Ozone : A Tool to Enhance Water/Wastewater Treatment Efficiency

Ozone is one of the most powerful commercially available oxidants and is commonly used for municipal water, sewage and wastewater treatment.

In addition to its oxidizing capabilities, it is an environmentally friendly method of water treatment.

Pollutants, colored substances, odors and microorganisms are directly destroyed by oxidation, without creating harmful chlorinated by-products or significant residues. 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. The recent advances in Ozone generation technology make it feasible to generate substantial concentrations of ozone for a
multitude of applications.

Advantages of ozone:

• Ozone eliminates bacteria, viruses and most other organic and inorganic contaminants.
• Ozone can significantly reduce levels of dangerous chemicals, such as chlorine.
• Ozone acts as a microflocculant aiding in the removal of minerals such as in the removal of minerals such as iron and manganese.
• Ozone leaves neither chlorinated by products nor unpleasant chemical tastes or odors.
• Ozone is generated on site and on demand from air/oxygen and power.
• No storage and handling of chemicals.

How does ozone gets generated?

 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.

The generation of ozone is a relatively simple process. Air or pure oxygen (dry) is used as the feed-gas source and is passed to the ozone generator (at a set flow rate), at which point the air is charged with high voltage. The air is made up of diatomic oxygen (O2) and nitrogen (N2). Diatomic oxygen is a molecule composed of two oxygen atoms (O) held together by four equally shared electrons. As the air/oxygen is drawn through the ozone generator, the high voltage splits some oxygen molecules into oxygen atoms. Some of these atoms then quickly react with oxygen molecules to form ozone: (O1 ) + (O2 ) = (O3 ).

The energy source for production is generated by electrical discharge in a gas that contains oxygen. Ozone is produced when oxygen (O2) molecules are dissociated by an energy source into oxygen a toms and subsequently collide with an oxygen molecule to form an unstable gas, ozone (O3), which is used to disinfect wastewater. 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 stainless steel (a material which has induced corrosion in the gas phase) serves as the other boundary to the discharge gap.

Ozone produced commercially for oxidation reactions always is produced as a gas, from air at concentrations between 1.0 and 2.0 air at concentrations between 1.0 and 2.0 percent by weight, or from oxygen at concentrations greater than 2 percent and up to 8 percent (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.

How does ozone work? 

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

Traditionally, chlorine has been used as a disinfectant. However, ozone and UV radiation are also alternative methods of disinfection. Disinfection must accomplish the dual objective of inactivating pathogenic organism while not harming water users (human or environmental) or plant workers. Ozone and ultraviolet radiation are both more effective disinfectants than chlorine, but both are energy intensive and expensive although prices are decreasing.
During the twentieth century, the growing need for reliable water coupled with environmental concerns about discharge of wastewater into fragile ecosystems and the increasing costs and energy requirements of wastewater treatment has spurred progress in water reclamation and reuse. However the constant pressure by regulatory bodies to look for suitable Technology to meet stringent norms has forced the Industry to consider ozone. Today after drinking water, waste water have found maximum number of ozone users in India
The Location of ozone use in Waste water treatment plant (WWTP) will determine the goal of treatment. Ozone finds use in each of these stages of WWTP. Ozone during preliminary stage is used for detoxification. Ozone at secondary stage is used for sludge reduction, and ozone during the tertiary stage is more common and used for disinfection, micro pollutant removal, COD reduction and de-coloration. The location of ozone is dependant on the goal of ozonation.
Numerous pilot tests studies held using ozone as additional treatment step proven ozone as an effective solution for eliminating various non-biodegradable persistent pollutants, COD, colour etc. This means that residual pollutants present in water/wastewater can be sufficiently removed with environmentally friendly and economically feasible doses of ozone.