1. Ways of generating high salt wastewater
1.1 Wastewater from seawater substitution discharge
The so-called seawater substitution refers to the direct replacement of freshwater resources used in certain situations without desalination treatment of seawater.
In industry, seawater can be widely used as boiler cooling water and applied in industries such as thermal power, nuclear power, petrochemicals, metallurgy, and steel plants. The annual consumption of seawater cooling water in developed countries has exceeded 100 billion cubic meters. At present, the annual utilization of seawater in China is over 6 billion cubic meters. Qingdao Power Plant began using seawater as industrial cooling water in 1936 and has a history of over 60 years. At present, 12 coastal enterprises in the power, chemical, textile and other industries in Qingdao use 837 million cubic meters of seawater annually. Tianjin's annual utilization of seawater reaches 1.8 billion cubic meters. 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 utilized seawater in different ways. For industries such as printing and dyeing, building materials, alkali production, rubber, and seafood processing, seawater can also be used as industrial production water.
Urban domestic water supply. In urban life, seawater can replace freshwater as toilet flushing water. At present, the penetration rate of seawater flushing in Hong Kong is over 70%, and the future plan is to increase the penetration rate to 100%, making it a city in the world that uses seawater as flushing water. In some units in cities such as Dalian, Tianjin, Qingdao, Yantai, etc., there are also practices of using seawater for toilet flushing, but the scale is relatively small.
1.2 Industrial production wastewater
Some industries, such as printing and dyeing, papermaking, chemical and pharmaceutical industries, generate high salt content organic wastewater during production.
1.3 Other high salt wastewater
Ship ballast water
Minimize wastewater generated during water production
Domestic sewage generated on large ships
The inhibitory principle of inorganic salts on microorganisms
2.1 Inhibition principle
The main toxic substances in saline wastewater are inorganic toxins, namely high concentrations of inorganic salts.
The impact of toxic substances on wastewater biological treatment is related to the type and concentration of the toxic substances, which can generally be divided into three categories: stimulating effect, inhibitory effect, and toxic effect as the concentration increases.
The toxic effect of high concentration inorganic salts on wastewater biological treatment is mainly caused by the increased environmental osmotic pressure, which destroys the cell membrane and enzymes inside the microorganisms, thereby disrupting their physiological activities.
① Microorganisms grow well under osmotic pressure. Microorganisms in NaCl solution with a mass of 5-8.5g/L, while red blood cells in NaCl solution with a mass of 9g/L maintain their morphology and size, and grow well At low osmotic pressure (ρ (NaCl)=0.1g/L), a large number of water molecules in the solution infiltrate into the microbial body, causing the microbial cells to expand, and in severe cases, rupture, leading to microbial death; ③ Under high osmotic pressure (ρ (NaCl)=200g/L), a large amount of water molecules from microorganisms infiltrate into the extracellular space, causing cell wall separation
2.2 Survival rate of freshwater microorganisms at different salinities
Microorganisms living in freshwater environments or freshwater treatment structures inoculated into high salt environments only partially survive. This is a selection of salinity for microorganisms. The survival rate of freshwater microorganisms is defined as 100%, and when the salinity exceeds 20g/L, its survival rate is below 40%. Therefore, when the salinity exceeds 20g/: L, it is generally believed that different freshwater microorganisms cannot be used for treatment.
5.4 Adding Antagonists
Antagonistic effect refers to the situation where the toxic effect of a toxin is reduced due to the presence or increase of another substance.
It can be seen from the figure that the toxic effect of one toxin decreases with the increase of low concentration of another substance, and after reaching a good state, the reaction rate decreases with further increase of antagonist concentration.
At present, research has found that K has an antagonistic effect on Na, reducing the toxic effects of Na salts on microorganisms. Potassium absorption and sodium excretion function
The main principle may be the Na+/K+reverse transport function. Although bacterial growth requires a high sodium environment, the concentration of Na inside the cell is not high. For example, the H+proton pump mediated by halophilic bacteria has the function of Na+/K+reverse transport, which has the ability to absorb and concentrate K+and discharge Na+to the outside of the cell K+, as a compatible solute, can regulate osmotic pressure to achieve balance inside and outside the cell, with a concentration of up to 7mol/L, to maintain the same water activity inside and outside. For example, halophilic anaerobic bacteria, halophilic sulfur reducing bacteria, and halophilic archaea use intracellular accumulation of high concentrations of K+to counteract the high osmotic environment outside the cell. For example, the Na+/K+reverse carrier in yeast can eliminate excess salt from the body and improve its salt tolerance
5.5 Choose appropriate processing technology
Different processing techniques affect the salt tolerance range of microorganisms. The following are the limits of NaCl concentration in several reported biological treatment methods
Sludge treatment
Activated sludge process
Biological filter
Self purification
Two-stage contact oxidation method
NaCI（mg/L）
5000~10000
8000~9000
10000~40000
ten thousand
25000~35000
It is generally believed that the salt tolerance of biofilm process is greater than that of suspended activated sludge process. In addition, adding a salt tolerant segment can greatly improve the salt tolerance range of subsequent aerobic segments.
Design requirements for biological treatment of high salt wastewater
6.1 Add a salinity control tank
Salinity changes have a significant impact on stable systems, manifested as a sharp decrease in treatment efficiency and significant loss of sludge. A regulating tank should be established during design to ensure relative stability of salinity. Conductivity monitoring devices can be installed at the inlet and outlet of the regulating pool to enhance online control and feedback of salinity, preventing salt shock from causing processing system failure.
6.2 Reduce sludge load
Salinity reduces the rate of biodegradation, therefore the design load should be relatively reduced. Many studies have shown that the sludge index decreases in high salt environments, so there is no need to worry about sludge expansion caused by low loads.
6.3 Increase sludge concentration
The high salt treated sludge has poor coagulation and severe sludge loss. Therefore, high sludge concentration should be ensured in the design. This is also a means to improve processing efficiency. When designing sludge concentration tanks, additional sludge storage can be ensured to quickly replenish sludge when it is lost.
6.4 Increase the retention time in the clarification tank
High salt content affects the coagulation properties, therefore an extended residence time is beneficial for the sedimentation of sludge.
6.5 Increase aeration rate
Microorganisms adapt to high salt environments by increasing aerobic respiration rates, resulting in additional oxygen consumption during respiration. Increasing the concentration of dissolved oxygen in water is beneficial for the metabolism of microorganisms. Provide physiological requirements for adapting to high salt environments.
