Stoichiometric relationship in the coagulation of melanoidins-dominated molasses wastewater

Batch experiments of coagulation/flocculation of biologically treated molasses wastewater were conducted to investigate the stoichiometric relationship between the concentration of melanoidins-dominated organics and the dosage of hydrolyzing metal salts (ferric chloride and aluminum sulfate). Wastewater samples were first fractionated by ultrafiltration. Jar tests were conducted to evaluate coagulation efficiency by measuring zeta potential, removal rates of color and chemical oxygen demand (COD). Experimental results indicate that the dissolved organic fraction accounts for predominant portion of the organic compounds present in biologically treated molasses effluent. A stoichiometric relationship exists between the concentration of melanoidins-based organic compounds and coagulant demand. When the change in solution conditions is proportional to that in organic concentration, such stoichiometric relationship still exists. On the other hand, no stoichiometry was observed between the concentration of melanoidins-dominated organics and the metal dosage when substantial changes in the nature of organics or solution conditions occur. The optimal dosage of ferric chloride expressed in terms of the ratio of metal to organic carbon removed was calculated as 0.73-0.81 g Fe3+/g COD.