The secondary sedimentation tank is one of the core structures of the activated sludge wastewater treatment system, undertaking the key function of mud-water separation. However, sludge floating frequently occurs in the secondary sedimentation tank during actual operation, leading to deterioration of effluent water quality and even affecting the stable operation of the entire treatment system in severe cases. Although the problem of sludge floating seems complicated, it can be classified into two main categories: bulk sludge floating and fine particle sludge floating (sludge washout). This paper systematically sorts out the causes of various sludge floating phenomena and corresponding countermeasures, providing a reference for operation and management personnel. I. Bulk Sludge Floating Fist-sized sludge lumps occasionally float up in the sedimentation tank, which is mainly caused by two factors. 1. Denitrification Sludge Phenomenon CharacteristicsThe floating sludge is light in color and sometimes rust-colored.Cause AnalysisWhen the nitrification degree in the aeration tank is relatively high, nitrogenous compounds are converted into nitrates through ammonification and nitrification, resulting in a high concentration of NO₃-N. If the sludge level rises in the sedimentation tank due to excessively low return ratio or poor sludge return, the sludge cannot be renewed for a long time. The sludge at the bottom of the sedimentation tank is in an anoxic state, triggering the denitrification reaction of nitrates. Generated nitrogen (N₂) forms tiny air bubbles attached to the sludge, eventually causing bulk sludge floating. Solutions 1. Increase the return ratio to renew the sludge in the secondary sedimentation tank timely and reduce the height of the sludge layer; 2. Shorten the sludge retention time and discharge more sludge to lower the sludge concentration; 3. Appropriately reduce the dissolved oxygen (DO) level in the aeration tank. The above measures can weaken nitrification and reduce the source of nitrates. 2. Septic Sludge Phenomenon CharacteristicsDifferent from denitrification sludge, the septic sludge is black in color and accompanied by a strong putrid odor.Cause AnalysisDead zones exist in the secondary sedimentation tank where sludge accumulates. Long-term sludge accumulation leads to anaerobic septic decomposition, producing gases such as H₂S, CO₂ and H₂. These air bubbles adhere to the sludge and drive it to float upward. Solutions 1. Eliminate sludge accumulation in dead zones, for example, regularly perform aeration and agitation with compressed air in dead zones; 2. Increase sludge return to avoid long-term sludge retention; 3. Optimize the tank structure in the design stage to reduce areas prone to sludge accumulation. II. Fine Particle Sludge Floating (Sludge Washout) Fine sludge particles continuously flow out with the effluent, commonly known as "sludge washout". The causes are complex, involving system operating load, sludge status, water quality changes and other aspects. Main Causes of Sludge Washout 1. Excessive System LoadSudden increase in treatment load (water volume and pollutant concentration) shortens the hydraulic retention time of the secondary sedimentation tank. Activated sludge fails to settle and flows out with the effluent. Meanwhile, increased influent concentration enhances the activity of activated sludge and deteriorates its settleability, resulting in turbid effluent and sludge washout. 2. Filamentous Sludge BulkingExcessive proliferation of filamentous bacteria causes sludge bulking. The sludge presents a loose structure with an extremely slow sedimentation rate, leading to sludge washout before complete sedimentation. Filamentous sludge bulking is one of the most common problems in the operation of activated sludge processes. 3. Low-Load Long-Term OperationLong-term operation under low load conditions causes sludge aging and microbial endogenous respiration, resulting in floc disintegration and fragmentation, which also triggers sludge washout. Both excessively high and low load can lead to sludge loss in effluent. 4. Sudden Changes in Environmental FactorsAbnormal changes in environmental conditions may induce sludge washout: • Low water temperature reduces microbial activity and deteriorates sludge settleability; • Excessive aeration produces high shear force and breaks sludge flocs; • Sharp fluctuation of pH value interferes with the normal metabolism of microorganisms; • Entry of toxic and inert substances inhibits sludge activity. 5. Sudden Fluctuation of Influent Water QualitySudden changes in influent pH, toxic pollutants and other indicators, as well as the inflow of toxic and inert substances, can cause sludge poisoning or activity loss, further resulting in floc disintegration and sludge washout. 6. Nutrient Imbalance and Sludge AgingSludge aging occurs due to nutrient deficiency or excessive oxygenation. Microorganisms conduct self-consumption, weakening the floc structure and reducing sedimentation performance. 7. Excessively High Influent Ammonia NitrogenExcessively high influent ammonia nitrogen and low C/N ratio break the balance of sludge colloid mechanism and cause floc disintegration and sludge washout. 8. Excessively High Tank Water TemperatureWhen the tank temperature exceeds 40℃, microbial metabolism is disordered, impairing sludge flocculation and sedimentation. 9. Mechanical FactorsExcessively high rotating speed of mechanical aeration impellers generates strong shear force, breaking floc particles and producing a large number of fine particles that flow out with water. Comprehensive Solutions for Sludge Washout The core principle for addressing sludge washout is to identify the root cause first and then take targeted measures: 1. In case of sludge poisoning, immediately cut off the inflow of toxic wastewater and pollution sources; 2. For nutrient deficiency and sludge aging, appropriately add nutrients (carbon source, nitrogen source, phosphorus source) and implement sludge rejuvenation measures; 3. In case of load fluctuation, adjust operating parameters and reasonably control aeration rate and sludge return ratio; 4. For filamentous sludge bulking, adopt measures such as dosing chemical agents, adjusting sludge retention time, and adding anaerobic/anoxic selection zones. III. Quick Reference for Operation and Management Key Points Dissolved Oxygen (DO) Management • Continuously low DO + high influent load → Incomplete organic matter degradation, turbid effluent with dark chromaticity; • Continuously high DO + low influent load → Microbial endogenous oxidation of sludge, light sludge with poor settleability, light sludge floating out with effluent. Secondary Sedimentation Tank Management Floating sludge is mostly caused by anaerobic decomposition of bottom sludge, mainly for two reasons: insufficient sludge return flow, or sludge accumulation in dead zones due to damaged sludge scrappers. Special Reminder The troubleshooting of sludge floating shall follow the principle of "Observation First, Detection Second, Adjustment Last". The type of sludge floating can be initially judged by observing the appearance characteristics such as color, size and odor of floating sludge. Combined with water quality detection data (DO, NO₃-N, pH, MLSS, etc.), the problem can be confirmed, and targeted adjustment schemes can finally be formulated.
