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Reasons and solutions for sludge floating in the secondary sedimentation tank

Author: Site Editor     Publish Time: 2023-10-16      Origin: Site

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Reasons and solutions for sludge floating in the secondary sedimentation tank

I believe that when working in a sewage disposal plant, one is often troubled by various problems in the secondary sedimentation tank. Among them, the phenomenon of sludge floating in the secondary sedimentation tank directly affects the discharge of effluent water quality. When the sludge expands, the SVI value will seriously exceed the standard, and then the SS value of the tank effluent will also increase significantly. At the same time, the COD, BOD, and TN values of the effluent will also exceed the standard. If immediate measures are not taken to control the loss of activated sludge, it is likely to lead to a sharp decrease in the number of microorganisms in the biological reaction tank, a decline in system performance, or even collapse.

Factors affecting the generation of floating sludge in the secondary settling tank: 

Under anoxic conditions, the activated sludge residence time increases, which will increase the possibility of nitrogen production from denitrification. When there is excess organic matter in the incoming water quality, the ammonia nitrogen content of the incoming water quality will increase, further increasing the possibility of denitrification. At the same time, insufficient dissolved oxygen, low pH value, long sludge age, etc. will also promote the occurrence of denitrification reaction. When the nitrification reaction occurs, the sludge is driven by N2 to accelerate the floating of the sludge. Therefore, the main factors affecting sludge floating are summarized as follows:

1) Amount of incoming water

When the incoming water volume suddenly increases, the hydraulic load on the surface of the secondary clarifier increases and the upward flow rate increases, which affects the normal settlement of activated sludge. A large incoming water impact load will destroy the stability of the sludge in the secondary sedimentation tank, affect the normal settlement of activated sludge, and accelerate the fluctuation of the mud layer in the secondary sedimentation tank.

2) Temperature

As the temperature rises, nitrogen in the  tank is produced along with denitrification, and the denitrification rate is related to the nitrate concentration, temperature, carbon source, and sludge concentration of the sewage.

3) pH value

In sewage disposal, if the pH value is too high or too low, it is not conducive to the survival of microorganisms and is harmful to the normal biological treatment effect. Especially when the pH value is lower than 5, it will have a greater impact on the sludge expansion in the tank, which will accelerate the floating of activated sludge in the settling tank, leading to the loss of activated sludge and directly affecting the quality of the effluent water.

4) Dissolved oxygen

The dissolved oxygen is high, the metabolism is fast, the sludge activity is good in the short term, and the organic matter is decomposed quickly. However, over time, dissolved oxygen will flow away with the water, and in the biological reaction aerobic pool, it will directly appear as large-area bubbles on the surface. When the dissolved oxygen is too low, the sludge in the settlement tank will be gray due to lack of oxygen; if it is deprived of oxygen for too long, it will be black, often accompanied by small bubbles, and the effluent water quality will be larger and yellowish in color. , the smell of sewage is strong, and there are few and small bubbles on the surface.

5) Residence time of sludge in the secondary settling tank

When the sludge concentration is high, the lower the dissolved oxygen (DO), the longer the residence time of the sludge in tank, and the higher the denitrification reaction rate in the tank. During the denitrification process The amount of nitrogen produced is large. At the outlet of the tank, as the water pressure decreases, the nitrogen saturation concentration decreases, and nitrogen is released, causing floating sludge in the tank.

In order to avoid floating mud, the following measures can be taken:

1) Ensure the relative stability of water volume

When designing a sewage treatment plant, a control well and a regulating tank are set up to control the opening angle of the valve so that the amount of water entering the sewage disposal systems remain relatively stable. By taking the above measures, the smooth operation of the system can be ensured, and major impact on the system can be avoided, thereby ensuring the stability of the entire system operation.

2) Control the temperature/pH within the appropriate range

Generally, the suitable temperature range for aerobic activated sludge is between 20 and 35°C. In cold northern areas, measures such as controlling DO, adjusting sludge concentration, sludge return volume, and sludge age need to be taken to prevent sludge expansion. The pH value at the sewage inlet is generally between 7 and 8, which is conducive to microbial processing of organic matter. If the pH value of the incoming water to the sewage wastewater treatment plant is lower than 5, the pH value should be adjusted by adding chemicals.

3) Increase dissolved oxygen concentration in incoming water

The main body of activated sludge is bacterial jelly. When the oxygen supply is insufficient, filamentous bacteria and fungi will multiply in large numbers, resulting in sludge expansion, flooding of the secondary sedimentation tank, and abnormal water effluent. Oxygen can inhibit most denitrifying bacteria themselves, and some components of these bacteria need to be synthesized under aerobic conditions. Increasing the dissolved oxygen concentration of incoming water is one of the effective measures to avoid floating sludge.

4) Maintain appropriate sludge concentration

When the sludge settling performance and stability become poor, sludge expansion will occur. With the loss of sludge, the sludge concentration in the tank decreases. When the sludge return rate remains unchanged, the sludge concentration in the biological reaction tank will also decrease. The degree of decrease depends on the loss of activated sludge and the settling performance of the sludge. The decrease in MLSS value will affect the metabolism of microorganisms in the biological reaction tank, and ultimately affect the treatment effect of the entire sewage treatment system.

Increasing the sludge return rate is one of the means to maintain activated sludge concentration. When the sludge return rate in thetank increases, the amount of water flowing into the sedimentation tank from the biological reaction tank will also increase, resulting in an increase in the surface load of the sedimentation tank, shortening the sludge settlement time, and making the sludge settlement more incomplete.

Based on practical experience, the external return variable frequency pump can be turned on to increase the external return sludge ratio. By controlling the opening time of the external return pump, the external sludge return ratio can be controlled to be around 100%; by controlling the opening time of the sludge pump in the primary sedimentation tank , control the amount of sludge discharged from the primary sedimentation tank; by controlling the opening time of the remaining sludge pump, the remaining sludge can be discharged in a timely manner.

Through the above measures, the sludge concentration and activity in the biological reaction tank are controlled well, and the sludge age is controlled within a suitable range.

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