Although AO method and AAO method belong to the biological sewage treatment method, the comparative study of the two is rarely reported, and this paper makes a comparative analysis of the two sewage treatment processes through the comparison of water quality before and after the process improvement of sewage treatment plant in Jiyang County, which provides a certain basis for the selection of urban sewage treatment plant process.
1. Comparative analysis of AO and AAO removal performance
1. Comparison of COD removal performance
The removal ability of sewage treatment process to remove organic matter is one of the main indicators to characterize the process efficiency, and the size of COD directly reflects the amount of organic matter content in sewage.
The results showed that there was no significant difference in COD in influent COD between the two processes, and there was a significant difference in effluent COD and removal rate, and the removal of COD in AAO process was significantly better than that of AO process.
The reason is that in the AO process, the denitrification reaction of the hypoxic section can consume part of the organic matter in the sewage, but most of the organic matter is removed by aerobic degradation, and the hydraulic residence time of the aerobic section of the AO process of the first phase of the Jiyang County Sewage Treatment Plant is short, the volume of the aeration tank is small, and the aeration amount is not enough, resulting in poor organic matter removal effect.
In the AAO process, most of the organic matter is converted into PHB by polyphosphorus bacteria in the anaerobic section and stored in cells, and some organic matter is removed by denitrification reaction in the anoxic section.
Studies have shown that the COD removal rate of the anaerobic section of the AAO process can reach more than 80%, while the removal rate of the hypoxic section is less than 10% on average.
2. Comparison of nitrogen removal 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 are denitrification and denitrification removal.
The results showed that there was no significant difference in TN in the influent and removal rates of the two processes, and the TN removal rate in the effluent was significantly different, and the removal of TN in the AAO process was significantly better than that of the AO process.
In the denitrification and denitrification process, nitrate nitrogen is the main substance in the total nitrogen effluent, and the removal rate of nitrate nitrogen in the hypoxic section can be higher than 90%. Some studies have pointed out that controlling the nitrate concentration in effluent in hypoxic areas of 1mg/L~2mg/L can maximize the removal rate of TN and make full use of COD to improve the denitrification ability of hypoxic areas. The aerobic zone mixture contains a large amount of nitrate nitrogen, which flows back into the hypoxic zone through internal circulation and undergoes denitrification reaction in the hypoxic zone.
The HRT of the anoxic section of the AO process of Jiyang County sewage treatment plant is too short, only 1.8h, less than 3.46h of the AAO process, and the internal reflux ratio is 50%~100%, which is less than 150%~250% of the AAO process, resulting in the nitrogen removal function is not as good as AAO. Moreover, the denitrification effect of AO process is not as stable as that of AAO process, and is greatly affected by external factors (temperature, C/N ratio, etc.).
3. Comparison of phosphorus removal performance
Excessive phosphorus content in water will also lead to the proliferation of microorganisms, vigorous growth of plankton, and eutrophication. The emergence of denitrification phosphorus removal technology is a breakthrough in the traditional biological phosphorus removal theory, which can not only solve the contradictions existing in traditional processes, but also help to achieve sustainable treatment of sewage.
The results showed that there was no significant difference in TP in the influent water and the removal rate of the two processes, and the TP removal rate in the effluent was significantly different, and the removal of TP in the AAO process was significantly better than that of the AO process.
The reason is that the AO process of the first phase of Jiyang County Sewage Treatment Plant does not set up an anaerobic phosphorus release section, and in the process of biological phosphorus removal, polyphosphorus bacteria can only ensure that there is a good phosphorus absorption effect in the anoxic section and aerobic section by sufficient phosphorus release in the anaerobic section.
The phosphorus removal of the AAO process is mainly completed by polyphosphorus bacteria. Generally speaking, polyphosphorus bacteria take in more phosphorus in the hypoxic and aerobic sections than they release in the anaerobic section. Studies have shown that the ratio of average phosphorus absorption and average phosphorus release in the AAO process is 1.28, and the phosphorus absorption in the hypoxic section is higher than that in the aerobic section.
4. Summary of removal performance
In summary, the AAO process for the removal of organic matter, nitrogen and phosphorus is significantly better than the AO process, especially the phosphorus removal, because the AO process has no anaerobic section, only a small part of the phosphorus can be removed through the assimilation of microorganisms, so do not choose this process or increase chemical phosphorus removal if there are requirements for phosphorus removal.
2. The effect of temperature on AO and AAO denitrification and phosphorus removal
1. The effect of temperature on COD removal in both processes
Temperature had little effect on the removal of COD in the AAO process, and the removal rate of COD could reach more than 85% even if the temperature was lower than 5 degrees Celsius, which indicated that temperature had little effect on the conversion of organic matter by polyphosphobacteria.
The removal of COD by the AO process often decreases between 5-15 degrees Celsius and then increases again as the temperature increases. This may be due to the fact that when the climate is alternating between cold and warm, the number and structure of bacteria in the system will change, and the dominant population in the system will gradually change from one temperature-preferring bacterial group to another-temperature favoring bacterial flora, thus affecting the processing capacity of organic matter.
2. The effect of temperature on the denitrification of the two processes
The effect of temperature on the denitrification of both processes is more obvious - the TN removal rate increases significantly with the increase of temperature in both processes. In particular, the TN removal rate increases almost linearly when the temperature is above 15 degrees Celsius.
On the one hand, the reason for the temperature enhancement of the nitrogen removal performance of the two processes is that the increase of temperature is conducive to the growth and reproduction of activated sludge microorganisms and improves the efficiency of nitrogen assimilation. On the other hand, the increase in temperature also enhances the metabolic activity of nitrifying bacteria and denitrifying bacteria in the system, and enhances the denitrification and denitrification ability 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 decreases significantly, and the activity of nitrifying bacteria is also greatly reduced. When the temperature drops below 5 degrees Celsius, the vital activity of nitrifying bacteria almost stops.
It is worth mentioning that sometimes even at low temperature (below 5 degrees), the TN removal rate of the two systems is not less than 40%, which means that the two systems mainly rely on the assimilation of microorganisms in activated sludge to remove nitrogen at low temperature.
3. The effect of temperature on phosphorus removal in the two processes
From the actual situation, the phosphorus removal rate of AAO process increased correspondingly with the increase of temperature, especially after the temperature was higher than 20 degrees Celsius, the TP removal rate tended to be stable.
This is because the key to biological phosphorus removal is to rely on the phosphorus removal activity of polyphosphorus bacteria, and the increase of temperature is conducive to increasing the activity of polyphosphorus bacteria and improving the phosphorus removal rate.
However, the removal of phosphorus by AO process is not strong and has no obvious correlation with temperature. This is because there is no anaerobic section in the AO process, there is no condition for the survival of polyphosphorus bacteria, and the removal of phosphorus only depends on the assimilation of microorganisms, so temperature has little effect on the assimilation of microorganisms in this process.
3. Effect of influent C/N ratio on nitrogen and phosphorus removal of AO and AAO
1. The effect of influent C/N ratio on COD removal in the two processes
For the AAO process, the removal rate of COD remained basically stable regardless of the C/N ratio.
The data show that most of the COD is synthesized into intracellular storage PHA by polyphosphorus bacteria in the anaerobic area, with an average utilization rate of 75%-85%, about 10% of the COD enters the anoxic zone, and almost no remaining biodegradable organic matter enters the aerobic zone, so the process can realize the full use of influent carbon sources and is less affected by the impact of organic matter load.
The C/N ratio has a certain effect on the removal of COD in the AO process. According to the data of Jiyang County Sewage Treatment Plant, after the C/N ratio is greater than 10, the COD removal rate decreases slightly with the increase of the C/N ratio, and the organic matter load has an impact on the system.
2. The effect of the C/N ratio of influent water on the denitrification of the two processes
With the increase of C/N ratio, the removal rate of TN decreased almost linearly, and the concentration of organic matter had a serious impact on the nitrification process.
This is because nitrifying bacteria are autotrophic bacteria, and the concentration of organic matter is not its growth limiting factor, and the high concentration of organic matter will make the heterotrophic bacteria with rapid proliferation rate multiply, giving priority to the use of oxygen in the water.
In the AAO process, experimental data show that when the C/N ratio of influent water increases from 5 to 9, the TN removal rate increases steadily, and when the C/N ratio is 8.9, the TN removal rate is as high as 83.2%, but when the C/N ratio increases from 9 to 14, the TN removal rate does not increase but decreases.
When the C/N ratio increases to a certain value and the TN removal rate reaches the highest, the TN removal rate decreases with the increase of the C/N ratio.
The main reason is the same as the AO process, the increase in the C/N ratio leads to fewer and fewer autotrophic bacteria in the system, and the nitrification efficiency is reduced, resulting in a decrease in the total nitrogen removal rate. Data show that the minimum theoretical C/N ratio to achieve complete denitrification without storing an internal carbon source is 2.86, but the actual required value is much greater than this.
3. The effect of influent C/N ratio on phosphorus removal in the two processes
The C/N ratio had a great influence on the phosphorus removal effect of the AAO process, and the experimental data showed that when the C/N ratio of influent water increased from 5 to 9, the TP removal rate gradually increased.
This is mainly due to the insufficient carbon source in the influent when the C/N ratio is low, and the reflux sludge contains a large amount of nitrate, which consumes a large amount of COD, resulting in insufficient phosphorus release in the anaerobic area and a decrease in the phosphorus removal rate of the system.
When the C/N ratio of influent water increased from 9 to 14, the removal rate of total phosphorus decreased, especially when the C/N ratio was greater than 11, the total phosphorus removal rate decreased almost linearly.
This is because at relatively high organic load, the organic matter in the influent cannot be fully utilized by polyphosphorus bacteria in the anaerobic section, and the remaining excess organic matter will promote the growth of polyphosphorus bacteria, resulting in a decrease in the proportion of polyphosphorus bacteria in activated sludge and affecting the phosphorus removal effect.
The C/N ratio of influent water had little effect on the phosphorus removal effect of AO process, mainly because the removal of phosphorus by AO process was only through microbial assimilation, while C/N ratio had little effect on assimilation.
4. Effect of influent C/P ratio on AO and AAO nitrogen and phosphorus removal
1. The effect of influent C/P ratio on COD removal in the two processes
Experimental data show that the removal rate of COD by AO process is not regular with the C/P ratio, and there is no obvious correlation, which shows that the C/P ratio is not the main factor affecting the organic matter removal effect of AO process.
For the AAO process, the COD removal rate is higher than 85% regardless of the change in the influent C/P ratio.
Relevant studies pointed out that more than 79% of COD is consumed in the anaerobic zone for the synthesis of PHA, the internal storage of cells, while 6%-11% of COD in the hypoxic zone is used for cell growth and denitrification consumption, and there is almost no COD consumption in the aerobic zone, because after cell death, refractory substances such as cell walls enter the mixture, increasing COD.
2. The effect of the C/P ratio of influent water on the denitrification of the two processes
There was no obvious law on the effect of C/P ratio on the nitrogen removal effect of AO process, and the TN removal rate fluctuated greatly. This may be due to the fact that factors other than the C/P ratio have a better influence on the denitrification of the AO process than the C/P ratio.
The C/P ratio had no obvious effect on the denitrification effect of AAO process, and although the C/P ratio changed greatly, the TN removal rate was relatively stable.
This is mainly because the C/P ratio of general sewage is relatively high, and excessive COD entering the hypoxic area will inhibit the absorption of phosphorus, while the C/N ratio in the hypoxic area is always higher than the minimum value of actual demand.
3. The effect of influent C/P ratio on phosphorus removal in the two processes
In the AAO process, when the C/P ratio is lower than 80, the removal rate of phosphorus fluctuates significantly. When the C/P ratio is higher than 80, the phosphorus removal rate is stable at more than 85%, and the phosphorus concentration in the effluent is less than 0.5mg/L, and the phosphorus removal rate of the system is basically no longer affected by other factors, indicating that when the C/P ratio of influent water is higher than 80 in the AAO system, stable and efficient effluent water quality can be achieved.
This is because the carbon source provided by the influent water is higher than the amount of carbon required to release phosphorus in the anaerobic zone at high C/P ratio, so the phosphorus removal rate is higher. When the C/P was low, the phosphorus absorption capacity of polyphosphorus bacteria decreased due to the limitation of COD, resulting in low phosphorus removal efficiency.
For AO process, there was no obvious law in the effect of C/P ratio on phosphorus removal effect, which indicated that C/P ratio had little effect on microbial assimilation.
