Water Disinfection Using UV Radiation—A S ri L ankan Experience

Abstract

Abstract UV radiation has been established as an effective bactericide and viricide. UV radiation penetrates the cell wall and is absorbed by cellular nucleic acids. Radiation absorption prevents replication, thus killing the cell. However, the effectiveness of UV radiation in killing spore forming bacteria such as Giardia lamblia has not yet been established. UV radiation is not a chemical agent, so it produces no toxic residuals. A special lamp is used to transfer electromagnetic energy to the target organism's cells. Mercury arc lamps are the most commonly used type of lamp because about 85% of the energy output is of the 253.7 nm wavelength, which is within the optimum germicidal range of 250–270 nm. The effectiveness of radiation is a direct function of the energy dose absorbed by the organism, which is measured as the product of the lamp's intensity and the time of exposure. Intensity is the rate at which photons are delivered to the target organism. It is governed by the lamp's power, the depth of water, and the presence of substances that absorb the energy, such as suspended solids, color, turbidity, and soluble organic matter. The radiation dose absorbed by the water is the water's UV demand (which is similar to the chlorine demand) and is quantified as the absorption of UV energy (wavelength 253.7 nm) in a given depth of water. The energy dose also depends on the exposure time of the water to the UV radiation, although the required time is only seconds, rather than minutes, for effective disinfection. However, water that has high concentrations of substances like suspended solids, color, turbidity, and soluble organic matter may not receive an adequate radiation dose for effective disinfection.