Method for enhanced biological denitrification in a sewage treatment plant

Abstract

The application discloses a method for enhancing biological denitrification in a sewage treatment plant, which comprises the following steps: setting a bypass closed biological reactor in the sewage treatment plant, and backflowing effluent from a secondary sedimentation tank to the closed biological reactor, and then closing the water inlet; opening an air inlet pipe of the closed biological reactor, and then closing the air inlet pipe after specific proportion of carbon dioxide and hydrogen are introduced; opening an exhaust pipe after hydrogen reaction in the closed biological reactor is completed; and opening an effluent pipe of the closed biological reactor, and then flowing the effluent into a biological denitrification process of the sewage treatment plant. The application utilizes backflow of effluent from the secondary sedimentation tank and coupling of hydrogen / carbon dioxide to enhance in-situ and ex-situ biological denitrification in the sewage treatment plant, and realizes the effect of simultaneous denitrification and carbon sequestration in the sewage treatment plant.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of biological denitrification of sewage, and particularly relates to a method for enhancing biological denitrification in a sewage treatment plant. BACKGROUND

[0002] Traditional biological denitrification technology mainly converts nitrogen-containing compounds in water into nitrogen gas through the processes of ammonification, nitrification and denitrification, so as to achieve the purpose of denitrification. In the denitrification process, denitrifying bacteria can be generally divided into autotrophic denitrifying bacteria and heterotrophic denitrifying bacteria. The autotrophic denitrifying bacteria utilize inorganic electron donors for denitrification, and have obvious advantages under low C / N conditions. Common inorganic electron donors include sulfur-reducing compounds, hydrogen, pyrite and the like. Hydrogen is a clean energy and can be obtained by various ways such as electrolysis of water, and autotrophic denitrification based on hydrogen has been proved to be an effective way of denitrification. However, due to the low solubility of hydrogen in water, the mass transfer efficiency of hydrogen is low, and the denitrification rate is slow, which limits its application. The heterotrophic denitrifying bacteria need organic carbon sources as electron donors to complete cell synthesis and denitrification process. However, the carbon-nitrogen ratio of the influent in sewage treatment plants is generally low, which seriously affects the denitrification efficiency of sewage. The sewage treatment plant has to additionally add organic carbon sources to improve the denitrification efficiency. At present, the organic carbon sources used for denitrification and denitrification mainly include acetic acid, methanol and glucose and the like. These high-cost organic carbon sources have become a burden for the operation of sewage treatment plants. Therefore, it is very important to find and develop an economic and efficient denitrification and denitrification technology.

[0003] Due to the proposal of "carbon neutralization" and "carbon peak", the fixation and resource utilization of carbon dioxide have attracted people's attention. At present, the main methods for CO2 recovery at home and abroad include physical adsorption method, membrane absorption method, physical absorption method and chemical absorption method. Although the above methods can capture CO2, they have problems such as poor selectivity, high requirement for equipment, generation of by-products, high energy consumption, secondary pollution and the like. Biological method is an economic and effective carbon fixation method, which is often used for CO2 fixation in sewage treatment plants. In the biological treatment method, CO2 can be converted into products such as acetic acid by homoacetogenic bacteria, and can also be utilized by hydrogenotrophic microorganisms. However, the accumulation of volatile fatty acid products will cause the pH in the biological reactor to continuously decrease, and thus inhibit the metabolic activity of microorganisms, which is not conducive to the long-term stable operation of the system. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application provides a method for enhancing biological denitrification in a sewage treatment plant, a bypass closed bioreactor is arranged in the sewage treatment plant, effluent from a secondary sedimentation tank is backflowed to the closed bioreactor, and then water inlet is closed; an air inlet pipe of the closed bioreactor is opened, and specific proportions of carbon dioxide and hydrogen are introduced, and then the air inlet pipe is closed; after hydrogen reaction in the closed bioreactor is completed, an air outlet pipe is opened; an effluent pipe of the closed bioreactor is opened, and effluent flows into an anoxic tank of the sewage treatment plant.

[0005] The sewage treatment plant should have an anoxic tank or a biological denitrification process section after ammonia and nitrogen nitrification reaction, such as an anaerobic-anoxic-aerobic process, an anoxic-aerobic process, a sequencing batch reactor, a biofilm process, etc.

[0006] The bypass closed bioreactor is a separately arranged reactor, which is composed of a reaction container, an air inlet pipe, an air outlet pipe, a water inlet pipe, a water outlet pipe, a hydrogen sensor, a carbon dioxide sensor, a gas pressure sensor, etc. The bypass closed bioreactor needs to be inoculated with activated sludge containing acid-producing bacteria and denitrifying bacteria, and the inoculation mass ratio of the acid-producing bacteria and the denitrifying bacteria is 1:1-2:1.

[0007] The hydrogen introduced into the bypass closed bioreactor is introduced by the air inlet pipe and the air outlet pipe is closed, and the change of the headspace gas components of the reactor is monitored by the hydrogen sensor, the carbon dioxide sensor and the gas pressure sensor. The hydrogen can be introduced again after the hydrogen reaction in the closed bioreactor is completed.

[0008] The acid-producing bacteria convert hydrogen and carbon dioxide into acetic acid, and the carbon dioxide can be produced by the microorganisms in the bypass closed bioreactor or introduced from the external carbon dioxide gas of the sewage treatment plant.

[0009] The effluent from the secondary sedimentation tank is backflowed to the closed bioreactor, which can be backflowed from the effluent of the secondary sedimentation tank of the sewage treatment plant or from the effluent of the process section after ammonia and nitrogen nitrification reaction.

[0010] The specific proportions of carbon dioxide and hydrogen are introduced, and the initial volume ratio of hydrogen and carbon dioxide needs to be controlled in the range of 0.1:1-1:1, and the hydrogen partial pressure is controlled in the range of 0.1-0.5 atm.

[0011] The effluent from the secondary sedimentation tank or the process section after ammonia and nitrogen nitrification reaction is backflowed to the closed bioreactor, and the backflow ratio can be regulated according to the ratio of chemical oxygen demand concentration and nitrate concentration in the closed bioreactor, which is 4:1-250:1.

[0012] Hydrogen autotrophic denitrification is an effective denitrification and nitrogen removal pathway, but its practical application is limited due to the low solubility and mass transfer efficiency of hydrogen. Introducing H2 into anaerobic system is an effective way to improve the production of acetic acid and realize the resource utilization of carbon dioxide. The H2 added into the anaerobic digestion system can convert CO2 into volatile fatty acids through homoacetogenesis, but the long-term accumulation of volatile fatty acids will cause the continuous decrease of pH, which reduces the acid production efficiency of the system. The present application can solve the problems of in-situ fixation and resource utilization of carbon dioxide released by the bioreactor by controlling the ratio of hydrogen and carbon dioxide to improve the homoacetogenesis rate and mass transfer efficiency of H2 in the bioreactor; on the other hand, by recycling the effluent of the secondary sedimentation tank to supplement nitrate into the bioreactor, the problem of continuous decrease of pH in the bioreactor is solved, the stability of the system is improved, the in-situ and ex-situ biological denitrification of the sewage treatment plant is strengthened, and the effect of simultaneous denitrification and carbon sequestration in the sewage treatment plant is realized. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 The figure is the daily acetic acid production under different pH conditions, the ratio of carbon dioxide and hydrogen is 0.5:1, and no nitrate is added. The daily acetic acid production of the pH=5 group in Example 1 is always lower than that of the other three groups. When running to the 19th day, the acetic acid production rate of the pH=5 group began to decrease, and when running to the 22nd day, the daily acetic acid production decreased to 5.186±0.98 mg / d, which was only 8.78%, 7.98% and 7.04% of the pH=6, 7 and 8 groups.

[0014] Figure 2 (a) is the pH change diagram of the influent and effluent of the bioreactor with only CO2 / H2 (volume ratio of 0.5:1) added, in Example 2, the continuous accumulation of acetic acid in the early stage of the bioreactor caused the influent pH to decrease from 7.39 to 4.96, and the acetic acid concentration was 1133.33-3166.67 mg / L. Figure 2 (b) is the pH change diagram of the influent and effluent of the bioreactor with CO2 / H2 (volume ratio of 0.5:1) and sodium nitrate added, the mass concentration ratio of acetic acid and sodium nitrate added is 4:1. In Example 3, the influent pH of the bioreactor is higher than 5.5, and after stable operation, the influent and effluent pH is maintained at about 6.0 and 5.0, respectively.

[0015] Figure 3 The figure is the hydrogen consumption rate of the bioreactor with only CO2 / H2 (volume ratio of 0.5:1) added, in Example 2, the hydrogen consumption rate increased continuously in the first 11 days of operation, and reached a maximum of 3.53±0.11 mmol / L on the 11th day. At this time, a large amount of volatile fatty acids accumulated in the bioreactor, such as Figure 2As shown, a pH below 5.5 in the bioreactor has a certain inhibitory effect on the growth and metabolism of homoacetic bacteria. In Example 2, the hydrogen consumption rate continuously decreased from day 14 onwards, reaching only 1.66 ± 0.12 mmol / L at the end of the experiment (day 20), which was only 47.48% of that on day 11. In Example 3, the hydrogen consumption rate remained relatively stable throughout the entire operation, generally around 3.5 mmol / d, which was higher than that of Example 2.

[0016] Figure 4 (a) is a graph showing the daily acetic acid production of a bioreactor with only CO2 / H2 added (ratio of 0.5:1). In Example 2, the daily acetic acid production increased continuously for the first 6 days of operation, reaching a maximum of 2.06 ± 0.13 mmol / d on the 6th day, which is 83.74% of the theoretical value. After that, the acetic acid production decreased significantly, and almost stopped producing acid by the 20th day of operation. Figure 4 (b) is a graph showing the daily acetic acid production of a bioreactor that simultaneously adds CO2 / H2 and nitrate (the ratio of carbon dioxide to hydrogen is 0.5:1, and the mass concentration ratio of acetic acid to sodium nitrate is 4:1). After the addition of nitrate, the daily acetic acid production gradually increases, reaching a peak of 1.87 ± 0.1 mmol / L on day 10.

[0017] Figure 5 This is a graph showing the changes in influent and effluent alkalinity (calculated as CaCO3) in a bioreactor where CO2 / H2 and nitrate are added simultaneously (carbon dioxide to hydrogen ratio of 0.5:1, acetic acid to sodium nitrate mass concentration ratio of 4:1). Figure 5 As shown, on day 16 of operation, the alkalinity of the bioreactor influent and effluent were 330.3 and 230.23 mg / L, respectively. Subsequently, the alkalinity of the effluent continued to increase, reaching 257.21 mg / L at the end of the experiment. The introduction of nitrates may have promoted hydrogen-induced autotrophic denitrification in the bioreactor, generating a certain amount of alkalinity during this process. Therefore, the alkalinity of the bioreactor gradually increased. The coupling of hydrogen and nitrates provided a certain amount of alkalinity to the bioreactor, improving its pH buffering capacity and creating a suitable environment for the growth of homoacetic bacteria, which is beneficial for achieving the wastewater treatment plant's carbon dioxide emission reduction target.

[0018] Figure 6 (a) A graph showing the H2 utilization rate of bioreactors (acetic acid and sodium nitrate ratio of 4:1) with simultaneous addition of different CO2 / H2 ratios and nitrates. Figure 6As shown in (a), after the first four cycles of acclimatization, the hydrogen utilization rates on the first day of the fifth cycle for the pure H2, CO2 / H2(0.25), CO2 / H2(0.5), and CO2 / H2(1) groups were 8.42±6.0%, 38.4±5.0%, 49.0±5.0%, and 68.0±9.0%, respectively. The results show that the hydrogen utilization rates of the pure H2, CO2 / H2(0.25), CO2 / H2(0.5), and CO2 / H2(1) groups increased by 3.56, 4.82, and 7.10 times, respectively. Figure 6 (b) is a graph showing the nitrogen removal rate of the denitrification unit: Similar to the hydrogen utilization rate, the nitrogen removal rate also increases with the increase of carbon dioxide partial pressure. The nitrogen removal rates on the first day for the pure H2, CO2 / H2 (0.25), CO2 / H2 (0.5), and CO2 / H2 (1) groups were 17.8±2.2%, 28.5±6.1%, 38.1±3.4%, and 68.0±5.1%, respectively, which were 0.6, 1.1, and 3.1 times higher than those of the pure H2 group. This experimental result shows that the coupling of hydrogen and nitrate can enhance in-situ and ex-situ denitrification, and the denitrification effect is enhanced as the ratio of carbon dioxide to hydrogen increases. Detailed Implementation

[0019] This invention discloses a method for enhanced biological nitrogen removal in wastewater treatment plants, which involves introducing CO2 / H2 and nitrate into an anaerobic system. This invention utilizes hydrogen to promote the homoacetogenesis process in the bioreactor, improving the utilization rate of H2 by acid-producing bacteria and enhancing the ex-situ denitrification effect. By adding nitrate to the bioreactor, the problem of continuous pH decline in the bioreactor is solved, enhancing the stability of the acid-producing system. This invention investigates the effects of CO2 / H2 dosage ratio and nitrate addition on acid production and denitrification in the bioreactor. In this case, hydrogen is introduced through an inlet pipe and discharged through an outlet pipe. The hydrogen partial pressure at the top of the bioreactor is controlled within the range of 0.1 atm to 0.5 atm using hydrogen sensors, carbon dioxide sensors, and gas pressure sensors.

[0020] This invention provides five embodiments:

[0021] Example 1 – Bioreactors under different pH conditions (no nitrate added, carbon dioxide to hydrogen volume ratio 0.5:1); Example 2 – Bioreactor with only CO2 / H2 added (carbon dioxide to hydrogen volume ratio 0.5:1); Example 3 – Bioreactor with simultaneous addition of CO2 / H2 and nitrate (carbon dioxide to hydrogen volume ratio 0.5:1, acetic acid to sodium nitrate mass concentration ratio 4:1); Example 4 – Bioreactor with simultaneous addition of different CO2 / H2 ratios and nitrate (acetic acid to sodium nitrate mass concentration ratio 4:1); Example 5 – Denitrification unit using the acidified liquid of the bioreactor as the carbon source. In these examples, CO2 is the sole carbon source for the bioreactor.

[0022] In Example 1, operating conditions were set at pH = 5, 6, 7, and 8, with a carbon dioxide to hydrogen ratio of 0.5:1, and no nitrate was added. The daily acetic acid yield in the pH = 5 group was consistently lower than the other three groups. By day 19, the acetic acid yield in the pH = 5 group began to decline, and by day 22, the daily acetic acid yield had dropped to 5.186 ± 0.98 mg / d, only 8.78%, 7.98%, and 7.04% of the pH = 6, 7, and 8 groups, respectively. By day 28, the acid production process in the pH = 5 group was severely inhibited, with acetic acid production almost zero, while the other three groups continued to produce acid. This phenomenon indicates that pH is one of the important conditions affecting acid production in bioreactors, and that lower pH conditions are unfavorable for long-term acid production in bioreactors.

[0023] In Example 2, during the initial operation, acetic acid continuously accumulated in the bioreactor, causing the influent pH to drop from 7.39 to 4.81, while the optimal pH for most acid-producing microorganisms is 5–8. The hydrogen consumption rate in the bioreactor decreased from 3.18 mmol / L at the beginning of operation to 1.68 mmol / L, indicating that the activity of homoacetogenic bacteria in the bioreactor was severely inhibited. Figure 4 As shown in (a), the actual daily yield of acetic acid in Example 2 reached a maximum of 2.46 mmol / d on day 6. Afterward, the actual daily yield of acetic acid decreased significantly, with the actual daily yield on day 12 being only 30.05% of that on day 6. At the end of the experiment, the actual daily yield of acetic acid was almost zero, indicating that the bioreactor cannot stably produce acid in the long term simply by adding carbon dioxide and hydrogen.

[0024] In Example 3, after simultaneously introducing CO2 / H2 and nitrate into the bioreactor, the pH of the bioreactor influent decreased from 7.39 to 5.89 within 0–4 days. From day 6 onwards, the pH of the bioreactor influent continuously increased, and after stabilization, the influent pH remained around 6.0–6.4. This phenomenon indicates that the introduction of nitrate can solve the problem of continuous pH decline during acid production, providing a possibility for long-term and stable acid production in the bioreactor. Figure 3 As shown, the hydrogen consumption rate in Example 3 remained stable throughout the entire experimental run, reaching 3.55 ± 0.24 mmol / d at the end of the experiment, which was 2.11 times that of Example 2. Figure 4 As shown, the actual acetic acid yield in Example 3 fluctuated around 1.2 mmol / d, and its acid production effect was more stable compared to Example 2, which only added CO2 / H2. Figure 5 As shown, the alkalinity of the effluent in Example 3 continuously increased during the later stages of operation, reaching 230.23 mg / L on day 16 and 250.21 mg / L at the end of the experiment. Compared to Example 2, which only added carbon dioxide and hydrogen, Example 3, which simultaneously added carbon dioxide, hydrogen, and nitrate, exhibited smaller pH fluctuations and a more stable hydrogen consumption rate. This indicates that the coupling of hydrogen and nitrate is beneficial for improving the utilization rate of hydrogen in the bioreactor, enhancing its pH buffering capacity, and promoting long-term stable acid production.

[0025] In Example 4, a total of 5 experimental groups were set up: CO2 / H2 (0.25), CO2 / H2 (0.5), CO2 / H2 (1), pure H2, and pure CO2. In the first three groups, the volume ratios of CO2 and H2 were 0.25, 0.5, and 1, respectively. Figure 6 (a) It can be seen that the hydrogen utilization rate of the CO2 / H2(1) group was the highest on the first day, reaching 68.0±9.0%, which is 7.10 times higher. This result shows that a higher CO2 / H2 ratio is more conducive to improving the hydrogen utilization rate and mass transfer efficiency in the system.

[0026] In Example 5, the acidified liquid from the bioreactor was refluxed as a carbon source to the denitrification unit, and the process lasted for 25 days. Figure 6 (b) The CO2 / H2(1) group showed the fastest nitrogen removal rate and the nitrogen removal rate could reach 100%, which indicates that: (1) the acidified liquid of the bioreactor has the potential to be used as a carbon source for denitrification; (2) a higher CO2 / H2 ratio is conducive to improving the stability of the bioreactor and the denitrification rate of the denitrification reactor, strengthening the in-situ and ex-situ denitrification of the denitrification reactor, and realizing the purpose of carbon source resource utilization and wastewater denitrification.

Claims

1. A method for enhanced biological nitrogen removal in a wastewater treatment plant, characterized in that: A bypass closed bioreactor is set up in the wastewater treatment plant. The effluent from the secondary sedimentation tank is returned to the closed bioreactor, and then the inlet is closed. The air inlet pipe of the closed bioreactor is opened, and a specific ratio of carbon dioxide and hydrogen is introduced, and then the air inlet pipe is closed. After the hydrogen reaction in the closed bioreactor is completed, the exhaust pipe is opened, and the effluent pipe of the closed bioreactor is opened, and the effluent flows into the biological denitrification process of the wastewater treatment plant. The bypass closed bioreactor is a separately set reactor, consisting of a reaction vessel, an air inlet pipe, an exhaust pipe, a water inlet pipe, a water outlet pipe, a hydrogen sensor, a carbon dioxide sensor, and a gas pressure sensor; the bypass closed bioreactor is inoculated with activated sludge containing acid-producing bacteria and denitrifying bacteria, and the inoculation mass ratio of acid-producing bacteria to denitrifying bacteria is 1:1 to 2:

1. The acid-producing bacteria convert hydrogen and carbon dioxide into acetic acid. The carbon dioxide is produced by the microbial metabolism of the bypass closed bioreactor or introduced from the external carbon dioxide gas of the sewage treatment plant. The effluent from the secondary sedimentation tank is returned to the closed bioreactor, either by the effluent from the secondary sedimentation tank of the wastewater treatment plant or by the effluent from the process section after the ammonia nitrogen nitrification reaction. When the effluent from the secondary sedimentation tank is returned to the closed bioreactor, the return ratio is adjusted to 4:1 to 250:1 according to the ratio of chemical oxygen demand concentration to nitrate concentration in the closed bioreactor. The process involves introducing a specific ratio of carbon dioxide and hydrogen, with the initial volume ratio of hydrogen to carbon dioxide controlled within the range of 0.1:1 to 1:1, and the hydrogen partial pressure controlled within the range of 0.1 to 0.5 atm. The effluent pH in the closed bioreactor is ≥5. After being treated by sedimentation or membrane separation, the effluent flows into the inlet of the biological denitrification process in the wastewater treatment plant.

2. The method according to claim 1, characterized in that: The wastewater treatment plant is equipped with an anoxic tank or a biological denitrification process following ammonia nitrification.

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