In Multi Effect Distillation (vacuum evaporation) seawater is evaporated under reduced pressure. Because of the vacuum, water boils at low temperatures. The steam is condensed and collected as drinking water, while salt and impurities remain behind. The technique uses heat efficiently and produces very pure water. It is used successfully in shipping.
MED systems run under vacuum and require stable temperatures, clean heat exchangers and functioning vacuum pumps. Deposits can immediately reduce heat transfer, which is why pre filtration, descaling and regular cleaning are essential. The process is sensitive to pressure and temperature deviations, so continuous monitoring is necessary. On ships the system uses the engine’s waste heat, while on land it often relies on industrial process heat.
Thermal desalination requires more energy than reverse osmosis but becomes significantly more efficient when waste heat is used. The brine is concentrated and warm, so controlled discharge is important to avoid stressing local ecosystems. Material consumption is low because the system does not require membrane replacement.
MED is most sustainable in places where heat is already available and does not need to be generated additionally.