The unit structure of microorganism is cell, and the cell wall is equivalent to semi-permeable membrane. When the chloride concentration is less than or equal to 2000mg/L, the osmotic pressure that the cell wall can withstand is 0.5-1.0 atmosphere. Even if the cell wall and cytoplasmic membrane have certain toughness and elasticity, the osmotic pressure that the cell wall can withstand will not be greater than 5-6 atmosphere. However, when the chloride concentration in the aqueous solution is above 5000mg/L, the osmotic pressure will increase to about 10-30 atmospheres. Under such a high osmotic pressure, a large number of water molecules in the microbial body will penetrate into the in vitro solution, resulting in cell water loss and plasma wall separation, and in serious cases, the microbial death. Engineering experience data show that: when the chloride concentration in wastewater is greater than 2000mg/L, the microbial activity will be inhibited, COD removal rate will be significantly decreased; When the chloride concentration in the wastewater is greater than 8000mg/L, the sludge volume will expand, a lot of foam will appear on the surface, and the microorganisms will die one after another.