Reuse of Coagulation Sludge: Removal of Color and COD from Soy Sauce Wastewater by Coagulation

Effluents from soy sauce industry are characterized by high COD and color. Although significant reduction in COD is achieved through biological treatment processes, a substantial amount of color remains. Pretreatment by coagulation with polyaluminum chloride and lime is effective for color removal. But this method is expensive due to the high doses and sludge production. In this paper, sludge produced from soy sauce wastewater coagulation was reused in coagulation process. Tests varying the reused dosage of sludge and pH were conducted to analyze their influences on the decolorization and COD of the wastewater. Satisfactory results were attained working with 50 mL reused sludge per liter wastewater and adjusting pH to 12. In this way, 93% color elimination, 45% COD removal, 40% dose reduction and 28% sludge production decrease were obtained. Microscopic examination of the floes has increased the understanding of reused sludge, which physically swept the colloidal particles from the suspension.