Fouling mechanisms and energy appraisal in microfiltration pretreated by aluminum-based electroflocculation

Although membrane fouling has been much investigated, solutions are far from being complete. The effects of aluminum-based electroflocculation pretreatment on filtration-energy consumption and fouling mechanisms were investigated in dead-end microfiltration. Silica-CMP suspensions were pretreated by electroflocculation at various operation times (0-4 min, constant electric current of 0.4 A) followed by different slow mixing times (0-30 min) and filtration without any sedimentation step. A new method for filtration-energy appraisal was proposed to define the optimum conditions for operation. Fouling mitigation by electroflocculation was found to be dependent on fouling intensity, dominant fouling mechanism, suspension pH and electroflocculation operation time. Filtration energy was minimized by 90% in a pH range of 6-6.5. Scanning electron micrographs of the fouled membrane surface showed the important role played by the sweep-coagulation mechanism in mitigating fouling. When internal fouling was the dominant mechanism, the amorphous aluminum-hydroxide solids formed a layer that filtered out the primary particles, protecting the membrane pores from plugging. Aluminum-hydroxide particles also reduced the hydraulic resistance of the cake when the external fouling mechanism dominated. Significant energy reduction was observed, even without the slow mixing step, as a result of the local flocculation conditions near the membrane surface. Additional energy savings were obtained due to the significantly higher initial flux restoration rates (>90%) imposed by treating the suspension with electroflocculation.