Ozonated WaterEuropeans were the first to discover the many benefits of ozone in the treatment of drinking water. But from the Netherlands where it was initially used in the late 1800s, to Nice in France where the first ozonation plant was built, ozone as a method of water purification has spread to the other parts of the world.
In the United States, ozonation was initially utilized as a means to remove color and control the foul taste and odor of water. However, with the U.S. Food and Drug Administration deeming it safe, ozone has been used in more applications, specifically in the areas of water disinfection and oxidation, and food processing and storage.
Basic Facts About OzoneDiscovered by Christian Friedrich Schonbein in 1840, Ozone is a triatomic molecule represented by the chemical formula O3. It consists of oxygen (O2), which is found in the air we breathe, and an additional oxygen atom. Hence, ozone can also be referred to as trioxygen.
The term ozone is derived from the Greek word ozein or ogeiv, which means “to smell.” This is actually quite apt considering that ozone gives off a characteristic pungent odor akin to the smell of air after a huge thunderstorm. Because of this distinctive smell, ozone is readily detectable at low concentrations within the range of 0.2 to 0.5 ppm.
Ozone is light blue in color in its gaseous form, condenses to a dark blue liquid at minus 112 degrees Celsius (161 K), and is a violet-black solid at temperatures lower than minus 193 degrees Celsius (80 K). Exposure to ozone in air can prove to be destructive to organic materials such as plastics, while persons exposed to ozone concentrations of about 0.1 to 1 μmol/mol can experience headaches, and eye and lung irritation.
With its three-oxygen composition, ozone is considered an unstable gas that can rapidly degenerate back to the ordinary oxygen plus one oxygen state (O3 - O = O2). When ozone makes this transition, a free oxygen atom or a free radical is produced. That extra oxygen atom serves as an oxidizing agent, attaching itself to any matter that it can come into contact with.
Ozone is part of the natural composition of the air and is created when lightning strikes, or when the ultraviolet rays of the sun react with the atmosphere, thereby producing the protective ozone layer of the Earth. To eliminate pollutants in the water source however, ozone is artificially produced by generators that convert oxygen using ultraviolet radiation or an electric discharge field.
Ozone, Oxidation, and Water PurificationSecond only to the hydroxyl free radical, ozone is one of the most powerful and fast-acting oxidants used in water treatment. But what exactly does it mean when ozone acts as an oxidizing agent on something?
Technically speaking, oxidation (used together with the term reduction in a reduction-oxidation process) is defined as the chemical reaction relating to the transfer of electrons from one element to another. In simpler terms however, oxidation is also used to describe the interaction of oxygen atoms with other substances. In binding itself to any substance within its vicinity, the oxygen causes it to oxidize or turn into something else.
A classic example is the process of oxidizing iron. The oxygen causes a slow burning of the original material, and the end result is iron oxide which we know more commonly as rust. Oxidation is also what happens when a copper penny turns green, and when fresh fruit spoils and changes into a brownish color.
In water disinfection, oxygen free radicals from ozone can oxidize all forms of bacteria, viruses, yeast spores, mold, and other organic materials. Aside from eliminating biological contaminants, ozone’s oxidizing properties can also reduce the concentration of iron, manganese, and sulfur in untreated water.
In air and at temperatures ranging from 20 to minus 50 degrees Celsius, ozone can have a half-life of 3 days to 3 months, while dissolved in water, its half-life significantly reduces to a mere 8 to 30 minutes. On the upside, ozone is 12 times more soluble in water than pure oxygen. So when it reverts to oxygen, there are a lot more oxygen free radicals in the water than what would have been possible otherwise.
The Process of OzonationOzone as a disinfectant can only be feasible if it is produced on-site, and then immediately added to the water.
In water purification systems, ozone production is done by passing oxygen through ultraviolet light or electrical discharge. Providing the required energy for such processes are UV-type ozone generators for ultraviolet radiation (simulating the sun’s rays that create ozone in the atmosphere), and CD-type (corona discharge) generators to produce the electrical discharge field. UV ozonators are ideal for disinfecting small quantities of water, while CD generators are more commonly used for the bulk production of ozone.
A typical ozonation system consists of an air pump that produces a continuous supply of oxygen, a UV- or CD-type generator that converts the oxygen into ozone, and an in-line mixing system that sends the ozone into a diffuser which in turn, creates ozone saturated bubbles. The entire system is then attached to the storage tank containing the water to be treated.
Other methods used in producing ozone for commercial applications include electrolytic discharge generators and cold plasma.
The Advantages and Disadvantages of OzoneBecause it is produced on demand and does not require the transport, storage, and handling of hazardous chemicals, ozone is a relatively cost-effective method of disinfecting large quantities of raw water, particularly in areas where the supply of electricity is abundant. As such, it has become a widely-used treatment process utilized by many municipal water companies.
A major benefit of ozone technology is its capability to eliminate all forms of waterborne pathogens on contact including the harmful protozoa Giardia lamblia and Cryptosporidium. In fact, ozone can remove unwanted microorganisms thousands of times faster than chlorine and bromine. In addition, ozone does not remain in the treated water, nor does it leave any other residue or form organochlorine compounds. The resulting water is free of any unpleasant tastes and odors.
On the other hand, ozone also oxidizes iron, manganese, and sulfur if these are present in the water and when this happens, insoluble metal oxides and elemental sulfur can form. And while ozonation has far more fewer by-products than other water purification treatment methods, recent evaluations indicate that some brominated by-products such as aldehydes, ketones, and carboxylic acids may be carcinogenic.
To remove these insoluble particles and potentially hazardous by-products, as well as dissolved minerals and salts that cannot be eliminated by the ozonation process, post-filtration of the water is necessary. For a more complete water purification system, an activated carbon filter works best with ozone technology.