Tubular membrane photoreactor for the tertiary treatment of urban wastewater towards antibiotics removal: application of different photocatalyst/oxidant combinations and ozonation

Abstract

A tubular membrane photoreactor, operated in continuous mode and low residence times (τ from 3.9 to 12.2 s), was applied for the removal of 14 antibiotics (sulfonamides, fluoroquinolones, macrolides, cephalexin, and trimethoprim) in demineralized water (DW) and secondary-treated urban wastewater (UWW). In this setup, the oxidant (hydrogen peroxide (H2O2), persulfate (S2O82−) or ozone (O3)) is permeated through the membrane pores and delivered to the catalyst (titanium dioxide (TiO2) or silver molybdate (Ag2MoO4)), embedded in the membrane shell side, and to the annular reaction zone (ARZ) where the DW or UWW fortified with the target antibiotics (50 µg L−1 each) flows. The antibiotics removal in the tested processes - photolysis (UVC), photocatalysis (UVC/TiO2 or UVC/Ag2MoO4), photochemical oxidation (UVC/H2O2 or UVC/S2O82−), their combinations (UVC/TiO2/H2O2, UVC/TiO2/S2O82‾, UVC/Ag2MoO4/H2O2 and UVC/Ag2MoO4/S2O82‾) and O3 - were evaluated at steady-state conditions by ultra-high performance liquid chromatography followed by quadrupole time-of-flight high-resolution mass spectrometry (UHPLC-QTOF-HRMS). In the DW, the treatment efficiency followed the sequence: UVC < UVC/TiO2 ≈ UVC/Ag2MoO4 < UVC/H2O2 < UVC/TiO2/H2O2 ≈ UVC/Ag2MoO4/H2O2 < UVC/S2O82‾ < UVC/TiO2/S2O82‾ < UVC/Ag2MoO4/S2O82‾ < O3. In the UWW, the O3 process proved again to be the most efficient (12 antibiotics removed >90%), followed by UVC/Ag2MoO4/S2O82‾ (five antibiotics removed >60%). UHPLC-QTOF-HRMS tentatively identified seven by-products stemming from macrolides for O3 treatment. This is a pioneer study on the application of Ag2MoO4 combined with H2O2 or S2O82- for advanced treatment of UWW, laying the groundwork for future research. The membrane reactor proves to be an effective technological approach for the O3 process.