The AAO process, also known as anaerobic anoxic aerobic biological treatment method, is based on the A/O phosphorus removal process. It adds an anoxic reactor after the anaerobic reactor, and then refluxes the mixed liquid in the aerobic reactor to the anoxic reactor for denitrification and denitrification. I believe that all water enthusiasts are familiar with this process, which can remove both phosphorus and nitrogen. Who doesn't love it! However, despite its powerful functionality, there are also many problems that come with it. A while ago, the editor visited an AAO process sewage treatment plant. After smoking a pack of cigarettes, the teacher Fu Wei talked about the problems encountered during the commissioning process of the AAO sewage treatment plant and his accumulated commissioning experience over the years.
Before debugging the AAO process, it is necessary to carefully inspect the structures, equipment, etc. The master has been working in the sewage treatment plant for so many years and found that the following problems are quite common:
(1) Unclean construction waste in structures and pipelines has caused blockages in water pumps and aeration systems, affecting sludge discharge;
(2) The sealing of reserved holes, pipeline expansion joints, and cable penetrations is poor, and there is leakage after water supply, which affects the debugging work;
(3) The leakage at the joint between the water outlet weir and the wall is severe, even causing the weir mouth to not discharge water, which cannot meet the design requirements;
(4) The improper placement angle or position of the agitator or thruster results in energy waste and insufficient local flow rate, leading to local sludge deposition.
Therefore, before the water commissioning of the sewage treatment plant, a detailed inspection must be carried out to ensure that all structures, pipeline lines, and mechanical and electrical equipment can operate according to the design requirements.
Process parameter control of dissolved oxygen during debugging process
At the initial stage of activated sludge cultivation, microorganisms did not grow and oxygen demand was low, so the air supply was reduced, and even some air could be released through the air drain valve to prevent excessive foam on the aeration tank.
If the amount of foam is too large, affecting the commissioning operation, intermittent aeration can be used. Generally, the stop aeration time is controlled between 4~8 hours. At the same time, observe the color and smell of the sludge in the aeration tank. The normal color of the sludge is yellow brown, with a muddy smell. When there is insufficient oxygen supply or anaerobic, the sludge color becomes black, and there is a phenomenon of sludge floating, aeration must be carried out at this time. In domestic sewage, there are a large number of microorganisms in the biological phase. When the number of bell worms and branch accumulating worms in the aeration tank increases, and various protozoa such as ciliates and fixed ciliates appear, it indicates that the activated sludge is basically mature. As the activated sludge grows, some protozoa such as rotifers and nematodes can be found after about two to three weeks, indicating that the activated sludge is completely mature. The calculation of sludge growth requires a process, and the calculation of sludge growth can be roughly based on 40% of the influent BOD5 (including endogenous respiration and oxidation consumption), plus the total amount of SS pollution intercepted in the influent, which is the total amount of activated sludge in the biological system. These sludge are distributed in the anaerobic tank, AO tank, and secondary sedimentation tank. When the amount of activated sludge in the aeration tank reaches a concentration of 1000-1500mg/L and a settling ratio of about 10%, the sludge cultivation process is basically completed.
In the initial stage of trial operation, the reflux ratio can be controlled to 100-200% to ensure timely reflux of sludge in the secondary sedimentation tank. When microorganisms reach a certain stage of growth, adjust the reflux ratio to below 100%. When SVI is between 50-100ml/g, it can reduce the external reflux ratio to 50% -60%. In addition, based on the sedimentation curve, reflux ratio control is carried out while ensuring that nitrification and phosphorus release do not occur in the secondary sedimentation tank. The optimal control management for denitrification requires the control of dissolved oxygen. For the AAO process, the dissolved oxygen in the anaerobic tank and aerobic section can ensure the full release and absorption of phosphorus by polyphosphate accumulating bacteria; Controlling the dissolved oxygen values in the anoxic and aerobic stages is an important factor affecting nitrification and denitrification. (1) Generally, the DO level in the aerobic zone is controlled between 1.5 and 2.5 mg/L. If the DO in the aerobic zone decreases, it indicates insufficient aeration; (2) The DO in the anoxic section should be controlled below 0.5mg/L. If the DO is high, it indicates that the internal reflux ratio is too high; (3) The DO in the anaerobic tank should be controlled below 0.2mg/L. When the DO is too high, check whether the external reflux ratio configuration is reasonable or whether the stirring intensity is too high, which may cause the oxygen in the air to be restored to the water. F/M (BOD load) control was implemented during the early stages of wastewater treatment plant commissioning. Due to insufficient activated sludge, the BOD load exceeded 0.3kgBOD/kgMLVSS. d, resulting in low BOD removal rate and less than 30% denitrification efficiency.
When the BOD load gradually approaches 0.3kgBOD/kgMLVSS. d, the BOD removal rate can reach 90%, the nitrification efficiency is significantly improved, and the denitrification effect can reach 70%.
When the sludge load of the system continues to decrease to 0.15kgBOD/kgMLVSS. d, the denitrification efficiency does not change much because the ratio of organic matter to nitrogen is constant.
When the BOD load is less than 0.1kgBOD/kgMLVSS. d, the BOD removal rate and denitrification efficiency actually decrease. This is due to the low organic matter in the influent, the microorganisms are in a state of starvation and aging, the activated sludge flocs disintegrate, the flocculation performance deteriorates, and the settling performance deteriorates, resulting in turbid effluent.
Therefore, it is necessary to maintain a moderate sludge load in actual operation. The BOD load variation of the general AAO process is controlled within the range of 0.10-0.30kgBOD5/kg/MLVSS. d, which belongs to the low load operation process. Within this range, the BOD removal rate can reach 90%, and the denitrification efficiency is over 70%. Control of sludge age
The control of sludge age is crucial for the AAO process.
When the inflow and water quality are constant, it is necessary to reasonably control the discharge of residual sludge and adjust the concentration of MLSS.
Usually, during winter operation, MLSS is controlled at around 3500mg/L, sludge load is around 0.1kgBOD5/kg/MLVSS.d, and SRT is controlled at around 12 days;
During summer operation, MLSS is controlled at a low concentration of around 2000mg/L, with a sludge load of around 0.18kgBOD5/kg/MLVSS. d, and SRT is controlled for less than 8 days for better performance.
At the end of the AAO process debugging plan experience, the master also came up with his own debugging plan and summarized it for mThe commissioning of AAO sewage treatment plant requires the design of appropriate data, collection of sewage component content, development of standardized operating procedures, training on relevant operations based on the condition of operating personnel, development of equipment dashboard control, and the design of sewage treatment plant commissioning plan based on equipment operation and process training for sewage treatment.
During the commissioning of AAO sewage treatment plant, it is necessary to check whether the angle and distribution of the grille comply with relevant national regulations according to the requirements of the equipment, and turn on the grille cleaner to operate the grille. The bolt connections in the composition of the water pump should be intact, and the operation and power distribution function of the water pump should be checked. Check the installation dimensions of the sand water separator, confirm if there are any issues with the power distribution equipment, inspect the operation of the fan, check the operation and installation of various devices, and ensure that the on-site equipment is debugged to the optimal level. During the commissioning of AAO sewage treatment plant, various microorganisms should be coordinated according to different environmental and process conditions, including anaerobic, anoxic, and aerobic conditions, and attention should be paid to removing organic matter. During the commissioning of AAO sewage treatment plants, several influencing factors should be considered, including temperature, pH value, dissolved oxygen, C/N ratio, sludge age, nitrate, and other factors in the design. The temperature of AAO sewage treatment plant can increase the proliferation rate and activity of bacteria, and temperature treatment is carried out according to the aerobic, anoxic, and anaerobic stages. The temperature range for aerobic nitrification reaction is generally between 5 and 35 degrees Celsius. The reaction rate can be accelerated according to the temperature increase, and the appropriate temperature range for active nitrification reaction can be selected. The denitrification reaction during the anoxic stage occurs at 5-27 degrees Celsius and shows a linear correlation with temperature. During the anaerobic stage of the AAO sewage treatment plant, attention should be paid to the pH range of the biological phosphorus removal system and the appropriate pH value should be controlled. If the pH value is not suitable, it needs to be adjusted in advance to avoid sludge poisoning and reduce the pH reaction of nitrate nitrifying bacteria. Pay attention to the role of anaerobic and nitrate in the commissioning of AAO sewage treatment plant, and achieve the synthesis of nitrate reductase. Attention should be paid to the reproductive characteristics and growth rate of nitrifying bacteria in the setting of AAO sewage treatment plants. Attention should also be paid to the decomposition of degrading bacteria and the system situation.
