Discussion on DO control in activated sludge biological pond

For the DO wastewater treatment plant in the activated sludge method biological pond, the blower is used to provide sufficient dissolved oxygen to the biological pond to ensure that the microorganisms decompose organic matter and the nitrification of ammonia nitrogen. As the air needs to be compressed and passed to the bottom of the biochemical aeration tank, this process consumes A large amount of energy is also the main source of energy consumption in the sewage plant. Therefore, many sewage plants hope to achieve precise control on the control of the blower to realize the energy saving and consumption reduction of the sewage plant, but how to achieve it during operation?

From the perspective of energy saving, every operator hopes to obtain a minimum dissolved oxygen control parameter while ensuring the stable quality of the effluent water, and then according to this minimum dissolved oxygen control value, the air output of the fan can be controlled. Adjust, so as to achieve the purpose of energy saving and consumption reduction. This control method is to obtain a minimum dissolved oxygen control value. The use of oxygen in the push-flow biological aeration tank is gradually increased along the process. The ideal working condition is to reach the final exit position, and the microorganisms have been realized In order to avoid the waste of dissolved oxygen caused by the full degradation of organic matter, the more traditional value of the biological tank is to control about 2mg/L at the outlet of the aeration tank, that is, to maintain a certain amount of surplus. To resist the impact of load changes without causing too much dissolved oxygen waste. Therefore, adjusting the supply of aeration air volume according to this value is a more traditional dissolved oxygen control method.

With the process requirements for the removal of nitrogen and phosphorus, the control of dissolved oxygen in the biological pond becomes more and more complicated. The traditional single DO index control can no longer meet the precise control of the blower under the new index process requirements. In the A2O process, the internal reflux returns the aerated nitrification solution to the anoxic zone of denitrification. The rich dissolved oxygen in the nitrification solution should avoid affecting the anoxic environment required by the anoxic zone. The nitrification solution should bring back as little free oxygen as possible in the aerobic zone. The internal reflux point of the general nitrification solution is set at the end of the aerobic zone, so the dissolved oxygen control value at the outlet of the original 2mg/L aerobic tank means that there is still 2mg/L of dissolved oxygen remaining in the water. The value will appear to be relatively high. The nitrification solution with high dissolved oxygen returns to the hypoxic zone, and there will be a process of releasing oxygen at the front end of the hypoxic zone. This process will take up a part of the hypoxic zone to complete, which reduces the lack of oxygen. The space for the denitrification reaction in the oxygen zone leads to a decrease in the progress of the denitrification reaction, and the removal effect of total nitrogen becomes poor. But at what value is the DO control more reasonable, and how to ensure the degradation of organic matter and the nitrification of ammonia nitrogen in the aeration tank under low dissolved oxygen at the outlet?

Before determining the optimal value of dissolved oxygen at the outlet, it is first necessary to understand the main role of dissolved oxygen in the biological aeration tank. According to the basic principles of biological degradation of sewage treatment, dissolved oxygen is mainly used in two reactions, one is The degradation process of organic matter is a process in which heterotrophic bacteria convert organic matter in sewage into CO2 under aerobic conditions. The other process is a process in which nitrifying bacteria convert ammonia nitrogen in sewage into NO3-N under aerobic conditions. The corresponding equations are as follows:

 C5H7O2N+5O2→5CO2+2H2O+NH3+energy

 NH4＋＋2O 2→NO3+2H ++H 2O

Compared with heterotrophic bacteria, the nitrifying ability of nitrifying bacteria is weaker. Dissolved oxygen will be first consumed by heterotrophic bacteria as degrading organic pollutants, and then used by the nitrifying bacteria in the nitrification process. From the utilization process of the dissolved oxygen in the biological aeration tank, we can know that the time when the dissolved oxygen is no longer consumed by microorganisms is the stage after the ammonia nitrogen is converted to nitrate nitrogen. When the ammonia nitrogen drops to zero, the dissolved oxygen produced by the blast aeration Oxygen is no longer consumed, and the dissolved oxygen in the biological pond starts to rise.

In the traditional activated sludge theory, the organic pollutants in the water, that is, the carbon source, are degraded by aerobic heterotrophic microorganisms. Therefore, in the design of many sewage plants, the aerobic section is relatively large, mainly It is necessary to consider that the organic matter in the influent water needs to be fully degraded to ensure that the COD of the effluent reaches the emission standard value. However, with the large-scale adoption of phosphorus and nitrogen removal processes, the phosphorus release process of phosphorus accumulating bacteria in the anaerobic zone and anoxic zone, and the denitrification and denitrification process of denitrifying bacteria will consume carbon sources in the water. The aerobic tank designed by the traditional activated sludge process is based on the removal of all the carbon sources in the influent water in the aerobic tank. This design calculation ignores the carbon source consumed by anaerobic and hypoxia. Therefore, the aerobic tank designed according to the traditional activated sludge method is often larger than the actual aerobic volume required. In practical applications, many sewage plants will feel that the dissolved oxygen in the aerobic tank at this stage is always higher, or the choice of blower The type is always large, it is difficult to control the actual air volume required, and it is impossible to effectively reduce the dissolved oxygen at the outlet of the aerobic tank to the desired control.

In addition to this reason, the quality of the influent water at this stage is affected by the rainwater from the combined rain and sewage pipe network of the urban pipe network, and the sewage pipe network has been in disrepair over the years, and the groundwater in the area with high groundwater level has been infiltrated seriously, resulting in water ingress. The BOD value is far lower than the design index value of the urban domestic sewage used in the design, resulting in a very large difference between the organic matter degraded by the microorganisms in the design of the sewage plant and the actual influent organic matter, which causes the air volume used in the design to be much higher than the actual The required air volume. Therefore, the influent water quality is too low, which also causes an excess of dissolved oxygen in the wastewater treatment plant. Especially the selection of blowers to meet the high standard of influent water quality also increases the difficulty of effective process control of dissolved oxygen.

The fans at this stage adopt air levitation and magnetic levitation fans, and there is a wide range of frequency adjustment space. It seems that this problem can be solved from the operation, but in actual operation, it can not really achieve consumption reduction. The main reason is that the fan has a surge zone under complex multi-machine operating conditions. In order to avoid damage to the fan caused by the surge effect, the control area of the fan is often reduced to a very narrow range, especially even in the control area. To meet the control of low dissolved oxygen, the dissolved oxygen at the outlet end of the aerobic tank is too high. In order to achieve better biological denitrification control, many sewage plants have to add a gas release valve at the end of the aeration main pipe of the biological tank to discharge excess The air supply is completely contrary to the concept of energy saving and consumption reduction.

These factors have caused the dissolved oxygen in the biological aeration tank to be too high, and the process operation problems caused by the excessively high dissolved oxygen have become more and more prominent. The rich dissolved oxygen in the back stage of the biological aeration tank is caused by the degradation of organic matter and ammonia nitrogen. The aging of activated sludge is serious, and more and more sewage plants have a large number of biological foam and sludge expansion problems, and the high dissolved oxygen at the end causes the free dissolved oxygen in the internally refluxed nitrification solution to cause anoxic denitrification. The size of the zone is reduced, the reaction time is shortened, and the effective carbon source in the water is consumed, which leads to the incomplete denitrification reaction. The sewage plant needs to add more additional carbon sources to ensure the stable operation of biological denitrification, which consumes The medicament increases the operating cost.

From these aspects, the control of dissolved oxygen in biological aeration tanks is restricted by the new process control targets at this stage, and standards must be re-established, especially for the general use of anaerobic tanks and anoxic tanks. The carbon source removal in the previous stage requires a new accounting to check the effective volume of the aerobic tank, re-determine the system from the aeration air volume, reduce the selection of the blast air volume, lower the fan selection, reduce energy consumption, and at the same time The operation in the hypoxic zone provides a more optimized operation guarantee.

Control discussion