On the electromagnetic spectrum U.V. rays are between X rays and the violet rays of the visible spectrum, and among the range of rays whose wavelengths are between 100 and 400 nm.
U.V. treatment
The U.V. spectrum can be divided into 4 bands :
- UVA: 400 to 315 nm: skin pigmentation.
- UVB: 315 to 280 nm: production of antirachtic vitamin D.
- UVC: 280 to 200 nm: this area of the U.V. spectrum is used for its microbicide capacity with a maximum germicidal efficiency of around 260 nm.
- UV < 200 nm: the rays from this part of the U.V. spectrum lead to ozone production from oxygen, and are absorbed by most substances including air.
Compared to chemical disinfection processes, the ultraviolet rays technique - whose bactericidal properties have been known for many years - has the added advantage that it does not produce any toxicity nor does it generate any sapid by-products (such as chlorine). On the other hand, its effect is not long-lasting.
This process is already widely used in Europe, and its use is growing throughout the rest of the world.
Water is irradiated by a generator using a mercury lamp
Ultraviolet rays are produced by mercury lamps whose germicidal efficiency is in the region of 30 to 35%.
Disinfection takes place in a treatment chamber where the water is irradiated by one or several U.V. generators. Each generator is prevented from coming into contact with the water by a quartz shield that is permeable to U.V. rays.
Disinfection is achieved by circulating a thin annular film of water (less than 50 mm thick) around these shields.
The hydraulic flow rate into the chamber must be of a piston type and turbulent in order to ensure correct treatment homogeneity (same exposure time for all germs).