The performance of a cyclic electrodialysis process (with sealed membranepairs and periodically reversing flow and electrical polarity) was compared with a steady-state process when both were operating under fouling conditions.
A 2000 mg/l sodium chloride solution was circulated through the bench-scale test cells for 20 hours in each run.
Iron (II) was introduced into the feed as ferrous chloride at concentrations of 0, 1 and 5 ppm.
IONAC MC 3142 cationic membranes and IONAC MA 3148 anionic membranes were used in each type of process, with the flow channels defined by polyethylene-mesh separators through which the fluid velocity was 1.25 cm/set.
Potentials of 20 and 30 volts were applied across each stack, which consisted of 13 or 14 active membranes plus 2 membranes in each of the electrode rinse compartments.
Reddish-brown deposits were visible on all of the test membranes in all steady and unsteady-state runs having iron in their feed streams. Deposits for the steady-state runs were uneven and showed clear evidence of flow channelling. In the unsteady-state runs the deposits were observed as a fainter, even, coloring over the whole surface of the membrane.
However there is clear evidence that the deposition process is partly irreversible so that the polarity switching does not prevent build up of this type of fouling.
Furthermore, some fouling occured inside the sealed membrane-pairs.
Photographs of these various deposits will be presented.
The effect of deposits on separation efficiency and on pressure drop are reported and discussed in the light of a 'charged film' model.