Characteristics and mechanism of hybrid ozonation-coagulation process in wastewater reclamation

Because of the limited dissolved organic matter removal efficiency in conventional pre-ozonation-coagulation process, the hybrid ozonation-coagulation (HOC) process was developed for wastewater reclamation in this study. In this process, coagulation and ozonation could synchronize within a single unit. Compared with the pre-ozonation-coagulation process and traditional coagulation process at the same coagulants dosage, the HOC process exhibited higher treatment efficiency, especially better organic matter removal performance at each pH value, which were 37.96% and 39.66%, respectively. In order to obtain the removal mechanism of dissolved organic matters by HOC process, ozone decomposition at two pH values either with or without AlCl₃·6H₂O was monitored in this study. The results showed that higher pH and the addition of coagulant could accelerate ozone decay. In addition, para-chlorobenzoic acid (p-CBA) was chosen as the hydroxyl radical (·OH) probe compound to indirectly calculate the ·OH exposure. The results revealed that the HOC process was involved in ·OH reaction and coagulant could promote ozone decomposition due to its enhancing effect on ·OH production during the ozonation process. Furthermore, the ozone depletion occurred more quickly at high pH due to its reaction with hydroxide ions (OH^-), leading to the formation of ·OH through a series of chain reactions. Moreover, the effects of tert-butanol, a well-known ·OH inhibitor on ozone decay and p-CBA decomposition were also investigated. The results indicated that ozone depletion and p-CBA decomposition were significantly inhibited by tert-butanol, which further proved that ozone was decomposed into ·OH radicals through chain reactions. In the HOC process, Al-based coagulants and hydrolyzed products could act as catalysts to enhance ozone decomposition into ·OH which is a powerful oxidant to improve the organics removal efficiency.