The UV-C radiation, which has a wavelength of approximately 254 nanometres, alters the DNA of micro-organisms. Radiation is emitted by low or high-pressure mercury vapor lamps. The water to be disinfected is passed through the stainless steel exposure chamber with UV-C lamp inside. Water enters one end of the chamber, flows through the chamber around the lamps and exits the other end. The degree of inactivation is directly related to the applied UV dose. The effectiveness of UV systems depends on the quality of the irrigation water radiation intensity and exposure times. Ultraviolet systems are sensitive to high water turbidity and organic matter content, both of which decrease UV-C light permeability called the T10 transmission. The system should be designed to provide a dose greater than 100 mJ cm-2. For selective disinfection (microbes, moulds, and bacteria), dose is 80 mJ cm–². For total disinfection (including viruses), the minimum UV-C dose is 250 mJ cm-2. To disinfect the Pepino mosaic virus, which occurs in tomato crops, a dose of 150 mJ cm-2 is required. In advanced systems computer controls and protects the whole disinfection process and ensures that the chosen UV-C dose is delivered. Due to the necessity of obtaining the highest permeability of UV-C light, proper pre-filtration by rapid sand filters is necessary, clear water can also be added to the water to treat.