Ozone is an effective disinfectant for treating municipal and industrial wastewater, enabling the end user to meet EPA pre-treatment standards
Ozone is formed naturally in the atmosphere as a colorless gas having a very pungent odor. Chemically, ozone is the triatomic, allotropic form of oxygen having the chemical symbol O3 and a molecular weight of 38. Under standard atmospheric temperature and pressure, it is an unstable gas that decomposes into molecular oxygen.
This very powerful oxidant, with a redox potential of 2.07, has many commercial and industrial applications. It is used commonly in potable and non-potable water treatment, and as an industrial oxidant. The considerable oxidizing power of ozone and its molecular oxygen by-products make it a first choice for oxidation or disinfection.
In 1785, Van Marum noticed that air near his electrostatic machine acquired a characteristic odor when electric sparks were passed. In 1801, Cruickshank observed the same odor at the anode during the electrolysis of water. In 1840, Shonbein named the substance, which gave off this odor, "ozone" from the Greek word "ozein" — to smell. In 1857, Siemens designed an ozone generator that has since evolved into the present day, cylindrical dielectric type that makes up most of the commercially available ozone generators in use, and which has sometimes been called the "Siemens Type" ozone generator.
The first drinking water plant began operations in Nice, France, in 1906. Since Nice has been using ozone since that time, it generally is referred to as the birthplace of ozonation for drinking water treatment.
Since then, ozone rapidly has gained public acceptance in the United States, with the introduction of the modern ozone generation equipment. This new technology makes it feasible to generate substantial concentrations of ozone for a multitude of applications. Recently, ozone has been granted G.R.A.S. status (generally recognized as safe) by the U.S. Food and Drug Administration (FDA).
Ozone has several significant advantages over the many various chemical alternatives:
- Ozone can be generated on-site;
- Ozone is one of the most active, readily available oxidizing agents;
- Ozone rapidly decomposes to oxygen, leaving no traces;
- Reactions do not produce toxic halogenated compounds;
- Ozone acts more rapidly, and more completely, than do other common disinfecting agents;
- Ozone reacts swiftly and effectively on all strains of viruses.
Ozone acts by direct or indirect oxidation, by ozonolysis, and by catalysis. The three major action pathways occur as follows:
1) Direct oxidation reactions of ozone, resulting from the action of an atom of oxygen, are typical first order, high redox potential reactions.
2) In indirect oxidation reactions of ozone, the ozone molecule decomposes to form free radicals (OH) which react quickly to oxidize organic and inorganic compounds.
3) Ozone may also act by ozonolysis, by fixing the complete molecule on double linked atoms, producing two simple molecules with differing properties and molecular characteristics.
Large quantities of ozone are produced commercially in a modern ozone generator, in the same manner that ozone is formed naturally by the discharge of electricity during a thunderstorm.The passage of a high voltage, alternating electric discharge (A.C.) through a gas stream containing oxygen will result in the breakdown of molecular oxygen to atomic oxygen. Some of the atoms of oxygen thus liberated reform into ozone, while others recombine to form oxygen. In order to control the electrical discharge, and maintain a "corona" or silent discharge in the gas space, a dielectric space or discharge gap is formed, using a dielectric material such as glass or ceramic.
A ground electrode, constructed usually in 316L stainless steel (a material which has demonstrated high resistance to ozone-induced corrosion in the gas phase) serves as the other boundary to the discharge gap. This can be accomplished in many ways, but the most frequently employed geometry is that of the cylindrical dielectric (or Siemens type) ozone generator. The cylindrical dielectric is more space efficient than other shapes and consequently more economical to manufacture.
Ozone produced commercially for oxidation reactions always is produced as a gas, from air at concentrations between 1.0 and 2.0% by weight, or from oxygen at concentrations greater than 2% and up to 8% (or greater) by weight. Since ozone is highly reactive, and has a short half-life, it cannot be stored as a gas and transported. Consequently, ozone always is generated on site for immediate use.
When ozone is applied as a gas for drinking water treatment, it is done primarily because of its oxidative strength. This powerful oxidation potential allows ozone to be effective in the reduction or elimination of color, aftertaste and odor. More importantly, ozone will effectively destroy bacteria and inactive viruses more rapidly than any other disinfectant chemical.
Ozone also will oxidize heavy metals. Iron and manganese can be reduced to very low, safe levels in water supplies through ozone oxidation. This same process is used to liberate organically bound heavy metals, which otherwise are not easily removed.
When properly applied at the start of a water treatment process, ozone will not lead to the formation of halogenated compounds such as Trihalomethanes (THMs), which are formed when chlorine is added to the raw water containing humic materials. Once a THM is formed, it is quite difficult, if not impossible, to oxidize — even with ozone. Thus, ozone can be used as an oxidant, where it is applied at the latter stages of water treatment.
There are more than 2,000 installations worldwide that use ozone to treat drinking water. Ozone is an effective disinfectant for treating municipal and industrial wastewater, enabling the end user to meet EPA pre-treatment standards. Ozone is effective in treating numerous complex, toxic chemical. But for some compounds, it may be necessary to combine ozone treatment with ultraviolet light or ultrasound to increase reaction time. Quantities of ozone required to treat a specific chemical compound, and the required contact time, will vary.
Treating wastewater with ozone, again primarily for disinfection, was a key focus of its application in the United States during the late 1970s and early 1980s. This approach is again being considered as a means to avoid using chlorine as the primary disinfectant.