Cost effective electrodialytic seawater desalination

The share of electrodialysis in seawater desalination is very small in contrast to reverse osmosis. It is stated that ED may compete with RO in the range of feed water salinity up to 8-10 g/L only because ED desalination cost is proportional to the amount of salt, which must be carried through the membrane. It is assumed in the papers comparing RO and ED that exergy loss in RO is equal to 0.983 while in ED to 12.87 kWh/m(3) in the process of seawater desalination. The cited results were however achieved assuming 35 atm excess pressure in RO and 0.8 V excess voltage in an electrodialysis stack. Diminishing the aforementioned DeltaP value would indeed result in a decrease in RO flux and, as a consequence, an increase in desalination costs. The voltage drop in ED may be however diminished by applying low-resistance membranes and decreasing an intermembrane distance in an ED stack. Electrodialytic desalination of seawater was investigated in a laboratory using ED stack (developed by the author) equipped with 0.19 mm spacer. The pressure drop vs. linear velocity in the ED channel is small because of a special shape of a spacer net. Asahi Glass CMV and AMV membranes were applied. Seawater was desalinated in a cascade of two electrodialyzers at current density 300-600 A/m(2) in the first stage and 300 A/m(2) in the second. The total energy consumption estimated for industrial plant was equal to 6.6-8.7 kWh/m(3) of desalinated water (TDS 0.45 g/L) depending on the first stage current density. The desalinated water cost was equal to 1.05 $/m(3) assuming 0.06 $/kWh.