electrocoagulation pretreatment prior to Reverse Osmosis membranes

This study refined the hardness removal (together with chemical oxygen demand (COD) and turbidity) from the produced water by a pilot-scale electrocoagulation (EC) system to mitigate the scaling and fouling of Reverse Osmosis (RO) membranes. In preliminary studies, effects of initial pH, current density and electrolysis time on pollutant removal were investigated. Response surface method (RSM) was employed further to evaluate individual/ interactive influences of parameters on pollutant recovery. Analysis of Variance (ANOVA) showed a satisfactory agreement of the predicted and experimental data. The refined operating conditions were pH of 7.36, current density of 5.90 mA/cm2, and reaction time of 30.94 min to maximize the hardness removal at 85.81%, COD at 66.64%, and turbidity at 93.80%. The equivalent coagulant dose was 23.76 mg/L (as Fe3+). The permeate flux was around 22 L/(m2 h) and the water recovery rate reached 87.83%. The operating refinement of EC processes by RSM was a promising alternative for the pretreatment of produced water prior to RO membranes. The product water from the batch EC–RO system was suitable for reinjection or beneficial reuse. © 2014 Elsevier B.V. All rights reserved.

For the first time, a pilot-scale electrocoagulation process was developed for simultaneous removal of hardness, COD, and turbidity, with the related experimental design being supported by the RSM. This was critical for mitigating membrane scaling and fouling. A preliminary research was employed to obtain desired operating conditions: initial pH 7, current density 5.56 mA/cm2 and electrolysis time 30 min. RSM with an eighteen-run CCD design was performed and second-order regression models were generated. ANOVA was conducted to validate the significant consistency between experimental values and predicted ones (hardness removal: R2 0.9782, F value 39.80, p-value b0.0001; COD removal: R2 0.9752, F value 34.97, p-value b 0.0001; turbidity removal: R2 0.9865, F value 65.15, p-value b0.0001). The pH and current density had the most significant effects on pollutant recovery, while reaction time was least significant. The 3D surface graph and contour plots indicated that the recovery efficiencies would increase with pH and current density. The models derived from the EC process provided optimum Table 7 Validation experiments for the refined operating conditions. Parameter Refined value Experimental value COD removal (%) Turbidity removal (%) Hardness removal (%) pH Current density (mA/cm2) Electrolysis time (min) 66.64 93.80 85.81 7.36 5.90 30.94 65.97 92.48 85.36 5.56 Fig. 8. Permeate flux trends during RO filtration: comparison between EC pretreated water and raw produced water.