The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the...
What Mother Nature performs, science tries to duplicate and then works to improve upon. Ozone—created by a natural phenomenon—provides many benefits to mankind that science had to harness the energy and recreate the process. This seemingly simple process that takes place naturally in the environment has taken the industrial society a long time to perfect.
When stimulated by either an electrical charge or ultraviolet (UV) light (specific wavelength) the oxygen molecule (O2) breaks up and temporarily joins other oxygen molecules, forming ozone (O3) or other levels of ozone depending on the charge and feed source. These outside valences or oxygen molecules are not happy in this new arrangement and seek to disengage themselves, creating an abundance of available oxidizing power.
Ozone was discovered in 1840 through the process of electrolysis. The development moved about in the laboratory until 1857 when Werner Von Siemans developed a process for general and industrial use of ozone. Experimentation and development continued in the search to improve on the generation of this fragile molecule that does so much, but readily decomposes back to oxygen.
There are several ways to produce ozone—carona discharge, UV light, photo chemical and cold plasma have all been improved by science and technology and the processes are now relatively inexpensive.
Ozone, unlike most other chemicals, has no natural resource or method of storage. Ozone is generated onsite, and due to its rapid decomposition, cannot be stored for extended periods of time.
The generation method of carona discharge requires the energetic excitement of molecular oxygen to redistribute itself into atomic oxygen in the form of O3. The “silent arc discharge” known also as carona discharge or brush discharge has become the preferred method of ozone generation when outputs above 1 gram per hour are required. A carona discharge generator can take on many forms, sizes and ozone outputs. The required components of a basic carona discharge ozone generator are:
The reliability, performance and efficiency of the ozone generator depend on several factors. Because more than 80% of the applied electrical energy is converted to heat, the materials used in constructing the generator must be heat resistant. The heat generated must also be removed quickly and efficiently from the area or the heat will accelerate the decomposition of the ozone generated. Some ozone generators are water-cooled. For small ozone generators, this method can be costly.
Another method of cooling is through airflow or refrigeration. With the proper use of airflow, heat is reduced and utility costs are not increased. Many manufacturers use heat-sink devices in their design around the ozone components to direct heat away from the unit.
The generation of ozone is a process of balance and equilibrium because ozone is being generated and destroyed concurrently. Feed-gas decontamination is critical for good ozone generation. Heat, particulate matter, moisture, feed-gas flow (volume), pressure, vacuum, water conditions and other variables will affect the ozone quality and percentage of concentration. This is one reason of many that clean dry air or oxygen should be used in the generation of ozone. The U.S. Environmental Protection Agency suggests that minimum moisture content below -60 Dew point (frost point) should be maintained with the feed source.
Clean air, free of particulate matter, provides for maximum oxygen content in the volume of air being supplied as a feed gas. Dry air, to maximize volume flow, also eliminates the potential for nitrous oxide development when the carona arc energizes the feed gas. Nitrogen oxide, converted to nitric acid, is detrimental to the operating equipment and catalytically destroys the ozone.
The selection of the dielectric material is critical in the performance, output and life of the generator. When considering the dielectric, it should be rated based on the continuous electron bombardment necessary to generate the desired ozone output. This same concern must be applied when selecting the electrodes.
To provide a sufficient voltage potential between the electrodes to generate the ozone, a transformer is incorporated into the system to step up the voltage and operate between 10,000 and 25,000 volts at low amperage. This voltage can vary based on the design characteristics for the ozone application. Some transformers are of the dry type where others can be encased in oil or filled with silicone or other heat-control material to maintain low operating temperatures. The line voltage to the transformers can be 120 VAC, 230 VAC or 440 VAC in single phase or 3 phase at 50 to 60 Hz.
Ozone is a gas that is virtually colorless and has an acidic odor.
The gas has an electrochemical oxidation potential that is quite high and is superior to chlorine or other sanitizing products. The high oxidizing potential allows ozone to break down organic compounds that chlorine cannot.
The pungent odor makes the presence of ozone immediately noticeable but not necessarily harmful. Most people can detect about 0.01 ppm in the air, well within the general comfort level of individuals. Symptoms that are experienced with concentrations at 0.01 to 1 ppm are headaches, irritation, burning of the eyes or respiratory discomfort. When compared to the same exposure to chlorine, you will find that an exposure to 1,000 ppm of ozone for 30 seconds would be mildly irritating but the same exposure to chlorine is often fatal.
Ozone will attack and decompose organic and inorganic materials. The oxidation of inorganic material helps in the process of treating soluble soil, making it insoluble so that it can be precipitated out of solution. This attribute is the basis for top-end wastewater treatment.
Potential ozone applications, at present, seem unlimited. The benefits are untold in environmental issues, economic issues and health benefits. Now that we have produced the ozone, we harness and apply it for practical benefits. One of the simplest ways is to use a venturi injector. This device, set into the stream of water that is in use and to be infused with the ozone, creates a pressure differential from the inlet to the outlet side of the device.
This pressure differential creates a low-pressure vacuum in the outlet flow of the water. This vacuum is the suction for the ozone feed line into the water flow. This type of application should be designed properly so that the loss of ozone in the vacuum does not inhibit the designed function of the ozone. The venturi system requires a water flow under pressure.
Another method is the use of a sparger, which allows the ozone bubble into the water under pressure for dispersion in the water. This method provides for large and small bubbles of ozone to be applied and allows for off gassing unless destroyed.