1, The mechanism of sludge expansion
Sludge swelling is a common abnormal phenomenon in activated sludge process, mainly caused by the following reasons:
Excessive reproduction of filamentous bacteria
-The dominant factor is the proliferation of filamentous bacteria (such as Fusarium graminearum, sulfur producing bacteria, etc.) in the sludge flocs, forming a loose mesh structure that hinders floc settling.
-Trigger condition:
-Low dissolved oxygen (DO<1.0 mg/L);
-High carbohydrate content (BOD ₅/N/P imbalance);
-The sludge load (F/M) is too high or too low;
-Abnormal pH value (<6.0 or>9.0).
non-filamentous bulking
-Accumulation of viscous substances: Excessive secretion of extracellular polymeric substances (EPS) by microbial flocs in sludge leads to a decrease in floc density.
-Environmental impact: sudden change in inlet water temperature, impact of toxic substances (such as heavy metals, surfactants).
2, The phenomenon of sludge swelling
Physical performance
-The interface between the sludge and water in the secondary sedimentation tank rises, and the sludge volume index (SVI) significantly increases (>150 mL/g, and in severe cases>300 mL/g);
-The sedimentation rate of sludge slows down, the supernatant becomes turbid, and carries small flocs.
Operational impact
-Sludge loss leads to a decrease in the concentration of sludge in the aeration tank (MLSS);
-The effluent COD and SS exceed the standard, resulting in a decrease in treatment efficiency;
-In severe cases, it can cause system crashes.
3, Solution measures
Based on the type and cause of expansion, the following targeted measures can be taken:
1. Control of filamentous bacterial expansion
-Process adjustment
-Increase DO concentration: Maintain DO in the aeration tank at 2.0-3.0 mg/L to suppress the competitive advantage of filamentous bacteria;
-Optimize F/M value: Control the sludge load at 0.2~0.4 kg BOD ₅/(kg MLSS · d) by discharging sludge or adjusting the inflow load;
-Improve nutrient ratio: Add nitrogen and phosphorus nutrients (such as urea and phosphoric acid) to ensure BOD ₅: N:P ≈ 100:5:1.
-Chemical intervention
-Add coagulants such as polyaluminum chloride (PAC) and ferric chloride (FeCl) to enhance the cohesiveness of flocs;
-Oxidant: Sodium hypochlorite (NaClO) or hydrogen peroxide (H ₂ O ₂) is added to selectively inhibit filamentous bacteria.
-Biological regulation
-Introducing advantageous bacterial strains: adding specialized microbial agents (such as photosynthetic bacteria and Bacillus) to compete and inhibit filamentous bacteria;
-Sludge washing: Dilute the concentration of filamentous bacteria through the secondary sedimentation tank sludge reflux system.
2. Control of non filamentous bacterial expansion
-Reduce influent load: minimize the impact of high concentration organic wastewater and avoid excessive adsorption of organic matter by sludge;
-Adjust pH value: Add alkaline solution (such as NaOH) or acidic solution (such as H ₂ SO ₄) to maintain pH between 6.5 and 8.5;
-Strengthening pre-treatment: adding grilles and sedimentation tanks to remove substances such as oil and suspended solids that may cause viscous expansion.
3. Emergency measures
-Adding inert substances such as diatomaceous earth and fly ash to increase sludge density;
-Temporary sludge dewatering: rapidly reducing the amount of sludge in the system through centrifuges or filter presses;
-Recovery of stuffiness: Stop water inflow and continue aeration for 24-48 hours, consuming excessive organic matter and EPS.
4, Prevention and monitoring
Daily monitoring indicators
-SVI: Daily monitoring, with a warning threshold set at 120-150 mL/g;
-Microscopic observation: Regular microscopic examination of filamentous bacteria abundance (controlled below level 3);
-DO, pH, F/M values: Real time monitoring of process parameter fluctuations.
Long term management strategy
-Stabilize the incoming water quality and avoid impact loads;
-Regularly discharge sludge and maintain a reasonable sludge age (SRT);
-Optimize the design of the aeration system to ensure even distribution of dissolved oxygen (DO).
By scientifically analyzing the type of expansion and accurately adjusting the process parameters, the sedimentation performance of sludge can be effectively restored, ensuring the stable operation of the sewage treatment system.
