Chlorine dioxide (ClO2) is a neutral compound of chlorine that effectively disinfects by oxidation. An unstable gas with a green yellowish color and strong, chlorine-like odor, chlorine dioxide is produced by activating sodium chlorite with an oxidizing agent or an acid source.
Not withstanding its name and chemical formula, chlorine dioxide bears little resemblance to the elementary chlorine. While chlorine reacts (often violently) with many substances, chlorine dioxide is weaker and more selective in its reactions. For instance, ClO2 does not react with water, ammonia-nitrogen,elementary amines, and other organic compounds present in the water. What itdoes react with, is the cell membrane of the microorganism, “stealing” its electrons,and essentially destroying that cell which causes the microbe to die.
In water disinfection, chlorine dioxide has been found out to be superior for the control of the Legionella bacteria and more effective than pure chlorine against many waterborne pathogens such as viruses, bacteria, and the the cyst sof Giardia and the oocysts of Cryptosporidium.
Chlorine dioxide has several significant properties that are well worth noting: it is highly soluble in water -- about 10 times more soluble than chlorine, it remains effective over a broad pH range, it does not lose its disinfecting capability over time, and it rapidly sterilizes at relatively low concentrations and at an ambient temperature.
On the other hand, chlorine dioxide is almost impossible to transport because it is explosive under severe pressure, hence, it is usually generated on site and as needed.