Ozone (O3) is an unstable gas made of three oxygen atoms; it may be produced from oxygen gas or air with electric discharge, UV radiation and eventually also some chemical reactions. At ambient conditions ozone is a colorless gas, with a pungent smell. Man can already perceive ozone at concentrations in the 0.02 - 0.05 ppmrange; this concentration is 1/20 of what is considered safe for a 15' exposure and 1/4 of what may be accepted for working environment. While is formed, ozonehas a purple color, is highly unstable and tend to decompose back to oxygen above all with increasing temperature. For this reasons, ozone must be producedwhere it will be used, as it cannot be stored for long periods.
Ozone is a very strong oxidizer and may react with many organic substances that have double or triple bonds; this characteristic is very useful for water and air treatment applications. Ozone effect on bacteria, yeast and viruses has been known for long time (Sonntag, 1890). The first potable water ozone installations has beenrealized in 1906 (Nice). Today many cities (Amsterdam, Moscow, Paris, Turin, Florence, Bologna, Ferrara) use ozone for surface water treatment in theirpotable water production plants. Ozone is definitely a stronger sterilizer compared with chlorine based products, both on bacteria and viruses; more thanthis, ozone does not leave taste on treated water (Viebahn, 1977) and generates less dangerous sterilization by-products (see alogenated hydrocarbons).
Effective June 26th 2001, FDA accepts ozone use in the food industry processes; this is another demonstration of ozone compatibility with human activity. Ozone hasbeen extensively used for bottled water production since 1982; all Olympic pools must include ozone treatment, effective since 1984.
Ozone is not stable and therefore is important to know its decomposition time. For ozone in water half-time (time needed to reduce by 1/2 concentration) is dependent on many parameters. pH relations is as follows (T=21ºC, P=1 bar):
Also temperature is relevant. At pH=7:
Decomposition is much slower in the gas phase:
Maximum allowable concentration for working environment, for exposure 8 h/day - 5 d/week is 0,1 ppm. A concentration of 1 ppm may be tolerated for short periods (e.g. 15'). Concentrations of 100-1000 ppm may kill an adult even in very short time. Anyway, ozone tolerance is really dependant on the specific individual and is also influenced by ambient condition, above all temperature. It is highly recommended to be very cautious when room temperature is high.
Due to its high oxydizing potential, ozone can attach bacteria cells and large molecules that are essential for bacteria, yeasts, molds and viruses life cycle. This powerful sanitizing potential is widely used for potable water and waste water disinfection.
This table compares ozone effect vs. traditional chlorine based disinfectants.
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