Zero Liquid Discharge (ZLD) is a treatment process through which all the water contaminants are removed from it. The focus of ZLD is to reduce economically wastewater and produce clean water that is suitable for reuse.
Working Principle of ZLD
Zero Liquid discharge can be achieved through various different processes such as Evaporation, ED / EDR, FO & MD, etc. The ZLD technology includes pre-treatment & evaporation of the industrial effluent until the dissolved solids precipitate out. As said earlier along with evaporation, Electrodialysis (ED / EDR), Forward osmosis (FO) & Membrane distillation (MD) technologies result in high water recovery.
ZLD system is generally comprised of two steps which are represented. Pre-Treatment plays an important role in ZLD. Neutralization - pH correction, coagulation, Flocculation, Ultrafiltration are part of pre-treatment.
Evaporators
Evaporators treat wastewater by heating it to evaporate volatile solvents like water from the solution, and then cool and condense it to produce freshwater. The purpose is to concentrate non-volatile solutes like inorganic salts and organic compounds and leave behind a more concentrated wastewater stream.
Evaporators are of four types
• MVR - Mechanical Vapour Recompression
• Multiple Effect Evaporators
• Atmospheric Evaporators
• HDH Evaporators - Humidification - Dehumidification Evaporators
After distillation or Evaporation, the solid waste generated is then fed to crystallizers which boils all the water until all the impurities crystallize and are filtered out as a solid
Advantages
• Minimizes wastewater discharge
• Maximizes water recovery
• Creates a valuable product for sale and further processing
• Removes many environmental issues
• Reduces dependence on local water sources
• Significantly reduces the risk of sanctions within the legislative standards for discharged waste
