Marketing Service

In the past forty years of reform and opening up, with the rapid development of China's economy and the continuous development of basic industries, the environmental pollution caused by improper discharge of industrial wastewater has become increasingly serious, and industrial wastewater must be properly treated. High salinity wastewater is a kind of toxic and refractory industrial wastewater. The source, composition and characteristics of high salinity wastewater are introduced in this article. Three treatment technologies of high salinity wastewater are summarized: conventional treatment technology, concentration technology and zero discharge technology.

If high-salt wastewater is directly diluted, on the one hand, it will cause waste of water resources, on the other hand, it will have adverse effects on the environment: accelerating eutrophication of rivers and lakes, leading to the collapse of soil ecosystems, producing odors affecting water quality, changing the color and visibility of water bodies, and forming a large number of suspensions. With the development of industry, more and more high-salt wastewater is produced, whose composition is more and more complex and concentration is higher and higher. Therefore, it is imperative to study the effective treatment methods of high salt wastewater.

1. Source and Composition of High Salt Wastewater

High salt wastewater refers to wastewater containing organic matter and total dissolved solids (TDS) of at least 3.5% (mass concentration). This wastewater comes from a wide range of sources. First of all, in the chemical, pharmaceutical, petroleum, paper, dairy processing, food cans and other industrial production processes, a large number of wastewater will be discharged.It not only contains a large number of high concentration of organic pollutants,but also a large amount of plasma,such as calcium,sodium,chlorine and sulfate. In order to make full use of water resources, many coastal cities directly use seawater as industrial production or cooling water. In some places, sea water is used for fire fighting, flushing toilets and roads.Although this part of the sewage contains no large amount of toxic substances,it is difficult to deal with the large amount of water with salt content.

2. Characteristics of High Salt Wastewater

The organic matter of high salinity organic wastewater varies greatly according to different production processes. The types and chemical properties of organic matter vary greatly, but most of the salts are Cl-, SO42-, Na+, Ca2+. These ions are essential nutrients for microbial growth and play an important role in promoting enzymatic reaction, maintaining membrane balance and regulating osmotic pressure during microbial growth. However, if these ions concentration are too high, they will inhibit microorganisms. Toxic effects. High salt concentration, high osmotic pressure and microbial cell dehydration lead to cell protoplast separation; salting out can reduce dehydrogenase activity; high chloride ion is toxic to bacteria; With high salt concentration, increased wastewater density, active fouling slurry is easy to float and discharge,which can seriously affect the purification effect of biological treatment system.

Huang Xinwen with other people studied the effects of several common inorganic salts in wastewater on microbial treatment system. The experimental results show that when inorganic salt concentration is too high, excessive inorganic salt will cause microorganism death, reduce sludge volume, increase effluent suspended matter and reduce COD removal rate in activated sludge system. Therefore, this kind of concentrated salt wastewater needs to be treated separately.

3. High Salt Wastewater Treatment Technology

3.1 Conventional Technology

3.1.1 Electrolysis

High salinity wastewater has high conductivity. It can realize the redox reaction of toxic and harmful substances by electrolysis, that is, strong current between anode and cathode can be generated to remove pollutants in water. Electrolysis can effectively reduce COD in wastewater. The process has strong adaptability to sewage and good removal effect, but its operation cost is high. The treatment of high salinity organic wastewater from lac synthetic resin production by electrolytic flocculation can not only effectively reduce the transparency of COD in wastewater, but also have higher removal rates of BOD, TP and TN.

3.1.2 Ion Exchange Metho

The key of ion exchange method is ion exchange resin. It is a polymer with functional groups, network structure and insoluble polymers. Amino and hydroxyl groups in this polymer can bind metal ions in high salt wastewater.Ion exchange process can be used as a pretreatment process to remove various metal ions and achieve the purpose of effective desalination. The disadvantage is that the solid suspension in wastewater can block the resin and make the ion exchange resin lose its function. Tang Shuh and other people used ion exchange resin to treat chromium-containing wastewater. After treatment, the content of Cr in wastewater decreases from 1540 mg/L to 0.5 mg/L, which meets the national discharge standard.

3.1.3 Incineration Method

Incineration means spraying high salt wastewater into a high temperature incinerator. Toxic and harmful substances in wastewater are decomposed into water, gas and inorganic salt ash by high temperature oxidation. When treating high-salt wastewater by incineration, it is necessary to prevent the blockage of atomizing nozzles and subsequently purify the polluted gases generated during incineration. Wang Wei and other people apply incineration methods to treat high concentration organic and saline wastewater to prove the feasibility of this method, which can effectively treat and discharge the wastewater, waste gas and solid waste generated in the process.

3.1.4 Biochemical Treatment

Biochemical treatment refers to the use of microorganisms widely existing in nature to oxidize, decompose and adsorb organic substances in wastewater to achieve the purpose of purifying water quality. Biochemical treatment has the advantages of economy, high efficiency and harmlessness. However, inorganic salts in wastewater with high salinity have strong inhibitory effect on microorganisms. The domestication of salt-tolerant microorganisms is the focus and difficulty of biochemical treatment. A moderate halophilic bacterium was isolated from brine of salt pond in Daokou Salt Farm, Weihai City, Shandong Province. The high salt tannery wastewater containing 9.3% salt and 1738 mg/L CODcr was treated by the bacteria. After 216 hours, CODcr removal rate reached 98%.

3.2 Concentration Technology

High-salt wastewater treatment costs are high and energy consumption is high. Therefore, reducing the treatment quantity of high-salt wastewater (increasing salt content, increasing concentration and reducing the amount of treated water) can not only reduce the cost of treatment, but also benefit the recovery and utilization of salt in high-salt wastewater. High salinity wastewater concentration technology includes membrane separation and evaporation.

3.2.1 Membrane Separation Method

Membrane separation refers to the selective osmotic separation, purification and concentration of mixtures of different components in high salt wastewater by using membrane, so as to achieve the reduction of wastewater treatment. The key of this method is to select suitable filter membranes, which can be divided into microfiltration membranes (MF), ultrafiltration membranes (UF), nanofiltration membranes (NF) and reverse osmosis membranes (RO) according to their pore size. According to whether the external pressure is increased, it can be divided into positive osmosis membrane technology and reverse osmosis membrane technology. Membrane separation method has the advantages of low energy consumption, strong adaptability and good selectivity, but the filter membrane is easy to be blocked and corroded by substances in high salt wastewater, and needs frequent cleaning or replacement. In actual industrial production, reverse osmosis membrane is one of the most widely used. It can recover up to 60% of fresh water, and the concentration of treated high salt wastewater can be doubled.

3.2.2 Evaporation method

The evaporation method is to evaporate the high-salt wastewater by heating to concentrate the high-salt wastewater and achieve the purpose of reduction treatment. In the process of industrial high salt wastewater treatment, multi-effect evaporators, i.e. multiple evaporators in series, are often used. The concentration effect of multi-effect evaporation process is influenced by many factors, such as temperature difference of heat transfer, steam pressure of heating and so on. Multi-effective evaporation technologies were used to centralize desalination of oilfield wastewater, and the salt content of concentrated wastewater could reach more than 8%.

3.3 Zero Emission Technology

The concentrated high-salt wastewater has a high salinity content, which is difficult to treat and has a great impact on the environment after discharge. Therefore, zero-discharge technology is needed to fundamentally solve the problem of high-salt wastewater treatment. Zero-emission technology is based on evaporation and concentration technology. The key of this technology is crystallization, which separates soluble salts from high salt wastewater to form crystalline salts.

The crystallization process includes cooling crystallization and thermal crystallization, in which cooling crystallization is the basis of thermal crystallization. In the cooling crystallization process, the mother liquor after evaporation concentration is separated by cooling crystallization, and needs to be re-heated and evaporated repeatedly back to the front end. The process flow is long, the energy consumption is high and the efficiency is low. Thermal crystallization process is to introduce special equipment to make concentrated mother liquor continue to be heated and concentrated, and then cooling crystallization to form supersaturated solution. The process can achieve 100% separation of salt.

3.4 Comparison of High Salt Wastewater Treatment Technologies

On this basis, various high salt wastewater treatment technologies were introduced in detail, and on this basis, the advantages and disadvantages of various treatment technologies and their applications were compared and analyzed.

4. Conclusion

Based on the principles of harmlessness, reduction and resource recovery, this paper comprehensively considers the simplicity, efficiency, cost and return on investment of the treatment technology. At present, industrial wastewater with high salinity has the characteristics of high salinity and biological toxicity. Simple application of a process is often difficult to meet the needs. In practical applications, complex processes suitable for biological treatment of salt, evaporation and concentration, membrane separation and membrane biological treatment have been widely used because of their advantages of cost and efficiency.