Research on Denitrification Performance of Two-stage Upflow Biological Aerated Filter
Release time：
2020-07-16
Biological aerated filter (BAF) is a new type of biofilm wastewater treatment process developed in Europe and the United States in the late 1980s, and has experienced downflow, downflow two-stage, upflow, and upflow two stages. The four process forms of
Biological aerated filter (BAF) is a new type of biofilm wastewater treatment process developed in Europe and the United States in the late 1980s, and has experienced downflow, downflow two-stage, upflow, and upflow two stages. The four process forms of the type biological aerated filter have gradually developed from a single structure to a comprehensive structure. It combines the contact oxidation process and the feed water fast filter process to remove organic matter in the water. It can also achieve the denitrification effect through nitrification and denitrification [1, 2, 3]. It has high volume load, large hydraulic load, The hydraulic retention time is short, the required infrastructure investment is small, the effluent water quality is good, the operation energy consumption is low, and the operation cost is low.
The author uses a two-stage upward-flow biological aerated filter (UBAF) to treat urban sewage. By controlling the operating conditions, various factors affecting the denitrification effect of the two-stage UBAF are studied.
1 Test device and method
1.1 Test device
The two-stage UBAF process flow used in the test is shown in Figure 1. The filler added in the two-stage UBAF is ceramsite, and its performance parameters are shown in Table 1. The A-stage biological aerated filter mainly removes a small part of the ammonia nitrogen and organic matter in the raw sewage, and the B-stage biological aerated filter mainly removes the remaining COD and ammonia nitrogen. The two biological aerated filters both adopt the upward flow operation mode, and their structural design parameters are exactly the same. The main material is plexiglass, the design size is D 0.25 m×2.5 m, and the filler layer height is 1.50 m. The bottom is provided with a backwash air supply pipe, a vent pipe, and a perforated water distribution pipe.
Figure 1 Test device
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1.2 Starting method and film hanging
uses inoculation to hang the film, the inoculation solution is taken from the raw water mixture of a sewage plant. After continuous suffocating for 24 h at an aeration rate of 15-18 L/h, empty the filter column and repeat twice. On the 3rd day, a small flow of water (conducive to the growth and fixation of nitrifying bacteria) was run at a filtration rate of 0.55 m/h (a flow rate of about 16 L/h) and aeration rate of 16 L/h. On the 5th day, the filtration rate was increased to 0.75 m/h (flow rate is about 21 L/h), and the aeration rate is increased to 31 L/h. During this period, the DO of each column was detected, and the effluent DO was above 4 mg/L. After 26 days, the filtration rate was increased to 0.89 m/h, and the air-water ratio was 3:1. At this time, the COD, NH4+-N, and turbidity were all removed. The biofilm on the surface of the filter material was peeled off. Microscopic examination revealed a large number of filamentous bacteria in the biofilm, as well as miniature animals such as bellworms, nematodes, amoeba, and rotifers.
1.3 Test method and water quality
The two reactors are fed with water from the bottom, with gas and water in the same direction. The hydraulic load of section A is controlled to be 0.81 m/h and the gas-water ratio is 3:1. The gas-water ratio of section B is studied under the same hydraulic load as 3:1 and 2 respectively. :1, 1:1, the operation of the reactor. The various water quality indicators in the test were monitored according to the standard methods provided in the literature [4], including: DO, instrument method; NO3--N, ultraviolet spectrophotometry; NO2--N, N-(1-naphthyl)- Ethylenediamine photometric method; NH3-N, Nessler's reagent photometric method; COD, potassium dichromate method; TN, potassium persulfate digestion ultraviolet spectrophotometry. The test water comes from a distribution well at the inlet of a sewage plant. The raw water quality during the test is shown in Table 2.
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2 Results and discussion
2.1 The influence of temperature on the denitrification performance of UBAF
When the filtration rate is 0.8 m/h, the gas-water ratio is 2:1, and the system is operating stably, examine the removal effect of the reactor on ammonia nitrogen and total nitrogen at 7-10, 10-20, 21-28 ℃ respectively , The results show that: water temperature is the main factor affecting microbial growth and biological metabolic activity. When the water temperature is 7~10 ℃, the average removal rate of NH4+-N and TN by UBAF is 69.19% and 25.35%, respectively. The mass concentrations of NH4+-N and TN in the effluent are 11.8 and 40.32 mg/L; water temperature is 10~20 At ℃, the average removal rates of NH4+-N and TN by UBAF are 80.15% and 33.68%, respectively. The mass concentrations of NH4+-N and TN in the effluent are 7.48 and 41.09 mg/L respectively; when the water temperature is 21-28 ℃, UBAF is The removal rate of NH4+-N was significantly increased, with an average removal rate of 89.16%, and the average removal rate of TN was also increased to 38.75%. The mass concentrations of NH4+-N and TN in the effluent were 4.93 and 37.68 mg/L, respectively. This shows that water temperature has a great influence on the removal of NH4+-N and TN by UBAF. The higher the water temperature, the better the nitrification and denitrification effects of UBAF; on the contrary, the worse. Moreover, under low temperature conditions, the removal rate of UBAF to NH4+-N and TN is relatively low, and the change of water temperature has a great influence on the denitrification effect; at room temperature, the removal rate of NH4+-N and TN increases, and the change of water temperature has an effect on the denitrification effect. The effect is small; when the water temperature is higher, the removal rate of NH4+-N and TN is obviously increased, and the change of water temperature has little effect on the nitrogen removal effect of the biological aerated filter. This is because the suitable growth temperature for most nitrifying bacteria is between 25 and 30 ℃. When the temperature is lower than 25 ℃ or higher than 30 ℃, the growth of nitrifying bacteria slows down, and when the water temperature is lower than 15 ℃, the denitrification rate is significantly reduced. In addition, the reproduction speed of nitrifying bacteria is several orders of magnitude lower than that of heterotrophic bacteria. Under low temperature conditions, the reproduction speed is lower, which affects the nitrification effect, resulting in a decrease in the removal rate of NH4+-N by UBAF; The rate also decreases, and the corresponding TN removal rate also decreases.
2.2 The influence of hydraulic load on the denitrification performance of UBAF
When the air-water ratio is 2:1, the water temperature is 16-25 ℃, the influent NH4+-N is 28.56-57.29 mg/L, and the TN is 44.2-75.36 mg/L, the influence of hydraulic load on the removal of TN by UBAF is investigated. The results show that when the hydraulic load is increased from 0.8 m/h to 1.2 m/h, the average removal rate of NH4+-N by UBAF decreases from 87.48% to 84.94%, a decrease of 2.54%, and the average removal rate of TN decreases from 36.40% Decrease to 32.38%, a decrease of 4.02%; when the hydraulic load increases from 1.2 m/h to 1.8 m/h, the average removal rate of NH4+-N by UBAF is 78.70%, down by 6.24%, and the average removal rate of TN is 26.67%, a decrease of 5.71%. It can be seen that hydraulic load has a greater impact on the denitrification performance of UBAF. With the increase of hydraulic load, the removal rate of NH4+-N and TN by UBAF gradually decreases, and the decline is getting larger. Analysis believes that, on the one hand, the generation period of nitrifying bacteria is longer, and with the increase of hydraulic load, the rapid renewal of biofilm is not conducive to the attachment and proliferation of nitrifying bacteria, and the thickness of the biofilm formed is relatively thin. It facilitates the transfer of oxygen to the inside of the biofilm, destroys the internal anaerobic environment, and is not conducive to the progress of the denitrification reaction; on the other hand, the increase in hydraulic load leads to an increase in organic load. At a higher concentration of organic matter, the degradation of organic matter Heterotrophic bacteria have an absolute advantage and inhibit the proliferation and activity of autotrophic nitrifying bacteria.
2.3 The influence of organic load on the denitrification performance of UBAF
When the filtration rate is 0.8 m/h, the air-water ratio is 2:1, the water temperature is 16-25 ℃, and the system is operating stably, the influence of the organic volume load on the NH4+-N removal effect of UBAF is shown in Figure 2.
Figure 2 The effect of organic load on the denitrification performance of UBAF
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It can be seen from Figure 2 that as the organic volume load of the system increases, the removal rate of NH4+-N and TN by UBAF gradually decreases. It can be seen that when the organic volume load increases, the organic volume load has a significant inhibitory effect on the removal of NH4+-N. At this time, the area where the heterotrophic bacteria degrade organic matter will move up along the height of the filter material, and the living space of the heterotrophic bacteria is also It expands accordingly, compressing the activity space of nitrifying autotrophic bacteria. Moreover, because the specific growth rate of heterotrophic bacteria is much greater than that of nitrifying autotrophic bacteria, heterotrophic bacteria are often preferred in the competition for dissolved oxygen and nutrient substrates. Using the oxygen in the water to multiply under the condition of abundant organic substrates, the proliferation of nitrifying autotrophic bacteria is restricted. The higher the organic volume load, the stronger the inhibition of nitrifying autotrophic bacteria by heterotrophic bacteria, which makes the nitrification performance of UBAF show a greater decline. As the organic volume load increases, the nitrification performance of the system decreases, and the concentration of nitrate nitrogen decreases. The nitrate nitrogen available for denitrifying bacteria as electron acceptors decreases, and the growth of denitrifying bacteria is inhibited, making the system's denitrification performance decline.
2.4 The influence of ammonia nitrogen volumetric load on the denitrification performance of UBAF
When the filtration rate is 0.8 m/h, the air-water ratio is 2:1, the water temperature is 16-25 ℃, and the system is operating stably, the effect of the NH4+-N volumetric load on the NH4+-N removal effect of UBAF is shown in Figure 3.
Figure 3 The influence of ammonia nitrogen volumetric load on the denitrification performance of UBAF
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It can be seen from Figure 3 that the removal rate of NH4+-N by UBAF decreases with the increase of the volume load of influent NH4+-N. This is because nitrifying bacteria are chemical autotrophic bacteria, which have a slower growth rate, a long generation cycle, and are more sensitive to changes in environmental conditions. When the NH4+-N volume load is high, the high NH4+-N concentration will inhibit the growth of nitrifying autotrophic bacteria and affect the nitrification performance of UBAF. The decline in nitrification performance reduces the nitrate nitrogen available for denitrifying bacteria as electron acceptors, the growth of denitrifying bacteria is inhibited, and the removal rate of TN gradually decreases. It can be seen that the increase in NH4+-N volume load will affect the UBAF system The denitrification effect of this product has a more adverse effect.
2.5 The influence of gas-water ratio on the denitrification performance of UBAF
When the filtration rate is 0.8 m/h, the water temperature is 16～25℃, and the influent NH4+-N mass concentration is 27.89～41.36 mg/L, the influence of gas-water ratio on the removal of NH4+-N and TN by UBAF is investigated, and the results show : When the gas-water ratio is 1:1, the DO in the effluent is 0.77~1.35 mg/L, the average removal rate of NH4+-N and TN by UBAF is 79.34% and 29.77% respectively; the gas-water ratio increases to 2:1 When the effluent DO is 1.76~2.65 mg/L, the average removal rate of NH4+-N and TN by UBAF is 86.83% and 35.44% respectively; when the gas-water ratio increases to 3:1, the DO in the effluent is 2.32~ At 3.35 mg/L, the average removal rate of NH4+-N and TN by UBAF was 87.98% and 33.89%, respectively.
It can be seen that with the increase of gas-water ratio, the removal rate of NH4+-N by UBAF is on the rise. This is because sufficient dissolved oxygen in water is beneficial to the oxidation of ammonia nitrogen. Air-water ratio is the main operating condition to control DO, and DO increases with the increase of air-water ratio. According to the double-membrane theory, the oxygen transfer rate is determined by the resistance of the gas-liquid stagnant membrane. The larger the gas-water ratio, the lower the mass transfer resistance between membranes and the higher the dissolved oxygen concentration in the biofilm, which increases accordingly The activity and biodegradation rate of aerobic microorganisms. However, when the gas and water are relatively large, the dissolved oxygen penetrates the biofilm deeper, the facultative and anaerobic layer of the biofilm is thin, and it is difficult to form anoxic zone inside, and a large amount of ammonia nitrogen is converted into nitrate nitrogen and nitrite nitrogen. Therefore, the denitrification effect is poor, the removal rate of TN is relatively low, and the TN concentration of the effluent is relatively high; and when the gas-water ratio is relatively small, the anaerobic layer in the biofilm becomes thicker, and the denitrification effect becomes better; but when the gas-water ratio is 1 :1, the TN removal effect becomes worse due to the incomplete nitrification.
3 Conclusions and recommendations
(1) Water temperature has a greater influence on the denitrification effect of UBAF. When the water temperature is less than 10 ℃, the average removal rate of UBAF to NH4+-N and TN is 69.19% and 25.35%, respectively; when the water temperature is 10 ~ 20 ℃, the average removal rate of UBAF to NH4+-N and TN is 80.15% and 33.68% ; When the water temperature is greater than 20 ℃, the average removal rates of NH4+-N and TN are 89.16% and 38.75%, respectively. The higher the water temperature, the better the denitrification effect of UBAF.
(2) When the water temperature is 16-25 ℃ and the air-water ratio is 2:1, when the hydraulic load increases from 0.8 m/h to 1.2 m/h, the average removal rate of NH4+-N by UBAF drops by 2.54%. The average removal rate of TN decreased by 4.02%; when the hydraulic load increased from 1.2 m/h to 1.8 m/h, the average removal rate of NH4+-N by UBAF decreased by 6.24%, and the average removal rate of TN decreased by 5.71% . With the increase of hydraulic load, the denitrification effect of UBAF shows a downward trend.
(3) The gas-water ratio has a great influence on the denitrification effect. When the hydraulic load is 0.8 m/h, the water temperature is 16-25 ℃, and the gas-water ratio is 1:1, the average removal rate of NH4+-N and TN by UBAF When the gas-water ratio is increased to 2:1, the average removal rate of NH4+-N and TN by UBAF is 86.83% and 35.44%; when the gas-water ratio is increased to 3:1, UBAF is effective for NH4+-N The average removal rate of TN is 87.98% and 33.89%.
(4) The two-stage UBAF has a poor removal effect on TN. In order to increase its removal effect on TN and reach the first level standard in the Pollutant Discharge Standard for Urban Sewage Treatment Plants (GB 18918-2002), the author recommends adding The oxygen filter performs denitrification to achieve a good denitrification effect.
