Once the ozone has been dissolved, time is necessary for it to act on the contaminants in the water. Some oxidation reactions occur so quickly they are limited only by the rate at which the ozone is transferred into the water. Other reactions, such as disinfection, require both an ozone residual and proper contact time. The concentration of dissolved ozone (C measured in mg/1 or ppm) multiplied by the contact time (T measured in minutes) provides what is known as CT value. Each application has a different CT value, which depends on a variety of factors, including the type and level of contaminants in the water and the actual treatment goal(s). The required contact time can be accomplished by holding the water in a contact vessel, plumbing, or a combination of these. Contact vessels are available in a variety of types and sizes, each with its own function and efficiency rating. Contact vessels for inorganic contaminants are usually single chamber, countercurrent floe units because baffles are not normally required. If disinfection is the goal, baffled contractors should be used, which are capable of retaining the ozone/water mixture longer to achieve the required CT value.

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