Treated wastewater reuse for crop irrigation: a comprehensive health risk assessment†

The use of treated effluent/wastewater (TWW) for crop irrigation is gaining prominence globally due to growing freshwater scarcity. However, there are still questions about the safety of such a practice. This study sought to investigate and evaluate the health risks associated with the use of TWW for crop irrigation by assessing the potential risks arising from pathogens, heavy metals/potentially toxic elements (PTEs), micropollutants or pharmaceuticals and antibiotic resistance genes (ARGs), using tomato, carrot and cabbage as test crops. The levels of copper bioaccumulated in TWW irrigated crops were 25 mg kg⁻¹ for tomato, 30 mg kg⁻¹ for carrot and 20 mg kg⁻¹ for cabbage, while those of the control (tap water) were 30 mg kg⁻¹ for tomato, 40 mg kg⁻¹ for carrot and 65 mg kg⁻¹ for cabbage, respectively. Arsenic, cadmium and lead levels were below the detection limit for all treatments. The hazard quotient (HQ) and hazard index (HI) of copper and zinc were below 1 (adults) for TWW irrigated crops. Escherichia coli, Clostridium perfringens, coliform and thermotolerant bacteria were not detected on the fruits of tomato plants irrigated with TWW. All analysed pharmaceuticals were below the limit of detection except gabapentin, which was 3 μg kg⁻¹ in TWW irrigated tomatoes. tetA, ermB, bla_TEM, sul2, sul3 and qnrS genes were found in the metagenomic DNA extracted from TWW- and tap-irrigated cabbage. The results indicate no potential non-carcinogenic health risk for adult consumers and no microbial contamination of the tomato fruits under TWW irrigation. No difference was observed in the presence and distribution of the ARGs between TWW- and tap-irrigated crops, suggesting no contribution to the diffusion of different ARGs due to irrigation. Altogether, these findings highlight that health risk assessment of TWW for crop irrigation should focus on the quality of the TWW and on soil characteristics, which may contribute to risk exposure of different types of contaminants.