Treatment of effluents loaded with sulfide from unhairing-liming tannery unit by coagulation-flocculation process: Parameters effect and statistical approach

• Comprehensive analysis of sulfide-rich wastewater treatment from tanneries. • Optimization of coagulation-flocculation for efficient sulfide and COD removal. • Demonstration of the use and efficiency of aluminum sulfate as a coagulant . • Quantification of parameter effects using the Analysis of Variance (ANOVA) test. Effluents from tannery unhairing-liming units contain high concentrations of sulfide ions (S²⁻) due to the use of sodium sulfide (Na 2 S), making them highly toxic and hazardous to both the environment and human health. This study aims to characterize and treat wastewater from a tannery unhairing-liming unit using the coagulation-flocculation process, with aluminum sulfate as the coagulant and praestol as the flocculant. Several parameters, including pH, coagulant dosage, flocculant dosage, stirring speed, and stirring time, were studied to determine the most effective treatment conditions. The influence of these parameters was analyzed using analysis of variance (ANOVA) to assess their statistical impact on sulfide and chemical oxygen demand (COD) removal. The result revealed that wastewater from the unhairing-liming unit is heavily loaded with sulfides and organic matter, significantly exceeding discharge standards. Under optimal conditions pH 7, a coagulant dose of 0.4 g/L, a flocculant dose of 1 mg/L, a coagulation time of 10 min, a flocculation time of 20 min, and stirring speeds of 200 rpm for coagulation and 55 rpm for flocculation—COD removal reached 93.7 %, while sulfide reduction was 86.9 %. Coagulation-flocculation at pH 6–7 minimizes H₂S gas formation, reducing toxic emissions significantly. Among the tested parameters, pH and coagulant dosage had the most significant impact on pollutant removal efficiency. These findings suggest that the coagulation-flocculation process is an effective method for treating tannery effluents, with potential for significantly improving the quality of industrial wastewater, although additional treatment steps may be necessary to meet environmental discharge standards fully.