1.Chlorine- The most common disinfection method is some form of chlorine or its compounds such as chloramines or chlorine dioxide. Chlorine is a strong oxidant that kills many micro-organisms. Because chlorine is a toxic gas, there is a danger of a release associated with its use. This problem is avoided by the use of sodium hypochlorite, which is a relatively inexpensive liquid that releases free chlorine when dissolved in water. We can also use TCP/ Bleaching powder. Handling the solid, however, requires greater routine human contact through opening bags and pouring than the use of gas cylinders which are more easily automated. These disinfectants are widely used despite their respective drawbacks. A major drawback to using chlorine gas or sodium hypochlorite is that they react with organic compounds in the water to form potentially harmful levels of the chemical by-products trihalomethanes (THMs) and halo acetic acids, both of which are carcinogenic and regulated by the U.S. Environmental Protection Agency (EPA). The formation of THMs and haloacetic acids is minimized by effective removal of as many organics from the water as possible before disinfection. Although chlorine is effective in killing bacteria, it has limited effectiveness against protozoans that form cysts in water. (Giardia lamblia and Cryptosporidium, both of which are pathogenic). Hence water purification plants must have a very good filtration system.
2.Chlorine dioxide is another fast-acting disinfectant. It is, however, rarely used, because it may create excessive amounts of chlorate and chlorite, both of which are regulated to low allowable levels. Chlorine dioxide also poses extreme risks in handling: not only is the gas toxic, but it may spontaneously detonate upon release to the atmosphere in an accident.
3.Chloramines are another chlorine-based disinfectant. Although chloramines are not as effective as disinfectants, compared to chlorine gas or sodium hypochlorite, they are less prone to form THMs or haloacetic acids. It is possible to convert chlorine to chloramine by adding ammonia to the water along with the chlorine: The chlorine and ammonia react to form chloramines. Water distribution systems disinfected with chloramines may experience nitrification, wherein ammonia is used a nitrogen source for bacterial growth, with nitrates being generated as a byproduct.