Continuous UV-C/HO and UV-C/Chlorine applied to municipal secondary effluent and nanofiltration retentate: Removal of contaminants of emerging concern, ecotoxicity, and reuse potential.

As global effects of water scarcity raise concerns and environmental regulations evolve, contemporary wastewater treatment plants (WWTPs) face the challenge of effectively removing a diverse range of contaminants of emerging concern (CECs) from municipal effluents. This study focuses on the assessment of advanced oxidation processes (AOPs), specifically UV-C/HO and UV-C/Chlorine, for the removal of 14 target CECs in municipal secondary effluent (MSE, spiked with 10 μg L of each CEC) or in the subsequent MSE nanofiltration retentate (NF, no spiking). Phototreatments were carried out in continuous mode operation, with a hydraulic retention time of 3.4 min, using a tube-in-tube membrane photoreactor. For both wastewater matrices, UV-C photolysis (3.3 kJ L) exhibited high efficacy in removing CECs susceptible to photolysis, although lower treatment performance was observed for NF. In MSE, adding 10 mg L of HO or Cl enhanced treatment efficiency, with UV-C/HO outperforming UV-C/Chlorine. Both UV-C/AOPs eliminated the chronic toxicity of MSE toward Chlorella vulgaris. In the NF, not only was the degradation of target CECs diminished, but chronic toxicity to C. vulgaris persisted after both UV-C/AOPs, with UV-C/Chlorine increasing toxicity due to potential toxic by-products. Nanofiltration permeate (NF) exhibited low CECs and microbial content. A single chlorine addition effectively controlled Escherichia coli regrowth for 3 days, proving NF potential for safe reuse in crop irrigation (<1 CFU/100 mL for E. coli; <1 mg L for free chlorine). These findings provide valuable insights into the applications and limitations of UV-C/HO and UV-C/Chlorine for distinct wastewater treatment scenarios.