Advanced Treatment of Effluents Loaded with Sulfide from Unhairing-Liming Tannery Unit:A Combination of Chemical and Biological Processes

This study investigates a hybrid treatment process for highly polluted tannery effluents from the unhairing-liming stage, which are characterized by elevated sulfide and Chemical Oxygen Demand (COD) levels, posing significant risks to human health and the environment. The process integrates chemical precipitation with Ferric Chloride (FeCl₃) and biological treatment in a Sequencing Batch Reactor (SBR). At an optimal FeCl₃ concentration of 1.2 mol.L⁻¹ and pH 8.5, the chemical stage achieved 90.11% sulfide removal and 88.33% COD removal through sulfide precipitation and organic matter coagulation but failed to meet Moroccan discharge standards. Subsequent SBR treatment, operating at a high organic load (1.5 kg COD.d⁻¹.m⁻³), improved removal efficiencies to 99.97% for sulfides and 99.60% for COD, ensuring full regulatory compliance. One-way ANOVA confirmed the statistical significance of the combined process (p < 0.001). XRD and SEM analyses identified gypsum (CaSO₄.2H₂O) as the primary precipitate, which exhibited favorable morphology for sludge management. The FeCl₃–SBR system provides a robust, sustainable solution for small- and medium-scale tanneries, particularly in developing countries.