Water is a scarce resource. Due to Urbanization and Globalization water usage would increase in the coming years. Industries like power, oil and gas, pharma and chemical generate large volume of wastewater which contains solid waste, organic waste, toxic and non-toxic chemicals which affect the environment. In fact many government and industries have come up with initiatives to reduce water waste by installing ZLD, which in turn will reduce water wastage and help attain a sustainable environment.
The key driver for zero liquid discharge is a growing concern about the impact of such wastewater discharge on the environment. Such concern is resulting in increased regulation and limitation of waste water discharges. Some organizations are also targeting ZLD for their waste because they can sell the solids that are produced or reuse them as a part of their industrial process. For example, salt is the by-product we get at the last stage of ZLD. This salt can be used in textile process.
In MEE sequences of vessels are used. Here water is boiled in the vessels, each held at a lower pressure than the last, because the boiling temperature of the water decreases as pressure decreases the vapour boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat. In MEE the first vessel is fed with the high pressure steam to evaporate the wastewater. The evaporated water in the form of steam produced after boiling the effluent in first vessel is used to evaporate effluent in the second vessel at atmospheric pressure. Evaporated water from the second vessel is used to evaporate waste water in the third stage under vacuum. Finally evaporated water from the third stage is condensed in the steam condenser using cooling water on the other side. Condensate from the entire three vessels is collected in condensate receiving tanks, which is pure water and hence reused in the process. Concentrated mass from each effect is collected in the crystallizer where, inorganic salts are precipitated along with the organic contaminants. This mass is filtered in activated carbon filter and the filtrate is recycled back to process.