Irrigation effects of municipal effluents treated with advanced oxidation processes: Bioaccumulation and potential health risks

Advanced oxidation processes (AOPs) like ozonolysis, UV-photolysis, and TiO₂-photocatalysis have proven effective in treating secondary municipal wastewater effluent to meet unrestricted agricultural reuse standards. However, concerns arise due to residual byproducts such as catalysts and reactive oxygen species (ROS) in the treated effluents, potentially impacting crop and human health. The current work explored the impacts of AOPs-treated effluents on crop physiological traits and the potential microbial and chemical health risks associated with the consumption of such crops. Lettuce and Swiss chard were irrigated using secondary municipal wastewater effluents treated with TiO₂-photocatalysis (Ti-TE), UV (UV-TE), ozone (O-TE) and municipal tap water (MTW) as control. The effect of the different irrigation streams on crop growth was evaluated. The results revealed significant phytotoxic stress on the examined crops, with Ti-TE showing more pronounced effects than UV-TE and O-TE. Notably, TiO₂ deposition in stomata and leaf coverage led to significant chlorophyll fluorescence and gas exchange stresses on the crops. Ti-TE caused the highest percentage of electrolyte leakage in Swiss chard (34.22 %) and lettuce (25.29 %), followed by O-TE (19.07 % in Swiss chard and 20.20 % in lettuce) and UV-TE (12.90 % in Swiss chard) and 19.57 % in lettuce). Interestingly, Swiss chard exhibited higher tolerance to phytotoxicity stress from Ti-TE, UV-TE, and O-TE than lettuce. No pathogens were detected on the crop leaves, with E. coli reading < 1 MPN and no faecal coliform reaction observed. However, the levels of titanium found on the edible leaves (Swiss chard: 2.7, lettuce: 1.46, g kg^-1 dry-leaf weights) indicated potential health risks associated with consuming Ti-TE-irrigated crops. Conversely, O-TE and UV-TE demonstrated a promising approach to maintaining environmental integrity and food quality standards, highlighting their potential in safe agricultural wastewater treatment practices.