Membrane and Desalination Technologies - TCE Moodle Website

544 J.P. Chen et al.
The distance between membranes (i.e. the cell thickness) needs to be as small as possible
in order to minimize the electrical resistance and save energy consumption. The spacer
maintains the shape of the membranes, controls the solution flow distribution in the stack,
and reduces the film thickness on the membrane, which in turn limits the effects of concentration
polarization. It is important to have a uniform flow distribution in the manifold
channels to prevent internal leakage, particularly from the ion-concentrating cell to the iondepleting
cell. The feed water (wastewater) is usually retained for about 10–20 days in a
single stack or stage. Both sheet flow and tortuous path design are used in ED.
In the sheet flow design, the solution flows in a relatively straight line from the entrance to
the exit ports, which are located on opposite sides. Therefore, the supply ducts for the dilute
and brine must be in line with the holes in the spacer gaskets, the membranes, and the
electrode cell. An array of membranes is held between the electrodes and the process streams
are kept separated. If the length-to-width ratio of the spacer is large enough, the entrance and
exit ports can be located at the corners.
In the tortuous path design, a spacer gasket creates a labyrinthine flow and extends the path
length within the cell. The solution takes several 180 turns between the entrance and exit
ports located in the middle of the spacer. The main objective is to provide a long residence
time for the solution in the cell despite the requirement of high linear velocity to limit the
polarization effect. A tortuous path spacer gasket is shown next.
The end plate of the stack is a hard and plastic frame that contains the electrode compartment.
The cathode is made of stainless and the anode is made of platinum-coated tantalum,
niobium, or titanium. The whole setup is compressed together with bolts between the two end
flow plates. The perimeter gaskets of the gasket spacers are tightly pressed into the membranes
to form the cells.
The membranes of ED units are subject to fouling and scaling, and thus some pretreatment
of the feed water is usually necessary. Precipitation of scale can be facilitated in the ED
process by changes in pH that occur near the membranes as a result of the transport of H + and
OH - ions. However, as there is no flux of water through the membranes, ED can treat water
with a higher level of suspended solids than RO. As nonionic solids, e.g. silica, are not
concentrated by this process, these components are of less concern. Thus, inorganic acids
such as H2SO4 can be fed to the concentrate stream to maintain a low pH and thus minimize
scaling and fouling.
3.3.3. Ion-Exchange Membrane
In the ion-exchange membranes, the charged groups are attached to the polymer backbone
of the membrane material. A cationic membrane with fixed negative groups will exclude the
anions but is permeable to the cations. Similarly, an anionic membrane with fixed positive
groups will exclude cations but is permeable to anions. Cation-selective membranes consist
of sulfonated polystyrene, whereas anion-selective membranes consist of polystyrene with
quaternary ammonia. There are two types of IX membranes: homogeneous membranes and
heterogeneous membranes.