Residual Ozone Removal from Water

In most ozone water treatment applications, achieving the treatment objective requires meeting or exceeding a certain residual target to ensure disinfection or oxidation. This is achieved by monitoring the residual ozone concentration 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 residual ozone 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 indoors, 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.

1. Decomposition of Ozone by Ultraviolet Light (UV)

One method to decompose and destroy ozone is by UV light. Ultraviolet systems are highly effective for destroying ozone in process water. Chemicals or heat are unwanted processes in most high purity water treatment processes. This makes UV 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.

2. Adding Chemicals to Remove Residual Ozone

Another method to remove the residual of ozone is by adding certain chemicals. These chemicals include: hydrogen peroxide, sodium bisulfite and calcium thiosulfate. Outside of ultra pure water system applications, 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.

3. Ozone Removal with Activated Carbon

For small quantities of ozone, activated carbon is an effective treatment method for removing small residuals from water. The carbon catalytically decomposes the residual of ozone.

Breaking down and removing residual ozone is important in ozone water treatment applications. Here are three ways to decompose and remove ozone.