Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. Microorganisms are destroyed or deactivated, resulting in termination of growth and reproduction. When microorganisms are not removed from drinking water, drinking water usage will cause people to fall ill. Disinfectants should not only kill microorganisms. Disinfectants must also have a residual effect, which means that they remain active in the water after disinfection. A disinfectant should prevent pathogenic microorganisms from growing in the plumbing after disinfection, causing the water to be re-contaminated Sodium hypochlorite solution sometimes known as liquid Chlorine kills bacteria, algae, and many other microorganisms in swimming pools, drinking water, waste water, sewage treatment and many other industrial wastes. When applied to industrial waste water, Sodium hypochlorite reduce odors by neutralizing sulphur hydrogen gas (SH) and ammonia (NH3). It is also used to detoxify cyanide baths in metal industries. Hypochlorite can be used to prevent algae and shellfish growth in cooling towers. In water treatment and swimming pools, Sodium hypochlorite is used to disinfect water. In households, hypochlorite is used frequently for the purification and disinfection of the house. It has the advantage that microorganisms cannot build up any resistance to it. Sodium hypochlorite is effective against Legionella bacteria and bio film, in which Legionella bacteria can multiply. Advantages of Sodium hypochlorite as a disinfectant includes easy to transport, store and dosage is simple. Sodium hypochlorite produces residual disinfectant. Other disinfectants that can be used include UV sterilization with the following advantages: - UV does not alter taste, odour, colour or pH of the water - UV does not require the addition of chemicals - UV does not impart toxic by-products into the water - UV systems are compact and easy to install - UV systems require very little maintenance - Running costs are often lower than those of a household light bulb UV sterilization deactivate notorious pathogens such as Giardia Lambia and Cryptosporidium. When a micro-organism is exposed to UV-C, the nuclei of the cells are modified, due to photolytic processes. In result, cell division and, by extension, reproduction is prevented.