How wastewater treatment with ozone works
Ozone is a known powerful disinfectant and oxidizing agent. Traditionally, ozone has been used in drinking water treatment for disinfection purposes and for removal of color, odor and taste. Recently, the use of ozonation in wastewater treatment is considered in order to remove organic micropollutants that are resistant to biological degradation.
During ozone treatment, the required ozone dose is influenced by the nature and characteristics of the wastewater matrix. High concentration of dissolved organics and other inorganic species may remain after biological treatment, causing an ozone demand in the wastewater. On the other hand, reaction of ozone with dissolved organic matter generates secondary oxidant called hydroxyl (OH) radicals that may contribute to the removal of ozone-refractory micropollutants. Depending on the treatment goal, ozone may be added after extensive biological treatment or at the end of the treatment train as a polishing step to minimize the ozone dose required. Another factor affecting ozone reaction is the pH of the wastewater. Low pH favors direct reaction with molecular ozone, whereas high pH results in increased ozone decomposition producing OH radicals. The produced OH radicals is seen as an advantage since they react non-selectively with trace organic compounds in the wastewater.
The transformation of organic micropollutants is dependent on its chemical structure particularly on the reactivity of a specific functional group or substituent. Ozone reacts directly with electron-rich sites such as phenols, tertiary amines and carbon to carbon double bonds. On the other hand, those micropollutants exhibiting low ozone reactivity can be effectively removed by hydroxyl radical mechanism. Although complete mineralization cannot be expected, ozone treatment contributes to biodegradability enhancement by transforming organic micropollutants into smaller and simpler molecules that can then be removed by subsequent biological treatment.