Comparison of the chemical precipitation, UV/H₂O₂ and Fenton processes to optimize removal of chronic toxicity from kraft mill effluents

Secondary Treatment Effluents (STE) from Kraft mill effluents are discharged into aquatic ecosystems with high color and chronic toxicity contents owing to the recalcitrance of compounds in the effluents. The goal of the study was to evaluate the chemical precipitation, UV/H₂O₂, and the Fenton processes (H₂O₂/Fe²⁺) for chemical oxygen demand (COD) and for removing chronic toxicity from STE. A circumscribed central composite model and a response surface methodology were used to evaluate the effects of variables such as Al₂(SO₄)₃, Fe(II), and H₂O₂ concentration and pH range for each treatment. The optimal conditions were 984.2 mg Al₂(SO₄)₃/L and pH 5.2 for chemical precipitation; 51.4 mM H₂O₂ and pH 5.1 for UV/H₂O₂; and 5.5 mM Fe(II): 25 mM H₂O₂ concentration and pH 2.8 for H₂O₂/Fe²⁺. Under such optimal conditions, COD removal was 84.7, 80.0, and 93.6%, with reaction times of 57, 75, and 10 min for the chemical precipitation, UV/H₂O₂, and H₂O₂/Fe²⁺ methods, respectively. This study recorded chronic toxicity in STE and sludge formed during chemical precipitation with maximum reductions in percentages of Allometric Growth Rate (AGR) of 11.5 for STE without dilution (100%, p < 0.05). For chemical precipitation sludge, the maximum reduction of AGR was 3.4% for a dilution of 75%. We concluded that all the assessed treatments effectively removed chronic toxicity in the treated effluents.