Operation management of wastewater treatment plant aeration tanks and solutions to common problems

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This post was last edited by yinkuilin6868 on 2016-2-25 08:52. The operation of the aeration tank is a key aspect in wastewater treatment; it directly affects the quality of the water discharged from the wastewater treatment plant as well as the level of production costs. If not managed properly, the wastewater treatment system may fail. Below are some insights regarding the operation and management of aeration tanks in the activated sludge process: In a completely mixed aeration tank, the water inflow to the tanks operating in parallel can be made uniform and the loads equal by adjusting the inlet valves. The staged aeration principle requires uniform water inlet at multiple points along the length of the aeration tank, so that microorganisms can fully utilize their ability to decompose organic matter under conditions with a more even distribution of nutrients. In activated sludge systems, regardless of the operating mode used and in line with the requirements for treatment efficiency and effluent quality, several important parameters such as sludge load, sludge age, and sludge concentration must be taken into account when carrying out process control. Adjusting the sludge load rate must take into account the flocculation and sedimentation properties of the sludge, and efforts should be made to avoid the load range of 0.5–1.5 kgBOD5/(kg·MLSS·d), as this range results in poor sludge sedimentation performance and an increased risk of sludge bulking. Since the sludge age represents the average number of days that newly added sludge stays in the aeration tank and indicates the composition of microorganisms in the activated sludge, microorganisms with a generation time longer than the sludge age cannot reproduce within the system. Therefore, during carbon and nitrogen removal processes, it is necessary to take into account the sludge age determined by the sludge growth rate of nitrifying bacteria at a certain temperature; by controlling the amount of excess sludge discharged using the sludge age, better treatment results can be achieved. To some extent, the sludge concentration determines the safety of the activated sludge process operation. It has a high sludge concentration and strong capacity to withstand shock loads. With a constant organic load, the aeration time is relatively short. With a constant aeration time, the load rate is low. Furthermore, sludge concentration is proportional to the oxygen demand; excessive sludge concentration reduces the oxygen absorption rate. Additionally, an increased amount of returned sludge, combined with the characteristics of the water quality that result in a high sludge index, makes sludge bulking more likely to occur. Therefore, the sludge concentration should be controlled at 2500–3000 mg/L. In summary, by keeping the sludge load, sludge age, and sludge concentration within optimal ranges and making adjustments according to actual conditions, microorganisms can grow in a regular and balanced manner. As a result, the activated sludge exhibits good sedimentation properties, enabling stable purification effects. The maximum oxygen demand in the activated sludge process occurs at the beginning of the aeration tank, where the wastewater and sludge start to mix; therefore, by ensuring the required dissolved oxygen level at the outlet, the oxygen demand at the inlet can be met. At the inlet of a completely mixed aeration tank, ensuring the required dissolved oxygen level at the outlet is sufficient to meet the oxygen demand at the inlet. In a completely mixed aeration tank, the dissolved oxygen is uniform throughout the entire tank. The effluent from processes such as A/O and A2/O for biological nitrogen removal also comes from the aerobic stage. Therefore, the dissolved oxygen at the outlet of the aeration tank should be controlled. It is generally believed that an oxygen concentration of 0.5 mg/L is sufficient to sustain microbial metabolic activity. However, when the dissolved oxygen level is below 2 mg/L, it facilitates the growth of filamental bacteria, resulting in smaller activated sludge flocs and poor settling properties. Taking all these factors into account, the dissolved oxygen level in the effluent from the aeration tank should be around 2 mg/L. The sludge settling ratio and the sludge concentration in the aeration tank mixture can indicate the amount of sludge required for the proper operation of the aeration tank. The settling ratio is generally kept between 20% and 30%, while the sludge concentration varies depending on the mode of operation; when it is below these limits, less sludge needs to be removed, and when it is above these limits, more sludge must be removed. Although this method is somewhat rough, it is easy to implement and facilitates management. The aeration tank is operating normally; the activated sludge is in a flocculated structure, brownish-yellow in color, and has no unusual odors. It exhibits good adsorption and sedimentation properties, with a distinct interface between the sludge and water during sedimentation. Microscopic examination shows good growth of bacterial flocs. The indicator organisms include attached and grape-shaped ciliates such as rotifers, infusoria, and testate infusoria, along with a small amount of filamentous bacteria and other organisms. Abnormal phenomena such as sludge bulking occur in the parameters of sedimentation ratio and mixed liquor sludge concentration. The oxygen requirement is a function of microbial metabolism. Low dissolved oxygen hinders normal metabolic processes, while high levels accelerate the oxidation of organic matter, leading to sludge aging. This not only increases operating costs but also facilitates denitrification in the sludge in the secondary sedimentation tank. The sludge index can reflect the looseness and coagulation properties of activated sludge. An excessively low sludge index indicates that the sludge particles are small, contain a high amount of inorganic substances, and lack activity and adsorption capacity. A high sludge index indicates that the sludge is difficult to settle and separate, meaning it is expanding or has already expanded. Under normal operation, the settlement ratio is around 30%, and the dissolved oxygen level is 0.5–2.0 mg/L. The sludge index ranges from 80 to 120 L/g, and operators can use this value to assess the condition of the sludge in the aeration tank. During the transition period from spring to summer, when the water temperature is between 15°C and 30°C, Pellicularia, one of the microorganisms that cause filamentous swelling, proliferates at the fastest rate. If the dissolved oxygen level in the tank is low at this time, filamentous bacteria will multiply extensively in the aeration tank, leading to sludge bulking. Therefore, during this period it is necessary to increase the aeration volume or reduce the water inflow rate in order to ease the load, or to appropriately lower the sludge concentration so as to reduce the oxygen demand. Additionally, sludge accumulation in the dead corners of the secondary sedimentation tank during summer can also lead to anaerobic fermentation; it is therefore important to remove this sludge promptly and thoroughly to prevent it from floating to the surface and being carried away with the water, which could affect the quality of the effluent. In autumn, summer, and winter, sludge denitrification or sludge disintegration may occur; operators should take specific and effective preventive and control measures based on the underlying causes. In the treatment of wastewater using the activated sludge process, the purification efficiency is highest when the water temperature is between 20 and 30 degrees. If the water temperature can be maintained at 7 to 8 degrees, measures such as increasing the concentration of activated sludge and reducing the sludge load can be taken to ensure the quality of the secondary effluent. In phosphorus and nitrogen removal process systems, the impact caused by low water temperature can be mitigated by extending the aeration time or taking other measures to raise the water temperature. There are several points to note during operation. The return flow rate in a combined aeration tank is determined during the trial run by testing based on the opening degree of the gate valves and the rotational speed of the impellers; during normal operation, this value can be used as a reference for control, or the stability of the sedimentation area can also be utilized for control, as long as the return flow rate does not affect the sedimentation area. When there is a large difference between the temperature of the compressed air after ventilation and the outdoor temperature (especially in winter), condensation water tends to form inside the air pipes, which hinders air flow and affects proper aeration. Therefore, it is necessary to regularly drain the condensation water and moisture, and to close the gate valve immediately after drainage in order to prevent air loss. During the operation of the aeration tank, when a large number of white bubbles appear on the surface of the tank, it indicates that the sludge concentration in the mixture inside the tank is too low. This situation can occur at the initial stage of cultivating activated sludge, or when the concentration of returned sludge is low and the amount of sludge returned is small. At this point, efforts should be made to increase the sludge concentration to 2–3 grams per liter. However, when a large number of brownish-yellow bubbles or bubbles of other colors appear on the surface of the aeration tank, it may be due to too high carbon content in the influent water, excessive growth of filamental bacteria, or a high amount of surfactants in the water. In such cases, it is necessary to reduce the sludge concentration and decrease aeration to gradually alleviate the situation. When the aeration impeller is in operation, attention should be paid to the immersion depth. During normal operation of the impeller, no water droplets should splash as a result of waves generated around it hitting the tank walls, and there should be no decrease in current flow. To this end, a separately constructed aeration tank allows the outlet gate valve to be lowered, raising the water level in the tank and preventing the impeller from coming out of the water surface as well as leaf blockage. The gate of the return window in the combined aeration tank cannot be raised too high; otherwise, the flow exiting from the return window will not be able to disrupt the swirl, which could lead to the blades rising above the water surface and becoming clogged. Attention should be paid to adjusting the window opening degree at any time. During the long-term operation of the aeration tank, the sludge accumulated in some dead corners should be removed. Furthermore, all types of aeration heads can become clogged and damaged by sludge; therefore, they should be regularly cleaned, inspected, and replaced. The ordinary steel components in the tank should be treated to prevent corrosion. At the same time, proper measures must be taken to ensure the integrity of the air supply pipes and to carry out regular maintenance, in order to avoid air loss and insufficient oxygen supply, which could lead to energy waste.