In the complex system of wastewater treatment, the anoxic tank plays a crucial role in nitrogen removal, serving as a key component of the biological treatment process. However, many wastewater treatment plants encounter the issue of floating sludge in the anoxic tank during operation. These floating sludges not only impair treatment efficiency but may also trigger a series of chain reactions, causing significant headaches for operators. Today, we will delve into the causes and hazards of anoxic tank sludge floating, along with targeted solutions to help safeguard stable wastewater treatment operations. **I. The Core Mission and Normal State of the Anoxic Tank** To understand the sludge floating issue, it is essential to first grasp the role of the anoxic tank in the wastewater treatment process. The anoxic tank is the core unit of the biological nitrogen removal process. Under anoxic conditions, denitrifying bacteria utilize organic matter in the wastewater as a carbon source to convert nitrate nitrogen into nitrogen gas, thereby achieving nitrogen removal and reducing the total nitrogen concentration in the effluent to prevent water eutrophication. Under normal circumstances, the sludge in the anoxic tank should remain suspended or settled, thoroughly mixing with the wastewater, allowing microorganisms to carry out efficient metabolic activities. At this point, the tank surface remains clear, with no noticeable sludge accumulation, and the effluent quality consistently meets standards. However, when sludge floating occurs, it indicates a disruption in system balance. **II. Frequent Sludge Floating: Eight Key Causes** (1) Denitrification Sludge Floating: The "Trouble" of Nitrogen Gas
Denitrification sludge is one of the most common causes of floating sludge in anoxic tanks.
During the denitrification process, denitrifying bacteria produce a significant amount of nitrogen gas. If the nitrogen gas cannot be promptly released from the sludge, it will adhere to the surface of sludge particles, forming tiny bubbles that reduce the sludge's density, ultimately causing it to float to the water surface. This situation typically occurs when the denitrification reaction is overly intense. For example, when the influent has ample carbon sources and high nitrate nitrogen concentrations, denitrifying bacteria proliferate rapidly, generating large quantities of nitrogen gas in a short time. Additionally, poor sludge settling performance makes it easier for nitrogen gas to carry sludge to the surface.
(2) Sludge bulking: "Rampant growth" of filamentous bacteria Sludge bulking is another critical factor causing floating sludge. When environmental conditions in the anoxic tank change—such as abnormal pH, nutrient imbalance, excessively low or high dissolved oxygen—filamentous bacteria proliferate and become dominant. The growth of these bacteria loosens the sludge structure, impairing its settling performance and ultimately leading to sludge flotation. Sludge bulking is categorized into filamentous bulking and non-filamentous bulking. Filamentous bulking is more common, involving excessive growth of filamentous bacteria like Zoogloea ramigera and Thiothrix. This leads to increased sludge volume, reduced density, and poor settling. Non-filamentous bulking, on the other hand, is often caused by abnormal increases in bound water within the sludge, resulting in heightened viscosity and difficulty in settling.
(3) Mixer operational failure: The "consequences" of uneven mixing The mixer plays a crucial role in the anoxic tank by ensuring thorough mixing of sludge and wastewater, facilitating microbial contact with organic matter and nitrate nitrogen, and promoting denitrification reactions. If the mixer malfunctions—such as insufficient mixing intensity, uneven mixing, or equipment damage—it can lead to sludge deposition at the tank bottom, localized oxygen deficiency, and anaerobic reactions producing gases like methane, which carry sludge to the surface. Additionally, mixer failure prevents sludge from remaining uniformly suspended, leaving some sludge in a stagnant state for extended periods. This reduces microbial activity, ultimately causing sludge death and disintegration, resulting in floating sludge.
(4) Sludge Aging: A "Signal" of Activity Decline Sludge aging refers to the reduced microbial activity and metabolic capacity in sludge, leading to deteriorated settling and dewatering performance. When sludge remains in the anoxic tank for an excessively long period or experiences excessively low sludge loading, microorganisms may enter a decline phase due to nutrient deficiency, resulting in sludge aging. Aged sludge has a loose structure, broken flocculants, and fine sludge particles that are easily carried away by water flow or attached to bubbles and float to the surface. Additionally, as microorganisms in aged sludge die, they release intracellular substances, increasing sludge viscosity and further exacerbating floating sludge issues.
(5) Inadequate pre-treatment: The invasion of "impurities" is the first hurdle in sewage treatment. If pre-treatment is not done well, a large amount of suspended solids, oils, difficult to degrade organic matter and other impurities will enter the anoxic tank. These impurities will adhere to the surface of the sludge, changing its properties and reducing its settling performance. For example, the oil and fat in the inflow will wrap around the surface of the sludge particles, forming an oil film that hinders the contact between the sludge and sewage, and affects the metabolic activity of microorganisms. At the same time, oil and fat can also reduce the density of sludge and make it easy to float up. In addition, a large amount of suspended solids can increase the concentration of sludge, leading to excessive sludge load and causing sludge swelling and floating.
(6) Difficult to treat wastewater: the "nightmare" of microorganisms. When anaerobic tanks treat difficult to treat wastewater, such as chemical wastewater, pharmaceutical wastewater, printing and dyeing wastewater, due to the large amount of toxic and harmful substances and difficult to degrade organic matter in the wastewater, it will have an inhibitory effect on microorganisms, leading to a decrease in microbial activity, deterioration of sludge settling performance, and ultimately forming floating sludge. The water quality of difficult to treat wastewater is complex, with variable components and significant fluctuations in parameters such as pH, temperature, and salinity, which can disrupt the stable environment of anoxic tanks and affect the normal growth and metabolism of microorganisms. In addition, the presence of recalcitrant organic matter can reduce the degradation efficiency of microorganisms, leading to the accumulation of organic matter in sludge and resulting in sludge swelling and floating.
(7) Excessive inflow SS: Overloaded sludge with excessive suspended solids (SS) can impose a heavy burden on the anoxic tank. A large amount of suspended solids will mix with sludge, resulting in high sludge concentration and increased sludge load. When the sludge load exceeds the microbial processing capacity, the metabolic activity of microorganisms will be inhibited, the settling performance of sludge will decrease, and it is easy to form floating sludge. Meanwhile, excessive suspended solids will adsorb onto the surface of the sludge, altering its structure and properties, increasing its viscosity and making it less prone to settling. In addition, organic matter in suspended solids can be decomposed by microorganisms, consuming a large amount of dissolved oxygen and nutrients, leading to deterioration of environmental conditions in anoxic tanks and further exacerbating the problem of floating mud.
3, Floating mud poses significant hazards, and it cannot be ignored that the floating mud in anoxic tanks not only affects the appearance of sewage treatment, but also seriously endangers the treatment effect and stable operation of the system.
(1) Reducing the treatment efficiency of floating sludge will decrease the contact area between sludge and sewage, and microorganisms will not be able to fully utilize the organic matter and nitrate nitrogen in sewage, resulting in a decrease in denitrification reaction efficiency and an increase in total nitrogen concentration in the effluent. Meanwhile, the microbial activity and metabolic capacity in the floating mud decrease, which can also affect the removal efficiency of pollutants such as COD and BOD.
(2) If the sludge floating on the blocked equipment and pipelines cannot be cleaned up in a timely manner, it will enter the subsequent treatment units such as sedimentation tanks, filtration tanks, etc. with the water flow, blocking the equipment and pipelines and affecting the normal operation of the treatment system. For example, if the sludge clogs the discharge pipe of the sedimentation tank, it will result in the inability to discharge the sludge in a timely manner, reduce the effective volume of the sedimentation tank, and decrease the sedimentation effect.
(3) To address the issue of floating sludge, it is necessary to increase the dosage of chemicals, enhance mixing intensity, and increase the frequency of sludge discharge, all of which will increase the operating costs of sewage treatment. In addition, floating mud can also lead to increased equipment wear, shorten equipment service life, and increase equipment maintenance and replacement costs. (
4) The organic matter and microorganisms in the floating mud that affect the effluent quality will be discharged into the natural water body with the effluent, causing secondary pollution to the water body. If the effluent quality does not meet the standard, it will not only be punished by the environmental protection department, but also pose a threat to the surrounding ecological environment and residents' health. 4, Targeting the root cause of the floating mud in the anoxic pool, we can take corresponding measures to restore the stable operation of the system.
(1) Control denitrification reaction, alleviate nitrogen floating sludge, adjust carbon source dosage: Based on the influent nitrate nitrogen concentration and denitrification reaction requirements, reasonably control the carbon source dosage to avoid excessive carbon source causing excessive denitrification reaction. The carbon source dosage can be adjusted in real-time by monitoring the inlet and outlet water quality online.
Optimize operating parameters: appropriately reduce the dissolved oxygen concentration in the anoxic tank, controlling it between 0.2-0.5mg/L, to provide a suitable anoxic environment for denitrifying bacteria. At the same time, adjust the sludge reflux ratio and mixed liquid reflux ratio to ensure sufficient mixing of sludge and nitrate nitrogen. Increase the frequency of sludge discharge: timely discharge of aged sludge and floating sludge, reduce the residence time of sludge in the anoxic tank, and lower the risk of nitrogen carrying sludge upward.
(2) Inhibit the proliferation of filamentous bacteria, solve sludge swelling, and adjust nutrient ratios: ensure that the proportion of nutrients such as carbon, nitrogen, and phosphorus in the influent is appropriate, generally C: N: P=100:5:1. When there is an imbalance of nutrients, timely supplementation of corresponding nutrients is necessary to avoid the proliferation of filamentous bacteria due to a single nutrient. Control dissolved oxygen concentration: Keep the dissolved oxygen concentration in the anoxic tank stable within an appropriate range, avoiding excessive or insufficient dissolved oxygen. Excessive dissolved oxygen can lead to the proliferation of aerobic bacteria, competing with filamentous bacteria for nutrients; Low dissolved oxygen can promote the growth of filamentous bacteria. Adding chemical agents: In the early stage of sludge expansion, an appropriate amount of chemical agents such as copper sulfate, ferric chloride, etc. can be added to inhibit the growth of filamentous bacteria. But attention should be paid to the dosage of the medication to avoid excessive inhibition of microorganisms. Replacing sludge: When the sludge swells severely and cannot be solved by conventional methods, it is possible to consider replacing some or all of the sludge, introducing activated sludge, and restoring the normal operation of the system.
(3) Repair the mixer to ensure even mixing. Regularly inspect and maintain the mixer: Establish a regular inspection and maintenance system for the mixer to promptly detect and resolve any malfunctions. For example, regularly cleaning the debris on the blades of the mixer, checking the operation of the motor, reducer, and other components of the mixer to ensure its normal operation. Adjust the mixing intensity: Adjust the mixing intensity of the mixer according to the actual situation of the anoxic tank to ensure that the sludge and sewage are fully mixed. The mixing intensity can be adjusted by changing the speed and blade angle of the mixer. Optimize mixer layout: If the mixer layout is unreasonable and results in uneven mixing, the position and quantity of the mixer can be readjusted to improve the mixing effect.
(4) Updating sludge and improving activity to control sludge retention time: By adjusting the sludge reflux ratio and residual sludge discharge, the retention time of sludge in the anoxic tank is controlled, usually 10-20 days. Avoid prolonged sludge retention time, which can lead to sludge aging. Increasing sludge load: Appropriately increasing the concentration of organic matter in the influent or reducing the sludge concentration in the anoxic tank can increase the sludge load, provide sufficient nutrients for microorganisms, and promote their growth and metabolism. Adding nutrients: When the nutrients in the water are insufficient, timely add appropriate amounts of nitrogen, phosphorus and other nutrients to the aerobic and anoxic tanks to meet the growth needs of microorganisms.
(5) Strengthen pretreatment, intercept impurities, optimize pretreatment process: Based on the characteristics of the incoming water quality, select appropriate pretreatment processes such as grids, sedimentation tanks, settling tanks, air flotation tanks, etc., effectively remove suspended solids, oils, difficult to degrade organic matter and other impurities in the incoming water. Strengthen pre-treatment operation management: Regularly clean pre-treatment equipment such as grilles and sedimentation tanks to ensure the normal operation of the equipment. At the same time, monitor the quality of the pre treated effluent and adjust the pre treatment process parameters in a timely manner to ensure the effectiveness of the pre treatment.
(6) Targeted pre-treatment of difficult to treat wastewater: pre-treatment of difficult to treat wastewater, such as hydrolysis acidification, advanced oxidation, adsorption, etc., reduces the toxicity of wastewater and improves its biodegradability. For example, using Fenton oxidation method to treat chemical wastewater can effectively decompose recalcitrant organic compounds and improve the biodegradability of the wastewater. Domestication of Microorganisms: Gradually increasing the proportion of difficult to treat wastewater inflow, allowing microorganisms to gradually adapt to the water quality characteristics of wastewater, and domesticating microbial communities with specific degradation capabilities. During the domestication process, it is necessary to closely monitor the activity of microorganisms and the treatment effect, and adjust operating parameters in a timely manner. Optimize process combination: Adopt anaerobic anoxic aerobic (A ²/O), sequencing batch reactor (SBR) and other process combinations to improve the treatment efficiency of difficult to treat wastewater. These processes can fully utilize the metabolic characteristics of different microorganisms to achieve effective degradation of recalcitrant organic compounds.
(7) Reduce influent SS, alleviate sludge burden, and strengthen source control: Starting from the source of wastewater generation, reduce the discharge of suspended solids. For example, companies are required to strengthen wastewater pretreatment during the production process to reduce the generation of suspended solids. Optimize the pretreatment process: increase the treatment capacity of the pretreatment process, such as using efficient sedimentation tanks, filtration tanks, etc., to further remove suspended solids in the influent. Adjust operating parameters: appropriately increase the sludge concentration in the anoxic tank, improve the adsorption capacity of sludge, and reduce the impact of suspended solids on sludge. At the same time, increase the frequency of sludge discharge and promptly discharge sludge containing a large amount of suspended solids.
5, Prevention first, building a long-term stable mechanism is not only to address the issue of floating mud, but more importantly, to establish a prevention mechanism to reduce the occurrence of floating mud problems from the source.
(1) Strengthen water quality monitoring and establish a comprehensive water quality monitoring system to monitor real-time water quality parameters of inflow, outflow, and various treatment units, such as COD, BOD, total nitrogen, total phosphorus, pH value, dissolved oxygen, sludge concentration, etc. Through data analysis, timely detection of water quality trends, taking measures in advance, and avoiding the occurrence of floating mud problems.
(2) Optimize operational management, establish scientific and reasonable operational management systems, and standardize operational processes. Operators must strictly follow the operating procedures, regularly check the operation status of the equipment, and adjust the operating parameters in a timely manner. At the same time, strengthen the training of operators to improve their professional skills and emergency response capabilities.
(3) Establish a regular maintenance system for equipment, and regularly inspect, maintain, and repair key equipment such as mixers, aeration equipment, and water pumps. Timely replace worn parts to ensure the normal operation of the equipment and avoid floating mud problems caused by equipment failures.
(4) Develop emergency plans for floating mud issues, clarify emergency response procedures and division of responsibilities. When floating mud problems occur, effective measures can be taken quickly to reduce the impact on the treatment system. At the same time, regular emergency drills should be organized to improve the emergency response capabilities of operators. 6, Conclusion: The problem of floating sludge in anoxic tanks is a common problem in the process of sewage treatment, with complex causes and serious harm. But as long as we have a deep understanding of the causes of floating mud, take targeted solutions, and establish long-term prevention mechanisms, we can effectively control the problem of floating mud and ensure the stable operation of the sewage treatment system. In practical operation, we need to continuously explore and optimize operational management methods based on the specific situation of sewage treatment plants, improve sewage treatment efficiency, reduce operating costs, and contribute to protecting the water environment and promoting sustainable development. I hope the analysis and suggestions in this article can provide useful references for environmental practitioners, and let's work together to protect clear water sources! Recommended Search for Xiao Wang's Environmental Notes
