Stage 1 (5–10 days): Acclimation Stage
Introduce water to the biological reactor and start the submersible mixer.
Continuously introduce water until the designed effective water depth is reached. Homogenize the inoculated sludge in the biological reactor. Initiate aeration using the blower aeration system, gradually increasing the aeration rate during continuous water introduction after sludge inoculation until the maximum aeration rate is reached. Activate internal recirculation, fill the aeration tank with wastewater, then stop the water flow, leaving only the aeration equipment on to oxygenate the tank for 1–2 days.
Continuously introduce water into the secondary sedimentation tank. When the tank is half full, shut off the internal recirculation in the biological reactor and start the sedimentation tank scraper and sludge return pump. This allows the activated sludge settled in the secondary sedimentation tank to be quickly collected during the initial acclimation stage and returned to the biological treatment tank. Adjust the sludge return rate by observing the returned sludge; generally, it should be controlled between 50% and 100%. Once the secondary sedimentation tank reaches its normal operating water level, observe the activated sludge condition and control the influent flow until indistinct flocculent matter appears. At this point, appropriately increase and change the water to replenish nutrients. During this stage, adjust the influent flow rate and blower operating time based on the actual influent volume, water quality, and aerobic oxygen demand.
When the secondary sedimentation tank begins to overflow, temporarily suspend subsequent wastewater treatment processes (advanced treatment, disinfection) and discharge the wastewater directly, bypassing these processes.
After the biological treatment tank reaches its normal operating water level, continuously monitor the dissolved oxygen (DO) concentration (using a dissolved oxygen meter) and suspended solids (MLSS) concentration in the biological treatment tank to determine whether the aeration rate is sufficient and make appropriate adjustments. During activated sludge acclimation, the dissolved oxygen concentration should meet the following scenarios:
a) Low dissolved oxygen concentration in the influent and return sludge, requiring a high level of oxygenation;
b) Anoxic influent, requiring sufficient dissolved oxygen to quickly convert it into an oxygenated environment;
c) Nutrient-rich wastewater, requiring a large amount of dissolved oxygen to support microbial growth.
During sludge acclimation, the minimum dissolved oxygen concentration should ensure that the DO concentration at the effluent outlet of the biological treatment tank is not less than 1.0 mg/L.
In the first stage of acclimation, due to the low concentration of activated sludge, a large amount of biochemical metabolic foam may form during aeration. Generally, no treatment measures are required, and the foam gradually decreases as the bacteria acclimate. If necessary, measures such as spraying water droplets can be used to remove the foam.
Stage 2 (10–20 days): Growth Phase
During this phase, while monitoring dissolved oxygen, the 30-minute settling ratio (SV30) and nutrient parameters of the activated sludge should also be monitored.
At the beginning of this phase, the sludge-water mixture color is similar to that of the influent. As aeration continues, particle size increases, settling performance improves, and the color gradually turns dark brown. During this phase, the activated sludge settling ratio can reach 5–20%.
Nutrient testing ensures conditions for microbial growth. During the acclimation process, the BOD:N:P ratio should be maintained at approximately 100:5:1. If this ratio cannot be achieved, nutrients should be added to adjust it.
Stage 3 (20–30 days): Stabilization Phase
During this phase, the acclimation process is essentially complete. Key parameters of the sludge-water mixture are monitored, analyzed, and recorded for reference during normal system operation. When the activated sludge concentration reaches the specified range and becomes relatively stable, the acclimation process can be considered essentially complete. After biological and sedimentation treatment, the effluent suspended solids should meet the standards. Excess sludge should be discharged according to actual operating conditions.
Stage 4 (30 days): Transition Phase
The purpose of this phase is to record operational parameters, including key control metrics such as the activated sludge 30-minute settling ratio (SV30), biomicroscopic examination results, sludge return ratio, and excess sludge discharge. This provides a reference for normal system operation.
When the influent concentration is low and sludge growth is poor, the sludge return ratio should be increased; conversely, when sludge bulking occurs, the ratio should be decreased. During this phase and throughout normal operation, the sludge return ratio must be strictly controlled. Insufficient sludge return may result in:
Inadequate activated sludge to treat pollutants, usually occurring 1–2 weeks before system startup;
Sludge remaining too long in the sedimentation tank, causing anaerobic reactions, floating sludge, and odors;
A thick sludge layer in the secondary sedimentation tank, increasing suspended solids in the effluent;
With sufficient dissolved oxygen, nitrification occurs in the biological treatment tank, possibly leading to denitrification in the sedimentation tank and increased sludge volume.
After the fourth stage and completion of the acclimation process, all operating parameters of the activated sludge should be within the design control range and relatively stable.