Reverse Osmosis and Electrodialysis

Briefly and simply, it is like brewing coffee with a paper filter. But instead of a paper filter, a semi-permeable membrane is used to get freshwater out of seawater. Under 50 to 60kg/cm2 of pressure, freshwater will permeate this membrane while salt will not. Electrodialysis uses a more complicated theory. This method removes freshwater by applying an electric current to seawater in which two special membranes, cation (positive ion) exchange membranes and anion (negative ion) exchange membranes, are lined up alternately.

In order to choose which technology to apply, we need to consider various factors. The first and biggest is cost. Generally speaking, it costs US$1-2 to get 1m3 of freshwater from a desalination plant with a capacity of 4,000m3 per day. In the Middle East countries, where oil is inexpensive and is used to generate electricity rather cheaply, the distillation process is most frequently used since they can utilize low pressure steam from steam-power stations. The reverse osmosis process is more popular in the rest of the world. The desalination cost for brackish water differs substantially depending on the level of salinity, but on the whole, it is less expensive than for seawater.

Of the freshwater made through various desalination processes, 63% is used for water supply, 25% for industry and 4% for power generation. In Abu Dhabi, UAE, they long ago started to use desalinated water for water supply and to re-use waste water for greening the city. This re-use of water changed a dreary, desert town into a verdurous city with many street trees casting cool shade and green lawns covering once heated land. Greening the city has brought various birds to the town and has lowered the average temperature by 7 to 8 degrees. Also in Muscat, Oman, a greening project which re-uses desalinated water has recently begun. This may suggest rather eloquently that the desalination of water can also contribute to an improved environment. (Source UNEP)

Reverse osmosis Schematic (below): When seawater and freshwater border on each other with a semi-permeable membrane in-between, freshwater starts to go through such a membrane toward seawater (see Figure 1). When it stops moving and is equilibrated, the pressure gap between seawater and freshwater is called "osmosis pressure" (see Figure 2). If a pressure higher than this "osmosis pressure" is put on the seawater, freshwater is strained out of the seawater through the membrane toward the freshwater side (see Figure 3).

In a chamber between an anode and a cathode, several cells are filled with brackish water. The cells are created by placing anion exchange membranes and cation exchange membranes one after another. When direct electric current is applied between the anode and the cathode, sodium ions move toward the cathode and chloride ions toward the anode. As sodium ions can only permeate cation exchange membranes while chloride ions can only permeate anion exchange membranes, this creates alternate cells of concentrated brackish water and diluted brackish water. Freshwater is obtained by taking out only the diluted brackish water. (Source UNEP)