Indirect Electrochemical Treatment of Saline Dyestuff Wastewater

Abstract

Dyestuff wastewater is always found to contain highly-concentrated sodium chloride. In many cases, biological processes are not available to dispose saline dyestuff wastewater. In this paper, both direct and indirect electrochemical oxidation of saline dyestuff wastewater were carried out and the accessibility of indirect electrochemical oxidation by electrochemically generated hypochlorite was investigated. It was found that the indirect electrochemical process removed COD by 87% and color by 100% in 50 minutes while the direct electrochemical process removed COD by 47% and color by 50% in 5 hours. The rate-limiting step for hypochlorite evolution at the anode was found to be diffusion-controlled. For the two processes, COD removal kinetics could be simplified as the pseudo first-order kinetics. The rate constants of COD removal were 0.038 min−1 and 0.0018 min−1 for the indirect and direct oxidation respectively. Current efficiency of the indirect process was 1 while that of the direct process was 0.16; cost efficiency of the two processes was 8000 mg-COD (kWh)−1 and 1000 mg-COD (kWh)−1 respectively. It can be concluded that the indirect electrochemical process is a promising method for the disposal of saline dyestuff wastewater.