Page 47 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 2 7

Conrentrote Inlet
Electrode Feed Electrode Waste

Cation Transfer Membrane
Demineralized Flow Spacer
Anion Transfer Membrane
Concentrate Flow Spacer
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Electrode Feed Electrode Waste

Concentrate Outlet
Figure 2.8 Elrrtrodialysisstack (by kindpermission oflonics)

assembled. By the same token, the design allows both membrane sheets and the
spacers that separate them to be removed and replaced on an individual basis.
Indeed, ED suppliers are able to tailor the membrane spacer to the feedwater
specification and the duty to which the technology is being put, allowing a
degree of flexibility that other membrane module designs are unable to provide
(Scction 2.4.2). Ceramic rectangular pressure-sealed plate-and-frame modules
also exist, an example being the Keram module by TAMI.
Spirally wound membranes (Fig. 2.9) have the advantage of simple and robust
construction whilst providing a reasonable membrane area per unit volume, and
find use in reverse osmosis and nanofiltration plant. As with many membrane
modules, the membrane element comprises two membranes sandwiched
together, in this case forming a rectangular “bag” sealed on three sides. The
open side is presented to a central perforated tube which collects the permeate
product from inside the membrane element. The membrane element is rolled up
to produce a spiral which forms a cylindrical shape, one end of which is
presented to the feedwater. The fluid path length then equates to the length of the
cylinder, and spacers are used to maintain separation of the membrane leaves
both in the retentate, where the spacers also serve the important task of
turbulence promotion, and in the permeate channels. A number of membrane
elements, up to seven for very large plant, can make up a single module by