Design and application of zero-discharge technology for high-salt wastewater
High-salinity wastewater water quality
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+.
Design of zero-discharge technology for high-salinity wastewater
Mixed salt process technology
The mixed salt process is also a commonly used treatment technology in the design of zero-discharge technology for high-salinity wastewater. This technology uses ultrafiltration, evaporation crystallization, and mixed salt drying to treat high-salinity wastewater, obtaining mixed salt and condensate.
First, use an ultrafiltration membrane to simply treat the 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 ultrafiltration membrane has a very small pore size, it can remove suspended solids and some macromolecules from the wastewater. The wastewater entering the ultrafiltration component, under the influence of the pressure on both sides of the ultrafiltration membrane, forms an ultrafiltration liquid after the liquid flows out of the ultrafiltration membrane, and another part of the liquid flows out as a 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 an ultrafiltration membrane, 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 the 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 substances affect the boiling point of the solution. If it is suppressed and maintained at the boiling point, the evaporation rate will be greatly reduced or even stop evaporation. Therefore, organic substances 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 salt. At this time, the enterprise needs to package these mixed salts for reuse. 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.
Salt separation process design
The salt separation process is based on the mixed salt process to fully separate the solid waste mixed salt to form single-substance crystalline salts, that is, inorganic salts that meet the purity standards, while 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 the 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, and the pH value can reach about 8.5～10. 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 the ultrafiltration membrane is about 95%, and further ozone catalytic oxidation treatment is needed.
2. Ozone catalytic oxidation treatment
The main function of this step is to remove organic substances from high-salinity wastewater and avoid the problem of bubbles preventing evaporation in the subsequent evaporation crystallization process. Based on the heterogeneous ozone catalytic technology, undegraded organic substances are decomposed. In addition to using ozone as an oxidant for direct oxidation, this system mainly uses the method of generating hydroxyl radicals on the surface of a solid catalyst to remove organic substances 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 refractory organic substances in 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
Commonly used salt separation processes currently 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 semi-transparent membrane, which can selectively retain divalent salts, thus separating divalent salts from monovalent salts. At the same time, using a nanofiltration membrane to separate water with high salt content into nanofiltration concentrate and effluent, the effluent is mainly sodium chloride, and the concentrate is 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 step, 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 crystalline salts, they must be dried. After entering their respective dryers, a vacuum paddle dryer is used to process the sodium sulfate and sodium chloride crystals. High-vacuum exhaust processing and steam jacket indirect heating of the material are used to obtain elemental crystalline salts. During this process, the temperature must be controlled; it cannot be too high. If the temperature is too high, the oxidized material is prone to producing powder. The crystalline salt is added from the top middle of the dryer. Using the stirring action of the rake teeth and indirect steam heating, the moisture in the material is vaporized, and then the vaporized moisture is extracted by a vacuum pump to achieve the purpose of drying the crystalline salt.
After treatment with the high-salt wastewater zero-discharge process, the water quality of the crystalline salt and condensate must meet industrial use standards. The various substances in the sodium sulfate crystalline salt and sodium chloride crystalline salt must meet the relevant indicators before 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 significant 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 achieve the recovery and utilization of crystalline salt, mainly using the salt separation process. This article analyzes and discusses the design and application of the high-salt wastewater zero-discharge process, especially the implementation process of the mixed salt process technology and the salt separation process technology, in order to achieve zero-discharge treatment of high-salt wastewater.
