Difficulties in controlling aeration volume in sewage treatment

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Issue Time: Nov 26,2024

Summary

The power consumption of the blast aeration system generally accounts for about 60% of the power consumption of the entire plant, and is the key to energy saving in the entire plant. The most fundamental energy-saving measure is to improve aeration control efficiency and reduce oxygen waste, thereby reducing air volume.

Difficulties in controlling aeration volume in sewage treatment

Preface

Air volume control is the most significant energy-saving method for aeration systems. According to a survey of 12 treatment facilities in the United States by the U.S. Environmental Protection Agency, 33% of electricity can be saved when controlling air volume using dissolved oxygen (DO) as an indicator. According to the relationship between fan air volume and energy consumption, it can be seen that the power consumption changes greatly with the air volume, so the energy saving effect of air volume control is significant, and the greater the power, the more obvious the effect. Of course, the air volume cannot be reduced arbitrarily, and it will be affected by many factors. Influence.

From the perspective of the treatment process, the aeration system must be controlled, because if the aeration system is improperly operated and the aeration volume is too small, sludge corrosion may occur in the secondary sedimentation tank due to lack of oxygen, that is, anaerobic decomposition of the sludge at the bottom of the tank. , producing a large amount of gas, causing the sludge to float. When the aeration time is long or the aeration volume is too large, a high degree of nitrification will occur in the aeration tank, resulting in a higher nitrate concentration in the mixed solution. At this time, a large amount of N2 may be produced in the sedimentation tank due to denitrification, causing the sludge to float.

In addition, whether the distribution of aeration volume is balanced and stable is also an important reason that affects the treatment effect and energy consumption. When the aeration system is running, due to various interferences, the distribution of the aeration volume will change. For example, if the aeration head in one place is blocked, the gas flow will decrease. At the same time, it will also cause the flow in other places to increase. On the contrary, the aeration head will If it is damaged, the gas flow will increase greatly, and the flow in other places will also decrease sharply. These will make the biological reaction unbalanced and the treatment quality will decrease. In order to achieve the treatment effect, the aeration volume has to be adjusted. At this time, the change of dissolved oxygen at a certain point cannot accurately reflect the treatment status of the biological pool, making the control using dissolved oxygen as an indicator unstable and increasing energy consumption.

Industry status

After summarizing the operation of most sewage treatment plants, it was found that the investment in automation equipment was low, the energy consumption was high, and most of the systems failed to meet the design operation requirements when they were put into production, or changed to a partially automatic and partially manual operation state after a period of operation, especially the aeration system. The main reasons for the analysis are as follows:

1. Inadequate training of the automatic control system. Many sewage treatment plant operators have not received systematic training from the control system supplier. In addition to basic operations, there is no theoretical explanation of the aeration system adjustment technology, which makes managers have to re-explore at work.

2. Operation experience is not utilized. A very important point of sewage treatment plants is that after long-term operation, daily rules can be summarized and are relatively stable. For managers, these rules are often more useful than expensive automatic control equipment. However, in the construction of sewage plants, many designs do not leave managers with sufficient adjustment space, and these useful experiences also lack ways to apply to the construction of other sewage facilities.

Control strategy

Difficulties in DO Control

The variability of sewage water quality and the complexity of biochemical reactions in biological treatment systems determine that the detection and control of dissolved oxygen (DO) in sewage treatment is a large lag system. The detection results and parameter processing and adjustment are often delayed for several hours or even days, resulting in the discharge of a large amount of unqualified water. The characteristic of this system is that the operation and management of the sewage biological treatment system is quite technically difficult, requiring managers to have a good foundation of environmental engineering knowledge and quite rich operation and management experience.

In addition, the dissolved oxygen index cannot directly reflect the oxygen demand of the biological reaction. It only reflects the residual degree of oxygen in the reaction tank, and the gas volume cannot be directly calculated based on its value and changes.

Although traditional PID control is widely used in engineering, it can only solve the regulation problem of linear systems. PID in the aeration system can realize the control of flow, but its control ability on water quality treatment effect is limited. When controlling dissolved oxygen (DO), the setting of PID parameters needs to be adjusted continuously according to the actual situation such as season and water quality changes. From the perspective of control theory, the biological treatment process of sewage has the characteristics of large lag, nonlinearity, randomness and multivariate, and the established model is also empirical and conditional. Therefore, the classical control method based solely on the theoretical model cannot meet the needs of dissolved oxygen (DO) regulation, resulting in frequent adjustment of blowers and valves, large overshoot, reduced equipment life and high energy consumption.

Importance of flow control

Air mass flow is an indicator that directly affects the effect of aeration treatment. From an engineering perspective, a large reaction pool often requires many sets of aeration equipment, including air pipes, aeration heads or aerators. In actual operation, whether these equipment can work stably and whether they can detect and suppress faults in time will affect the stability and balance of the aeration process, the biological reaction effect and power consumption. Unstable flow distribution will disrupt the true meaning of dissolved oxygen detection parameters, making dissolved oxygen control, which is prone to oscillation, even more difficult to control.

Aeration tanks are usually hundreds or thousands of square meters of flowing water pools. The air pipeline transports compressed air to the aeration equipment at the bottom of the pool through the main pipe and branch pipes. For example, air is transported from A to B, C, D, E, and F respectively. In the design of the aeration system, the aeration volume should be evenly distributed as needed. In fact, due to pipeline pressure loss, there are differences in air pressure and flow at positions B and F. When the total air volume is adjusted due to changes in water quality or water volume, the pressure difference and flow difference between positions B and F will also change, which will cause a deviation in aeration distribution, and this deviation is also variable; in addition, when the system is in operation, if the aeration facilities at a certain position (such as D) are blocked or leaking, it will cause changes in the pressure and flow at that position, and at the same time, it will cause the pressure and flow of the entire air pipeline to be redistributed, and the air flow at other points (B, C, E, F) will also change accordingly, causing a deviation in aeration distribution. The uneven aeration distribution in the above operation is often hidden and difficult to detect on the water surface. Uneven aeration distribution makes it more difficult to dissolve oxygen. Because in engineering, dissolved oxygen can only be detected at a certain point (usually the outlet of the aeration tank), and cannot reflect the distribution of oxygen. One condition for dissolved oxygen control is that the dissolved oxygen value truly reflects the environmental state of the biological reaction in the aeration tank. When the aeration distribution is uneven, this condition is not true and the control effect will not be ideal.

Therefore, air flow control is a very important part of aeration control. If flow detection equipment and regulating valves are installed at positions B, C, D, E, and F, and control links are established, flow deviations will be corrected during operation, and dissolved oxygen control will be more effective.

Analysis Conclusion

The characteristics of the aeration system are as follows:

1) The sewage input is a random variable, and its external environment has many uncertain factors, so it is difficult to establish an accurate mathematical model of the aerated biological system;

2) The parameters of the aeration system are high in dimension, strongly coupled, and highly nonlinear;

3) There is a large time lag in dissolved oxygen, and the system balance is difficult to achieve in a short time;

4) The sewage treatment process requires a large amount of practical experience and knowledge of skilled operators;

5) The stability and uniformity of the aeration flow distribution is the basis for controlling the treatment effect and energy saving.

Therefore, the control of the aeration system should be improved from two aspects. One is to solve the problem of the balance and stability of the air flow in the aeration tank, and the other is to seek a control strategy suitable for dissolved oxygen control of the air flow.