Composite carbon source and application thereof in nitrogen removal by denitrification of sewage

Abstract

The invention relates to a composite carbon source and application thereof in sewage denitrification nitrogen removal, and belongs to the technical field of sewage treatment.The composite carbon source is obtained by forming a gel network through polyvinyl alcohol and sodium alginate, then crosslinking the gel network with calcium ions in calcium chloride to form a three-dimensional framework, and wrapping modified composite powder and polylactic acid in the three-dimensional framework. The three-dimensional skeleton can provide certain mechanical strength, so that the internal carbon source can resist water flow scouring, the porous structure can also enable the internal carbon source to be slowly released, more pores are formed after Na-montmorillonite and iron-carbon powder are compounded, the loading rate of flaky nano-hydroxyapatite on the montmorillonite intercalated iron-carbon powder is further improved, and the adsorption capacity of the montmorillonite intercalated iron-carbon powder is improved. Hydroxyl groups in the flaky nano-hydroxyapatite can form hydrogen bonds with Na-montmorillonite and hydroxyl groups on a three-dimensional skeleton, the interface bonding force is improved, the mechanical strength of the composite carbon source is further improved, polylactic acid serves as a slow-release carbon source and is gradually hydrolyzed under the action of microorganisms, and the release period of the composite carbon source under continuous impact of high-speed water flow is prolonged.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sewage treatment, and relates to a composite carbon source and application thereof in sewage denitrification. BACKGROUND

[0002] Carbon source plays a crucial role in the biological denitrification process, and it is an indispensable substance in the reduction of nitrate nitrogen, the promotion of microbial denitrification, and the growth and reproduction of bacteria. In the sewage environment with low C / N, in order to ensure the smooth progress of the microbial denitrification process, additional carbon source usually needs to be added to the system. Single carbon source has some deficiencies in some aspects. Mixing different carbon sources to prepare a composite carbon source can combine the advantages of different carbon sources and often achieve better results. In addition to considering the shape and surface structure of the solid material, the preparation of the composite carbon source also needs to improve the carbon release stability of the material and prolong the carbon release period of the material.

[0003] The Chinese invention patent application with the publication number CN116969592B discloses a composite carbon source and a preparation method and application thereof. The byproduct crude glycerol of biodiesel is purified and compounded with other carbon sources to solve the problem of difficult utilization of crude glycerol waste, and the organic matter in biodiesel is utilized to increase the utilization rate and selectivity of microorganisms to carbon source. Compared with single carbon source, the crude glycerol composite carbon source has higher utilization rate and selectivity. By compounding with other carbon sources, the growth and metabolism of microorganisms can be promoted, which is beneficial to the denitrification of sewage.

[0004] The Chinese invention patent application with the publication number CN113264588B discloses a composite carbon source for sewage treatment. Molasses is used as one of the carbon sources for sewage treatment, and its main components are sucrose, vitamins, proteins, minerals, surface active substances and growth promoting factors, etc. It also has the ability of resisting acid and alkali and resisting oxidation. Humic acid fermentation liquid is applied to the liquid carbon source of denitrifying bacteria agent. The energy density of the humic acid fermentation liquid is low, the digestion and absorption are fast, the price is low, it has strong chelating ability to divalent cations, can promote the reproduction of functional bacteria and improve the reaction rate, and has the advantages of high utilization efficiency and green environmental protection.

[0005] The composite carbon sources in the above-mentioned solutions do not involve solid-state forming or enhanced structure design, so in the high hydraulic load scene such as fluidized bed, the carbon source has insufficient anti-scouring ability, and when subjected to continuous impact of high-speed water flow, the carbon source is easily lost with the water flow, resulting in a decrease in the utilization rate of the carbon source. SUMMARY

[0006] The application aims to provide a composite carbon source and application thereof in wastewater denitrification, which is obtained by forming a three-dimensional skeleton structure through chemical cross-linking between calcium ions and sodium alginate and coating a supported composite powder inside, thereby improving long-acting slow-release property and mechanical strength of the composite carbon source, and improving utilization rate of the composite carbon source in a high hydraulic load scene such as a fluidized bed.

[0007] The application aims to provide a composite carbon source and application thereof in wastewater denitrification, which is obtained by forming a three-dimensional skeleton structure through chemical cross-linking between calcium ions and sodium alginate and coating a supported composite powder inside, thereby improving long-acting slow-release property and mechanical strength of the composite carbon source, and improving utilization rate of the composite carbon source in a high hydraulic load scene such as a fluidized bed. The application aims to provide a composite carbon source and application thereof in wastewater denitrification, which is obtained by forming a three-dimensional skeleton structure through chemical cross-linking between calcium ions and sodium alginate and coating a supported composite powder inside, thereby improving long-acting slow-release property and mechanical strength of the composite carbon source, and improving utilization rate of the composite carbon source in a high hydraulic load scene such as a fluidized bed. Step one: mixing ferric chloride and biomass powder, and then placing the obtained precursor powder in a tube furnace to be calcined and pyrolyzed under nitrogen protection, thereby obtaining iron-carbon powder.

[0008] Step two: intercalating sodium-based montmorillonite and iron-carbon powder to obtain montmorillonite intercalated iron-carbon powder, and then generating flaky nano-hydroxyapatite on the montmorillonite intercalated iron-carbon powder by a coprecipitation method, thereby obtaining modified composite powder.

[0009] Step three: forming a gel by using polyvinyl alcohol and sodium alginate, and then cross-linking the gel by using calcium ions and boric acid as cross-linking agents, and coating the modified composite powder and polylactic acid, thereby obtaining the composite carbon source.

[0010] Further, the preparation process of the iron-carbon powder is as follows: Placing the precursor powder in a tube furnace, and then increasing the temperature to 600-1000 DEG C at a temperature increasing rate of 10 DEG C / min under nitrogen protection, and pyrolyzing for 2-4 h, thereby obtaining iron-carbon powder.

[0011] Further, the preparation process of the precursor powder is as follows: Adding ferric chloride and an ethanol aqueous solution with a concentration of 50 vol% into a reaction kettle, uniformly mixing, and then adding biomass powder, stirring at 500-700 r / min for 3-5 h, filtering, and drying, thereby obtaining the precursor powder.

[0012] Further, the amount ratio of ferric chloride, the ethanol aqueous solution, and the biomass powder is 10-15 g:500-600 mL:100-130 g.

[0013] Further, the preparation process of the modified composite powder is as follows: Adding calcium nitrate tetrahydrate, montmorillonite intercalated iron-carbon powder, and deionized water into a reaction kettle, adding diammonium hydrogen phosphate and deionized water at 40-50 DEG C, dropwise adding sodium hydroxide solution, adjusting the pH value to 11, stirring for 7-8 h, filtering, washing, drying, and grinding, thereby obtaining the modified composite powder.

[0014] Further, the calcium nitrate tetrahydrate, the montmorillonite intercalation iron carbon powder, the diammonium hydrogen phosphate and the deionized water are in a ratio of 13-15g:16-22g:4.4-5.2g:80-110mL.

[0015] Further, the preparation process of the montmorillonite intercalation iron carbon powder is as follows: The sodium-based montmorillonite and the deionized water are added into a reaction kettle, stirred for 2-3h, then the iron carbon powder and the sodium hydroxide solution with a concentration of 6wt% are uniformly mixed and added into the reaction kettle, stirred for 5-6h, filtered, washed, dried, ground, and the montmorillonite intercalation iron carbon powder is obtained.

[0016] Further, the sodium-based montmorillonite, the deionized water, the iron carbon powder and the sodium hydroxide solution are in a ratio of 16-22g:500-700mL:18-22g:500-700m.

[0017] Further, the preparation process of the composite carbon source is as follows: The deionized water, the polyvinyl alcohol and the sodium alginate are added into a reaction kettle, heated at 500-700r / min and 95-100℃ for 2-3h, cooled to room temperature, then the polylactic acid and the modified composite powder are added, stirred and mixed, then poured into a mold, frozen into shape at-20℃, demolded, placed into a saturated boric acid solution containing 5wt% calcium chloride, crosslinked at 0-4℃ for 22-24h, filtered and washed, and the composite carbon source is obtained.

[0018] Further, the deionized water, the polyvinyl alcohol, the sodium alginate, the polylactic acid, the modified composite powder and the saturated boric acid solution containing 5wt% calcium chloride are in a ratio of 100-150mL:8-10g:1-2g:24-26g:16-20g:400-500mL.

[0019] The application of a composite carbon source in the denitrification of sewage.

[0020] The beneficial effects of the application are as follows: 1、In the application, the polyvinyl alcohol and the sodium alginate form a gel network, then a three-dimensional skeleton coating layer is formed through the double crosslinking action of calcium ions and boric acid, and the modified composite powder and the polylactic acid are wrapped inside to obtain the composite carbon source.

[0021] The three-dimensional framework can provide certain mechanical strength, so that the internal carbon source can resist water flow scouring, and the porous structure can also enable the internal carbon source to be released slowly. After the sodium-based montmorillonite and the iron-carbon powder are compounded, more pores are formed, thereby improving the loading rate of the sheet-shaped nano-hydroxyapatite on the iron-carbon powder intercalated in the montmorillonite. The hydroxyl groups in the sheet-shaped nano-hydroxyapatite can form hydrogen bonds with the sodium-based montmorillonite and the hydroxyl groups on the three-dimensional framework, so that the interfacial bonding force is improved, and the mechanical strength of the composite carbon source is further improved. The polylactic acid as a slow-release carbon source is gradually hydrolyzed under the action of microorganisms, so that small molecule carbon sources can be continuously provided, and the release period of the composite carbon source under the continuous impact of high-speed water flow is prolonged.

[0022] 2. In the present application, the iron chloride and biomass powder are mixed and then pyrolyzed, which not only increases the specific surface area of the iron-carbon powder, but also uniformly dopes the iron element. The nano zero-valent iron accelerates the denitrification process through electron transfer. Then, the sodium-based montmorillonite is compounded on the iron-carbon powder. The porous structure of the iron-carbon powder and the layered structure of the sodium-based montmorillonite are complementary, which increases the specific surface area and provides a large number of attachment points for the denitrifying bacteria, which is beneficial to the enrichment of the denitrifying bacteria on the surface. At the same time, the iron-carbon powder and the compounded sodium-based montmorillonite can adsorb small molecules in sewage, which is convenient for the bacteria to utilize and remove nitrogen sources in sewage, and improves the efficiency of denitrification of the composite carbon source.

[0023] 3. In the present application, the sheet-shaped nano-hydroxyapatite is generated on the surface of the montmorillonite intercalated iron-carbon powder. The sheet-shaped nano-hydroxyapatite is gradually dissolved in the process of denitrification acid production, releasing calcium ions and hydroxyl ions to maintain the stability of the microenvironment pH, which is conducive to the reproduction of denitrifying bacteria. It can also complex with heavy metal ions to reduce the inhibition of free heavy metal ions on enzyme activity. The interlayer cations of sodium-based montmorillonite can further buffer hydrogen ions through ion exchange, and also remove heavy metal ions through interlayer adsorption, which together reduces the inhibition of denitrifying bacteria activity, thereby improving the efficiency of denitrification. DETAILED DESCRIPTION

[0024] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined object, the specific embodiments, features and effects according to the present application are described in detail as follows.

[0025] Example 1: The present embodiment provides a kind of composite carbon source, which is prepared by the following steps: S1: 12.5g of iron chloride and 550mL of 50vol% ethanol aqueous solution are added to a reaction kettle, mixed uniformly, then 27.5g of rice husk powder, 35g of corn cob powder and 52.5g of reed rod powder are added, stirred at 600r / min for 4h, filtered, and vacuum dried at 75℃ for 25h to obtain a precursor powder.

[0026] S2: The precursor powder is placed in a tube furnace, and heated to 800℃ at a heating rate of 10℃ / min under nitrogen protection for 3h. The redox reaction between carbon, carbon monoxide and hydrogen and iron ions reduces the iron ions to nano-iron, and iron-carbon powder is obtained.

[0027] S3: 20g of sodium-based montmorillonite and 600mL of deionized water are added to the reaction kettle and stirred for 2.5h. Then 20g of iron-carbon powder and 600mL of 6wt% sodium hydroxide solution are uniformly mixed and added to the reaction kettle, stirred for 5.5h, filtered, washed with deionized water until neutral, dried, ground, and montmorillonite intercalated iron-carbon powder is obtained.

[0028] S4: 14g of calcium nitrate tetrahydrate, 20g of montmorillonite intercalated iron-carbon powder and 50mL of deionized water are added to the reaction kettle, 4.8g of diammonium hydrogen phosphate and 45mL of deionized water are added at 45℃, a 1mol / L sodium hydroxide solution is added dropwise, the pH value is adjusted to 11, and stirring is carried out at 600r / min for 7.5h. The calcium nitrate tetrahydrate and diammonium hydrogen phosphate generate flaky nano-hydroxyapatite by coprecipitation and are loaded on the montmorillonite intercalated iron-carbon powder. Centrifugal filtration is carried out, the precipitate is washed with deionized water until neutral, dried at 55℃ for 13h, and ground to obtain a modified composite powder.

[0029] S5: 125mL of deionized water, 9g of polyvinyl alcohol and 1.5g of sodium alginate are added to the reaction kettle, heated at 600r / min and 97℃ for 2.5h, cooled to room temperature, 25g of polylactic acid and 18g of modified composite powder are added, stirred and mixed, then poured into a 1.5cm×1.5cm×1.5cm square mold, frozen into shape at -20℃, demolded, placed in 450mL of saturated boric acid solution containing 5wt% calcium chloride, crosslinked at 2℃ for 23h, and a crosslinking reaction occurs between calcium ions and sodium alginate to form a three-dimensional network gel. Boric acid can also help polyvinyl alcohol crosslink. Filtration is carried out, and the composite carbon source is obtained by washing with deionized water four times.

[0030] Example 2: A composite carbon source is provided, which is prepared by the following steps: S1: 10g of iron chloride and 500mL of 50vol% ethanol aqueous solution are added to the reaction kettle, uniformly mixed, and then 25g of rice husk powder, 30g of corn cob powder and 45g of reed rod powder are added, stirred at 500r / min for 3h, filtered, and vacuum dried at 70℃ for 24h to obtain a precursor powder.

[0031] S2: The precursor powder is placed in a tube furnace, and heated to 600℃ at a heating rate of 10℃ / min under nitrogen protection, pyrolyzed for 2h, and the iron ions are reduced to nano-iron through the redox reaction between carbon, carbon monoxide and hydrogen and the iron ions, to obtain iron-carbon powder.

[0032] S3: 16g of sodium-based montmorillonite and 500mL of deionized water are added to a reaction kettle, stirred for 2h, and then 18g of iron-carbon powder and 500mL of a 6wt% sodium hydroxide solution are uniformly mixed and added to the reaction kettle, stirred for 5h, filtered, washed with deionized water until neutral, dried, ground, and montmorillonite intercalated iron-carbon powder is obtained.

[0033] S4: 13g of calcium nitrate tetrahydrate, 16g of montmorillonite intercalated iron-carbon powder and 40mL of deionized water are added to a reaction kettle, 4.4g of diammonium hydrogen phosphate and 40mL of deionized water are added at 40℃, a 1mol / L sodium hydroxide solution is added dropwise, the pH value is adjusted to 11, and stirring is carried out at 500r / min for 7h, the calcium nitrate tetrahydrate and diammonium hydrogen phosphate are co-precipitated to form flaky nano-hydroxyapatite and loaded on the montmorillonite intercalated iron-carbon powder, centrifugal filtration is carried out, the precipitate is washed with deionized water until neutral, dried at 50℃ for 12h, and ground to obtain a modified composite powder.

[0034] S5: 100mL of deionized water, 8g of polyvinyl alcohol and 1g of sodium alginate are added to a reaction kettle, heated at 500r / min and 95℃ for 2h, cooled to room temperature, 24g of polylactic acid and 16g of modified composite powder are added, stirred and mixed, then poured into a 1.5cm×1.5cm×1.5cm square mold, frozen into shape at -20℃, demolded, placed in a 400mL saturated boric acid solution containing 5wt% calcium chloride, crosslinked at 0℃ for 22h, and a crosslinking reaction occurs between the calcium ions and the sodium alginate to form a three-dimensional network gel, filtered, washed with deionized water three times, and a composite carbon source is obtained.

[0035] Example 3: A composite carbon source is provided, which is prepared by the following steps: S1: 15g of iron chloride and 600mL of a 50vol% ethanol aqueous solution are added to a reaction kettle, uniformly mixed, and then 30g of rice husk powder, 40g of corn cob powder and 60g of reed rod powder are added, stirred at 700r / min for 5h, filtered, and vacuum dried at 80℃ for 26h to obtain a precursor powder.

[0036] S2: The precursor powder is placed in a tube furnace, and heated to 1000℃ at a heating rate of 10℃ / min under nitrogen protection, pyrolyzed for 4h, and the iron ions are reduced to nano-iron through the redox reaction between carbon, carbon monoxide and hydrogen and the iron ions, to obtain iron-carbon powder.

[0037] S3: 22g of sodium-based montmorillonite and 700mL of deionized water are added to a reaction kettle, stirred for 3h, and then 22g of iron-carbon powder and 700mL of a 6wt% sodium hydroxide solution are uniformly mixed and added to the reaction kettle, stirred for 6h, filtered, washed with deionized water until neutral, dried, ground, and montmorillonite intercalated iron-carbon powder is obtained.

[0038] S4: 15g of calcium nitrate tetrahydrate, 22g of montmorillonite intercalated iron-carbon powder and 60mL of deionized water are added to a reaction kettle, 5.2g of diammonium hydrogen phosphate and 50mL of deionized water are added at 50℃, a 1mol / L sodium hydroxide solution is added dropwise, the pH value is adjusted to 11, and stirring is carried out at 700r / min for 8h, the calcium nitrate tetrahydrate and diammonium hydrogen phosphate generate flaky nano-hydroxyapatite through the coprecipitation method and are loaded on the montmorillonite intercalated iron-carbon powder, centrifugal filtration is carried out, the precipitate is washed with deionized water until neutral, dried at 60℃ for 14h, ground, and a modified composite powder is obtained.

[0039] S5: 150mL of deionized water, 10g of polyvinyl alcohol and 2g of sodium alginate are added to a reaction kettle, heated at 700r / min and 100℃ for 3h, cooled to room temperature, 26g of polylactic acid and 20g of modified composite powder are added, stirred and mixed, then poured into a 1.5cm×1.5cm×1.5cm square mold, frozen into shape at-20℃, demolded, placed in a 500mL saturated boric acid solution containing 5wt% calcium chloride, crosslinked at 4℃ for 24h, a crosslinking reaction occurs between calcium ions and sodium alginate to form a three-dimensional network gel, filtered, washed with deionized water 5 times, and a composite carbon source is obtained.

[0040] Comparative Example 1: On the basis of Example 1, no iron chloride is added in step S1 to prepare a precursor powder without iron, and the precursor powder without iron is used to replace the precursor powder in step S2, and the remaining steps remain unchanged to prepare a composite carbon source.

[0041] Comparative Example 2: On the basis of Example 1, the iron-carbon powder prepared in step S2 is used to replace the modified composite powder in step S5, and the remaining steps remain unchanged to prepare a composite carbon source.

[0042] Comparative Example 3: On the basis of Example 1, the montmorillonite intercalated iron-carbon powder prepared in step S3 is used to replace the modified composite powder in step S5, and the remaining steps remain unchanged to prepare a composite carbon source.

[0043] The composite carbon sources prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests: Denitrification and nitrogen removal performance test: using a static experimental system, the prepared composite carbon sources were compared with the sludge control group (ck group) without adding carbon source, and their denitrification rate, denitrification rate, etc. were analyzed. The experimental steps are as follows: ① Weighing: 10 g of each composite carbon source was weighed; ② Solution preparation: prepare about 50 mg / L nitrate simulated wastewater and add a certain concentration of potassium dichromate; ③ Soaking: respectively place the composite carbon sources in the same specification static treatment device, add 400 mL of simulated wastewater, seal with rubber plug, and number 1# (Example 1), 2# (Example 2), 3# (Example 3), 4# (Comparative Example 1), 5# (Comparative Example 2), 6# (Comparative Example 3); ④ Inoculation of sludge and bacteria: inoculate 30 mL of enriched and washed activated sludge, and add the cultured bacteria at a ratio of 40:1 (activated sludge:bacteria); ⑤ Add control group: add another group with the same amount of activated sludge (use organic matter in sludge without additional composite carbon source) as the control group (ck group, numbered 7#); ⑥ Stirring: place 1#-7# in a magnetic stirrer at 25°C and 150 r / min for uniform stirring to make the sludge suspended, and control the dissolved oxygen of each device system below 0.5 mg / L; ⑦ Filtration and sampling: every 24 h, filter the water sample by covering the bottle opening with three layers of gauze, and filter the collected water sample through 45 μm filter paper, a total of 30 d; ⑧ Sampling: measure NO3 - -N——UV spectrophotometry (HJ / T346-2007).

[0044] Compression strength performance test: the composite carbon source sample was pressed vertically and uniformly by the Aidberg push-pull gauge on the stress surface, and the value was recorded when the composite carbon source sample was deformed to break, which was the compression strength.

[0045] Table 1 Performance test results Item Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 ck group 1d denitrification rate (%) 30.8 30.5 30.6 30.2 25.5 28.9 15.2 5d denitrification rate (%) 61.3 61.5 61.8 58.5 50.2 56.1 25.1 14d denitrification rate (%) 79.5 78.6 78.1 70.1 65.3 69.5 28.5 30d denitrification rate (%) 95.8 95.1 95.6 85.2 77.2 83.4 30.8 Maximum denitrification rate (mg / g-h) 0.083 0.081 0.082 0.038 0.031 0.035 0.027 Compressive strength (N) 301 295 297 294 185 220 / As shown in Table 1, the compression strength of Examples 1-3 is greater than that of Comparative Examples 2 and 3. In step S5 of Comparative Example 2, the iron-carbon powder prepared in step S2 is used instead of the modified composite powder, and in step S5 of Comparative Example 3, the montmorillonite intercalated iron-carbon powder prepared in step S3 is used instead of the modified composite powder. It may be because the three-dimensional framework can provide certain mechanical strength, and the hydroxyl groups in the sheet-shaped nanometer hydroxyapatite can form hydrogen bonds with the sodium-based montmorillonite and the hydroxyl groups on the three-dimensional framework, improving the interfacial bonding force and further improving the mechanical strength of the composite carbon source, prolonging the release period of the composite carbon source under the continuous impact of high-speed water flow.

[0046] The 5d denitrification rate, 14d denitrification rate, 30d denitrification rate and maximum denitrification rate in examples 1-3 are all greater than those in comparative examples 1-3 and the ck group, and the effect in the comparative examples is poor, which may be because the nano zero-valent iron accelerates the denitrification process through electron transfer, the porous structure of the iron-carbon powder and the layered structure of the sodium-based montmorillonite complement each other, increasing the specific surface area and providing a large number of attachment points for the denitrification bacteria, which is conducive to the enrichment of the denitrification bacteria on the surface, and at the same time, the iron-carbon powder and the composite sodium-based montmorillonite can adsorb small molecules in the sewage, which is conducive to the efficient utilization and removal of nitrogen sources in the sewage by the bacteria, thereby improving the efficiency of the composite carbon source denitrification and denitrification.

[0047] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been disclosed as above with a preferred embodiment, it is not intended to limit the present application. Any person skilled in the art can make some changes or modifications to the above disclosed technical content to obtain equivalent embodiments with equivalent changes, without departing from the technical solution of the present application. Any modification, equivalent change and modification of the above embodiments, which does not depart from the technical solution of the present application, is still within the scope of the present application.

Claims

1. A composite carbon source, characterized in that, Prepared by the following steps: Step 1: Mix ferric chloride and biomass powder, then place the resulting precursor powder in a tube furnace and calcine and pyrolyze it under nitrogen protection to obtain iron-carbon powder. Step 2: Sodium-based montmorillonite and iron-carbon powder are intercalated and composited to obtain montmorillonite-intercalated iron-carbon powder. Flake-like nano-hydroxyapatite is generated on the montmorillonite-intercalated iron-carbon powder by co-precipitation method to obtain modified composite powder. Step 3: A gel is formed using polyvinyl alcohol and sodium alginate. Then, calcium ions and boric acid are used as crosslinking agents to crosslink the gel and coat the modified composite powder and polylactic acid to obtain a composite carbon source.

2. The composite carbon source according to claim 1, characterized in that, The preparation process of the iron-carbon powder in step one is as follows: The precursor powder was placed in a tube furnace and heated to 600-1000℃ at a heating rate of 10℃ / min under nitrogen protection, and pyrolyzed for 2-4 hours to obtain iron-carbon powder.

3. The composite carbon source according to claim 2, characterized in that, The preparation process of the precursor powder is as follows: Ferric chloride and a 50 vol% aqueous ethanol solution were added to a reaction vessel and mixed thoroughly. Then, biomass powder was added and stirred at 500-700 r / min for 3-5 h. The mixture was then filtered and dried to obtain precursor powder.

4. A composite carbon source according to claim 3, characterized in that, The ratio of ferric chloride, ethanol aqueous solution, and biomass powder is 10-15g: 500-600mL: 100-130g.

5. A composite carbon source according to claim 1, characterized in that, The preparation process of the modified composite powder in step two is as follows: Calcium nitrate tetrahydrate, montmorillonite intercalated iron-carbon powder, and deionized water were added to a reaction vessel. Diammonium hydrogen phosphate and deionized water were added at 40-50℃. Sodium hydroxide solution was added dropwise to adjust the pH to 11. The mixture was stirred and reacted for 7-8 hours. The mixture was then filtered, washed, dried, and ground to obtain the modified composite powder.

6. A composite carbon source according to claim 5, characterized in that, The ratio of the amounts of calcium nitrate tetrahydrate, montmorillonite intercalated iron carbon powder, diammonium hydrogen phosphate and deionized water is 13-15g: 16-22g: 4.4-5.2g: 80-110mL.

7. A composite carbon source according to claim 6, characterized in that, The preparation process of the montmorillonite intercalated iron-carbon powder in step two is as follows: Sodium-based montmorillonite and deionized water were added to a reaction vessel and stirred for 2-3 hours. Then, iron-carbon powder and a 6 wt% sodium hydroxide solution were mixed evenly and added to the reaction vessel and stirred for 5-6 hours. The mixture was then filtered, washed, dried, and ground to obtain montmorillonite-intercalated iron-carbon powder. The ratio of sodium-based montmorillonite, deionized water, iron-carbon powder, and sodium hydroxide solution is 16-22g: 500-700mL: 18-22g: 500-700mL.

8. A composite carbon source according to claim 1, characterized in that, The preparation process of the composite carbon source in step three is as follows: Deionized water, polyvinyl alcohol, and sodium alginate were added to a reaction vessel and heated at 500-700 r / min and 95-100℃ for 2-3 hours. After cooling to room temperature, polylactic acid and modified composite powder were added and stirred. The mixture was then poured into a mold and frozen at -20℃. After demolding, the mixture was placed in a saturated boric acid solution containing 5wt% calcium chloride and crosslinked at 0-4℃ for 22-24 hours. After filtration and washing, the composite carbon source was obtained.

9. A composite carbon source according to claim 8, characterized in that, The ratio of deionized water, polyvinyl alcohol, sodium alginate, polylactic acid, modified composite powder, and saturated boric acid solution containing 5 wt% calcium chloride is 100-150 mL: 8-10 g: 1-2 g: 24-26 g: 16-20 g: 400-500 mL.

10. The application of a composite carbon source according to any one of claims 1-9 in wastewater denitrification.