The concept of zero discharge was initially enforced in the United States in the 1970s due to industrial wastewater affecting river water quality. Subsequently, Australia's first industrial wastewater zero-discharge project was also mandatory due to policy regulations. This shows that the guiding role of policies on zero discharge is very prominent. In recent years, the continuous tightening of environmental regulations has put forward higher requirements for the treatment and disposal of high-salinity wastewater, which is particularly prominent in China's coal chemical and thermal power industries.

Zero liquid discharge (ZLD) and resource recovery process requirements for high-salt wastewater aim to maximize the separation and recovery of various substances under technically and economically feasible conditions, such as water reuse, salt crystallization, or acid-base production. Single-salt solutions primarily focus on concentration and recovery, while complex salt solutions prioritize salt separation and resource recovery. Currently, a pretreatment → concentration → evaporation crystallization system is commonly used to treat high-salt wastewater, achieving zero or near-zero discharge, with the resulting salt solids disposed of or recovered.

▲ Typical Zero Liquid Discharge Process Diagram

The key factor determining the cost of ZLD is the wastewater treatment capacity of the evaporation crystallization system. If high-concentration pretreatment can be achieved before the wastewater enters the evaporation crystallization stage, the cost of zero liquid discharge for high-salt wastewater will be significantly reduced. Numerous concentration technologies exist, and based on the treatment object and applicable range, high-salt wastewater concentration technologies are mainly divided into thermal concentration and membrane concentration technologies. Early ZLD systems primarily used thermal concentration technologies for brine concentration, including mechanical vapor compression (MVC) and the currently more widely used mechanical vapor recompression (MVR). Other thermal desalination technologies, such as multi-stage flash (MSF) and multi-effect distillation (MED), are mainly used in seawater desalination and have no reported applications in ZLD processes.

MVC technology has been used for decades, with continuous development of heat recovery devices. However, high energy consumption and the need for high-quality electricity remain the biggest limitations to its widespread application. Typically, 20-25 kWh of electricity is consumed per ton of water treated, which has become the benchmark for other ZLD concentration technologies and the direction for energy saving and emission reduction in other technologies. In addition to high energy consumption, MVC also has high investment costs, requiring the use of high-quality and expensive materials, such as titanium and stainless steel, to prevent corrosion by boiling brine.

▲ MVR Principle Diagram

To reduce the energy consumption of MVC, RO and MVC processes are often coupled in practical engineering. Using RO for pre-concentration can significantly reduce energy consumption, and the synergistic effect of the two achieves zero liquid discharge of high-salt wastewater. The addition of RO can save 58-75% of energy and 48-67% of operating costs. However, there are two major limitations to applying RO to zero discharge: membrane scaling/fouling and relatively low concentration capacity.

New membrane concentration technologies include membrane distillation, forward osmosis, and electrodialysis, which are used as further concentration processes for RO concentrate, with the effluent then entering the crystallization process. The advantages, limitations, and energy consumption analysis of various membrane concentration technologies are shown in the table below.

Membrane distillation is a process where water vapor is driven through a hydrophobic microporous membrane by vapor pressure difference (temperature difference) and then condensed into pure water. Membrane distillation theoretically has a 100% retention rate; low operating temperature, waste heat can be utilized; low operating pressure; low equipment investment; almost no membrane fouling problems, and long service life.

Forward osmosis utilizes the osmotic pressure of concentrated brine to allow water molecules in the wastewater to pass through the forward osmosis membrane into the salt side, achieving the separation of water and pollutants. The brine is then desalinated by reverse osmosis to achieve water resource recovery. The two core technical issues of forward osmosis are: the selection of forward osmosis membrane materials and structure; and the selection of the draw solution. Yale University developed NH3/CO2 as a thermal draw solution, which can be regenerated at a moderate temperature of 60°C, significantly reducing energy consumption compared to MVC.

On April 30, 2019, a paper titled "Membrane-less and Non-Evaporative Desalination of Hypersaline Brines by Temperature Swing Solvent Extraction" was published in Environ. Sci. Technol. Lett., a top journal in the environmental field, instantly attracting widespread attention from the industry. The article introduces a novel solvent extraction desalination method designed by researchers at Columbia University in New York, which can efficiently and cost-effectively extract freshwater from saltwater. It is called "Temperature Swing Solvent Extraction" or TSSE. This method uses a solvent with temperature-dependent water solubility. The solvent is added to the saltwater, allowing it to float above the denser salt-containing liquid. At room temperature, water from the saltwater is absorbed into the solvent. After this stage, the solvent is extracted and heated at 70°C. The solvent's "temperature swing" properties then separate it from the water, and the resulting desalinated water settles to the bottom and is collected. Compared to thermal and membrane methods, this method is energy-efficient, has low investment costs, and can desalinate high-salt industrial wastewater with salinity more than seven times that of seawater. However, it is currently still in the laboratory research stage. How to scale up and achieve stable operation is the key to the further development of this technology. Let's wait and see.

This system solution is a closed-loop system that can recycle wastewater and energy, saving energy and reducing maintenance costs. Evaporator technology is one of the top choices for achieving zero wastewater discharge in enterprises. It can reuse up to 98% of the recyclable distillate through the evaporator. The distillate is of high quality, so it can be directly fed back into the production cycle or discharged directly. The evaporator is simple to use, highly flexible and safe, and is specifically designed for 24/7 operation.

• Suitable for difficult-to-degrade industrial wastewater, stable performance

• Repetitive use of heat achieves extremely low energy consumption

• Distillation and concentration output does not require the use of additional pumps

• Advanced control devices and touch screen ensure high and intuitive operational comfort

• Built-in on-demand cleaning system

• Stronger corrosion protection

• For wastewater with more serious pollution and foaming

Its innovative technology cleverly combines the principles of reverse osmosis and forward osmosis to achieve efficient and low-cost salt concentration. This technology has broad application prospects in the fields of salt concentration and zero discharge. The reason why general RO is difficult to concentrate to a higher multiple of salt solution is mainly because the external pressure of reverse osmosis has reached the limit of osmotic pressure and can no longer be further concentrated. Using the principle of osmotic pressure, a small amount of salt is added to the outlet of the reverse osmosis membrane to reduce the pressure difference between the two sides of the membrane, so that on the one hand, further concentration can be achieved, and on the other hand, the external pressure can be reduced, saving energy and reducing consumption.

• The solution can be concentrated to near saturation (26% for NaCl)

• Increase process recovery rate to 75%

• Save energy, 5-10 kWh per cubic meter of recovered liquid

• No thermal concentrator required

• Reduce saltwater treatment costs by up to 50%

Flex EDR is an advanced electrodialysis reversal (EDR) system that utilizes Saltworks' new generation of ion exchange membranes. It requires less pretreatment, tolerates oil and high organics, selectively removes ions, and requires no chemical additives. It has been applied in industrial fields such as flue gas desulfurization wastewater chloride removal and water reuse, selective salt separation of high-salt wastewater in coal chemical industry, lithium separation and recovery, and desalination of produced water in enhanced oil recovery in oil fields.

• Durable Design

  Based on ion exchange membranes and stacks with high elasticity and extensibility, it can withstand oil, organic matter, oxidants (bleach), acids (pH 0), alkalis (pH 12), and suspended solids (<30 μm).

• Selective Ion Removal

  Removes monovalent ions without soda softening, changing the scaling water chemistry, and recovering valuable salts.

• Ultra-high Concentration, Flexible Operation

  Concentrates concentrated water to 180,000 mg/L, and can be combined with reverse osmosis to optimize the system.

• Intelligent self-cleaning strategy maintains high salt load and high water recovery rate

• High degree of modularity, easy to scale up, or add to existing processes

Using vacuum membrane distillation technology with hollow fiber membranes, the water recovery rate can reach 97%, and energy consumption can be reduced by 80% compared to traditional thermal methods. Suitable for the treatment of high-salt wastewater such as mining wastewater, RO concentrate, and brackish water.

The treatment of saline wastewater is difficult and costly. Hollow fiber vacuum membrane distillation (HF-VMD) technology provides a unique solution. This process recovers and reuses the maximum amount of high-quality pure water from challenging industrial wastewater. The KMX system has higher recovery rate, brine saturation, and capacity factor than traditional evaporator/crystallizer systems, and significantly reduces energy demand. The system has low maintenance requirements, small footprint, modularity, scalability, and applicability to various industrial wastewaters.

• No crystallization required, one-step process converts concentrated brine into wet salt cake

• 40-60% higher recovery rate than RO, up to 97%

• Low maintenance costs, no heat transfer interface or scaling on membrane surface

• Low energy consumption, 80% lower energy consumption than evaporator or crystallization processes

• Low cost, salt recovery and reuse, approximately 1-3 years to recover costs