Ozonation

The process which takes advantage of the oxidizing properties of ozone is known as ozonation. Ozone can be used in a variety of applications including the treatment of drinking water, bottled water, beverages, wastewater , industrial wastes, air pollutants, swimming pool water, and cooling tower water. In addition, there are a number of proprietary processes that use ozone, ranging from carpet cleaning to the making of gourmet ice cubes. Ozonation consists of four fundamental tasks: drying and cleaning the oxygen-containing feed-gas; generating ozone in a silent corona discharge generator; bringing the ozone into contact with the material being treated; and finally destroying remaining ozone prior to releasing the waste gas to the atmosphere . All ozonation processes require ozone generation and contacting, but not all applications require feed-gas preparation and off-gas treatment.

Ozone can be generated by three processes: electrical discharge, photochemical reaction with ultraviolet radiation , and electrolytic reactions. The latter two methods produce very low concentrations of ozone, limiting their application. Electrical discharge generators are currently the most economical choice for producing large quantities of ozone. Feed-gas must contain oxygen and be free of contaminants such as particles, hydrocarbons , and water vapor. For optimum production of ozone, the feed-gas should have a dew point of at least -37°F (-40°C) and preferably -73°F (-60°C). Small ozonation systems can use air which is dried before entering the ozone generator. Larger systems may find pure oxygen is economically viable. The temperature of the feedgas and the generator is one of the most important parameters affecting the production of ozone. High temperatures reduce the concentration of ozone and greatly decrease the life of ozone in the gas phase. The recycling of ozone process gases increases the concentration of nitrogen , which leads to decreases in ozone production, and, in the presence of water vapor, to the production of corrosive nitric acid .

Contacting ozone with the material to be treated is complicated by several factors: ozone is reactive and disappears, so it must be generated at the site where it is used, and the waste ozone in the exhaust gas is toxic. When choosing ozone contacting devices for an aquatic system, two types of reactions must be taken into account. In mass-transfer limited reactions ozone is being consumed faster than it can be transferred to solution. In reaction rate limited reactions ozone is in surplus in solution but the material being oxidized is rate limiting, so that ozone is wasted. Bubble-diffuser systems are commonly used in water treatment because they are good compromises for satisfying the need to control both mass-transfer and rate limited reactions. However, in some aquatic applications in-line dissolution and contacting may be the optimum technique for ozonating the water in question.

Exhaust gas or off-gas requires treatment to remove traces of ozone remaining in the gas after contacting. Thermal destruction is one of the most commonly used methods for removing ozone from the waste gas. Other methods for destroying ozone include catalytic destruction, activated carbon adsorption , and zeolites.

See also Industrial waste treatment; Water treatment

[Gordon R. Finch ]

RESOURCES

BOOKS

Langlais, B., D. A. Reckhow, and D. R. Brink, eds. Ozone in Water Treatment: Application and Engineering. Chelsea, MI: Lewis Publishers, 1991.

Rice, R. G., and A. Netzer, eds. Handbook of Ozone Technology and Applications. Vol. 1. Ann Arbor, MI: Ann Arbor Science Publishers, 1982.

Rice, R. G., and A. Netzer, eds. Handbook of Ozone Technology and Applications. Vol. 2. Boston: Butterworth Publishers, 1984.