Page 78 - Membranes for Industrial Wastewater Recovery and Re-Use

58 Menibranes for Industrial Wastewater Recovery and Re-use

diluate resistance and the resistance of the ion exchange membranes, although
determined to some extent by the salt concentration, is normally low - between 2
and 10 f2 cm2 for most commercial ion exchange membranes in 0.5 eq mP3 NaCl
(Strathmann, 1984). It therefore follows that the main contribution to the
electrical resistance is from the diluate cell.
The specific energy demand is increased, by 5-lo%, by electrodialysis reversal
(EDR) operation. In this operational mode, which is very common in ED
applications, the current is periodically reversed such that the concentrated
stream becomes the diluate stream and vice versa. This adds to process
complexity and reduces recovery but also virtually eliminates problems with
scaling, since reversing the polarity of the electrodes produces a concomitant pH
shift that suppresses the build-up of scale during the high-pH cathode electrode
compartment.

Ancillary mass transfer promotion
Mass transfer can be promoted simply through changing the spacer material.
Membrane spacers for ED technologies are either sheet flow, which promote plug
flow of the water throughout the width of the channel, or tortuous path, which
provide an extended path length and thus a greater linear flow velocity for the
same volumetric throughput (Fig. 2.24). Tortuous path spacers yield higher
Reynolds numbers but at the expense of a higher pressure drop across the stack.
Most commercial ED systems are now based on sheet flow spacers.
Certain technologies use additional devices to promote mass transfer.
Examples include the mechanically enhanced, high-shear processes (Section
2.1.4) which employ motors to increase the shear at or near the membrane
surface. The additional energy consumption of such devices relates directly the
shear applied, and a key part of the design of such devices is optimisation so as to
impart the maximum shear for the minimum energy expenditure. As already
stated (Section 2.1.4), coarse bubble aeration may also be employed to enhance
mass transfer and in immersed membrane processes, such as the submerged
membrane bioreactor (Stephenson et al., 2000). In such cases it represents a
significant, sometimes the most significant, contribution to the overall energy
demand and is dependent on the air flow rate, the nozzle diameter and the
submersion depth. Other turbulence promotion devices, such as pulsed and
vortex flow (Section 2.1.4) have yet to be commercialised.

(a) (b)
Figure 2.24 Electrodialysis spacers: (a) sheetflow (Eurodia) and (b) tortuouspath (Ionics)