Effect of Current Density and Contact Time of Electro-Coagulation on the Characteristics of Activated Sludge and Membrane Filtration

Abstract

Objectives : The effect of current density and contact time of electro-coagulation on membrane fouling was investigated. In order to elucidate the reason why the membrane fouling was reduced by electro-coagulation, the changes in the characteristics of activated sludge were examined before and after electro-coagulation.Methods : A series of electro-coagulation of activated sludge suspensions was carried out with current density of 2.5, 6, 12, 24 A/m2 for 0, 2, 6, 12 hours. After membrane filtrations of the activated sludge suspensions, the membrane fouling was compared before and after the electro-coagulation. Characteristic changes in the activated sludge were also analyzed quantitatively.Results and Discussion : Total fouling resistances (Rt) of the electro-coagulated activated sludge decreased as current density and contact time increased. For example, Rt decreased 15%, 63% and 86% under the condition of current density of 24 A/m2 and 2, 6, 12 hours of contact time. Particularly, three activated sludge suspensions having very different initial MLSS concentrations showed a similar reduction of Rt, indicating that the membrane fouling was reduced not dominantly by particulates but by soluble foulants. The MLSS increased as current density increased, but MLVSS had not changed accordingly, which means that inorganic solids such as Al(OH)3 and AlPO4 produced during electro-coagulation were the main reasons for the increased MLSS concentration. These solids could play a key role in the reduction of membrane fouling by means of a dynamic membrane. As current density increased, the EPS (extracellular polymeric substances) and soluble COD concentration were reduced, which led to the reduction of membrane fouling. Soluble TN and TP were not significantly changed after electro-coagulation.Conclusions : The fouling materials, particularly, EPS and soluble COD decreased as current density and contact time increased, which played a key role in the reduction of membrane fouling. In addition, the insoluble solids produced during the electro-coagulation would act as a dynamic membrane, which led to fouling reduction.