Green Chemistry and the Ten Commandments

Green Ozone for Water Disinfection

Another disinfection agent that is superior to chlorine in some respects is ozone, O 3.
An ozone-based water disinfection system is outlined in Figure 7.8. This reactive form
of elemental oxygen is made by passing an electrical discharge at approximately 20,000
volts through air that has been filtered, cooled, dried, and pressurized:

3O 2 (g)

Electrical

discharge 2O^2 (g) (7.17.5)

Refrigerator

and

deisiccator

Dried

air

Ozone

generation,

3O 2     → 2O 3

20,000  volts

O 3     in  air

Water   in

Water

contacting

ozone

Disinfected

water

Filteredair

Compressor

Figure 7.8. A schematic diagram of a system to generate ozone by electrical discharge through refrigerated,
dried air followed by disinfection of water by the ozone product.

The ozone contacts water in a contact chamber for 10–15 minutes. Although the low water
solubility of ozone limits its disinfective ability, there is no formation of organochlorine
disinfection byproducts. And ozone is more effective than chlorine against viruses.
Because ozone lasts for only a short period of time in water, some chlorine must be
added to maintain residual disinfection in the water distribution system.
The use of ozone for water disinfection is a virtually ideal example of the practice
of green chemistry. The only raw material required is air, which is available without
cost everywhere. The ozone is produced only as needed, where it is needed, and it is not

Chap. 7. Water, The Ultimate Green Solvent 187