Zero liquid discharge


Zero Liquid Discharge is a treatment process designed to remove all the liquid waste from a system. The focus of ZLD is to reduce wastewater economically and produce clean water that is suitable for reuse, thereby saving money and being beneficial to the environment. ZLD systems employ advanced wastewater/desalination treatment technologies to purify and recycle virtually all of the wastewater produced.
Also ZLD technologies help plants meet discharge and water reuse requirements, enabling businesses to:
The conventional way to reach ZLD is with thermal technologies such as evaporators, multi effect distillation and mechanical vapor compression ) and crystallizers and recover their condensate. Thus, ZLD plants produce solid waste.

ZLD discharge system overview

ZLD technology includes pre-treatment and evaporation of the industrial effluent until the dissolved solids precipitate as crystals. These crystals are removed and dewatered with a filter press or a centrifuge. The water vapor from evaporation is condensed and returned to the process.
In the last decades though, there has been an effort from the water treatment industry to revolutionize the high water recovery and ZLD technologies. This has led to processes like electrodialysis, forward osmosis and membrane distillation. A quick overview and comparison can be seen by the following table,
Table 1, Specific Energy Consumptions of Brine Treatment Technologies, Multistage Flash, Multi-Effect Distillation, Mechanical Vapor Compression, Electrodialysis, Forward Osmosis ED/EDR SEC depends on the salinity of the feed as higher salinities require higher SECs, 2) FO SEC depends on the Draw Solution and the Regeneration Method. Most papers assume the use of thermolytic salts and their regeneration at a 60oC temperature. 90% of the thermal energy needed can be acquired by waste heat if it’s available, 3) MD SEC depends on the configuration. Most common MD configuration in the studies is Direct Contact MD the total electrical equivalent was taken using the following, Total El. Equivalent = El. Energy + 0.45 x Thermal Energy due to modern power plant efficiency.
Brine Treatment TechnologyElectrical Energy Thermal Energy Total El. Equivalent Typical Size Investment max TDS
MSF3.6877.538.56<75,0001,800250,000
MED2.2269.5233.50<28,0001,375250,000
MVC14.86014.86<3,0001,750250,000
ED/EDR6.7306.73//150,000
FO0.47565.429.91//200,000
MD2.03100.8547.41//250,000

Configuration

Despite the variable sources of a wastewater stream, a ZLD system is generally comprised by two steps
  1. Pre-Concentration; Pre-concentrating the brine is usually achieved with membrane brine concentrators or electrodialysis. These technologies concentrate the stream to a high salinity and are able to recover up to 60–80% of the water.
  2. Evaporation/Crystallization; The next step with thermal processes or evaporation, evaporates all the leftover water, collect it, and drives it for reuse. The waste that is left behind then goes to a crystallizer which boils all the water until all the impurities crystallize and are filtered out as a solid.