Iron-based composite solid carbon source for denitrification of wastewater with low carbon-to-nitrogen ratio and its preparation method

Abstract

This invention discloses an iron-based composite solid-phase carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio and its preparation method. This invention relates to the field of wastewater treatment technology and aims to solve the problem that traditional mixed solid-phase carbon sources suffer from unstable carbon release rates under fluctuations in hydraulic retention time / hydraulic load, easily leading to a sharp drop in denitrification efficiency. The method involves co-dissolving polyvinyl alcohol and sodium alginate to form a cross-linking carrier solution; mixing zero-valent iron powder, PHBV powder, and natural cellulose powder and adding them to the carrier solution to form a homogeneous slurry; freezing the slurry and then solidifying it in an acidic cross-linking solution containing calcium ions; followed by washing and vacuum drying to obtain the iron-based composite solid-phase carbon source. The carbon source obtained by this invention has the characteristics of stable carbon release, high denitrification efficiency, and strong resistance to fluctuations in hydraulic load, making it particularly suitable for deep denitrification treatment of wastewater with a low carbon-to-nitrogen ratio.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio and its preparation method. Background Technology

[0002] With the large-scale discharge of wastewater effluent from wastewater treatment plants, eutrophication of water bodies is becoming increasingly serious. Nitrate nitrogen accounts for as much as 78%-95% of the effluent, making it a primary target for deep denitrification. However, the generally low carbon-to-nitrogen ratio in effluent severely restricts heterotrophic denitrification efficiency.

[0003] To address the problem of insufficient carbon sources, liquid carbon sources (such as methanol and sodium acetate) or solid carbon sources are often added. While liquid carbon sources are fast-acting, they suffer from difficulties in dosing control, high operating costs, and significant safety hazards. Solid carbon sources (such as natural cellulose and biodegradable polymers) can release carbon sources slowly, but each has obvious drawbacks: natural cellulose (such as straw and sawdust) is easily biodegradable, has a short carbon release cycle, and is prone to loss; synthetic biodegradable polymers (such as PHBV) are expensive, resulting in high costs when used alone. Therefore, the two are often mixed to balance cost and performance. However, traditional mixed solid carbon sources exhibit unstable carbon release rates under fluctuations in hydraulic retention time / hydraulic load, which can easily lead to a sharp drop in denitrification efficiency.

[0004] Zero-valent iron (ZVFe) can act as an electron donor to promote denitrification due to its low redox potential, and its corrosion products, Fe(II) / Fe(III), also have adsorption and co-precipitation effects. However, directly adding ZVFe powder to water bodies leads to rapid aggregation and oxidative deactivation, and it fails to provide the organic carbon source required by microorganisms. Therefore, it is necessary to propose an iron-based composite solid carbon source for denitrification of wastewater with low carbon-to-nitrogen ratios and its preparation method to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio and its preparation method, so as to solve the problem that the carbon release rate of traditional mixed solid carbon sources is unstable under fluctuations in hydraulic retention time / hydraulic load, which easily leads to a sharp drop in denitrification efficiency.

[0006] In a first aspect, the present invention provides a method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio, comprising the following steps: Step 1: Dissolve polyvinyl alcohol and sodium alginate in distilled water, heat and stir at 90℃-100℃ for 1-3 hours, and obtain a uniform PVA-SA mixed solution after cooling. Step 2: Mix zero-valent iron powder, PHBV powder and natural cellulose powder evenly to obtain a solid mixture; Step 3: Add the solid mixture to the PVA-SA mixed solution and stir thoroughly to form a uniform suspension slurry; Step 4: Inject the slurry into the mold and freeze it at -15℃ to -25℃ for 10 to 14 hours; after demolding, immerse the frozen blank in an acidic crosslinking liquid containing calcium ions and cure it at 2℃ to 10℃ for 20 to 28 hours; after taking it out, wash the surface to obtain wet composite carbon source particles. Step 5: Vacuum dry the wet composite carbon source particles at a temperature of 40℃-50℃ to constant weight to obtain an iron-based composite solid carbon source.

[0007] Furthermore, in step one, the degree of saponification of the polyvinyl alcohol is ≥99%, and the degree of polymerization is 1700-2000; the viscosity of the sodium alginate in a 1.0% aqueous solution at 20℃ is 80mPa·s-120mPa·s.

[0008] Further, in step one, the mass-to-volume ratio of polyvinyl alcohol, sodium alginate, and distilled water is (30-34g):(3-5g):400mL.

[0009] Furthermore, in step two, the particle size of both the PHBV powder and the natural cellulose powder is 150-250 mesh; the zero-valent iron powder is spherical with a particle size of 0.5μm-5μm.

[0010] Furthermore, in step two, the natural cellulose powder is selected from at least one of wheat straw powder, sawdust powder, and corn cob powder.

[0011] Furthermore, in step two, based on the total solid mass of the PVA-SA mixed solution, the amount of zero-valent iron powder added is 15wt%-25wt%, the amount of PHBV powder added is 10wt%-20wt%, and the amount of natural cellulose powder added is 15wt%-25wt%.

[0012] Furthermore, in step four, the acidic crosslinking solution containing calcium ions is a saturated boric acid solution containing 3wt%-5wt% calcium chloride.

[0013] Furthermore, in step four, the mold is a cubic mold, and the size of the carbon source particles after molding is from 0.5cm×0.5cm×0.5cm to 2cm×2cm×2cm.

[0014] Secondly, the present invention provides an iron-based composite solid carbon source prepared by the above-described method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio.

[0015] The beneficial effects of this invention are as follows: The iron-based composite solid carbon source for denitrification of low C / N ratio wastewater and its preparation method utilize zero-valent iron as an electron donor to promote denitrification, and its corrosion product Fe(II) acts as an electron shuttle, improving electron utilization efficiency and denitrification rate, and mitigating the inhibitory effect of nitrite accumulation on microorganisms. PHBV and natural cellulose provide a stable and slow-release organic carbon source. All three are encapsulated and fixed by PVA-SA hydrogel, forming a synergistic system of "electron-driven + slow-release carbon source," enabling the carbon source to maintain stable denitrification performance under dynamic hydraulic conditions and exhibiting strong resistance to fluctuations. This invention uses a dual curing process of freeze-molding and calcium ion crosslinking to form a hydrogel carrier with a three-dimensional network porous structure. This structure effectively fixes zero-valent iron and organic solid particles, preventing loss, and also regulates the dissolution rate of organic matter, significantly reducing the fluctuation range of chemical oxygen demand and dissolved organic carbon release compared to traditional solid carbon sources. This invention partially replaces expensive PHBV with relatively inexpensive natural cellulose and zero-valent iron, and utilizes environmentally friendly PVA and SA as molding carriers, significantly reducing raw material costs while ensuring performance. All components are non-toxic or biodegradable, posing no risk of secondary pollution. The product obtained by this invention is a regular solid granule, easy to add, recover, and replace, suitable for various wastewater treatment scenarios such as constructed wetlands, biofilters, and anaerobic reactors, and particularly suitable for deep denitrification treatment of wastewater with low carbon-to-nitrogen ratios. Attached Figure Description

[0016] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0017] Figure 1 This is a comparison chart showing the effect of total nitrogen removal rate as hydraulic retention time on the carbon sources prepared in the examples and comparative examples in a simulated constructed wetland.

[0018] Figure 2 The images show a comparison of the morphology of the iron-based composite carbon source prepared in Example 1 before and after leaching using scanning electron microscopes; where (a1)-(e1) are before leaching and (a2)-(e2) are after leaching. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. The technical solutions provided by various embodiments of this invention will be described in detail below with reference to the accompanying drawings.

[0020] This invention provides a method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio, comprising the following steps: Step 1: Dissolve polyvinyl alcohol and sodium alginate in distilled water, heat and stir at 90℃-100℃ for 1-3 hours, and then cool to obtain a homogeneous PVA-SA mixed solution.

[0021] Specifically, the degree of saponification of polyvinyl alcohol is ≥99%, and the degree of polymerization is 1700-2000; the viscosity of a 1.0% aqueous solution of sodium alginate at 20℃ is 80 mPa·s-120 mPa·s. The mass-volume ratio of polyvinyl alcohol, sodium alginate, and distilled water is (30-34 g): (3-5 g): 400 mL.

[0022] Step 2: Mix zero-valent iron powder, PHBV powder and natural cellulose powder evenly to obtain a solid mixture.

[0023] Specifically, the particle size of both PHBV powder and natural cellulose powder is 150-250 mesh; the zero-valent iron powder is spherical with a particle size of 0.5μm-5μm. The natural cellulose powder is selected from at least one of wheat straw powder, sawdust powder, and corn cob powder. Based on the total solid mass of the PVA-SA mixed solution, the addition amount of zero-valent iron powder is 15wt%-25wt%, the addition amount of PHBV powder is 10wt%-20wt%, and the addition amount of natural cellulose powder is 15wt%-25wt%.

[0024] Step 3: Add the solid mixture to the PVA-SA mixed solution and stir thoroughly to form a uniform suspension slurry.

[0025] Step 4: Inject the slurry into the mold and freeze it at -15℃ to -25℃ for 10 to 14 hours; after demolding, immerse the frozen preform in an acidic crosslinking liquid containing calcium ions and cure it at 2℃ to 10℃ for 20 to 28 hours; after removing it, wash the surface to obtain wet composite carbon source particles.

[0026] Specifically, the acidic crosslinking solution containing calcium ions is a saturated boric acid solution containing 3wt%-5wt% calcium chloride. The mold is a cubic mold, and the size of the carbon source particles after molding is from 0.5cm×0.5cm×0.5cm to 2cm×2cm×2cm.

[0027] Step 5: Vacuum dry the wet composite carbon source particles at a temperature of 40℃-50℃ to constant weight to obtain an iron-based composite solid carbon source.

[0028] The technical solution of the present invention will be further described below through several embodiments.

[0029] Example 1: Iron-based composite carbon source (wheat straw carrier, referred to as FPS).

[0030] Step 1: Weigh 32g of PVA with a saponification degree of 99% and a polymerization degree of 1800 and 4g of SA with a viscosity of 100mPa·s, add 400mL of distilled water, stir magnetically at 95℃ for 2 hours, and cool to room temperature to obtain a mixed solution of 8% PVA and 1% SA.

[0031] Step 2: Weigh out 20g of spherical zero-valent iron powder with a particle size of 0.5μm-5μm, 15g of 200-mesh PHBV powder, and 20g of 200-mesh wheat straw powder, and mix them evenly in a mortar.

[0032] Step 3: Gradually add the solid mixture from Step 2 to the PVA-SA solution from Step 1, and mechanically stir for 1 hour to form a uniform black slurry.

[0033] Step 4: Pour the slurry into a 1cm×1cm×1cm cube silicone mold and freeze at -20℃ for 12 hours. After demolding, immerse the frozen block in water at 4℃. -4% Soak in the crosslinking solution for 24 hours. After removal, rinse three times with ultrapure water to obtain wet carbon source particles.

[0034] Step 5: Place the wet particles in a vacuum drying oven at 45℃ and dry until constant weight to obtain the finished carbon source particles, then seal and store them.

[0035] Example 2: Iron-based composite carbon source (wood chip carrier, denoted as FPW).

[0036] The only difference between this embodiment and Embodiment 1 is that in step two, 20g of wheat straw powder is replaced with an equal mass of 200-mesh wood powder. All other steps and parameters are exactly the same.

[0037] Comparative Example 1: Pure PHBV carbon source (denoted as PHBV).

[0038] This comparative example uses only PHBV as the carbon source. 55g of 200-mesh PHBV powder was weighed and directly encapsulated and cured using the same steps one, three, four, and five as in Example 1, without adding zero-valent iron or natural cellulose.

[0039] Comparative Example 2: Iron-free composite carbon source (wheat straw carrier, denoted as PS).

[0040] The only difference between this comparative example and Example 1 is that in step 2, zero-valent iron powder is not added, and the amount of PHBV powder is increased to 35g, while the amount of wheat straw powder remains at 20g.

[0041] Comparative Example 3: Iron-free composite carbon source (wood chip carrier, denoted as PW).

[0042] The only difference between this comparative example and Example 2 is that in step 2, zero-valent iron powder is not added, and the amount of PHBV powder is increased to 35g, while the amount of wood chip powder remains at 20g.

[0043] Performance testing experiment.

[0044] Experimental setup: Six identical vertical flow constructed wetland simulation devices were constructed and labeled as Control (no external carbon source), PHBV, PS, PW, FPS, and FPW, respectively.

[0045] Carbon source addition: The corresponding carbon sources are added to the main reaction zones of the unit, with the initial C / N ratio controlled to be approximately 5. Calculations show the following amounts: PHBV group 1000g, PS group 200g, PW group 250g, FPS group 220g, FPW group 270g.

[0046] Operating conditions: The influent is simulated wastewater effluent from a wastewater treatment plant. The concentration of particulate matter (P<0.05) was 8.7 mg / L, and total nitrogen (TN) was 8.9 mg / L. After the system was started and stabilized, it was operated in three stages with hydraulic retention times of 3 days, 2 days, and 1 day, respectively. Samples were taken for testing after each stage had been running stably for a sufficient period of time.

[0047] Test indicators: Total nitrogen (TN) and nitrate nitrogen in effluent ), nitrite nitrogen ( ), ammonia nitrogen ( ) and total phosphorus (TP).

[0048] Experimental results and analysis.

[0049] Denitrification performance stability: such as Figure 1As shown, throughout the entire hydraulic retention time range, the FPS and FPW groups consistently maintained the highest (>85%) and most stable TN removal rates, with fluctuations of less than 10%. The PS and PW groups showed good denitrification effects (approximately 80%) at HRTs of 3 days and 2 days, but the removal rate significantly decreased (>15%) when the HRT was shortened to 1 day. The PHBV group performed the worst, with a significant decrease in removal rate when the HRT was shortened. The Control group had the lowest removal rate (24-30%). This indicates that the iron-based composite carbon source of this invention possesses excellent resistance to hydraulic load fluctuations.

[0050] Nitrite control: Nitrite levels appeared in all experimental groups when HRT was shortened. Accumulation was observed, but the cumulative concentrations in the FPS and FPW groups remained below 0.05 mg / L, significantly lower than other groups, with a maximum of 0.2 mg / L, demonstrating that the addition of zero-valent iron effectively inhibited the accumulation of nitrite.

[0051] Phosphorus removal effect: The FPS and FPW groups showed excellent phosphorus removal capabilities, with a TP removal rate of up to 98%-99%, which is attributed to the adsorption and precipitation of phosphate by the corrosion products of zero-valent iron.

[0052] Structural stability: Figure 2 SEM images showed that the surface of the FPS carbon source before use was dense and porous, with particles such as zero-valent iron firmly embedded; after use, the surface microbial film grew abundantly, but the carrier skeleton remained intact without obvious structural collapse, confirming its good physical and biological stability.

[0053] Carbon release characteristics: Static carbon release experiments on carbon sources show that the COD release curves of the FPS and FPW groups are stable, and the standard deviation of the daily release concentration fluctuation is reduced by about 55% compared with the PS / PW group, showing more stable sustained release performance.

[0054] Therefore, the iron-based composite solid carbon source provided by this invention, through unique component design and freeze-ion crosslinking process, successfully integrates the advantages of zero-valent iron and organic slow-release carbon source, and prepares a new type of wastewater treatment material with high denitrification efficiency, stable performance and reasonable cost, which has broad application prospects in the field of deep treatment of wastewater with low carbon-nitrogen ratio.

[0055] The above description is only a preferred embodiment of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio, characterized in that, Includes the following steps: Step 1: Dissolve polyvinyl alcohol and sodium alginate in distilled water, heat and stir at 90℃-100℃ for 1-3 hours, and obtain a uniform PVA-SA mixed solution after cooling. Step 2: Mix zero-valent iron powder, PHBV powder and natural cellulose powder evenly to obtain a solid mixture; Step 3: Add the solid mixture to the PVA-SA mixed solution and stir thoroughly to form a uniform suspension slurry; Step 4: Inject the slurry into the mold and freeze it at -15℃ to -25℃ for 10 to 14 hours; after demolding, immerse the frozen blank in an acidic crosslinking liquid containing calcium ions and cure it at 2℃ to 10℃ for 20 to 28 hours; after taking it out, wash the surface to obtain wet composite carbon source particles. Step 5: Vacuum dry the wet composite carbon source particles at a temperature of 40℃-50℃ to constant weight to obtain an iron-based composite solid carbon source.

2. The method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio according to claim 1, characterized in that, In step one, the degree of saponification of the polyvinyl alcohol is ≥99%, and the degree of polymerization is 1700-2000; the viscosity of the sodium alginate in a 1.0% aqueous solution at 20℃ is 80mPa·s-120mPa·s.

3. The method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio according to claim 1, characterized in that, In step one, the mass-to-volume ratio of polyvinyl alcohol, sodium alginate, and distilled water is (30-34g):(3-5g):400mL.

4. The method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio according to claim 1, characterized in that, In step two, the particle size of both the PHBV powder and the natural cellulose powder is 150-250 mesh; the zero-valent iron powder is spherical with a particle size of 0.5μm-5μm.

5. The method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio according to claim 1, characterized in that, In step two, the natural cellulose powder is selected from at least one of wheat straw powder, sawdust powder, and corn cob powder.

6. The method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio according to claim 1, characterized in that, In step two, based on the total solid mass of the PVA-SA mixed solution, the amount of zero-valent iron powder added is 15wt%-25wt%, the amount of PHBV powder added is 10wt%-20wt%, and the amount of natural cellulose powder added is 15wt%-25wt%.

7. The method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio according to claim 1, characterized in that, In step four, the acidic crosslinking solution containing calcium ions is a saturated boric acid solution containing 3wt%-5wt% calcium chloride.

8. The method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio according to claim 1, characterized in that, In step four, the mold is a cubic mold, and the size of the carbon source particles after molding is from 0.5cm×0.5cm×0.5cm to 2cm×2cm×2cm.

9. An iron-based composite solid carbon source prepared by the method for preparing an iron-based composite solid carbon source for denitrification of wastewater with a low carbon-to-nitrogen ratio as described in any one of claims 1 to 8.

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