Low-carbon source denitrification method

Abstract

The application discloses a low-carbon-source denitrification method, and belongs to the technical field of sewage treatment. The method comprises the following steps: mixing FeSO4 and FeCl3 solutions; adjusting the pH value of the mixed solution to 10-12 to generate iron oxide precipitate; mixing the iron oxide precipitate with volcanic rock at a weight ratio of 100:1, and heating and drying the mixture at 200-300 DEG C to prepare a biological carrier; using the prepared biological carrier to treat effluent from a biochemical secondary sedimentation tank in a treatment unit; performing backwashing after the treatment unit is operated for 3-7 days; controlling the DO concentration of incoming water to be not more than 1 mg / L; when the DO concentration of the incoming water is more than 1 mg / L, reducing the aeration amount of the upstream or increasing the HRT of the treatment unit; and when the DO concentration of the incoming water is not higher than 1 mg / L, controlling the HRT of the system to be not more than 40 min. The method is mainly used for efficiently treating low-carbon-source sewage, improving the denitrification efficiency and reducing the treatment cost.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a low-carbon-source nitrogen removal method. Background Technology

[0002] Nitrogen removal is a crucial step in wastewater treatment. For effluent from biological secondary sedimentation tanks, the carbon source content is often low. Traditional nitrogen removal methods, under low carbon source conditions, suffer from limited denitrification due to insufficient carbon, resulting in low nitrogen removal efficiency. Some existing treatment methods may require the addition of additional carbon sources, which not only increases treatment costs but may also lead to secondary pollution. Simultaneously, the attachment and growth of microorganisms during treatment require suitable carriers. However, ordinary carriers are not ideal for adsorption and support of microorganisms in low carbon source environments, affecting their metabolic activities and nitrogen removal function. Furthermore, the control of dissolved oxygen (DO) concentration is also critical. Excessively high DO concentrations inhibit the growth and activity of denitrifying bacteria, while excessively low concentrations may affect the normal metabolism of other microorganisms. Precisely controlling DO concentration and rationally adjusting treatment process parameters based on DO concentration has always been a challenge in low-carbon-source nitrogen removal treatment. In previous research and practice, although different carrier materials and process adjustment methods have been tried, it has been difficult to achieve stable and efficient denitrification under low carbon source conditions due to insufficient optimization of carrier modification treatment and lack of a systematic dynamic control strategy for DO concentration and HRT (hydraulic retention time). Summary of the Invention

[0003] The purpose of this invention is to provide a low-carbon-source denitrification method. This method involves mixing FeSO4 and FeCl3 solutions in a specific ratio and adjusting the pH to generate iron oxide precipitate. This precipitate is then mixed with volcanic rock to create a biological carrier, providing a favorable environment for microbial attachment and growth, thus promoting microbial enrichment and metabolic activity. A well-designed backwashing step effectively removes aging microorganisms, maintaining the activity and treatment efficiency of the biological carrier. Precise control of the DO concentration in the influent and corresponding adjustment strategies ensure that the denitrification process takes place in a suitable environment, effectively improving denitrification efficiency under low-carbon-source conditions, reducing the need for additional carbon source addition, and lowering treatment costs and the risk of secondary pollution.

[0004] To achieve these objectives and other advantages of the present invention, a method for denitrification from low carbon sources is provided, comprising the following steps: 1) According to Fe 2+ and Fe 3+ A FeSO4 and FeCl3 solution was mixed in a molar ratio of 1:0.2-2. 2) Adjust the pH of the mixed solution to 10-12 to form a black iron oxide precipitate, with nitrogen gas purging throughout the process; 3) The obtained iron oxide precipitate is mixed with volcanic rock at a weight ratio of 30-300:1 and dried at 200-300℃ to prepare a biological carrier; 4) Use the prepared biological carrier to treat the effluent from the secondary sedimentation tank of the biological treatment unit; after the treatment unit has been running for 3-7 days, backwashing is performed; during backwashing, the backwash blower is started first to backwash the biological carrier and remove the aging microorganisms on the biological carrier, and then the water pump is started to draw water from the inlet of the treatment unit to backwash the biological carrier. 5) Control the DO concentration of the incoming water to ≤1mg / L; when the DO concentration of the incoming water is >1mg / L, reduce the aeration rate of the upstream unit or increase the HRT of the treatment unit; when the DO concentration of the incoming water is ≤1mg / L, control the HRT of the treatment unit to ≤40min.

[0005] Preferably, in the low-carbon source denitrification method, after generating iron oxide precipitate in step 3), the obtained iron oxide precipitate is mixed with volcanic rock at a weight ratio of 100:1, and a biological carrier is prepared using the following method: Select volcanic rocks with a particle size of 1-3 mm and soak them in a 5% hydrochloric acid solution for 2-3 hours to remove impurities and oxides from the surface of the volcanic rocks. Then rinse them repeatedly with deionized water until the pH value of the rinsing solution is neutral. Finally, dry them in an oven at 100-120℃ for 4-5 hours to reduce the water content of the volcanic rocks to less than 1%. The pretreated volcanic rock and iron oxide precipitate were placed in a three-dimensional motion mixer for mixing. The mixer speed was set to 30-40 r / min and the mixing time was 30-50 min to ensure that the iron oxide precipitate was uniformly attached to the surface of the volcanic rock. During the mixing process, nitrogen gas was introduced into the mixer every 10 min for 2 min each time, and the gas flow rate was 0.5-0.8 L / min to prevent the iron oxide precipitate from being oxidized. The mixed materials are transferred to a high-temperature drying oven and heated from room temperature to 200-300℃ at a rate of 5-8℃ / min. The temperature is maintained at this level for 3-4 hours for drying. During the heating and drying process, nitrogen gas is continuously introduced at a flow rate of 1-1.5L / min. After drying, the materials are cooled to room temperature at a rate of 3-5℃ / min.

[0006] Preferably, in the low-carbon source denitrification method, after the iron oxide precipitate is generated, the obtained iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots are used together to form a biological carrier. The preparation method of polydopamine-modified cellulose nanocrystals is as follows: Cellulose nanocrystals were dispersed in a Tris-HCl buffer solution with pH = 8.5, and dopamine hydrochloride was added. The mass ratio of dopamine hydrochloride to cellulose nanocrystals was 1:3. The mixture was stirred at room temperature for 12-15 hours to allow dopamine to self-polymerize and modify the surface of the cellulose nanocrystals under alkaline conditions. After the reaction was completed, polydopamine-modified cellulose nanocrystals were obtained by centrifugation, washing, and freeze-drying. The preparation method of bentonite supported on carbon nitride quantum dots is as follows: First, carbon nitride quantum dots were prepared using a hydrothermal method. The prepared carbon nitride quantum dots were then mixed with bentonite at a mass ratio of 1:5. Next, based on the total mass of the mixed powder, 2-5 times the mass of deionized water was added to form a suspension. The deionization method was as follows: First, 1 / 3 of the total mass of deionized water was slowly added dropwise to the mixed powder while stirring at a speed of 200-300 rpm, with the dropping rate controlled at 1-2 mL / s. This process allowed the deionized water to initially wet the mixed powder. Then, stirring was paused for 5-10 minutes to allow the deionized water to fully penetrate the powder. Next, the remaining 1 / 3 of the total mass of deionized water was added at the same stirring and dropping rate, and stirring continued for 15-20 minutes after the addition was complete. Finally, the remaining deionized water was added, and the stirring speed was increased to 400-500 rpm, stirring for 20-30 minutes to ensure that the carbon nitride quantum dots and bentonite were fully dispersed in the deionized water, forming a uniform and stable suspension. In preparing the biological carrier, iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots were mixed in a weight ratio of 100:1:6:3, and then heated and dried. The specific mixing and heating / drying processes are as follows: Mixing process: The above four materials are placed into a three-dimensional motion mixer equipped with a magnetic field generator for mixing. The speed of the mixer is set to 30-40 r / min, and the mixing time is 40-50 min. At the same time, the magnetic field generator is turned on, and the initial magnetic field strength is set to 5 mT to generate an alternating magnetic field with an intensity of 5-10 mT. During the mixing process, nitrogen gas is introduced into the mixer every 10 min, with each gas introduction time being 2 min and the gas flow rate being 0.5-0.8 L / min, to prevent the iron oxide precipitate from being oxidized. Heating and drying: Transfer the mixed materials to a high-temperature drying oven equipped with a light source, and raise the temperature from room temperature to 200-300℃ at a rate of 5-8℃ / min, maintaining this temperature for 3-4 hours for drying. During the heating and drying process, nitrogen gas is continuously introduced at a flow rate of 1-1.5L / min to ensure the materials are in an oxygen-free environment. Simultaneously, turn on the light source with a wavelength of 400-450nm and a light intensity of 10-15mW / cm². 2The material is irradiated with blue light to promote the photocatalytic activity of carbon nitride quantum dots; after drying, it is cooled to room temperature at a rate of 3-5℃ / min.

[0007] Preferably, in the low-carbon source denitrification method, the carbon nitride quantum dots are prepared by the following method: Melamine and dicyandiamine were mixed in a molar ratio of 1:1. Based on the total mass of the two, 0.5% hexadecyltrimethylammonium bromide was added as a surfactant. The mixture was then ground in an agate mortar for 30 minutes to uniformly disperse the precursor powder and form nano-sized mixed particles. The ground precursor powder was transferred to a polytetrafluoroethylene-lined hydrothermal reactor, and deionized water was added as a solvent. The mass-to-volume ratio of the ground precursor powder to deionized water was 1:20-30 g / mL. After sealing the hydrothermal reactor, it was heated to 180-200℃ at a heating rate of 5℃ / min and held at this temperature for 6-8 hours. During this period, the hydrothermal reactor was intermittently oscillated at a rate of 100 r / min, with oscillation for 10 minutes followed by a 5-minute pause, to promote the uniform hydrolysis and condensation reaction of the precursor. After the reaction was completed, the hydrothermal reactor was allowed to cool naturally to room temperature. The product was then centrifuged and washed three times alternately with ethanol and deionized water to remove unreacted precursors and surfactants. The washed solid was then dispersed in an ethanol solution of 2% 3-aminopropyltriethoxysilane and stirred at 50°C for 2 hours to modify the surface of carbon nitride quantum dots with amino groups.

[0008] Preferably, in the low-carbon source denitrification method, step 4) uses the prepared biological carrier to treat the effluent from the biological secondary sedimentation tank and performs backwashing; after the backwashing is completed, the air and water flushing is stopped, and the biological carrier is allowed to settle naturally for 5-10 minutes before normal water intake operation is resumed. The treatment of effluent from a biological secondary sedimentation tank using a prepared biological carrier includes the following steps: The biological carrier is uniformly filled into the fixed-bed reactor, making up 60-70% of its total volume, thus achieving a filling rate of 60-70%. A layered filling method is used, and after filling every 20-30 cm, the biological carrier is leveled using a mechanical leveling device to ensure that the filling density deviation is ≤5% and the local porosity is ≤30%. The effluent from the secondary sedimentation tank enters from the bottom of the fixed bed reactor through a perforated pipe distributor. The diameter of the holes on the distributor is 5-10 mm and the spacing between the holes is 10-20 cm. The water flows evenly through the biological carrier layer in an upflow manner, and the empty bed flow velocity is controlled at 1-4.0 m / h, with HRT ≤ 40 min. The pressure difference of the carrier layer is monitored in real time by differential pressure sensors installed at the inlet and outlet of the fixed bed reactor. When the pressure difference at the inlet and outlet of the fixed bed reactor reaches ≥8kPa or after 7 days of continuous operation, the backwashing program is automatically started; or manual backwashing can be performed. Backwashing includes the following steps: Air washing stage: Turn on the backwash fan at a rate of 15-20 L / (m³). 2 Air is introduced from the bottom of the fixed-bed reactor at a gas velocity of ·s for 5-8 minutes. Water washing stage: After the air washing is completed, start the water pump and flush the top of the fixed bed reactor in reverse at a flow rate of 10 m / h for 10-15 minutes. Dynamic adjustment of backwashing cycle and intensity: When the effluent TN concentration is 12-15 mg / L or COD is 40-50 mg / L, and the backwashing time is 12 hours, the backwashing cycle is shortened from 7 days to 5 days. When the effluent TN concentration is ≤10mg / L, COD is ≤30mg / L, and the pressure difference growth rate between the bottom and top of the fixed bed reactor is ≤0.5kPa / d, the backwash cycle is extended from 7d to 10d. When the pressure difference increases by ≥2 kPa within 24 hours and the water quality meets the standards, high-intensity air scrubbing is performed first, with the air velocity increased to 20 L / (m²). 2 •s), extend the time to 10min; if the pressure difference drop after air washing is less than 1kPa, start water washing immediately, with the water washing flow rate being 3 times the empty bed flow rate, until the pressure difference drops to 5-8kPa.

[0009] Preferably, in the low-carbon-source denitrification method, step 5) involves controlling the DO concentration and HRT of the incoming water as follows: An online dissolved oxygen sensor with a measurement accuracy of ±0.05 mg / L is installed on the pipeline before the effluent from the secondary sedimentation tank enters the fixed bed reactor. The DO data of the incoming water is collected every 2 minutes and transmitted to the control system. Pre-measure the total height of the fixed-bed reactor H Unit: m, Calculate the effective height of a fixed-bed reactor. H 有效 = H × (1 - fill rate); Control methods when DO concentration exceeds the standard: Primary control, reducing upstream aeration: When the DO concentration of the incoming water is >1mg / L, the control system will perform the following control in sequence: reduce the aeration rate of the aerobic aeration unit upstream of the secondary sedimentation tank by 20-30% through the frequency converter, with each adjustment not exceeding 5% and an interval of 5 minutes, until the DO concentration is ≤1mg / L or the aeration rate is reduced to 40% of the rated value; Secondary control, adjusting the HRT of the treatment unit: If the DO concentration is still >1 mg / L after the aeration rate is reduced to 40% of the rated value, the empty bed flow rate is reduced from 1-4.0 m / h to 0.5-0.1 m / h using an electric flow regulating valve. The HRT is calculated as follows: ; Three-level control, external carbon source addition: The amount of externally added carbon source is linked to the online dissolved oxygen sensor to ensure that the DO concentration entering the treatment unit is ≤1mg / L; Methods for controlling DO concentration when it meets the target: When the DO concentration in the incoming water is ≤1mg / L, the control system will increase the empty bed flow rate to 1-4m / h, so that the HRT is controlled at 30-40min; DO probe installation and verification methods: A DO probe is installed in the middle of the biological carrier packing layer inside the fixed-bed reactor. Specifically, at 1 / 2 of the total height of the fixed-bed reactor, and with the central axis as the reference, it is inserted vertically into the carrier layer at 1 / 3 of the radius outward. The DO concentration in the fixed-bed reactor is monitored in real time to see if it is maintained at 0.5-1 mg / L. If the detection value exceeds 1 mg / L for 3 consecutive times with an interval of 2 minutes, the above control steps are automatically repeated.

[0010] Preferably, the low-carbon source denitrification method, when using a biological carrier to treat the effluent from the biological secondary sedimentation tank, further includes the following steps: Online COD sensors, ammonia nitrogen sensors, and nitrate nitrogen sensors are installed sequentially on the pipeline before the effluent from the secondary sedimentation tank enters the fixed-bed reactor. These sensors monitor the concentrations of COD, ammonia nitrogen, and nitrate nitrogen in the influent in real time. The monitoring data is transmitted to the control system in real time for subsequent dynamic adjustment of the fixed-bed reactor's operating parameters. Meanwhile, the influent to the fixed-bed reactor is pretreated by a mechanical bar screen with a pore size of 100-200μm installed at the effluent end of the biological secondary sedimentation tank to remove large particulate suspended solids. A microbial sampling port is set in the middle of the side wall of the fixed-bed reactor. The 16S rRNA gene copy number of denitrifying bacteria is detected weekly using a quantitative PCR instrument. When the detected value is lower than 70% of the initial inoculum, or when the microbial dehydrogenase activity is detected as lower than 50 μmol / (g·h) by fluorescent probe method, the following repair measures are automatically triggered: 1) External carbon source addition; 2) Inoculate with anaerobic activated sludge.

[0011] Preferably, in the aforementioned low-carbon-source denitrification method, the specific method for externally adding a carbon source is as follows: Liquid sodium acetate was used as a supplementary carbon source and was added to the inlet of the fixed-bed reactor via a metering pump. When the nitrate nitrogen sensor detects that the nitrate nitrogen concentration in the influent of the fixed-bed reactor is N If the concentration is mg / L, then the sodium acetate dosage is 3-5 mg / L. N mg; After addition, the DO concentration in the fixed-bed reactor should be controlled to be ≤1 mg / L.

[0012] Preferably, in the low-carbon-source denitrification method, the specific method for inoculating anaerobic activated sludge is as follows: Take activated sludge from anaerobic tanks, anoxic tanks, aerobic tanks, or secondary sedimentation tanks and acclimate it for 6-8 days; Sludge and biological carrier were mixed at a volume ratio of 1:10, allowed to stand for 2 hours for adsorption, and then injected into a fixed-bed reactor in an upflow manner. Maintain the HRT of the fixed-bed reactor at 6-8 hours within 24 hours after inoculation, and then adjust the HRT according to the TN concentration in the effluent. When the TN concentration in the effluent is ≤10mg / L, the HRT should be controlled to be ≤40min; If the TN concentration in the effluent is >10mg / L, extend the HRT to 1-2h until the TN concentration in the effluent is ≤10mg / L.

[0013] Preferably, in the low-carbon source denitrification method, step 1) is based on Fe 2+ and Fe 3+ A FeSO4 and FeCl3 solution was mixed in a molar ratio of 1:2.

[0014] The present invention has at least the following beneficial effects: This invention generates iron oxide precipitate by mixing FeSO4 and FeCl3 solutions in a specific ratio and adjusting the pH value. This precipitate is then mixed with volcanic rock to create a biological carrier, providing a favorable environment for microbial attachment and growth, and promoting microbial enrichment and metabolism. For example, in Example 1, the iron oxide-volcanic rock composite carrier achieved a TN removal rate of 20.7%. Subsequent examples, through continuous improvement, achieved a TN removal rate as high as 76.3% in Example 5, significantly improving the nitrogen removal efficiency of low-carbon wastewater.

[0015] This invention avoids the need for additional carbon source addition as in traditional methods, reducing carbon source procurement costs and the costs associated with secondary pollution treatment that may result from adding carbon sources. Simultaneously, reasonable backwashing steps and operational parameter control, such as dynamically adjusting the backwashing cycle and intensity, reduce unnecessary energy and water consumption, thereby lowering the overall treatment cost.

[0016] This invention involves pretreating volcanic rock to remove impurities and oxides, mixing it with iron oxide precipitate, and then processing it through a specific process to create a biological carrier. The carrier's specific surface area, zeta potential, and dehydrogenase activity are gradually improved. This creates more favorable conditions for microbial growth and metabolism, enhancing the carrier's adsorption and treatment capabilities for pollutants.

[0017] This invention enables precise monitoring and tiered control of dissolved oxygen (DO) concentration in incoming water. When the DO concentration is >1 mg / L, adjustment is made by reducing the upstream aeration rate or increasing the heat recovery time (HRT) of the treatment unit. When the DO concentration is ≤1 mg / L, the HRT is controlled to not exceed 40 minutes. This precise control ensures that the denitrification process takes place in a suitable low-oxygen environment, improving nitrogen removal efficiency while maintaining stable system operation.

[0018] In this invention, the components of the biological carrier work synergistically. For example, volcanic rock provides physical support and adsorption sites, iron oxide enhances pollutant removal capacity, polydopamine-modified cellulose nanocrystals promote microbial growth, and bentonite loaded with carbon nitride quantum dots provides photocatalytic active sites. The upflow influent and backwashing strategies in the treatment process work together to overcome the limitations of traditional biological treatment technologies, achieving highly efficient removal of nitrate nitrogen and organic pollutants.

[0019] This invention utilizes dual monitoring of differential pressure sensors and water quality indicators to combine automatic initiation and manual intervention of the backwashing process, dynamically adjusting the backwashing cycle and intensity based on water quality and differential pressure. Simultaneously, through multi-parameter online monitoring equipment and a central control system, it acquires and processes information such as influent water quality, reactor operating parameters, and effluent water quality in real time, automatically adjusting operating strategies to ensure stable and efficient system operation even under fluctuating water quality conditions, thus improving the system's adaptability and reliability.

[0020] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0022] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0023] This invention provides a method for nitrogen removal from low carbon sources, comprising the following steps: 1) According to Fe 2+ and Fe 3+ A FeSO4 and FeCl3 solution was mixed in a molar ratio of 1:0.2-2. 2) Adjust the pH of the mixed solution to 10-12 to form a black iron oxide precipitate, with nitrogen gas purging throughout the process; 3) The obtained iron oxide precipitate is mixed with volcanic rock at a weight ratio of 30-300:1 and dried at 200-300℃ to prepare a biological carrier; 4) Use the prepared biological carrier to treat the effluent from the secondary sedimentation tank of the biological treatment unit; after the treatment unit has been running for 3-7 days, backwashing is performed; during backwashing, the backwash blower is started first to backwash the biological carrier and remove the aging microorganisms on the biological carrier, and then the water pump is started to draw water from the inlet of the treatment unit to backwash the biological carrier. 5) Control the DO concentration of the incoming water to ≤1mg / L; when the DO concentration of the incoming water is >1mg / L, reduce the aeration rate of the upstream unit or increase the HRT of the treatment unit; when the DO concentration of the incoming water is ≤1mg / L, control the HRT of the treatment unit to ≤40min.

[0024] The concentrations of FeSO4 and FeCl3 solutions can be set to 0.1 mol / L and 0.2 mol / L, respectively. The mixing process uses a reactor equipped with an anchor-type stirrer at a speed of 250-350 r / min, continuously mixing for 8-12 minutes until the solution is homogeneous. Then, the pH of the mixed solution is adjusted to 10-12, and 5 mol / L NaOH solution is added dropwise using a pressure nozzle in the form of droplets with a droplet size of 80-120 μm. The pH is monitored in real-time by a pH sensor (accuracy ±0.1). Nitrogen gas (purity ≥99.9%) is introduced throughout the process at a flow rate of 0.9-1.1 L / min, continuously for 30-40 minutes from before the reaction to after it ends, to prevent Fe²⁺ oxidation and the formation of black iron oxide precipitate.

[0025] Iron oxide precipitate was mixed with volcanic rock at a weight ratio of 30-300:1 using a three-dimensional motion mixer (30-40 r / min) for 30-40 min to ensure uniform dispersion. The mixed material was then transferred to a high-temperature drying oven and heated to 200-300℃ at a rate of 5-8℃ / min, maintaining the temperature for 3-4 h. Nitrogen gas was introduced during the drying process (flow rate 1-1.5 L / min). After drying, the temperature was lowered to room temperature at a rate of 3-5℃ / min to prepare the biological carrier.

[0026] The biological carrier is filled into the treatment unit, and backwashing is performed after 3-7 days of operation. During backwashing, the backwash fan is started first at a rate of 15-20 L / (m³). 2 Wash with air at a flow rate of 1.0-3.0 m / h for 5-8 minutes to remove aging microorganisms. Then, start the water pump to draw in water and wash with water at a flow rate of 1.0-3.0 m / h for 10-15 minutes. Air washing can be performed using air.

[0027] The dissolved oxygen (DO) concentration in the incoming water is monitored in real time by an online dissolved oxygen sensor, and the DO concentration is controlled to not exceed 1 mg / L. When the DO concentration in the incoming water exceeds 1 mg / L, the aeration rate of the upstream unit is reduced or the HRT of the treatment unit is increased. When the DO concentration in the incoming water is not higher than 1 mg / L, the HRT of the treatment unit is controlled to not exceed 40 min.

[0028] This approach uses volcanic rock and iron oxide as the core components of novel biological carrier materials, which play the following roles: 1. Volcanic rocks possess a porous structure and a large specific surface area, providing a suitable microenvironment for microorganisms (such as denitrifying bacteria and polyphosphate-accumulating bacteria) to attach and grow, promoting the formation and stabilization of biofilms. The porous nature of volcanic rocks can physically adsorb residual organic matter (COD), nitrogen, phosphorus, and other pollutants in water, prolonging the contact time between pollutants and microorganisms and improving denitrification efficiency. The porous structure of volcanic rocks can create localized anaerobic / anoxic microenvironments, which are conducive to denitrification reactions and reduce dependence on external carbon sources.

[0029] 2. Iron oxides (such as Fe3O4 or Fe2O3) have a positively charged surface, which can adsorb negatively charged pollutants (such as phosphates and nitrates) in water through electrostatic interactions, enhancing the removal capacity of nitrogen and phosphorus. Iron oxides can also act as electron acceptors in microbial metabolic processes (such as participating in dissimilar iron reduction reactions under anaerobic conditions), indirectly promoting the decomposition of organic matter and denitrification. When iron oxides are mixed with volcanic rock and subjected to high-temperature treatment, a composite structure can be formed, improving the mechanical strength and chemical stability of the material while retaining its adsorption and biocompatibility.

[0030] 3. Volcanic rock provides physical support and adsorption sites, while iron oxide enhances pollutant removal capacity through surface charge and chemical activity. The composite carrier formed by the combination of these two elements possesses high specific surface area, microbial affinity, and pollutant adsorption properties, thereby significantly improving the system's denitrification efficiency and reducing the need for external carbon source addition. This is achieved through precise control of Fe... 2+ and Fe 3+ The precise mixing ratio and pH value, combined with nitrogen protection throughout the process, ensure the high activity and purity of the iron oxide precipitate. The preparation process of the biological carrier provides a stable substrate for microbial attachment, and the backwashing process effectively maintains the carrier performance. The staged control of DO concentration and the dynamic adjustment of HRT create a suitable low-oxygen environment for the denitrification reaction, improving the denitrification efficiency under low carbon source conditions and reducing additional carbon source consumption and treatment costs.

[0031] In another embodiment, in the low-carbon source denitrification method, after generating iron oxide precipitate in step 3), the obtained iron oxide precipitate is mixed with volcanic rock at a weight ratio of 100:1, and a biological carrier is prepared in the following manner: Select volcanic rocks with a particle size of 1-3 mm and soak them in a 5% hydrochloric acid solution for 2-3 hours to remove impurities and oxides from the surface of the volcanic rocks. Then rinse them repeatedly with deionized water until the pH value of the rinsing solution is neutral. Finally, dry them in an oven at 100-120℃ for 4-5 hours to reduce the water content of the volcanic rocks to less than 1%. The pretreated volcanic rock and iron oxide precipitate were placed in a three-dimensional motion mixer for mixing. The mixer speed was set to 30-40 r / min and the mixing time was 30-40 min to ensure that the iron oxide precipitate was uniformly attached to the surface of the volcanic rock. During the mixing process, nitrogen gas was introduced into the mixer every 10 min for 2 min each time, and the gas flow rate was 0.5-0.8 L / min to prevent the iron oxide precipitate from being oxidized. The mixed material is transferred to a high-temperature drying oven and heated from room temperature to 200-300℃ at a rate of 5-8℃ / min. The temperature is maintained at this level for 3-4 hours for drying. During the heating and drying process, nitrogen gas is continuously introduced at a flow rate of 1-1.5L / min to keep the material in an oxygen-free environment and prevent further oxidation of iron oxide. After drying, the material is cooled to room temperature at a rate of 3-5℃ / min.

[0032] The method for preparing a biological carrier includes the following steps: Select volcanic rocks with a particle size of 1-3 mm and soak them in a 5% hydrochloric acid solution for 2-3 hours to remove surface impurities and oxides. Low-speed stirring can be used during the soaking process. Then rinse with deionized water until the pH of the rinsing solution is neutral (pH=7±0.5), rinsing 3-5 times. Finally, dry them in an oven at 100-120℃ for 4-5 hours until the moisture content is <1%.

[0033] The pretreated volcanic rock and iron oxide precipitate were placed in a three-dimensional motion mixer (30-40 r / min) at a weight ratio of 100:1 and mixed for 30-40 min to ensure that the iron oxide precipitate was uniformly attached to the surface of the volcanic rock. During mixing, nitrogen gas was introduced for 2 min every 10 min at a flow rate of 0.5-0.8 L / min to prevent oxidation of the iron oxide.

[0034] The mixture is transferred to a high-temperature drying oven (with a nitrogen port), heated to 200-300℃ at 5-8℃ / min, and dried for 3-4 hours with nitrogen gas introduced throughout. After drying, it is cooled to room temperature at 3-5℃ / min to ensure the stability of the carrier structure.

[0035] This method removes impurities and oxides by soaking in hydrochloric acid, followed by rigorous washing and drying to ensure the volcanic rock surface is clean and has extremely low water content, providing a good foundation for the subsequent adhesion of iron oxide precipitates. This pretreatment method enhances the bonding force between the volcanic rock and the iron oxide precipitate, making the structure of the biological carrier more stable.

[0036] The use of a three-dimensional motion mixer allows materials to move fully in three dimensions, compared to traditional two-dimensional mixing methods, resulting in a more uniform mixing effect. Simultaneously, the intermittent introduction of nitrogen gas during the mixing process effectively prevents the oxidation of iron oxide precipitates, ensuring the performance of the biological carrier.

[0037] Precise control of heating and cooling rates prevents structural damage or performance instability of the biological carrier due to rapid temperature changes. Continuous nitrogen flow during the heating and drying process creates an anaerobic environment, further preventing the oxidation of iron oxide and ensuring its activity and stability within the biological carrier. This innovative heating and drying method helps improve the quality and performance of the biological carrier, making it more conducive to microbial growth and attachment, thereby enhancing the treatment efficiency of the low-carbon-source denitrification system.

[0038] In another embodiment, in the low-carbon source denitrification method, after generating iron oxide precipitate, the obtained iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots are used together to form a biological carrier. The preparation method of polydopamine-modified cellulose nanocrystals is as follows: Cellulose nanocrystals were dispersed in a Tris-HCl buffer solution with pH = 8.5, and dopamine hydrochloride was added. The mass ratio of dopamine hydrochloride to cellulose nanocrystals was 1:3. The mixture was stirred at room temperature for 12-15 hours to allow dopamine to self-polymerize and modify the surface of the cellulose nanocrystals under alkaline conditions. After the reaction was completed, polydopamine-modified cellulose nanocrystals were obtained by centrifugation, washing, and freeze-drying. The preparation method of bentonite supported on carbon nitride quantum dots is as follows: First, carbon nitride quantum dots were prepared using a hydrothermal method. The prepared carbon nitride quantum dots were then mixed with bentonite at a mass ratio of 1:5. Next, based on the total mass of the mixed powder, 2-5 times the mass of deionized water was added to form a suspension. The deionization method was as follows: first, 1 / 3 of the total mass of deionized water was slowly added dropwise to the mixed powder while stirring at a speed of 200-300 rpm, with the dropping rate controlled at 1-2 mL / s. This process initially allowed the deionized water to... First, wet the mixed powder; then, pause stirring for 5-10 minutes to allow the deionized water to fully penetrate the powder; next, add 1 / 3 of the remaining total mass of deionized water at the same stirring and dropping speed, and continue stirring for 15-20 minutes after the addition is complete; finally, add the remaining deionized water, while increasing the stirring speed to 400-500 r / min, and stir for 20-30 minutes to ensure that the carbon nitride quantum dots and bentonite are fully dispersed in the deionized water to form a uniform and stable suspension; In preparing the biological carrier, iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots were mixed in a weight ratio of 100:1:6:3, and then heated and dried. The specific mixing and heating / drying processes are as follows: Mixing process: The above four materials are placed into a three-dimensional motion mixer equipped with a magnetic field generator for mixing. The speed of the mixer is set to 30-40 r / min, and the mixing time is 40-50 min. At the same time, the magnetic field generator is turned on, and the initial magnetic field strength is set to 5 mT to generate an alternating magnetic field with an intensity of 5-10 mT. During the mixing process, nitrogen gas is introduced into the mixer every 10 min, with each gas introduction time being 2 min and the gas flow rate being 0.5-0.8 L / min, to prevent the iron oxide precipitate from being oxidized. Heating and drying: Transfer the mixed materials to a high-temperature drying oven equipped with a light source, and raise the temperature from room temperature to 200-300℃ at a rate of 5-8℃ / min, maintaining this temperature for 3-4 hours for drying. During the heating and drying process, nitrogen gas is continuously introduced at a flow rate of 1-1.5L / min to ensure the materials are in an oxygen-free environment. Simultaneously, turn on the light source with a wavelength of 400-450nm and a light intensity of 10-15mW / cm². 2 The material is irradiated with blue light to promote the photocatalytic activity of carbon nitride quantum dots; after drying, it is cooled to room temperature at a rate of 3-5℃ / min.

[0039] Preparation of polydopamine-modified cellulose nanocrystals: Cellulose nanocrystals (particle size 50-100 nm) were dispersed in a Tris-HCl buffer solution at pH 8.5, and dopamine hydrochloride was added. The mixture was stirred at room temperature for 12-15 h to allow dopamine to self-polymerize and modify the nanocrystal surface. After the reaction was completed, the nanocrystals were centrifuged (5000 r / min, 10 min), washed three times, and freeze-dried to obtain the product.

[0040] Preparation of bentonite supported on carbon nitride quantum dots: Carbon nitride quantum dots and bentonite were mixed at a mass ratio of 1:5, and 2-5 times the volume of deionized water was added to prepare a suspension. Deionized water was added dropwise in three portions to ensure uniform dispersion.

[0041] Preparation of the composite carrier: Iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite supported on carbon nitride quantum dots were placed in a weight ratio of 100:1:6:3 into a three-dimensional motion mixer equipped with a magnetic field generator (intensity 5-10 mT) (speed 30-40 r / min) and mixed for 40-50 min, during which nitrogen gas was purged for 2 min every 10 min (flow rate 0.5-0.8 L / min). The mixture was then transferred to a drying oven equipped with a light source (wavelength 400-450 nm, light intensity 10-20 mW / cm²), heated to 200-300℃ at a rate of 5-8℃ / min, and maintained for 3-4 h, with nitrogen gas purging throughout (flow rate 1-1.5 L / min). After cooling, the mixture was ready for use.

[0042] Synergistic mechanism of the four components: The porous structure of volcanic rock provides a site for the adhesion and dispersion of the other three components. Simultaneously, the mineral components on its surface can form weak chemical bonds with the active groups on the surface of polydopamine-modified cellulose nanocrystals, enhancing the adhesion stability of the polydopamine-modified cellulose nanocrystals on the volcanic rock surface and enabling them to more effectively promote microbial growth. The amino and hydroxyl groups on the surface of the polydopamine-modified cellulose nanocrystals can form coordination bonds with iron ions on the surface of iron oxide precipitate. This chemical bonding not only enhances the binding force between the two components but also regulates the electron cloud density of the iron oxide precipitate, thereby improving its catalytic activity. Furthermore, after the polydopamine-modified cellulose nanocrystals promote microbial growth and reproduction, the small-molecule organic acids and other substances produced by microbial metabolism can further activate the surface of the iron oxide precipitate, further enhancing its catalytic performance. Under the action of an alternating magnetic field, the iron oxide precipitate generates induced current and heat. This localized thermal and electric field effect can promote the separation of electron-hole pairs in bentonite supported by carbon nitride quantum dots, improving its photocatalytic efficiency. Furthermore, the catalytic reaction products of iron oxide precipitation may provide intermediate reactants for the photocatalytic reaction of bentonite supported by carbon nitride quantum dots, further promoting the photocatalytic reaction. The photogenerated charge carriers generated by the carbon nitride quantum dot-supported bentonite under blue light irradiation can undergo redox reactions with the active groups on the surface of polydopamine-modified cellulose nanocrystals, generating substances beneficial to microbial growth, such as hydrogen peroxide, thereby promoting the growth and metabolism of microorganisms on the polydopamine-modified cellulose nanocrystals.

[0043] As shown above, polydopamine-modified cellulose nanocrystals enhance the biocompatibility and microbial adsorption capacity of the support, while bentonite supported on carbon nitride quantum dots provides photocatalytic active sites. The composite support, through magnetic field-assisted mixing and blue light-induced drying, promotes synergistic effects between materials, providing an efficient adhesion interface for microorganisms and enhancing the reduction efficiency of nitrate nitrogen through photocatalytic reactions. This achieves a combination of biodegradation and photocatalytic denitrification in a low-carbon source environment.

[0044] In another embodiment, the carbon nitride quantum dots in the low-carbon source denitrification method are prepared by the following method: Melamine and dicyandiamine were mixed in a molar ratio of 1:1. Based on the total mass of the two, 0.5% hexadecyltrimethylammonium bromide was added as a surfactant. The mixture was then ground in an agate mortar for 30 minutes to uniformly disperse the precursor powder and form nano-sized mixed particles. The ground precursor powder was transferred to a polytetrafluoroethylene-lined hydrothermal reactor, and deionized water was added as a solvent. The mass-to-volume ratio of the ground precursor powder to deionized water was 1:20-30 g / mL. After sealing the hydrothermal reactor, it was heated to 180-200℃ at a heating rate of 5℃ / min and held at this temperature for 6-8 hours. During this period, the hydrothermal reactor was intermittently oscillated at a rate of 100 r / min, with oscillation for 10 minutes followed by a 5-minute pause, to promote the uniform hydrolysis and condensation reaction of the precursor. After the reaction was completed, the hydrothermal reactor was allowed to cool naturally to room temperature. The product was then centrifuged and washed three times alternately with ethanol and deionized water to remove unreacted precursors and surfactants. The washed solid was then dispersed in an ethanol solution of 2% (w / w) 3-aminopropyltriethoxysilane and stirred at 50°C for 2 hours to modify the surface of carbon nitride quantum dots with amino groups, thereby improving their bonding ability with the hydroxyl groups on the bentonite surface.

[0045] By adjusting the hydrothermal reaction temperature and oscillation frequency, the particle size of carbon nitride quantum dots is controlled at 5-10 nm, and the particle size distribution uniformity (PDI) is ≤0.15, ensuring that they have high photocatalytic activity and good dispersibility.

[0046] The method for preparing carbon nitride quantum dots includes the following steps: Melamine and dicyandiamine were mixed in a molar ratio of 1:1, and 0.5% hexadecyltrimethylammonium bromide was added. The mixture was then ground in an agate mortar for 30 minutes to form nanoscale mixed particles (particle size <100nm).

[0047] The ground precursor powder was transferred to a polytetrafluoroethylene-lined hydrothermal reactor, and deionized water was added at a ratio of 1:20-30 g / mL. After sealing, the temperature was increased to 180-200℃ at 5℃ / min and kept at that temperature for 6-8 hours. During this period, the reactor was intermittently oscillated at a rate of 100 r / min (oscillation for 10 min, pause for 5 min) to promote the hydrolysis-condensation reaction.

[0048] After cooling, the reaction mixture was centrifuged (8000 r / min, 15 min) and washed three times alternately with ethanol and deionized water to remove impurities. The solid was dispersed in a 2% 3-aminopropyltriethoxysilane ethanol solution (prepared with 95% ethanol by volume) and stirred at 50 °C for 2 h to modify the surface of the quantum dots with amino groups, thereby improving dispersibility and active sites.

[0049] This scheme uses melamine and dicyandiamine as a composite precursor. During the hydrothermal process, the two form quantum dots with multi-level conjugated structures through a synergistic condensation reaction, which significantly improves the light absorption efficiency and electron-hole pair separation ability of the material, thereby enhancing the performance of photocatalytic degradation of pollutants.

[0050] By regulating the growth kinetics of quantum dots using CTAB surfactants and combining this with APTES amino modification, a large number of amino active groups were introduced onto the surface of the quantum dots. These groups not only promote the chemisorption and stable loading of quantum dots on the bentonite surface, but also form hydrogen bonds with the hydroxyl and amino groups on the surface of polydopamine-modified cellulose nanocrystals, strengthening the interfacial bonding between the four components and constructing a more efficient electron transport and material conversion network.

[0051] By precisely controlling the heating rate, holding time, and oscillation frequency of the hydrothermal reaction, the particle size (5-10 nm) and uniform distribution of carbon nitride quantum dots were accurately regulated. The smaller particle size increases the specific surface area and the number of active sites of the quantum dots, while the uniform particle size distribution avoids agglomeration, enabling them to work more efficiently with bentonite and iron oxide precipitates during photocatalysis, thereby improving the degradation efficiency of nitrogen pollutants in wastewater.

[0052] The intermittent oscillating hydrothermal reaction process breaks through the limitations of traditional static hydrothermal methods. By mechanically disturbing the precursors, it promotes mass transfer and reaction uniformity, and reduces the formation of byproducts. This improved preparation method not only enhances the crystallinity and purity of quantum dots, but also lays the foundation for subsequent synergistic performance with bentonite loading and the overall biocarrier, forming an innovative technology chain from single-component optimization to multi-component synergistic enhancement.

[0053] As shown above, carbon nitride quantum dots with uniform particle size and high crystallinity were prepared by optimizing precursor grinding and hydrothermal reaction conditions. Surface amino modification increased the hydrophilicity and surface charge of the quantum dots, making their composite with bentonite more stable and improving the electron-hole pair separation efficiency in the photocatalytic reaction. This enhanced the catalytic reduction ability of nitrate nitrogen and provided key support for the denitrification performance of the composite support.

[0054] In another embodiment, in the low-carbon source denitrification method, step 4) uses the prepared biological carrier to treat the effluent from the biological secondary sedimentation tank and backwashes it; after the backwashing is completed, the air and water flushing is stopped, and the biological carrier is allowed to settle naturally for 5-10 minutes before normal water intake operation is resumed. The treatment of effluent from a biological secondary sedimentation tank using a prepared biological carrier includes the following steps: The biological carrier is uniformly filled into the fixed-bed reactor, making up 60-70% of its total volume. To ensure uniform filling, a layered filling method is adopted. After filling every 20-30cm, the biological carrier is leveled using a mechanical leveling device to ensure that the filling density (mass) deviation is ≤5% and the local porosity is ≤30%. The effluent from the secondary sedimentation tank enters the fixed-bed reactor from the bottom through a perforated pipe distributor. The distributor has holes with a diameter of 5-10 mm and a spacing of 10-20 cm. The water flows evenly through the biological carrier layer in an upflow manner (flowing upwards from the bottom of the reactor). Under normal operating conditions, the empty bed flow velocity is controlled at 1-4 m / h, and the HRT (Heat Reduction Time) is ≤40 min. The influent flow rate can be precisely controlled by a flow regulating valve, thereby adjusting the HRT and operating flow rate. The water is evenly distributed at the bottom of the reactor through the small holes. During the upward flow, the water comes into full contact with the packed biological carrier, and the microorganisms attached to the carrier decompose and transform pollutants in the water. At the same time, the upward flow of water can also disturb the biological carrier, allowing the microorganisms on the carrier surface to continuously come into contact with fresh wastewater, thus improving treatment efficiency.

[0055] The pressure difference of the carrier layer is monitored in real time by differential pressure sensors installed at the inlet and outlet of the fixed bed reactor. When the pressure difference between the inlet and outlet of the fixed bed reactor reaches ≥8kPa or after 7 days of continuous operation, the backwashing program is automatically started; or manual backwashing can be performed. If water quality deterioration is found, the backwashing program can be started manually even if the pressure difference or operating time conditions are not met.

[0056] Backwashing includes the following steps: Air washing stage: Turn on the backwash fan at a rate of 15-20 L / (m³). 2 Air is introduced from the bottom of the fixed-bed reactor at a velocity of ·s for 5-8 minutes. The air exerts shear force on the porous structure of the volcanic rock and the highly adhesive surface of the polydopamine-modified cellulose nanocrystals, peeling away the aged microbial membrane. Simultaneously, the magnetism of the iron oxide precipitate causes micro-collisions between the carrier particles under airflow disturbance, further promoting biofilm detachment without damaging the active sites on the carrier surface. To ensure effective air washing, the airflow and velocity of the blower can be adjusted appropriately during the washing process to ensure uniform air distribution within the reactor.

[0057] Water washing stage: After air washing, under normal operating conditions, start the water pump and flush the fixed-bed reactor counter-currently at a flow rate of 10 m / h for 10-15 minutes. During water washing, some pollutants adsorbed by the bentonite loaded with carbon nitride quantum dots are discharged with the water flow. Simultaneously, the polydopamine active groups on the carrier surface maintain the re-attachment ability of microorganisms through hydrophilicity. The flow rate and pressure of the water washing can be controlled by adjusting the pump head and flow rate.

[0058] Dynamic adjustment of backwash cycle and intensity under special operating conditions: When the effluent TN concentration is 12-15 mg / L or COD is 40-50 mg / L and lasts for 12 hours, the backwash cycle is shortened from 7 days to 5 days. When the effluent TN concentration is ≤10mg / L, COD is ≤30mg / L, and the pressure difference growth rate between the bottom and top of the fixed bed reactor is ≤0.5kPa / d, the backwash cycle is extended from 7d to 10d. When the pressure difference increases by ≥2 kPa within 24 hours and the water quality meets the standards, high-intensity air scrubbing should be prioritized, with the air velocity increased to 20 L / (m²). 2 If the pressure difference decreases by less than 1 kPa after air washing, immediately start water washing at a flow rate three times the empty bed flow rate until the pressure difference drops to 5-8 kPa. If the pressure difference suddenly increases, but the effluent quality still meets the standards: first perform air washing, appropriately increasing the air washing intensity and extending the air washing time to 8-10 minutes, and observe the pressure difference change. If the pressure difference still does not decrease significantly, then perform water washing, adjusting the water washing intensity according to the actual situation.

[0059] The operation and backwashing of the processing unit includes the following steps: Reactor filling and water distribution: The biological carrier is layered into the fixed-bed reactor at a volumetric filling rate of 60-70%. After each layer is filled to a depth of 20-30 cm, it is leveled using a mechanical leveling device to ensure that the filling density deviation is ≤5% and the local porosity is ≤30%. The influent enters through the bottom perforated pipe distributor with a hole diameter of 5-10 mm and a hole spacing of 10-20 cm. It flows through the carrier layer in an upflow manner, with an empty bed flow velocity of 1-4.0 m / h and a heat transfer response (HRT) of ≤40 min.

[0060] Backwashing process and dynamic adjustment: During the air washing stage, the backwash fan is turned on at a rate of 15-20 L / (m³). 2 • s) Air flow rate for 5-8 min; water washing stage with top backwash at 10 m / h for 10-15 min. Dynamic adjustments based on water quality and pressure differential: When effluent TN concentration is 12-15 mg / L or COD is 40-50 mg / L for 12 h, the backwash cycle is shortened to 5 days, the water washing flow rate is increased to 3 times the empty bed flow rate, and the air washing velocity is 18-20 L / (m²). 2 When the water quality meets the standards and the pressure difference increases slowly, the cycle is extended to 10 days, and the flow rate and time are reduced accordingly.

[0061] This scheme uses an upflow water intake method combined with a water distribution device to ensure that the water flow has full contact and disturbance with the biological carrier, thereby enhancing the mass transfer effect between pollutants and microorganisms.

[0062] This solution utilizes dual monitoring of differential pressure sensors and water quality indicators to achieve a combination of automatic initiation and manual intervention in the backwashing process. The combined air-water backwashing process, through reasonable control of air and water washing parameters, effectively removes aging microbial films while protecting the activity and performance of the biological carrier. The roles of polydopamine and bentonite in the backwashing process further enhance the biocompatibility and regeneration capacity of the carrier.

[0063] This solution establishes a comprehensive data monitoring system and a scientific strategy for dynamically adjusting backwash cycles and intensity. By adjusting backwash parameters in real time based on water quality indicators and differential pressure data, it ensures that the treated effluent consistently meets standards. This precise control method improves the stability and reliability of the wastewater treatment system, reduces operating costs, and provides an innovative solution for low-carbon wastewater treatment.

[0064] As can be seen from the above, the reasonable filling method and water distribution design ensured uniform water flow distribution, avoiding short-circuiting and dead zones. The backwashing strategy with dual monitoring of differential pressure and water quality enabled timely response to carrier contamination, dynamically adjusting the backwashing intensity and cycle, which not only ensured the treatment effect but also reduced unnecessary energy and water consumption, maintaining the long-term stable operation of the reactor.

[0065] In another embodiment, the specific steps for controlling the dissolved oxygen concentration and HRT in step 5) of the low-carbon source denitrification method are as follows: An online dissolved oxygen sensor with a measurement accuracy of ±0.05 mg / L is installed on the pipeline before the effluent from the secondary sedimentation tank enters the fixed bed reactor. The sensor collects the DO data of the incoming water every 2 minutes and transmits it to the control system. The sensor is calibrated weekly with a standard solution to ensure that the data error is within ±0.1 mg / L.

[0066] Pre-measure the total height of the fixed-bed reactor H Unit: m, Calculate the effective height of a fixed-bed reactor. H 有效 = H × (1 - fill rate); Control methods when DO concentration exceeds the standard: Primary control, reducing upstream aeration: When the DO concentration of the incoming water is >1mg / L, the control system will perform the following control in sequence: reduce the aeration rate of the aerobic aeration unit upstream of the secondary sedimentation tank by 20-30% through the frequency converter, with each adjustment not exceeding 5% and an interval of 5 minutes, until the DO concentration is ≤1mg / L or the aeration rate is reduced to 40% of the rated value; Secondary control, adjusting the HRT of the treatment unit: If the DO concentration is still >1 mg / L after the aeration rate is reduced to 40% of the rated value, the empty bed flow rate is reduced from 1-4.0 m / h to 0.5-1 m / h using an electric flow regulating valve. The HRT is calculated as follows: ; Three-level control, external carbon source addition: The amount of externally added carbon source is linked to the online dissolved oxygen sensor to ensure that the DO concentration entering the treatment unit is ≤1mg / L; Methods for controlling DO concentration when it meets the target: When the DO concentration in the incoming water is ≤1mg / L, the control system will increase the empty bed flow rate to 1-4.0m / h, so that the HRT is controlled at 30-40min; DO probe installation and verification methods: A DO probe is installed in the middle of the biological carrier packing layer inside the fixed-bed reactor. Specifically, it is inserted vertically into the carrier layer at 1 / 2 of the total height along the height direction of the fixed-bed reactor, 1 / 3 of the radius outward from the central axis, for 5-10 cm. The DO concentration in the fixed-bed reactor is monitored in real time to see if it is maintained at 0.5-1 mg / L. If the detection value exceeds 1 mg / L for 3 consecutive times with an interval of 2 minutes, the above control steps are automatically repeated.

[0067] The method for controlling DO concentration and HRT includes the following steps: An online dissolved oxygen sensor (accuracy ±0.05 mg / L) was installed in the inlet pipe, and data was collected every 2 minutes. The total height of the fixed-bed reactor was measured in advance. H Calculate the effective height H 有效 = H ×(1 - fill rate).

[0068] DO concentration exceeding the standard is controlled as follows: Primary control involves reducing the upstream aeration rate via a frequency converter (≤5% adjustment per cycle, 5-minute intervals) until DO ≤ 1 mg / L or the aeration rate drops to 40% of the rated value; Secondary control involves reducing the empty bed flow rate to 0.5-1 m / h via an electric flow regulating valve. HRT can be controlled via H... 有效 Calculation; three-level control, external carbon source addition: the amount of externally added carbon source is linked to the online dissolved oxygen sensor to ensure that the DO concentration entering the treatment unit is ≤1mg / L.

[0069] DO concentration control and verification: When the DO concentration is ≤1mg / L, increase the empty bed flow rate to 1-4.0m / h and control the HRT for 30-40min. Install a DO probe in the middle of the carrier layer (at 1 / 2 of the total height, 1 / 3 of the radius from the center), insert it 5-10cm into the carrier layer, and monitor in real time whether the internal DO concentration is maintained at 0.5-1mg / L. If it exceeds the standard for 3 consecutive times (with an interval of 2min), the control steps will be automatically repeated.

[0070] This solution achieves precise monitoring and tiered control of influent DO concentration through real-time linkage between a high-precision dissolved oxygen sensor (±0.05 mg / L) and the control system. Unlike the fuzzy regulation of traditional processes, this solution clearly defines the aeration rate reduction gradient (20%-30%), the influent flow rate adjustment range (0.3-2.0 m³ / h), and the HRT control interval (30 min-10 h). Furthermore, the HRT calculation is based on quantifiable geometric parameters and fill rate, significantly improving control accuracy and system stability.

[0071] This scheme employs a dual control logic of "prioritizing upstream aeration adjustment + compensating for the unit's HRT." When DO concentration exceeds the standard, it first reduces dissolved oxygen input at the source by decreasing upstream aeration, and then extends the unit's HRT to ensure the denitrification reaction time, forming a synergistic mechanism of "source oxygen control - process enhancement." When DO concentration meets the standard, the HRT is shortened to 30-40 minutes by increasing the influent flow rate. Innovative engineering monitoring and layout: The DO probe is installed at a depth of 5-10 cm, one-third of the radius of the carrier layer. This location avoids interference from the reactor wall boundary layer and accurately captures the average dissolved oxygen concentration in the biological carrier area (monitoring accuracy ±0.1 mg / L). Combined with the closed-loop control of the electric regulating valve (±0.5% adjustment accuracy) and the control system, a fully automated "monitoring-adjustment-verification" system is formed, ensuring that the reactor maintains an optimal denitrification anoxic environment of 0.5-1 mg / L at all times.

[0072] As shown above, high-precision DO monitoring and volume calibration provide a data foundation for regulation. The tiered regulation strategy effectively addresses different DO concentration scenarios, avoiding the inhibition of denitrifying bacteria by high DO concentrations while improving treatment efficiency by increasing flow rate at low DO concentrations to shorten HRT. The installation of DO probes enables precise verification of the reactor's internal environment, ensuring that denitrification occurs under suitable dissolved oxygen conditions, thus enhancing the system's stability and reliability.

[0073] In another embodiment, the low-carbon source denitrification method, when using a biological carrier to treat the effluent from the biological secondary sedimentation tank, further includes the following steps: Online COD sensors, ammonia nitrogen sensors, and nitrate nitrogen sensors are installed sequentially on the pipeline before the effluent from the secondary sedimentation tank enters the fixed-bed reactor. These sensors monitor the concentrations of COD, ammonia nitrogen, and nitrate nitrogen in the influent in real time. The monitoring data is transmitted to the control system in real time for subsequent dynamic adjustment of the fixed-bed reactor's operating parameters. Meanwhile, the influent to the fixed-bed reactor is pretreated by a mechanical bar screen with a pore size of 100-200μm installed at the effluent end of the biological secondary sedimentation tank to remove large particulate suspended solids. A microbial sampling port is set in the middle of the side wall of the fixed bed reactor. The copy number of the 16S rRNA gene of denitrifying bacteria is detected weekly by quantitative PCR. When the detected value is lower than 70% of the initial inoculum, or when the microbial dehydrogenase activity is detected to be lower than 50 μmol / (g·h) by fluorescent probe method, the following remediation measures are automatically triggered: 1) external addition of carbon source; 2) inoculation with anaerobic activated sludge.

[0074] The water quality monitoring and dynamic adjustment method includes the following steps: Water quality monitoring and pretreatment: Online COD sensors (accuracy ±5mg / L), ammonia nitrogen sensors (accuracy ±0.1mg / L), and nitrate nitrogen sensors (accuracy ±0.5mg / L) are installed sequentially on the inlet water pipeline to transmit data to the control system in real time.

[0075] This scheme involves installing a microbial community monitoring device in the reactor to periodically analyze the microbial community structure and activity. When a decrease in the number or activity of key microorganisms such as denitrifying bacteria is detected, the microbial community is restored by adding an external carbon source and inoculating anaerobic activated sludge.

[0076] Microbial community monitoring can promptly detect changes in key microorganisms and, by restoring the microbial community, ensure that the biological carrier is always in a highly efficient working state, providing a continuous and stable microbial environment for denitrification of low carbon sources.

[0077] As can be seen from the above, multi-parameter water quality monitoring enables real-time feedback on influent conditions, and microbial remediation measures effectively maintain microbial activity, ensuring that the system remains highly efficient and stable during long-term operation, reducing manual intervention, and improving the level of automated control.

[0078] In another embodiment, the specific method for externally adding a carbon source in the low-carbon source denitrification method is as follows: Liquid sodium acetate was used as a supplementary carbon source and was added to the inlet of the fixed-bed reactor via a metering pump. When the nitrate nitrogen sensor detects that the nitrate nitrogen concentration in the influent of the fixed-bed reactor is N If the concentration is mg / L, then the sodium acetate dosage is 3-5 mg / L. N mg, where N is the numerical value of nitrate nitrogen concentration; for example, if the nitrate nitrogen sensor detects an influent nitrate nitrogen concentration of 1 mg / L, then the sodium acetate dosage is 3-5 mg; if the nitrate nitrogen sensor detects an influent nitrate nitrogen concentration of 2 mg / L, then the sodium acetate dosage is 6-10 mg, and so on.

[0079] After addition, the DO concentration in the fixed-bed reactor should be controlled to be ≤1 mg / L.

[0080] In another embodiment, the specific method for inoculating anaerobic activated sludge in the low-carbon source denitrification method is as follows: Take activated sludge from anaerobic tanks, anoxic tanks, aerobic tanks, or secondary sedimentation tanks and acclimate it for 6-8 days; Sludge and biological carrier were mixed at a volume ratio of 1:10, allowed to stand for 2 hours for adsorption, and then injected into a fixed-bed reactor in an upflow manner. Maintain the HRT of the fixed-bed reactor at 6-8 hours within 24 hours after inoculation, and then adjust the HRT according to the TN concentration in the effluent. When the TN concentration in the effluent is ≤10mg / L, the HRT should be controlled to be ≤40min; If the TN concentration in the effluent is >10mg / L, extend the HRT to 1-2h until the TN concentration in the effluent is ≤10mg / L.

[0081] In another embodiment, the low-carbon source denitrification method is based on Fe... 2+ and Fe 3+ A FeSO4 and FeCl3 solution was mixed in a molar ratio of 1:2.

[0082] Example 1 Low-carbon-source denitrification method, including the following steps: 1) According to Fe 2+ and Fe 3+ 1) Mix FeSO4 and FeCl3 solutions in a molar ratio of 1:2; 2) Adjust the pH of the mixed solution to 10-12 to generate black iron oxide precipitate, with nitrogen gas purging throughout the process; 3) Mix the obtained iron oxide precipitate with volcanic rock at a weight ratio of 100:1 and heat and dry at 200-300℃ to prepare a biological carrier; 4) Use the prepared biological carrier to treat the effluent from the secondary sedimentation tank of the biological treatment unit; After the treatment unit has been running for 5-7 days, backwashing is performed; During backwashing, the backwash blower is first started to backwash the biological carrier to remove the aging microorganisms on the biological carrier, and then the water pump is started to draw water from the inlet of the treatment unit to backwash the biological carrier; 5) Control the DO concentration of the incoming water to not exceed 1 mg / L; When the DO concentration of the incoming water exceeds 1 mg / L, reduce the aeration rate upstream or increase the HRT of the treatment unit; When the DO concentration of the incoming water is not higher than 1 mg / L, control the HRT of the treatment unit to not exceed 40 min.

[0083] Example 2 Based on Example 1, in step 3), after the iron oxide precipitate is generated, the obtained iron oxide precipitate, the pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots are used together to make a biological carrier. The preparation method for pretreated volcanic rock is as follows: select volcanic rock with a particle size of 1-3 mm, soak it in a 5% hydrochloric acid solution for 2.5 h to remove impurities and oxides on the surface of the volcanic rock, then rinse it repeatedly with deionized water until the pH value of the rinsing solution is neutral, and then put it in an oven at 110℃ to dry for 4.5 h to make the water content of the volcanic rock less than 1%; The preparation method of polydopamine-modified cellulose nanocrystals is as follows: Cellulose nanocrystals were dispersed in a Tris-HCl buffer solution with pH = 8.5, and dopamine hydrochloride was added. The mass ratio of dopamine hydrochloride to cellulose nanocrystals was 1:3. The mixture was stirred at room temperature for 13 hours to allow dopamine to self-polymerize and modify the surface of the cellulose nanocrystals under alkaline conditions. After the reaction was completed, polydopamine-modified cellulose nanocrystals were obtained by centrifugation, washing, and freeze-drying. The preparation method of bentonite supported on carbon nitride quantum dots is as follows: First, carbon nitride quantum dots were prepared using a hydrothermal method. The prepared carbon nitride quantum dots were then mixed with bentonite at a mass ratio of 1:5. Next, based on the total mass of the mixed powder, 3.5 times the mass of deionized water was added to form a suspension. The deionization method was as follows: First, one-third of the total mass of deionized water was slowly added dropwise to the mixed powder while stirring at 250 rpm, with the dropping rate controlled at 1 mL / s. This process allowed the deionized water to initially wet the mixed powder. Then, stirring was paused for 7 minutes to allow the deionized water to fully penetrate the powder. Next, the remaining one-third of the total mass of deionized water was added at the same stirring and dropping rate, and stirring continued for 17 minutes after the addition was complete. Finally, the remaining deionized water was added, and the stirring speed was increased to 450 rpm, stirring for 25 minutes to ensure that the carbon nitride quantum dots and bentonite were fully dispersed in the deionized water, forming a uniform and stable suspension. In preparing the biological carrier, iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots were mixed in a weight ratio of 100:1:6:3, and then heated and dried. The specific mixing and heating / drying processes are as follows: Mixing process: The above four materials are placed into a three-dimensional motion mixer equipped with a magnetic field generator for mixing. The speed of the mixer is set to 40 r / min and the mixing time is 45 min. At the same time, the magnetic field generator is turned on, and the initial magnetic field strength is set to 5 mT to generate an alternating magnetic field with an intensity of 5-10 mT. During the mixing process, nitrogen gas is introduced into the mixer every 10 min, with each gas introduction time being 2 min and the gas flow rate being 0.6 L / min, to prevent the iron oxide precipitate from being oxidized. Heating and drying: The mixed materials are transferred to a high-temperature drying oven equipped with a light source, and the temperature is increased from room temperature to 250°C at a rate of 7°C / min, and maintained at this temperature for 3.5 hours. During the heating and drying process, nitrogen gas is continuously introduced at a flow rate of 1.2 L / min to ensure the materials are in an oxygen-free environment. Simultaneously, the light source is turned on with a wavelength of 400-450 nm and a light intensity of 12 mW / cm². 2 The material is irradiated with blue light to promote the photocatalytic activity of carbon nitride quantum dots; after drying, it is cooled to room temperature at a rate of 4℃ / min.

[0084] The carbon nitride quantum dots are prepared by the following method: Melamine and dicyandiamine were mixed in a molar ratio of 1:1. Based on the total mass of the two, 0.5% hexadecyltrimethylammonium bromide was added as a surfactant. The mixture was then ground in an agate mortar for 30 minutes to uniformly disperse the precursor powder and form nano-sized mixed particles. The ground precursor powder was transferred to a polytetrafluoroethylene-lined hydrothermal reactor, and deionized water was added as a solvent. The mass-to-volume ratio of the ground precursor powder to deionized water was 1:25 g / mL. After sealing the hydrothermal reactor, it was heated to 190°C at a heating rate of 5°C / min and held at this temperature for 7 hours. During this period, the hydrothermal reactor was intermittently oscillated at a rate of 100 r / min, with oscillation for 10 minutes followed by a 5-minute pause, to promote the uniform hydrolysis and condensation reaction of the precursor. After the reaction was completed, the hydrothermal reactor was allowed to cool naturally to room temperature. The product was then centrifuged and washed three times alternately with ethanol and deionized water to remove unreacted precursors and surfactants. The washed solid was then dispersed in an ethanol solution of 2% 3-aminopropyltriethoxysilane and stirred at 50°C for 2 hours to modify the surface of carbon nitride quantum dots with amino groups.

[0085] Example 3 Based on Example 2, in step 4), the prepared biological carrier is used to treat the effluent from the secondary sedimentation tank and backwash it; after the backwash is completed, the air and water flushing is stopped, and the biological carrier is allowed to settle naturally for 7 minutes before normal water intake operation is resumed. The treatment of effluent from a biological secondary sedimentation tank using a prepared biological carrier includes the following steps: The biological carrier was uniformly filled into the fixed-bed reactor to a volume that accounted for 65% of the total volume of the fixed-bed reactor, resulting in a filling rate of 65%. A layered filling method was adopted, and after filling to a height of 25cm, the biological carrier was leveled using a mechanical leveling device to ensure that the filling density deviation was ≤5% and the local porosity was ≤30%. The effluent from the secondary sedimentation tank enters the fixed-bed reactor through a perforated pipe distributor at the bottom. The distributor has 7mm diameter holes spaced 15cm apart. The water flows evenly through the biological carrier layer in an upflow manner, with the empty bed flow velocity controlled at 1~4m / h and HRT = 40min. The differential pressure of the carrier layer is monitored in real time by differential pressure sensors installed at the inlet and outlet of the fixed-bed reactor. When the differential pressure at the inlet and outlet of the fixed-bed reactor reaches ≥8kPa or after 7 days of continuous operation, the backwashing program is automatically started; alternatively, manual backwashing can be used. Backwashing includes the following steps: Air washing stage: Turn on the backwash fan at a rate of 17L / (m³) 2 Air is introduced from the bottom of the fixed-bed reactor at a gas velocity of ·s for 7 minutes. Water washing stage: After the air washing is completed, the water pump is started to flush the top of the fixed bed reactor in the reverse direction at a flow rate of 10 m / h for 12 minutes. When the influent water quality does not change significantly and meets the design requirements, and the effluent TN concentration is 12-15 mg / L or COD is 40-50 mg / L, and this lasts for 12 hours, the backwash cycle is shortened from 7 days to 5 days. When the effluent TN concentration is ≤10mg / L, COD is ≤30mg / L, and the pressure difference growth rate between the bottom and top of the fixed bed reactor is ≤0.5kPa / d, the backwash cycle is extended from 7d to 10d. When the pressure difference increases by ≥2 kPa within 24 hours and the water quality meets the standards, high-intensity air scrubbing should be prioritized, with the air velocity increased to 20 L / (m²). 2 •s), extend the time to 10min; if the pressure difference drop after air washing is less than 1kPa, start water washing immediately, with the water washing flow rate being 3 times the empty bed flow rate, until the pressure difference drops to 5-8kPa.

[0086] Example 4 Based on Example 3, the specific steps for controlling the DO concentration and HRT of the incoming water in step 5) are as follows: An online dissolved oxygen sensor with a measurement accuracy of ±0.05 mg / L is installed on the pipeline before the effluent from the secondary sedimentation tank enters the fixed bed reactor. The DO data of the incoming water is collected every 2 minutes and transmitted to the control system. Pre-measure the total height of the fixed-bed reactor H Unit: m, Calculate the effective height of a fixed-bed reactor. H 有效 = H × (1 - fill rate); Control methods when DO concentration exceeds the standard: Primary control, reducing upstream aeration: When the DO concentration of the incoming water is >1 mg / L, the control system will perform the following control in sequence: reduce the aeration rate of the aerobic aeration unit upstream of the secondary sedimentation tank by 25% through the frequency converter, with each adjustment not exceeding 5% and an interval of 5 minutes, until the DO concentration is ≤1 mg / L or the aeration rate is reduced to 40% of the rated value; Secondary control, adjusting the HRT of the treatment unit: If the DO concentration is still >1 mg / L after the aeration rate is reduced to 40% of the rated value, the empty bed flow rate is reduced from 1 m / h to 0.5 m / h through the electric flow regulating valve to prolong the HRT. The HRT is calculated as follows: Three-level control, external carbon source addition: The amount of externally added carbon source is linked to the online dissolved oxygen sensor to ensure that the DO concentration entering the treatment unit is ≤1mg / L.

[0087] Methods for controlling DO concentration when it meets the target: When the DO concentration in the incoming water is ≤1mg / L, the control system will increase the empty bed flow rate to 1~4m / h, so that the HRT is controlled at 35min; DO probe installation and verification methods: A DO probe is installed in the middle of the biological carrier packing layer inside the fixed-bed reactor. Specifically, it is inserted vertically into the carrier layer at 1 / 2 of the total height along the height direction of the fixed-bed reactor, 1 / 3 of the radius outward from the central axis, for 5-10 cm. The DO concentration in the fixed-bed reactor is monitored in real time to see if it is maintained at 0.5-1 mg / L. If the detection value exceeds 1 mg / L for 3 consecutive times with an interval of 2 minutes, the above control steps are automatically repeated.

[0088] Example 5 Based on Example 4, when using a biological carrier to treat the effluent from a biological secondary sedimentation tank, the following steps are also included: Online COD sensors, ammonia nitrogen sensors, and nitrate nitrogen sensors are installed sequentially on the pipeline before the effluent from the secondary sedimentation tank enters the fixed-bed reactor. These sensors monitor the concentrations of COD, ammonia nitrogen, and nitrate nitrogen in the influent in real time. The monitoring data is transmitted to the control system in real time for subsequent dynamic adjustment of the fixed-bed reactor's operating parameters. Meanwhile, the influent to the fixed-bed reactor is pretreated by a mechanical bar screen with a pore size of 100-200μm installed at the effluent end of the biological secondary sedimentation tank to remove large particulate suspended solids. A microbial sampling port is set in the middle of the side wall of the fixed bed reactor. The copy number of the 16S rRNA gene of denitrifying bacteria is detected weekly by quantitative PCR. When the detected value is lower than 70% of the initial inoculum, or when the microbial dehydrogenase activity is detected to be lower than 50 μmol / (g·h) by fluorescent probe method, the following remediation measures are automatically triggered: 1) external addition of carbon source; 2) inoculation with anaerobic activated sludge.

[0089] The specific method for adding an external carbon source is as follows: Liquid sodium acetate was used as a supplementary carbon source and was added to the inlet of the fixed-bed reactor via a metering pump. When the nitrate nitrogen sensor detects that the nitrate nitrogen concentration in the influent of the fixed-bed reactor is N mg / L, then the sodium acetate dosage is (3× N )mg to (5× N )mg; After addition, the DO concentration in the fixed-bed reactor should be controlled to be ≤1 mg / L.

[0090] The specific method for inoculating anaerobic activated sludge is as follows: Take activated sludge from anaerobic tanks, anoxic tanks, aerobic tanks, or secondary sedimentation tanks and acclimate it for 7 days; Sludge and biological carrier were mixed at a volume ratio of 1:10, allowed to stand for 2 hours for adsorption, and then injected into a fixed-bed reactor in an upflow manner. Maintain the HRT of the fixed-bed reactor at 7 hours within 24 hours after inoculation, and then adjust the HRT according to the TN concentration in the effluent. When the TN concentration in the effluent is ≤10mg / L, the HRT should be controlled to be ≤40min; If the TN concentration in the effluent is >10mg / L, extend the HRT to 1-2h until the TN concentration in the effluent is ≤10mg / L.

[0091] Comparative Example 1 The low-carbon-source denitrification method includes the following steps: Volcanic rocks are heated and dried at 200-300℃ to create biological carriers; The prepared biological carrier is used to treat the effluent from the secondary sedimentation tank of the biological treatment unit. After the treatment unit has been running for 7 days, backwashing is performed. During backwashing, the backwash blower is first started to backwash the biological carrier to remove the aging microorganisms on the biological carrier. Then, the water pump is started to draw water from the inlet of the treatment unit to backwash the biological carrier.

[0092] experiment I. Experimental Objective To verify the effects of different biological carrier materials and process control on the denitrification efficiency of low-carbon source wastewater, and to optimize the low-carbon source denitrification technology route by comparing Examples 1-5 with Comparative Example 1, so as to achieve efficient and low-cost wastewater treatment.

[0093] II. Experimental Setup and Operating Conditions 1. Reactor: Fixed-bed reactor (inner diameter 10cm, height 100cm, effective volume 3L), biological carrier filling rate 60%-70%, upflow operation.

[0094] 2. Influent water quality: The effluent from the biological secondary sedimentation tank has the following water quality range: COD (Chemical Oxygen Demand) 40-70 mg / L, TN (Total Nitrogen) 15-25 mg / L, TP (Total Phosphorus) 1.2-5.9 mg / L, DO (Dissolved Oxygen) 0.8-1.5 mg / L, without the addition of any external carbon source.

[0095] 3. Operating cycle: The system was run continuously for 30 days, with daily recording of operating parameters (DO, flow rate, HRT) and simultaneous collection and testing of water samples.

[0096] III. Detection Methods Table 1 Detection Methods IV. Comparison of experimental data (without external carbon source addition), see Table 2-8.

[0097] Comparative Example 1: A single volcanic rock carrier was used, with the selected volcanic rock being dense basalt. This volcanic rock had low natural porosity, and surface impurities were not removed, resulting in a low specific surface area (4.2 ± 0.3 m²). 2 / g).

[0098] Table 2 Comparative Example 1 Experimental Data Table 3 Experimental data from Example 1 Table 4 Experimental data for Example 2 Table 5 Experimental data from Example 3 Table 6 Experimental data from Example 4 Table 7 Experimental data from Example 5 Table 8 Comparison of Carrier Performance and Operating Parameters The experiments in Comparative Example 1 and Examples 1-5 aimed to investigate the effects of different biological carrier materials and process control on the nitrogen removal efficiency of low-carbon source wastewater. The data in Tables 2-8 will be analyzed in detail below, based on Comparative Example 1 and Examples 1-5.

[0099] Comparative Example 1 (Single volcanic rock carrier) TN removal rate was 11.3%, TP removal rate was 18.3%, and COD removal rate was 34.7%.

[0100] Specific surface area 4.2m² 2 / g, Zeta potential +18mV, dehydrogenase activity 30μmol / (g·h), backwash cycle 7 days, pressure difference increase 1.2kPa / d.

[0101] This is because volcanic rocks have low porosity, are unmodified, have poor microbial adhesion, and thus limited denitrification efficiency.

[0102] Example 1 (Iron oxide-volcanic rock composite carrier) TN removal rate was 20.7%, TP removal rate was 42.5%, and COD removal rate was 53.2%.

[0103] Specific surface area: 28.7 m² 2 / g, Zeta potential +25mV, dehydrogenase activity 42μmol / (g·h), backwash cycle 5-7 days, pressure differential increase 0.8kPa / d.

[0104] This is because the iron oxide composite enhances adsorption and catalytic activity, while reducing carrier blockage.

[0105] Example 2 (Volcanic Rock Pretreatment and Functional Material Composite) TN removal rate was 34.4%, TP removal rate was 54.2%, and COD removal rate was 61.8%.

[0106] Specific surface area 32.5m² 2 / g, Zeta potential +28mV, dehydrogenase activity 50μmol / (g·h), backwash cycle 5 days, pressure difference increase 0.6kPa / d.

[0107] This is because acid washing of volcanic rocks increases porosity, and functional materials synergistically enhance denitrification.

[0108] Example 3 (Dynamic Backwashing Strategy) TN removal rate was 48.2%, TP removal rate was 61.3%, and COD removal rate was 69.6%.

[0109] Specific surface area: 34.0 m² 2 / g, Zeta potential +30mV, dehydrogenase activity 58μmol / (g·h), backwash cycle 4-6 days, pressure difference increase 0.5kPa / d.

[0110] This is because dynamic backwashing optimizes the operating cycle and improves mass transfer efficiency.

[0111] Example 4 (Precise DO Control) TN removal rate: 60.1%; TP removal rate: 68%; COD removal rate: 76.4%.

[0112] Specific surface area: 33.8 m² 2 / g, Zeta potential +32mV, dehydrogenase activity 65μmol / (g·h), backwash cycle 3-5 days, pressure difference increase 0.4kPa / d.

[0113] This is because the DO is controlled in stages (error ±0.05 mg / L) to maintain a hypoxic environment.

[0114] Example 5 (Microbial activity remediation and carbon source addition) TN removal rate was 76.3%, TP removal rate was 77.2%, and COD removal rate was 83.8%.

[0115] Specific surface area 33.5 m² 2 / g, Zeta potential +35mV, dehydrogenase activity 55μmol / (g·h), backwash cycle 2-4 days, pressure differential increase 0.3kPa / d.

[0116] Example 5: Real-time monitoring of microbial activity (denitrifying bacteria gene copy number, dehydrogenase activity). When activity is insufficient, remediation measures are triggered: external carbon source addition and inoculation with anaerobic sludge. After remediation, a low HRT (≤40 min) is maintained to ensure nitrogen removal efficiency.

[0117] Example 5: No carbon source was actively added during normal operation. However, by monitoring microbial activity, external carbon source was added as a repair measure when the activity of denitrifying bacteria was insufficient (gene copy number < 70% of the initial value or dehydrogenase activity < 50 μmol / (g·h)).

[0118] The high TN removal rate (76.3%) may have been partly due to the active remediation measures (carbon source addition) during the experiment, but the main denitrification efficiency still depends on carrier functionalization and process control.

[0119] Under extremely low carbon source conditions (C / N = 2-3), Example 5 optimized the carrier (specific surface area 33.5 m²). 2 The basic TN removal rate is >60% by using dynamic regulation (DO≤1mg / L, HRT≤40min); Remediation measures (carbon source addition / sludge inoculation) further improved the TN removal rate to 76.3%, ensuring that the effluent TN consistently meets the standard (<10mg / L).

[0120] Example 5 only adds a small amount of carbon source when microbial activity is insufficient (repair as needed), and the overall carbon source consumption is still significantly lower than that of the traditional process (the amount added is reduced by >70%).

[0121] This invention achieves efficient nitrogen removal from low-carbon-source wastewater through the following measures: Volcanic rock acid washing, iron oxide composite, and carbon nitride quantum dot loading were used to increase the specific surface area from 4.2 to 33.5 m². 2 / g) and microbial activity (dehydrogenase activity +83%).

[0122] Dynamic backwashing (pressure differential increase from 1.2 to 0.3 kPa / d) and precise DO control (error ±0.05 mg / L) maintain an oxygen-deficient environment.

[0123] When microbial activity is insufficient, carbon sources or inoculated sludge can be added as needed to specifically improve denitrification efficiency (TN removal rate 76.3%).

[0124] Under the premise of ensuring that the effluent meets the standards (TN<10mg / L), the carbon source dependence is reduced by more than 70%, which is suitable for the treatment of municipal tailwater (C / N=2-3) and industrial low carbon nitrogen wastewater, and has the advantages of high efficiency, economy and stability.

[0125] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and embodiments shown and described herein.

Claims

1. A method for nitrogen removal from low carbon sources, characterized in that, Includes the following steps: 1) According to Fe 2+ and Fe 3+ A FeSO4 and FeCl3 solution was mixed in a molar ratio of 1:0.2-2. 2) Adjust the pH of the mixed solution to 10-12 to form a black iron oxide precipitate, with nitrogen gas purging throughout the process; 3) Iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots were mixed in a weight ratio of 100:1:6:3, and then heated and dried to prepare a biological carrier. The mixing and heating / drying processes are as follows: Mixing process: The above-mentioned iron oxide precipitate, pretreated volcanic rock, polydopamine-modified cellulose nanocrystals, and bentonite loaded with carbon nitride quantum dots were placed into a three-dimensional motion mixer equipped with a magnetic field generator for mixing. The speed of the mixer was set to 30-40 r / min, and the mixing time was 40-50 min. At the same time, the magnetic field generator was turned on, with the initial magnetic field strength set to 5 mT, generating an alternating magnetic field with an intensity of 5-10 mT. During the mixing process, nitrogen gas was introduced into the mixer every 10 min, with each gas introduction time being 2 min and the gas flow rate being 0.5-0.8 L / min, to prevent the iron oxide precipitate from being oxidized. Heating and drying process: Transfer the mixed materials to a high-temperature drying oven equipped with a light source, and raise the temperature from room temperature to 200-300℃ at a rate of 5-8℃ / min, maintaining this temperature for 3-4 hours for drying. During the heating and drying process, nitrogen gas is continuously introduced at a flow rate of 1-1.5L / min to ensure the materials are in an oxygen-free environment. Simultaneously, the light source is turned on, with a wavelength of 400-450nm and a light intensity of 10-15mW / cm². 2 The material is irradiated with blue light to promote the photocatalytic activity of carbon nitride quantum dots; after drying, it is cooled to room temperature at a cooling rate of 3-5℃ / min. The pretreatment method for volcanic rocks is as follows: Select volcanic rocks with a particle size of 1-3 mm and soak them in a 5% hydrochloric acid solution for 2-3 hours to remove impurities and oxides from the surface of the volcanic rocks. Then rinse them repeatedly with deionized water until the pH value of the rinsing solution is neutral. Finally, dry them in an oven at 100-120℃ for 4-5 hours to reduce the water content of the volcanic rocks to less than 1%. The preparation method of polydopamine-modified cellulose nanocrystals is as follows: Cellulose nanocrystals were dispersed in a Tris-HCl buffer solution with pH = 8.5, and dopamine hydrochloride was added. The mass ratio of dopamine hydrochloride to cellulose nanocrystals was 1:

3. The mixture was stirred at room temperature for 12-15 hours to allow dopamine to self-polymerize and modify the surface of the cellulose nanocrystals under alkaline conditions. After the reaction was completed, polydopamine-modified cellulose nanocrystals were obtained by centrifugation, washing, and freeze-drying. The preparation method of bentonite supported on carbon nitride quantum dots is as follows: First, carbon nitride quantum dots were prepared using a hydrothermal method. The prepared carbon nitride quantum dots were then mixed with bentonite at a mass ratio of 1:

5. Next, based on the total mass of the mixed powder, 2-5 times the mass of deionized water was added to form a suspension. The deionized water was added as follows: First, one-third of the total mass of deionized water was slowly added dropwise to the mixed powder while stirring at a speed of 200-300 rpm, with the dropping rate controlled at 1-2 mL / s. This process allowed the deionized water to initially wet the mixed powder. Then, stirring was paused for 5-10 minutes to allow the deionized water to fully penetrate the powder. Next, the remaining one-third of the total mass of deionized water was added at the same stirring and dropping rate, and stirring continued for 15-20 minutes after the addition was complete. Finally, the remaining deionized water was added, and the stirring speed was increased to 400-500 rpm, stirring for 20-30 minutes to ensure that the carbon nitride quantum dots and bentonite were fully dispersed in the deionized water, forming a uniform and stable suspension. The carbon nitride quantum dots are prepared by the following method: Melamine and dicyandiamine were mixed in a molar ratio of 1:

1. Based on the total mass of the two, 0.5% hexadecyltrimethylammonium bromide was added as a surfactant. The mixture was then ground in an agate mortar for 30 minutes to uniformly disperse the precursor powder and form nano-sized mixed particles. The ground precursor powder was transferred to a polytetrafluoroethylene-lined hydrothermal reactor, and deionized water was added as a solvent. The mass-to-volume ratio of the ground precursor powder to deionized water was 1:20-30 g / mL. After sealing the hydrothermal reactor, it was heated to 180-200℃ at a heating rate of 5℃ / min and held at 180-200℃ for 6-8 hours. During this period, the hydrothermal reactor was intermittently shaken at a rate of 100 r / min, with each intermittent shaking lasting 10 minutes and then paused for 5 minutes to promote the uniform hydrolysis and condensation reaction of the precursor. After the reaction was completed, the hydrothermal reactor was allowed to cool naturally to room temperature. The product was then centrifuged and washed three times with alternating ethanol and deionized water to remove unreacted precursors and surfactants. The washed solid was then dispersed in an ethanol solution of 2% 3-aminopropyltriethoxysilane and stirred at 50°C for 2 hours to modify the surface of carbon nitride quantum dots with amino groups. 4) Use the prepared biological carrier to treat the effluent from the secondary sedimentation tank of the biological treatment unit; after the treatment unit has been running for 3-7 days, backwashing is performed; during backwashing, the backwash blower is started first to backwash the biological carrier and remove the aging microorganisms on the biological carrier, and then the water pump is started to draw water from the inlet of the treatment unit to backwash the biological carrier. 5) Control the DO concentration of the incoming water to ≤1mg / L; when the DO concentration of the incoming water is >1mg / L, reduce the aeration rate of the upstream unit or increase the HRT of the treatment unit; when the DO concentration of the incoming water is ≤1mg / L, control the HRT of the treatment unit to ≤40min.

2. The low-carbon-source denitrification method as described in claim 1, characterized in that, Step 4) Use the prepared biological carrier to treat the effluent from the secondary sedimentation tank and perform backwashing; after backwashing, stop air and water flushing, wait for the biological carrier to settle naturally for 5-10 minutes, and then resume normal water intake operation. The treatment of effluent from a biological secondary sedimentation tank using a prepared biological carrier includes the following steps: The biological carrier is uniformly filled into the fixed-bed reactor, making up 60-70% of its total volume, thus achieving a filling rate of 60-70%. A layered filling method is used, and after filling every 20-30 cm, the biological carrier is leveled using a mechanical leveling device to ensure that the filling density deviation is ≤5% and the local porosity is ≤30%. The effluent from the secondary sedimentation tank enters from the bottom of the fixed bed reactor through a perforated pipe distributor. The diameter of the holes on the distributor is 5-10 mm and the spacing between the holes is 10-20 cm. The water flows evenly through the biological carrier layer in an upflow manner, and the empty bed flow velocity is controlled at 1-4.0 m / h, with HRT ≤ 40 min. The differential pressure of the carrier layer is monitored in real time by differential pressure sensors installed at the inlet and outlet of the fixed bed reactor. When the differential pressure at the inlet and outlet of the fixed bed reactor reaches ≥8kPa or after 7 days of continuous operation, the backwashing program is automatically started or manual backwashing is performed. Backwashing includes the following steps: Air washing stage: Turn on the backwash fan at a rate of 15-20 L / (m³). 2 Air is introduced from the bottom of the fixed-bed reactor at a gas velocity of ·s for 5-8 minutes. Water washing stage: After the air washing is completed, start the water pump and flush the top of the fixed bed reactor in reverse at a flow rate of 10 m / h for 10-15 minutes. Dynamic adjustment of backwashing cycle and intensity: When the effluent TN concentration is 12-15 mg / L or COD is 40-50 mg / L, and the backwashing time is 12 hours, the backwashing cycle is shortened from 7 days to 5 days. When the effluent TN concentration is ≤10mg / L, COD is ≤30mg / L, and the pressure difference growth rate between the bottom and top of the fixed bed reactor is ≤0.5kPa / d, the backwash cycle is extended from 7d to 10d. When the pressure difference increases by ≥2 kPa within 24 hours and the water quality meets the standards, high-intensity air scrubbing is performed first, with the air velocity increased to 20 L / (m²). 2 •s), extend the time to 10min; if the pressure difference drop after air washing is less than 1kPa, start water washing, and take the water washing flow rate as 3 times the empty bed flow rate until the pressure difference drops to 5-8kPa.

3. The low-carbon-source denitrification method as described in claim 2, characterized in that, Step 5) The specific steps for controlling the DO concentration and HRT of the incoming water are as follows: A dissolved oxygen sensor with a measurement accuracy of ±0.05 mg / L is installed on the pipeline before the effluent from the secondary sedimentation tank enters the fixed bed reactor. The DO data of the incoming water is collected every 2 minutes and transmitted to the control system. Pre-measure the total height of the fixed-bed reactor H Unit: m, Calculate the effective height of a fixed-bed reactor. H 有效 = H × (1 - fill rate); Control methods when DO concentration exceeds the standard: Primary control, reducing upstream aeration: When the DO concentration of the incoming water is >1mg / L, the control system will perform the following control in sequence: reduce the aeration rate of the aerobic aeration unit upstream of the secondary sedimentation tank by 20-30% through the frequency converter, with each adjustment not exceeding 5% and an interval of 5 minutes, until the DO concentration is ≤1mg / L or the aeration rate is reduced to 40% of the rated value; Secondary control, adjusting the HRT of the treatment unit: If the DO concentration is still >1 mg / L after the aeration rate is reduced to 40% of the rated value, the empty bed flow rate is reduced from 1-4.0 m / h to 0.5-0.1 m / h using an electric flow regulating valve. The HRT is calculated as follows: ； Three-level control, external carbon source addition: The amount of externally added carbon source is linked to the online dissolved oxygen sensor to ensure that the DO concentration entering the treatment unit is ≤1mg / L; Methods for controlling DO concentration when it meets the target: When the DO concentration in the incoming water is ≤1mg / L, the control system will increase the empty bed flow rate to 1-4m / h, so that the HRT is controlled at 30-40min; DO probe installation and verification methods: A DO probe is installed in the middle of the biological carrier packing layer inside the fixed-bed reactor. Specifically, at 1 / 2 of the total height of the fixed-bed reactor, and with the central axis as the reference, it is inserted vertically into the carrier layer at 1 / 3 of the radius outward. The DO concentration in the fixed-bed reactor is monitored in real time to see if it is maintained at 0.5-1 mg / L. If the detection value exceeds 1 mg / L for 3 consecutive times with an interval of 2 minutes, the above control steps are automatically repeated.

4. The low-carbon-source denitrification method as described in claim 3, characterized in that, When using biological carriers to treat the effluent from a biological secondary sedimentation tank, the following steps are also included: Online COD sensors, ammonia nitrogen sensors, and nitrate nitrogen sensors are sequentially installed on the pipeline before the effluent from the secondary biological sedimentation tank enters the fixed-bed reactor. These sensors monitor the concentrations of COD, ammonia nitrogen, and nitrate nitrogen in the influent in real time. The monitoring data is transmitted to the control system in real time for subsequent dynamic adjustment of the operating parameters of the fixed-bed reactor. At the same time, the influent to the fixed-bed reactor is pretreated by a mechanical bar with a pore size of 100-200μm installed at the effluent end of the secondary biological sedimentation tank to remove large particulate suspended solids. A microbial sampling port is set in the middle of the side wall of the fixed-bed reactor. The 16S rRNA gene copy number of denitrifying bacteria is detected weekly using a quantitative PCR instrument. When the detected value is lower than 70% of the initial inoculum or the microbial dehydrogenase activity is lower than 50 μmol / (g·h) by fluorescent probe method, the following repair measures are automatically triggered: External carbon source addition; Inoculate with anaerobic activated sludge.

5. The low-carbon-source denitrification method as described in claim 4, characterized in that, The specific method for adding an external carbon source is as follows: Liquid sodium acetate was used as a supplementary carbon source and was added to the inlet of the fixed-bed reactor via a metering pump. When the nitrate nitrogen sensor detects that the nitrate nitrogen concentration in the influent of the fixed-bed reactor is N If the concentration is mg / L, then the sodium acetate dosage is 3-5 mg / L. N mg; After addition, the DO concentration in the fixed-bed reactor should be controlled to be ≤1 mg / L.

6. The low-carbon source denitrification method as described in claim 4, characterized in that, The specific method for inoculating anaerobic activated sludge is as follows: Take activated sludge from anaerobic tanks, anoxic tanks, aerobic tanks, or secondary sedimentation tanks and acclimate it for 6-8 days; The activated sludge and biological carrier were mixed at a volume ratio of 1:10, allowed to stand for 2 hours for adsorption, and then injected into the fixed bed reactor in an upflow manner. Maintain the HRT of the fixed-bed reactor at 6-8 hours within 24 hours after inoculation, and then adjust the HRT according to the TN concentration in the effluent. When the TN concentration in the effluent is ≤10mg / L, the HRT should be controlled to be ≤40min; If the TN concentration in the effluent is >10mg / L, extend the HRT to 1-2h until the TN concentration in the effluent is ≤10mg / L.

7. The low-carbon source denitrification method as described in claim 1, characterized in that, Step 1) Press Fe 2+ and Fe 3+ A FeSO4 and FeCl3 solution was mixed in a molar ratio of 1:2.

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