With the development of China's economy and technological advancements, the types and output of chemical products are gradually increasing, and a large amount of high-salinity wastewater is also generated annually.

The chemical industry is a broad category, including basic chemicals, petroleum, metallurgy, energy, fine and daily chemicals, pharmaceuticals, pesticides, environmental protection, military industry, etc. The raw materials, intermediate products, and derivative products in the production process of chemical products have the same chemical properties, while the wastewater generated in the production process contains various types and concentrations of salt substances. Most of the wastewater is high-salinity wastewater, and these salt compounds are composed of various inorganic ions such as Fe2+ and Na+.

Current Status of Wastewater in Various Industries

Coal Chemical Industry:

In recent years, China's coal chemical industry has developed rapidly. A large amount of high-salt wastewater is generated in the coal chemical production process, and its composition is also very complex. Under the current requirements of green and environmental protection development, coal chemical enterprises are also facing the challenge of treating high-salt wastewater.

Reverse osmosis is often used to treat coal chemical wastewater. This method is currently relatively mature and is a desalinization method with lower cost and better effect at this stage. Its principle is to separate the solvent and solute in the wastewater and recycle the water to reduce resource waste.

Experts have studied the complexing nanofiltration method, using complexing agents to react with heavy metal ions in wastewater to form complexes, and then using nanofiltration membranes to retain heavy metal ions, achieving good results.

Pharmaceutical Chemical Industry:

The pharmaceutical chemical industry is also one of the major sources of high-salinity wastewater. This is because acidic and alkaline materials are used more frequently in pharmaceutical chemical production, and a large amount of inorganic salts are produced during neutralization reactions. In addition, a large amount of inorganic salts are also used for washing in pharmaceutical chemical production, resulting in the generation of a large amount of high-salinity wastewater with high salt content and COD.

Based on this, the MVR evaporation system is currently commonly used to treat high-salinity wastewater, removing most of the salt content. First, a more suitable living environment is created for microorganisms, and then a biochemical system and iron-carbon device are used to treat high-concentration wastewater. Currently, the iron-carbon device and biochemical treatment system are a system with good application effects in the treatment of high-salinity wastewater in the pharmaceutical chemical industry, with the advantages of long-term effectiveness and low consumption.

Metallurgical Wastewater:

Metallurgical wastewater is also one of the major sources of high-salinity wastewater. High hardness, high salt content, and complex composition are its main characteristics. The treatment of metallurgical wastewater usually adopts a combination of multiple technologies to achieve solid-liquid separation.

Experts have studied the metallurgical wastewater generated in zinc smelting projects and adopted a combined treatment process of double alkali method-clarification-ultrafiltration-brackish water reverse osmosis-resin softening-seawater reverse osmosis-MVR. The results show that this process has a relatively stable treatment effect and low energy consumption, but the addition of NaCLO needs to be carefully controlled during the process. In order to prevent the brackish water reverse osmosis process from polluting the membrane, a separate acid-base pool needs to be installed to maintain the stable pH value of the solution to avoid affecting the stable operation of the system.

When experts studied high-salt cyanide wastewater, ozone was first added to the wastewater sample to remove some COD and cyanide, and part of the wastewater was recycled. The remaining wastewater was then treated with chemicals to remove heavy metal ions and reduce wastewater hardness. After obtaining high-concentration saline wastewater, reverse osmosis was used for volume reduction and concentration, and the concentrated liquid was treated to remove cyanide. Finally, the MVR process was used for evaporation and crystallization. The results show that this method has a good treatment effect and low energy consumption and can be promoted for use.

Dye and Pesticide Industry:

Dye and pesticide production also generates a large amount of high-COD, high-salt, and toxic wastewater. After pretreatment with advanced catalytic oxidation coupled with biological treatment, most of the refractory organic matter and toxic and harmful substances in the wastewater can be removed and recycled. However, after recovering about 60% of the wastewater through reverse osmosis membrane, a large amount of high-salinity water will be formed, requiring special treatment.

In recent years, China's textile industry has developed rapidly, driving the development of industries such as man-made fibers, dyes, auxiliaries, and adhesives. These types of enterprises also generate a large amount of high-salinity wastewater during production.

High-Salinity Wastewater Treatment Technology

The treatment of high-salinity wastewater has always been a challenge in the industry. It is necessary to ensure that the wastewater meets the national discharge standards after treatment, and also to ensure that the derived substances will not cause secondary pollution to the environment.

Electrolytic oxidation: Due to the excellent electrical conductivity of high-salinity wastewater, the electrolytic oxidation method can be used for degradation. A series of oxidation-reduction reactions will be generated after electrolysis, and then substances insoluble in water will be generated. After precipitation, the water is recycled, which can effectively reduce the COD in the water. The treatment of high-salinity wastewater using the electrolysis method is greatly related to the type and concentration of organic and inorganic salts in the wastewater. For example, if the wastewater contains a large amount of Cl-, it needs to be discharged at the anode, and after the reaction, ClO- is generated to achieve the purpose of degrading COD. However, when treating organic salts, they need to be deeply oxidized to harmless CO2 to effectively remove COD.

Evaporation treatment technology: The advantage of this technology is that the recovered freshwater has good water quality. The distillation desalination technology used in the treatment of wastewater in the chemical industry originates from seawater desalination technology and low-temperature multi-effect distillation technology, and has the advantages of low energy consumption and energy saving. For the treatment of high-salinity wastewater, it is mainly suitable for wastewater with low COD content and solid-liquid separation can be achieved through evaporation and crystallization. The evaporation and crystallization process consists of multiple evaporators. First, the high-salinity wastewater is evaporated to produce a concentrated liquid, and then the concentrated liquid is placed on a rotating thin-film evaporator for heating. After most of the water evaporates, a supersaturated salt solution is obtained. Finally, it is cooled to below 40℃ to form salt mud, thus completing the separation of soluble salt substances in high-salinity wastewater.

Membrane separation technology: It can be completed at room temperature, belongs to the physical separation process, there is no chemical reaction, the operation is simple, and the energy consumption is low. At present, the application range of membrane separation technology is also very wide and tends to be mature. Commonly used ones include nanofiltration, microfiltration, and ultrafiltration. Microfiltration and ultrafiltration are mostly used for the separation of suspended substances, but soluble substances cannot be separated, while nanofiltration can achieve the separation of divalent ions. In practical applications, it should be reasonably selected according to specific requirements. At present, nanofiltration membrane separation technology is often used in the treatment of high-salinity wastewater in the coal chemical industry, but the disadvantage is that the concentration factor is low, generally only reaching about 3 times, which affects the treatment effect.

Adsorption Coupling Process: In the treatment of high-salinity wastewater, the adsorption method utilizes the physical and chemical adsorption of solid adsorbents to treat highly toxic substances and refractory biological pollutants in wastewater. Activated carbon is an excellent adsorption material with a unique crystal lattice structure and a large number of oxygen-containing functional groups on its surface, resulting in a very strong adsorption capacity. When impurities in the water are adsorbed into the microporous structure, chelates are formed, purifying the water body.

For example, in the Fenton oxidation process, macromolecular organic matter is converted into low-molecular-weight organic matter, increasing the biodegradability of the organic matter, or directly converted into CO2 for easier adsorption. When used in the treatment of high-salinity wastewater, the adsorption method can add activated carbon to the Fenton reagent to enhance the adsorption effect.