Desalination Technologies

In 2002 there were a total of approximately 12,500 desalination plants around the world in 120 countries. The largest users of desalination are in the Middle East, accounting for 70 per cent of worldwide capacity, followed by the United States, Algeria, China, and Australia. Worldwide, the desalination market is growing at over 7 per cent per year.

A number of desalination technologies exist, each with relative merits and challenges, yet all share high input energy requirements. The high input energy is due to the minimum thermodynamic energy barrier that must be overcome to separate dissolved salt from a solution. The minimum energy barrier is often characterized as 0.75 kWh of energy to produce 1,000 litres of fresh water from seawater. Desalination technologies fall into three categories: thermal, membrane, and chemical.

Membrane Processes

Reverse Osmosis (RO) is often presented as the most energy efficient commercialized desalination technology. RO is a separation process in which saltwater is pressurized and forced through a semi-permeable membrane that filters salt ions from the pressurized solution. RO membranes are made in a variety of configurations by a handful of specialized manufacturers. Pre-treatment of the feed water is critical to ensure that membrane surfaces remain clean. Pre-treatment for seawater desalination commonly involves the addition of antiscalant chemicals, ultrafiltration for fine particle removal, chlorination to control organics, and de-chlorination to prevent membrane damage. RO represents over half of the global desalination market with most new plants using this technology.

Electrodialysis (ED) and Electrodionization (EDI) involve forcing ions from saltwater through ion exchange membranes under an electrical field. A salt molecule is comprised of a positive and negative ion bound together. ED and EDI devices consist of a cell of compartments separated by alternating positive and negative ion exchange membranes which allow only ions with the same charge to pass through them. An electric current is applied to the cell forcing positive and negative ions in opposite directions and through the membranes. Ions are then essentially trapped in concentration compartments and flushed out while alternating product compartments are desalted. EDI differs slightly from ED in that ion exchange resins are used to enhance the transfer of ions. These resins also continually regenerate through water splitting, resulting in relatively constant output water quality over time. EDI is commonly used to make ultrapure water for industrial processes.

Thermal Processes

Multi Stage Flash (MSF) involves introducing heated saltwater into a lower pressure container. A portion of the water rapidly boils (flashes) into steam with the remaining water passing through additional stages, each at a lower pressure. The generated steam is condensed on heat exchangers running through each stage. MSF is very reliable; however, scaling and corrosion of the pipes and machine parts are some of the main concerns with this process. MSF plants produce over 85 per cent of all desalinated water in the world.

Multiple Effect Distillation (MED) consists of several consecutive cells (effects) at decreasing pressure and temperature. Within each effect, saltwater is sprayed on a bundle of tubes carrying steam. The water vapour generated is fed into the tubes running through the next effect while the brine and distillate are collected. MED shares similar reliability, scaling, and corrosion characteristics with MSF but may have better efficiency.

Vapour Compression (VC) is another thermal process that uses the same principles of reduced boiling points with lower pressure. In this process, the heat for evaporating the water comes from the compression of vapour rather than the direct exchange of heat from steam produced in a boiler.

Chemical Processes