Let’s discuss how to enhance the phosphorus and nitrogen removal capabilities of AA/O!
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1. The principle of the A2/O process: In the first anaerobic tank, phosphorus is released, increasing the concentration of P in the wastewater; meanwhile, soluble organic matter is absorbed by cells, resulting in a decrease in the BOD concentration in the wastewater； Additionally, some of the NH3—N is removed due to cellular synthesis, resulting in a decrease in the NH3—N concentration in the wastewater, whereas the NO3-—N content remains unchanged. In the anoxic tank, denitrifying bacteria use the organic matter in the wastewater as a carbon source to reduce the large amounts of NO3–N and NO2–N brought in by the recirculated mixture to N2, which is then released into the air. As a result, the BOD5 concentration continues to decline, the NO3–N concentration drops significantly, while there is little change in phosphorus levels. In the aerobic tank, the concentration of organic matter continues to decline after being biologically degraded by microorganisms ； Organic nitrogen is ammonified and then nitrified, resulting in a significant decrease in the NH3–N concentration; however, as the nitrification process progresses, the concentration of NO3–N increases, and P will also decline at a relatively fast rate due to the excessive uptake by polyphosphate-accumulating bacteria. Therefore, the A2/O process can simultaneously carry out functions such as the removal of organic matter, nitrification and nitrogen removal, as well as the removal of excess phosphorus. For nitrogen removal to be possible, NH3–N must be completely nitrified, and the aerobic tank is capable of performing this function ； The anoxic tank performs the denitrification function. 2. Characteristics of the A2/O process: (1) The organic combination of three different environmental conditions – anaerobic, anoxic, and aerobic – along with various types of microbial communities enables it to remove organic matter as well as carry out nitrogen and phosphorus removal. (2) Among the processes for simultaneous nitrogen and phosphorus removal along with organic matter removal, this process has the simplest flow structure, and its total hydraulic retention time is also shorter than that of other similar processes. (3) Under alternating anaerobic-anoxic-aerobic operation, filamental bacteria do not proliferate in large numbers; the SVI is generally less than 100, and sludge bulking does not occur. (4) The phosphorus content in sludge is high, generally above 2.5%. (5) The anaerobic-anoxic tank only requires gentle stirring to ensure mixing, as long as it does not increase dissolved oxygen levels. (6) The sedimentation tank must be kept free from anaerobic and hypoxic conditions, in order to prevent phosphorus-accumulating bacteria from releasing phosphorus and thus deteriorating the quality of the effluent, as well as to avoid nitrogen gas being produced through denitrification, which could interfere with the sedimentation process. (7) The denitrification efficiency is affected by the reflux ratio of the mixed liquid, while the phosphorus removal efficiency is influenced by the DO and nitrate oxygen carried in the returned sludge; therefore, the efficiency of both denitrification and phosphorus removal cannot be very high. 3. Influencing factors of the A2/O process (1) The impact of biodegradable organic matter in wastewater on nitrogen and phosphorus removal. Soluble organic matter that can be rapidly biodegraded in the mixtures of the biological reaction tanks has the greatest impact on nitrogen and phosphorus removal. In the anaerobic stage, such organic substances are absorbed, reducing their concentration; at the same time, polyphosphate-accumulating bacteria release phosphorus, enabling more intense phosphorus absorption in the aerobic stage, thereby achieving the goal of phosphorus removal. If there is little rapidly biodegradable organic matter in the wastewater, polyphosphate-accumulating bacteria are unable to release phosphorus properly, resulting in the aerobic stage also being unable to absorb more phosphorus. Experimental studies have shown that for optimal phosphorus removal, the ratio of dissolved phosphorus to dissolved BOD5 in the feed water to the anaerobic section should be less than 0.06. In the anoxic zone, when the BOD5 concentration in the wastewater is high and there is an adequate amount of soluble organic matter that can be rapidly biodegraded, meaning the C/N ratio in the wastewater is high, the denitrification rate of NO3–N is at its maximum. In this case, a hydraulic retention time HRT of 0.5 to 1.0 hours is sufficient in the anoxic zone ； If the C/N ratio is low, the HRT in the anoxic zone requires 2–3 hours. It can be seen that the C/N ratio in wastewater has a significant impact on the efficiency of nitrogen and phosphorus removal. For urban wastewater with low BOD5 concentrations, when the C/N ratio is low, the nitrogen removal rate is not high. Generally, when the COD/KN in wastewater is greater than 8, the total nitrogen removal rate can reach 80%. (2) Influence of sludge age Өc: The sludge age in the A2/O process system is influenced by two factors. One of these is the generation time of nitrifying bacteria; as a result, Өc is somewhat longer than that in conventional activated sludge processes ； On the other hand, since phosphorus removal is primarily achieved through the excess sludge disposal system, it is not advisable for the Өc value in the A2/O process to be too high. Taking these two aspects into consideration, the Өc in the A2/O process is generally 15~20 days. (3) Effect of dissolved oxygen (DO) in the A2/O process system: In the aerobic stage, an increase in DO accelerates the nitrification rate of NH4+–N, but this growth trend slows down once DO exceeds 2 mg·L-1. Therefore, a higher DO level is not necessarily better. Because the DO level in the aerobic zone is too high, dissolved oxygen is carried to the anaerobic and anoxic zones via sludge return and mixed liquor return, resulting in incomplete anaerobiosis in the anaerobic zone and affecting the release of phosphorus-accumulating bacteria as well as the denitrification of NO3–N in the anoxic zone. (4) Effect of sludge load rate Ns: In the aerobic tank, Ns should be below 0.18 kg BOD5/(kg MLSS·d); otherwise, the number of heterotrophic bacteria will **exceed that of nitrifying bacteria, thereby suppressing the nitrification process. In the anaerobic tank, Ns should be greater than 0.10 kg BOD5/(kg MLSS·d); otherwise, the phosphorus removal efficiency will decline sharply. Therefore, in the A2/O process, the sludge load rate Ns has a narrow range. (5) Impact of the KN/MLSS load rate: High concentrations of NH4+—N can inhibit nitrifying bacteria; therefore, the KN/MLSS load rate should be less than 0.05 kg KN/(kg MLSS·day), otherwise it will affect the nitrification of NH4+—N. (6) Influence of sludge return ratio and mixed liquor return ratio on denitrification efficiency: The denitrification efficiency is closely related to the mixed liquor return ratio; a higher return ratio yields better results, but it increases operational costs, and vice versa. The appropriate mixed liquor reflux ratio for the A2/O process is generally 200%. Generally, the sludge return ratio ranges from 25% to 100%. If it is too high, too much DO and nitrate oxygen will be introduced into the anaerobic tank, affecting its anaerobic conditions (DO)
(1) When the NO2--N value is too high and deviates from the normal stable level, it indicates that the internal recirculation ratio is too low; in such cases, the internal recirculation ratio should be increased； If the DO level in the oxygen-deficient section is greater than 0.5 mg/L, first check whether the internal recirculation ratio is too high; if so, reduce it appropriately by adjusting the number of internal recirculation pumps in operation. (2) If both the total phosphorus and BOD levels are normal but the ammonia nitrogen level is high, it indicates that there is insufficient DO in the aerobic zone. Check the DO level in this zone to see if it is below 2 mg/L. If it lies between 1.5–2.0 mg/L, it is not sufficient for nitration to take place; in such cases, the air supply should be increased to keep the DO level between 2–3 mg/L. It’s also possible that industrial wastewater present in the incoming wastewater is inhibiting nitration. If the BOD5 and NO3-N levels in the aerobic tank are low during this process, the BOD5 of the inflowing water will also be low; in such cases, it is necessary to use an inter-domain primary sedimentation tank to increase the inflow volume. (3) The suspended solids concentration in the aeration tank is high, and the sludge age in the aerobic tank is long; appropriate measures should be taken to accelerate the discharge of excess sludge. Based on the total nitrogen content of the influent water, the internal recirculation ratio is adjusted; generally, a ratio of 300%-500% yields the best denitrification results, and this ratio can be adjusted as appropriate. (4) While ensuring that denitrification and secondary phosphorus release do not occur in the secondary sedimentation tank, the external recirculation ratio should be controlled between 50-100%, in order to prevent too much NO3-N from being returned to the anaerobic tank and thereby affecting the phosphorus removal efficiency. When phosphorus removal is the primary goal, if the NO3-N concentration in the anaerobic section exceeds 4 mg/L, the external recirculation ratio must be reduced; the optimal recirculation ratio should be determined during the tuning process. (5) Keep the DO in the anaerobic zone below 0.2 mg/L, in the anoxic zone below 0.5 mg/L, and control the DO in the aerobic zone between 2–3 mg/L to ensure biological nitrification. (6) Maintain the pH value above 7.0; if it is below 6.0, lime or sodium carbonate should be added to increase alkalinity. (7) Use the online bio-redox potential OPR meter to monitor the OPR value. Generally, in a mixed solution containing NO3-N, the higher its concentration, the higher the OPR value; whereas in the presence of PO43-P, the OPR decreases as the concentration of PO43-P increases. To ensure effective nitrogen and phosphorus removal, the OPR in the anaerobic zone should be below -250 MV, it should be controlled at -100 MV in the anoxic zone, and above 40 MV in the aerobic zone. (8) During operation, it was found that ； An increase in the OPR value in the anaerobic zone indicates that the phosphorus removal efficiency has declined; the reasons for this need to be analyzed. Generally, too much NO3-N being introduced through external recirculation, or excessive air aeration due to high-intensity submersible agitators, can lead to an increase in this concentration. An increase in the OPR value in the anoxic zone suggests that too much DO is being introduced through internal recirculation. If the OPR value decreases in the aerobic zone, it indicates insufficient aeration, resulting in a lower DO concentration.