Although AO method and AAO method both belong to the biological wastewater treatment method, there are few reports on the comparative research between the two. This article compares the water quality before and after the process improvement of Jiyang County sewage treatment plant, and analyzes the two sewage treatment processes, providing a certain basis for the selection of urban sewage treatment plant processes.

1、 Comparative analysis of AO and AAO removal performance

(1) Comparison of COD removal performance

The removal ability of organic matter in sewage treatment process is one of the main indicators to characterize the efficiency of the process, and the size of COD directly reflects the amount of organic matter content in sewage.

Through data processing, a significant difference test was conducted on the COD concentration and COD removal rate in the inflow and outflow of the two processes. The results showed that there was no significant difference in COD in the inflow of the two processes, but there was a significant difference in COD and removal rate in the effluent. The AAO process showed significantly better COD removal than the AO process. The reason is that in the AO process, the denitrification reaction in the anoxic section can consume a part of the organic matter in the wastewater, but most of the organic matter is removed through aerobic degradation. However, in the first phase of the Jiyang County Sewage Treatment Plant, the hydraulic retention time in the aerobic section of the AO process is short, the aeration tank volume is small, and the aeration amount is insufficient, resulting in poor organic matter removal efficiency.

In the AAO process, most of the organic matter is converted into PHB by polyphosphate accumulating bacteria in the anaerobic stage and stored in cells. Some organic matter is removed through denitrification reaction in the anaerobic stage. When the wastewater enters the aerobic stage, the COD concentration is basically close to the discharge standard, and further degradation will occur in the aerobic stage.

Research has shown that the COD removal rate in the anaerobic section of the AAO process can reach over 80%, while the average removal rate in the anaerobic section is less than 10%.

(2) Comparison of denitrification performance

In recent years, with the increasing eutrophication of environmental water quality and the continuous improvement of sewage discharge standards, finding an effective denitrification process has become one of the important issues in the design of sewage treatment plants. Both AAO process and AO process have biological denitrification function, and the denitrification principle of both processes is the same, both of which are denitrification.

By conducting significant differences in TN concentration and TN removal rate in the inlet and outlet water of two processes, the results showed that there was no significant difference in TN in the inlet water of the two processes, but there was a significant difference in TN and removal rate in the outlet water. The AAO process had significantly better TN removal than the AO process.

In the denitrification process, nitrate nitrogen is the main substance in the total nitrogen of the effluent, and the removal rate of nitrate nitrogen in the anoxic stage can be higher than 90%. Research has shown that controlling the nitrate concentration in the effluent of the anoxic zone to 1mg/L~2mg/L can maximize the removal rate of TN and fully utilize COD to enhance the denitrification capacity of the anoxic zone. The mixed solution in the aerobic zone contains a large amount of nitrate nitrogen, which is refluxed back to the anoxic zone through internal circulation and undergoes denitrification reaction in the anoxic zone.

The HRT of the oxygen deficient section in the AO process of Jiyang County Sewage Treatment Plant is too short, only 1.8 hours, which is less than 3.46 hours of the AAO process, and the internal reflux ratio is 50% to 100%, which is less than 150% to 250% of the AAO process, resulting in a lower denitrification function than AAO. Moreover, the denitrification effect of AO process is not as stable as AAO process, and is greatly affected by external factors such as temperature and C/N ratio.

(3) Comparison of phosphorus removal performance

Excessive phosphorus content in water can also lead to the proliferation of microorganisms, vigorous growth of plankton, and eutrophication. The emergence of denitrification phosphorus removal technology is a breakthrough in traditional biological phosphorus removal theory. It can not only solve the contradiction problems in traditional processes, but also facilitate the sustainable treatment of wastewater.

By conducting significant differences in TP concentration and TP removal rate in the inlet and outlet water of two processes, the results showed that there was no significant difference in TP in the inlet water of the two processes, but there was a significant difference in TP and removal rate in the outlet water. The AAO process showed significantly better removal of TP than the AO process.

The reason is that the AO process of the first phase of the Jiyang County sewage treatment plant does not have an anaerobic phosphorus release section. In the biological phosphorus removal process, phosphorus accumulating bacteria can only ensure good phosphorus absorption effect in the anaerobic and aerobic sections by fully releasing phosphorus in the anaerobic section. This process only removes phosphorus through the assimilation of microorganisms.

The phosphorus removal in the AAO process is mainly completed by phosphorus accumulating bacteria. Generally speaking, phosphorus accumulating bacteria absorb more phosphorus in the anaerobic and aerobic stages than they release in the anaerobic stage. Research has shown that the ratio of average phosphorus uptake to average phosphorus release in the AAO process is 1.28, and the phosphorus uptake in the anoxic section is higher than that in the aerobic section.

(4) Summary of removal performance comparison

In summary, the AAO process is significantly better than the AO process in removing organic matter, nitrogen, and phosphorus, especially in phosphorus removal. Since the AO process does not have an anaerobic stage, only a small portion of phosphorus can be removed through the assimilation of microorganisms. Therefore, those who have requirements for phosphorus removal should not choose this process or add chemical phosphorus removal.

2、 The Effect of Temperature on AO and AAO Nitrogen and Phosphorus Removal

(1) The influence of temperature on COD removal in two processes

The temperature has little effect on the removal of COD in the AAO process. Even if the temperature is below 5 degrees Celsius, the COD removal rate can reach over 85%, indicating that the temperature has little effect on the transformation of organic matter by polyphosphate accumulating bacteria.

The removal of COD by AO process often decreases between 5-15 degrees Celsius with increasing temperature, and then increases again. This may be due to the change in the number and structure of microbial communities within the system during the climate transition between warm and cold. The dominant population in the system gradually evolves from microbial communities that prefer one temperature to microbial communities that prefer another temperature, thereby affecting the ability to handle organic matter.

(2) The influence of temperature on denitrification in two processes

The temperature has a significant impact on the denitrification of both processes - both processes show a significant increase in TN removal rate with increasing temperature. Especially in the AO process, when the temperature is above 15 degrees Celsius, the TN removal rate increases almost linearly.

The reason for the enhanced denitrification performance of the two processes by temperature is, on the one hand, that the increase in temperature is conducive to the growth and reproduction of activated sludge microorganisms, improving the efficiency of nitrogen assimilation; On the other hand, the increase in temperature also enhances the metabolic activity of nitrifying and denitrifying bacteria in the system, enhancing the denitrification and denitrification capacity of the system.

It is generally believed that the most suitable growth temperature for nitrifying bacteria is 25-30 degrees Celsius. When the temperature is less than 15 degrees Celsius, the nitrification rate significantly decreases, and the activity of nitrifying bacteria also decreases significantly. When the temperature drops below 5 degrees Celsius, the life activity of nitrifying bacteria almost stops.

It is worth mentioning that sometimes even at low temperatures (below 5 degrees), the TN removal rate of the two systems will not be lower than 40%, indicating that the two systems mainly rely on the assimilation of microorganisms in the activated sludge for nitrogen removal at low temperatures.

(3) The influence of temperature on phosphorus removal in two processes

From the actual situation, as the temperature increases, the AAO process correspondingly improves the phosphorus removal rate, especially when the temperature is above 20 degrees Celsius, the TP removal rate tends to stabilize.

This is because the key to biological phosphorus removal relies on the phosphorus removal activity of phosphorus accumulating bacteria, and an increase in temperature is conducive to increasing the activity of phosphorus accumulating bacteria and improving the phosphorus removal rate.

However, the regularity of phosphorus removal by AO process with temperature variation is not strong, and there is no significant correlation. This is because the AO process has no anaerobic stage, and there is no condition for the survival of phosphorus accumulating bacteria. The removal of phosphorus only relies on the assimilation of microorganisms, so temperature has little effect on the assimilation of microorganisms in this process.

3、 The effect of influent C/N ratio on AO and AAO denitrification and phosphorus removal

(1) The effect of influent C/N ratio on COD removal in two processes

For the AAO process, regardless of the C/N ratio, the COD removal rate remains stable.

There is data indicating that most of the COD is utilized by polyphosphate accumulating bacteria in the anaerobic zone to synthesize intracellular storage PHA, with an average utilization rate between 75% -85%. About 10% of the COD enters the anaerobic zone, and almost no remaining easily biodegradable organic matter enters the aerobic zone. Therefore, this process can achieve full utilization of the influent carbon source and is less affected by the impact of organic load.

The C/N ratio has a certain impact on the removal of COD in AO process. According to the data from the sewage treatment plant in Jiyang County, when the C/N ratio is greater than 10, the COD removal rate slightly decreases as the C/N ratio increases, and the organic load has an impact on the system.

(2) The effect of influent C/N ratio on denitrification in two processes

As the C/N ratio increases in the A/O process, the TN removal rate almost linearly decreases, and the concentration of organic matter has a serious impact on the rate of nitrification process.

This is because nitrifying bacteria are autotrophic bacteria, and organic matter concentration is not a limiting factor for their growth. If the organic matter concentration is too high, heterotrophic bacteria with fast proliferation speed will rapidly reproduce, prioritizing the use of oxygen in water. Autotrophic bacteria do not have an advantage, their activity is inhibited, and it affects the progress of nitrification reaction.

In the AAO process, experimental data shows that when the influent C/N ratio increases from 5 to 9, the TN removal rate steadily increases. When the C/N ratio is 8.9, the TN removal rate reaches 83.2%. However, when the C/N ratio increases from 9 to 14, the TN removal rate does not increase but decreases.

As the C/N ratio increases within a certain range, the TN removal rate also steadily increases. However, when the C/N ratio reaches a certain value and the TN removal rate reaches its maximum, the TN removal rate decreases as the C/N ratio increases.

The main reason is similar to the AO process, where an increase in C/N ratio leads to a decrease in the number of autotrophic bacteria in the system, resulting in a decrease in nitrification efficiency and a decrease in total nitrogen removal rate. According to data, the minimum theoretical C/N ratio for achieving complete denitrification without storing internal carbon sources is 2.86, but the actual required value is far greater than this number.

(3) The effect of influent C/N ratio on phosphorus removal in two processes

The C/N ratio has a significant impact on the phosphorus removal efficiency of AAO process. Experimental data shows that as the influent C/N ratio increases from 5 to 9, the TP removal rate gradually increases.

This is mainly because when the influent C/N ratio is low, the influent carbon source is insufficient, and the reflux sludge contains a large amount of nitrate, which consumes a large amount of COD, leading to insufficient phosphorus release in the anaerobic zone and a decrease in the system's phosphorus removal rate.

When the influent C/N ratio increases from 9 to 14, the removal rate of total phosphorus decreases, especially when the C/N ratio is greater than 11, the removal rate of total phosphorus almost linearly decreases.

This is because at relatively high organic loads, the organic matter in the influent cannot be fully utilized by polyphosphate accumulating bacteria in the anaerobic section, and the excess organic matter will promote the growth of polyphosphate accumulating bacteria, resulting in a decrease in the proportion of polyphosphate accumulating bacteria in the activated sludge and affecting the phosphorus removal effect.

The C/N ratio of influent has little effect on the phosphorus removal efficiency of AO process, mainly because the removal of phosphorus by AO process is only through the assimilation of microorganisms, and the C/N ratio has little effect on assimilation.

4、 The effect of influent C/P ratio on AO and AAO denitrification and phosphorus removal

(1) The effect of influent C/P ratio on COD removal in two processes

The experimental data shows that the regularity of COD removal rate by AO process with the change of C/P ratio is not strong, and there is no significant correlation. It can be seen that the C/P ratio is not the main factor affecting the organic matter removal efficiency of AO process.

For the AAO process, regardless of how the influent C/P ratio changes, its COD removal rate is higher than 85%.

Related studies have shown that over 79% of COD is consumed in the anaerobic zone for the synthesis of intracellular storage PHA, while 6% -11% of COD is used for cell growth and denitrification in the anaerobic zone. There is almost no consumption of COD in the aerobic zone because after cell death, difficult to degrade substances such as cell walls enter the mixed solution, causing an increase in COD.

(2) The effect of influent C/P ratio on denitrification in two processes

The effect of C/P ratio on the denitrification efficiency of AO process has no obvious pattern, and the fluctuation range of TN removal rate is relatively large. This may be due to the fact that factors other than the C/P ratio have a greater impact on denitrification in the AO process than the C/P ratio.

The effect of C/P ratio on the denitrification efficiency of AAO process is not significant. Although the C/P ratio changes significantly, the TN removal rate is relatively stable.

This is mainly because the C/P ratio of general sewage is relatively high, and excessive COD entering the anaerobic zone will inhibit the absorption of phosphorus. In the anaerobic zone, the C/N ratio is always higher than the minimum actual demand value. Denitrifying bacteria will use excessive external carbon sources to quickly carry out denitrification, without affecting the removal of TN.

(3) The effect of influent C/P ratio on phosphorus removal in two processes

In the AAO process, when the C/P ratio is below 80, the phosphorus removal rate fluctuates significantly. When the C/P ratio is higher than 80, the phosphorus removal rate stabilizes at 85% or above, and the phosphorus concentration in the effluent is less than 0.5mg/L. The phosphorus removal rate of the system is basically not affected by other factors, indicating that stable and efficient effluent quality can be achieved when the inlet C/P ratio is higher than 80 in the AAO system.

This is because when the C/P ratio is high, the carbon source provided by the influent is higher than the amount of carbon source required for phosphorus release in the anaerobic zone, resulting in a higher phosphorus removal rate. When the C/P ratio is low, due to the limitation of COD, the phosphorus absorption ability of phosphorus accumulating bacteria decreases, resulting in low phosphorus removal efficiency.

For the AO process, the influence of C/P ratio on its phosphorus removal efficiency has no significant pattern, indicating that the effect of C/P ratio on microbial assimilation is not significant.

Source: Environmentally friendly water treatment