Zero liquid discharge (ZLD)
Zero liquid discharge can be defined as an efficient wastewater management system that ensures no discharge of industrial wastewater into the environment. It involves wastewater treatment process of recycling followed by recovery and reuse for industrial purposes.
An efficient ZLD system is characterized as following:
- Able to handle waste flow and contamination variation
- Easy to make adjustments with chemical variations
- Recovery of liquid
- Able to handle waste flow and contamination variation
- Easy to make adjustments with chemical variations
- Recovery of liquid waste is up to 95%
- Able to retrieve and treat valuable by products such as salt and brine from waste
- Production of solid dry cake for disposal
- waste is up to 95%
- Able to retrieve and treat valuable by products such as salt and brine from waste
- Production of solid dry cake for disposal
Contents of Zero Liquid Discharge
Zero liquid discharge is a closed loop cycle with no discharge. Methods usually involved in this system include thermal process (evaporation), membrane processes (reverse osmosis), electro-dialysis, forward osmosis and membrane distillation. A typical ZLD is made up of following contents:
- Physiochemical and biological pretreatment
- Membrane processes such as Reverse Osmosis (RO)
- Thermal processes including evaporator and crystallizer
Benefits of Zero Liquid Discharge
A typical ZLD has following benefits:
- Discharge of wastewater is avoided through recycling
- The cost of the operation is reduced through the recovery of salt and water
- Sustainability of industry and environment is promoted
- The ZLD system helps in the recovery of the environment
- Water can be saved during the operations and used for other activities such as agriculture and domestic utilities
- The sludge produced can be further used in cement industry
Working of a Typical ZLD Treatment System
Although specific processes vary, a typical ZLD system includes the following processes as shown in figure 1.
1.Pre-treatment and Conditioning
Simple things are removed through filtration or precipitation in the pretreatment step that would otherwise foul or scale the forthcoming treatment steps. Metals, hardness and silica are precipitated out in a clarifier. Bulk suspended solids are removed in the process of coagulation process. Coagulates such as alum and polyaluminum chloride are most widely used coagulates. After the process of coagulation is complete, water enters the flocculation chamber.
Water and flocculated material then move to the gravity settler for a slow settling process. Material in feed is an important factor that decides whether there is need for additional reactors or chemistry for further reduction of metals or silica. Hence, for a successful ZLD system, important consideration should be given to the pretreatment step.
2.Phase One Concentration
The pretreated water is moved through the reverse osmosis step where majority of the dissolved solids are captured. On the other hand brine concentrators also reduce the dissolved solids but they can handle high salt concentration of brine as compared to RO. Electrodialysis is another membrane process that can be used at this stage of ZLD. All these processes combined can concentrate the stream and remove up to 80% of the water content.
3.Evaporation/Crystallization
After the completion of concentration step, solid is generated in this step. Dissolved oxygen, carbon dioxide and other non-condensable gases can be released during this step through deaeration. The leftover waste is then subjected to crystallizer where all the water is evaporated and boiled until water impurities crystallize out and filtered as solid.
4.Recycled Water Distribution / Solid Waste Management
The treated water can be reused in the industry provided it is purified enough to be reused in the processes. The solid waste generated will be subjected to dewatering process to yield a solid cake.
Implementation of ZLD Around the Globe
Geographical location plays an important role in the implementation of ZLD in different countries around the world. At present majority of ZLD systems in the world are functional in USA. Most of the systems are operating in power generation sector treating cooling tower blowdown and flue gas desulfurization (FGD) wastewater.
China, having water intensive power plants owned by largest state own agencies in water scarce areas, has considered the implementation of ZLD systems as a sustainable option. In addition to this, boom in coal to chemical plants also require ZLD systems for the preservation of water and ecosystem.
Due to the escalating water scarcity issue in India, all the textile plants generating wastewater greater than 25 m3 were required to install ZLD systems under the water conservation policies of the Indian government. India has also extended the application to different industries including beverage, food, pharmaceutical, power, textile, chemical and steel.
Pakistan being a developing country has a great scope for ZLD to cater for its water scarcity issue and conserve the water resources and environment. Industrial sector including textile, food, beverages, power generation and petrochemical are main areas for ZLD application. However, factors such as capital, operation cost and land need full consideration for efficient and full-scale application in the country.
Pioneering water sustainability, the integration of Zero Liquid Discharge (ZLD), the eco-friendly efficacy of Biocleaner, and the precision of Real-Time Monitoring converge to redefine wastewater management, ensuring responsible practices and optimal purification outcomes.