Comparison between heterogeneous and homogeneous solar driven advanced oxidation processes for urban wastewater treatment: Pharmaceuticals removal and toxicity

The release of toxic contaminant of emerging concern from urban wastewater treatment plants (UWTPs) into the environment calls for more effective (tertiary) treatment methods. In this manuscript, homogeneous solar-driven advanced oxidation processes (AOPs), namely H2O2/sunlight, solar photo-Fenton (Fe+2/H2O2/sunlight) and solar photo-Fenton with ethylenediamine-N,N'-disuccinic acid (EDDS) complex (Fe+2/H2O2/EDDS/sunlight) were compared to a new heterogeneous process (supported nitrogen-doped TiO2 (N-TiO2)/sunlight), with the aim of contributing to fill the gap between lab scale tests and full scale applications as well as to provide a sustainable solution for tertiary treatment in small UWTPs. Process efficiency was evaluated in terms of effluent toxicity and degradation of a mixture of three pharmaceuticals (namely carbamazepine (CBZ), diclofenac and trimethoprim), at initial concentration of 200 μg/L each, in deionized water (DW) and real wastewater (WW). Fe2+/H2O2/EDDS/sunlight was found to be the most effective process (98% removal of CBZ from WW in 60 min, 5.6 kJ/L as cumulative solar energy per unit of volume). Conventional solar photo Fenton was drastically and negatively affected by water matrix, due to the spontaneous neutral pH and iron precipitation in real WW. Although N-TiO2/sunlight process was not so affected by water matrix, it was found to be less efficient (30% removal of CBZ in 180 min, 13.3 kJ/L) than Fe2+/H2O2/EDDS/sunlight process. Toxicity values were found to be lower in WW compared to DW matrix. Class weight scores for WW samples showed a toxicity reduction up to the no acute toxicity level for N-TiO2/sunlight and Fe2+/H2O2/EDDS/sunlight treatments, while H2O2/sunlight and Fe2+/H2O2/sunlight increased the final effluent toxicity up to slightly acute levels.