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/m3 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 ∆P 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/m2 in the first stage and 300 A/m2 in the second. The total energy consumption estimated for industrial plant was equal to 6.6–8.7 kWh/m3 of desalinated water (TDS 0.45 g/L) depending on the first stage current density. The desalinated water cost was equal to 1.05 $/m3 assuming 0.06 $/kWh.

Two-stage electrodialysis in countercurrent flow mode was proposed for seawater desalination. Electrodialyzer with small intermembrane distances and a new type of separator were applied. Applied solutions allowed diminishing problems that are the result of major differences in ED concentrate and diluate concentrations and the small conductivity of potable water grade diluate. Only 0.145 kWh DC per 1 kg of salt transferred through membranes was found in the first stage of ED stand in laboratory tests. This value is even smaller than what was obtained in Japanese salt production plants, despite the fact that it was achieved with rather worse membranes and a smaller concentration of diluate. When desalting to 0.45 g/L, the total energy consumption per 1 kg of salt transferred through membranes was equal to 0.258 kWh/kg in the case of AMV and CMV membranes and 0.173 kWh/kg in the case of lowresistance membranes (at 300 A/m2). These values are several times smaller than the lowest value indicated in literature (0.59 kWh/kg). Energy consumption as small as 6.6 kWh/m 3 w hen desalting to 0.45 g/L was found at current density 300 A/m2. Assuming 0.06 $/kWh, 320 $/m2 of installed membrane and maintenance cost equal to 25% of investment costs minimum desalination cost equal to 1.05 $/m3 was found at current density 600 A/m2 in the first stage electrodialysis. The values of energy consumptions and the total cost of desalination are similar to those gained in reverse osmosis and thermal methods. Further optimization of electrodialytic desalination may lead to decrease in power consumption equal to or less than value of 5 kWh/m3. Therefore, it was found that electrodialysis is fully suitable for seawater desalination and may compete with reverse osmosis and thermal methods.