The impact of chloride ions on bacteria in wastewater treatment and countermeasures

With the booming water treatment industry, technological innovation has brought about more diversified wastewater filtration and purification solutions. Wastewater biotreatment is one of them, but the negative effects of high concentrations of chloride ions must be guarded against. This article details the impact of chloride ions on bacteria in wastewater treatment and countermeasures.

Mechanism of Chloride Ion Impact

　　The toxic effect of high concentrations of chloride ions on wastewater biotreatment is mainly to destroy the cell membrane and enzymes in the microbial cells through increased environmental osmotic pressure, thereby disrupting the physiological activities of microorganisms.

　　Microorganisms grow well under isotonic pressure, such as microorganisms in a 5-8.5 g/L NaCl solution; under hypotonic pressure (p(NaCl)=0.1 g/L), a large amount of water molecules in the solution enter the microorganisms, causing the microbial cells to swell, and in severe cases, rupture, leading to microbial death; under hypertonic pressure, (p(NaCl)=200 g/L), a large amount of water molecules in the microorganisms permeate to the outside (i.e., dehydration), causing cell plasmolysis.

　　The unit structure of microorganisms is the cell, and the cell wall is equivalent to a semi-permeable membrane. When the chloride ion concentration is less than or equal to 2000 mg/L, the osmotic pressure that the cell wall can withstand is 0.5-1.0 atmospheres. Even if the cell wall and cell membrane have a certain toughness and elasticity, the osmotic pressure that the cell wall can withstand will not exceed 5-6 atmospheres.

　　However, when the chloride ion concentration in the aqueous solution is above 5000 mg/L, the osmotic pressure will increase to about 10-30 atmospheres. Under such high osmotic pressure, a large amount of water molecules in the microorganisms will permeate into the external solution, causing cell dehydration and plasmolysis, and in severe cases, microbial death. Engineering experience data shows that when the chloride ion concentration in wastewater is greater than 2000 mg/L, the activity of microorganisms will be inhibited, and the COD removal rate will decrease significantly; when the chloride ion concentration in wastewater is greater than 8000 mg/L, sludge volume expansion will occur, a large amount of foam will appear on the water surface, and microorganisms will die successively.

Manifestations of Inhibition of Sludge Activity

　　When the chloride ion concentration in the biochemical system changes drastically, the carbonization and nitrification performance of the sludge will quickly weaken or even disappear, leading to a significant decrease in COD removal rate and nitrite accumulation in the nitrification process. Even increasing the dissolved oxygen in the wastewater will not have a significant effect. In other words, activated sludge has a certain tolerance to chloride ion concentration. When the chloride ion concentration exceeds a certain value, the system degradation capacity decreases until the system loses its treatment capacity.

　　Sudden changes in chloride ions have a greater impact on the system than gradual changes. As the chloride ion concentration increases, the organic matter degradation rate decreases, so low F/M (the ratio of nutrients to activated sludge by mass) is more suitable for the treatment of wastewater containing chloride ions.

　　Chloride ions change the composition of microorganisms in the sludge, changing the sedimentation and effluent SS, resulting in serious sludge loss, decreased activated sludge concentration, increased sludge index, and decreased 30-minute settling rate.

　　From the microscopic examination results of activated sludge, at low salinity, the biophase is relatively rich, with a wide variety of filamentous bacteria, bacterial flocs, and protozoa, and the activated sludge particles are large, the bacterial flocs are closed, and the flocs have a certain degree of compactness. As the chloride ion concentration of the influent water increases, when the chloride ion suddenly changes from the original 150 mg/L to 1000 mg/L, filamentous bacteria and protozoa basically do not exist, and the bacterial flocs become more compact, and the flocs become smaller and abnormally compact at this time. The degradation of organic matter in wastewater is mainly completed by the combined action of a large number of microorganisms in the wastewater. The increase in chloride ions leads to a decrease in the number of microorganisms in the activated sludge, thereby reducing the organic matter degradation rate.

How to Eliminate the Impact of Chloride Ions?

　　1. Acclimation of Activated Sludge
　　By gradually increasing the chloride ion content of the biochemical influent, microorganisms will balance the osmotic pressure inside the cells or protect the protoplasm inside the cells through their own osmotic pressure regulation mechanism. These regulatory mechanisms include aggregating low-molecular-weight substances to form a new extracellular protective layer, regulating their own metabolic pathways, and changing gene composition, etc.
　　Therefore, normal activated sludge can treat high-chloride wastewater through a certain period of acclimation within a certain range of chloride ion concentration. Although acclimation can improve the chloride ion resistance range and treatment efficiency of the system, the chloride ion tolerance range of microorganisms in acclimated activated sludge is limited, and it is sensitive to environmental changes. When the chloride ion environment suddenly changes, the adaptability of microorganisms will immediately disappear. Acclimation is only a temporary physiological adjustment of microorganisms to adapt to the environment and does not have genetic characteristics. This sensitivity of adaptability is very unfavorable for wastewater treatment.
　　The acclimation time of activated sludge is generally 7-10 days. Acclimation can improve the tolerance of sludge microorganisms to salt concentration. In the early stage of acclimation, the concentration of activated sludge decreases because the increase in salt solution is toxic to microorganisms, causing some microorganisms to die, which is manifested as negative growth. In the later stage of acclimation, microorganisms that have adapted to the changed environment begin to reproduce, so the concentration of activated sludge increases. Taking the COD removal of activated sludge in 1.5% and 2.5% sodium chloride solutions as an example, the COD removal rates in the early and late stages of acclimation are: 60%, 80% and 40%, 60%, respectively.

　　2. Diluting Wastewater with High Chloride Ion Concentration
　　To reduce the chloride ion concentration entering the biochemical system, the influent can be diluted to make the chloride ions lower than the toxic value, and the biological treatment will not be inhibited. Its advantages are simple methods, easy operation and management; the disadvantages are increased treatment scale, infrastructure investment and operating costs. For wastewater treatment plants, due to the large influent volume and continuous operation, even if the online instrument measures a high chloride ion concentration at a certain time, the operability of targeted dilution is poor. Therefore, this method is more suitable for factories and enterprises that produce wastewater with high chloride ion concentration.

　　3. Selection of Reasonable Process Flow
　　Select different treatment processes for different chloride ion concentrations, and appropriately select anaerobic processes to reduce the range of chloride ion tolerance in the subsequent aerobic stage.

　　4. Increase the DO of the biological system
　　Properly increase the dissolved oxygen in the biological system to ensure the activity of activated sludge.

　　5. Discharge excess sludge
　　Increase the discharge of excess activated sludge to ensure that sludge growth is in the logarithmic growth phase, thereby improving pollutant removal efficiency.

　　6. Add nutrients
　　While increasing dissolved oxygen, the metabolism of sludge accelerates. To ensure sludge metabolism, sufficient nutrition should be ensured. If necessary, a certain amount of nutrients can be added to ensure sludge activity.