From private domestic wells to public water-purification systems of all sizes, ozonation is playing an increasing role in the oxidation of iron, manganese and sulfide (the "troublesome trio" in water treatment, which are subsequently removed by physical filtration), and in reducing taste and odor problems. Ozonation is also effective at disabling the full spectrum of microbiological contaminants (including the protozoans Giardia and Cryptosporidium), viruses, and bacteria. Ozonation systems also raise and stabilize the pH of water.

Ozone is a naturally-occurring gas created from oxygen molecules, which are broken into atoms by ultraviolet (UV) light from the sun or the corona discharge during lightning storms. The distinctive "clean rain" smell is a result of nature-produced ozone. (Ozone derives from the Greek word ozein, which means to smell.) Some single oxygen atoms regroup back into diatomic oxygen molecules and some single oxygen atoms will regroup into loosely-bonded triatomic oxygen, or ozone. As a function of this loose bonding arrangement, ozone is a very strong oxidant and an ideal, chemical-free purification and disinfection agent.

In ozonation systems, ozone is created with an electric discharge field as in the CD-type (corona discharge) ozone generator or by using a UV source. Ozone is an unstable compound that leaves no residue. Its only byproduct is pure oxygen. It must be generated onsite and constantly introduced into the water to be effective.

Ozone has a relatively short half life of about 30 minutes in water, which means that every half hour the ozone concentration is reduced to half its initial concentration. The rate of degradation is a function of water chemistry, pH and water temperature. Ozone dissolves about 12 times more readily in water than pure oxygen, providing far more dissolved oxygen in water than would otherwise be possible.

Ozone has greater disinfection effectiveness against bacteria and viruses than chlorination; also, chlorination creates harmful byproducts known as trihalomethanes (THMs). The EPA monitors and enforces the levels of THMs in public water supplies. Ozonation prior to chlorination lowers THM formation potential and ozonation is commonly used as pretreatment to eliminate THM precursors.

One of the major disadvantages of ozonation is that it provides no disinfection residual as required under the Safe Drinking Water Act. Because the dissolved ozone will revert to oxygen, municipal plants that use ozonation typically add a small chlorine residual after ozonation to prevent the possibility of the water picking up bacteria in the lengthy distribution system. But if chlorine is added to ozonated water, all the chorine will be free chlorine, rather than chloramine, or combined chlorine, which forms when the chlorine has not completely oxidized the contaminants and which imparts the distinctive odor. Chlorine added to ozonated water has nothing to oxidize and therefore becomes free chlorine.

Outside public treatment systems, the main demands for ozonation are in storage cisterns for private wells and for purifying rainwater catchment supplies.


Stephen Wiman has a background in earth science (Ph.D. in geology) and is the owner of Good Water Company in Santa Fe. He may be reached at 505-471-9036 and skwiman@goodwatercompany.com.