(1) Multi-effect Distillation (MED):
MED is one of the earliest types of distillation used for seawater desalination. There are different stages the water goes through. In each stage, water is heated by steam in tubes. Some of the water evaporates and that steam is then used in the tubes of the next stage, heating and evaporating water, effectively recycling energy usage. However, the salt in the water seriously affects the efficiency of heat exchange. This method has been gradually replaced with the Multi-stage technique.
(2) Multi-stage Flash (MSF):
MSF sends water into an evaporation chamber with low-pressure and boiled in an instance to make it evaporate. The water vapor created is then used to obtain clean water. The remaining seawater is then placed again into another evaporation chamber and placed under low-pressure. There does not need to be a large amount of energy used in this method and the water quality is acceptable. This method originated in the Middle East and has spread all over the world.
(3) Vapor Compression (VC):
Seawater is evaporated into water vapor and then compressed. Due to the increase in pressure, the temperature of the water vapor is increased and is used as the heat source. Energy consumption using VC is much lower than the two aforementioned techniques. However, the compressor is not heat-resistant, meaning that development is limited.
(4) Reverse Osmosis (RO):
Seawater must be treated before using this method in order to remove unwanted particles and microorganisms. The seawater is then pressurized at 900psi and put through a semipermeable membrane to filter out any salt. The water quality depends on the amount of pressure, the concentration of salt in the water, and the membrane itself. Using a second or third membrane can improve water quality. Unfortunately, RO is not perfect as the semipermeable membrane can become blocked by microorganisms, regardless of ultraviolet sterilization.
Electrodialysis consumes a significant amount of energy when treating highly concentrated seawater. It is mainly used for brine desalination.
Many seawater desalination plants are built with power plants to reduce costs. This means that desalination plants waste large amounts of heat and vapour due to the power plant. The Total Dissolved Solids (TDS), which measures the combined content of organic and inorganic substances in liquid, in the water processed by these plants is between 50-250ppm. The TDS for water put through membranes is between 250-500ppm.
The World Health Organization (WHO) has stated that drinking water should have a TDS of less than 50ppm. However, most countries go against the WHO’s recommendations and have set the TDS to 250ppm or less due to the fact that it is too expensive to produce water of higher quality. All plants will also treat desalinated water. For example, they may mix the desalinated water with fresh water or they may add chemicals to it to meet water quality standards. The treated water can be used as drinking water for humans, or it can be used for irrigation and industrial purposes. It is then pumped through pipelines and travels a few kilometers away from the plants.
Aside from producing pure water, desalination plants also produce brine, which is highly concentrated salt water. If the concentration is not too high, it will be dumped back into the sea, damaging the sea around the plant. Brine is not waste and can be refined to make products.