What Is High-Salinity Wastewater? A Wastewater Treatment Glossary
High-salinity wastewater is a specific type of industrial wastewater, mainly from chemical plants and from the extraction and processing of oil and natural gas. It contains pollutants such as suspended solids, organic matter, heavy metals, harmful chemicals and nutrients, and can have a huge impact on the environment and on ecosystems.
To address the question of how to treat high-salinity wastewater, we take a detailed look at one treatment method: distillation desalination.
Distillation is the oldest and most widely used desalination method. Distillation-based desalination of industrial wastewater has basically been developed from seawater desalination technology. Distillation is the process of heating saline water until it boils and evaporates, then condensing the steam into fresh water. It was the earliest desalination method adopted, and its advantages include simple structure, easy operation and good quality of the fresh water obtained. There are many types of distillation, such as multi-effect evaporation, multi-stage flash, vapor compression distillation and membrane distillation.
Multi-effect evaporation evaporates heated brine in several evaporators connected in series, where the steam produced by one evaporator serves as the heat source for the next and is condensed into fresh water. Low-temperature multi-effect distillation is one of the most energy-efficient distillation methods. Thanks to its energy savings, low-temperature multi-effect distillation technology has developed rapidly in recent years, with plant sizes growing and costs falling; its main trends are to raise the water output of a single unit, use inexpensive materials to lower construction cost, increase the operating temperature and improve heat transfer efficiency.
Taking seawater desalination as an example, feed seawater is heated to a certain temperature and then introduced into a flash chamber. Because the pressure in the chamber is controlled below the saturation vapor pressure corresponding to the temperature of the hot brine, the hot brine becomes superheated as it enters and partially vaporizes rapidly, lowering the temperature of the brine itself; the steam produced is condensed to give the fresh water required. Multi-stage flash is based on this principle: the hot brine flows in turn through several flash chambers of progressively lower pressure, evaporating and cooling stage by stage while the brine becomes more concentrated, until its temperature approaches, but remains above, that of natural seawater.
Multi-stage flash (MSF) is one of the more mature technologies in the seawater desalination industry, developed to overcome the serious scaling drawbacks of multi-effect evaporation. Since its introduction, MSF has been widely applied and developed; it offers simple and reliable equipment, high operational safety, good anti-scaling performance, wide operating flexibility and the ability to use low-grade heat and waste heat. It suits large and very large desalination plants and is mainly used in the Gulf countries.
Vapor compression condensation desalination preheats the brine, which then enters the evaporator and partially evaporates inside it. The secondary steam produced is compressed by a compressor to raise its pressure and is then introduced into the heating side of the evaporator. The steam is condensed and drawn off as product water, thus recycling the heat energy. When used as a desalination and recovery process for circulating cooling water, it concentrates and discharges the harmful components in the cooling water and recovers more than 95% of the blowdown water as condensate, which is returned to the system as make-up water for circulating water and boilers. This process places extremely high demands on equipment materials and consumes a large amount of heat during operation, with the drawback of very high capital and operating costs; it can therefore only be adopted by power plants in particularly water-scarce regions.
Selection of a biological treatment flow for high-salinity wastewater. The biological treatment flow for high-salinity wastewater is basically the same as an ordinary biological treatment flow, mainly comprising an equalization tank, an aeration tank, a secondary clarifier, sludge return, excess sludge dewatering and nutrient dosing.
(1) Equalization tank. The main factor to consider for a saline wastewater equalization tank is the variation in salt concentration. Besides production fluctuation cycles and shock loads, emphasis should be placed on how the salt concentration in the water varies and how to adjust for it, such as a reduction in low-salinity water or a shock of excessively saline influent.
(2) Aeration tank. The choice of aeration tank should differ according to the type of salt in the wastewater. For biological treatment of wastewater with high CaCl2 content, conventional aeration should be used. Calcium ions increase the floc strength of activated sludge; high CaCl2 can raise the ash content of the sludge to 40%-50%, increase sludge density and allow the sludge concentration in the aeration tank to exceed 5,000 mg/L. Therefore, aeration methods with greater lifting force should be used, such as conventional aeration, deep-shaft aeration and fluidized-bed aeration. Diffused aerators with larger bubbles and stronger lifting force should also be selected. Fine-bubble and variable-orifice aerators should not be used, to prevent the aeration pores from being blocked by inorganic salts and hindering agitation in the tank. When the flow is less than 1,000 m3, jet aeration can also be used; jet aeration has high oxygen transfer efficiency and is not prone to clogging the aeration equipment. The aeration intensity should also be greater than in ordinary biological treatment, around 10 m3/(m2·h), or a central tube can be used to increase lifting and mixing capacity. Under high-salinity conditions the increased oxygen transfer rate is favorable for high sludge concentrations; as long as the zoogloea does not disintegrate, the sludge will not float and be washed out even if filamentous bacteria appear. Attention should be paid to the dosing position of phosphorus nutrients, because the calcium phosphate precipitate formed not only affects performance but also causes scaling that easily clogs pipelines.
When using the SBR process to treat high-salinity wastewater, because SBR combines aeration and settling in one unit, the settling time must be fully considered in the design, especially when treating wastewater with a high concentration of sodium salts. Wastewater containing sodium salts settles poorly, so the settling time should be correspondingly extended; moreover, to reduce the disturbance of the decanter to the settled sludge, the decanting depth should be correspondingly reduced. When treating wastewater with large salinity fluctuations, an equalization tank is still required.
The biofilm process is an ideal process for treating high-salinity wastewater, such as the biological aerated filter process and the contact oxidation process. When treating wastewater with high calcium salt content, attention must be paid to the choice of packing or filter media, and a larger backwash intensity and duration should be designed for the biological aerated filter. Packing with a higher void ratio should also be used in the contact oxidation tank, and its installation should take into account ease of removal and washing so as to prevent calcium carbonate formed during treatment from clogging the packing. In biological treatment of wastewater with high NaCl content, the sludge ash content is lower than for CaCl2 wastewater, while saline wastewater has a higher density; when sludge bulking occurs or the aeration tank is shocked and the sludge disintegrates, the zoogloea floats and is washed out more easily than in CaCl2 wastewater. Therefore, the biofilm process is best adopted for biological treatment of wastewater with high NaCl content.
(3) Secondary clarifier. The surface loading of the secondary clarifier should have a certain margin, mainly because the higher density of the wastewater hinders sludge settling, especially for NaCl-containing wastewater. For larger flows, particularly CaCl2 wastewater, a peripheral-driven scraper is best used to accommodate the high concentration and density of the sludge. When conventional activated sludge is used to treat high-CaCl2 wastewater, the sludge return rate should be appropriately increased to reduce the shock caused by wastewater fluctuations and to improve system stability.
When treating wastewater with a high calcium ion concentration, because the inorganic fraction of the activated sludge is high and its organic removal capacity is relatively low, operating under a low loading rate gives a higher pollutant removal rate than under high loading. However, extended aeration is not very suitable for high-salinity wastewater, because the long sludge age and long hydraulic retention time cause the activated sludge to age easily and its flocculation performance to deteriorate, ultimately affecting effluent quality.
(4) Sludge dewatering. Because the excess sludge from biological treatment of CaCl2 wastewater contains much calcium salt, it dewaters readily and no flocculant is needed. After thickening, the sludge concentration can exceed 50 g/L. The excess sludge quantity is similar to that of ordinary wastewater treatment, and design parameters can follow ordinary sludge dewatering.
In May 2025, Dai Junbiao, a guest researcher at the Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, working with the team of Professor Tang Hongzhi of Shanghai Jiao Tong University, successfully constructed a new engineered bacterial strain capable of degrading five organic pollutants simultaneously. Verified with real industrial wastewater samples, the strain demonstrated efficient degradation of combined pollutants in high-salinity wastewater, offering a Chinese solution to the salt inhibition effect that limits conventional microbial treatment technologies. [2]
In the same year, in its work of awarding contracts for major environmental protection technology and equipment innovation tasks, the Ministry of Industry and Information Technology set up task lists focusing on difficult issues such as the treatment and reuse of high-salinity wastewater, including complete technology and equipment for resource recovery of waste salt from high-salinity concentrate. [9]
