Why do high-concentration saline wastewater have a particularly large impact on microorganisms?
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Why do high-concentration saline wastewater have a particularly large impact on microorganisms? Let’s first describe an experiment on osmotic pressure: Two salt solutions of different concentrations are separated by a semipermeable membrane. Water molecules from the solution with the lower concentration will pass through the semipermeable membrane into the solution with the higher concentration, while water molecules from the solution with the higher concentration will also pass through the membrane into the solution with the lower concentration, but in smaller quantities. As a result, the liquid level on the side of the solution with the higher concentration rises. Osmosis stops when the pressure generated by the difference in liquid levels between the two sides becomes sufficient to prevent further water flow; this pressure is what is known as osmotic pressure.
Generally, the higher the salt concentration, the greater the osmotic pressure. The situation of microorganisms in saline solutions is similar to the osmotic pressure experiment. The basic structural unit of microorganisms is the cell; the cell wall functions as a semipermeable membrane. When the chloride ion concentration is 2000 mg/L or less, the osmotic pressure that the cell wall can withstand is 0.5–1.0 atmospheres. Even taking into account the certain toughness and elasticity of both the cell wall and the cytoplasmic membrane, the osmotic pressure that the cell wall can tolerate does not exceed 5–6 atmospheres. However, when the chloride ion concentration in the aqueous solution exceeds 5000 mg/L, the osmotic pressure rises to approximately 10–30 atmospheres. Under such high osmotic pressure, water molecules within the microorganisms penetrate into the external solution in large quantities, causing the cells to lose water and undergo plasmolysis; in severe cases, the microorganisms die. In daily life, people use table salt (sodium chloride) to pickle vegetables and fish in order to sterilize and preserve food, applying this very principle.
Engineering experience data shows that when the chloride ion concentration in wastewater exceeds 2000 mg/L, microbial activity is suppressed, and the COD removal efficiency drops significantly； When the chloride ion concentration in wastewater exceeds 8000 mg/L, it causes the sludge volume to expand, large amounts of foam to form on the surface, and the microorganisms die one after another. Through long-term domestication, microorganisms gradually adapt to growing and reproducing in high-concentration saline solutions. People have already domesticated microorganisms capable of adapting to chloride or sulfate concentrations of over 10,000 mg/L. However, the principles of osmotic pressure tell us that microorganisms adapted to growing and reproducing in high-concentration saltwater have cell fluids with a very high salt concentration. Once the salt concentration in the wastewater is low or very low, water molecules from the wastewater will flood into the microorganisms, causing their cells to swell; in severe cases, this leads to cell rupture and death. Therefore, microorganisms that have undergone long-term domestication and can gradually adapt to growing and reproducing in high-concentration saline solutions require a salt concentration in the biochemical feedwater to remain at a fairly high level; it cannot fluctuate greatly, otherwise a large number of these microorganisms will die.
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The explanation is very comprehensive; in simple terms, it’s sterilization using high salt levels
Very detailed explanation, thank you to the original poster