Ozone

(endorsed 2005)

Ozone is used as an oxidant and disinfectant in the treatment of drinking water. It can oxidise metals and organic compounds, including algal toxins, tastes and odours. Ozonation does not produce a residual so cannot be used to maintain a disinfection residual in the distribution system.

General description

Ozone ($\text{O}_{3}$) is an unstable blue or colourless gas with a pungent odour. It can be liquefied at –12°C, and has a boiling point of –112°C and a freezing point of –192°C. It is more soluble in water than oxygen. As a strong oxidant and disinfectant, ozone effectively inactivates bacteria, viruses and protozoa (Cryptosporidium and Giardia), controls tastes and odours, and breaks down organic contaminants and algal toxins. Ozone also aids in coagulation and flocculation, by breaking down organic chains and starting microflocculation. Ozone does not produce halogenated disinfection by-products, except in bromide-rich waters where bromate ion is generated.

Disadvantages of ozone are that it is relatively costly and does not produce a persistent disinfectant residual (and therefore cannot be used to maintain a disinfection residual in the distribution system).

Also, ozone produces biodegradable organic material that increases biofouling problems in the water distribution system. This biodegradable material can be achieved by using biologically activated carbon (BAC) filters after ozone treatment.

Ozone in water is highly corrosive; therefore, only it can only be used with certain materials, such as 316 and 305 stainless steel, glass and Teflon. Ozone is produced on site using electrical discharges in the presence of oxygen. The maximum concentration of ozone generated is 50 g/$\text{m}^{3}$ and the maximum practical solubility of ozone in water is approximately 40 mg/L.

Chemistry

Ozone is produced on site, as described above, and is highly unstable in the gaseous phase. Ozone has a half-life of 20–100 hours in clean vessels, at room temperature.

Two types of reactions occur when ozone is added to water:

• direct oxidation (a slow and extremely selective reaction favoured by low pH conditions)

• auto-decomposition to the hydroxyl radical (a reaction catalysed by hydroxyl radicals, organic radicals, hydrogen peroxide, ultraviolet light or high concentrations of hydroxide ion; and favoured by high pH conditions or high concentrations of organic matter).

Ozone breaks down more slowly in water that has a high concentration of bicarbonate or carbonate. Therefore, an ozone residual will last longer in highly buffered water with low pH.

The reaction of ozone with contaminants in the water requires a sufficient contact time and a high transfer efficiency coefficient, which can be provided by well-designed ozone contactors and mixing devices. The gas vented from the contactors contains ozone, which has to be destroyed or re-injected before the air is released to the atmosphere.

Typical values used in Australian drinking water treatment

Ozone is a very strong oxidant that is moderately soluble in water. Typical concentrations used in drinking water are 0.5–5 mg $\text{(O}_{3}\text{)/L}$, depending on the organic content of the water. The required dose should be determined through bench-scale ozone demand tests or pilot-plant testing, using available C.t (concentration × contact time) data for the inactivation of various microorganisms. The contact time required for ozone inactivation of microorganisms varies from seconds to minutes (the longer time being required for inactivation of protozoan cysts) and is temperature dependent; it is significantly shorter than the contact time required for chlorine or chloramines.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process and impurities in the air or oxygen used to generate the ozone. The following chemicals may be present in ozone:

• acetylene

• argon

• carbon dioxide

• carbon monoxide

• hydrocarbons

• nitrous oxide

Residual and by-product formation in drinking water

When employed in drinking water treatment, ozone should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

Ozone can react with bromide to form brominated ozone, which includes bromate ion ($\text{BrO}_{3}^{-}$). If natural organic material is present, nonhalogenated organic disinfection by-products are formed. These include aldehydes (formaldehyde being dominant), ketoacids and carboxylic acids. If both natural organic material and bromide are present, hypobromous acid is formed, together with brominated organohalogen compounds.

Status

Ozone was endorsed by the NHMRC for use as a drinking water treatment chemical in 2005.

References

APHA (1998). APHA 4500-O3, Ozone (residual), in Standard Methods for the Examination of Water and Wastewater, 20th Edition, American Public Health Association, Washington, DC.

Water Treatment Plant Design (1990). American Water Works Association, 3rd Ed. McGraw-Hill Companies, Inc.