Laserfiche WebLink
<br />Electrodialysis <br /> <br />N <br />00 <br />. ,:::.. <br />o <br /> <br />Electrodialysis removes salt from saline feedwaters by <br />passing electrical current through positive and negative ion <br />permeable membranes as illustrated in Figure 7. Unlike de- <br />salting processes involving high temperature or low temperature <br />phase changes, the electrodialysis process uses energy at a <br />rate proportional to the quantity of salts to be removed. <br />Consequently, primary application of this technology is to soft, <br />warm waters having 1000-5000 mg/~ of total dissolved solids. <br />Product water is usually 300-500 mg/~. <br /> <br />Feedwater entering a electrodialysis stack is divided into <br />a brine flow and a product flow. Two electrodes on either side <br />of the system created a positive potential to which the anions <br />migrate and negative potential attracting cations. The brine <br />and product flows are separated by an ion selective membrane <br />allowing either cations or anions to pass but not both. The <br />membranes are arranged as shown in Figure 7 to remove salts in <br />the brine compartment. Passing individual flows through <br />successive treatments allows production of product water at <br />various levels of quality. Scaling and corrosion are major <br />problems in electrodialysis systems and, therefore, require <br />special attention, often by acidifying the brine side of the <br />membranes. <br /> <br />The hydraulic limitations of the ED method follows about <br />the same format as discussed earlier. The BPR, determined by, <br /> <br />BPR '" <br /> <br />l- (TDS /TDS.) <br />P l <br /> <br />............................... (61) <br /> <br />(900/Cai) - 1 <br /> <br />must be greater than or equal to 0.15 due to electrical and <br />chemical factors. Brine volume is determined by Eq. 48 and <br />total intake rate by Eq. 60. <br /> <br />The costing model for ED plants is summarized in Table 5. <br />As noted previously, the performance of the ED process as well <br />as the costs are dependent on both the feed and product water <br />quality. Estimates of construction, land and electrical costs <br />are functions of the number of ED stages which involve first <br />determining a "rating factor", RF, by: <br /> <br />RF '" <br /> <br />0.575{Na. + K. + Cl.} <br />l l l <br /> <br />+ 0.014375 x (T-4.44) .... (62) <br /> <br />TDS. <br />l <br /> <br />where, <br /> <br />RF '" <br />Na. = <br />l <br /> <br />plant rating factor; <br />feedwater concentration of sodium, mg/~; <br /> <br />42 <br />