Efficient organic removal from hypersaline reverse osmosis concentrates through a hybrid microbubble ozonation-coagulation process: A two-stage removal procedure caused by two-sided effect of salinity

Removal of organic matter in hypersaline reverse osmosis concentrate (ROC) poses significant challenges. In this study, the hybrid microbubble ozonation-coagulation (HOC) process was established for actual hypersaline ROC treatment from energy chemical industry. In this HOC process, the hypersaline environment facilitated the formation of microbubbles, which enhanced ozone mass transfer and ensured an adequate dissolved ozone concentration. Efficient organic removal was achieved through a two-stage procedure: a rapid-removal stage dominated by coagulation (≤ 30 min) and a slow-removal stage dominated by ozone oxidation (> 30 min). Moreover, salinity exhibited two-sided effect on oxidation and coagulation in the HOC process. In the first stage of the treatment process, the alkaline conditions in hypersaline environment promoted oxidation and coagulation through increased •OH production and polymerized Al species generation. However, as the pH decreased owing to coagulant hydrolysis, excessive anions in hypersaline environment inhibited both oxidation and coagulation processes by quenching •OH and promoting large floc generation in the second stage. Furthermore, the two-stage organic removal mechanism was elucidated from the perspectives of oxidative transformation and floc entrapment. In the first stage, high-coagulability organics were directly removed through enhanced coagulation. Meanwhile, low-coagulability organics were oxidized into high-coagulability structures, which were removed via coagulation. In the second stage, organic matter was mainly removed through molecular ozone oxidation, while the coagulation process was inhibited. This study unveiled the two-sided effect of hypersaline environment on oxidation and coagulation, and provided new approaches for enhanced organic removal in the ozone-based process for hypersaline wastewater.