Integrated Multi-Barrier Treatment of Hydroponic Wastewater Using A/O-MBR, Ferrate-Assisted GDM, and UVC Disinfection

An integrated oxic/anoxic–oxic membrane bioreactor (MBR), ferrate [Fe(VI)]-assisted gravity-driven membrane (GDM), and UVC system was developed as a compact multi-barrier treatment train for hydroponic waste nutrient solution (WNS). MBR performance was systematically evaluated in terms of permeate flux, transmembrane pressure (TMP), nutrient transformation, and long-term operational stability. Over 150 days of operation, the MBR systems achieved stable organic matter removal (>90% COD removal; effluent <15–20 mg/L) and efficient NH4 + –N oxidation (>95%). The anoxic/oxic configuration further improved nitrogen control by suppressing NO 3 − –N accumulation through partial denitrification. GDM filtration provided stable low-energy polishing under passive hydrostatic conditions for 70 days, with stabilized flow rates ranging from 0.03 to 0.12 mL/min depending on membrane pore size (0.2–1.2 μm). When coupled with Fe(VI), the GDM unit enabled targeted phosphate capture through in situ ferric floc formation and membrane retention, reducing PO 4 3− –P concentrations to sub-mg/L levels at 133 µM Fe(VI). Despite effective membrane filtration, culturable microorganisms persisted in the MBR and GDM permeates at concentrations exceeding 10 6 CFU/mL, necessitating downstream disinfection. Subsequent UVC irradiation at 20 mJ cm −2 further reduced viable microbial concentrations to 10 1 –10 3 CFU/mL, demonstrating effective terminal microbial control. Acute Daphnia magna toxicity assays (24–48 h) provided an additional ecotoxicological verification step and suggested favorable discharge safety of the treated effluent, with no observable immobilization detected in the UVC-treated 0.45 µm GDM filtrate. Overall, the integrated A/O-MBR–Fe(VI)/GDM–UVC system provided complementary biological, physicochemical, membrane-based, photolytic, and ecotoxicological treatment barriers, demonstrating strong potential for sustainable hydroponic wastewater treatment and reuse-oriented water quality management.