Water Journal : Water Journal April 2011
membranes & desalination 96 APRIL 2011 water technical features Abstract Electrodialysis has been used economically and reliably on brackish surface and groundwaters worldwide, as well as on municipal and industrial wastewaters. It is successful because it can tolerate higher levels of turbidity than other membrane processes, is very resistant to fouling, and is not affected by pH levels of 0 to 10. Capital costs are competitive or slightly higher, but operating costs are lower. Introduction A massive research effort has been put into desalination by the pressure-driven process of reverse osmosis (RO), which is well established. In RO, pressure is applied to overcome osmotic forces, pushing water through a semi-permeable membrane that rejects salts and larger species. It is an expensive desalination procedure, both in capital and energy costs, and requires pre-treatment of the feed water to minimise the scaling and fouling that reduce productivity and shorten membrane life. A number of other membrane processes that require little or no applied pressure also exist. The earliest and most developed of these, electrodialysis (ED), makes use of electrical energy to drive ions through ion-selective membranes that only allow passage of ions of opposite charge to that of the fixed sites on the membrane. It was the first of the membrane desalination processes to be used on a large scale, having its origins a decade before RO, so it has a long and proven history (Wilson, 1960). However, its rate of expansion has been much slower than that of competing technologies and its application is generally restricted to brackish salinities because of the increasing equipment and operational costs involved. ED differs from RO in that the feed water becomes the product stream, with only the salt being transferred rather than the entire volume of product water. It therefore should have a higher intrinsic recovery than pressure-driven processes. There are other emerging membrane approaches that can be also considered: options that include forward or direct osmosis, membrane distillation and dual nanofiltration/RO systems (Bolto et al., 2007a, 2007b, 2010). Principle of Electrodialysis In ED dissolved salts are transferred through ion-selective membranes in a multi-compartment assembly by a direct current electric field (Strathmann, 2004). The ions transfer from a less concentrated stream to a more concentrated one. The cell assembly consists of an electrode pair and a stack of cation and anion permeable membranes arranged alternately (Figure 1). The positively charged ions migrate towards the cathode and the negatively charged ions go to the anode. The migration of the ions is restricted, as the cations cannot pass through the positively charged membranes and the anions cannot pass through the negatively charged membranes. Thus alternate compartments end up containing water that is either desalted or more concentrated. The Electrodialysis Stack An ED stack comprises two electrodes, typically platinised titanium or graphite, separated by as many as 200 pairs of alternating cation- and anion-selective membranes, each separated by a polymeric spacer designed to maximise the efficiency of the membrane stack. Most practical membranes are reinforced with a synthetic fibre cloth and are about 0.5mm thick. A DC voltage is applied between the electrodes and the resulting DC current causes electrolysis and the movement of ions through the membranes. 50% of all ions are typically removed in a single stage, so multiple stages are required for further removal. Thus, with a feed of 2000mg/L, a single- stage EDR may produce 1000mg/L while a three-stage system may be required to achieve a product stream of 250mg/L. The brine or salt concentrate stream (Figure 1) into which the ions migrate is recirculated and the salts accumulate until precipitation occurs, at which point feed water is used to maintain the brine at this set point with the overflow or blowdown going to waste or further processing. Electrodialysis Reversal ED with polarity reversal (EDR) has been employed to drive off scales, typically calcium and magnesium carbonates, sulphates or phosphates depending on feed composition, from the membranes and electrodes, eliminating or at least reducing the need for the addition of acid or anti-scalants (Katz, 1983; Strathmann, 2004). When the polarity is reversed, the directions of ion movement are also reversed, which necessitates a simultaneous reversal of the flow streams, so a more sophisticated flow control is needed. The reversal occurs at intervals of several times an hour. EDR enables the brine stream to be operated under conditions of supersaturation with respect to solubility-limiting species like calcium carbonate and sulphate (Meller, 1984). R Taylor, B Bolto A review of a robust process with wider applications ELECTRODIALY SIS -- A MATURE MEMBRANE DESALTING PROCESS Figure 1: Principle of electrodialysis (Astom Corporation, 2004: Used with permission).
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