12.2 J Thermal desalination
Thermal desalination, or distillation, is the earliest form of water purification and is still one of the main commercial desalination methods today. It is a mature technology that has been used extensively in the desalting of seawater since the late 1950s, particularly in the Middle East and in other water-stressed coastal regions. The thermal process is capable of producing very large amounts of freshwater from seawater, though the amount of energy required has traditionally made it a very expensive process because the feedwater has to be heated. To improve economic efficiency, however, most of the desalination plants operating in the Middle East and elsewhere are dual-purpose facilities, producing both water and electricity.
Distillation basically involves heating saline feedwater to evaporation point in order to separate out the salt and other dissolved minerals. The purified vapour is then condensed, collected and stored. The three most common thermal processes are described in the table below:
1 multistage flash (MSF)
2 multiple effect distillation (MED)
3 vapour compression (VC)
Feedwater passes through a series of chambers with successively lower temperature and pressure, to rapidly vaporize (‘flash’) the water into steam. The vapour then rises and is condensed by tubes of cooler inflowing feedwater.
Feedwater passes through a number of evaporators in a series; vapour from one series is then used to evaporate water in the next series.
Feedwater is evaporated, the vapour is compressed, and then the heated compressed vapour is used as a heat source to evaporate additional feedwater.
Some thermal desalination plants are hybrid facilities, which means that more than one of the above technologies is utilized in the same plant.
Thermal desalination plants are simple, robust and reliable, though initial capital costs are high. Reports suggest that capital costs of desalination by MSF and MED are around twice that of RO desalination systems. However, since the introduction of commercialized distillation in the 1950s, there has been an accumulation of experience in process technology, design and construction practices, which means that operational problems, such as scaling, fouling and corrosion, have already been solved. The simple construction of MSF plants, for example, involves only a small number of connection pipes and no moving parts apart from conventional boilers. This reduces leakage and cuts down on maintenance. Additionally, many plants are highly automated and require a limited number of staff to operate, except during routine inspections, maintenance and cleaning of pipes and pumps.
Pre-treatment of feedwater
Unlike RO, feedwater in thermal processes requires limited pre-treatment because there are no membranes to foul or damage. In this respect, nor do they generate waste from backwashing of fouled filters. Apart from the addition of anti-scalants, distillation does not normally require the addition of chemicals to pre-treat the source water. The limited use of chemicals also reduces the serious environmental effects of the disposal of hot brine concentrate back into the sea. As most distillation methods recover around 30–50% of the source water, brine disposal involves very large quantities. Higher recoveries would be less economical, energy- wise, due to a higher boiling point caused by the high salinity of seawater.
Product Water
Thermal desalination produces high-quality water with a range of 1.0–50 ppm/TDS, compared with product water from RO plants ranging from 10–500 ppm/TDS. Plants that produce water for domestic use employ post-treatment processes similar to those applied in the RO process to ensure health standards for drinking water. Pure distilled water is used mostly in industrial processes, including power plant boilers, for example.
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ppm/TDS is one of the standard ways of measuring water purity. TDS stands for total dissolved solids, which are measured in ppm or parts per million. Total dissolved solids are all the inorganic and organic materials, including minerals, salts and metals, in a given amount of water. Most tap water has a measurement of 150–420 ppm TDS; water with over 500 ppm is generally considered unfit for consumption.
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