Seeking advice on denitrification and phosphorus removal processes

This post was last edited by zhaolijun on 2015-10-29 at 15:25. The process that enables simultaneous nitrogen and phosphorus removal is the A/A/O process. And anaerobic/anoxic/aerobic processes. Oxic end water is recycled to the anoxic zone for nitrogen removal, while sludge from the secondary sedimentation tank is recycled to the anaerobic zone for phosphorus removal. I hope this is helpful to you.

Exploration of Methods for Controlling Phosphorus and Nitrogen Removal in the A2O Wastewater Treatment Process ________________________________________ (2005-11-23 10:07:49) 1. Introduction to the A2O Process for Wastewater Treatment. The A2O biological nitrogen and phosphorus removal process is a combination of traditional activated sludge processes, biological digestion and reverse digestion processes, as well as biological phosphorus removal processes. Its process flow diagram is shown in Figure 1; the biological tank is divided into an anaerobic section, an anoxic section, and an aerobic section through the use of aeration devices, propellers (in the anaerobic and anoxic sections), and recirculation channels. Within this process flow, BOD5, SS, and nitrogen and phosphorus in various forms will be removed one by one. In the activated sludge of the A2O biological nitrogen and phosphorus removal system, the microbial community is mainly composed of nitrifying bacteria, denitrifying bacteria, and polyphosphate-accumulating bacteria. In the aerobic zone, nitrobacilli convert the ammonia nitrogen and organic nitrogen present in the inflowing water into ammonia nitrogen, which is then transformed into nitrate through biological nitrification ； In the anoxic zone, denitrifying bacteria convert the *AO acids brought in by backflow into nitrogen through biological denitrification, which is then released into the atmosphere, thereby achieving nitrogen removal ； In the anaerobic zone, polyphosphates bacteria release phosphorus and absorb readily degradable organic substances such as lower fatty acids ； In the aerobic stage, phosphorus-accumulating bacteria absorb excess phosphorus, and phosphorus is removed through the discharge of excess sludge. All three types of bacteria mentioned above are capable of removing BOD5, but it is actually denitrifying bacteria that play the primary role in this process. Figure 1: Flow diagram of the biological treatment section in the A2O process. 2. Determination of monitoring and control parameters for the A2O tank: The efficiency of the A2O process in removing phosphorus and nitrogen from wastewater depends on factors such as DO, the internal recirculation ratio r, the external recirculation ratio R, the sludge age SRT, wastewater temperature, and pH value. Generally, the DO in anaerobic tanks is below 0.2 mg/L, the DO in anoxic tanks is below 0.5 mg/L, while the DO in aerobic tanks is above 2.0 mg/L ； The pH value of the sludge mixture is greater than 7 ； The SRT is 8–15 days. However, the A2O biological phosphorus and nitrogen removal process is essentially a combination of a series of biological redox reactions, and the ORP (oxidation-reduction potential) in the mixed liquor at various stages of the A2O biological tank can comprehensively reflect the changes in various parameters within the tank. The higher the DO in the mixture, the higher the ORP value as well ； And when phosphate ions and free phosphorus are present, the ORP decreases as the concentrations of phosphate ions and free phosphorus increase. In the typical A–A–O biological phosphorus and nitrogen removal process, the ORP in the anaerobic stage should be less than –250 mV, it should be around –100 mV in the anoxic stage, and above 40 mV in the aerobic stage. If the ORP in the anaerobic section increases, it indicates that the DO level is too high. This may be due to an excessive reflux ratio that introduces more oxygen, as well as too much nitrogen from the recycled sludge; it can also be related to intense stirring that leads to oxygenation through air. If the ORP in the oxygen-deficient zone increases, it indicates that the DO level is too high; this may be due to an excessive reflux ratio that introduces more oxygen, as well as excessive stirring intensity that leads to oxygenation through air. Based on the effects of the changes in various parameters in the A2O tank, as described above, on the phosphorus and nitrogen removal processes in wastewater treatment, appropriate testing instruments should be selected to monitor these parameter changes during wastewater treatment, thereby providing a basis for the operational control of wastewater treatment plants. In the typical A2O process, the data that need to be monitored include: For the influent water: flow rate Q, COD, COD5, pH, and water temperature T. In the anaerobic section of the A2O tank: dissolved oxygen DO and redox potential ORP. In the anoxic section of the A2O tank: dissolved oxygen DO and redox potential ORP. In the aerobic section of the A2O tank: dissolved oxygen DO, redox potential ORP, and MLSS. For the effluent water: COD and COD5. Based on the recommended typical instrument configuration and process control characteristics, we propose using ORP and DO as the main control parameters to manage the aeration system, internal recirculation system, external recirculation system, and excess sludge discharge system. This approach enables effective phosphorus and nitrogen removal, helps to reduce BOD5 levels in wastewater, and simultaneously saves energy to ensure the efficient and stable operation of the entire system. 3. Control methods for the A2O wastewater treatment process. In the traditional control of the A2O tank in this wastewater treatment process, PID regulation is used for the DO level (air supply volume), and PID regulation is also applied to MLSS (recirculation ratio); in both cases, it involves control of a single parameter related to a single variable. However, the wastewater treatment process is one with a very large time lag, and there are many parameters that influence it; it is therefore impossible to control the entire process based on just one specific parameter. In the wastewater treatment process, the control of the aeration system in biological tanks and the control of the wastewater recirculation system are both extremely complex control processes. It is difficult to meet the control requirements using an independent single closed-loop control. With the continuous development of control technologies, and the accumulation of a large amount of operational data during sewage treatment processes, it has become possible to use look-up tables in the control process, thereby enabling fuzzy control. (1) Automated control of the aeration system: Depending on factors such as season, quality of incoming water, temperature of incoming water, volume of incoming water, DO levels in the aerobic tank, as well as COD, BOD5, NH3-N, TOP, TKN, and SS levels in the effluent, different air supply amounts are determined. This involves setting the opening degree of the air control valves, as well as determining the number of blowers to be used and their rotation speeds. It automatically adjusts the process control to achieve fuzzy control. The control system determines the initial control scheme by monitoring values such as DO, water inflow volume, pH, as well as outlet COD, BOD5, NH3-N, TOP, TKN, and SS through online (or manually measured) input parameters, and uses these initial values to control the valve opening degree and the blower speed ； After operating stably for a period of time, if the parameter values detected by the on-line instruments do not meet the requirements, the output value is automatically adjusted based on whether the detected values are too high or too low ； If certain requirements are met over a specific period of time, this period can be regarded as the control lag. The set of values that are closest to these requirements is recorded in a database, creating a control record; we refer to this database as the expert database for control systems. If the optimal operating method is determined manually, it can also be entered into the computer to be included in the expert database. The optimal control methods for different influent conditions are stored in a database, and the computer performs the necessary calculations to create an expert database and expert system for an aeration control system specific to that wastewater treatment plant. The control block diagram is as follows: Figure 2: Block diagram of the control method for the aeration system in the A2O tank. During the operation of the control system, data is continuously collected; at the same time, operators also gather relevant operational data. This data is entered into a computer, where it is analyzed to determine N different adjustment methods for automatic control or to provide operators with suggestions regarding which control mode to use. (2) Automated control of the phosphorus and nitrogen removal system: Depending on factors such as season, quality and volume of the incoming water, DO, ORP, and pH values in various sections of the biological tanks, as well as COD, BOD, TON, and TOP values of the outgoing water, the system automatically collects and stores data, makes adjustments accordingly, and determines the appropriate internal and external recirculation ratios. All of this data is fed into an expert database, which is used to determine the number of pumps that need to be activated for internal and external recirculation, as well as the speed of variable-frequency pumps and the number of underwater propellers that need to be used; table lookups are performed to achieve fuzzy control. The control system determines the initial control scheme based on online measurements such as DO, water inflow volume, pH, ORP, MLSS, as well as manually entered parameters, and uses these initial values to control the number of pumps operating for internal and external recirculation as well as the operating speed of variable-frequency pumps ； After operating stably for a period of time, if the parameter values detected by the on-line instruments do not meet the requirements, the output value is automatically adjusted based on whether the detected values are too high or too low ； If certain requirements are met over a certain period of time, this period can be determined as the control lag ； If the optimal operating mode is determined manually, it can also be entered into the computer and stored in the control system database. The optimal control methods for different feedwater conditions are stored in a database; the computer performs the necessary calculations to create an expert database and expert system specific to that wastewater treatment plant’s sludge return control system. Fuzzy control does not require deterministic values; its main control process is illustrated in the following diagram: Figure 3: Control block diagram of the recirculation system in the A2O process. 4. Conclusion Due to the complexity of controlling wastewater treatment processes, any control method relies on long-term operational experience. There are few examples of successful application of traditional PID control methods; generally, an operating mode is fixed manually depending on different seasons. By utilizing fuzzy control methods and practical experience, an expert database for A2O process control is established; the computer handles the lookup process automatically, thereby generating relatively accurate control plans. This helps to ensure that the water quality meets the required standards, reduces the workload on operators, and at the same time lowers the professional requirements for those who operate the system ； Furthermore, more precise control of the system is achieved, improving the operational efficiency of the equipment and reducing energy consumption, thereby minimizing operating costs.