In most ozone water treatment applications achieving the treatment objective requires meeting or exceeding a certain
residual target to insure disinfection or oxidation. This is achieved by monitoring the residual concentration of ozone in
the water or in the vent gas and increasing or decreasing the ozone generator output.
In some ozone water treatment applications the process requires keeping the ozone residual in the treated water below a certain limit. While ozone has a short half life in water at typical conditions found in water treatment, it can persist for 1-2 hours. If down stream corrosion or process contamination are a concern, measures need to be taken to reduce or eliminate the residual of ozone.
In some applications such as drinking water treatment ozone residual can create an unsafe working environment. In northern climates, some of the treatment processes might be housed in doors, but open to the ambient air in the building, residual ozone in the water from these processes can enter the work area. Where this off gassing can cause a health and safety concern removal of the ozone residual or quenching is employed.
There are three basic approaches to decomposing ozone in water: UV radiation, chemicals and activated carbon.
Ultraviolet Light (UV)
Ultraviolet systems are highly effective for destroying ozone in process water. As chemicals or heat are unwanted processes in most high purity water treatment processes, UV is the method of choice to break down the ozone in a simple flow-through physical process. This is the preferred method where the addition of chemicals or other materials would not be desirable, for example in the food, beverage, pharmaceutical or semiconductor industries.
Certain chemicals can also be added to the water to remove the residual ozone. These chemicals include: hydrogen peroxide, sodium bisulfite and calcium thiosulfate. Outside of ultra pure water system application, chemical additions of these sort can be used. A good example is in drinking water treatment. The reaction with sulfite or thiosulfate occurs in seconds. The dosage rate for sulfite is about 2.2 g sodium sulfite/g of ozone. The ratio for sodium thiosulfate is 2.1 g thiosulfate/g ozone. As with any chemical, material data safety sheets should be consulted before use. Sulfite can produce fumes of sulfur dioxide, it is also an oxygen scavenger. Chemical treatment is the typical method used in drinking water treatment applications.
For small quantities of ozone, activated carbon is an effective treatment method for removing small residuals from water. The carbon catalytically decomposes the residual ozone.