Ozone is an activated form of oxygen with three atoms per molecule instead of the usual two atoms. This extra oxygen atom makes ozone a highly energetic oxidiser and a very efficient natural purifying agent. It is generated in nature through two ways; by ultra violet solar radiation striking the earth's upper atmosphere and by lightning discharging through the air. These two natural methods of generating ozone are reflected in the way ozone is generated commercially.
Ultra Violet Ozone Generation
Ozone is produced using ultra violet light when air is passed over a UV lamp. The light emitted from the lamp splits a small percentage of the oxygen molecules in the air stream. Light is measured on a scale called the electromagnetic spectrum and wavelength increments are measured in nanometers. Ultra violet generators use a mostly 185 nanometer lamp as that wavelength is most efficient for producing ozone. The concentration of ozone available in the output gas of an ultra violet ozone generator is comparatively low � between 0.01 and 0.1% by weight � and concentration directly affects ozone's solubility in water. At the same time ultra violet ozone generators do not require the feed gas air preparation of corona discharge systems. However, ozone output can fluctuate according to changes in humidity and because UV lamps gradually lose their intensity.
Corona Discharge Ozone Generation
This method of producing ozone is based on passing a stream of dried air or oxygen through an electric field. Multiple high voltage electrical discharges cause the oxygen molecules to dissociate and reassemble as ozone. The concentration of the ozone produced by corona discharge varies between 1% and 15-20% depending upon the feed gas used and the electrical configuration of the field. Air preparation systems are either air dryers or oxygen concentrators and as well as increasing the amount of ozone production they ensure that corrosion, due to the production of nitric acid by moisture reacting with nitrogen oxides produced in the generator, is eliminated.
Ozone effectively oxidises a variety of waterborne contaminants without leaving an undesirable residual, adding to total dissolved salts or causing radical shifts in pH. Ozone is used to achieve a number of water treatment goals, including disinfection and oxidation of both inorganic and organic contaminants. Ozone is also an effective pre-treatment for improving the performance of subsequent water treatment processes.
Disinfection is broadly defined as the inactivation of bacteria, viruses and parasites and ozone very effectively eliminates many of these contaminants. E. Coli, Staphylococcus, Legionella pneumophila, the protozoa Giardia and Cryptosporidium are examples of microorganisms that are oxidisable by ozone.
Oxidation of inorganics such as ferrous compounds and manganese in water is another application of ozone. The problems normally associated with the presence of these metals are more aesthetic than health related. However, standards have been established for both in their soluble states because the growth of iron- and manganese-oxidising bacteria can cause a deterioration of overall water quality.
Oxidation of organics that cause colour, taste and odour problems, most due to the presence of humic substances, is an important water treatment application for ozone. The use of ozone on organics is generally followed by filtration with granular activated carbon.