High-salinity wastewater refers to wastewater with significantly higher salinity compared to conventional domestic water and surface water. High-salinity wastewater is mostly discharged from industrial enterprises.

After testing the high-salinity wastewater discharged from a certain industrial enterprise, it was found that the salt content in the wastewater liquid reached more than 1%. In addition to salt, the wastewater also contains a relatively large amount of organic heavy metals, oils, and some substances with strong radioactivity and hazards. In addition, the TDS in high-salinity wastewater is high, the components include NaCl, Na2SO4, and both COD and chroma are relatively high, and it contains some impurity ions, such as Mg2+, Ca2+, and NH4+.

2.1 Salt mixing process technology

In the zero-discharge process design of high-salinity wastewater, salt mixing is also a commonly used treatment technology. This technology uses ultrafiltration, evaporation crystallization, and salt mixing drying technologies to treat high-salinity wastewater, and obtains mixed salts and condensate.

First, ultrafiltration membranes are used to simply treat high-salinity wastewater. This process uses the pressure difference between the two sides of the ultrafiltration membrane as the driving force, using the principle of mechanical sieving to separate the solution and substances. Because the pore size of the ultrafiltration membrane is very small, suspended solids and some macromolecules in the wastewater can be removed. The wastewater entering the ultrafiltration component, under the influence of the pressure on both sides of the ultrafiltration membrane, the liquid flows out of the ultrafiltration membrane to form ultrafiltration liquid, and another part of the liquid flows out as concentrated liquid. In the ultrafiltration process, a relatively stable and balanced state is mainly presented, which ensures the efficiency and quality of the ultrafiltration process. In addition, in the process of filtering high-salinity wastewater with ultrafiltration membranes, substances with larger molecules will be removed, such as total silicon, suspended matter, etc.; the remaining small molecules and salts are introduced into the next evaporation crystallization process with the wastewater, thus achieving 95% recovery.

Second, in the evaporation crystallization environment, the main task is to carry out mixed salt evaporation crystallization treatment. Because high-salinity wastewater has a relatively high COD content, the ultrafiltration membrane cannot remove all organic substances. During evaporation crystallization, bubbles are easily formed, and defoamers need to be added to ensure that the evaporation process continues. The organic matter affects the boiling point of the solution. If it is suppressed and kept boiling, the evaporation rate will be greatly reduced or even stop evaporation. Therefore, organic matter will have some adverse effects on the mixed salt evaporation crystallization treatment. After evaporation crystallization treatment, substances such as sodium sulfate and sodium chloride will be dried to form solid mixed salts. At this time, the enterprise needs to package these mixed salts for secondary use. The application of this zero-discharge treatment technology for high-salinity wastewater improves the utilization rate of high-salinity wastewater and processes some beneficial substances, resulting in some solid mixed salt waste. However, this technology does not truly achieve zero discharge and needs innovation and research and development of mixed salt technology.

2.2 Salt separation process design

Salt separation technology is based on the mixed salt technology to fully separate the solid waste mixed salt to form single crystalline salts, that is, inorganic salts that meet the purity standards, and the wastewater meets the water quality requirements of primary reclaimed water. In this way, both water and crystalline salts can be reused, truly achieving the zero-discharge goal of high-salinity wastewater. Taking specific coal chemical high-salinity wastewater as an example, the salt separation process design scheme is as follows: ultrafiltration—ozone oxidation—nanofiltration—evaporation crystallization—drying.

(1) Ultrafiltration membrane pretreatment. After high-salinity wastewater passes through the ultrafiltration membrane, suspended matter, total silicon, and macromolecules are removed; the remaining small molecules and salts enter the evaporation crystallization system with the wastewater. In the ultrafiltration membrane process, the inlet water pressure reaches 1 MPa, the pH value can reach about 8.5～10, and the system is also equipped with an independent cleaning system, and the cleaning operation and ultrafiltration operation can be carried out simultaneously. The water yield after ultrafiltration membrane is about 95%, and further ozone catalytic oxidation treatment is needed.

(2) Ozone catalytic oxidation treatment. The main function of this process is to remove organic matter from high-salinity wastewater and avoid the problem of bubbles preventing evaporation during subsequent evaporation crystallization. Based on the non-homogeneous ozone catalytic technology, undegraded organic matter is decomposed. In addition to using ozone as an oxidant for direct oxidation, this system mainly uses the way that ozone produces hydroxyl radicals on the surface of the solid catalyst to remove organic matter and achieve oxidation. In this process, the oxidation-reduction potential can reach more than 2.8 V, which can achieve a good oxidation effect and thoroughly degrade the refractory organic matter in the wastewater. In the implementation of the ozone catalytic oxidation process, it is mainly divided into four processes: the first is the adjustment pool process, the second is the primary ozone catalytic oxidation process, the third is the secondary ozone catalytic oxidation process, and the fourth is the water release pool process. After this process, the COD concentration of the effluent can be reduced to less than 100 mg/L, the total COD removal rate reaches about 80%, and the chroma is less than 10 times.

(3) Nanofiltration membrane system treatment. Currently commonly used salt separation processes include thermal salt separation technology and nanofiltration salt separation technology. Thermal salt separation technology uses the advantages of variable temperature crystallization for salt separation; nanofiltration salt separation technology uses the advantages of selective retention in nanofiltration membranes and the separation and extraction of inorganic salts for salt separation. For example, in some coal chemical wastewater, anions are usually dominated by chloride ions and sulfate ions, and monovalent cations are mainly sodium ions. After the ozone catalytic oxidation treatment, the effluent enters the nanofiltration membrane stage to treat the separated Na2SO4 and NaCl. In this process, the most important role is played by a translucent membrane, which can selectively retain divalent salts, so that divalent salts and monovalent salts can be separated. At the same time, the nanofiltration membrane is used to separate the water with high salt content into nanofiltration concentrate and effluent, with the effluent mainly sodium chloride and the concentrate mainly sodium sulfate.

(4) Sodium chloride and sodium sulfate crystallization treatment. After nanofiltration membrane treatment, the next step is sodium chloride and sodium sulfate crystallization treatment to extract sodium chloride and sodium sulfate crystals. Because the amount of effluent is relatively large, a three-effect evaporation method is needed to evaporate the solution to obtain crystals and form sodium chloride crystals. In this process, the mother liquor is discharged into the original water to improve the purity of sodium chloride crystals; the remaining condensate is used after ion detection.

(5) Crystal salt drying treatment. After obtaining sodium sulfate and sodium chloride crystal salt, it also needs to be dried. After entering the respective dryers, a vacuum target dryer is used to treat the sodium sulfate and sodium chloride crystals. The elemental crystal salt is obtained by high-vacuum exhaust treatment and steam jacket indirect heating of the materials. During this process, the temperature must be controlled, and the temperature cannot be too high. If the temperature is too high, the oxidized materials are prone to powdering. The crystal salt is added from the middle of the top of the dryer, and the water in the material is vaporized by the stirring action of the rake teeth and the indirect heating of steam. Then, the vacuum pump extracts the vaporized water to achieve the purpose of crystal salt drying.

After the high-salt wastewater zero-discharge process treatment, the water quality of the crystal salt and condensate water must meet industrial use standards. As shown in Table 2, various substances in sodium sulfate crystal salt and sodium chloride crystal salt must meet the relevant indicators so that they can be put into industrial use.

In summary, high-salt wastewater is a type of industrial wastewater discharged by industrial enterprises, which has a very large impact on the natural environment. Therefore, it is necessary to treat high-salt wastewater to protect the natural environment. High-salt wastewater zero-discharge process technology has been widely used in many industrial enterprises. Under the application of the zero-discharge concept, the high-salt wastewater zero-discharge treatment concept can better realize the recycling of crystal salt, mainly using the salt separation process. This article analyzes and discusses the design and application of high-salt wastewater zero-discharge technology, especially the implementation process of mixed salt technology and salt separation technology, in order to achieve zero-discharge treatment of high-salt wastewater.