Ozonation

Background

Ozone, a triatomic form of oxygen (O3), is the product smelled near an electric spark or lightning strike. Ozone is the strongest oxidant of the common disinfecting agents. It was used as early as 1893 for drinking water treatment. Today, it is the most commonly used disinfection process in Europe. One important reason is that only oxygen is added to organic compounds, so few of the products are considered to be health hazards. A wider spectrum of organisms is destroyed by ozone than by chlorine, and removal of taste and odors is excellent. The reactions are rapid compared to those for chlorine, but only after an initial demand for ozone is satisfied.

Even though chlorine has been used successfully in the U.S. for many years, there is rapidly growing sentiment to switch to ozone. The word cancer scares people, and chlorination does produce chemicals that are more carcinogenic than the organic compounds initially present. Such compounds accumulate in fatty structures such as the membranes of cells, so concentrations tend to be higher in organisms than in the water in which they grow. Nevertheless, there are many unanswered questions about true health hazards, concentrations that cause cancer, and costs that complicate the decision to convert to ozone.

More about ozone

Chemistry

The understanding of the actual mechanisms of ozonation is not complete. Ozone, being a strong oxidizing agent reacts directly with constituents found in solution and is also capable of forming highly reactive chemical agents that can contribute to additional oxidizing reactions. The most common of these reactive agents is the hydroxyl free radical. Conditions that contribute to the formation of these reagents vary with water quality. The actual disinfection occurs as the oxidation reactions damage and destroy critical components of microorganisms. Like chlorination, ozonation also forms a residual, however due to its short life span it is negligible for preventive measures in distribution systems.

Application Methods

Ozone is used in the same manner as chlorine. The major difference is that ozone is unstable so cannot be produced and transported to the point of use. It must be generated at the point of use. For water treatment, ozone is produced by an electrical corona discharge or ultraviolet irradiation of dry air or oxygen. Ozone can injected or diffused into the water supply stream.

Advantages and Disadvantages

Ozone is extremely active as a disinfectant. The benefits are the strength of the disinfection and the lack of potentially harmful by-products like trihalomethanes (THMs). A wider range of organisms is killed by ozonation than by chlorination. It also achieves excellent removal of taste and odors. The reactions, in general, are more rapid than that of chlorination processes.

The drawbacks of ozone, like chlorine, is that it may not kill cysts and some other large organisms so these should be eliminated by filtration or other procedures prior to treatment. A major disadvantage is that due to its instability, ozone must be generated before use and the equipment and operating costs can be quite high. Ozone has an active residual measured in minutes. This lack of long residual is a significant drawback for its use in large distribution systems.

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Page created by Steve Boglarski and Shyam Telikicherla, Fall 1995