Analysis of the causes of failures in the activated sludge treatment system for papermaking wastewater plants

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In activated sludge treatment systems used in the papermaking industry, there are many operational abnormal points associated with various structures within common systems. Summarizing and analyzing these abnormal points is a key issue that needs to be addressed and mastered; this greatly helps in improving our system’s ability to make judgments. The following provides a detailed analysis of the common faults in the entire system, aiming to help front-line operators and managers gain a better understanding of system failures. 1. Causes of abnormal fluctuations in pH value Abnormal fluctuations in pH value are mainly related to the discharge of acidic and alkaline substances at the production site. In large papermaking companies, acidic and alkaline wastewater mainly arises from the regular cleaning of equipment. 2. Analysis of abnormalities in inflow volume and water quality: The inflow volume generally remains constant. However, in the event of an accidental discharge, production lines also generate large amounts of wash water during the cleaning of tanks, equipment, etc. In case of accidents, vast quantities of raw materials and chemicals are also discarded. When such wastewater enters wastewater treatment plants, it often has a significant impact on the system. This is also the reason why the COD levels in the effluent from primary sedimentation tanks and biological treatment tanks tend to be relatively high. Since most of the raw materials in the wastewater are fibers and amylose, particularly given that amylose has a relatively large molecular weight and poor physicochemical precipitability, the presence of various dispersants also affects the coagulation efficiency of PAC and PAM in the physicochemical treatment stage. These are the effects of the flow rate and quality of the incoming wastewater on the system. 3. Analysis of the impact of excessive sediment in the adjustment tank: The excessive sediment in the adjustment tank originates from unfiltered calcium carbonate particles and pulp fibers present in the daily water supply. Due to the central placement of the mixing device in the equalization tank, the centrifugal force generated by its mixing causes a large amount of sediment to accumulate in the four corners of the tank, which is the main reason for the excessive accumulation of sludge in it. Since no dedicated aeration device is installed in the well of the adjustment tank, the settled sludge undergoes hydrolysis and acidification in an anaerobic environment. This inadvertently performs a pre-treatment on the incoming wastewater; by adjusting the hydrolysis and acidification reactions in the sludge layer, the large molecular pulp fibers and amylose are hydrolyzed and acidified into small molecular, easily degradable organic substances, with a degradation rate of about 10% for these organic substances. By comparing the pH value of the wastewater entering the adjustment tank with that of the wastewater exiting it, it can be seen that the pH value of the wastewater leaving the adjustment tank is about 0.5 to 1.0 units lower than that of the wastewater entering the tank. This is strong evidence of the hydrolytic acidification reaction occurring in the tank due to sludge accumulation, and the extent of this hydrolytic acidification reaction can also be determined through this difference in pH value. Typically, this difference is significantly higher in summer than in winter. The effect of temperature on the hydrolytic acidification reaction is evident. 4. Analysis of the reasons for poor flocculation effects in the physical-chemical treatment zone. The poor flocculation effects in this zone are related to various factors, including excessive amounts of dispersants in the incoming wastewater, dosages of flocculants and coagulants that are not within the optimal range, the influence of pH value, issues related to mixing speed, the relationship between flow rate and load, the amount of suspended particles in the wastewater, and an excessive presence of substances that are difficult to flocculate. Regarding the above influencing factors, a detailed fault analysis will be conducted below. (1) Effect of dispersants: A large paper manufacturing company added dispersants in order to ensure even distribution of the pulp on the paper machine blanket and prevent clumping. Dispersants and flocculants are two types of chemicals that have opposite effects. The white water resulting from pressing and filtering in the paper machine is rich in dispersants; through the waste wastewater, this dispersant-rich wastewater flows into the wastewater treatment plant, thereby affecting the dosing area where flocculants are added for physical-chemical precipitation. Typically, after the addition of flocculants and coagulants, the suspended particles do not aggregate into larger flocs; what is seen are small particles that are difficult to aggregate. Such small particles are highly susceptible to disturbance by water currents, and their settling performance in the primary sedimentation tank is not ideal. Under high load conditions, they tend to escape from the primary sedimentation tank, thereby affecting subsequent biochemical treatment processes. (2) The dosing amounts of flocculants and coagulants are not within the optimal range. The dosage of flocculants and coagulants, as well as the ratio between the two, are extremely important; they are key to improving the settling efficiency of flocs and reducing the amount of flocculants and coagulants required. When insufficient amount of the coagulant PAC is added, the primary slow mixing tank fails to produce a fine mixture with clear interstitial water; in particular, the clarity of this interstitial water is key to determining on-site whether the addition of coagulants such as PAC meets the required standards. On the contrary, adding an excessive amount of flocculants and coagulants also fails to achieve the best flocculation effect. When an excessive amount of coagulant and flocculant is added, the flocs formed in the mixture are relatively large. However, the same problem persists: the interstitial water between these large flocs does not become clearer just because an excessive amount of chemicals has been added. Primarily, when an excessive amount of chemical is added, although large flocculent flocs are formed, these flocs break easily under the hydraulic mixing action in the slow-mixing tank. The broken flocs then have poor flocculation properties, and as a result, they become turbid particulate matter in the interstitial water. Looking at the composition of the entire chemical dosing area, it can be seen that there is a pH adjustment tank in front of the rapid mixing tank; only after the wastewater has had its pH adjusted can coagulants and flocculants be added to achieve the best results. After adding PAC or similar flocculants to the rapid mixing tank, rapid stirring does not affect the flocculation effect; on the contrary, it helps to distribute the added flocculants throughout the wastewater quickly, thereby laying the foundation for the formation of the first stage of flocs in the slow mixing tank. The purpose of slow stirring in the slow mixing tank is also to allow the formed flocs to coagulate and grow further, while ensuring that the flow shear force does not destroy these flocs. The coagulant added to this wastewater treatment plant is PAM (polyacrylamide), and the designed point for its addition is in the first slow mixing tank. At this time, the presence of the second slow mixing tank provides a site for further growth of the flocs as a result of the action of the coagulant. 5. Analysis of the causes of operational failures in the primary sedimentation tank: The operational failures of the primary sedimentation tank mainly relate to its role in carrying out the separation of sludge from water after physical and chemical flocculation of wastewater following the addition of chemicals. If this separation process is not effective, it will inevitably affect the proper functioning of the subsequent biochemical treatment system. The sludge-water separation efficiency in the primary sedimentation tank is poor; aside from potential issues arising from the construction design, this is mainly due to the inappropriate use of coagulants and flocculants added in the chemical treatment section. The result of insufficient flocculation due to inadequate dosing, and the result of excessive dosing leading to overly large flocs that break apart during sludge-water separation in the primary sedimentation tank, are the same. When the retention time in the primary sedimentation tank was sufficient to allow the flocs to settle completely, the effluent from this tank was clear and free of undissolved particles. Conversely, if this retention time was insufficient, it could lead to the flocs escaping from the tank, thereby affecting the subsequent biochemical treatment processes. Sludge discharge failures in primary sedimentation tanks are mostly caused by malfunctions in the sludge discharge equipment; common reasons include faults in the sludge pumps, delayed sludge discharge, and an insufficient increase in sludge discharge volume due to the presence of a large amount of suspended particles in the incoming wastewater. When checking whether there is excessive sludge accumulation in the primary sedimentation tank, the focus should be on whether the sludge scraper operates smoothly. If the scraper shakes or slips while moving, it is necessary to verify whether there is indeed an issue of excessive sludge accumulation in the primary sedimentation tank. 6. Analysis of common faults during the operation of biological filters. The faults that occur during the operation of biological filters are mainly due to the impact of fluctuations in water flow on the biofilm that has been formed. If the biological filter does not function optimally, it will have a significant impact on the subsequent activated sludge system. The common causes of failure in biological filters are analyzed as follows. (1) Reasons for biological filter failure caused by abnormal pH values: Similar to activated sludge, biofilms are more resistant to abnormal fluctuations in pH values than the activated sludge process, but they still suffer significant damage when exposed to wastewater with abnormal pH levels. As the microorganisms on the surface of the biofilm die and flake off due to the impact, the microorganisms inside are also affected by this impact and flake off as well, ultimately causing the entire biofilm to flake off. The flaked biofilm easily clogs the gaps in the filter media of biological filters, causing poor water flow and resulting in overflow. After biofilms peel off due to fluctuations in pH, they require a certain amount of time for automatic repair; usually, it takes about a week to recover once the effects of these pH fluctuations are eliminated. (2) Poor biofilm growth: Typical cases of poor biofilm growth are mainly when the biofilm grows too thick or too thin. Additionally, it is quite common for the thickness of biofilm growth to be uneven. In addition to being related to the organic matter content in the water fed into the biological filter, it is also related to whether there is an adequate supply of nutrients in that water. When the organic content in the inflowing water is too low, the biofilm becomes thin ； When the organic content of the influent water is too high, the biofilm grows rapidly, and excessively thick biofilms commonly form on the filter media. In actual operation, it was found that when the biofilm is too thick, its removal efficiency of organic matter is not necessarily higher than that of a thinner biofilm. The reason for this is that the efficiency of the biofilm in removing organic matter from wastewater depends on the effective contact area between the biofilm and the wastewater, rather than on the thickness of the biofilm. The deficiency of nutrients in wastewater can be understood in the same way as the demand for nutrients during the operation of the activated sludge process. To this end, nutrients need to be added at the inlet stage of the biological filter. Since the amount of nutrients to be added must take into account the requirements of microorganisms in biological filters and activated sludge processes, it is important to choose an appropriate value for the organic content in those nutrients. In the wastewater treatment plant of this large paper manufacturing company, the value of organic matter in the influent water used to calculate the amount of nutrients to be added before the biological tower is taken from the organic matter content in the effluent from the primary sedimentation tank. Such a calculation method inevitably results in unused nutrients in the biological filter; these nutrients then flow into the activated sludge system for reuse. On the surface, there seems to be no problem with adding such nutrients. However, in practice, it has been found that due to the excessive amount of nutrients present in the biological filter, and because the biological filter does not have a roof, large amounts of algae grow on the surface of the filter media. This also reduces the treatment efficiency of biological filters, typically by 10% in terms of removal efficiency. The reason for this is that the algae that grow there do not possess the ability to degrade organic substances in wastewater; they only require nutrients and sunlight as energy for growth and reproduction. There is not much understanding regarding the milky-white discoloration that occurs in biofilms; based on the quality of the incoming water and the overall operation of the system, it is generally believed that this is related to the swelling of filamental microorganisms within the biofilms. Microscopic observation reveals that the milky white mycelial aggregates are composed of numerous filamentous fungal cells. There are almost no protozoa around the filamental cells, which is also understandable in the context of biotic community analysis, as filamental cells do not provide food for protozoa like bacterial aggregates do, such as free bacteria and small bacterial aggregates. Since filamentous bacteria also possess the ability to purify water, it can be seen that when these bacteria multiply in large numbers within a biofilm, the overall efficiency of the biological filter in removing organic pollutants from wastewater does not decrease significantly; rather, there is only a slight reduction, typically around 10% lower than that of a normal biofilm. In terms of the morphological appearance observed under a microscope for such filiform teeth, there are filiform bacteria that exhibit characteristics similar to those seen during filamentous bacterial swelling in the activated sludge method, namely being long and thin, rigid, and not easily bendable ； There are also those with a soft structure, which are filamentous cells unique to biofilms. In short, the massive proliferation of filamentous microorganisms inevitably results in the formation of milky-white, scattered bacterial clusters on the biofilm in biological filters. Unlike zoogloea, which adheres to the filter media via the adsorption effect of the anaerobic layer, these milky-white filamentous microorganisms primarily rely on their dense surface structure to withstand the impact of wastewater, thereby remaining attached to the filter media without detaching. Although the degradation capacity of wastewater organic matter does not decrease significantly after biofilms produce milky-white microbial cells, it can be observed that such filamental cells readily lead to the outbreak of filamentous bulking in subsequent activated sludge systems. The main reason is that during normal operation, a certain amount of such white biofilm peels off and flows into the subsequent aeration tanks. In the initial stage, due to the inability to adapt to the new environment, the filamental mycelia struggle to survive when moving from the biofilm environment to the aeration tank; as a result, no outbreaks occur in the aeration tank. However, over time, some of these filamental microorganisms that break off and enter the aeration tank will eventually be able to adapt to the new environment, thus forming a dominant population within the aeration tank and affecting the overall performance of the activated sludge system. 7. Analysis of common fault causes during the operation of aeration tanks (1) Causes of scum formation on the surface of the aeration tank liquid When analyzing the scum that forms on the surface of the aeration tank liquid, we should be aware of a fundamental principle: the scum forms because its specific gravity is lower than that of the mixture in the aeration tank, which is why it floats on the surface. Therefore, when trying to determine why scum forms in the aeration tank, this aspect can be used as a starting point. From the perspective of practical operation, it is mainly due to bubbles mixed in the scum. As for the reason why bubbles form scum, it is merely the factor that causes the scum to float; the fundamental reason behind why bubbles are able to lift the scum is what needs to be determined. It can be considered that the formation of bubbles is inevitable due to aeration. Therefore, the main issue is whether activated sludge has the ability to adsorb and encapsulate bubbles. If such a capability is present, the bioflocs with trapped air bubbles will naturally float to the surface of the liquid; this is why we can observe scum on the surface of an aeration tank. The ability of activated sludge to adsorb bubbles is primarily determined by the activated sludge itself. If it secretes an excessive amount of its own viscous substances, the viscosity of the activated sludge will increase significantly, which in turn enhances its ability to adsorb small bubbles. Looking back, let’s analyze the causes of bubble formation. As mentioned earlier, aeration can generate a large number of small bubbles, which are then absorbed by the viscous activated sludge, ultimately leading to the formation of scum. Additionally, the bubbles are formed as a result of gases released when activated sludge decomposes organic matter; these gases include carbon dioxide and hydrogen. In practice, such bubbles tend to cause the sticky activated sludge to form flocculent sludge after adsorption. Similarly, if denitrification occurs in activated sludge due to an imbalanced carbon-to-nitrogen ratio, scum caused by bubbles will also be generated. Identifying what type of bubbles cause the scum on the liquid surface is essential for adjusting the process in operational management. The commonly used identification method here is the activated sludge settling ratio. After observing the settling ratio of the activated sludge, when examining the scum on the liquid surface in the cylinder, it becomes clear that there are bubbles within this scum. These bubbles originate from the aeration process. Although we cannot visually detect the presence of bubbles in the scum with the naked eye, if they can be observed under a microscope, it can be concluded that these bubbles are produced when the activated sludge decomposes organic matter on its own. The difference here is an indicator for comparing the size of the bubbles. Another aspect of comparison involves gently stirring the scum on the liquid surface; if this scum sinks again after being stirred, it can be assumed that the bubbles within it originate from the decomposition of organic matter by the activated sludge itself, or from the denitrification process occurring in the activated sludge. And when stirring the scum on the liquid surface still does not result in any noticeable sinking of the scum, it is generally assumed that the bubbles contained within this scum originate from the aeration of the activated sludge. This involves determining the origin of bubbles within the floating scum on the liquid surface, based on the phenomena that occur after agitation of the liquid surface by flotation. (2) The cause of the earthy odor in activated sludge: During inspections of the biochemical tank, a distinct earthy odor can be detected. This is mainly due to the unique smell produced by activated sludge during the decomposition of organic matter as well as its own reproduction and metabolism. In activated sludge, in addition to living bacteria, there are also dead bacteria, and the activated sludge thus possesses the characteristics of sludge. Due to aeration, various odors present in the activated sludge are also brought to the surface as a result of aeration, and this is the main reason for the earthy smell detected when inspecting the area around the aeration tank. Here, there is no denial of the earthy odor produced by activated sludge; on the contrary, such an earthy odor is necessary to confirm the proper functioning of the activated sludge. When no earthy odor can be detected in the biochemical tank or another odor is present, then there is a problem with the activated sludge system, and further investigation is required. The intensity of the fishy odor in activated sludge is related to temperature and the degree of reaction of the activated sludge. In summer, the earthy odor from the biochemical tanks increases due to higher temperatures, which is why this odor is more noticeable during the summer ； In winter, it’s the opposite. At the same time, the earthy odor of activated sludge is also related to the intensity of the reactions in the biochemical system; when the concentration of activated sludge is kept too high, the biochemical reactions intensify as well, resulting in a stronger earthy odor. In practice, it has been found that the intensity of the earthy odor can indicate whether the activated sludge system is operating in a good condition. Generally, during periods of high activated sludge load or when the activated sludge is in an aged state, it is difficult to detect a strong fishy odor from the activated sludge. This is very helpful for making a comprehensive assessment of the operating conditions of activated sludge. In addition to the earthy smell of activated sludge in the biochemical tank, other odors can also be detected, especially acidic or alkaline smells. The main reason for this is the inflow of wastewater with excessively high or low pH values into the biochemical system, which causes abnormal fluctuations in the pH value of the overall activated sludge mixture within that system. Under such circumstances, it is easy to detect an acidic or alkaline smell around the biochemical system, and it becomes very necessary to adjust the biochemical system; otherwise, it may cause unnecessary damage to it. (3) Foam problems in aeration tanks: The causes of foam problems in aeration tanks are diverse, and their analysis is relatively complex. However, certain patterns can be identified based on practical experience, and analysis can be conducted from the following aspects. 1) The scum in the aeration tank is to some extent derived from foam. As foam continues to accumulate, the scum will become thicker, and the subsequent problem is the development of anaerobic conditions and blackening within the scum. The dissipation of the decomposed scum and the formation of new scum determine the final thickness of the scum layer through these opposing processes; meanwhile, the scum that dissolves becomes the main cause of turbidity in the effluent from the secondary sedimentation tank, as well as the reason for the increased levels of organic matter in this effluent. Based on the above analysis, we should pay close attention to the formation of foam in the biochemical tank, in order to prevent it from contributing to the formation of scum on the surface of the tank’s liquid. 2) The formation of scum in the aeration tank is related to high viscosity of the activated sludge; similarly, it can be seen that foam formation is also associated with the viscosity of the activated sludge. 3) The color of the foam formed on the surface of the biochemical tank can also provide many clues to people. When brownish foam appears, it is possible to determine whether the foam is caused by the aging of activated sludge by considering the fragility of the foam and its accumulation rate. The previously discussed criteria can be used to determine whether activated sludge has aged, such as the sedimentation ratio of the activated sludge, SVI value, F/M ratio, and sludge age. Here we would like to point out that when activated sludge suffers from severe aging, brownish foam appears on the surface of the biological reactor. In addition to its brown color, this foam is characterized by being easy to break apart, prone to accumulating as scum, and having a low viscosity. In addition to the common brownish foam, another common type of foam is white foam. In addition to its white color, the main distinguishing features of white foam include its high viscosity, tendency to accumulate, and the fact that it does not produce sediment. The formation of such foam usually provides us with clear indicators of faults in the activated sludge system. Another highly practical indicator that needs to be highlighted here is that the formation of such white foam is associated with sudden high shock loads on the activated sludge system. In terms of the mechanism, it can be said that wastewater with high organic concentrations can also produce foam with a high accumulation capacity when there is sufficient aeration, just as large amounts of foam tend to accumulate in areas with water jumps in discharge tanks. It is therefore conceivable that the organic matter concentration in the effluent (taking COD as an example) usually does not exceed 100 mg/L, and foam can accumulate to a certain extent under the effect of water jumps. In contrast, the organic matter content in the wastewater entering the biological treatment tank (also using COD as an example) is typically higher than 500 mg/L; such wastewater with a high load and high impact tends to cause foam accumulation once it enters the biological treatment tank due to aeration. In summary, when activated sludge is subjected to high-volume, high-load wastewater streams, it produces large amounts of white, sticky foam; the surface of this foam does not contain any brownish sludge particles. The absence of such brownish sludge means that the activated sludge does not enter an aged state as a result of the high load, and therefore there is no decomposed activated sludge attached to the white, sticky foam. On the contrary, the microorganisms in the activated sludge are in a logarithmic growth phase, resulting in very high activity, and no free microbial flocs form to adhere to the surface of the white, sticky foam. 4) Regarding the foam problems caused by sludge aging and shock loads, which were fully discussed in the previous point, it is also necessary to distinguish them from some special cases. The main distinctions that need to be made are between the foam generated by the flow of detergent, and the foam produced after the breakdown of activated sludge – these two specific types of foam. When detergent enters the biochemical system, we observe white foam that is sticky, although its stickiness is not as strong as that of the white foam produced when the load is too high. Furthermore, the foam caused by detergents and surface dispersants takes on a slight color when exposed to sunlight. This is because such detergents, surface dispersants, and surfactants are mostly derived from petroleum and contain oily components, which result in colored reflections in the foam under sunlight. To further determine whether the foam is caused by detergents, surface dispersants, or surfactants, verification can be carried out in the physical-chemical zone at the upstream end of the biochemical tank; the focus is on checking whether foam is generated at the outlet weir of the primary sedimentation tank. Usually, due to high loads and an excessive amount of organic matter in the wastewater, as long as the water jump is not too pronounced, foam generally does not accumulate. However, the foam caused by detergents, surface dispersants, surfactants, etc., generates a large amount of foam even under slight hydraulic gradients; sometimes foam also forms at the axis of the mixer in the chemical dosing area. This is different from the foam that results from the high organic content in wastewater. The foam caused by activated sludge poisoning can be distinguished from the foam resulting from activated sludge aging based on its color. After poisoning, the activated sludge disintegrates rapidly and is easily pushed to the surface by aeration to form foam. However, this foam has a dull color, mostly gray, and does not contain the active sludge that remains attached to the foam, as is the case with the foam formed when activated sludge ages and still retains a vivid appearance. Of course, the best method for auxiliary diagnosis is to combine microscopic observation of protozoa, which makes confirmation easier. 8. Analysis of common fault causes during the operation of the secondary sedimentation tank. Most faults in the secondary sedimentation tank are caused by poor operation of the aeration tank, as the aeration tank and the secondary sedimentation tank are closely connected. As the area where sludge is separated from the activated sludge in the biochemical system, the proper functioning of the secondary sedimentation tank directly affects the quality of the water discharged after sludge separation. Therefore, it is necessary to analyze the causes of common faults. (1) Scum on the surface of the secondary sedimentation tank: The secondary sedimentation tank itself produces little scum; it mainly comes from the aeration tank. Because the scum generated in the aeration tank tends to enter the secondary sedimentation tank and float to the surface there, and since the secondary sedimentation tank does not have mixing capabilities, this makes it easier for scum to form. Of course, compared to aeration tanks. Denitrification of activated sludge occurs more easily in the secondary sedimentation tank, ultimately leading to a large amount of activated sludge floating to the surface and forming scum. This is because denitrification requires relatively anaerobic or hypoxic conditions, whereas such conditions do not exist in the aeration tank. This is why denitrification of active sludge occurs easily in the secondary sedimentation tank, especially when the dissolved oxygen at the outlet of the aeration tank is too low and the carbon-nitrogen ratio is severely imbalanced. (2) Sludge floating is observed in the effluent from the secondary sedimentation tank. The formation of this sludge is mainly related to the aging of the activated sludge; as the aged activated sludge breaks down, fine flocs remain suspended in the water, and if they do not settle in time, they flow out of the secondary sedimentation tank and become the sludge floating there. Of course, in many cases, sludge suspension is the result of multiple factors, such as excessive hydraulic load, which prevents the activated sludge flocs in the mixture from settling before being discharged from the tank under normal conditions. (3) Rising of activated sludge flocs in the secondary sedimentation tank: When conducting inspections of the secondary sedimentation tank, we usually check the water conditions near the inlet of the tank, as that is where the first signs of proper or improper sedimentation can be observed. If a large number of active sludge flocs are observed rising at the water inlet, the problem is usually quite serious. If these aggregated activated sludge flocs fail to settle in the secondary sedimentation tank in time, they will inevitably flow out of it. This is fatal for the entire activated sludge system, as once such aggregates exit the secondary sedimentation tank, the number of microorganisms in the aeration tank drops sharply; as a result, it takes a long time for the aeration tank to recover, and the quality of the effluent discharged will definitely exceed acceptable standards. So what causes the activated sludge flocs to rise in groups in the secondary sedimentation tank? In practice, it has been found that such situations are closely related to filamentous bacterial bulking in activated sludge, which is also why filamentous bacterial bulking poses such a significant problem for us. Therefore, after filamentous bacterial bulking occurs in the activated sludge, if it is then subjected to a hydraulic load shock, it is very likely that the clusters of activated sludge flocs will rise. (4) The excessive growth of moss at the outlet weir of the secondary sedimentation tank is mainly due to the high concentration of nutrients in the water exiting the tank, which allows algae to grow in large quantities in areas with light. By combining the detection of nutrients in the effluent from the secondary sedimentation tank, it is possible to determine whether algae growth is caused by an excess of residual nutrients. 9 Analysis of operational faults in the three-set sedimentation tanks: In this large paper manufacturing plant, there are generally not many faults with the three-set sedimentation tanks, as their role is to provide an additional layer of assurance for the sedimentation process in the second set of sedimentation tanks. But the problem mainly lies in the physicochemical reaction zone at the front end of the three sedimentation tanks, where some operators are prone to making mistakes. When operating in the physical-chemical reaction section of this area, the main objective is to enhance the micro-sediment particles entrained in the secondary sedimentation effluent, so that they can be removed more effectively in the tertiary sedimentation tank through the addition of chemicals. However, problems often arise with dosing, as the chemicals used are exactly the same as those in the physicochemical stage before the primary sedimentation tank. This leads to a problem: since the suspended particles in the wastewater treated in the physical-chemical treatment stage before the primary sedimentation tank are mostly similar to clay particles and carry a negative charge, the particles formed after adding PAC in that treatment area are very small; it is only by adding the coagulant PAM (which also has a negative charge) that the flocs become larger in size. The reason for this is that PAC acts as the framework for floc formation; through its flocculating effect, it aggregates the particulate matter in wastewater, allowing a large number of negatively charged colloidal particles or particles with clay-like properties to be adsorbed onto the polymer groups of PAC. However, the disadvantage of PAC is that it cannot continue to adsorb other flocs onto those that have already formed, thereby increasing the volume of the floc mass. As a result, the flocs formed after adding PAC are usually small in size. In such cases, to determine whether the addition of PAC is appropriate, it is necessary to check whether there are clear water gaps between the flocs in the overall mixture – in other words, whether there are distinct inter-particle water zones. Such observations are very helpful in assessing the effectiveness of adding PAC. There is also theoretical support for adding PAM after PAC to enhance the enlargement of the coagulated flocs. PAM is an organic polymer coagulant with a very high molecular weight; upon hydrolysis, it generates a large number of flocculation monomers, giving it a strong ability to adsorb and capture particulate matter in water. It is usually used in combination with PAC, and the negative-type PAM refers to the monomers resulting from the ionization and hydrolysis of PAM (which carry a negative charge). As mentioned earlier, after the positively charged groups from PAC are adsorbed, the groups contained within a single PAC chain will almost all be covered with negatively charged substances similar to clay particles. When highly negatively charged PAM is added, the framework formed around the PAM can adsorb a large number of PAC chains. Once a PAM core group has adsorbed a large amount of PAC chains, large floc particles can be formed. Here we can clearly see that the particulate matter in the negatively charged water is adsorbed by the positively charged PAC added, forming long-chain flocs; then, the negatively charged PAM coagulant is added, which further adsorbs the positively charged PAC onto these long-chain flocs, thereby enhancing flocculation. This is the general process by which large flocs are formed. To ensure the quality of the water leaving the third sedimentation tank, it is necessary to precipitate and remove the activated sludge flocs flowing out of the second sedimentation tank by adding coagulants. We found that adding a small amount of coagulant simply does not have any flocculating effect. The reason for this is that the flocs emerging from the secondary sedimentation tank are mostly disintegrated activated sludge. As for activated sludge, it consists of floc particles carrying a strong negative charge. However, once the activated sludge disintegrates for various reasons, it becomes quite difficult to re-agglomerate these disintegrated flocs, as without the adhesive substances inherent in the microorganisms present in the activated sludge, the negatively charged floc particles cannot aggregate effectively into larger flocs. This is particularly evident in the flocculation-breaking activated sludge particles that are carried away by the water flow in the effluent from the secondary sedimentation tank. Therefore, insufficient addition of PAC in the physical-chemical zone at the front section of the three-settling tanks will make it difficult to achieve good flocculation effects. Many operators, upon seeing this situation, assume that the amount of PAM added is insufficient; as a result, they increase the dosage of PAM. This leads to an abundance of negatively charged PAM molecules, which cannot coagulate with the negatively charged flocs present in the activated sludge responsible for flocculation. As a consequence, not only are no flocculated flocs formed, but the particles in the activated sludge become even more stable, resulting in absolutely no flocculation effect. To solve this problem, it is necessary to increase the dosage of PAC significantly, using PAC as a framework for flocs. Since the amount of flocculated particles that emerge from the secondary sedimentation tank is sometimes not large, when there are few such particles in the water body, there are fewer opportunities for them to collide with each other and form flocs. These scattered flocculated particles can only have their flocculation improved by increasing the dosage of PAC, which is used as a flocculating framework. Practice has also shown that determining the electrical properties of the sludge in the secondary sedimentation tank is very helpful in making choices regarding the addition of flocculants and coagulants, as well as in determining the appropriate dosage.