A sulfur-carbon-limestone-ceramic composite filler-based constructed wetland simultaneous nitrogen and phosphorus removal system and an application method thereof

Abstract

This invention discloses a simultaneous nitrogen and phosphorus removal system for constructed wetlands based on a sulfur-bone char-limestone-ceramsite composite packing material and its application method. It relates to the fields of wastewater ecological treatment and constructed wetland enhancement. This invention constructs a quaternary composite packing material system of sulfur-bone char-limestone-ceramsite, integrating three mechanisms: autotrophic denitrification, heterotrophic denitrification, and crystallization precipitation, to achieve efficient simultaneous removal of nitrogen and phosphorus from wastewater. In an anaerobic environment, sulfur acts as an electron donor to drive autotrophic denitrification, eliminating the need for an external carbon source. Hydroxyapatite in the bone char induces phosphorus formation and precipitation for phosphorus removal, and its residual organic matter further enhances heterotrophic denitrification. Ceramsite provides biofilm attachment sites to ensure synergistic reactions. Limestone replenishes alkalinity, neutralizes acidic substances, stabilizes the substrate pH to suit microbial activity, and optimizes packing porosity and hydraulic mass transfer efficiency. Furthermore, molten sulfur acts as a binder to prevent bone char pulverization and loss, improving packing stability and extending system operating cycles.

Description

Technical Field

[0001] This invention relates to the field of wastewater ecological treatment and constructed wetland enhancement technology, specifically to a simultaneous nitrogen and phosphorus removal system for constructed wetlands based on sulfur-bone char-limestone-ceramsite composite filler and its application method. Background Technology

[0002] With increasingly stringent requirements for rural sewage treatment and urban wastewater discharge standards, efficient nitrogen and phosphorus removal under low C / N ratio conditions has become a critical issue that urgently needs to be addressed. Constructed wetlands, with their advantages of low investment, ease of operation, and eco-friendliness, are widely used for wastewater purification. However, in the long term, the following major technical bottlenecks still exist: The scarcity of organic carbon sources leads to low nitrogen removal efficiency. Nitrate nitrogen removal in constructed wetlands primarily relies on heterotrophic denitrification, a process requiring sufficient organic carbon sources as electron donors. However, current advanced treatment of domestic wastewater in my country often results in a low carbon-to-nitrogen ratio (C / N), making it difficult to meet the demands of nitrogen removal reactions. Phosphorus removal relies on chemical agents or adsorption-saturated packing materials. Common phosphorus removal pathways include chemical precipitation (addition of iron and aluminum salts) and physical adsorption (zeolite, ceramsite, steel slag, etc.), but these suffer from high operating costs or difficulties in regeneration after adsorption saturation. Single packing materials have limited functionality, making it difficult to achieve synergistic nitrogen and phosphorus removal. Traditional packing materials such as ceramsite and gravel mainly provide physical support, with low chemical or biological activity, and cannot support multi-mechanism synergistic reactions.

[0003] In recent years, sulfur-driven autotrophic denitrification (SAD) technology has attracted widespread attention. This process uses sulfur as an electron donor and is mediated by sulfur-oxidizing bacteria (such as *Thiobacillus denitrificans*) to reduce nitrates to nitrogen gas, achieving denitrification under low-carbon or carbon-free conditions. Natural or modified pyrite compounds (Fe–S complexes, such as FeS and FeS2) have shown particularly good reactivity. However, natural pyrite minerals suffer from uneven particle distribution, slow dissolution rates, and low specific surface areas in wetland applications, limiting their reactivity in biofilm systems. Furthermore, if the sulfur oxidation products (SO4)... 2- Excessive accumulation may also lead to system acidification and sulfate contamination.

[0004] On the other hand, bone char has a structure similar to hydroxyapatite, which can lower the nucleation barrier and provides good phosphorus removal performance. It can also slowly release a small amount of organic carbon, providing supplementary electron donors for heterotrophic denitrification. However, bone char is prone to loss, has poor mechanical strength, and is not stable enough when used alone.

[0005] Therefore, there is an urgent need for a new type of wetland filler that combines mechanical strength, chemical activity, and biocompatibility to achieve the organic unity of sulfur autotrophic denitrification, bone char phosphorus removal, and slow carbon source release. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, this invention provides a simultaneous nitrogen and phosphorus removal system for constructed wetlands based on a sulfur-bone char-limestone-ceramsite composite filler and its application method. This system overcomes the defects of insufficient carbon source, filler adsorption saturation, and asynchronous nitrogen and phosphorus removal in existing constructed wetland nitrogen and phosphorus removal processes, and can achieve efficient synergistic removal of nitrogen and phosphorus without the addition of external carbon source.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A simultaneous nitrogen and phosphorus removal system for constructed wetlands based on sulfur-bone char-limestone-ceramsite composite filler is provided. The system is characterized by comprising a reactor, wherein the reactor is sequentially filled from bottom to top with a first gravel layer, a composite filler layer, and a second gravel layer; an inlet is provided at the bottom of the reactor, and an outlet is provided on the side wall of the reactor. The composite filler layer is composed of sulfur powder as an electron donor, bone char powder as a phosphorus removal agent and slow-release carbon source, limestone powder as a pH buffer, and porous ceramic particles as a biofilm carrier and framework. The sulfur powder, bone char powder, and limestone powder are uniformly coated on the surface of the porous ceramic particles by using molten sulfur as a binder, forming an integral spherical particle with an outer reactive layer encapsulating the internal biological carrier. This achieves a dynamic balance between the acid production process of sulfur autotrophic denitrification and the alkali-requiring process of bone char phosphorus removal on the same filler particle surface.

[0008] Furthermore, the mass fraction of the components in the composite filler is: 30% sulfur powder, 12-18% bone char powder, 7-13% limestone powder, and 45% ceramsite.

[0009] Furthermore, the particle size of the bone char powder and limestone powder is 100-200 mesh.

[0010] Furthermore, the ceramsite is a porous ceramsite with a particle size of 6-10 mm.

[0011] Furthermore, the composite filler is a spherical particle with a particle size of 6~12mm.

[0012] Furthermore, the specific method for preparing the composite filler is as follows: S1: Melt sulfur powder at 130℃, add bone char powder and limestone powder to the molten sulfur powder, and stir at 130℃ for 5~10 minutes to obtain composite dressing; S2: Add ceramsite to the composite dressing at 130℃, and continue stirring at 130℃ for 15 minutes to make the composite dressing evenly coat the ceramsite. S3: Cool the ceramic particles with the composite dressing uniformly coated on the surface at 20~25℃ to obtain the composite filler.

[0013] Furthermore, before adding ceramsite to the composite dressing, the ceramsite is cleaned and dried.

[0014] This invention also provides an application of a simultaneous nitrogen and phosphorus removal system for constructed wetlands based on a sulfur-bone char-limestone-ceramsite composite filler. Several such simultaneous nitrogen and phosphorus removal systems are set up, and urban sewage effluent flows through the system to simultaneously remove total nitrogen and total phosphorus from the sewage without the addition of external carbon sources or chemical phosphorus removal agents.

[0015] The beneficial effects of this invention are as follows: This invention constructs a quaternary composite packing system of sulfur-bone char-limestone-ceramsite, integrating three mechanisms: autotrophic denitrification, heterotrophic denitrification, and crystallization precipitation, to achieve efficient and simultaneous removal of nitrogen and phosphorus from wastewater. In an anaerobic environment, sulfur acts as an electron donor to drive autotrophic denitrification, eliminating the need for an external carbon source. Hydroxyapatite in the bone char induces phosphorus formation and precipitation for phosphorus removal, and its residual organic matter further enhances heterotrophic denitrification. Ceramsite provides biofilm attachment sites to ensure synergistic reactions. Limestone replenishes alkalinity, neutralizes acidic substances, stabilizes the substrate pH to suit microbial activity, and optimizes packing porosity and hydraulic mass transfer efficiency. Furthermore, molten sulfur acts as a binder to prevent bone char pulverization and loss, improving packing stability and extending system operating cycles. The system's components are widely available and inexpensive, requiring no external chemical reagents, and are easy to operate and maintain. The proportions can be adjusted according to water quality and treatment scale, making it suitable for deep treatment of rural domestic sewage and urban effluent. It exhibits strong resistance to shock loads and stable nitrogen and phosphorus removal rates over long-term operation. Meanwhile, the mixture rich in hydroxyapatite generated during the phosphorus removal process can be recycled as a slow-release phosphate fertilizer, achieving resource reuse and combining environmental and economic benefits. Compared with existing technologies, this invention solves the problems of traditional wetland carbon source dependence and system acidification, with better overall performance and broad application prospects. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the structure of the constructed wetland simultaneous nitrogen and phosphorus removal system in Embodiment 2 of the present invention; Figure 2 For the 45-day operation process of different systems in Example 2 Concentration change graph; Figure 3 For the 45-day operation process of different systems in Example 2 Concentration change graph; The components are: 1. Reactor; 2. First gravel layer; 3. Composite packing layer; 4. Second gravel layer; 5. Inlet; 6. Outlet. Detailed Implementation

[0017] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0018] Example 1 The composite filler was prepared using the following method: S1: Weigh 30 parts of industrial-grade sublimed sulfur powder (purity ≥98%) and place it in a 130℃ constant temperature oil bath reaction device for heating and melting; slowly add 100~200 mesh bone char powder and limestone powder that have been crushed and sieved to the molten sulfur, wherein the bone char powder is 12 parts and the limestone powder is 13 parts, stir for 10 minutes to make the components evenly dispersed, and obtain the composite dressing; S2: At 130℃, add washed and dried ceramsite with a particle size of 6~10mm to the composite dressing, and continue stirring at 130℃ for 15min to make the composite dressing evenly coat the ceramsite. S3: Cool the ceramic particles with a uniformly coated composite material at 20-25 degrees Celsius to obtain a composite filler with a particle size of 6-12 mm.

[0019] In specific implementation, the bone char powder and limestone powder added to the molten sulfur can also be: 15 parts bone char powder and 10 parts limestone powder; or 18 parts bone char powder and 7 parts limestone powder. Example 2 The composite packing material prepared in Example 1 was loaded into reactor 1 to prepare a simultaneous denitrification and phosphorus removal system for artificial wetlands. To facilitate the verification of denitrification and phosphorus removal, reactor 1 in this example is a cylindrical reactor 1 with an effective volume of 4L. 1L of gravel was filled from bottom to top as the first gravel layer 2, 2L of the composite packing material prepared in Example 1 was filled as the composite packing layer 3, and 1L of gravel was filled as the second gravel layer 4. In practice, reactor 1 can be any existing artificial wetland container, and several containers can be put into the pretreated water body for use.

[0020] The bottom of reactor 1 is equipped with an inlet 5. To achieve a realistic simulation of an artificial wetland, four outlets 6 are provided on the side wall of reactor 1. When using this denitrification and phosphorus removal system for simulation experiments, a subsurface flow type artificial wetland is simulated, and water samples are taken from the uppermost outlet for testing.

[0021] In the anaerobic environment of constructed wetland systems, sulfur, acting as an electron donor, is oxidized to sulfate by sulfur-autotrophic denitrifying bacteria, while nitrate is reduced to nitrogen gas, thus achieving denitrification under inorganic carbon source conditions. The chemical reactions are as follows: ; Bone char component hydroxyapatite induces a sufficient concentration of Ca in wastewater. 2+ Phosphorus reacts with calcium to form a sparingly soluble calcium phosphate precipitate, thus achieving phosphorus removal. The chemical reaction is as follows: ; Furthermore, the residual organic matter in bone char can slowly release carbon sources, further driving heterotrophic denitrification. Ceramsite, as a supporting matrix, provides biofilm attachment sites, allowing autotrophic and heterotrophic denitrification and precipitation to proceed synergistically within the same system, thus achieving simultaneous nitrogen and phosphorus removal. Limestone, as a pH adjuster, can replenish the system's alkalinity through dissolution and ionization, neutralizing the alkalinity generated during sulfur autotrophic denitrification, effectively alleviating system acidification and maintaining the pH of the constructed wetland matrix environment within a suitable range for microbial activity. This process not only ensures the metabolic activity of sulfur autotrophic denitrifying bacteria but also provides a stable chemical environment for bone char-induced calcium phosphate precipitation, promoting Ca2+ precipitation. 2+ The combination reaction with [the other compound] further enhances the simultaneous removal efficiency of nitrogen and phosphorus. Simultaneously, limestone particles dispersed in the pores of the packing enhance the pore connectivity of the matrix, optimize hydraulic mass transfer efficiency, and synergistically construct a fully functional composite packing system with sulfur, bone char, and ceramsite.

[0022] To verify the effectiveness of this system (sulfur-bone char-limestone-ceramsite), simulated urban wastewater with a COD of 60 mg / L was used. The concentration of the active ingredient was 16 mg / L, the total phosphorus (TP) was 4 mg / L, the hydraulic retention time was 24 h, and the system was operated continuously for 45 days at (22±1)℃. An anaerobic environment was maintained throughout the operation, and no external carbon sources or chemical agents were added.

[0023] Comparative examples were also set up, including: Gravel group: 4L of gravel was filled in reactor 1; Sulfur-bone char-ceramsite group: The difference between the composite packing in reactor 1 and Example 1 was that only 12 parts of bone char powder were added to the molten sulfur; Sulfur-limestone-ceramsite group: The difference between the composite packing in reactor 1 and Example 1 was that only 13 parts of limestone powder were added to the molten sulfur; Sulfur-ceramsite group: The difference between the composite packing in reactor 1 and Example 1 was that ceramsite was directly added to the molten sulfur; Bone char-ceramsite group: The difference between the composite packing in reactor 1 and Example 1 was that 12 parts of bone char powder were directly mixed with ceramsite.

[0024] For the water discharged from each group within 45 days Concentration and Concentration was measured. Concentration results as follows Figure 2 As shown, Concentration results as follows Figure 3 As shown, by Figure 2 and Figure 3It is evident that the sulfur-bone char-limestone-ceramsite composite filler prepared in this invention can achieve efficient and stable synergistic removal of nitrate nitrogen and phosphorus in constructed wetland systems without the addition of external organic carbon sources. In the effluent... The average concentration was below 0.05 mg / L, with an average removal rate exceeding 99.7%; the average TP removal rate was over 85%, significantly better than other composite packing groups. Simultaneously, no obvious hydrogen sulfide odor was detected during system operation, and the effluent pH remained between 7.2 and 7.9, indicating that the limestone effectively buffered the acid produced during sulfur autotrophic denitrification. Observation of the packing structure revealed that no granulation or pulverization occurred on the particle surface, maintaining its intact morphology and exhibiting good mechanical stability.

Claims

1. A simultaneous nitrogen and phosphorus removal system for constructed wetlands based on sulfur-bone char-limestone-ceramsite composite filler, characterized in that, The reactor includes a first gravel layer, a composite packing layer, and a second gravel layer, which are sequentially filled from bottom to top. The reactor has an inlet at the bottom and an outlet on the side wall. The composite filler layer is composed of sulfur powder as an electron donor, bone char powder as a phosphorus removal agent and slow-release carbon source, limestone powder as a pH buffer, and porous ceramic particles as a biofilm carrier and framework. The sulfur powder, bone char powder, and limestone powder are uniformly coated on the surface of the porous ceramic particles by using molten sulfur as a binder, forming an integral spherical particle with an outer reactive layer encapsulating the internal biological carrier. This achieves a dynamic balance between the acid production process of sulfur autotrophic denitrification and the alkali-requiring process of bone char phosphorus removal on the same filler particle surface.

2. The constructed wetland simultaneous nitrogen and phosphorus removal system based on sulfur-bone char-limestone-ceramsite composite filler according to claim 1, characterized in that, The mass fraction of the components in the composite filler is: 30% sulfur powder, 12-18% bone char powder, 7-13% limestone powder, and 45% ceramsite.

3. The constructed wetland simultaneous nitrogen and phosphorus removal system based on sulfur-bone char-limestone-ceramsite composite filler according to claim 2, characterized in that, The particle size of the bone char powder and limestone powder is 100-200 mesh.

4. The constructed wetland simultaneous nitrogen and phosphorus removal system based on sulfur-bone char-limestone-ceramsite composite filler according to claim 3, characterized in that, The ceramsite is a porous ceramsite with a particle size of 6-10 mm.

5. The constructed wetland simultaneous nitrogen and phosphorus removal system based on sulfur-bone char-limestone-ceramsite composite filler according to claim 4, characterized in that, The composite filler consists of spherical particles with a particle size of 6-12 mm.

6. The constructed wetland simultaneous nitrogen and phosphorus removal system based on sulfur-bone char-limestone-ceramsite composite filler according to claim 5, characterized in that, The specific method for preparing the composite filler is as follows: S1: Melt sulfur powder at 130℃, add bone char powder and limestone powder to the molten sulfur powder, and stir at 130℃ for 5~10 minutes to obtain composite dressing; S2: Add ceramsite to the composite dressing at 130℃, and continue stirring at 130℃ for 15 minutes to make the composite dressing evenly coat the ceramsite. S3: Cool the ceramic particles with the composite dressing uniformly coated on the surface at 20~25℃ to obtain the composite filler.

7. The constructed wetland simultaneous nitrogen and phosphorus removal system based on sulfur-bone char-limestone-ceramsite composite filler according to claim 6, characterized in that, Before adding ceramsite to the composite dressing, the ceramsite should be cleaned and dried.

8. The application of the constructed wetland simultaneous nitrogen and phosphorus removal system based on sulfur-bone char-limestone-ceramsite composite filler as described in any one of claims 1 to 6, characterized in that, Several artificial wetland simultaneous nitrogen and phosphorus removal systems are set up, and urban sewage effluent flows through the system to simultaneously remove total nitrogen and total phosphorus from the sewage without the addition of external carbon sources and chemical phosphorus removal agents.

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