In the daily operation of a wastewater treatment plant, the activated sludge system is quite a "delicate" component. Even a slight overload of wastewater can easily cause it to malfunction or fail, and in severe cases, it may stop working altogether. Today, we’ll discuss how to revive an activated sludge system that has failed due to high-load shock.
First, it's essential to understand what high-load shock means. Essentially, it refers to a sudden surge in the concentration of pollutants like organic matter and ammonia nitrogen in the influent within a short period, or a sudden increase in water volume that exceeds the treatment capacity of the activated sludge. Under such conditions, the microorganisms in the sludge suffer—while they were thriving, they are abruptly overloaded with "food," either disrupting their metabolism or failing to adapt to the drastic environmental changes, leading to mass die-offs. The failure of the sludge is also evident, such as a sharp decline in effluent quality with超标 COD and ammonia nitrogen levels; darkened, foul-smelling sludge in the aeration tank; deteriorated settling performance with abnormal increases or decreases in SV30 values; and even sludge floatation in the secondary sedimentation tank.
Upon discovering that the sludge has been overwhelmed by high-load shock and "shut down," the first step is not to panic but to immediately cut off the source of the shock. At this point, promptly adjust the influent—either divert the high-concentration wastewater to prevent further entry into the aeration tank or reduce the influent volume to give the sludge system a chance to recover. If the plant has an emergency storage tank, transfer the shock wastewater there for temporary storage until the subsequent treatment capacity can catch up, then slowly draw it back. The core objective here is to minimize losses and prevent the sludge system from deteriorating further under the shock.
Next, it's time to "reduce the burden" on the sludge, allowing the surviving microorganisms to recover. The most straightforward approach is to increase aeration intensity and raise the dissolved oxygen concentration in the aeration tank. Under high-load shock, microorganisms work overtime to decompose organic matter, leading to a sharp rise in oxygen demand. A low-oxygen environment accelerates their death. Maintaining a DO level of 2–4 mg/L provides sufficient oxygen for the microorganisms, helping them survive the crisis. Additionally, partially removing severely damaged sludge and replenishing it with fresh, highly active sludge—akin to "blood transfusion"—can expedite recovery. If the plant lacks backup sludge, borrowing from nearby well-operated wastewater treatment plants proves particularly effective during emergencies.
Next, it is necessary to regulate the water quality environment in the aeration tank to create a comfortable living condition for microorganisms. High-load shock often leads to severe fluctuations in pH levels, and environments that are too acidic or alkaline can inhibit microbial activity. At this point, acid-base regulators should be added to maintain the pH within the suitable range of 6.5 to 8.5. Additionally, the ratio of nutrients in the water must be monitored, as microbial growth requires carbon, nitrogen, and phosphorus in a proportion of approximately 100:5:1. If the balance is disrupted, nutrient agents such as urea and potassium dihydrogen phosphate should be added to ensure microorganisms have sufficient "food" for reproduction.
Once the sludge system shows slight improvement, the influent load can be gradually increased to restore it. This process must be carried out step by step, avoiding sudden increases in both flow rate and concentration, as this could easily cause secondary shock. The load can be raised by 10% to 20% daily while closely monitoring key indicators such as SV30, MLSS, effluent COD, and ammonia nitrogen. If the effluent quality remains stable and the sludge's settling performance improves, it indicates the recovery direction is correct. If the indicators deteriorate again, the load must be reduced promptly and the system consolidated for a few more days.
Finally, there is a crucial point to note: during the recovery phase, daily monitoring must be conducted closely to track changes in all relevant data. Daily measurements should include MLSS, MLVSS, DO, and pH in the aeration tank, as well as SV30, SVI in the secondary sedimentation tank, along with COD, ammonia nitrogen, total nitrogen, and total phosphorus in both influent and effluent. These data provide a clear indication of sludge recovery progress, enabling timely adjustments to operational strategies. Additionally, once the system fully stabilizes, a thorough review of the causes behind the high-load impact is necessary—whether issues stemmed from the pretreatment stage or illegal discharge of high-concentration wastewater by upstream facilities. Targeted preventive measures, such as upgrading pretreatment processes and installing online monitoring equipment, should be implemented to avoid recurrence of similar incidents in the future.
The recovery of an activated sludge system is akin to the process of healing when one falls ill—first mitigate the damage, then regulate the condition, gradually replenish, and finally implement preventive measures. As long as each step is properly executed with patience, even sludge rendered ineffective by high-load impact can regain its activity and continue aiding in wastewater treatment.
