Surface or sub-surface water contains a lot of bacteria and other microorganisms. In municipal water treatment plants, chlorine has been the only general disinfectant used so far. This has changed in the last few decades as almost all the municipal water treatment plants have gone in for ozone as the primary disinfectant. While chlorine produces carcinogenic chloro by-products, ozone does not produce any harmful by-products as a result of ozonation. Besides, the half life ozone is so low (about 20 mints. in sterile water with zero ozone demand), that there is no residual ozone in water and hence no residual toxicity in potable water.

Surface water is generally coloured by natural organic materials such as humic, fulvic and tannic acids. These compounds result from the decay of vegetative materials and are generally related to phenol like compounds; having conjugated carbon/carbon double bonds. When the series of double bonds cross over about twenty, the colour absorption shows up in the visible spectrum. Ozone breaks this organic double bonds, and as more of these double bonds are broken, the colour disappears. Surface water can usually be decolourized when treated with 2 to 4 ppm of ozone. Chlorination of humic material leads to chlorophenols which are carcinogenic, have bad odour and bad taste. Most of the odour can be removed by treatment with ozone. Even some sulphur compounds such as hydrogen sulphide, mercaptans or organic sulphides can be oxidised to sulphates with ozone. Chlorine reacts with organic materials to form chloro compounds like chloroform, carbon tetrachloride, chloromethane and others, generally known as Trihalomethanes (THMs). Ozone reacts with organics to break them down into simpler compounds. Combined with activated charcoal, these can be removed. This water can then be treated with low level of Chlorine, say 0.2 - 0.3 ppm to maintain sanitation in the distribution system. This way no THMs will be formed. The THMs have been implicated as carcinogens in the development of kidney, bladder and colon cancer.

Ozone is used as a primary disinfection to take care of the disinfection in the primary stage. However, the drawback for ozone in this particular case is its very short half life. As a result, there is no ozone in water while it is being transported in pipelines to the point of consumption. In order to control bacterial breeding in the water under transportation, some amount of chlorine is injected enough to maintain the required residual disinfectant level in the water till it reaches the point of consumption. The result is that the water has far lower level of harmful carcinogenic compounds, partly because ozone reduces the organics in water and partly because there is less amount of chlorine used than otherwise.

The application of ozone in municipal water treatment is quite simple. After the water is pumped from the source, it undergoes various stages of treatment to reduce the colloidal and suspended particles. Ozone is injected in to this water using injectors or/and diffusers inside a large contact tank to mass transfer ozone in to water. The contact tank serves the purpose of giving enough residual time to complete the disinfection and oxidation reactions.