Chemical oxygen demand (COD) reduction in wastewater from the textile industry by coagulation-flocculation and adsorption

The aim of this work was to test the effectiveness of coagulation-flocculation using ferric chloride (FeCl₃) and adsorption on a biochar (OSB) prepared in the laboratory and their combination on textile industry wastewater treatment. By coagulation-flocculation, increasing the dose of FeCl₃ significantly improved the reduction of chemical oxygen demand (COD) levels as well as turbidity in the wastewater. The COD value (1520 mg O₂/L) decreased as the coagulant dose increased, reaching a value of 240 mg/L corresponding to an optimum FeCl₃ dose of around 2500 mg/L. When adsorption was applied to the biochar after 4 h of agitation, the COD reduction efficiency was 83.65%. Application of the kinetic models confirmed that chemisorption of the pollutants presented by the COD on the surface of OSB was predominant. COD reduction efficiency improved significantly with increasing adsorbent dose and wastewater initial pH variations. Combining the two processes, starting with coagulation-flocculation followed by adsorption on OSB, was a cost-effective technique. This satisfactory result was based on the fact that this combination enables a lower dose of coagulant (2500 to 100 mg/L) and adsorbent (4 to 2 g/L) to be used than that required when treating by each process alone.