Ozonation-adsorption treatment train for organic load removal and ecotoxicity assessment of agro-industrial effluent

The Brazilian agro-industrial sector relies heavily on pesticides, leading to effluents that can contaminate soils and water bodies. Due to the low biodegradability of these effluents, conventional treatment methods have limited efficiency. Therefore, this study evaluated the removal of organic load and ecotoxicity using a combined application of ozonation and adsorption with granular activated carbon (O₃/GAC) process applied to a real seed-washing effluent from Rio Grande do Sul. The effluent was initially characterized by physicochemical analyses, revealing a high organic load (COD: 411.67 mg/L; TOC: 239.12 mg/L). The treatment consisted of 15 min ozonation followed by 30 min adsorption. Operational conditions were optimized using a Box-Behnken experimental design, evaluating pH (4, unadjusted, 10), adsorbent dosage (4, 10 and 20 g/L), and applied ozone dosage (0.8, 1.3 and 2.0 g/L) as factors, with responses being chemical oxygen demand (COD) and total organic carbon (TOC) removal. Ecotoxicity was assessed through Lactuca sativa germination bioassays. Results showed COD removal between 20.24% and 65.59%, with best performance at alkaline pH (∼10) and intermediate ozone dosage (1.3 g/L) and AC dose (10 g/L). TOC removal was lower (up to 31.21%), possibly due to matrix complexity and difficulty achieving full mineralization of organic compounds. Higher COD removal is associated with partial ozone oxidation, which fragments complex molecules into less oxidizable intermediates. Despite the synergistic action of ozonation and adsorption, residual ecotoxicity remained significant. Although hypocotyl length yielded similar EC₅₀ values for the raw (32.9%) and treated (38.5%) effluents, the treated effluent exhibited a markedly lower Growth Index (GI = 50.8%) compared to the raw effluent (71.2%). These results reinforce the importance of considering effluent nature and complexity when defining operational conditions. While the integrated process reduced organic load, residual toxicity suggests the need for additional steps, such as longer contact times, biological post-treatment, or complementary adsorbents/advanced oxidation processes (AOPs), to remove toxic intermediates and improve the environmental safety of the treated effluent.