Ozone is the triatomic form of oxygen, this is it is composed of three oxygen atoms.  Ozone's chemical symbol is O₃.  Under normal conditions ozone is unstable and quickly decomposed to the more stable gaseous oxygen, O₂.  Because ozone is unstable and cannot be stored successfully, it must be generated at the point of application. 

Most simply, ozone can be generated by passing oxygen, or air containing oxygen, through an area having an electrical discharge or spark.  You may have noticed a clean smell in the air after a thunder and lightning storm.  The clean smell was most likely caused by ozone formed by lightning bolts passing through the atmosphere. 

To generate a sufficient quantity of ozone for a wastewater treatment plant, ozonators developing a corona discharge are used.  These ozonators have two large area metal electrodes separated by a dielectric and an air gap.  An alternating electric current is applied to the electrodes creating an electrical discharge.  At the same time air or oxygen is passed through the air gap.  As the air or oxygen flows through the air gap, and the electrical discharge, a portion of the oxygen is converted to ozone.  The dielectric is necessary to spread the electric discharge over the entire electrode area and avoid producing an intensive single arc. 

A side product from the corona discharge is the generation of a large amount of heat.  The air or oxygen flow in the air gap is not great enough to cool the electrodes.  Since high temperatures cause ozone to very rapidly decompose to oxygen, it is necessary to provide a cooling system for the electrodes.  At present, two types of cooling systems are used;  they are (1)  air cooled, and (2)  water cooled. 

The formation of oxides of nitrogen also takes place in the corona discharge.  Oxides of nitrogen react with water to form nitric acid which would in time attack the materials inside the ozonator.  To avoid this problem and extend the useful life of the ozonator, the air or oxygen flowing through the air gap in the ozonator must be moisture free.  This is accomplished by cooling the compressed gas to remove the moisture before allowing it to enter the air gap. 

The concentration of the ozone leaving the ozonator is approximately 1 to 2% by weight and is applied to the wastewater to be disinfected.  As with chlorination, the effectiveness of disinfection is depended on the concentration of the disinfectant, thorough mixing and contact time.  To satisfy the mixing and contact time requirements, three general types of contactors are usually used:  (1)  packed bed, (2)  sparged column, and (3)  sparged column with mixing.  The most efficient contactor design will vary from treatment plant and may be different from the best for another wastewater with different conditions. 
 
 

Advantages and Disadvantages of Ozonation

The advantages of ozonation include: 

1. eliminates odors 
2. reduces oxygen demanding matter, turbidity and surfactants 
3. removes most colors, phenolics and cyanides 
4. increases dissolved oxygen 
5. production of n o significant toxic side products 
6. increases suspended solids reduction 

   
    

The disadvantages of ozonation include: 

1. high capital cost 
2. high electric consumption 
3. highly corrosive, especially with steel or iron and even oxidizes Neoprene 

   
    

To minimize the disadvantages of ozonation in wastewater treatment plants, some innovations have been developed:  use and recycle of oxygen feed to air gap and improved design of ozone contactors. 

These innovations, of course, are efforts to increase the effectiveness of ozonation systems while minimizing costs associated with ozone generation.  Typically, once-through air feed/air cooled systems require about 6 to 9 kwh/lb while recycled oxygen feed/air cooled systems require about 2.5 to 3.5 kwh/lb.  It can be seen from these power consumption figures that if pure oxygen is readily available, the cost of ozone generation can be cut dramatically. 
 
 

Disinfection Using Ozone

Ozone is thirteen times more soluble in water than oxygen.  When first introduced into wastewater, very little disinfection occurs.  The ozone is rapidly consumed, satisfying the ozone demand of inorganic salts and organic matter dissolved in the wastewater.  The disinfecting properties of the ozone come into play only after the ozone demand is satisfied.  When the demand is satisfied, research studies indicate, ozone brings about disinfection 3100 times faster than chlorine.  It has also been found that disinfection occurs within contact times of 3 to 8 seconds.  Typical ozone dosages needed to reach the disinfection stage vary with the quality of the effluent.  Dosages between 5 to 15 mg/L are commonly cited for disinfection of secondary wastewater effluents.  Ozone also exhibits excellent virocidal properties at these dosages but with longer contact time of about 5 minutes needed.  It has also been found that any residual ozone in the effluent of the contactor disappears in a matter of seconds outside the contactor. 
 
 

Other Uses of Ozone in Wastewater Treatment

1. Ozone has the ability to remove solids from wastewater by oxidation and physical floatation. A Foam develops when wastewater is ozonated.  It has been found that this foam traps a significant amount of solids and nutrient material such as phosphates and nitrates. 

   
    

2. pH has been found to increase very slightly because of ozonation.  This is probably the affect of carbon dioxide being driven out of the solution by the gas feed in the ozone contactor. 

   
    

3. Color and turbidity are reduced by addition of ozone.  This is brought about by chemical oxidation of the substance causing the color or turbidity. 

   
    

4. Some minor nitrification occurs, but not at levels high enough to consider ozonation as an effective nitrification process. 

   
    

    

Safety of Ozone

The Maximum Allowable Concentration (MAC) of ozone in air, as established by the American Council of Governmental Industrial Hygienists is 0.1 ppm by volume for continuous human exposure.  The threshold odor of ozone is 0.01 ppm.  This means a person working near an ozone-handling area should be able to detect the presence of ozone at levels far below the MAC.  The odor of ozone has been described as similar to that of cloves, new mown hay, nitric acid, etc., depending on the concentration.  Concentrations greater than 1 ppm are extremely pungent and are considered unsafe for prolonged human exposure, and therefore should be avoided.