Comparison of O3-BAC, UV/O3-BAC and TiO2/UV/O3-BAC processes for removing organic pollutants in secondary effluents

Three processes combining different oxidations and subsequent biological activated carbon (BAC), i.e. O3-BAC, UV/O3-BAC and TiO2/UV/O3-BAC were employed to treat secondary effluents and compared. The effects of ozone dosage and empty bed contact time (EBCT) in BAC unit on dissolved organic carbon (DOC) removal were investigated. The results showed that the presence of UV and TiO2 improved ozone utilization and biodegradability of the effluent. DOC removal increased with ozone dosage and EBCT in BAC, however, 3 mg/L ozone dosage with 15 min oxidation time and 15 min EBCT in BAC were more economical and efficient. The synergetic effect between oxidation and BAC is in the order: TiO2/UV/O3-BAC > UV/O3-BAC > O3-BAC. Among three oxidation processes, organic pollutants with molecular weight (MW) < 10,000 Da were mostly removed by TiO2/UV/O3, and those with MW > 10,000 Da were mostly removed by O3 alone. GC-MS analysis showed dibutyl phthalate, bis(2-ethylhexyl) phthalate, 4-bromo-3-chloroaniline and other phenol derivatives were prevalent organic pollutants found in the secondary effluent. After TiO2/UV/O3 treatment, some aromatic compounds including 2,4-dichloro-benzenamine, 4-bromo-3-chloroaniline and 3,5-dimethoxy-acetophenone disappeared and some small molecules were formed, but the kinds and concentration of organic pollutants including phthalate esters were greatly reduced by the subsequent BAC unit.