Let us first describe an osmotic pressure experiment: use a semi-permeable membrane to separate two salt solutions of different concentrations. The water molecules of the low-concentration salt solution will pass through the semi-permeable membrane into the high-concentration salt solution, and the water molecules of the high-concentration salt solution will also pass through the semi-permeable membrane into the low-concentration salt solution, but the number is smaller, so the liquid level on the high-concentration salt solution side will rise. When the height difference of the liquid levels on both sides produces enough pressure to prevent the water from flowing again, the osmosis will stop. At this time, the pressure generated by the height difference of the liquid levels on both sides is the osmotic pressure. Generally speaking, the higher the salt concentration, the greater the osmotic pressure.
The situation of microorganisms in salt water solutions is similar to the osmotic pressure experiment. The unit structure of microorganisms is cells, and the cell wall is equivalent to a semi-permeable membrane. When the chloride ion concentration is less than or equal to 2000mg/L, the osmotic pressure that the cell wall can withstand is 0.5-1.0 atmospheres. Even if the cell wall and cytoplasmic membrane have a certain toughness and elasticity, the osmotic pressure that the cell wall can withstand will not be greater than 5-6 atmospheres. However, when the chloride ion 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 amount of water molecules in the microorganism will penetrate into the extracorporeal solution, causing cell dehydration and plasmolysis, and in severe cases, the microorganism will die. In daily life, people use salt (sodium chloride) to pickle vegetables and fish, sterilize and preserve food, which is the application of this principle.
Engineering experience data show that when the chloride ion concentration in wastewater is greater than 2000mg/L, the activity of microorganisms will be inhibited and the COD removal rate will drop significantly; when the chloride ion concentration in wastewater is greater than 8000mg/L, it will cause the sludge volume to expand, a large amount of foam will appear on the water surface, and the microorganisms will die one after another.
However, after long-term domestication, microorganisms will gradually adapt to growing and reproducing in high-concentration salt water. At present, some people have domesticated microorganisms that can adapt to chloride ion or sulfate concentrations above 10000mg/L. However, the principle of osmotic pressure tells us that the salt concentration of the cell fluid of microorganisms that have adapted to growing and reproducing in high-concentration salt water is very high. Once the salt concentration in the wastewater is low or very low, a large number of water molecules in the wastewater will penetrate into the microorganisms, causing the microbial cells to swell, and in severe cases, rupture and die. Therefore, microorganisms that have been domesticated for a long time and can gradually adapt to growing and reproducing in high-concentration salt water require that the salt concentration in the biochemical influent always be kept at a fairly high level, and cannot fluctuate, otherwise the microorganisms will die in large numbers.
Post time: Feb-28-2025