A method for full-process resource utilization of residual sludge

Abstract

The application provides a method for full-process resource utilization of residual sludge, and belongs to the technical field of solid waste resource utilization. The method comprises the following steps: adjusting the residual sludge to be alkaline, adding red mud to perform anaerobic fermentation, and performing solid-liquid separation to obtain a fermentation liquid containing volatile fatty acids and fermentation residues; washing, drying, grinding, screening and pyrolyzing the fermentation residues to prepare biochar, modifying the biochar by a prussian blue modification method through a reaction of ferric chloride and potassium ferrocyanide, adding the modified biochar to the fermentation liquid containing volatile fatty acids to adsorb ammonia nitrogen and phosphate, separating to obtain purified fermentation liquid, and then using the purified fermentation liquid as a carbon source for denitrification. The application uses red mud to strengthen sludge acid production, converts the residues into modified biochar for fermentation liquid purification, and reuses the purified liquid as a carbon source, so that the resource utilization of red mud and residual sludge is realized, and the input of additional carbon source and the cost of sludge disposal are reduced.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization technology, and in particular to a method for the whole-process resource utilization of residual sludge. Background Technology

[0002] With the continuous advancement of urbanization and the ongoing expansion of wastewater treatment capacity, the amount of residual sludge generated by urban wastewater treatment plants is increasing year by year. This residual sludge contains a large amount of organic matter, microbial cells, and nutrients, possessing significant potential for resource utilization. However, it has a high water content, poor stability, and is prone to putrefaction and foul odors. Improper disposal not only occupies substantial land resources but may also cause secondary environmental pollution. Currently, the main methods for treating and disposing of residual sludge include landfill, incineration, land application, and anaerobic digestion, but all of these methods suffer from high operating costs, limited resource recovery efficiency, or significant environmental risks. Therefore, how to achieve the reduction, stabilization, and resource utilization of residual sludge has become one of the most pressing technical challenges in the wastewater treatment field.

[0003] Meanwhile, with increasingly stringent wastewater discharge standards, higher requirements are being placed on nitrogen and phosphorus removal efficiency in wastewater treatment processes. Denitrification requires a sufficient supply of available organic carbon sources as electron donors, but the influent carbon-to-nitrogen ratio (C / N) of most urban wastewater treatment plants in my country is generally low, making it difficult to meet the carbon source demands of denitrifying bacteria. Existing research indicates that when the influent C / N ratio is below 3.4, additional exogenous carbon sources such as methanol, sodium acetate, or glucose are typically required to ensure good denitrification. While adding exogenous carbon sources can improve denitrification efficiency to some extent, it significantly increases the operating costs of wastewater treatment plants and may lead to further increases in sludge production, which is detrimental to the low-carbon and sustainable operation of wastewater treatment systems. Therefore, developing stable and low-cost endogenous carbon sources to replace traditional exogenous carbon sources has significant engineering application value.

[0004] Waste sludge contains abundant organic matter, which can be converted into volatile fatty acids (VFAs) under anaerobic conditions through hydrolysis and acidification, thus serving as a high-quality carbon source for denitrification. However, in traditional anaerobic fermentation of waste sludge, the stable structure of sludge flocs, the difficulty in breaking cell walls, and the low efficiency of organic matter release limit the hydrolysis and acidification processes, resulting in low VFA production and insufficient carbon source to meet the needs of wastewater treatment systems. Existing research often employs methods such as thermal pretreatment, acid-base pretreatment, ultrasonic pretreatment, and oxidative pretreatment to enhance acid production from sludge fermentation, but these methods generally suffer from high energy consumption, high operating costs, or complex processes, limiting their engineering applications. Furthermore, current research mainly focuses on improving the efficiency of acid production from sludge fermentation, with less attention paid to the subsequent resource utilization of fermentation residues. This results in a large amount of residual solids still requiring further treatment and disposal, making it difficult to achieve full-process resource utilization of sludge.

[0005] Furthermore, red mud is a major industrial solid waste generated during the alumina production process, typically producing approximately 1.0 to 2.0 tons of red mud for every ton of alumina produced. With the rapid development of the alumina industry, my country's annual red mud emissions have continued to increase, resulting in a massive accumulated stockpile. Red mud contains abundant metallic elements such as iron, aluminum, calcium, and silicon, and is highly alkaline, with a pH typically ranging from 10 to 12.5. Due to its fine particles and high alkalinity, long-term stockpiling of red mud not only occupies significant land resources but also easily causes groundwater and soil pollution, becoming a major obstacle to the green development of the alumina industry. Existing research indicates that red mud contains abundant iron and alkaline components, which can, to some extent, improve the anaerobic fermentation environment of sludge, promote the breakdown of sludge slurry structure and the release of organic matter, thereby improving sludge hydrolysis and acidification efficiency. Simultaneously, the metal oxides in red mud may also participate in electron transfer processes, promoting the degradation of organic matter and the generation of volatile fatty acids. Therefore, co-treating red mud with excess sludge not only improves the efficiency of anaerobic fermentation for acid production but also enables the synergistic resource utilization of industrial solid waste and municipal sludge, reducing environmental risks and treatment costs. However, existing research on enhanced anaerobic fermentation of excess sludge with red mud and the high-value utilization of fermentation residue is relatively limited, especially lacking a systematic process that can simultaneously achieve efficient acid production from sludge and resource utilization of fermentation residue.

[0006] Therefore, developing a method to enhance the anaerobic fermentation of residual sludge to produce acid using red mud and to realize the resource utilization of fermentation residue is of great significance for increasing the endogenous carbon source production of sludge, reducing the operating cost of sewage treatment, and realizing the synergistic resource utilization of various solid wastes. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a method for the full-process resource utilization of waste sludge. This method enhances the anaerobic fermentation of waste sludge with red mud to increase the production of volatile fatty acids, and prepares biochar from the fermentation residue. After Prussian blue modification, the biochar is used to adsorb nitrogen and phosphorus in the fermentation broth. The purified fermentation broth is then reused as a carbon source for denitrification. This forms a technical route of enhanced acid production with red mud, residue char production and modification, and fermentation broth reuse for denitrification, thereby realizing the resource utilization of red mud and waste sludge, while reducing the input of external carbon sources and the cost of sludge disposal.

[0008] To achieve the above objectives, the present invention provides a method for the full-process resource utilization of excess sludge, comprising the following steps: S1. The pH value of the remaining sludge is adjusted to alkaline using an alkaline reagent. Red mud is added to the remaining sludge and mixed. Hydrolysis and acidification fermentation is carried out under anaerobic conditions. After fermentation, the fermentation products are separated into solid and liquid components to obtain fermentation broth containing volatile fatty acids and fermentation residue. S2. The fermentation residue is washed, dried, ground and sieved in sequence, and then pyrolyzed under a nitrogen atmosphere to obtain fermentation residue-based biochar. S3. Add the fermentation residue-based biochar to an aqueous solution of ferric chloride and citric acid, add potassium ferrocyanide and citric acid aqueous solution dropwise and react, and perform solid-liquid separation, washing and drying on the reaction product in sequence to obtain Prussian blue modified fermentation residue-based biochar. S4. The Prussian blue-modified fermentation residue-based biochar is added to the nitrogen- and phosphorus-containing fermentation broth obtained in step S1 for adsorption treatment. After the adsorption is completed, the Prussian blue-modified fermentation residue-based biochar is separated to obtain the purified fermentation broth. S5. The purified fermentation broth obtained in step S4 is added to the denitrification denitrification system as a supplementary carbon source for denitrification denitrification; the fermentation residue is converted into modified biochar in steps S2 and S3, and after the fermentation broth is adsorbed and purified in step S4, it is reused for denitrification denitrification, thereby realizing the resource utilization of the fermentation residue.

[0009] Preferably, in step S1, the alkaline reagent is a sodium hydroxide solution, and the initial pH value of the residual sludge is adjusted to 10.0 using the sodium hydroxide solution; the mass ratio of the amount of red mud added to the suspended solids of the residual sludge is 0.1~3:1.

[0010] Preferably, in step S1, the temperature of the hydrolysis acidification fermentation is 35°C and the fermentation time is 2-7 days; before the hydrolysis acidification fermentation, nitrogen is introduced into the reactor to replace the air and the reactor is sealed, and the fermentation is carried out by shaking or stirring.

[0011] Preferably, in step S1, the solid-liquid separation includes centrifuging the fermentation product at a speed of 5000 r / min and filtering the resulting liquid phase.

[0012] Preferably, in step S2, the fermentation residue is washed with deionized water, and the washed fermentation residue is dried in a vacuum drying oven at 80°C for 12 hours. Then it is ground and sieved using a sieve with a pore size of 0.18 mm.

[0013] Preferably, in step S2, the sieved powder is placed in a pyrolysis reactor, heated to 600°C at a heating rate of 5°C / min under nitrogen gas, and kept at 600°C for 2 hours to obtain the fermentation residue-based biochar.

[0014] Preferably, step S3 includes: dissolving ferric chloride hexahydrate in an aqueous solution of citric acid to obtain the ferric chloride citric acid aqueous solution; dissolving potassium ferrocyanide trihydrate in an aqueous solution of citric acid to obtain the potassium ferrocyanide citric acid aqueous solution; heating the ferric chloride citric acid aqueous solution and adding the fermentation residue-based biochar, stirring and dispersing evenly, then adding the potassium ferrocyanide citric acid aqueous solution dropwise, and continuing to heat the reaction; after the reaction is completed, performing vacuum filtration, collecting the filter residue and washing it, and then drying the washed filter residue to obtain the Prussian blue modified fermentation residue-based biochar.

[0015] Preferably, in step S4, the pH value of the nitrogen- and phosphorus-containing fermentation broth is adjusted to 6.0~7.0, the dosage of the Prussian blue modified fermentation residue-based biochar is 19 g / L, and the broth is oscillated and adsorbed at 200 r / min for 180 min at 20~30℃.

[0016] Preferably, in step S5, the purified fermentation broth is added under the condition that the carbon-nitrogen ratio requirement of the denitrification system is met, so that the organic matter in the purified fermentation broth can act as an electron donor for denitrifying microorganisms, replacing part or all of the external carbon source, thereby realizing the resource recycling of fermentation residue.

[0017] Compared with the prior art, the present invention discloses at least the following technical effects: (1) This invention significantly improves the acid production efficiency of anaerobic fermentation of sludge and enhances the applicability of fermentation broth as a carbon source for denitrification. Utilizing the high alkalinity and abundant iron content of red mud, combined with alkaline pretreatment conditions, it can effectively destroy the floc structure and cell walls of residual sludge, promote the release of intracellular organic matter, accelerate the hydrolysis and acidification process of sludge, and increase the yield of volatile fatty acids. At the same time, the addition of red mud can improve the buffering capacity and SCOD / TN ratio of the fermentation system, making the carbon-nitrogen ratio of the fermentation broth more reasonable and better suited to the quality requirements of the carbon source in the denitrification process, facilitating the utilization by denitrifying bacteria, and reducing or even replacing the addition of traditional external carbon sources.

[0018] (2) This invention can efficiently remove nitrogen and phosphorus elements from fermentation broth, further improving the reuse quality of fermentation broth. The solid residue produced by anaerobic fermentation is washed, dried, and pyrolyzed to prepare biochar, which is then modified with Prussian blue. The resulting modified biochar has both a porous structure and abundant metal active sites, and has a good adsorption effect on ammonia nitrogen and phosphate in fermentation broth. It can effectively reduce nitrogen and phosphorus residues in fermentation broth and improve the reuse stability of fermentation broth.

[0019] (3) This invention realizes the joint resource utilization of red mud and residual sludge. Red mud is used to enhance sludge fermentation and acid production, while the fermentation residue is converted into functional biochar and used for the purification of fermentation liquid. The purified fermentation liquid is then reused for wastewater treatment and denitrification. The materials are recycled within the process, which reduces the input cost of external carbon sources and also lowers the cost of sludge end-of-pipe disposal, resulting in good economic and environmental benefits. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart of a method for the full-process resource utilization of residual sludge according to the present invention.

[0022] Figure 2 The graph shows the change in volatile fatty acid concentration in anaerobic fermentation broth of sludge with fermentation time under different amounts of red mud added, as provided by this invention.

[0023] Figure 3 The graph shows the change in carbon-nitrogen ratio of sludge fermentation broth with fermentation time under different amounts of red mud added, as provided by this invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] like Figure 1 As shown, this invention provides a method for the full-process resource utilization of excess sludge, comprising the following steps: S1. The pH value of the remaining sludge is adjusted to alkaline using an alkaline reagent. Red mud is added to the remaining sludge and mixed. Hydrolysis and acidification fermentation are carried out under anaerobic conditions. After fermentation, the fermentation products are separated into solid and liquid components to obtain fermentation broth containing volatile fatty acids and fermentation residue.

[0026] Specifically, the alkaline reagent is sodium hydroxide solution, which is used to adjust the initial pH of the residual sludge to 10.0. The mass ratio of red mud added to the suspended solids of the residual sludge is 0.1~3:1. Red mud itself is highly alkaline (pH usually reaches 10~12.5) and rich in iron. When used in conjunction with alkaline pretreatment on the residual sludge, it can effectively destroy the floc structure and cell walls of the sludge, promote the dissolution and release of intracellular organic matter, and provide sufficient substrate for the acidification process. At the same time, the metal oxides in red mud can participate in electron transfer during anaerobic fermentation, promoting the degradation of organic matter and the generation of volatile fatty acids. In addition, the addition of red mud can improve the buffering capacity of the fermentation system, increase the SCOD / TN ratio of the fermentation broth, and make the carbon-nitrogen ratio of the fermentation broth more reasonable.

[0027] Furthermore, the hydrolysis-acidification fermentation temperature is 35℃, and the fermentation time is 2-7 days. Before hydrolysis-acidification fermentation, nitrogen gas is introduced into the reactor to replace the air, and the reactor is sealed to ensure the maintenance of an anaerobic environment. During fermentation, shaking or stirring is used to cultivate the product, ensuring homogenization of the reaction system and mass transfer efficiency. After fermentation, the fermentation product is centrifuged at 5000 r / min, and the resulting liquid phase is filtered to effectively separate the fermentation broth from the fermentation residue.

[0028] S2. The fermentation residue is washed, dried, ground and sieved in sequence, and then pyrolyzed under a nitrogen atmosphere to obtain fermentation residue-based biochar.

[0029] Specifically, the fermentation residue was washed with deionized water to remove residual soluble impurities. The washed residue was then dried in a vacuum drying oven at 80°C for 12 hours, followed by grinding and sieving using a 0.18 mm sieve to obtain a precursor powder with uniform particle size. After the anaerobic fermentation stage, a large amount of easily degradable organic matter in the fermentation residue was consumed, resulting in biochar prepared from this residue with a higher degree of carbonization and a more developed pore structure. The sieved powder was placed in a pyrolysis reactor and heated to 600°C at a rate of 5°C / min under nitrogen purging. This temperature was maintained at 600°C for 2 hours, allowing the organic matter in the residue to carbonize in a high-temperature, inert environment. Simultaneously, metal components such as iron, aluminum, and calcium carried in the red mud were in situ loaded into the biochar framework, resulting in fermentation residue-based biochar with a preliminary porous structure.

[0030] S3. Add the fermentation residue-based biochar to an aqueous solution of ferric chloride and citric acid, add potassium ferrocyanide and citric acid aqueous solution dropwise and react. Separate the reaction product into solid and liquid, wash and dry it in sequence to obtain Prussian blue modified fermentation residue-based biochar.

[0031] Specifically, 1 mmol of ferric chloride hexahydrate was dissolved in 20 mL of 0.05 mmol / L citric acid aqueous solution to obtain a ferric chloride citric acid aqueous solution; 0.75 mmol of potassium ferrocyanide trihydrate was dissolved in 20 mL of 0.05 mmol / L citric acid aqueous solution to obtain a potassium ferrocyanide citric acid aqueous solution. The ferric chloride citric acid aqueous solution was heated to 55°C, and 0.35 g of fermentation residue-based biochar was added. After stirring and dispersing evenly at 200 rpm, the potassium ferrocyanide citric acid aqueous solution was added dropwise, and the reaction was continued for 2 h. Citric acid, as a complexing agent, can regulate the release rate of iron ions and promote the in-situ uniform growth of Prussian blue on the surface of biochar. After the reaction, the mixture was filtered, the filter residue was collected and washed with pure water, and the washed filter residue was dried in a vacuum drying oven at 100°C for 6 h to obtain Prussian blue-modified fermentation residue-based biochar. This modification process introduces a large number of metal active sites on the basis of the original pore structure of biochar, enabling the modified biochar to have both physical and chemical adsorption functions.

[0032] S4. The Prussian blue-modified fermentation residue-based biochar is added to the nitrogen- and phosphorus-containing fermentation broth obtained in step S1 for adsorption treatment. After the adsorption is completed, the Prussian blue-modified fermentation residue-based biochar is separated to obtain the purified fermentation broth.

[0033] Specifically, the pH of the nitrogen- and phosphorus-containing fermentation broth was adjusted to 6.0–7.0, with an optimal value of 6.25. Prussian blue-modified fermentation residue-based biochar was added at a dosage of 19 g / L, and adsorption was performed at 20–30 °C and a rotation speed of 200 r / min for 180 min. Under these conditions, the iron-active sites on the surface of the modified biochar effectively captured ammonia nitrogen and phosphate in the fermentation broth through electrostatic attraction and ligand exchange. Simultaneously, its porous structure provided ample adsorption space, thus achieving efficient removal of nitrogen and phosphorus from the fermentation broth. After adsorption purification, the carbon-to-nitrogen ratio of the fermentation broth was further improved, avoiding the additional denitrification load caused by directly reusing the original fermentation broth and improving the overall quality of the reused fermentation broth.

[0034] S5. The purified fermentation broth obtained in step S4 is added to the denitrification denitrification system as a supplementary carbon source for denitrification denitrification; the fermentation residue is converted into modified biochar in steps S2 and S3, and after the fermentation broth is adsorbed and purified in step S4, it is reused for denitrification denitrification, thereby realizing the resource utilization of the fermentation residue.

[0035] Specifically, purified fermentation broth is added under the condition of meeting the carbon-to-nitrogen ratio requirements of the denitrification system. The organic matter in the purified fermentation broth serves as an electron donor for the denitrifying microorganisms, replacing part or all of the external carbon source. The fermentation broth obtained after enhanced fermentation with red mud contains abundant volatile fatty acids, which have high availability as a carbon source. Excess nitrogen and phosphorus are removed through adsorption purification, without additionally increasing the denitrification load of the system and avoiding adverse effects on the effluent quality. This step reuses the purified fermentation broth in the wastewater treatment denitrification stage, realizing the resource recovery and reuse of fermentation residue.

[0036] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that any aspects not described in detail in the present invention are conventional practices in the art and are not the focus of the present invention. The raw materials and reagents used in the specific embodiments of the present invention are all commercially available products. The room temperature and ambient temperature mentioned in the specific embodiments of the present invention refer to 20-30°C.

[0037] Example 1 The residual sludge used in this embodiment was taken from the return sludge of the secondary sedimentation tank of a municipal wastewater treatment plant, and its total solids (TS) content was measured to be approximately 12 g / L. This sludge was injected into an anaerobic reactor with a working volume of 5 L, and the initial pH of the system was adjusted to 10.0 ± 0.5 using 1M NaOH solution. 6 g of red mud was weighed and added to the reactor, resulting in a red mud addition to suspended solids (SS) mass ratio of 0.1:1. After addition, the pH of the system was measured to be 10.59.

[0038] High-purity nitrogen gas was introduced into the reactor for approximately 5 minutes to displace the top air, after which the reactor was sealed to ensure the system was in an anaerobic state. The reactor was placed in a constant-temperature water bath, and the fermentation temperature was controlled at 35°C. The reactor was cultured using either shaking or stirring methods, with a shaking rate of 120 rpm, or a stirring culture period of 6 days. Sampling was performed as follows: samples were collected every 4 hours on day 1 of fermentation, and every 24 hours thereafter, with each sample volume approximately 30 mL. The collected samples were immediately centrifuged at 5000 rpm for 10 minutes. The supernatant was then filtered through a 0.45 μm aqueous filter membrane for parameter determination.

[0039] The peak concentration of volatile fatty acids (VFAs) in the fermentation broth of this embodiment was found to be 84.25 mg / L.

[0040] Example 2 The source and properties of the residual sludge used in this embodiment are the same as in Example 1 (TS approximately 12 g / L). This sludge was injected into a 5 L anaerobic reactor, and the initial pH was adjusted to 10.0 ± 0.5 with 1 M NaOH. 30 g of red mud was weighed and added to the reactor, resulting in a red mud addition to sludge suspended solids mass ratio of 0.5:1. After addition, the pH of the system was measured to be 11.12.

[0041] The nitrogen purging for deoxygenation, the closed reactor, the fermentation temperature, and the cultivation method were the same as in Example 1. The sampling and sample pretreatment methods were also the same as in Example 1.

[0042] The peak concentration of volatile fatty acids in the fermentation broth of this embodiment was measured to be 100.32 mg / L.

[0043] Example 3 The source and properties of the residual sludge used in this embodiment are the same as in Example 1 (TS approximately 12 g / L). This sludge was injected into a 5 L anaerobic reactor, and the initial pH was adjusted to 10.0 ± 0.5 with 1 M NaOH. 60 g of red mud was weighed and added to the reactor, resulting in a red mud addition to sludge suspended solids mass ratio of 1:1. After addition, the pH of the system was measured to be 11.65.

[0044] The nitrogen purging for deoxygenation, the closed reactor, the fermentation temperature, and the cultivation method were the same as in Example 1. The sampling and sample pretreatment methods were also the same as in Example 1.

[0045] The peak concentration of volatile fatty acids in the fermentation broth of this embodiment was 240.06 mg / L, and the peak value of the ratio of soluble chemical oxygen demand to total nitrogen (SCOD / TN) reached 23.1.

[0046] Example 4 The source and properties of the residual sludge used in this embodiment are the same as in Example 1 (TS approximately 12 g / L). This sludge was injected into a 5 L anaerobic reactor, and the initial pH was adjusted to 10.0 ± 0.5 with 1 M NaOH. 180 g of red mud was weighed and added to the reactor, resulting in a red mud addition to sludge suspended solids mass ratio of 3:1. After addition, the pH of the system was measured to be 11.94.

[0047] The nitrogen purging for deoxygenation, the closed reactor, the fermentation temperature, and the cultivation method were the same as in Example 1. The sampling and sample pretreatment methods were also the same as in Example 1.

[0048] The peak concentration of volatile fatty acids in the fermentation broth of this embodiment reached 530.27 mg / L, and the peak value of the soluble chemical oxygen demand to total nitrogen ratio (SCOD / TN) was 21.5. Moreover, the SCOD / TN ratio remained above 15 in the later stage of fermentation (96h to 144h), which met the requirements of external carbon source. The addition of red mud can significantly increase the concentration of VFAs in the anaerobic digester.

[0049] Example 5 This embodiment uses the fermentation residue obtained in Example 4 as raw material to prepare, modify, and verify the adsorption performance of biochar.

[0050] The fermentation residue obtained after solid-liquid separation following fermentation in Example 4 was washed three times with deionized water to remove residual soluble impurities. The washed residue was then transferred to an 80°C vacuum drying oven and dried for 12 hours until constant weight. The dried sample was then ground and passed through a 0.18 mm sieve. The powder passing through the sieve was collected for later use.

[0051] Weigh the above powder and place it in a corundum boat, then transfer it to a tubular pyrolysis furnace. Under an inert atmosphere maintained by introducing high-purity nitrogen (flow rate 100 mL / min), raise the furnace temperature to 600°C at a rate of 5°C / min, and hold at the set temperature for 2 hours. After pyrolysis, cool the furnace to room temperature, and remove the product to obtain fermentation residue-based biochar (denoted as RSBC).

[0052] 1 mmol of ferric chloride hexahydrate was dissolved in 20 mL of 0.05 mmol / L citric acid aqueous solution to obtain a ferric chloride citric acid solution. Separately, 0.75 mmol of potassium ferrocyanide trihydrate was dissolved in 20 mL of 0.05 mmol / L citric acid aqueous solution to obtain a potassium ferrocyanide citric acid solution. The ferric chloride citric acid solution was heated to 55°C in a constant temperature water bath, and 0.35 g of the prepared RSBC was added and dispersed evenly under stirring at 200 rpm. After the system temperature stabilized, the potassium ferrocyanide citric acid solution was slowly added dropwise using a constant pressure dropping funnel (dropping rate approximately 2 mL / min). After the addition was complete, the reaction was continued at 55°C for 2 h. After the reaction was completed, the solid product was collected by filtration and washed repeatedly with pure water until the filtrate was colorless. The washed sample was dried in a vacuum drying oven at 100°C for 6 h to obtain Prussian blue modified fermentation residue-based biochar (denoted as RSBC-PB).

[0053] Simulated ammonia nitrogen wastewater (initial concentration 50 mg / L) was prepared using ammonium chloride, and simulated phosphate wastewater (initial concentration 20 mg / L, with a small amount of sulfuric acid added to prevent precipitation) was prepared using potassium dihydrogen phosphate. 100 mL of the simulated wastewater was placed in a 250 mL Erlenmeyer flask, and RSBC-PB was added to achieve a concentration of 5 g / L. Adsorption was carried out at room temperature with shaking at 200 rpm for 180 min. After adsorption, a sample was taken and filtered through a 0.45 μm filter membrane. The concentrations of residual ammonia nitrogen and phosphate in the filtrate were measured. The adsorption removal rate and the equilibrium adsorption capacity of the adsorbent were calculated using the following formula: ; ; In the formula, This represents the initial concentration of the substance in the solution, expressed in mg / L. The concentration of the substance remaining in the solution after adsorption equilibrium is given in mg / L. The mass of the adsorbent is expressed in grams. This refers to the solution volume, in ml. The adsorption removal rate is expressed as % (%). This represents the equilibrium adsorption capacity of the adsorbent, expressed in mg / g.

[0054] Calculations show that RSBC-PB achieves ammonia nitrogen removal rate of approximately 84% and phosphate removal rate of approximately 81%.

[0055] Comparative Example 1 The same batch of residual sludge from the secondary sedimentation tank (total solids (TS) approximately 10 g / L) as in Example 1 was injected into a 5 L anaerobic reactor. The pH of the system was adjusted to 10.0 ± 0.5 using 1 M NaOH solution, without adding red mud. High-purity nitrogen gas was introduced into the reactor for approximately 5 minutes to displace the top air, after which the reactor was sealed to ensure the system was in an anaerobic state. The reactor was placed in a constant-temperature water bath, and the fermentation temperature was controlled at 35°C. The entire fermentation process was carried out using a shaking method at a shaking rate of 120 rpm. The total fermentation period was 6 days. The sampling procedure was the same as in Example 1: samples were taken every 4 hours on the first day of fermentation, and every 24 hours from the second day onwards, with approximately 30 mL taken each time. The samples were centrifuged at 5000 rpm for 10 minutes, and the supernatant was filtered through a 0.45 μm filter membrane before measuring various indicators.

[0056] The peak concentration of volatile fatty acids in the fermentation broth of the comparative sample was 89.58 mg / L, and the peak value of the ratio of soluble chemical oxygen demand to total nitrogen (SCOD / TN) was 18.7.

[0057] from Figure 2It can be seen that the accumulation of VFAs in each group was mainly concentrated in the early stage of fermentation (days 1-3). Examples 1-4 all reached peak concentrations around day 2-3 of fermentation, after which they tended to stabilize or slightly decrease. Specifically, Comparative Example 1 reached a peak of 89.58 mg / L on day 2, and then gradually decreased to approximately 57 mg / L on day 6, a decrease of about 36%. When the red mud addition was 0.1 g / g SS, the VFAs concentration trend was basically the same as that of Comparative Example 1, with a peak concentration of 84.25 mg / L on day 3. No significant increase was observed compared to Comparative Example 1, indicating that the lower red mud addition had a limited promoting effect on acid production. When the red mud addition was increased to 0.5 g / g SS, the peak VFAs concentration increased to 100.32 mg / L (day 2), an increase of about 12% compared to Comparative Example 1. Furthermore, the VFAs concentrations at each time point after day 3 of fermentation were higher than the corresponding time points in Comparative Example 1, initially demonstrating the promoting effect of red mud on the hydrolysis and acidification process. When the red mud dosage was 1 g / g SS, the peak concentration of VFAs jumped to 240.06 mg / L (day 2), approximately 2.7 times higher than the control group. Furthermore, the VFAs concentration at all time points throughout the fermentation cycle was significantly higher than both control group 1 and the low dosage group. When the red mud dosage was 3 g / g SS, the peak concentration of VFAs further increased to 530.27 mg / L (day 2), approximately 5.9 times higher than control group 1. The concentration remained at a high level on day 3 of fermentation, then slowly decreased, remaining at approximately 210 mg / L on day 6, about 3.7 times the level of control group 1 at the same time point. These results indicate a clear dose-response relationship between the red mud dosage and VFAs production. The higher the red mud dosage, the more significant the acid production promotion effect. Moreover, the high dosage maintained a high VFAs concentration in the fermentation broth throughout the entire cycle, which is beneficial for a continuous and stable supply when used as a carbon source for subsequent reuse.

[0058] from Figure 3It can be seen that the SCOD / TN ratio of each fermentation broth group showed a trend of first increasing and then decreasing. The SCOD / TN ratio of Comparative Example 1 reached a peak of 18.7 at 11 h of fermentation, and then continued to decrease, reaching 12.5 by 48 h, and then remained at around 12-13. This indicates that although conventional alkaline fermentation without red mud can achieve a certain carbon-nitrogen ratio in the early stage of fermentation, the carbon source consumption rate is faster as the fermentation time increases, making it difficult to maintain the carbon-nitrogen ratio. The example with added red mud showed different characteristics. When the red mud addition was 1 g / g SS, the SCOD / TN ratio reached a peak of 23.1 at 11 h of fermentation, which was about 23.5% higher than that of Comparative Example 1, and the SCOD / TN ratio at each time point throughout the monitoring period was higher than the corresponding level of Comparative Example 1, remaining at 14.5 by 48 h. When the red mud addition was 3 g / g SS, the SCOD / TN ratio reached a peak of 21.5 at 11 h of fermentation, which was about 15% higher than that of control group 1. In the early stage of fermentation (0~24 h), the SCOD / TN ratio was generally higher than that of the 1 g / g SS group, but in the middle and late stage (after 48 h), it was slightly lower than that of the 1 g / g SS group.

[0059] More notably, during the later stages of fermentation (48-144h), the SCOD / TN ratio of Comparative Example 1 remained consistently between 12.5 and 14.0, which was lower than the general requirements for carbon source quality in the denitrification process (SCOD / TN > 15). In contrast, the examples with red mud additions of 1 g / g SS and 3 g / g SS showed SCOD / TN ratios of 14.5 and 17.0 at 48h of fermentation, respectively, and remained around 14.5 and 15.0 at 144h. This demonstrates that with a red mud addition of 3 g / g SS, the carbon-to-nitrogen ratio of the fermentation broth remained above 15 throughout the 144h fermentation period, meeting the quality requirements for the added carbon source in the denitrification process.

[0060] In summary, the addition of red mud significantly promotes acid production during the anaerobic fermentation of waste sludge. The peak concentration of VFAs increases progressively with increasing red mud dosage, reaching its highest level at 3 g / g SS, approximately 5.9 times that of Comparative Example 1. Simultaneously, the addition of red mud effectively improves the carbon-to-nitrogen ratio and buffering capacity of the fermentation system. The SCOD / TN peak value of the fermentation broth is higher after adding red mud, and a high carbon-to-nitrogen ratio is maintained even in the mid-to-late stages, which is beneficial for a stable supply of carbon source for denitrification. Furthermore, the red mud dosage exhibits excellent performance within the range of 1–3 g / g SS, with the most significant carbon-to-nitrogen ratio optimization effect at 1 g / g SS and the most obvious VFA production advantage at 3 g / g SS, with the carbon-to-nitrogen ratio remaining above 15 throughout the fermentation cycle. In practical applications, the red mud dosage can be flexibly selected within the above range based on the specific requirements of the wastewater treatment system for carbon source deficit and denitrification load.

[0061] Therefore, by adopting the above-mentioned method for the full-process resource utilization of residual sludge, the anaerobic fermentation of residual sludge is enhanced by red mud to increase the production of volatile fatty acids, and the fermentation residue is prepared into biochar, which is modified by Prussian blue and used to adsorb nitrogen and phosphorus in the fermentation broth. The purified fermentation broth is reused as a carbon source for denitrification, thus forming a technical route of enhanced acid production by red mud, carbonization and modification of residue, and denitrification by reuse of fermentation broth. This realizes the resource utilization of red mud and residual sludge, while reducing the input of external carbon sources and the cost of sludge disposal.

[0062] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0063] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for the full-process resource utilization of excess sludge, characterized in that, Includes the following steps: S1. The pH value of the remaining sludge is adjusted to alkaline using an alkaline reagent. Red mud is added to the remaining sludge and mixed. Hydrolysis and acidification fermentation is carried out under anaerobic conditions. After fermentation, the fermentation products are separated into solid and liquid components to obtain fermentation broth containing volatile fatty acids and fermentation residue. S2. The fermentation residue is washed, dried, ground and sieved in sequence, and then pyrolyzed under a nitrogen atmosphere to obtain fermentation residue-based biochar. S3. Add the fermentation residue-based biochar to an aqueous solution of ferric chloride and citric acid, add potassium ferrocyanide and citric acid aqueous solution dropwise and react, and perform solid-liquid separation, washing and drying on the reaction product in sequence to obtain Prussian blue modified fermentation residue-based biochar. S4. The Prussian blue-modified fermentation residue-based biochar is added to the nitrogen- and phosphorus-containing fermentation broth obtained in step S1 for adsorption treatment. After the adsorption is completed, the Prussian blue-modified fermentation residue-based biochar is separated to obtain the purified fermentation broth. S5. The purified fermentation broth obtained in step S4 is added to the denitrification denitrification system as a supplementary carbon source for denitrification denitrification; the fermentation residue is converted into modified biochar in steps S2 and S3, and after the fermentation broth is adsorbed and purified in step S4, it is reused for denitrification denitrification, thereby realizing the resource utilization of the fermentation residue.

2. The method according to claim 1, characterized in that, In step S1, the alkaline reagent is a sodium hydroxide solution, which is used to adjust the initial pH value of the remaining sludge to 10.0; the mass ratio of the amount of red mud added to the suspended solids of the remaining sludge is 0.1~3:

1.

3. The method according to claim 1, characterized in that, In step S1, the temperature of the hydrolysis acidification fermentation is 35°C, and the fermentation time is 2-7 days. Before the hydrolysis acidification fermentation, nitrogen gas is introduced into the reactor to replace the air, and the reactor is sealed. During the fermentation process, the fermentation is carried out by shaking or stirring.

4. The method according to claim 1, characterized in that, In step S1, the solid-liquid separation includes centrifuging the fermentation product at a speed of 5000 r / min and filtering the resulting liquid phase.

5. The method according to claim 1, characterized in that, In step S2, the fermentation residue is washed with deionized water, and the washed fermentation residue is dried in a vacuum drying oven at 80°C for 12 hours. Then it is ground and sieved using a sieve with a pore size of 0.18 mm.

6. The method according to claim 5, characterized in that, In step S2, the sieved powder is placed in a pyrolysis reactor and heated to 600°C at a heating rate of 5°C / min under nitrogen gas, and kept at 600°C for 2 hours to obtain the fermentation residue-based biochar.

7. The method according to claim 1, characterized in that, Step S3 includes: dissolving ferric chloride hexahydrate in an aqueous solution of citric acid to obtain the ferric chloride citric acid aqueous solution; dissolving potassium ferrocyanide trihydrate in an aqueous solution of citric acid to obtain the potassium ferrocyanide citric acid aqueous solution; heating the ferric chloride citric acid aqueous solution and adding the fermentation residue-based biochar, stirring and dispersing evenly, then adding the potassium ferrocyanide citric acid aqueous solution dropwise, and continuing to heat the reaction; after the reaction is completed, performing vacuum filtration, collecting the filter residue and washing it, and then drying the washed filter residue to obtain the Prussian blue modified fermentation residue-based biochar.

8. The method according to claim 1, characterized in that, In step S4, the pH of the nitrogen- and phosphorus-containing fermentation broth is adjusted to 6.0-7.0, the dosage of the Prussian blue-modified fermentation residue-based biochar is 19 g / L, and the broth is oscillated and adsorbed at 200 r / min for 180 min at 20-30°C.

9. The method according to claim 1, characterized in that, In step S5, the purified fermentation broth is added under the condition that the carbon-nitrogen ratio requirement of the denitrification system is met, so that the organic matter in the purified fermentation broth can act as an electron donor for denitrifying microorganisms, replacing part or all of the external carbon source, thereby realizing the resource recycling of fermentation residue.

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