Sludge always likes to tinker with the secondary sedimentation tank in various ways. The more common ones are floating mud and turning mud.
The phenomenon of floating sludge is caused by denitrification of activated sludge in the secondary sedimentation tank, resulting in the sludge at the bottom of the tank floating up in blocks or the sludge particles in the tank floating up and overflowing with the effluent. The phenomenon of sludge turning is due to the poor settling performance of activated sludge. After entering the secondary sedimentation tank, the settling speed of the mud water mixture is too slow, which causes the sludge level in the tank to continue to rise. When there is a fluctuation in water volume, a large amount of activated sludge flows out from the effluent weir.
What are the adverse effects of dredging in the 01 secondary sedimentation tank?
Although it is not as common as the phenomenon of floating mud, there is indeed a phenomenon of secondary sedimentation tank turning mud during the operation of urban sewage treatment plants, especially in the cold winter and hot summer, where the secondary sedimentation tank turning mud is extremely frequent.
The specific effects of common sludge turning phenomena are as follows: 1. It causes a rapid decrease in the MLSS concentration of activated sludge in the biological reaction tank, which in turn destroys the treatment capacity of the biological reaction system, resulting in an increase in various effluent indicators of the secondary sedimentation tank, making it difficult to meet the inlet standards of subsequent deep treatment processes. 2. After turning the mud, the incoming water in the subsequent deep treatment section is mainly a mixture of mud and water, with COD, BOD5, NH3-N, TN, SS and other indicators severely exceeding the standard. The deep treatment process cannot treat it to the expected discharge standard, and a large amount of activated sludge can easily cause blockages in the deep treatment process, leading to more serious damage.
Why does the phenomenon of secondary sedimentation tank turning over occur?
1. Improper control of sludge return ratio and residual sludge discharge volume can lead to an increase in sludge accumulation at the bottom of the secondary sedimentation tank when the sludge return flow rate is too small, resulting in a continuous rise in sludge level and an increased risk of sludge overturning; When the sludge reflux rate is too large, although it can quickly reduce the sludge level in the secondary sedimentation tank, it can also cause an increase in the inflow and flow rate of the secondary sedimentation tank, and excessive impact load on the secondary sedimentation tank, which can also lead to sludge overturning. If the remaining sludge is not discharged in a timely manner, it will not only lead to excessive sludge age and severe aging in the entire biological treatment system; As the system runs, the mud level at the bottom of the secondary sedimentation tank will continue to rise. Once the mud level exceeds a certain warning line, if the amount of water entering the secondary sedimentation tank suddenly increases, the phenomenon of mud overturning will immediately occur. 2. Reduced sedimentation performance of activated sludge
The deterioration of the settling performance of activated sludge is caused by the following factors:
The water inlet load is too low. This can lead to a low BOD sludge load in the bioreactor tank, causing microorganisms in the activated sludge to undergo self oxidation due to a lack of nutrients, resulting in the loss of adsorption and coagulation of microbial flocs, causing sludge swelling and deteriorating its settling performance. The incoming water contains toxic substances. Microbial flocs will undergo varying degrees of disintegration, ultimately leading to the dissolution of activated sludge flocs and a decrease in sludge settling performance. The pH value of the incoming water is greater than 9 or less than 6. In this case, the settling ratio monitoring often shows slow settling of activated sludge, turbid supernatant, and even floating activated sludge flocs on the liquid surface. The aeration rate in the aerobic zone is too high. Excessive aeration can cause the disintegration or even self oxidation of activated sludge, making it difficult for the sludge to settle quickly after entering the secondary sedimentation tank. The concentration of sludge increases. To ensure normal operation in winter, sewage treatment plants often increase the concentration of activated sludge in advance, but as the sludge concentration increases, the settling performance of the sludge will rapidly decrease. Filamentous bacteria proliferate massively. Activated sludge will become excessively loose, with a decrease in density, and its settling performance will also deteriorate accordingly. The water temperature is too low. It will lead to the inhibition of the activity of microbial flocs and the deterioration of sludge settling performance; In addition, in low-temperature environments, microbial activity decreases, the time required to decompose organic matter increases, the time required to complete sedimentation and sludge water separation is prolonged, and the sedimentation rate of sludge in the secondary sedimentation tank is slow. 3. Unreasonable design and installation of secondary sedimentation tanks or mechanical failures are mainly reflected in the project construction and implementation stages, including excessive surface load design, uneven water distribution in the inlet holes, short flow in the inlet, improper installation of scraper suction machines, and uneven bottom of the secondary sedimentation tank affecting the scraping effect. Mechanical failure caused the sludge at the bottom of the secondary sedimentation tank to be unable to be removed, resulting in an increase in the sludge level in the tank. Eventually, the sludge flowed out through the effluent weir. The main mechanical failures include the shutdown of the scraper suction machine, damage to the scraper suction machine scraper, blockage of the suction hole, blockage of the sleeve valve, blockage of the sludge discharge pipe, and failure of the residual sludge pump.
4. Sudden increase in incoming water volume, impacted by fluctuations
A sudden increase in the inflow of sewage treatment plants can cause an increase in hydraulic load on the surface of the secondary sedimentation tank, increase the upward flow velocity, shorten the residence time of the mud water mixture in the secondary sedimentation tank, affect the normal sedimentation of activated sludge, and cause the sludge to flip over in time, resulting in a large amount of activated sludge overflowing from the effluent weir in the secondary sedimentation tank.
How to deal with mud overturning in the secondary sedimentation tank?
In fact, after clarifying the causes of the mud turning phenomenon, experienced operators can deduce effective solutions to deal with the mud turning in the secondary sedimentation tank based on the actual dominant factors, truly achieving targeted treatment.
For example, when the inlet water temperature is the main factor causing the phenomenon of mud turning, transparent cover plates can be installed at the top of the biological reaction tank and the secondary sedimentation tank. At the same time, insulation measures can be taken on the exterior walls of the tank that are higher than the ground to increase insulation and minimize the loss of heat in the sewage; For example, when the inflow load is too low, the concentration of activated sludge can be reduced to avoid the impact of low BOD sludge load on the settling performance of activated sludge. Of course, when dealing with the problem of sludge overturning in the secondary sedimentation tank, operators still need to clarify their countermeasures and eliminate unnecessary non dominant factors.
1. Clarify the response strategy, conduct inspections, monitoring, and verification of various design parameters of the secondary sedimentation tank to determine if they still meet the current operational requirements. Stop water and drain inspection, check whether the scraper plate at the bottom of the secondary sedimentation tank is damaged, and whether the scraper speed and outer edge line speed settings meet the current design specifications. Daily microscopic examination of microbial strains in each section of the bioreactor tank is conducted to determine whether the difficulty in settling activated sludge in the secondary sedimentation tank is caused by filamentous bacterial expansion. Continuous sampling and testing are conducted at the inlet of the sewage treatment plant to determine whether the sewage contains toxic and harmful substances. Continuously monitor pH and review recent recorded data to determine the normal range of pH values for sewage plant inflow throughout the year, and eliminate the influence of pH values. 2. Consider changing the current scraper to a suction machine
In actual operation, it has been found that every time the secondary sedimentation tank flips the mud, it is often the outer side of the outlet weir that flips the mud, while the inner side overflows with clean water. This is because the mud water interface of the secondary sedimentation tank section is parabolic and has a large curvature, and the mud level at the outer edge of the secondary sedimentation tank is significantly higher than that on the inner side when turning over the mud. Therefore, it is recommended to replace the current scraper with a suction machine if conditions permit, in order to reduce the mud level at the outer edge of the secondary sedimentation tank and make the mud level curve in the tank smooth, thereby reducing the risk of mud overturning.
3. Add fillers to the bioreactor tank
The organic combination of A2/O process and biofilm method has strong adaptability to changes in water quality and quantity, and has good sludge settling performance, making it easy for solid-liquid separation.
4. Add coagulant aid at the outlet of the biological reaction tank
This method can improve the settling performance of activated sludge entering the secondary sedimentation tank, treating the symptoms rather than the root cause, and has a high cost. It can only be used as an emergency plan. In summary, the poor settling performance of sludge is the fundamental reason for the sludge overturning in the secondary sedimentation tank. However, in the actual operation and management of sewage treatment plants, various factors such as sludge concentration, aeration rate, sludge reflux ratio, and residual sludge discharge are interrelated and affect each other, and the mechanism of phenomenon formation is extremely complex. Therefore, sewage treatment plants need to be tailored according to actual operating parameters and long-term operational experience.
