I. Phenomena and Hazards
Phenomena: Observation using a sludge microscope or the naked eye reveals that the sludge layer in the secondary sedimentation tank continuously rises, approaching or even exceeding the overflow weir; the effluent becomes turbid, containing fine suspended particles; in severe cases, large sludge sheets float to the surface, resembling "clouds."
Direct Hazards: Excessive SS (suspended solids) in the water: This is the most direct consequence, leading to substandard effluent quality. It carries away valuable bacteria: The lost active sludge reduces the treatment capacity of the biological treatment system.
It affects disinfection effectiveness: Turbid effluent consumes more disinfectant, increasing costs. It clogs the filter: If subsequent advanced treatment is required, it significantly increases the burden on the filtration units.
II. Reason
1: Deterioration of sludge settling performance. This is the most important and common reason.
Sludge bulking characteristics: Extremely high SV30 (30-minute settling ratio) values (often exceeding 50%), even reaching over 90%; loose sludge flocs, clear supernatant but blurred sludge-water interface.
Causes: Overgrowth of filamentous bacteria (such as microfilamentous bacteria), or excessive secretion of viscous substances by bacterial flocs (non-filamentous bacteria bulking), leading to loose sludge structure, low density, and inability to settle.
Sludge aging characteristics: In the SV30 test, the sludge surface settles slowly, resulting in a large final compacted volume; the supernatant is turbid with fine particles; the sludge is dark black and has an odor.
Causes: Excessively old sludge lacks activity and reduces decomposition capacity. Excessive aeration can also break up sludge particles, leading to poor settling performance.
Sludge poisoning characteristics: Sudden onset, rapid deterioration of settling performance, turbidity of the SV30 supernatant, and a sharp decrease in microbial count under a microscope.
Cause: The influent contains toxic substances (such as heavy metals, high concentrations of cyanide, phenols), which inhibit microbial activity.
Denitrification characteristics: Sludge floats in clumps in the secondary settling tank, accompanied by numerous tiny bubbles. After stirring, the bubbles carry the sludge to the surface.
Cause: In the anoxic environment at the bottom of the secondary settling tank, denitrifying bacteria convert nitrates (NO₃⁻) into nitrogen gas (N₂), and these small bubbles adhere to the sludge.
2: Hydraulic or Solid Load Impact
Untimely Sludge Removal Characteristics: Sludge surface rises slowly and steadily, MLSS (Mixed Lime Sludge Concentration) is too high.
Cause: The simplest and most direct cause. The total amount of sludge in the system exceeds the carrying capacity of the secondary sedimentation tank; it's "only inflow, no outflow," and the sludge naturally accumulates higher and higher. Sudden Increase in Influent Load Hydraulic Load Impact: Heavy rain or a large influx of water; the water flow is too fast, and the sludge doesn't have enough time to stay in the secondary sedimentation tank, so it's "washed" out.
Solid Load Impact: The sludge concentration in the biological treatment tank is already high; encountering high backflow, the amount of solids entering the secondary sedimentation tank instantaneously exceeds its design settling capacity.
Characteristics: Occurs simultaneously with water volume or water quality impacts.
Cause: Sludge Scraper/Suction Machine Malfunction
Characteristics: An abnormality occurs only in a certain secondary sedimentation tank.
Cause: The sludge scraper is not rotating, or the suction pipe is blocked, preventing the effective collection and removal of sludge deposited at the bottom of the tank.
III. On-site Emergency Response and Fundamental Solutions
Step 1: Emergency Response - Immediately Increase Sludge Discharge! This is the preferred emergency measure for dealing with high sludge levels. By increasing the sludge discharge volume, the sludge inventory in the secondary sedimentation tank can be quickly reduced. However, note that if sludge bulking is the cause, sludge discharge should be done in "small amounts, multiple times" to avoid system collapse.
Temporarily Adjust the Return Ratio: While increasing sludge discharge, the sludge return ratio can be appropriately increased to quickly pump the sludge from the secondary sedimentation tank back to the biological treatment tank, relieving pressure on the secondary sedimentation tank. However, the characteristics of the returned sludge must be closely monitored.
Add Coagulant to Aid Settling: Temporarily add a small amount of coagulant such as PAC (polyaluminum chloride) at the inlet of the secondary sedimentation tank to help fine sludge flocculate and settle, quickly improving the effluent quality.
Step Two: Measures: If the problem is determined to be sludge bulking/aging: The core approach is to adjust sludge discharge and control sludge age. During bulking, the discharge rate can be increased appropriately; during aging, sludge discharge must be increased to renew the sludge population.
Adjust the operating conditions of the biological treatment tank: Ensure dissolved oxygen (DO) is within a reasonable range (usually 2-4 mg/L) to prevent filamentous bacteria bulking due to hypoxia or sludge disintegration due to over-aeration.
Nutrient addition: Check the nutrient ratio (C:N:P). If denitrification is determined, increase the sludge return rate and shorten the sludge residence time in the secondary settling tank to prevent hypoxia. Optimize the denitrification efficiency of the biological treatment tank to ensure effective removal of nitrates at the aerobic zone outlet, reducing the total amount of nitrates entering the secondary settling tank. If the problem is determined to be a load shock, strengthen influent monitoring and communicate with the dispatcher to strive for balanced influent. If the problem is a hydraulic shock, the bypass pipe can be opened, diverting part of the water flow, if permissible. If the problem is determined to be equipment failure, immediately repair the sludge scraper and clean any blocked pipes.