The mechanism of electro-osmotic dewatering (EOD) is quite different from that of mechanical dewatering. Compared with the mechanical dewatering, EOD can be remarkably effective for hardly dewaterable suspensions of very fine particles and gelatinous materials. However, the gradual discontinuity of liquid state in the dewatering material, due to the formation of an unsaturated layer at the anode, causes the electrical blockage rendering EOD no longer effective. Mixing is believed to reduce such an effect. Therefore, a research is conducted to investigate this effect by having the anode rotating....[ Read more ]

The mechanism of electro-osmotic dewatering (EOD) is quite different from that of mechanical dewatering. Compared with the mechanical dewatering, EOD can be remarkably effective for hardly dewaterable suspensions of very fine particles and gelatinous materials. However, the gradual discontinuity of liquid state in the dewatering material, due to the formation of an unsaturated layer at the anode, causes the electrical blockage rendering EOD no longer effective. Mixing is believed to reduce such an effect. Therefore, a research is conducted to investigate this effect by having the anode rotating.

An experimental apparatus was specially designed for this study. It consisted of one acrylic plastic cylinder, one porous stainless steel plate and one circular stainless steel plate. The porous plate acted as the cathode, and the circular plate as the rotating anode. Bentonite sludge, with 9.lwt% initial solid content was used as the model sludge. The rotational speed varied from 0 to 300rpm. Three commercial impellers (axial flow impeller, high-shear dispersing impeller and high-shear radial flow impellers) were also used as rotating anodes and tested for their performances on EOD.

The experimental results showed that, by using a flat plate as the rotating anode, the water removed from the sludge increased significantly with the rotational speed initially and reached a plateau at 240rpm. This was believed due to the "fall-off" of the cake from the anode during the rotation. The solid content of the cake could be 23.0wt%, 60% better than the performance of normal EOD. Besides, the excessive suspension added on the top of the anode was found to have a beneficial effect for both water removal and suppression of drier cake formation. It was also found that a constant thickness of the cake formed (about 5mm) was more efficient in electro-osmotic dewatering. The combined effects of anode rotation and dry cake thickness control could remove 56% of the initial moisture within the suspension. It was 37% better than normal EOD. Different commercial impellers performed similarly at the rotational speed of 240rpm. In addition, the value of rpm=60 was found to be the most effective condition for dewatering in the case of using high shear dispersing impeller. At this optimal condition, the water removal enhancement was comparable with those obtained using the rotating plate with the dry cake thickness control. The interesting experimental results were well explained following the fundamental analysis of EOD.