A Better Disinfectant

Ozone is more effective than chlorine and derivatives and can deactivate chlorine-resistant pathogens — but it is not suitable for residual disinfection.

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Amid-size town approached Florida Gateway College inquiring about how to control disinfection byproducts and a rotten egg smell in the drinking water. Officials were planning a new treatment facility and were looking for alternative treatments.

The raw water sampling indicated that the new facility would have high levels of hydrogen sulfide and that total organics would be elevated. Because of its excellent disinfection and oxidation qualities, we began a discussion about ozone treatment.

Key benefits

Ozone is widely used in drinking water treatment. It is effective over a wide pH range, and the gas rapidly reacts with bacteria, viruses and protozoans. It has stronger germicidal properties than chlorination and has excellent oxidizing power with a short reaction time. Ozone treatment does not add chemicals to the water, and ozone can eliminate a wide variety of inorganic, organic and microbiological agents, as well as taste and odors.

Additional benefits, we explained, include its decanting abilities and its removal of micro-pollutants, such as pesticides. We also explained that since the city’s source water contained elevated levels of organic material, they would have to be aware of disinfection byproducts when using chlorine.

We also explained the disadvantages of ozone: Equipment and operating costs are higher, and ozonation provides no germicidal or disinfection residual to inhibit or prevent regrowth of bacteria in the distribution system (although the quantity of disinfectant required to provide a residual is significantly reduced). Ozonation byproducts are still being evaluated, and it is possible that some may be carcinogenic.

Ozone treatment byproducts may include brominated compounds, aldehydes, ketones and carboxylic acids. Specifically, the gas reacts with bromide to form bromate, a regulated contaminant in drinking water with a Maximum Contaminant Level of 10 ppb.

For this reason, a post filtration system that includes an activated carbon filter may be necessary. An ozone treatment system also may require pretreatment for hardness reduction or the addition of polyphosphate to prevent formation of carbonate scale in the distribution system.

Ozone is less water-soluble than chlorine, and therefore requires special mixing techniques. In addition, potential fire hazards and toxicity issues are associated with ozone generation.

Physical and chemical properties

Ozone was first used in water treatment in the late 1800s, and today it is more widely used in Europe and Asia than in the United States (although use here is increasing). Ozone is an unstable gas comprised of three oxygen atoms. It readily degrades back to oxygen, and during the transition a free oxygen atom (free radical) is formed. This free radical is highly reactive and short-lived; it normally will survive only for milliseconds.

Ozone is a colorless gas that has an odor similar to the smell of the air after a major thunderstorm. It is a powerful oxidizing agent that is toxic to most waterborne organisms. It is a strong broad-spectrum disinfectant: Ozone treatment is an effective way to inactivate protozoans that form cysts and to eliminate almost all other pathogens.

Ozone is produced by passing oxygen through ultraviolet light or a cold electrical discharge. It must be created on site and added to the water column by bubble contact.  Ozone has the advantage of producing fewer dangerous byproducts than chlorine disinfection, and it removes tastes and odors from raw water without producing any noxious odor.

Protecting the wells

In the area around the city in question there are numerous sinkholes, and the city engineer believes that surface water infiltrates the city wells. For this reason, there was concern that other pollutants, such as micro-pollutants, could enter the water wells.

We explained that micro-pollutants, such as pesticides that occur in surface water, are effectively oxidized with ozone treatment. Because ozone inhibits the formation of disinfection byproducts, removes hydrogen sulfide, removes organic and inorganic material and removes micro-pollutants, the community invested in an ozone water treatment facility. In 2011, the community won a best tasting water contest sponsored by its AWWA region.


John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.


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