Tertiary treatment of a municipal wastewater toward pharmaceuticals removal by chemical and electrochemical advanced oxidation processes

Abstract

This study focuses on the degradation of pharmaceuticals from a municipal wastewater after secondary treatment by applying various advanced oxidation processes (AOPs) and electrochemical AOPs (EAOPs) like UVC, H₂O₂/UVC, anodic oxidation (AO), AO with electrogenerated H₂O₂ (AO-H₂O₂), AO-H₂O₂/UVC and photoelectro-Fenton (PEF) using either UVC radiation (PEF-UVC) or UVA radiation (PEF-UVA). The municipal wastewater after secondary treatment was spiked with 5.0 mg L^-1 of trimethoprim (TMP) antibiotic. The efficiency of processes to remove TMP followed the order UVC < AO-H₂O₂ < PEF-UVA << AO ≈ PEF-UVC < AO-H₂O₂/UVC < PEF-UVA (pH = 2.8) < H₂O₂/UVC ≈ PEF-UVC (pH = 2.8), using neutral pH, except when identified. While the UVC radiation alone led to a very low TMP removal, the H₂O₂/UVC process promoted a very high TMP degradation due to the production of hydroxyl radicals (OH) by H₂O₂ cleavage. In the AO-H₂O₂/UVC process, the electrogeneration of H₂O₂ can avoid the risks associated with the transportation, storage and manipulation of this oxidant and, furthermore, OH at the anode surface are also formed. Nevertheless, low contents of H₂O₂ were detected mainly at the beginning of the reaction, leading to a lower initial reaction rate when compared with the H₂O₂/UVC system. In the PEF-UVC, the addition of iron at neutral pH led to the visible formation of insoluble iron oxides that can filter the light. At pH 2.8, the iron remained dissolved, thereby promoting the Fenton's reaction and increasing the organics removal. The UVA-driven processes showed limited efficiency when compared with those using UVC light. For all processes with H₂O₂ electrogeneration, the active chlorine species can be scavenged by the H₂O₂, diminishing the efficiency of the processes. This can explain the lower efficiency of AO-H₂O₂ when compared with AO. Moreover, the degradation of the MWWTP effluent spiked with 18 pharmaceuticals in μg L^-1 during AO process was assessed as well as the influence of the following operational variables on the process efficiency: (i) H₂O₂ concentration on H₂O₂/UVC, (ii) current density on AO, AO-H₂O₂, AO-H₂O₂/UVC, PEF-UVC and PEF-UVA, and (iii) pH on PEF-UVA.

Keywords: AOPs; EAOPs; Municipal wastewater; Pharmaceuticals; Tertiary treatment.