Organics removal in high strength petrochemical wastewater with combined microbubble-catalytic ozonation process

Ozonation is a well-known and widely applied advanced oxidation process (AOP) for industrial wastewater treatment, while the ozonation efficiency might be limited by low mass transfer, poor solubility, and rapid decomposition rate of ozone molecules in the aqueous phase. The present study aims to investigate the feasibility of combined microbubble-catalytic ozonation process (M-O₃/Fe/GAC) for improving the ozonation efficiency during treatment of petrochemical wastewater (PCW). M-O₃/Fe/GAC process optimization was carried out with different pH conditions, ozone dosages and catalyst loadings. The optimum operating conditions were identified as 50 mg L^-1 ozone dosage, real PCW pH (7.0-7.5) and 4 g L^-1 catalyst loading. Among different ozonation processes, M-O₃/Fe/GAC process achieved the highest chemical oxidation demand (COD) removal efficiency of 88%, which is 18% and 43% higher than those achieved by the microbubble and macrobubble ozonation processes, respectively. Phenolic compounds presented in PCW could be reduced by 63% within 15 min in M-O₃/Fe/GAC treatment process. Long-term continuous flow studies suggested M-O₃/Fe/GAC process to be the most cost-effective technology for PCW treatment with an operating cost of S$0.18 kg^-1 COD and S$0.4 m^-3 with good catalyst stability. Liquid size exclusion chromatography with organic carbon detection (LC-OCD) data suggested humic substances to be the dominant organic species in PCW, M-O₃/Fe/GAC could achieve significant humic substances removal and biodegradability enhancement in PCW. Kinetics and mechanism studies revealed that organics removal in M-O₃/Fe/GAC was 1.8 times higher than that in microbubble ozonation process, and hydroxyl radical (^●OH) was the dominant oxidant specie for organics removal in M-O₃/Fe/GAC process.