Why is high-salinity wastewater difficult to treat? On the one hand, it is due to a lack of technology, while on the other hand, it is due to a lack of economic feasibility and reliability.

If most of the wastewater is diluted and discharged, not only will it not truly reduce the amount of pollutants discharged, but it will also waste freshwater, especially the discharge of saltwater, which will inevitably lead to soil alkalinization and freshwater mineralization.

However, if the saltwater is separated into water and salt, and the salt is treated centrally, the effect of "zero discharge" of wastewater can be achieved, which avoids soil and water pollution and improves operating efficiency.

For this reason, "zero discharge" wastewater technology has become an important measure for industrial enterprises to achieve sustainable development of water resources.

01

What is high-salinity wastewater?

Sources and water quality characteristics of high-salinity wastewater

In China, there are three main sources of high-salinity wastewater:

1. Concentrated brine produced during seawater desalination

There are two main ways to treat high-salinity wastewater produced by seawater desalination: one is to use waste recycling to generate economic benefits and achieve true "zero discharge"; the other is to directly discharge high-salinity wastewater into the sewage treatment system, rivers, lakes, or oceans.

However, due to the lack of technology and economic costs in most coastal areas, the second treatment method is generally chosen in production.

2. High-salinity wastewater directly discharged during industrial production

Generally speaking, the inorganic salts in high-salinity wastewater mainly come from production wastewater and domestic sewage (containing potassium ions, calcium ions, sodium ions, chloride ions, sulfate ions, etc.), while some organic substances it contains mainly include glycerol and low-carbon chain compounds.

It is worth mentioning that in addition to the inorganic salt ions such as potassium, sodium, and calcium mentioned above, the inorganic salt ions contained in industrial wastewater from different fields vary greatly, and some high-salinity wastewater even contains some heavy metal elements.

3. Brine produced by the recycling of industrial production wastewater

For example, industries with large drainage volumes such as steel enterprises, coal chemical industry, and petroleum need to recycle most of the water for reuse during production in order to save energy and reduce emissions, and a certain concentration of brine will also be produced during reuse.

If this part of the concentrated brine is discharged without treatment, it will cause great environmental pollution. After treatment, different industrial wastewater will produce high-content wastewater, such as calcium, magnesium, potassium, sodium, chloride ions, carbonate ions, etc.

02

What are the methods for treating high-salinity wastewater?

Traditional biological treatment methods are difficult to implement

At present, there are dozens of methods for treating high-salinity wastewater, mainly including thermal methods, membrane methods, ion exchange methods, hydrate methods, solvent extraction methods, and freezing methods.

Among them, thermal methods and membrane desalination technologies are the main technologies currently used in large-scale industrial applications.

Thermal methods can be mainly divided into multi-stage flash (MSF), multi-effect evaporation (MED), and vapor compression distillation (VC). The seawater desalination technology in the 1990s was mainly multi-stage flash, especially in the Middle East, but MSF was later challenged by multi-effect evaporation and membrane technology [3].

Membrane desalination technology represented by RO technology is suitable for desalination of salt water on a large, medium, and small scale because it does not require a large amount of thermal energy.

For zero-discharge treatment of high-salinity wastewater, direct evaporation crystallization can achieve the purpose of zero discharge, but it is extremely energy-consuming and resource-intensive.

Membrane technology can further concentrate high-salinity wastewater into ultra-high-salinity wastewater, the freshwater part can be reused directly, and the concentrated ultra-high-salinity wastewater is then evaporated and crystallized to achieve zero discharge, which greatly reduces energy consumption and reasonably utilizes part of the water resources.

However, membrane technology has certain requirements for the water quality of the influent. Therefore, high-salinity wastewater must undergo pretreatment (chemical softening, filtration, ion exchange, etc.), which can effectively reduce membrane fouling and improve the service life of the membrane and the quality of the effluent.

03

Key technologies for zero discharge of high-salinity wastewater

3 stages: pretreatment, membrane treatment, evaporation crystallization

Combining the above analysis, the key technologies for zero discharge of high-salinity wastewater can be divided into three stages: pretreatment stage, membrane treatment stage, and evaporation stage.

1. Pretreatment

Hardness is divided into total hardness, temporary hardness, and permanent hardness.

Among them, total hardness refers to the total amount of Ca ²⁺ and Mg ²⁺ in the water.

Temporary hardness, also known as carbonate hardness, is mainly composed of Ca(HCO ₃)₂ 、Mg(HCO ₃)₂ ). Because this salt decomposes into precipitates from water after heating, it is called temporary hardness.

Permanent hardness, also known as non-carbonate hardness, mainly refers to CaSO ₄ 、MgSO ₄ 、CaCl ₂ 、MgCl ₂ 、Ca(NO ₃)₂ 、Mg(NO ₃)₂ and other salts in the water. This type of hardness cannot be removed by heating, so it is called permanent hardness.

Hardness is an important indicator of water quality, and the removal of hardness from water is called water softening. Currently, water softening mainly includes precipitation softening, enhanced crystallization technology, adsorption, and ion exchange methods.

Chemical softening method

Chemical softening methods mainly include traditional chemical softening methods and biodegradation urea-producing carbonate precipitation methods.

Traditional softening methods for pharmaceutical preparations are categorized into lime softening, lime-gypsum softening, and lime-soda ash softening methods. The drawbacks of these methods include potential secondary pollution and higher reagent costs, leading to increased overall expenses.

The biodegradation urea carbonate precipitation method primarily utilizes bioenzymes to decompose urea and other substances through a series of biochemical reactions to generate carbonate precipitates, which are then removed through filtration.

The disadvantage of this method is the relatively high concentration of ammonium ions generated during the reaction, leading to increased subsequent processing costs.

Enhanced Crystallization Technology

The use of fluidized beds to remove water hardness began in the 1990s. The basic principle of a fluidized bed is to use gas or liquid to keep solid particles in a suspended state of motion. A researcher used aeration in wastewater to increase the pH value of the wastewater to enhance crystallization. As a result, the removal rates of phosphate, Mg ²⁺ and Ca ²⁺ reached 65%, 51%, and 34%, respectively.

Currently, granular calcite (CaCO ₃ ) and quartz sand are mainly added to fluidized bed reactors. The advantages are not only the effective removal of calcium and magnesium ions but also the possibility of recycling the resulting calcium and magnesium precipitates.

Adsorption and Ion Exchange Methods

Ion exchange water softening is mainly used before membrane treatment to pre-remove Mg ²⁺ and Ca ²⁺ ions, either partially or completely.

Since the 20th century, research on low-cost, renewable adsorbents has been a key focus of adsorption and ion exchange.

Researchers abroad have achieved good results using alginate to adsorb Mg ²⁺ and Ca ²⁺ ions in water, and this method has been widely promoted. This non-toxic polysaccharide alginate is extracted from brown algae.

Similarly, chemically modified sugarcane molasses and mercerized cellulose have also been used to remove Mg ²⁺ and Ca ²⁺ from water, with significant removal effects.

Ion exchange resin is another material for water softening; it is a polymer with corresponding functional groups. When raw water is passed through an ion exchange resin adsorption column, the Mg ²⁺ and Ca ²⁺ in the water will exchange with the cations on the resin, achieving the purpose of removing water hardness.

Currently, researchers are developing many types of resins. Among them, Orica Watercare in the United States has developed a magnetic weak acid cation exchange resin that is very effective in removing hardness.

2. Membrane Technology

In the 1980s, reverse osmosis, ion exchange, microfiltration, ultrafiltration, and nanofiltration membranes gradually entered the stage of promotion and application. The emergence and application of membrane technology have comprehensively improved water treatment technology.

So far, with the comprehensive development of membrane technology, many new technologies have been derived. Among them, the new polyvinylidene fluoride (PVDF) hollow fiber hydrophobic membrane can achieve 99.9% desalination efficiency, and the COD of the effluent can be guaranteed within the range of 30-40 mg/L.

Similarly, a new membrane separation technology—vacuum membrane distillation—is applied to the reconcentration of high-concentration solutions and the removal of Mg ²⁺ and Ca ²⁺ and other substances.

Deep treatment technologies for low-hardness water mainly include RO/electrodialysis (EDI), reverse electrodialysis (EDR), electrodialysis (ED), and reverse electrodialysis (EDIR).

It is worth mentioning that RO/electrodialysis (EDI) (also known as packed bed electrodialysis) soft water technology refers to a water treatment process that removes calcium and magnesium ions from water under the action of an externally applied DC electric field. This technology has the characteristics of deep dehardening, continuous water production, and no need for regeneration reagents.

Nanofiltration (NF), Ultrafiltration (UF), Microfiltration (MF)

Because the nanofiltration operating range is between ultrafiltration and reverse osmosis membranes, it can retain nanometer-level (0.001 micron) substances, so it is called "nanofiltration." Its retention of organic molecules is about 200-800 MW, and its retention of dissolved salts is between 20%-98%. The removal rate of soluble monovalent ions is lower than that of multivalent ions. Nanofiltration is generally used to remove organic matter and pigments from surface water, hardness and radium from groundwater, and partially remove dissolved salts. It is used in food and pharmaceutical production for the extraction and concentration of useful substances.

Its advantages are low operating pressure and high throughput. Nanofiltration technology has obvious advantages and unique energy-saving effects in organic matter desalination and purification, and water softening.

Ultrafiltration can retain substances larger than 0.01 microns, allowing small molecules and dissolved solids (inorganic salts) to pass through, removing macromolecular organic matter, colloids, proteins, and microorganisms. Ultrafiltration utilizes the microporous sieving mechanism of ultrafiltration membranes and is mainly used in drinking water, industrial wastewater treatment, and high-purity water preparation. Microfiltration also uses the sieving mechanism of microfiltration membranes. Under pressure-driven conditions, it retains viruses and particles between 0.1-1 μm.

Microfiltration can retain particles larger than 0.1-1 micron, allowing macromolecules and dissolved solids (inorganic salts) to pass through, but it will retain suspended matter, bacteria, and high-molecular-weight colloids. The operating pressure of microfiltration membranes is generally 0.3-7 bar.

The microfiltration membrane separation mechanism is mainly sieving retention, with the advantages of low operating pressure and high membrane flux, but microfiltration membranes are generally easily polluted and have a short service life.

Ultrafiltration is used in the fields of medicine, chemical engineering, and water treatment. Microfiltration is mostly used for water pretreatment and is also used in medicine, chemical engineering, and electronics. Ultrafiltration and microfiltration are also used in the treatment of high-salt wastewater, but generally as pretreatment.

Reverse Osmosis (RO)

Reverse osmosis, also known as reverse osmosis, is a membrane separation operation that uses a pressure difference as the driving force to separate the solvent from the solution.

Currently, reverse osmosis technology is used in pre-desalination treatment with good results. After reverse osmosis treatment, 99.5% of magnesium and calcium components and 99% of salt in the water can be removed. It can reduce the load on ion exchange resin by more than 90%, and the amount of regenerant used for the resin can also be reduced by 90%.

Therefore, it not only saves costs but also benefits environmental protection. Reverse osmosis technology can also be used to remove particles, organic matter, and colloids in water, which has a good effect on reducing the pollution of ion exchange resin and extending its service life.

With the increasing maturity of membrane production technology and the gradual reduction of costs, reverse osmosis also plays a significant role in the treatment of high-salt wastewater. However, when the conductivity of high-salt wastewater is greater than 25000us/cm, the membrane flux will decrease rapidly, and membrane scaling will become more serious.

It is worth mentioning that, in the reverse osmosis process, combined with efficient crystallization technology, the amount of water treated by reverse osmosis can be increased, the service life of the membrane can be extended, and more high-salt wastewater can be treated.

Forward Osmosis (FO)

Because forward osmosis has a different operating principle from traditional membranes, it has special advantages.

For example, the membrane device is simple in structure and easy to operate; forward osmosis membranes apply low or even no pressure, saving energy and reducing operating costs; forward osmosis has a strong ability to separate pollutants and has a high salt rejection rate; the pollution of forward osmosis membranes is almost reversible, and the cleaning efficiency is high.

Under ideal conditions, forward osmosis membranes need to have a high rejection rate, good hydrophilicity, and high water flux in the active layer, while the support layer should have the characteristics of thin thickness, low tortuosity factor, high porosity, and high mechanical strength. It also needs to have strong anti-pollution ability and be applicable to multiple fields.

In early research, the forward osmosis membranes used were mainly reverse osmosis membranes and modified nanofiltration membranes. With further research, it was found that because reverse osmosis has a thicker porous support layer, its concentration polarization is very large, causing the water flux to decrease rapidly.

Membrane Distillation

Membrane distillation technology is a membrane separation technology that combines distillation and membrane methods.

The separation principle of vacuum membrane distillation is that one side is evacuated to use the pressure difference between the two ends to achieve mass transfer of steam, using the membrane to retain other substances in the solution, and condensing the liquid after distillation to achieve separation or concentration.

The vacuum membrane distillation process has a lower operating temperature compared to other membrane distillation processes, and a larger permeation flux, making it easy to utilize inexpensive heat sources such as geothermal energy, solar energy, and waste heat.

In recent years, research on the use of vacuum membrane distillation technology to treat seawater desalination brine has gradually increased.

Scholars have used polyethylene and polypropylene microporous membranes to study the vacuum membrane distillation of RO seawater desalination brine. Studies have shown that the maximum rejection rate of the membrane can be as high as 99.999%, so this technology can achieve efficient concentration of RO seawater desalination brine.

This technology uses the pressure difference between the two sides of the membrane to generate driving force, with the advantages of low mass transfer resistance, high thermal utilization efficiency, high separation efficiency, high membrane flux, and no permeate evaporation. However, this process also has scaling and membrane fouling problems when treating brine.

3. Final Evaporation Technology

The discharge of high-concentration brine will have an adverse impact on the environment. There are two main reasons for this impact: firstly, the brine concentration is high; secondly, the brine contains many components.

The effect of evaporation technology is, on the one hand, to compress the volume of high-concentration brine, causing the salt inside to crystallize; on the other hand, to form a circular industrial economy, providing the precipitated salt to manufacturers who use it as a raw material, achieving the goal of "zero discharge".

Natural Evaporation

The principle of natural evaporation is to use sunlight to remove the water from the high-concentration brine in the pool, thereby reaching the saturation crystallization point of the high-concentration brine, causing the salt to precipitate. This device is called an "evaporation pond".

The energy of this device comes from sunlight, so this device is more suitable for use in relatively dry areas with less annual rainfall and abundant solar radiation energy.

This facility has the following advantages: because the energy comes from sunlight, there is no service life limitation of the heat source, daily maintenance is relatively easy, the cost of treating high-concentration brine is also relatively low, and it can withstand load shocks.

The disadvantage is that the "evaporation pond" device is a non-closed device, and volatile components in the concentrated brine easily enter the atmosphere, causing air pollution.

At the same time, the anti-seepage engineering on the sides and bottom of the "evaporation pond" device is also crucial. If not handled properly, it is easy to cause serious pollution to the geotechnical body and groundwater sources;

Generally, the "evaporation pond" occupies a large area, and its use in areas with tight land resources will cause certain waste; in the operation of the "evaporation pond", the evaporated freshwater resources are difficult to utilize, causing certain waste.

Thermal Method

The thermal zero-discharge technology developed based on the thermal brine desalination system has low energy consumption. Multiple-effect evaporation is one of the three most commonly used brine desalination technologies today. Based on this technology, the multiple-effect evaporation-evaporation crystallization theory has been derived and is becoming increasingly widely used.

Foreign scholars have explored the "zero-emission" system using evaporation crystallization. The steam generated by evaporation is used to heat the water entering the evaporator, and its efficiency is far higher than that of conventional evaporation crystallization facilities.

Multiple-effect evaporation (MEE) is generally controlled at 3-6 effects. Too few are not energy-efficient enough, too many have insufficient temperature difference, and too long a system is prone to problems. The first-stage evaporator uses steam heating, and the subsequent evaporators use the secondary steam generated by the previous evaporator as a heat source, realizing the repeated use of thermal energy multiple times, which is multiple-effect evaporation.

Compared with multi-stage flash evaporation, multiple-effect evaporation has more serious scaling.

Multi-Stage Flash (MSF) is a process where hot brine is introduced into a flash chamber, causing it to superheat and rapidly partially vaporize. This lowers the temperature of the brine itself, and the resulting steam, when condensed, becomes the desired desalinated water.