In wastewater treatment processes, the activated sludge method is widely used due to its high efficiency and cost-effectiveness. The settling performance of activated sludge directly determines the solid-liquid separation efficiency of secondary sedimentation tanks, thereby affecting the stability of effluent quality. When the settling performance of activated sludge is poor, it often manifests as an abnormal increase in the sludge volume index (SVI), sludge bulking in secondary sedimentation tanks, floating sludge, and excessive suspended solids (SS) in the effluent. If not addressed promptly, it may lead to the paralysis of the entire wastewater treatment system. This article will begin with an analysis of the causes of poor activated sludge settling performance, systematically elaborate on targeted recovery measures, and propose long-term preventive strategies to provide technical references for the stable operation of wastewater treatment plants.
```Analysis of the Core Causes of Poor Settling Performance in Activated Sludge```
The root cause of poor settling performance in activated sludge is the abnormal flocculation structure of sludge or the reduced density difference between sludge and water, which prevents effective settling in the secondary sedimentation tank. Based on engineering practice, the core causes can be categorized into three major types: biological factors, operational factors, and environmental factors, as detailed below
(1) Biological Factors: Microbial Community Imbalance
Biological factors are the most common cause of poor sludge settleability, primarily related to microbial species, quantity, and metabolic state, with filamentous bulking and non-filamentous bulking being the most typical examples.
Floc bulking: Excessive proliferation of filamentous bacteria in activated sludge, where their hyphae intertwine to form a loose floc structure, hindering sludge-water separation. Common causes include:
Imbalance in the influent carbon-to-nitrogen (C/N) ratio, such as excessive carbon sources (e.g., high-concentration carbohydrate wastewater) or insufficient nitrogen/phosphorus nutrients, leads to enhanced competitive advantage for filamentous bacteria;
Dissolved oxygen (DO) concentration is too low (typically <2mg/L), especially in localized oxygen-deficient zones of the aeration tank. Filamentous bacteria (such as Nocardia and Thiothrix) thrive in low-oxygen environments and proliferate extensively;
The influent contains inhibitory substances (such as heavy metals and toxic organic compounds), which suppress the growth of normal flocculent bacteria. Filamentous bacteria, being more resistant to toxicity, gradually dominate.
2. Filamentous bulking: Also known as sticky bulking, the sludge flocs become loose and viscous due to adsorbing a large amount of water, resulting in a significant decrease in settling speed. The main causes include:
-The concentration of organic matter in the influent is too high (such as COD>3000mg/L), leading to excessive microbial proliferation and insufficient formation of compact structures in flocs;
-If the sludge age (SRT) is too long, microorganisms will enter the decline period, and excessive secretion of extracellular polymeric substances (EPS) will lead to enhanced hydrophilicity of flocs;
-The denitrification process is abnormal, and nitrate in the secondary sedimentation tank is reduced to nitrogen gas under anaerobic conditions. Bubbles attach to the sludge flocs, causing them to float up.
（2） Operational factors: Deviation of process control parameters
Improper control of operating parameters in sewage treatment systems can directly disrupt the normal metabolic environment of activated sludge and cause sedimentation problems.
-Abnormal aeration system: Uneven aeration leads to excessive fluctuations in DO concentration in the aeration tank (local peroxide or hypoxia), or high aeration intensity (such as air-water ratio>15:1), which breaks down sludge flocs and forms small particles;
-Improper control of reflux ratio: The sludge reflux ratio is too low (usually<50%), resulting in insufficient sludge concentration (MLSS) in the aeration tank and difficulty in floc formation; If the reflux ratio is too high (>150%), it may bring the loose sludge that has not settled in the secondary sedimentation tank back to the aeration tank, exacerbating the deterioration of sedimentation;
-Delayed sludge discharge: Insufficient discharge of remaining sludge leads to excessive sludge age, sludge aging, disintegration, or high MLSS (>5000mg/L), resulting in increased viscosity of the sludge water mixture in the aeration tank and increased settling resistance.
（3） Environmental factors: sudden changes in inflow water quality and external conditions
Sudden changes in external environment or incoming water quality that exceed the adaptability of activated sludge can easily lead to sedimentation problems.
-Fluctuations in influent water quality: such as sudden increases (>9) or drops (<6) in influent pH, which can damage the structure of microbial cell walls and lead to the disintegration of activated sludge; Sudden changes in inlet water temperature (such as temperature difference>5 ℃/d) can affect microbial metabolic rate, especially in low-temperature environments (<15 ℃), which can significantly reduce sludge settling rate;
-Impact load: In a short period of time, a large amount of high concentration wastewater (such as intermittent discharge of industrial wastewater) enters the system, or the suspended solids (SS) in the influent are too high (such as>500mg/L), and the activated sludge cannot be adsorbed and degraded in a timely manner, resulting in the "dilution" or inclusion of impurities in the flocs and a decrease in sedimentation performance;
-Abnormal operating environment of the secondary sedimentation tank: such as uneven water distribution in the inlet of the secondary sedimentation tank, high local flow velocity (>0.5m/h), and erosion of the sludge layer; Excessive accumulation of sludge in the secondary sedimentation tank (with a sediment layer thickness greater than 0.5m) can lead to anaerobic decay of the sludge, producing gases such as methane and hydrogen sulfide, which carry the sludge up to the surface.
2、 Targeted recovery measures for poor settling of activated sludge
In response to the settlement problems caused by different causes, it is necessary to adopt the approach of "precise diagnosis classification policy dynamic adjustment", implement recovery measures in stages, and avoid blind operation that exacerbates system deterioration.
（1） Emergency intervention: Quickly alleviate the worsening trend of settlement
When the sedimentation of activated sludge is poor and the effluent SS exceeds the standard (such as>30mg/L) or the sludge in the secondary sedimentation tank is severely drifted, emergency measures need to be taken first to control pollution and create conditions for subsequent recovery.
1. Strengthen the separation of mud and water:
-Add inorganic flocculants (such as polyaluminum chloride (PAC) and ferrous sulfate) to the aeration tank or secondary sedimentation tank, with the dosage adjusted according to the sludge concentration (usually 50-200mg/L), to promote the coagulation of sludge flocs and increase sedimentation rate through charge neutralization;
-If the sludge viscosity is too high, inert coagulants (such as fly ash and diatomaceous earth) can be added at a dosage of 5% -10% of MLSS to enhance floc density and reduce water adsorption;
-Temporarily reduce the inflow load, control the COD volumetric load below 0.3-0.5kg COD/(kgMLSS · d), reduce microbial metabolic pressure, and avoid further loosening of flocs.
2. Optimize the operation of the secondary sedimentation tank:
-Suspend or reduce sludge reflux, wait for the sludge in the secondary sedimentation tank to fully settle (usually 1-2 hours), and then discharge the upper clear liquid to avoid loose sludge circulation;
-If there is denitrification floating in the secondary sedimentation tank, a small amount of aeration can be added to the tank (such as opening the bottom aeration device) to maintain a DO concentration of 1-2mg/L and inhibit denitrification reaction;
-Clean up the accumulated sludge in the secondary sedimentation tank and thoroughly discharge the aged sludge at the bottom of the tank through a sludge discharge pump to avoid anaerobic decay affecting the overall sludge performance.
（2） System adjustment: Targeting the remediation of sludge performance based on its causes
After emergency measures to control pollution, it is necessary to adjust process parameters according to the causes and fundamentally restore the settling performance of activated sludge.
1. Recovery measures for filamentous bacterial expansion
-Balanced nutrient supply: Check the influent C/N/P ratio. If nitrogen/phosphorus is insufficient, add urea (nitrogen source) or potassium dihydrogen phosphate (phosphorus source) to the aeration tank to control the C/N ratio at 10-15:1 and C/P at 50-100:1, enhancing the competitiveness of flocculent bacteria;
-Increase dissolved oxygen concentration: Adjust the aeration system to ensure that the DO concentration in the aeration tank remains stable at 2-4mg/L, especially in the corners of the tank and areas with malfunctioning aerators. Timely maintenance or addition of aeration points is necessary to eliminate local oxygen deficiency zones;
-Shorten sludge age: Increase the amount of excess sludge discharge, control the sludge age at 5-8 days (adjusted according to water temperature, and can be appropriately extended to 10 days at low temperatures), and reduce the number of filamentous bacteria through the "washing" effect - filamentous bacteria have a longer growth cycle, and short sludge age can inhibit their excessive reproduction;
-Add antibacterial agents (use with caution): If filamentous bacteria (such as sulfur producing bacteria) multiply in large numbers, a small amount of chlorine based oxidant (such as sodium hypochlorite) can be added to the inlet of the aeration tank at a dosage of 0.5-1mg/L (calculated as effective chlorine) to inhibit filamentous bacterial activity, but microbial activity needs to be closely monitored to avoid excessive inhibition.
2. Recovery measures for non filamentous bacterial expansion
-Control the concentration of organic matter in the influent: By adjusting the influent valve or setting up a homogenization tank, the COD of the influent in the aeration tank can be controlled within 1000-2000mg/L. If the influent concentration is too high, methods such as diversion and dilution can be used to reduce the load;
-Optimize sludge age and discharge: If the sludge age is too long, increase the discharge of excess sludge, control MLSS at 3000-4000mg/L, adjust the sludge age to 8-12 days, promote normal microbial metabolism, and reduce EPS secretion;
-Strengthen aeration and stirring: appropriately increase the aeration intensity (air water ratio 12-15:1), enhance the mixing effect of mud and water, and avoid the flocs from adsorbing too much water due to standing, but it is necessary to avoid excessive aeration that breaks the flocs.
3. Recovery measures for operational/environmental factors
-Stable aeration system: Regularly inspect aerators (such as membrane aerators and perforated pipes), replace damaged parts, and ensure uniform aeration; Real time adjustment of aeration rate through DO online monitoring device to avoid DO fluctuations;
-Optimize the reflux ratio: Based on the MLSS and sedimentation conditions of the secondary sedimentation tank, control the reflux ratio at 70% -100% to ensure that the MLSS of the aeration tank is stable at 3000-4000mg/L, while avoiding the scouring of the sludge layer in the secondary sedimentation tank;
-Adjust the parameters of the inlet water environment: If the pH of the inlet water is abnormal, add acid (such as sulfuric acid) or alkali (such as sodium hydroxide) to the regulating tank to control the pH between 6.5-8.5; If the water temperature is too low, a heating device (such as steam heating) can be used to maintain the water temperature at 15-25 ℃ (the optimal temperature range for microorganisms);
-To cope with impact loads: An emergency regulating tank is installed at the inlet. When high concentration or toxic wastewater enters, it is first diluted and neutralized in the regulating tank, and then slowly pumped into the aeration tank. At the same time, the aeration intensity and nutrient addition are increased to help microorganisms adapt to load changes.
（3） Verification and consolidation: Ensure stable recovery effect
After taking recovery measures, it is necessary to continuously monitor key indicators, verify their effectiveness, and adjust parameters to avoid recurring problems.
-Monitoring indicators: Daily detection of SVI (normal range 50-150mL/g), MLSS, DO, influent C/N/P, effluent SS and other indicators. If SVI remains stable in the normal range for 3 consecutive days and effluent SS<10mg/L, it indicates that sludge settling has been restored;
-Parameter solidification: solidify the optimal aeration intensity, reflux ratio, sludge discharge rate, nutrient salt dosage and other parameters during the recovery process to form a standardized operation plan;
-Sludge domestication: If the inflow water quality is unstable for a long time, the activated sludge can be domesticated by gradually increasing the inflow load (by 10% -20% each time) to enhance its impact resistance.
