Learn about high salinity wastewater treatment and solutions

High-salinity wastewater is a significant environmental challenge, particularly in industrial settings where seawater or salt-rich byproducts are frequently produced. Understanding the production, effects, and treatment of high-salinity wastewater is essential for implementing effective solutions that meet regulatory standards and ensure environmental protection. This guide outlines the primary sources of high-salinity wastewater, the inhibitory effects of inorganic salts on microorganisms, and the latest biological treatment strategies used to manage saline wastewater.

1. Ways of producing high-salinity wastewater

1.1 Wastewater discharged by seawater substitution

The so-called seawater substitution is to directly replace freshwater resources used in certain occasions with seawater without desalination.

In industry, seawater can be widely used as boiler cooling water, and is applied to thermal power, nuclear power, petrochemical, metallurgy, steel mills and other industries. The annual seawater cooling water consumption in developed countries has exceeded 100 billion m3. At present, the annual utilization of seawater in my country is more than 6 billion m3. Qingdao Power Plant began to use seawater as industrial cooling water in 1936, and it has a history of more than 60 years. At present, 12 coastal enterprises in Qingdao’s power, chemical, textile and other industries use 837 million m3 of seawater annually. Tianjin uses 1.8 billion m3 of seawater annually. In addition, more than 70 coastal thermal power, nuclear power, chemical, petrochemical and other enterprises such as Qinhuangdao Thermal Power Plant, Huangdao Thermal Power Plant and Shanghai Petrochemical General Plant have directly used seawater in different ways. For industries such as printing and dyeing, building materials, alkali, rubber and seafood processing, seawater can also be used as industrial production water.

Urban domestic water. In urban life, seawater can replace fresh water as toilet flushing water. At present, the penetration rate of seawater flushing in Hong Kong is as high as over 70%, and the penetration rate is planned to increase to 100% in the future, making it the world’s first city to use seawater as toilet flushing water. In some units in cities such as Dalian, Tianjin, Qingdao, and Yantai, there are also practices of using seawater to flush toilets, but on a smaller scale.

1.2 Industrial production wastewater

Some industries, such as printing and dyeing, papermaking, chemicals and goods, produce organic wastewater with high salt content in production.

1.3 Other high-salt wastewater

Ship ballast water

Wastewater minimization Sewage generated in production

Domestic sewage generated on large ships

2. The inhibition principle of inorganic salts on microorganisms

2.1 Inhibition principle

The main poison in salty wastewater is inorganic poison, that is, high concentration of inorganic salts.

The effect of toxic substances on wastewater biological treatment is related to the type and concentration of the poison. Generally, as the concentration increases, it can be divided into three categories: stimulation, inhibition and toxicity.

The toxic effect of high-concentration inorganic salts on wastewater biological treatment is mainly to destroy the cell membrane of microorganisms and enzymes in the bacteria through the increased environmental osmotic pressure, thereby destroying the physiological activities of microorganisms.

① Microorganisms grow well under isotonic pressure. Microorganisms in NaCI solutions with a mass of 5~8.5g/L, and red blood cells in NaCI solutions with a mass of 9g/L do not change shape and size, and grow well; ② Under low osmotic pressure (ρ(NaCI)=0.1g/L), a large amount of water molecules in the solution penetrate into the microorganisms, causing the microbial cells to swell, and in severe cases, rupture, leading to the death of the microorganisms; ③ Under high osmotic pressure (ρ(NaCI)=200g/L), a large amount of water molecules in the microorganisms penetrate into the body, causing the cells to undergo plasmolysis.

2.2 Survival rate of freshwater microorganisms under different salinities‍

When different microorganisms living in freshwater environments or freshwater treatment structures are inoculated into high-salinity environments, only some of them survive. This is a kind of selection of salinity for microorganisms. The survival rate of freshwater microorganisms is defined as 100%. When the salinity exceeds 20g/L, its survival rate is less than 40%. Therefore, when the salinity exceeds 20g/:L, it is generally believed that it cannot be treated with different freshwater microorganisms.

3. Classification and utilization of salt-adapted microorganisms

Halophilic microorganisms: can tolerate a certain concentration of salt solution, but grow under salt-free conditions, and their growth does not require a large amount of inorganic salts.

Halophilic microorganisms: refers to bacteria that can grow under high-salinity conditions, and their growth is inseparable from the high-salinity environment. According to the range of good growth salinity, it can be divided into three categories.

Marine bacteria: Good growth salinity 1-3%

Moderate halophilic bacteria: Good growth salinity 3-15%

Extreme halophilic bacteria: Good growth salinity 15-30%

4. Problems encountered in biological treatment of high-salinity sewage

Poor adaptation to salinity

The traditional activated sludge method is used to treat saline wastewater with a salinity of less than 2%.

When the salinity environment changes to a freshwater environment, the adaptability of the sludge will disappear quickly.

Great impact of salinity changes

Salinity changes of 0.5-2% usually cause serious interference to the treatment system.

Sudden changes in salinity interfere with the system more than gradual changes in salinity. The impact of changing from high salt to no salt is greater than the impact of changing from a low salt environment to a high salt environment.

Slow degradation rate

As the salinity increases, the degradation rate of organic matter decreases, so low F/M is more suitable for the treatment of saline wastewater.

5. Countermeasures for biological treatment of high-salinity wastewater‍

5.1 Domestication of freshwater microorganisms

When microorganisms adapted to living in freshwater biological treatment facilities enter a salty environment of a certain concentration, they will balance the osmotic pressure in the cell or protect the protoplasm in the cell through their own osmotic pressure regulation mechanism. These regulation mechanisms include aggregating low molecular weight substances to form a new extracellular protective layer, regulating their own metabolic pathways, changing genetic composition, etc. Therefore, normal activated sludge can treat saline wastewater within a certain salinity range through domestication for a certain period of time.

Although sludge can improve the salt tolerance range of the system and improve the treatment efficiency of the system through domestication, the microorganisms in the domesticated sludge have a limited tolerance range for salinity and are sensitive to environmental changes. When the salinity environment changes, the adaptability of microorganisms will disappear immediately. Domestication is only a temporary physiological adjustment for microorganisms to adapt to the environment and does not have genetic characteristics. This adaptive sensitivity is very unfavorable for the implementation of sewage treatment projects.

Studies have shown that salinity domestication can be used to treat saline wastewater under conditions of salinity less than 20g/L. However, the domestication salinity concentration must be gradually increased, and the system must be domesticated to the required salinity level in stages. Sudden high-salinity environment will cause failure of acclimatization and delay of start-up.

5.2 Dilute influent salinity

Since high salt becomes an inhibitor and poison to microorganisms, the influent should be diluted to make the salinity lower than the toxic threshold value, and the biological treatment will not be inhibited. This method is simple, easy to operate and manage; its disadvantages are increasing the treatment scale, increasing infrastructure investment, increasing operating costs, and wasting water resources.

5.3 Use of salt-adapted microorganisms

Inoculation or gene immobilization of salt-adapted microorganisms to treat high-salinity sewage is an effective treatment method. This method can treat more than 3% of high-salinity sewage, which is impossible to achieve with different acclimatization methods. Some of the salt-adapted bacteria selected for specific pollutant removal can have high specific degradation ability, greatly improving the treatment effect. The screening inoculum comes from the active substances in the ocean or estuary sediment, salt field substrate and other high-salinity environments. The screening often has certain procedures and genetic measures.

The disadvantages of this method are long start-up time and high initial start-up costs. However, it is a feasible method for biological treatment of high-salinity sewage. 5.4 Adding antagonists Antagonism refers to the situation where the toxic effect of a poison is reduced by the presence or increase of another substance. It can be seen from the figure that the toxic effect of a poison decreases with the increase of the low concentration of another substance, and after the good state, the reaction rate decreases with the further increase of the antagonist concentration. Current research has found that K will have an antagonistic effect on Na, reducing the toxic effect of Na salt on microorganisms. Potassium absorption and sodium excretion effect The main principle may be the Na+/K+ reverse transport function. Although bacteria need a high sodium environment to grow, the Na concentration in the cell is not high. For example, the light-mediated H+ proton pump of Halobacterium has the function of Na+/K+ reverse transport, that is, it has the ability to absorb and concentrate K+ and discharge Na+ to the extracellular space. K+, as a compatible solute, can adjust the osmotic pressure to achieve a balance between the inside and outside of the cell. Its concentration is as high as 7 mol/L to maintain the same water activity inside and outside. For example, halophilic anaerobic bacteria, halophilic sulfur-reducing bacteria and halophilic archaea accumulate high concentrations of K+ in the cell to resist the hypertonic environment outside the cell. For example, the Na+/K+ reverse carrier in yeast can excrete excess salt from the body and improve the salt tolerance of yeast.

5.5 Selecting a suitable treatment process

Different treatment processes affect the salt tolerance range of microorganisms. The following are the limiting amounts of NaCl concentration in several reported biological treatment methods.

Research generally believes that the biofilm process has a greater salt tolerance than the suspended activated sludge process. In addition, adding an anaerobic stage can greatly increase the salt tolerance range of the subsequent aerobic stage.

6. Design requirements for biological treatment of high-salinity sewage

6.1 Add a salinity regulating tank

Salinity changes have a great impact on the stability of the system, which is manifested as a sharp drop in treatment efficiency and a large amount of sludge loss. During the design, a regulating tank should be set up to ensure the relative stability of salinity. Conductivity monitoring devices can be set up at the inlet and outlet of the regulating tank to strengthen the online control and feedback of salinity to prevent salinity shock from causing the treatment system to fail.

6.2 Reduce sludge load

Salinity reduces the rate of biodegradation, so the design load should be relatively reduced. Many studies have shown that the sludge index decreases in a high-salinity environment, so there is no need to worry about sludge expansion caused by too low a load.

6.3 Increase sludge concentration

The sludge treated with high salt has poor coagulation and serious sludge loss. Therefore, a high sludge concentration should be guaranteed in the design. This is also a means to improve treatment efficiency. It is also possible to ensure additional sludge reserves when designing a sludge thickening tank, and quickly replenish it when sludge is lost.

6.4 Increase the retention time of the clarifier

High salt affects the coagulation capacity, so a longer retention time is conducive to the sedimentation of sludge.

6.5 Increase the aeration volume

The adaptation of microorganisms to high-salt environments is manifested by an increase in the aerobic respiration rate, so respiration will cause additional oxygen consumption. Increasing the dissolved oxygen concentration in water is beneficial to the metabolism of microorganisms. Provide their physiological requirements for adapting to high-salt environments.

Conclusion

Treating high-salinity wastewater presents unique challenges, but with advancements in biological treatment methods, it is possible to achieve efficient and sustainable wastewater management. By adopting strategies such as domestication of microorganisms, using salt-adapted strains, and optimizing treatment processes, industries can reduce the environmental impact of their operations while complying with regulations. With ongoing research and innovation in the field, the treatment of high-salinity wastewater will continue to improve, providing more effective solutions for industries and municipalities alike.