Ozone Water Treatment
Published on Jan 10, 2016
Ozone has been used in Europe for water treatment since early in the 20th century. Initial applications were to disinfect relatively clean spring or well water, but they increasingly evolved to also oxidize contaminants common to surface waters. Since World War II, ozonation has become the primary method to assure clean water in Switzerland, West Germany and France.
More recently, major fresh water and waste water treatment facilities using ozone water treatment methods have been constructed throughout the world.
Relatively, the use of ozone for water treatment and purification in the United States has been much more limited. However, the use of ozone has been increasing here in the US, particularly over the last decade as the negative effects of chlorination have become more apparent. For example, a modern water treatment plant in the USA has been built by the City of Los Angeles to use ozone for primary disinfection and microflocculation of as much as 600 million gallons of water per day.
TOzone - or O3 - is Mother Nature's purifier and disinfectant. The 3 stands for the three oxygen atoms that compose Ozone. The normal Oxygen we breathe is called O2, and is made up of only two chemically linked Oxygen atoms.
You may have noticed that a sudden summer storm leaves behind a very distinct smell, sort of a "fresh scent" which lasts for about an hour. In this case, you smell Ozone, which has been creating from lighting bolts during the electrical storm. Ozone is also created by the Sun's ultra violet rays.
Ozone is one of the most powerful water treatment compounds available to systems managers today. It is a technology that has been in continual commercial use for over 100 years and has distinct properties that allow disinfection of even heavily compromised water streams.With the 1996 reauthorization of the Safe Drinking Water Act, Ozone was named as among Abest available technology@ (BAT) for small system compliance to National Primary Drinking water Regulations as overseen by the US Environmental Protection Agency.
Ozone Small Potable Water Systems
Ozone, the strongest oxidant and disinfectant in commercial use has been employed in over 3,000 large scale municipal plants world-wide. In August 1997, and again in August 1998, the U.S. EPA identified ozone as a Small System Compliance Technology for existing National Primary Drinking Water Regulations related to revisions in the 1996 Safe Drinking Water Act. Survey data developed to support the inclusion of ozone as a "Compliance Technology" identified that over half of the more than 260 U.S. municipal ozone installations known to be operating in early 1998 are in systems treating less than 1 MG . An additional 363 community, non-community and single family ozone installations using ultraviolet generation and filtration process also were identified.
Engineering Aspects of Ozonation Systems
Because ozone is such a powerful oxidant/disinfectant, the trick to applying it to solve water treatment problems is to do so in a manner that is effective for water treatment, yet at the same time ensuring the safety of people in the vicinity. Ozone safety issues are handled quite easily by use of proper ambient ozone monitoring, tank venting and ozone destruction. In the case of systems driven solely by a pumping/injector system, Ozone may be produced under vacuum, which ensures no leakage of Ozone into the operating environment.
The five basic components of an Ozone system include
1. Gas Preparation - either drying gasto a suitable dewpoint or using oxygen concentrators.
2. A suitable electrical power supply.
3. A properly sized Ozone Generator(s)
4. An Ozone contacting system.
5. Ozone off-gasdestruction or suitable venting system.
For corona discharge ozone generation, it is critical to feed the generator a clean and dry oxygen- containing gas. Moisture in the feed gas causes two operating problems. First, the amount of ozone produced by application of a given electrical energy level is lowered as relative humidity rises. Consequently, it is usually cost-effective to dry the air to a recommended dew point of minus 65'C (-65'C = -76'F) or lower. Second, when ozone is generated using air in the presence of moisture, the small amount of nitrogen oxides react with the moisture to produce nitric acid.
Moist gas condensation at the cooling/heat transfer surfaces produces the corrosive compound which can soon cause corrosion problems in the ozone generation equipment, with concomitant increases in equipment maintenance requirements. Because of the high oxidative qualities of gas-phase ozone and the chance of moisture from a failing feed gas unit, small system managers should take extra care to make certain that all components in the ozone generator, ozone supply line, ozone gas to liquid mass transfer equipment and the contact vessel are ozone-compatible.For large scale ozonation systems, the equipment for cleaning and drying feed gases can become quite complex.
Ozone is a powerful oxidant with high disinfectant capacity. A study found that within a pH range of 6 to 10, at 3 to 10 C, and with ozone residuals between 0.3 to 2.0 mg/L, bacteriophage MS-2 (a surrogate test organism) and Hepatitis A virus were completely inactivated. Inactivations ranged from >3.9-log to >6-log, and occurred within very short contact periods (i.e., 5 seconds).
A 1992 research report describes treatment studies conducted on MS-2, poliovirus, and Giardia cysts. It found that MS-2 in natural waters are very sensitive to ozone in comparison to poliovirus type 3. In addition, Giardia muris and enteric viruses may be inactivated by ozone (as the primary disinfectant) with 5 minutes contact time and ozone residuals of 0.5 to 0.6 mg/L to 3-log and 4-log removals, respectively. The report concludes that design of ozone as a primary treatment should be based on simple criteria including ozone residual, competing ozone demands, and a minimum contact time to meet the required cyst and viral inactivation requirements, in combination with USEPA guidance recommendations.
Viral inactivation CT values for ozone were published in the original USEPA guidance manual for the SWTR.The EPA has reviewed survey data submitted by the International Ozone Association and found that ozonation has been applied at many drinking water treatment facilities in the U.S. with capacities greater than 100,000 gal/day and some smaller facilities, for disinfection as well as for other water treatment objectives. Applications at the smallest water system size category.
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