Principal

Disinfection with Ozone

by Stacie Kramer and Susanna Leung

Table of Contents:

Definition

Background

The Ozone Journey

Evaluation of Ozone as a Disinfectant

References

Definition:

Ozone is a form of oxygen with the molecular formula O₃. It forms when O₂ or clean dry air is exposed to a powerful electric current. In nature, it forms in the upper atmosphere when lightning passes through the air. Ozone is unstable and changes to O₂ shortly after its formation. It is a powerful oxidant and one of the most powerful disinfectants available in water treatment.

Background:

Ozone was discovered in 1783, by VanMarum but was not named until 1840. In 1857, Siemens developed the first electric discharge ozone generating device and by 1893, it was utilized for potable water disinfection in Oudshoorn, Netherlands. Ozone began gaining popularity in Europe as people realized its potential for disinfection and other uses. Since then, it has become a widely used disinfectant in all parts of Europe. The water treatment plant in Nice, France has continued to use ozone for disinfection since 1906.
Only recently has the use of ozone gained national attention in the United States. Although New York City's Jerome Park Reservoir began applying ozone in its treatment of water for taste and odor control since 1906, there were only 5 water treatment facilities using ozone in the United States in 1987 (3). In the past, America has relied more heavily on less expensive disinfectants such as free chlorine, chlorine dioxide, and chloramines. Recent legislature, such as the Safe Drinking Water Act Amendments, Surface Water Treatment Rule and other proposed bills place stricter rules on both the range and amount of disinfection needed and the concentrations of disinfection-by-products allowed in drinking water. Therefore, the use of free chlorine and other common disinfectants are becoming less cost-effective. Ozone is one of the few disinfectants that can effectively inactivate both Cryptosporidium and Giardia

Flow path of ozone in water treatment

The Ozone Journey:

Gas Preparation

Ozone is a highly unstable molecule with a relatively short half-life. It must therefore be generated on-site. Ozone can be generated from any source of gas which contains oxygen molecules. The most common sources for ozone generation are oxygen gas tanks or air in the atmosphere. Although the use of pure oxygen gas will result in a higher efficiency of ozone generation, it will also increase the initial cost of the gas source. On the other hand, using the atmosphere requires preparation of the gas source. The atmosphere must first be compressed, cleansed and dehumidified (3). Compression of the air serves to increase the concentration of ozone in the supply. The removal of foreign particulate such as dirt and dust is often accomplished through the use of filters. Air humidity is decreased usually by lowering the dew point by refrigeration. Most ozone generators require clean, dry gas for optimal conditions.

Generation

The clean, dry gas passes through a chamber with a high voltage electric current which discharges electrons into the air. The electrons convert the oxygen molecules to ozone molecules and oxygen atoms. The highly unstable oxygen atoms bond with hydrogen atoms in the air to form hydroxyl radicals. The gas is then injected in the water passing through the system and solubilized. The solubility of ozone in water has been found to obey Henry's Law and is a function of temperature and pH (3). The amount of ozone in the system can be regulated in the generator by adjusting the voltage of the current or the flowrate of the gas. The maximum ozone concentration produced by a generator is 50 g/m³ and the maximum solubility concentration of ozone in water is 40 mg/L (3).

Contact

The ozone-enriched water flows into a chamber to allow for sufficient contact time. During this contact time, ozone molecules decay to form oxygen molecules and free hydroxyl radicals. It is both the ozone and the highly reactive free radicals which oxidize organic molecules. Through this oxidation, toxic herbicides and pesticides are reduced to more environmentally friendly components (6). Non-biodegradable organics will reduce to smaller biodegradable parts. The reaction is capable of splitting proteins, carbohydrates and humic acids at double-bonded carbons to damage and destroy critical components of organisms found in the water. Since the free hydroxyl is so highly reactive, the contact time necessary is minimal, as compared to other disinfectants (6).

Destruction

Ozone has an average half-life of 20 minutes if it is not oxidized by particulates (5). Depending on the water quality at a given time, a portion of the dissolved ozone may pass through the system unoxidized. Ingestion of ozone is a serious concern because of its strong oxidizing capabilities. Safety precautions are usually taken to insure that the remaining ozone molecules are destroyed before they reach the distribution system. Regulations require the resulting concentration of ozone to be reduced to less than 0.002 mg/L before the water is released from the plant (6). Ozone degradation can be accelerated by the addition of hydrogen peroxide or passing the system through a UVc system. Contact time can also be lengthened to allow for further destruction.

Evaluation of Ozone as a Disinfectant:

References:

(1) American Water Works Association Research Foundation. "The Ozone Option". 1986.
(2) Asea Brown Boveri. "The Use of Ozone in Drinking Water Treatment"
(3) Haas, Charles N. "Disinfection". Water Quality and Treatment. Ed. Fredrick W. Pontius. 4th ed. AWWA McGraw-Hill Inc.: New York, 1990.
(4) Reynolds, Tom D., and Richards, Paul A. Unit Operations and Processes in Environmental Engineering. PWS: Boston, 1996.
(5) http://ies-energy,com/sustomer_programs/new_technologies/ozonation.html
(6) http://www.drydenaqua.com/Ozonation/ozonation.htm

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