have different perspective about boundaries between the sub-bands of the UV light. UV range is generally divided into four sub-bands.
UVA; This sub-band extends from 315-400 nm
UVB; This sub-band extends from 280-315 nm
UVC; This sub-band extends from 200-280 nm
Vacuum UV; This sub-band extends from 100-200 nm
UVA and UVC bands is the most effective to inactivate microorganisms. UVC radiation has germicidal bandwidth and high disinfection capability. Maximum wavelength is 260 nm. UV-A radiation is required quite a while contact time to be effective for disinfection. The ozone layer prevents significant quantities of UV-B and UV-C radiation. Consequently, artificial sources of UV is used for practical application of UV disinfection.
The earliest scientific observations of germicidal effects of UV radiation began 1800s who reported that bacterias were inactivated by sunlight. UV light generallyinactivates bacterias, viruses and protozoas by damaging material in DNA and RNA's. Additionally many microorganisms, which are resisting to chlorine attack, were inactivated with UV light. There is no change either taste and odour.
UV lamps have sleeves and ballasts. Ballasts are devices to limiting the amount of current in the electrical circuit. Sleeves protect th lamps from contact water. Sleeves are made of high quality quartz. The most common sources of UV are low and medium pressure lamps. A mercury arc lamp consists of sealed tube of UV and transmitting quartz with electrodes at the both ends. The electrodes are composed of tugsten to aid arc formation within the lamp. A gas discharge is struck with applying a high voltage to the electrodes. UV light is emitted from the lamp when mercury vapor. The wavelength of the emitted light is varied depend on the pressure in the tube. Low pressure UV lamps (1-10Pa) is mostly concentrated around 254nm. Output ranges of the medium pressure lamps from 254-265nm. Selection lamp is very important for the disinfection efficiency depend on the application.
Key Parameters of the UV Disinfection Systems
The nature and quality to be disinfected water is important to select appropriate UV system. UVT (UV Transmittance) is the measure of UV energy at a specific wavelength, which is also define as transmitted through water from the lamp. UVT is affected turbidity of the water (colloidal solids or different materials in water). Materials of the system can be affected by salinity content rate of the water . The salinity can be caused corrosion problems.
The another critical parameter is UV dose which can be defined as a product of UV intensity and exposure time in mathematically. The amount of reduction in the microorganisms population is directly depend on applied UV dose. Many microorganisms show different resistance to the UV-C light. Required UV dose is 400 J/m² to damage all pathogens. The cells irradiated with UV-C light can recover their DNA with repair mechanisms. This mechanism is called as photoreactivation (self-repair). Consequently, UV dose should be kept under the control when disinfect the water.