Analysis and Optimization Strategies for Common Problems in Activated Sludge System Operation
As the most widely used biological treatment technology in wastewater management, activated sludge process (ASPP) operates through microbial communities in aeration tanks that degrade pollutants via metabolic processes involving organic debris and inorganic substances. However, system malfunctions frequently occur due to variable influent quality, operational parameter imbalances, and environmental fluctuations, directly impacting treatment efficiency and effluent quality. This study systematically analyzes the causes, mechanisms, and solutions for 19 typical operational challenges, providing technical guidance for maintaining stable operations in wastewater treatment plants.
The sedimentation performance, density and microbial activity of activated sludge are the basis for the stable operation of the system. These problems are often the source of other abnormalities and need to be prioritized.
Sludge bulking is the most common malignant problem, which is manifested as the rapid deterioration of sludge settling performance (SVI> 150 mL/g), turbidity of supernatant, and even overflow of sludge surface in secondary sedimentation tank.
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Cause subdivision:
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o Filamentous bacterial swelling: Under the conditions of sufficient water and carbon source but lack of nitrogen (N) and phosphorus (P) (e.g., C/N> 20, C/P> 100), dissolved oxygen (DO) <0.5 mg/L, pH <6.5, filamentous bacteria (such as chlamydomycetes and sulfur-feeding bacteria) overproliferate due to their large specific surface area and strong competitive advantage, and entangle bacterial agglutinates to hinder sedimentation.
o Non-floc bulking: When dissolved organic matter (e.g., monosaccharides, organic acids) in influent exceeds 0.5 kgBOD₅/(kgMLSS·d), the bacterial flocs become "viscous gel-like" due to excessive water absorption and lose settling capacity. This phenomenon is exacerbated when combined with insufficient nitrogen/potassium nutrients or dissolved oxygen levels below 1 mg/L.
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solving strategies ：
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o Emergency measures: Add clay (50-100 mg/L), hydrated lime (to adjust pH to 7.0-8.0) or reflux digested sludge (to introduce dominant bacterial groups), and inhibit filamentous bacteria through physical weight gain or biological competition; in serious cases, 0.5-1.0 mg/L liquid chlorine can be added to selectively kill filamentous bacteria (the activity of bacterial flocs needs to be monitored).
o System regulation: degrade part of the easily degradable organic matter through pre-aeration (aeration for 1-2 hours) to reduce the F/M of the inlet water; increase the aeration intensity, and control the DO at 2-3 mg/L; supplement nitrogen source (such as urea) or phosphorus source (such as sodium phosphate), so that N and P meet BOD₅:N:P =100:5:1.
o Prevention mechanism: SVI and microbial phase (microscopic observation of filamentous bacteria abundance) were monitored weekly. When SVI> 120 mL/g, the operation parameters were adjusted in advance.
The sludge rises from the bottom of the secondary sedimentation tank to the surface, forming a black or gray floating mud layer, accompanied by foul odor.
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Cause subdivision:
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o Denitration and floating: the retention time of secondary sedimentation tank is too long (> 4 hours), the denitrifying bacteria in the sludge use nitrate as electron acceptor to produce N₂ bubbles, which are attached to the sludge flocs and make their density lower and float (the floating mud is mostly loose, and the bubbles will escape after sampling).
o Decay and floating: local hypoxia in the secondary sedimentation tank (such as sludge accumulation at the bottom of the tank, failure of the scraper), anaerobic decomposition of sludge produces gases such as H₂S and CH₄, resulting in black and smelly sludge and floating (the floating sludge has high viscosity and a strong smell of decay).
o Joint effect of sludge expansion: The expanded sludge has poor settling performance, so it can not be concentrated effectively in the secondary sedimentation tank and rises with the water flow.
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solving strategies ：
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o Shorten the hydraulic retention time of secondary sedimentation tank to 2-3 hours to optimize the uniformity of water distribution; check the operation status of sludge scraper and remove the accumulated mud at the bottom of the tank (the thickness of accumulated mud should be less than 0.3 m).
o If it is denitrification flotation, the DO at the end of the aeration tank can be reduced (controlled at 0.5-1.0 mg/L) to reduce the carrying amount of nitrate in the sludge; if it is humification flotation, the aeration device of the secondary sedimentation tank should be repaired (such as installing submersible stirrer) to maintain the DO at the bottom of the tank> 0.5 mg/L.
o Simultaneously solve the problem of sludge bulking (see above) and improve the sedimentation performance of sludge from the source.
The aeration tank or return sludge is black and has the odor of sulfide.
· Cause: When DO <0.5 mg/L in the aeration tank, anaerobic bacteria (such as desulfurizing vibrio) multiply in large quantities, reducing sulfate to H₂S, and H₂S combines with Fe²⁺ in the water to generate black FeS precipitation, which is attached to the surface of sludge and leads to black color.
· solving strategies ：
o Improve the oxygen supply capacity of the aeration system (such as increasing the number of aeration heads and improving the fan pressure), control the DO at 2-3 mg/L; check whether the aeration pipeline is leaking and clean the blockage of the aeration head.
o Increase the sludge return ratio (from 50% to 70%-100%) to accelerate sludge renewal and reduce the formation of anaerobic environment.
o If the influent contains high concentration sulfate (＞500 mg/L), the sulfate content should be reduced during pretreatment (e.g., Ca² + precipitation).
The sludge color becomes light gray and white, and the sedimentation performance decreases.
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cause of formation ：
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o Filamentous bacteria (such as Nocardia) or attached ciliates (such as trypanosomes) overproliferate and occupy the main body of sludge;
o The pH value of influent is less than 6.0, which inhibits the activity of heterotrophic bacteria and leads to loose sludge flocs.
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solving strategies ：
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o Add NaOH or Na₂CO₃ to adjust the pH of the inlet water to 6.5-7.5, and restore the suitable environment for microorganisms to survive.
o If filamentous bacteria are overgrown, the chlorine addition method of "sludge bulking" (0.3-0.5 mg/L) is referenced to inhibit the growth of filamentous bacteria.
o Increase the sludge load (F/M increased to 0.3-0.5 kgBOD₅/ (kgMLSS·d) to promote the dominant growth of bacterial flocs.
As the key unit of sludge and water separation, the operation state of secondary sedimentation tank directly determines the effluent quality. The common problems are concentrated in the mud surface, supernatant and sludge loss.
The mud surface exceeds the designed liquid level, and part of the sludge overflows with the effluent.
· Cause: Sludge bulking (SV> 90%, SVI> 200 mL/g) is the main cause, filamentous bacteria entanglement leads to the sludge can not be compressed, and the thickness of the sludge layer continues to rise; in addition, the scraper speed is too slow (<1 r/h) or the sludge discharge pipe is blocked, which also leads to the accumulation of mud surface.
· solving strategies ：
o The core is to solve sludge bulking (see above), supplemented by chemical enhancement: adding granular carbon (10-20 mg/L) or digested sludge (recovery ratio increased by 20%) to increase sludge density.
o Check the sludge discharge system, unclog the blocked pipeline, adjust the speed of the sludge scraper to 1-2 r/h, and ensure that the daily sludge discharge is 1/3-1/2 of the sludge in the pool.
Fine sludge flocs float on the water surface, and COD and BOD₅ of effluent increase.
· Cause: The concentration of toxic substances (such as heavy metals and phenols) in the water is too high (exceeding the microbial tolerance threshold), or the pH fluctuates sharply (less than 5.0 or more than 9.0), resulting in the death of microorganisms (such as rotifers and scyphozoa), the disintegration of sludge flocs (OUR <8 mgO₂/(gVSS·h), indicating that the microbial activity is significantly reduced.
· solving strategies ：
o In case of emergency, the toxic sludge in the water intake and discharge pool (50%-70% of the sludge discharge) was stopped, and nutrients such as glucose (50-100 mg/L) and urea were added to promote the reproduction of living microorganisms.
o Introduce domestic sewage (accounting for 30%-50% of the inlet) or healthy sludge (such as transferred from other sewage treatment plants) to accelerate the rehabilitation of sludge; if toxic substances are affected, detoxification devices (such as heavy metal chelation pool) should be added in the pretreatment stage.
The effluent carries a large number of fine sludge particles, and the SS exceeds the standard.
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cause of formation ：
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o The sludge is chronically deficient in nutrients (F/M <0.1 kgBOD₅/(kgMLSS·d)), and the microorganisms are disintegrated due to "hunger", and the flocs are broken (OUR <8 mgO₂/(gVSS·h));
o Inlet C/N imbalance (C/N <5), ammonia nitrogen is too high to inhibit microbial synthesis;
o The temperature of the pool is more than 40℃ (such as summer sun exposure or industrial waste water heat), and the protein of microorganisms is denatured;
o The rotation speed of the secondary sedimentation tank is too high (> 30 r/min), and the mechanical shear force destroys the floc.
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solving strategies ：
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o Add easily degradable carbon sources such as starch and acetic acid, or introduce high concentration BOD₅ wastewater (such as food wastewater), to increase F/M to 0.1-0.3 kgBOD₅/ (kgMLSS·d).
o Adjust the C/N to 10-20 (supplement methanol, glucose and other carbon sources); when the pool temperature is too high, add cooling tower or shade shed to control the temperature between 15-35℃.
o Reduce the stirrer speed to 10-20 r/min to reduce mechanical damage.
The supernatant was gray and black, and COD and BOD₅ exceeded the standard (OUR> 20 mgO₂/(gVSS·h), indicating that the microbial metabolism was vigorous but the load was too high.
· Cause: sludge load is too high (F/M> 0.5 kgBOD₅/(kgMLSS·d)), microorganisms can not completely degrade organic matter, resulting in the discharge of undecomposed organic debris with the effluent.
· solving strategies ：
o Reduce the inflow flow (reduce by 20%-30%) or divert part of the inflow to the regulating tank to reduce the instantaneous load;
o Reduce the sludge discharge (suspend the sludge discharge for 1-2 days), improve MLSS to 3000-4000 mg/L, and enhance the treatment capacity of the system;
o Extend the aeration time (from 6-8 hours to 8-10 hours) to ensure full decomposition of organic matter.
Aeration tank is the core place of microbial metabolism, and its environmental conditions (such as dissolved oxygen, foam, sludge state) directly affect the treatment efficiency.
The floatation residue is brown or gray, with a viscous texture and a thickness of more than 5 cm.
· Cause: The overgrowth of actinomycetes such as Nocardia and fibrillates, whose hyphae entwine each other to form a mesh structure, adsorbs organic debris and bubbles; the high concentration of detergent (such as LAS) in the water also forms a difficult to break scum.
· solving strategies ：
o Manual removal of scum (1-2 times per day) to avoid its return to the aeration tank (scum skimming device can be set up);
o Increase the amount of sludge discharge (sludge age is shortened from 15-20 days to 10-15 days) to reduce the accumulation of actinomycetes;
o If the detergent is excessive, a foam separation device (such as air flotation tank) should be added in the pretreatment stage.
The cause of foam problem should be judged according to color, viscosity and stability, and targeted treatment should be taken:
· White foamy liquid (fragile): This occurs when anionic detergents (e.g., LAS) exceed 10 mg/L in influent water, reducing surface tension and causing foam formation. Solutions: Increase spray water flow (5-10 L/h per square meter of aeration tank surface area) or add defoaming agents (e.g., polyether-based compounds at 5-10 mg/L dosage).
· Viscous foam (not easily broken): When sludge load is excessively high (F/M> 0.5 kgBOD₅/(kgMLSS·d)), incomplete organic matter degradation produces a large amount of intermediate products (such as polysaccharides), resulting in sticky foam. Solution: Reduce influent load (diversion or dilution) and extend aeration time.
· Tea/gray foam: sludge aging (sludge age> 30 days), after the decomposition of dead microorganisms, lipids released from the sludge surface attach to the bubble surface, forming colored foam. Solution: increase the amount of sludge discharge, control the sludge age to 15-20 days, promote sludge renewal.
The fluctuation of water intake or system imbalance will cause the indicators to deviate from the standard, so it is necessary to trace the source and adjust quickly.
pH <6.0, especially in the anaerobic pretreatment unit.
· Cause: The load of anaerobic reactor is too high (> 3.0 kgCOD/(m³·d)), and the metabolic rate of acid-producing bacteria exceeds that of methanogenic bacteria, resulting in the accumulation of volatile fatty acids (VFA) (VFA> 1500 mg/L) and the decrease of pH.
· solving strategies ：
o Reduce the anaerobic load (suspend the water intake or divert 50% of the water intake), and add NaHCO₃ to adjust the pH to 6.5-7.5;
o Increase the stirring intensity of anaerobic reactor to promote the contact between VFA and methanogens;
o In case of sudden load impact, granular activated carbon (100-200 mg/L) can be added to adsorb part of organic matter.
When MLSS> 4000 mg/L and F/M <0.1 kgBOD₅/(kgMLSS·d) in the aeration tank, the microbial metabolism is slow.
· Cause: low concentration of BOD₅ (＜100 mg/L) or high MLSS (e.g., excessive reduction of sludge discharge).
· solving strategies ：
o Introduce high concentration organic wastewater (such as municipal sewage and industrial wastewater mixed) to increase the inlet BOD₅ to 200-300 mg/L;
o Increase the sludge discharge to reduce MLSS to 2000-3000 mg/L and restore F/M to 0.2-0.4 kgBOD₅/(kgMLSS·d).
· Increased effluent coloration: This occurs when sludge flocculation (e.g., SVI> 150 mL/g) causes loss of colloidal organic matter, or when influent contains refractory colored substances (e.g., dyes). Solutions: Improve sludge settling performance (refer to sludge bulking control), and add PAC (50-100 mg/L) to enhance decolorization when necessary.
· Elevated effluent BOD₅ and COD levels: Caused by sludge poisoning (e.g., heavy metals, pesticides) or load shock leading to a sudden decline in microbial activity. Solutions: Emergency sludge discharge (30%-50%), introduction of healthy sludge with nutrient supplementation for revitalization; identification of toxic substances in influent water, and installation of additional pretreatment units.
· Inadequate influent (COD> 1000 mg/L): excessive microbial metabolic load and rapid DO consumption. Solution: increase MLSS to 3000-4000 mg/L (reduce sludge discharge), or dilute the influent (e.g., mix with low-concentration wastewater).
· Excessive inorganic reducing agents (e.g., S²⁻, Fe²⁺> 50 mg/L) in influent: The oxidation of reducing agents consumes a large amount of DO, resulting in oxygen deficiency in the system. Solution: Increase aeration intensity (raising DO to 3-4 mg/L) or oxidize reducing agents during pretreatment (e.g., stripping S²⁻ through aeration).
· Interference in COD determination: Cl⁻> 1000 mg/L or nitrite in the influent water affect the oxidation efficiency of potassium dichromate. Solution: Add HgSO₄ (Cl⁻ masking agent) or aminosulfonic acid (nitrite masking agent) during the measurement.
The stable operation of activated sludge systems relies on a dynamic equilibrium between "inlet water-microorganisms-environment", where any imbalance may trigger chain reactions. In practice, an "prevention-first, rapid response" management mechanism should be established: Daily monitoring of key parameters like DO, MLSS, and SVI; weekly analysis of microbial composition; monthly assessment of sludge activity (OUR) to promptly identify abnormal trends; and layered emergency measures —— addressing root causes (e.g., chemical dosing) for immediate control. Systematic adjustments (e.g., load control, DO regulation, nutrient ratios) resolve fundamental issues, while preventive mechanisms (e.g., enhanced pretreatment, parameter alerts) prevent recurrence. Only by integrating technical principles with practical experience can we achieve efficient and stable operation of activated sludge systems, ensuring continuous compliance with effluent quality standards.
