Biological aerated filter (BAF) is a new type of biofilm sewage treatment process developed in Europe and the United States in the late 1980s, and has experienced downflow, two-stage downflow, upflow, and two-stage upflow. There are four technological forms of biological aerated filter, gradually developing from a single structure to a comprehensive structure. It combines the contact oxidation process and the water supply fast filter process together to remove organic matter in water, and can also achieve [1, 2, 3]. It has high volume load and large hydraulic load. , short hydraulic retention time, less infrastructure investment required, good effluent quality, low operating energy consumption, low operating costs and many other advantages.

The author uses a two-stage upflow biological aerated filter (UBAF) to treat urban sewage. By controlling the operating conditions, various factors that affect the denitrification effect of the two-stage UBAF are studied.

1. Test device and method

1.1 Test device

The two-stage UBAF process used in the experiment is shown in Fig. 1. The filler added in the two-stage UBAF is ceramsite, and its performance parameters are shown in Table 1. The biological aerated filter in section A is mainly to remove a small part of ammonia nitrogen and organic matter in the raw sewage, and the biological aerated filter in section B is mainly to remove the remaining COD and ammonia nitrogen. The two biological aerated filters 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 height of the packing layer is 1.50 m. The bottom is equipped with a backwash air supply pipe, a vent pipe, and a perforated water distribution pipe.

Figure 1 Test device

1.2 Start-up method and film hanging

The inoculation film was adopted, and the inoculum solution was taken from the raw water mixture of a sewage plant. Continuously suffocate for 24 hours with an aeration rate of 15-18 L/h, then empty the filter column, and repeat twice. On the 3rd day, the water was fed with a small flow rate (beneficial to the growth and fixation of nitrifying bacteria), and the filtration rate was 0.55 m/h (the flow rate was about 16 L/h), and the aeration rate was 16 L/h. On the 5th day, the filtration rate was increased to 0.75 m/h (the flow rate is about 21 L/h), and the aeration rate is increased to 31 L/h. During the period, the DO of each column was detected, and the DO of the effluent 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, it had a good removal effect on COD, NH4+-N, and turbidity, and the biofilm on the surface of the filter material was peeled off. Microscopic examination found a large number of filamentous bacteria in the biofilm, as well as micro-animals such as bell worms, nematodes, amoebas, and rotifers.

1.3 Test method and water quality

The two reactors were fed with water from the bottom, and the gas and water flowed in the same direction. The hydraulic load of section A was controlled at 0.81 m/h, and the gas-water ratio was 3:1. :1, 1:1, the operation of the reactor. Various water quality indicators in the test were monitored according to the standard methods provided in the literature [4], wherein: DO, instrument method; NO3--N, ultraviolet spectrophotometry; NO2--N, N-(1-naphthyl)- Ethylenediamine photometry; NH3-N, Nessler's reagent photometry; COD, potassium dichromate method; TN, potassium persulfate digestion ultraviolet spectrophotometry. The test water came from a distribution well at the water inlet of a sewage plant. The raw water quality during the test is shown in Table 2.

2 Results and discussion

2.1 Effect of temperature on denitrification performance of UBAF

When the filtration rate is 0.8 m/h, the gas-water ratio is 2:1, and the system is running stably, the removal effects of the reactor on ammonia nitrogen and total nitrogen were investigated at 7-10, 10-20, and 21-28 °C respectively. , the results showed that water temperature was the main factor affecting microbial growth and biological metabolic activity. When the water temperature is 7-10 ℃, the average removal rates of NH4+-N and TN by UBAF are 69.19% and 25.35%, respectively, and the mass concentrations of NH4+-N and TN in the effluent are 11.8 and 40.32 mg/L respectively; when the water temperature is 10-20 At ℃, the average removal rates of NH4+-N and TN by UBAF were 80.15% and 33.68%, respectively, and the mass concentrations of NH4+-N and TN in the effluent were 7.48 and 41.09 mg/L respectively; when the water temperature was 21-28 ℃, UBAF The removal rate of NH4+-N increased significantly, with an average removal rate of 89.16%, and the average removal rate of TN also increased, reaching 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 effect of UBAF; otherwise, the worse it is. Moreover, under low temperature conditions, UBAF has a relatively low removal rate of NH4+-N and TN, 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 a great influence on the denitrification effect. The effect is small; when the water temperature is higher, the removal rate of NH4+-N and TN is significantly increased, and the change of water temperature has little effect on the denitrification effect of the biological aerated filter. This is because the suitable growth temperature for most nitrifying bacteria is between 25 and 30 °C. When the temperature is lower than 25 °C or higher than 30 °C, the growth of nitrifying bacteria slows down, and when the water temperature is lower than 15 °C, the denitrification rate decreases significantly. In addition, the reproduction speed of nitrifying bacteria is several orders of magnitude lower than that of heterotrophic bacteria, and the reproduction speed is lower under low temperature conditions, which affects the nitrification effect and leads to 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 Effect of hydraulic load on denitrification performance of UBAF

When the gas-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 loading on the removal of TN by UBAF is investigated. The results showed that: when the hydraulic load increased from 0.8 m/h to 1.2 m/h, the average removal rate of UBAF for NH4+-N decreased from 87.48% to 84.94%, a decrease of 2.54%, and the average removal rate for TN was from 36.40% decreased to 32.38%, a decrease of 4.02%; when the hydraulic load increased from 1.2 m/h to 1.8 m/h, the average removal rate of UBAF for NH4+-N was 78.70%, a decrease of 6.24%, and the average removal rate for TN 26.67%, a drop of 5.71%. It can be seen that the hydraulic load has a great influence on the denitrification performance of UBAF. With the increase of hydraulic load, the removal rate of NH4+-N and TN by UBAF decreases gradually, and the rate of decrease becomes larger and larger. According to the analysis, on the one hand, it is due to the long generation period of nitrifying bacteria, and the rapid renewal of biofilm with the increase of hydraulic load, which is not conducive to the attachment and proliferation of nitrifying bacteria, and the thickness of the formed biofilm is relatively thin, which is It is conducive to the transfer of oxygen to the interior of the biofilm, destroying its internal anaerobic environment, and is not conducive to the progress of denitrification reaction; on the other hand, the increase of hydraulic load leads to the increase of organic load. Under the higher concentration of organic matter, the degradation of organic matter Heterotrophic bacteria are in an absolute advantage, inhibiting the proliferation and activity of autotrophic nitrifying bacteria.

2.3 The effect of organic loading on the denitrification performance of UBAF

When the filtration rate is 0.8 m/h, the gas-water ratio is 2:1, the water temperature is 16-25 °C, and the system is running stably, the influence of organic volume load on the removal of NH4+-N by UBAF is shown in Figure 2.

Fig. 2 Effect of organic load on nitrogen removal performance of UBAF

It can be seen from Fig. 2 that with the increase of the organic volume load of the system, 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 range of heterotrophic bacteria degrading organic matter will move upward along the height of the filter material, and the living space of heterotrophic bacteria will also increase. Then it expands upwards, compressing the activity space of nitrifying autotrophic bacteria, and because the specific growth rate of heterotrophic bacteria is much higher than that of nitrifying autotrophic bacteria, in the competition for dissolved oxygen and nutrient substrate, heterotrophic bacteria often take priority Utilizing the oxygen in the water, they reproduce in large quantities under the condition that the organic substrate is relatively rich, so that the proliferation of nitrifying autotrophs is limited. The higher the organic volume load, the stronger the inhibition of heterotrophic bacteria on nitrifying autotrophic bacteria, resulting in a greater decline in the nitrification performance of UBAF. With the increase of organic volume load, the nitrification performance of the system decreases, the concentration of nitrate nitrogen decreases, the nitrate nitrogen available for denitrifying bacteria to be used as electron acceptors decreases, and the growth of denitrifying bacteria is inhibited, which makes the denitrification performance of the system decline.

2.4 The effect of ammonia nitrogen volume loading 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 °C, and the system is running stably, the influence of NH4+-N volume load on the NH4+-N removal effect of UBAF is shown in Figure 3.

Fig. 3 Effect of ammonia nitrogen volume loading on denitrification performance of UBAF

It can be seen from Figure 3 that the removal rate of NH4+-N by UBAF decreases with the increase of influent NH4+-N volume load. This is because nitrifying bacteria belong to chemoautotrophic bacteria, which have a small specific growth rate, a long generation cycle, and are sensitive to changes in environmental conditions. When the NH4+-N volume load was high, high NH4+-N concentration would inhibit the growth of nitrifying autotrophs and affect the nitrification performance of UBAF. The decline of nitrification performance reduces the nitrate nitrogen available for denitrifying bacteria to be used 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 of NH4+-N volume load will affect the UBAF system. The denitrification effect has a relatively adverse effect.

2.5 Effect of gas-water ratio on denitrification performance of UBAF

When the filtration rate was 0.8 m/h, the water temperature was 16-25°C, and the influent NH4+-N mass concentration was 27.89-41.36 mg/L, the effect of gas-water ratio on the removal of NH4+-N and TN by UBAF was investigated. The results showed that : When the air-water ratio is 1:1, the DO in the effluent is 0.77~1.35 mg/L, and the average removal rates of NH4+-N and TN by UBAF are 79.34% and 29.77% respectively; the air-water ratio increases to 2:1 When the air-water ratio increased to 3:1, the DO in the effluent was 2.32~ 3.35 mg/L, the average removal rates of NH4+-N and TN by UBAF were 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 tends to increase. This is because sufficient dissolved oxygen in water is conducive 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 two-phase stagnant membrane. The larger the gas-water ratio, the smaller the mass transfer resistance between the membranes, and the higher the dissolved oxygen concentration in the biofilm. The activity and biodegradation rate of aerobic microorganisms. But when the air and water are relatively large, the dissolved oxygen penetrates the biofilm deeply, the facultative oxygen and anaerobic layers of the biofilm are thin, and it is difficult to form an 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 concentration of TN in the effluent is high; when the air-water ratio is small, the anaerobic layer in the biofilm becomes thicker, and the denitrification effect becomes better; but when the air-water ratio is 1 : 1, the removal effect of TN becomes worse due to incomplete nitrification.

3 Conclusions and recommendations

(1) Water temperature has a great influence on the denitrification effect of UBAF. When the water temperature is less than 10 ℃, the average removal rates of NH4+-N and TN by UBAF are 69.19% and 25.35% respectively; when the water temperature is 10-20 ℃, the average removal rates of UBAF to NH4+-N and TN are 80.15% and 33.68% ; When the water temperature is higher 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 gas-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 decreases 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 UBAF to NH4+-N 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 showed 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 79.34% and 29.77%; when the gas-water ratio increased to 2:1, the average removal rates of UBAF for NH4+-N and TN were 86.83% and 35.44%; , TN average removal rate of 87.98%, 33.89%.

(4) The removal effect of two-stage UBAF on TN is not good. In order to increase its removal effect on TN and reach the first-level standard in the "Pollutant Discharge Standards for Urban Wastewater Treatment Plants" (GB 18918—2002), the author suggests adding Oxygen filter for denitrification to achieve the best denitrification effect.