Effects of Chloride Ions on Bacteria in Wastewater Treatment and Countermeasures

Microorganisms grow well under isotonic pressure, such as in NaCl solution with a mass of 5-8.5g/L. Under low osmotic pressure (p(NaCl)=0.1g/L), a large amount of water molecules in the solution penetrate into the microorganism, causing the microbial cells to swell and rupture in severe cases, leading to the death of the microorganism. Under high osmotic pressure (p(NaCl)=200g/L), a large amount of water molecules in the microorganism penetrate into the body (i.e., dehydration), causing the cell to undergo 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 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 microorganisms will penetrate into the solution outside the body, causing cell dehydration and plasmolysis, and in severe cases, the microorganisms will die. Engineering experience data show that when the chloride ion concentration in the 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 the 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.

Manifestations of inhibiting sludge activity

When the chloride ion concentration in the biochemical system changes drastically, the carbonization and nitrification properties of the sludge will quickly weaken or even disappear, resulting in a significant decrease in COD removal rate and accumulation of nitrite in the nitrification process. Even if the dissolved oxygen in the sewage is increased, the effect is not obvious. In other words, activated sludge has a certain tolerance to chloride ion concentration. When the chloride ion concentration exceeds a certain value, the degradation capacity of the system decreases until the system loses its processing capacity.

Sudden changes in chloride ions interfere more with the system than gradual changes in chloride ions. As chloride ions increase, the degradation rate of organic matter decreases, so low F/M (the ratio of nutrients to activated sludge in mass) is more suitable for the treatment of chloride ion-containing wastewater.

Chloride ions change the composition of microorganisms in sludge, change the sedimentation of sludge and effluent SS, resulting in serious sludge loss, a decrease in the concentration of activated sludge, an increase in the sludge index, and a decrease in the 30-minute sedimentation rate.

According to the results of the microscopic examination of activated sludge, the biological phase is relatively rich at low salinity, with a wide variety of filamentous bacteria, flocs, and protozoa. The activated sludge particles are large, the flocs are closed, and the flocs have a certain degree of compactness. With the increase of the chloride ion concentration of the incoming water, when the chloride ion suddenly increases from the original 150mg/L to 1000mg/L, the filamentous bacteria and protozoa basically do not exist, and the flocs become more dense. At this time, the flocs become small and abnormally compact. The degradation of organic matter in sewage is mainly completed by the joint action of a large number of microorganisms in sewage. The increase of chloride ions leads to a decrease in the number of microorganisms in the activated sludge, thereby reducing the degradation rate of organic matter.

How much chloride ion content in the sewage biochemical treatment system will affect microorganisms

(1) As the salinity increases, the growth of activated sludge is affected. The changes in its growth curve are manifested in: the adaptation period becomes longer; the growth rate of the logarithmic growth period becomes slower; the duration of the deceleration growth period becomes longer;

(2) Salinity enhances the respiration of microorganisms and the lysis of cells;

(3) Salinity reduces the biodegradability and degradability of organic matter. The removal rate and degradation rate of organic matter decrease. Although extending the aeration time can improve the removal efficiency of organic matter, after a certain time, the increase in the removal rate of organic matter with the increase of aeration time is slow. From an economic perspective, the method of increasing the removal rate of high-salt organic matter by extending the aeration time is not advisable;

(4) Inorganic salts enhance the sedimentation of activated sludge. As the salinity increases, the sludge index decreases;

 (5) Treating high-salt sewage, acclimating activated sludge is a necessary means for the success of the treatment system. The acclimation process of activated sludge is the process of gradually adapting the metabolism of microorganisms to the high-salt environment and allowing salt-tolerant bacteria to multiply in large numbers.

How to eliminate the influence of chloride ions?

01 Domestication of activated sludge

By gradually increasing the chloride ion content of biochemical influent, microorganisms will balance the intracellular osmotic pressure or protect the protoplasm in the cells through their own osmotic pressure regulation mechanisms. These regulation mechanisms include aggregating low molecular weight substances to form a new extracellular protective layer, regulating their own metabolic pathways, changing genetic composition, etc.

Therefore, normal activated sludge can treat high chloride ion wastewater within a certain chloride ion range through a certain period of domestication. Although activated sludge can improve the system's chloride ion tolerance range and improve the system's treatment efficiency through domestication, the microorganisms in the domesticated activated sludge have a limited tolerance range for chloride ions and are sensitive to environmental changes. When the chloride ion environment changes suddenly, the adaptability of microorganisms will disappear immediately. Domestication is only a temporary physiological adjustment for microorganisms to adapt to the environment and does not have genetic characteristics. This adaptive sensitivity is very unfavorable for sewage treatment.

The acclimation time of activated sludge is generally 7-10 days. Acclimation can improve the tolerance of sludge microorganisms to salt concentration. The concentration of activated sludge decreases in the early stage of acclimation because the increase of salt solution is toxic to microorganisms, causing some microorganisms to die, showing negative growth. In the later stage of acclimation, microorganisms that have adapted to the environment begin to reproduce, so the concentration of activated sludge increases. Taking the removal of COD by 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.

02  Dilute wastewater with high chloride ion concentration

In order to reduce the concentration of chloride ions entering the biochemical system, the influent can be diluted so that the chloride ions are lower than the toxic threshold value, and the biological treatment will not be inhibited. Its advantages are simple methods, easy operation and management; its disadvantages are increased treatment scale, infrastructure investment and operating costs. For Yangli Sewage Treatment Plant, due to the large influent volume and continuous operation, even if the chloride ion concentration is high at a certain time measured by online instruments, 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.

03  Choose a reasonable process flow

Choose different treatment processes for different concentrations of chloride ion content, and appropriately choose anaerobic process flows to reduce the range of tolerance of chloride ion concentration in the subsequent aerobic stage.

04  Increase the DO of the biochemical system and appropriately increase the dissolved oxygen in the biochemical system to ensure the activity of the activated sludge.

05  Discharge of residual sludge

Increase the discharge of residual activated sludge to ensure that the sludge grows in the logarithmic growth period to improve the removal efficiency of pollutants.

06  Add nutrient source

 While increasing dissolved oxygen, the metabolism of sludge is accelerated. In order to ensure the metabolism of sludge, sufficient nutrition should be ensured. If necessary, a certain nutrient source can be appropriately added to ensure the activity of sludge.