Introduction to high-salt wastewater

       High-salt wastewater refers to wastewater with a total salt content of at least 1%. It mainly comes from chemical plants and the collection and processing of oil and natural gas. This wastewater contains a variety of substances (including salt, oil, organic heavy metals and radioactive substances). There are many ways to produce salty wastewater, and the amount of water is increasing year by year. Removal of organic pollutants in salty sewage is of vital importance to the environment. The biological method is used for treatment, and high-concentration salt substances have an inhibitory effect on microorganisms. The use of physicochemical methods for treatment requires large investment and high operating costs, and it is difficult to achieve the expected purification effect.

       The organic matter of high-salinity organic wastewater varies greatly depending on the production process, and the types and chemical properties of the organic matter contained are quite different, but most of the salt substances contained in it are Cl-, SO42-, Na+, Ca2+ and other salt substances. Although these ions are essential nutrients for the growth of microorganisms, they play an important role in promoting enzyme reaction, maintaining membrane balance and adjusting osmotic pressure during the growth of microorganisms. However, if the concentration of these ions is too high, it will have inhibitory and toxic effects on microorganisms. The main manifestations are: high salt concentration, high osmotic pressure, and dehydration of microbial cells to cause cell protoplasm separation; salting out reduces dehydrogenase activity; Bacteria have toxic effects; high salt concentration, increased density of wastewater, and activated sludge are easy to float and lose, which seriously affects the purification effect of the biological treatment system.

       For high-salt wastewater treatment, we generally use the following process for treatment:

Multi-effect evaporation

       Multi-effect evaporation is to evaporate the heated salt water in multiple evaporators in series. The vapor evaporated from the previous evaporator is used as the heat source of the next evaporator and condenses into fresh water. Among them, low-temperature multi-effect distillation is one of the most energy-saving methods in distillation. Low-temperature multi-effect distillation technology has developed rapidly in recent years due to energy-saving factors. The scale of the device is increasing, and the cost is decreasing. The main development trend is to improve the single-machine water production capacity of the device, use cheap materials to reduce engineering costs, increase operating temperature, and improve heat transfer efficiency etc.

Vapor compression condensation

       The vapor compression condensation desalination technology is to preheat the brine, enter the evaporator and partially evaporate in the evaporator. The generated secondary steam is compressed by the compressor to increase the pressure and then introduced to the heating side of the evaporator. After the steam is condensed, it is drawn out as product water, thus realizing the recycling of heat energy. When it is used as a recycling cooling water desalination process, the harmful components in the cooling water can be concentrated and discharged, and more than 95% of the discharged sewage can be recovered in the form of condensate and returned to the system as circulating water and boiler make-up water. This process has extremely high requirements for equipment materials and consumes a lot of heat during operation. It has the disadvantages of one-time investment and extremely high operating costs. It can only be used in power plants in areas where water is particularly scarce.