What are the mainstream high salt wastewater treatment processes?

The concept of high salt wastewater is usually used for biochemical treatment of wastewater. High salt wastewater refers to wastewater with a mass fraction of organic matter and at least total dissolved solids (TDS) greater than 3.5%. In this type of wastewater, in addition to organic pollutants, it also contains a large amount of soluble inorganic salts, such as Cl+, Na+, SO42+, Ca2+, etc. The presence of these salts has a significant inhibitory effect on conventional biological treatment, which is generally the limit of biochemical treatment.

Source of high salinity wastewater
1. Produced by seawater desalination.
2. Chemical production: A large amount of high COD, high salt, and toxic wastewater generated during the production of chemical products such as dyes and pesticides due to incomplete chemical reactions or byproducts. For example, in the production of soda ash using the ammonia alkali method, the soluble salt content in the wastewater discharged from the system after ammonia evaporation treatment can generally reach 15% to 20%, most of which are CaCl2 and NaCl. In the coal chemical industry, the salt content of the concentrated wastewater discharged after thermal concentration process can reach over 20%.
3. Wastewater treatment: In the process of wastewater treatment, the addition of water treatment agents, acids, and alkalis, as well as the concentration of soluble salts generated from most water recovery, will increase the concentration, forming so-called "high salinity wastewater" that is difficult to biochemical treatment, and has greater environmental pollution than ordinary wastewater. The discharge of high salt wastewater mentioned above will cause serious pollution to the environment, such as soil compaction and inability of plants to continue growing. In addition, most high salinity wastewater is also high concentration organic wastewater, which can accelerate the eutrophication of natural water systems and increase environmental pressure.
How to treat high salt wastewater? This is the main focus of everyone's attention. Understanding the process of treating high salt wastewater and using process principles to guide treatment techniques can help design corresponding route plans for different situations (wastewater properties, effluent usage, water quality requirements, etc.). The process methods for treating high salinity organic wastewater include physical, chemical, and biological methods, generally aimed at reducing the COD and salt content of the wastewater.
Physical and chemical methods
1. The incineration method is suitable for high salinity wastewater with high calorific value and high COD content. It fully reacts with oxygen in the air at 800-1000 ℃, converting COD into gas and solid residues. It is generally suitable for wastewater with a COD value greater than 100g/L, and has high energy consumption.
2. Electrolytic high salt wastewater has high conductivity. During the electrolysis process, the organic electrolyte solution can undergo a series of oxidation-reduction reactions, generating substances that are insoluble in water. After precipitation or the generation of harmless gases, COD is reduced. This method is also related to the types of organic and inorganic salts, and in the presence of Cl -, it can discharge at the anode to generate ClO - for COD degradation. However, some experiments have shown that treating phenol wastewater through electrolysis only changes the form of COD and does not reduce the total amount of TOC present.
3. The commonly used mature membrane separation processes currently include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis. The membranes used for microfiltration and ultrafiltration have larger pore sizes, which have better retention effects on COD and suspended solids (SS), but cannot effectively remove salt from wastewater. Nanofiltration can intercept most divalent ions. Reverse osmosis (RO) can intercept monovalent ions and remove some dissolved organic matter, but it has certain limitations in water treatment applications. Electrodialysis technology is a relatively effective and commonly used desalination technique. According to different requirements, different membrane separation processes can be selected for treatment, but when the concentration of organic matter is high, the membrane is prone to contamination and the cost is high.
4. The evaporative crystallization process is suitable for processes with low COD values, and its main purpose is to separate the solid and liquid of high salt wastewater. At present, the commonly used processes are multi effect evaporation and mechanical compression evaporation. The bottleneck of the evaporation crystallization process lies in the high energy consumption, significant differences in the water quality of saline wastewater among enterprises, different treatment effects and costs, poor economic benefits, and secondary pollution. It is often used in the pretreatment stage.
5. The adsorption process of activated carbon has a unique lattice structure with many oxygen-containing functional groups on the surface, which can adsorb a large amount of inorganic and organic substances on the surface. At the same time, some organic substances enter the micropores inside the activated carbon to form chelates, thereby purifying water quality. The Fenton oxidation process can generate strong oxidative free radicals, which can cause organic matter to decompose, thereby improving biochemical activity or removing organic matter. Introducing activated carbon into the Fenton reagent system can increase the concentration of organic matter near the oxidation group and improve oxidation efficiency. Due to chemical reactions, activated carbon can continuously desorb and regenerate, recycle, and avoid secondary pollution. The high salinity in biological high salinity wastewater inhibits the metabolic function of microorganisms, and the biochemical treatment effect cannot meet the standard. Therefore, the biological process focuses on using halophilic bacteria to enhance the biochemical treatment effect of high salinity wastewater. Science popularization halophilic bacteria refer to bacteria that can grow in high salt environments, often found in high salt environments. Generally, bacteria that can survive well in water environments with a salinity of 2% -5% are called halotolerant bacteria. Bacteria that can survive in environments with a salinity of 3% -15% are moderate halophilic bacteria, generally fungi. Those that can survive in environments with a salinity of 15% -30% become extremely halophilic bacteria, generally archaea. They can maintain low water activity in the body under high salinity conditions, maintain enzyme activity, and grow into dominant bacterial strains in high salinity wastewater environments, which can degrade the COD of the wastewater to meet the discharge standards. At present, research on halophilic bacteria is still in the experimental stage. With the maturity of technology, biological methods have the characteristics of no secondary pollution, environmental protection, safety, adaptability to various changes, huge potential, and low cost, which can be widely applied in engineering practice. The purpose of biological methods is to degrade organic pollutants in water, and deep treatment of inorganic ions in high salinity wastewater needs to be carried out in conjunction with physical and chemical methods. Through the analysis and introduction of high salt wastewater treatment processes, it can be seen that there are many current process technologies, but they mainly focus on the research of physicochemical and biological aspects. Adhering to sustainable development and economically effective solutions to the problem of high salinity wastewater treatment, biological desalination technology may be the main research direction for future development.
Below is a brief introduction to some common biological desalination technologies.
1. Biological contact oxidation is a common biological desalination technology. The biofilm method has strong resistance to toxicity and impact, and can maintain sufficient sludge age. Its biological properties are relatively stable, and its volumetric loading capacity is strong. Compared with conventional activated sludge treatment methods, its hydraulic retention time is shorter. Scholars have used a two-stage contact oxidation process to treat high salinity wastewater, and the salinity of the wastewater can be reduced to 2.5 * 104mg/L or even lower, with a COD removal rate of about 95%.
2. Anaerobic technology and its improvement are common biological desalination technologies. This technology utilizes the adaptability of anaerobic bacteria, halophilic bacteria, and nitrifying bacteria to high salt environments to exert desalination effects. According to the investigation, if halophilic bacteria are placed in the SBR reactor and the sludge age is 18 days, the COD removal rate can reach up to 95%, and the ammonia nitrogen removal rate is not less than 6l%. However, the utilization of halophilic bacteria is still in the experimental stage.
3. SBR process is a new type of wastewater treatment technology, also known as sequencing batch activated sludge process, which uses intermittent aeration to purify high salinity wastewater with activated sludge. SBR has a distinct orderliness in operation and intermittency in operation. The core of this technology is the SBR reaction tank, which can be used for biodegradation, primary and secondary sedimentation, and can also integrate functions such as homogenization. Especially suitable for factories with significant changes in wastewater discharge and intermittent discharge characteristics. The SBR process is still under development, and some unique processes have been developed based on the basic process, such as ICEAS, CAST, DAT-IAT, UNITANK, MSBR, etc.
With the development of the chemical industry and the tightening of environmental protection governance, the discharge of high salt wastewater is becoming increasingly severe. Efficient and economical removal of inorganic salts from high salt wastewater has become a decisive factor in choosing a high salt wastewater treatment process. In response to the common forms of high salt wastewater (high salt wastewater containing organic matter), combining physicochemical and biological methods, the characteristics of no secondary pollution and low cost of biological methods with the high efficiency of physicochemical methods can broaden the development of high salt wastewater treatment.