Zero liquid discharge
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Zero Liquid Discharge (ZLD) is a treatment process with the goal of removing all the liquid waste from a system. The focus of ZLD is to economically reduce wastewater and produce clean water that is suitable for reuse, thus saving money and being beneficial to the environment. ZLD systems employ advanced wastewater 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:
- Meet stringent government discharge regulations
- Reach higher water recovery (%)
- Treat and recover valuable products from waste streams, such as potassium sulfate, caustic soda, sodium sulfate, lithium and gypsum
The conventional way to reach ZLD is with thermal technologies such as evaporators (multi stage flash (MSF), multi effect distillation (MED) and mechanical vapor compression (MCV)) and crystallizers and recover their condensate. 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 (ED/EDR), forward osmosis (FO) and membrane distillation (MD). A quick overview and comparison can be seen by the following table ,
Table 1, Specific Energy Consumptions (SECs) of Brine Treatment Technologies, Multistage Flash (MSF), Multi-Effect Distillation (MED), Mechanical Vapor Compression (MVC), Electrodialysis (ED/EDR), Forward Osmosis (FO), Membrane Distillation. The energy consumption values are the average of 13 comparative studies on ZLD technologies ranging from 2002 -2017. Clarifications are needed for ED/EDR, FO and MD. 1) 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 (DCMD) due to its simplicity. 90% of the thermal energy needed can be acquired by waste heat if it’s available and finally 4) 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 (according to relevant paper).
|Brine Treatment Technology||Electrical Energy (KWh/m3)||Thermal Energy (kWh/m3)||Total El. Equivalent (kWh/m3)||Typical Size (m3/d)||Investment ($/m3/d)||max TDS (mg/L)|
Despite the variable sources of a wastewater stream, a ZLD system is generally comprised by two steps
- Pre-Concentration; Pre-concentrating the brine is usually achieved with membrane brine concentrators or electrodialysis (ED). These technologies concentrate the stream to a high salinity and are able to recover up to 60–80% of the water.
- 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.
- Tong, Tiezheng; Elimelech, Menachem (2016-06-22). "The Global Rise of Zero Liquid Discharge for Wastewater Management: Drivers, Technologies, and Future Directions". Environmental Science & Technology. 50 (13): 6846–6855. doi:10.1021/acs.est.6b01000. ISSN 0013-936X. PMID 27275867.
- "Zero Liquid Discharge - ZLD". www.lenntech.com. Retrieved 2018-10-11.
- Charisiadis, Christos (2018-10-11). "ZLD Lenntech Booklet" (PDF). Lenntech.