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Desalination is the abstraction and treatment of brackish or seawater to remove dissolved salts and organics such as pesticides, producing freshwater for beneficial purposes including drinking water. 97% of the world’s water is seawater thus seawater desalination can provide a plentiful and secure supply of drinking water. Desalination is predominant in arid parts of the Middle East, North Africa, North America, as well as Mediterranean and Caribbean Islands with limited freshwater resources of their own.
There are two principal methods of desalination: thermal distillation and reverse osmosis. Distillation involves the heating of feed water to produce steam, leaving salt molecules behind. The steam is then condensed. Reverse Osmosis involves the passing of highly pressurised feed water through a semi-permeable membrane so that water molecules can pass through but molecules of impurities (including salt) are unable to pass through and are separated as a concentrate.
Energy Requirements
Distillation has a greater energy requirement compared with reverse osmosis (unless there is waste heat available) because of the energy needed to heat the feed water. Reverse osmosis does not require energy to heat water but pumping water at high pressures still necessitates significant energy consumption.
Energy consumption can be reduced by desalinating brackish water rather than fully saline seawater which has a higher osmotic pressure. Desalination of seawater also costs up to five times more than with brackish water (1). Solar power is considered too costly to supply the vast energy demands of a desalination plant except for very small systems. The energy requirement for thermal distillation can also be reduced if a source of pre-heated water (e.g. used power station cooling water) can be utilised. Recently Multi Effect Distillation which uses a vacuum to reduce the distillation temperature to 600C, has been used in the Middle East to produce potable water for US$ 0.70/m3. Only 300C of heat needs to be added because the summer seawater temperature is already 300C.
Energy Recovery Devices
Although the highly saline wastewater (concentrate) will not have the same pressurisation that was initially applied to the intake water, energy recovery devices can convert residual pressure in the concentrate, reusing it directly or converting it into mechanical energy. The International Desalinisation Association report energy usage in the range of 3 kWh/m3 for seawater reverse osmosis plants using energy recovery devices (2). Energy recovery is typically over 85%.
The Environment
The American Water Works Association suggests that 166 million litres/day (ml/d) of seawater is needed to make 95 ml/d of drinking water (1). The remaining 71 million litres is concentrate which needs disposing of. Traditionally this highly saline wastewater has been piped back in to the oceans, but concerns have been raised over whether this is adversely affecting ocean water quality. Other concerns include the impact of seawater intake pipes on marine organisms (especially small fish) and whether desalination plants, which for economies of scale are usually large-scale, will spoil coastal panoramas.
The Future
The membrane technology used in reverse osmosis desalination is a well established technology. Significant improvements have already been made in reducing the cost of membranes and improving their durability through reducing the fouling of the membranes but desalination remains a costly option. There are around 15 000 desalination plants already operational globally but whether desalination becomes more widespread, will depend on whether costs such as energy, membranes, concentrate disposal and chemical usage can be reduced further and what happens to the availability of other water sources.
Case Studies
Read about desalination in California, USA
Read about the proposed Thames Esutary desalination plant, London
References
(1) AWWA (2005) desalination fact sheet http://www.awwa.org/Advocacy/pressroom/Desalination.cfm
(2) Buros, O. K (2000) ABC’s of desalting. 2nd ed. IDA. Available at http://www.idadesal.org/PDFS/Publications/ABCs.pdf
(3) European Desalination Society (2005) http://www.edsoc.com/
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