Zero Liquid Discharge, abbreviated as ZLD, is a wastewater treatment process in which all generated wastewater is filtered and reused, not discharged into the environment.

Fresh water scarcity is one of the most important global challenges today, posing a major threat to economic growth, water safety and ecosystem health worldwide.

Urban areas and industrial plants consume significant amounts of fresh water while generating large amounts of wastewater. If not treated properly, the discharge of wastewater into the water environment causes serious pollution, adversely affecting the ecosystem and public health.

Wastewater recovery and reuse has become an increasing trend over the past decade due to increased demand for clean water. Reuse wastewater not only reduces the volume and environmental risks of wastewater, but also reduces the pressure on the ecosystem due to the scarcity of fresh water. Through reuse, wastewater is no longer seen as a potentially harmful waste to the environment, but as an additional resource that can be harnessed to achieve water sustainability.

What is Zero Liquid Discharge?

Zero Liquid Discharge (ZLD) is a method that uses the necessary technological processes to fully recover the wastewater generated during the production process and does not release liquid wastewater into the environment.

The ZLD Zero Liquid Discharge solution is an advanced wastewater treatment method that includes the technologies of ultrafiltration, reverse osmosis, evaporation/crystallization and fractional electrodeposition.

Description of the Zero Liquid Discharge ZLD system

History of ZLD (Zero Liquid Discharge)

Similar to other wastewater treatment technologies, ZLD systems began to be used very early, due to the need to solve environmental pollution problems in the world. According to documents on the water and environmental industry, ZLD solutions have been widely installed since the 1970s in the US for power plants. Since then, due to stricter regulations on wastewater treatment, this solution is also applied to many textile, textile, dyeing, chemical, and petrochemical industries in many places. in the world such as India, China, Europe, …

ZHDC organization:

ZDHC stands for Zero Discharge of Dangerous Chemicals. ZHDC is a multi-stakeholder organization founded in 2011, comprising more than 150 chemical brands, manufacturers and suppliers in the apparel, fashion and footwear sectors, aiming to eliminate harmful chemicals. hazardous substances from the production process. The mission of the ZDHC program is to orient brands and retailers in the textile and footwear industry to do well in the sustainable management of chemicals in the value chain. Through programs such as ZHDC MRSL, ZHDC Gateway, ZHDC connects cooperation, sets green standards and implements them, thereby gradually moving towards a commitment to not discharge hazardous chemicals.

ZDHC Zero Discharge Hazardous Chemical Program

In recent years, greater interest in the dual challenges of water scarcity and water pollution has revived global interest in ZLD. Stricter regulations, rising wastewater treatment costs, and the increasing value of fresh water are promoting ZLD as a beneficial or even necessary option for wastewater management. The global market for ZLD is estimated to reach an annual investment of at least US$100-200 million, spreading rapidly from developed countries in North America and Europe to emerging economies such as China, India and also in Vietnam.

Early ZLD systems were based on independent thermal processes, in which the wastewater was typically evaporated in a brine concentrator followed by a brine crystallizer or an evaporation pond. The distilled water condensing in ZLD systems is collected for reuse, while the produced solids are sent to landfill or recovered as valuable salt by-products. Such systems, which have been operating successfully for 40 years and are still being built, require considerable energy and capital.

Benefits of ZLD (Zero Liquid Discharge)

The purpose of the ZLD zero liquid discharge system is to reduce the volume of wastewater requiring further treatment, to treat it economically and to create a source of clean water suitable for reuse. ZLD technology is growing globally as an important wastewater management strategy to reduce water pollution and expand water supply. ZLD because of increasingly tightening wastewater treatment regulations, green initiatives, mandatory green certificates from customer partners, awareness of the surrounding community about the impact of industry on the ecological environment.

The main benefits of using the ZLD system are:

  • Reduce your business’ environmental impact by minimizing the sources of liquid waste, sludge and solid waste.
  • Reduce waste water, maximize water recovery
  • Save on transportation and external waste disposal costs.
  • Reduce water consumption by reusing the clean water obtained from the ZLD system. This advantage also leads to a reduced environmental impact for the business.
  • Reduce dependence on local water sources, improve self-sufficiency in water use.
  • Improve the company’s environmental responsibility and sustainability commitment to the community.
  • Comply with the strictest environmental regulations, avoiding penalties related to current environmental regulations for waste.
  • Tax savings applied to the treatment or discharge of waste.
  • Increased flexibility in the management of liquid wastewater generated by not being dependent on any external agents.
  • Easy to operate, high degree of automation.
  • The ability to reuse secondary heat from other processes, significantly reducing operating costs.

Commonly used technologies in the ZLD system

There are many different methods and techniques used to achieve ZLD, including:

  1. Thermal process (evaporation)
  2. Reverse osmosis: RO membrane technology
  3. Electrolysis (ED)
  4. Forward Osmosis (FO)
  5. Membrane Distillation (MD)

A ZLD system is made up of the following components:

  • pretreatment (physicochemical and biological)
  • RO filtration process
  • evaporator and crystallizer (thermal process)

Reverse osmosis RO, a membrane-based technology widely applied in desalination, has been incorporated into ZLD water reuse systems to improve energy and cost efficiency. However, although RO membrane is much more energy efficient than thermal evaporator, it is therefore often applied to feed water with limited salinity range. Accordingly, other salt concentration technologies that can treat feedwater with higher salinity, such as electrolysis (ED), forward osmosis (FO) and membrane distillation (MD), have recently been emerging as alternative ZLD technologies to further concentrate wastewater beyond RO.

A ZLD water reuse system using RO membrane technology

Working principle and process of the ZLD system

The operation of a ZLD system usually includes major steps such as pretreatment, concentration and heat treatment.

– Pretreatment is used to reduce TSS (Total Suspended Solids), COD (Chemical Oxygen Demand) and turbidity. After removing or significantly reducing TSS, COD and turbidity of the treated wastewater, other wastewater treatment processes can be used.

Concentration of  dissolved solids in a ZLD system is usually accomplished by membrane technologies, such as RO reverse osmosis, ED electrolysis, or by combining RO with ED. With the help of ED technology or the aforementioned combination of RO + ED, we can achieve recovery of more than 98% of treated water.

– Evaporation:  After pre-treatment, the wastewater is subjected to evaporation, which removes the water from the waste stream and concentrates the remaining contaminants. This is usually done using a multi-effect evaporator or a thermal evaporator.

– Crystallization:  In some ZLD systems, the concentrated waste stream must be crystallized, which removes additional water and results in a solid waste product that can be further treated or treated.

– Reuse:  Pure water from ZLD systems can be recycled back into the industrial process, reducing freshwater needs and helping to conserve water resources.

Energy recovery: ZLD systems often use energy-intensive processes, such as evaporation and crystallization. However, these processes can also generate heat and other forms of energy that can be recovered and reused, reducing the overall energy consumption of the system.

– Monitoring and control: ZLD systems are often equipped with monitoring and control systems that optimize the system’s performance and ensure that it is operating within design parameters.

– Heat treatment: the final step of the ZLD process usually includes the evaporator and crystallizer. The output of this process is the crystalline salts of NaCl, Na2, etc. Any unusable solid waste is buried in controlled landfills. Evaporated water can be reused as condensate.

Diagram illustrating the operating principle of the ZLD system

Advantages and disadvantages of the ZLD system


The ZLD zero liquid discharge system enables reuse of wastewater in many industries. The Zero Liquid Discharge system captures as much water as possible for reuse and recycles it as a valuable by-product. In addition to water recovery, the ZLD system also minimizes the environmental impact associated with wastewater treatment. It is also a great way to reduce operating costs for water treatment plants. ZLD liquid-free discharge systems are especially popular in countries where environmental concerns are strictly enforced.


Although ZLD wastewater reuse technology has some advantages in reducing the environmental impact of industrial plants and conserving water resources, it also has some disadvantages such as the treatment process which requires a large amount of water. energy and produces a significant amount of greenhouse gas (GHG).

– High capital and operating costs: ZLD systems are often expensive to install and maintain, which can be a significant expense for investors. The high energy consumption of some ZLD processes can also increase operating costs.

– Complexity: ZLD systems can be complex and require specialized expertise to operate and maintain. This can make it difficult for investors to find and retain highly qualified staff.

– Limited applicability: ZLD technology is not suitable for all types of industrial waste streams. It may not be effective for highly concentrated or toxic waste streams, and may require additional pretreatment steps before the waste can be treated.

– Generation of solid waste: The ZLD process often results in the production of solid waste, known as sludge, and must be disposed of properly. This may result in additional costs for handling or collection.

Space requirements: ZLD systems require a significant amount of space, which can be a limitation for plants with limited space and area for wastewater treatment.

– High energy consumption, corrosion, clogging of RO membrane, etc.

Overall, the ZLD system can be an effective technology for manufacturers to reduce their environmental impact and conserve water resources, but it also comes with significant costs and challenges. It is important that ZLD system suppliers as well as factories and enterprises carefully consider and calculate the costs and benefits of ZLD before deciding to invest in it.

Technical staff of Hoa Phat Eco guide the operation of Zero Liquid Discharge system for customers.

Operating costs of the ZLD system

The operating and investment costs of the ZLD system depend on the volume and characteristics of the treated wastewater. The capital cost of the entire system can vary depending on its size and characteristics.

Regarding the operating cost of the ZLD system, a specific example can be found in the following table:

Salt treatment Electricity cost (KWh/m3) Heat cost (kWh/m3) Total cost of energy (kWh/m3) Typical size (m3/d) Cost


max TDS (mg/L)
MSF 3.68 77.5 38.56 <75,000 1,800 250,000
MED 2.22 69.52 33.50 <28,000 1,375 250,000
MVC 14.86 0 14.86 <3,000 1,750 250,000
ED/EDR 6.73 0 6.73 / / 200,000
FO 0.475 65.4 29.91 / / 200,000
MD 2.03 100.85 47.41 / / 250,000

In terms of investment costs, in general, the pre-treatment stage does not cost much and is quite similar to primary wastewater treatment. Membranes are a bit more expensive as they offer higher recovery rates, but they are similar to membranes used in water treatment and water purification. The biggest cost will be the part that evaporates and crystallizes. Regarding the overall equipment cost, about 60-70% of the cost will be in the evaporation/crystallization category. The remaining 30-40% is the cost of pretreatment and RO. The larger the ZLD system, the more these parameters will fluctuate.

Application of the Zero Liquid Discharge system

The Zero Liquid Discharge system reuse technology will be a strategic solution in the current and future water treatment sector as it brings sustainable environmental values and addresses the urgent need for water pollution and climate change in the world.

In general, all industries and industrial plants that use clean water and generate wastewater in the production process can apply ZLD solutions for their wastewater treatment systems. Some typical applications of the ZLD system:

– Reuse of textile dyeing and textile wastewater

– Chemical and petrochemical industry

– Paper industry, leather tanning

– Food processing industry, breweries

– Blowdown cooling towers in heavy industry and power plants

– Flue gas desulfurization (FGD)

– Zero liquid discharge system for centralized industrial wastewater treatment.

– Remove membrane system (NF, MF, UF, RO)

– Integrated gasification combined cycle (IGCC)

– Oil refining wastewater, liquefied petroleum gas (GTL) and coal-to-chemical wastewater (CTX)

– Leachate landfill wastewater treatment

Install ZLD Zero liquid discharge system in textile factory

The company supply the ZLD wastewater reuse system:

Hoa Phat Eco is a company that provides water reuse solutions and ZLD systems with many years of experience and thorough understanding of this technology. Depending on the characteristics and scale of wastewater as well as the needs of each factory and enterprise, we will calculate and advise the appropriate solution so that customers can understand the technological process, operating costs, advantages and disadvantages. disadvantages, time to recover capital before deciding to invest. We hope that, each equipment, each item of water reuse project that customers spend money on, not only brings value in terms of environment and brand, but also brings value and economic efficiency. economic. Actively promote and develop production and business, contribute to the economic development of the country and always ensure a clean and green surrounding environment, accompany the community in protecting the sustainable ecosystem.


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