Treatment of textile dyeing factory wastewater by electrocoagulation with low sludge settling time: Optimization of operating parameters by RSM

Dyeing wastewater differs from textile wastewater because industrial dyeing wastewater except of color contains some dissolved additives such as sodium oxalate and acetic acid. This article deals with the treatment of industrial dyeing wastewater by electrocoagulation technique because the area needed for the treatment of industrial wastewater is a significant factor. In addition, effect of operational parameters such as initial pH (4-9), current density (15-35 mA/Cm-2) and EC time (20-60 min) was determined in low settling time of 30 min in comparison to other studies, which have settling time of 12 h or more. Electrocoagulation includes three stages of oxidation reaction at the anode, break emulsion particles and settling. In this study, Box-Behnken design as a response surface methodology (RSM) was used to investigate the effects of major operating variables and optimization conditions of electrocoagulation method on aluminum electrodes. Results showed that initial pH was important more than other parameters, and initial pH and current density had interaction. Furthermore, operational parameters are effective on settling velocity and low settling time is not appropriate for EC with Al electrodes. Under optimized conditions, initial pH of 5.5, current density of 15 mA/cm(2) and EC time of 23 min, the decolorization was predicted 98%. In the laboratory, decolorization under these conditions was obtained 97%. COD removal efficiency, sludge settling velocity (SSV), electrode consumption and energy consumption under the optimum conditions were measured, which were 40%, 0.004 cm/min, 1.3 kg/m(3) and 7.64 kWh/m(3), respectively. Under optimal conditions, 40% of settled waste can be retrieval; otherwise, more settling time is required. COD removal efficiency was low since wastewater contained dissolved additives such as sodium oxalate, polyvinyl alcohol, and acetic acid, which may be the cause of most COD of this wastewater.