Biological detention pond for synergistically enhancing denitrification through iron circulation

Abstract

The biological detention pond comprises a pond body, and a water storage layer, a gravel layer, an aerobic layer, an organic carbon source layer, a submerging layer, a quartz sand transition layer and a pebble drainage layer are sequentially arranged in the pond body from top to bottom in the vertical direction; wherein the submerged layer is filled with a mixture of quartz sand, biochar and hematite; meanwhile, denitrifying sludge, iron reduction sludge and Fammox sludge are inoculated in the submerged layer; a perforated water collecting pipe is arranged in the pebble drainage layer, one end of the perforated water collecting pipe is communicated with the pebble drainage layer, and the other end of the perforated water collecting pipe is communicated with the water inlet end of the raised water outlet pipe; an overflow pool is further arranged on one side of the pool body, the water outlet end of the lifting water outlet pipe is communicated with the overflow pool, and water discharged from the overflow pool converges into a municipal rainwater pipe network; and an overflow pipe is arranged at the top end of the tank body and is communicated with the overflow tank.

Description

Technical Field

[0001] The present invention relates to the technical field of rainwater treatment, and particularly relates to a bioretention pond with iron cycle synergistically enhancing denitrification. Background Art

[0002] With the acceleration of the urbanization process, the problem of urban non-point source pollution has become increasingly prominent. Bioretention facilities are the most commonly used single facilities in the construction of sponge cities, which have good removal effects on SS, heavy metals, pathogens, etc., but have low removal efficiencies for dissolved nitrogen and phosphorus. Nitrate and DON (dissolved organic nitrogen) are the difficult pollutants restricting the improvement of the denitrification efficiency of bioretention facilities.

[0003] Since both natural filter media and nitrate carry negative charges, it is difficult to remove nitrate by electrostatic adsorption. At the same time, the lack of easily degradable organic carbon sources in surface rainfall runoff limits the removal of nitrate through microbial denitrification. Existing technologies have found that setting up a submerged area at the bottom of bioretention facilities and adding solid organic carbon sources can significantly increase the denitrification rate and nitrate removal rate. However, since the carbon release rate of solid organic carbon sources varies with the total carbon amount and operation time, it is impossible to accurately match the carbon release amount of solid organic carbon sources with the demand for carbon sources by heterotrophic denitrification, thus leading to the generation of secondary pollution. On the other hand, how to achieve the removal of DON in the submerged area is still a major difficulty in current research. DON cannot be rapidly removed by ion exchange, and at the same time, the biological metabolism of DON is blocked by dissolved oxygen and microbial activity. DON has a high hydrophilicity and is easily transported with water to the submerged area, but this area lacks sufficient dissolved oxygen and related functional flora, so it is difficult to achieve the biological metabolism of DON. Summary of the Invention

[0004] Aiming at the above deficiencies existing in the prior art, the purpose of the present invention is to provide a bioretention pond with iron cycle synergistically enhancing denitrification, which can ensure the nitrate removal efficiency while realizing the biological metabolism of DON in the submerged area, reduce the generation of by-products, and extend the service life of the packing, so as to solve the problem of poor removal effects of the prior art on nitrate and DON.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A biological retention pond for iron cycle synergistic enhanced denitrification, comprising a pond body. Inside the pond body, there are successively arranged a water storage layer, a gravel layer, an aerobic layer, an organic carbon source layer, a submerged layer, a quartz sand transition layer and a pebble drainage layer from top to bottom in the vertical direction; wherein, a mixture of quartz sand, biochar and hematite is filled in the submerged layer; at the same time, denitrifying sludge, iron-reducing sludge and Feammox sludge are inoculated in the submerged layer; a perforated collector pipe is arranged in the pebble drainage layer, one end of the perforated collector pipe is communicated with the pebble drainage layer, and the other end thereof is communicated with the water inlet end of a raised outlet pipe; an overflow pond is also arranged on one side of the pond body, the water outlet end of the raised outlet pipe is communicated with the overflow pond, and the water discharged from the overflow pond flows into the municipal rainwater pipe network; an overflow pipe is arranged at the top of the pond body, and the overflow pipe is communicated with the overflow pond.

[0007] Preferably, gravel with a particle size of 8 mm to 12 mm is laid in the gravel layer, and the laying thickness of the gravel layer is 50 mm.

[0008] Preferably, the aerobic layer is composed of quartz sand and zeolite, and the laying thickness of the aerobic layer is 250 to 300 mm; wherein, calculated by volume percentage, quartz sand with a particle size of 1 mm to 2 mm is 30% to 50%, zeolite with a particle size of 0.5 mm to 1 mm is 30% to 50%, quartz sand with a particle size of 0.425 mm to 0.85 mm is 10% to 20%, and quartz sand with a particle size of 0.125 mm to 0.175 mm is 4% to 10%.

[0009] Preferably, the organic carbon source layer is composed of wood chips and quartz sand, and the laying thickness of the organic carbon source layer is 150 to 250 mm; wherein, calculated by volume percentage, wood chips with a particle size of 2 mm to 6 mm are 5% to 15%, quartz sand with a particle size of 1 mm to 2 mm is 15% to 25%, quartz sand with a particle size of 0.425 mm to 0.85 mm is 40% to 50%, quartz sand with a particle size of 0.175 mm to 0.425 mm is 5% to 15%, and quartz sand with a particle size of 0.125 mm to 0.175 mm is 10% to 20%.

[0010] Preferably, the submerged layer is composed of coconut shell biochar, quartz sand and hematite, and the laying thickness of the submerged layer is 250-350 mm; among them, calculated by volume percentage, the coconut shell biochar with a particle size of 0.85 mm-1.70 mm is 5%-15%, the quartz sand with a particle size of 1 mm-2 mm is 20%-30%, the quartz sand with a particle size of 0.425 mm-0.85 mm is 40%-50%, the quartz sand with a particle size of 0.175 mm-0.425 mm is 5%-10%, and the quartz sand with a particle size of 0.125 mm-0.175 mm is 10%-15%; among them, after the hematite and Feammox sludge are evenly mixed, a layer is laid flat every 5 cm in the submerged layer; the iron-reducing sludge (mainly containing iron-reducing bacteria) and the denitrifying sludge (mainly containing denitrifying bacteria) are inoculated on the top layer of the submerged layer.

[0011] Preferably, the quartz sand transition layer is composed of quartz sand with different particle sizes, and the laying thickness of the quartz sand transition layer is 50 mm; among them, calculated by volume percentage, the quartz sand with a particle size of 1 mm-2 mm is 25%-35%, the quartz sand with a particle size of 0.425 mm-0.85 mm is 35%-45%, the quartz sand with a particle size of 0.175 mm-0.425 mm is 5%-15%, and the quartz sand with a particle size of 0.125 mm-0.175 mm is 15%-25%.

[0012] Preferably, the pebble drainage layer is filled with pebbles, the pebble particle size is 6 mm-9 mm, and the laying thickness of the pebble drainage layer is 100 mm.

[0013] Preferably, the water outlet end of the raised outlet pipe is raised to half of the thickness of the organic carbon source layer and is flush with it.

[0014] Preferably, the permeability coefficient of the aerobic layer is 400-600 mm / h; the permeability coefficient of the organic carbon source layer is 300-500 mm / h; the permeability coefficient of the submerged layer is 300-500 mm / h; the permeability coefficient of the quartz sand transition layer is not less than 250 mm / h.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. The present invention particularly uses biochar as a filler in the submerged layer. The interfacial reaction between iron ions and biochar can strengthen its adsorption of nitrate and organic carbon; at the same time, it can also provide binding sites for iron oxidation and reduction products, avoiding excessive precipitation of iron ions on the surface of microbial cells or being discharged with water; the redox ability of biochar itself will also participate in the iron cycle and electron regulation.

[0017] 2. The present invention utilizes wood chips in the organic carbon source layer to promote the removal of nitrate through heterotrophic denitrification. Meanwhile, the ammonia nitrogen generated by DON deamination can react with hematite and be converted into N2 and Fe(II) through the Feammox pathway. The NO in the influent x - can also react with the generated Fe(II) and be converted into nitrogen gas and Fe(III) through iron autotrophic denitrification, realizing the iron cycle.

[0018] 3. The present invention utilizes the interaction between biochar and microorganisms. As an electron transfer mediator, biochar can promote the utilization of iron by microorganisms and carry out heterotrophic denitrification and nitrogen removal using the adsorbed organic carbon. Meanwhile, the rich pore structure inside biochar can not only increase the active microbial biomass but also form different ecological niches to increase microbial diversity, promoting the iron cycle and the denitrification process.

[0019] 4. The present invention discovers that compared with the traditional layered structure that is prone to a sudden drop in the permeability coefficient due to the migration of fine particles, through the particle size gradient control (0.125 - 2 mm) of the quartz sand transition layer and the skeleton support effect of the submerged biochar layer, the permeability coefficient of each layer is stabilized at 300 - 500 mm / h, which is improved compared with the conventional system and can effectively avoid the risk of waterlogging. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic structural diagram of the bioretention pond for iron cycle synergistic denitrification of the present invention.

[0021] In the figure: pond body 1, water storage layer 2, gravel layer 3, aerobic layer 4, organic carbon source layer 5, submerged layer 6, quartz sand transition layer 7, pebble drainage layer 8, perforated collector pipe 9, elevated outlet pipe 10, overflow pond 11, rainwater pipe network 12, overflow pipe 13. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The present invention will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the present invention belong to the scope of protection of the present invention.

[0023] Unless otherwise specified in specific cases, the numerical ranges listed herein include the upper and lower limit values, as well as all integers and fractions within the range, rather than the specific values listed when defining the range.

[0024] I. A bioretention pond for iron cycle synergistic enhanced denitrification

[0025] The bioretention pond described in the present invention comprises a pond body. Inside the pond body, there are successively arranged, from top to bottom along the vertical direction, a water storage layer, a gravel layer, an aerobic layer, an organic carbon source layer, a submerged layer, a quartz sand transition layer and a pebble drainage layer. Among them, the aerobic layer is filled with quartz sand and zeolite materials in a graded mixture, the organic carbon source layer is filled with quartz sand and wood chips in a graded mixture, and the submerged layer is filled with quartz sand, biochar and hematite in a graded mixture. At the same time, denitrifying sludge, iron-reducing sludge and Feammox sludge are inoculated in the submerged layer. A perforated collector pipe is arranged in the pebble drainage layer. One end of the perforated collector pipe is communicated with the pebble drainage layer, and the other end thereof is communicated with the water inlet end of a raised outlet pipe. An overflow pond is also arranged on one side of the pond body. The water outlet end of the raised outlet pipe is communicated with the overflow pond, and the water discharged from the overflow pond flows into the municipal rainwater pipe network. An overflow pipe is arranged at the top of the pond body, and the overflow pipe is communicated with the overflow pond.

[0026] After in-depth research on traditional bioretention systems, the present invention found that, firstly, traditional bioretention systems mainly rely on carbon-source-driven heterotrophic denitrification, and the denitrification effect is poor when the C / N ratio is low. In response to this, the present invention conceives a multi-path denitrification synergy mechanism combining iron redox cycling, Feammox, and iron autotrophic denitrification to improve the denitrification efficiency under low C / N ratio conditions. Secondly, traditional bioretention systems mainly use iron materials as electron donors or for coprecipitation phosphorus removal, and iron ions are prone to loss or passivation. Therefore, the present invention conceives to stabilize the denitrification process through biochar, promote the circulation of iron redox in the submerged area of the bioretention pond, and improve the denitrification efficiency and long-term stability. Thirdly, traditional bioretention systems are mainly dominated by heterotrophic denitrifying bacteria, and the microbial population is relatively single. The DON migrated with water to the submerged area is difficult to be degraded by microorganisms. Therefore, the present invention adopts a composite microbial system, including heterotrophic denitrifying bacteria, iron-reducing bacteria, Feammox bacteria, sulfur-reducing bacteria, etc., to form an efficient and sustainable denitrification system. Finally, the fillers in traditional bioretention systems are prone to clogging and the water flow distribution is uneven, resulting in short-circuiting phenomena. The present invention conceives to optimize the hydraulic characteristics, adopt a reasonable filler particle gradation, improve the permeability, prevent clogging, and increase the hydraulic retention time. Among them, the denitrifying sludge, iron-reducing sludge, and Feammox sludge all come from the activated sludge of a sewage treatment plant. There are relatively abundant heterotrophic denitrifying bacteria and other bacteria in this activated sludge. However, the sludge is a mixture, and the microorganisms contained in the sludge at this time are a mixture of various bacteria, and the abundances of autotrophic denitrifying bacteria, Feammox functional bacteria, etc. in the sludge are relatively low. In order to increase the abundance of functional bacteria with a certain specific function in these mixed bacteria, it is necessary to enrich and culture the activated sludge. The qualified standard for enrichment culture is that the removal effect of a certain type of pollutant in the influent by the sludge after enrichment culture reaches more than 90%. For example, the denitrifying sludge is sourced from the sludge of a sewage treatment plant, and the sulfur autotrophic denitrifying bacteria in the sludge are enriched by adding substrates mainly composed of thiosulfate and nitrate, so as to increase the abundance of autotrophic denitrifying bacteria in the sludge, and it is used as denitrifying sludge after enrichment culture. The Feammox sludge is also sourced from the sludge of a sewage treatment plant, and the Feammox bacteria and iron autotrophic denitrifying bacteria are enriched by adding substrates mainly composed of hematite and ammonium chloride, so as to obtain Feammox sludge. The iron-reducing sludge is also sourced from the sludge of a sewage treatment plant, and the sulfur and iron-reducing bacteria are enriched by adding substrates mainly composed of sodium pyruvate, sulfate, and ferric chloride, so as to obtain iron-reducing sludge. Inoculating and domesticating various sludges in the bioretention pond can increase the abundance of functional bacteria in the bioretention pond, enabling them to survive and cooperate in denitrification in this environment.

[0027] On this basis, the present invention adopts graded mixed filling for the aerobic layer, organic carbon source layer and submerged layer, that is: fillers of different particle sizes are used for mixed filling, especially the preparation of the rainwater runoff treatment matrix is strictly controlled, otherwise it will cause various problems. Uneven proportions may cause facility collapse or particle leakage; too fine a particle size will reduce hydraulic performance and increase the risk of clogging; and too coarse a particle size may weaken the pollutant removal capacity, reduce water retention, and destroy the oxygen-deficient environment of the submerged area. Therefore, a reasonable ratio is crucial to ensure matrix stability, permeability and pollutant removal effects. Figure 1 As shown, the biological retention pond of the present invention includes a pond body, which includes an aquifer, a gravel layer, an aerobic layer, an organic carbon source layer, a submerged layer, a quartz sand transition layer and a pebble drainage layer from top to bottom. A perforated water collecting pipe is arranged in the pebble drainage layer, and the perforated water collecting pipe is connected to a raised water outlet pipe. The water outlet end of the raised water outlet pipe is raised to half the thickness of the organic carbon source layer and is flush with it. An overflow pipe is arranged at the top of the pond body. A diversion slope is arranged at the edge of the pond body mouth.

[0028] During the long-term operation of the retention pond of the present invention, when the treatment effect of the rainwater biological retention pond decreases, maintenance can be achieved by simply renovating the aerobic layer matrix and the organic carbon source layer matrix.

[0029] Assume that the ratio of the surface area of the bioretention pond described in the present invention to the catchment area is 1:20, and the comprehensive runoff coefficient is taken as 0.7. When the present invention deals with low-intensity rainfall (defined as rainfall not exceeding 14.9 mm within 12 hours), the water flow velocity is appropriate and the retention time is relatively long, which is conducive to the full contact between pollutants and fillers. The gravel layer plays a role in evenly distributing water, provides a good infiltration channel, and prevents short-circuit flow. In the aerobic layer, quartz sand and zeolite with reasonable particle grading preferentially remove pollutants such as suspended particles, ammonia nitrogen, and dissolved phosphorus. At the same time, nitrification occurs during the rainfall interval, that is, microorganisms convert the adsorbed ammonia nitrogen into nitrate nitrogen under aerobic conditions. The organic carbon source layer further removes some particulate pollutants through the combination of quartz sand and wood chips, and provides an organic carbon source to promote the metabolism of heterotrophic denitrifying bacteria and iron- and sulfur-reducing bacteria. The synergistic effect of hematite (Fe2O3) and Feammox bacteria inside the submerged layer can further achieve the removal of DON and its deamination products. At the same time, the synergistic effect of sulfur autotrophic denitrifying bacteria and iron autotrophic denitrifying bacteria further improves the denitrification efficiency and reduces the risk of water eutrophication. The quartz sand transition layer can protect the fillers in the submerged area and prevent leakage. The pebble drainage layer diverts the treated rainwater and prevents waterlogging. During heavy rainfall or even rainstorms, the treatment capacity of the bioretention pond is greatly tested. The setting of the water storage layer can ensure that the bioretention pond absorbs and retains part of the rainwater in a short time, reduces the peak flow of surface runoff, and reduces the flood risk. The quartz sand gradation of each layer is reasonable, ensuring a high permeability and simultaneously performing gradient filtration on pollutants. Materials such as coconut shell biochar enhance the adsorption capacity for organic pollutants and can effectively purify rainwater even under short-term high-flow conditions. When the rainfall intensity exceeds the infiltration and storage capacity of the retention pond, some rainwater may be discharged from the overflow outlet. However, since the initial pollutants have been reduced, the pollution of the discharged water to the downstream water body is relatively low.

[0030] In some embodiments of the present invention, gravel with a particle size of 8 mm to 12 mm is laid in the gravel layer, and the laying thickness of the gravel layer is 50 mm. In the present invention, the laying thickness refers to the height of the gravel layer laid, and the same applies to other treatment layers.

[0031] In some embodiments of the present invention, the aerobic layer is composed of quartz sand and zeolite, and the laying thickness of the aerobic layer is 250-300 mm; wherein, calculated by volume percentage, the volume percentage of quartz sand with a particle size of 1 mm-2 mm can be 30%, 35%, 40%, 45%, 50%, etc., and all ranges and sub-ranges between the above values; the volume percentage of zeolite with a particle size of 0.5 mm-1 mm can be 30%, 35%, 40%, 45%, 50%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.425 mm-0.85 mm can be 10%, 15%, 20%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.125 mm-0.175 mm can be 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc., and all ranges and sub-ranges between the above values; it should be understood that in the embodiments, any of the above ranges can be combined with any other range, as long as the sum of the volume percentages of quartz sand and zeolite with different particle sizes is 100%.

[0032] In some embodiments of the present invention, the organic carbon source layer is composed of wood chips and quartz sand, and the laying thickness of the organic carbon source layer is 150-250 mm; wherein, calculated by volume percentage, the volume percentage of wood chips with a particle size of 2 mm-6 mm can be 5%, 10%, 15%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 1 mm-2 mm can be 15%, 20%, 25%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.425 mm-0.85 mm can be 40%, 45%, 50%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.175 mm-0.425 mm can be 5%, 10%, 15%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.125 mm-0.175 mm can be 10%, 15%, 20%, etc., and all ranges and sub-ranges between the above values; it should be understood that in the embodiments, any of the above ranges can be combined with any other range, as long as the sum of the volume percentages of quartz sand and wood chips with different particle sizes is 100%.

[0033] In some embodiments of the present invention, the submerged layer is composed of coconut shell biochar, quartz sand and hematite, and the laying thickness of the submerged layer is 250-350 mm; wherein, calculated by volume percentage, the volume percentage of coconut shell biochar with a particle size of 0.85 mm to 1.70 mm can be 5%, 10%, 15%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 1 mm to 2 mm can be 20%, 25%, 30%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.425 mm to 0.85 mm can be 40%, 45%, 50%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.175 mm to 0.425 mm can be 5%, 6%, 7%, 8%, 9%, 10%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.125 mm to 0.175 mm can be 10%, 11%, 12%, 13%, 14%, 15%, etc., and all ranges and sub-ranges between the above values; wherein, after the hematite and Feammox sludge are evenly mixed, a layer is laid flat every 5 cm in the submerged layer; the iron-reducing bacteria and denitrifying bacteria are inoculated on the top layer of the submerged layer. It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range, as long as the sum of the volume percentages of coconut shell biochar, quartz sand and hematite with different particle sizes is 100%.

[0034] In some embodiments of the present invention, the quartz sand transition layer is composed of quartz sand with different particle sizes, and the laying thickness of the quartz sand transition layer is 50 mm; wherein, calculated by volume percentage, the volume percentage of quartz sand with a particle size of 1 mm to 2 mm can be 25%, 30%, 35%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.425 mm to 0.85 mm can be 35%, 40%, 45%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.175 mm to 0.425 mm can be 5%, 10%, 15%, etc., and all ranges and sub-ranges between the above values; the volume percentage of quartz sand with a particle size of 0.125 mm to 0.175 mm can be 15%, 20%, 25%, etc., and all ranges and sub-ranges between the above values; It should be understood that in the implementation scheme, any of the above ranges can be combined with any other range, as long as the sum of the volume percentages of quartz sand with different particle sizes is 100%.

[0035] In some embodiments of the present invention, the pebble drainage layer is filled with pebbles, the pebble particle size is 6 mm to 9 mm, and the laying thickness of the pebble drainage layer is 100 mm.

[0036] In some embodiments of the present invention, the permeability coefficient of the aerobic layer is 400 - 600 mm / h; the permeability coefficient of the organic carbon source layer is 300 - 500 mm / h; the permeability coefficient of the submerged layer is 300 - 500 mm / h; the permeability coefficient of the quartz sand transition layer is not less than 250 mm / h.

[0037] II. Examples and Comparative Examples

[0038] Example 1

[0039] In this example, the structure of the bioretention pond is as Figure 1 shown, where:

[0040] The gravel particle size in the gravel layer is 8 - 12 mm.

[0041] The height of the aerobic layer is 250 mm, which is composed of the following materials mixed by volume ratio: quartz sand (1 - 2 mm) 40%, zeolite (0.5 - 1 mm) 40%, quartz sand (0.425 - 0.85 mm) 15%, quartz sand (0.125 - 0.175 mm) 5%.

[0042] The height of the organic carbon source layer is 150 mm, which is composed of the following materials mixed by volume ratio: wood chips (2 - 6 mm) 10%, quartz sand (1 - 2 mm) 20%, quartz sand (0.425 - 0.85 mm) 45%, quartz sand (0.175 - 0.425 mm) 10%, quartz sand (0.125 - 0.175 mm) 15%.

[0043] The height of the submerged layer is 250 mm, which is composed of the following materials mixed by volume ratio: coconut shell biochar (0.85 - 1.70 mm) 10%, quartz sand (1 - 2 mm) 25%, quartz sand (0.425 - 0.85 mm) 45%, quartz sand (0.175 - 0.425 mm) 7%, quartz sand (0.125 - 0.175 mm) 13%. Among them, after hematite and Feammox bacteria are evenly mixed, they are laid flat in a layer every 5 cm in the submerged layer; sulfur - reducing bacteria and denitrifying bacteria mainly exist in the top layer of the submerged layer.

[0044] The quartz sand transition layer is composed of the following materials mixed by volume ratio: quartz sand (1 - 2 mm) 30%, quartz sand (0.425 - 0.85 mm) 40%, quartz sand (0.175 - 0.425 mm) 10%, quartz sand (0.125 - 0.175 mm) 20%.

[0045] The pebble particle size in the pebble drainage layer is 6 - 9 mm.

[0046] The bioretention pond in this implementation case was tested with a constant head permeameter, and the following hydraulic conductivities were obtained: the hydraulic conductivity of the aerobic layer was 418 - 448 mm / h; the hydraulic conductivity of the organic carbon source layer was 344.4 - 347.2 mm / h; the hydraulic conductivity of the submerged layer was 333.9 - 349.7 mm / h; the hydraulic conductivity of the quartz sand transition layer was 347.2 - 357 mm / h.

[0047] Comparative Example 1

[0048] Based on Example 1, it was improved. The differences were as follows: in Comparative Example 1, the organic carbon source layer was not set, and the submerged layer was not inoculated with denitrifying sludge, iron-reducing sludge, and Feammox sludge.

[0049] The specific structure of the bioretention pond in Comparative Example 1 was as follows:

[0050] The gravel size in the gravel layer was 8 - 12 mm.

[0051] The height of the aerobic layer was 300 mm, which was composed of the following materials by volume ratio: quartz sand (1 - 2 mm) 40%, zeolite (0.5 - 1 mm) 40%, quartz sand (0.425 - 0.85 mm) 15%, quartz sand (0.125 - 0.175 mm) 5%.

[0052] The height of the submerged layer was 300 mm, which was composed of the following materials by volume ratio: quartz sand (1 - 2 mm) 35%, quartz sand (0.425 - 0.85 mm) 45%, quartz sand (0.175 - 0.425 mm) 7%, quartz sand (0.125 - 0.175 mm) 13%.

[0053] The quartz sand transition layer was composed of the following materials by volume ratio: quartz sand (1 - 2 mm) 30%, quartz sand (0.425 - 0.85 mm) 40%, quartz sand (0.175 - 0.425 mm) 10%, quartz sand (0.125 - 0.175 mm) 20%.

[0054] The pebble size in the pebble drainage layer was 6 - 9 mm.

[0055] The above are the differences between Comparative Example 1 and Example 1, and other structures are exactly the same as those in Example 1.

[0056] Comparative Example 2

[0057] Based on Example 1, it was improved. The difference was that the organic carbon source layer was not set. The structure of the bioretention pond in Comparative Example 2 was as follows: arranged vertically from top to bottom in sequence:

[0058] The gravel size in the gravel layer was 8 - 12 mm.

[0059] The height of the aerobic layer is 300 mm and it is composed of the following materials mixed by volume ratio: 40% quartz sand (1 - 2 mm), 40% zeolite (0.5 - 1 mm), 15% quartz sand (0.425 - 0.85 mm), and 5% quartz sand (0.125 - 0.175 mm).

[0060] The height of the submerged layer is 300 mm and it is composed of the following materials mixed by volume ratio: 10% coconut shell biochar (0.85 - 1.70 mm), 25% quartz sand (1 - 2 mm), 45% quartz sand (0.425 - 0.85 mm), 7% quartz sand (0.175 - 0.425 mm), and 13% quartz sand (0.125 - 0.175 mm). Among them, after hematite and Feammox bacteria are evenly mixed, they are laid flat in a layer every 5 cm in the submerged layer; sulfur - reducing bacteria and denitrifying bacteria mainly exist in the top layer of the submerged layer.

[0061] The quartz sand transition layer is composed of the following materials mixed by volume ratio: 30% quartz sand (1 - 2 mm), 40% quartz sand (0.425 - 0.85 mm), 10% quartz sand (0.175 - 0.425 mm), and 20% quartz sand (0.125 - 0.175 mm).

[0062] The pebble particle size in the pebble drainage layer is 6 - 9 mm.

[0063] The above are the differences between Comparative Example 2 and Example 1, and other structures are exactly the same as those in Example 1.

[0064] The rainwater pollutant removal tests of Example 1, Comparative Example 1, and Comparative Example 2 were carried out. Assuming that the facility service area ratio is 1:20, the runoff coefficient is 0.7, the rainfall duration is 2 h, the rainfall amount is 12 mm, and rainfall is simulated once every three days and the removal efficiency of total nitrogen is measured. The test results of total nitrogen removal are shown in Table 1.

[0065] Table 1 Average total nitrogen removal rate of simulated runoff pollution (%)

[0066]

[0067] As can be seen from Table 1, the present invention significantly improves the total nitrogen removal rate. Example 1: It is overall stable, indicating that the system operates stably and reliably; the removal rate remains at a high level during long-term operation after rain (such as on March 25th and March 28th), indicating that the system has the ability to resist shock loads. Comparative Example 1: The removal rate is maintained at about 60%, without obvious fluctuations, but there is no further room for improvement. Comparative Example 2: The initial effect is close to that of Example 1, but it drops rapidly, indicating that the microorganisms or carbon source supply in the system cannot continuously maintain a high denitrification ability. Therefore, the biological retention pond described in the present invention optimizes the hydraulic structure through multi-path collaborative denitrification, jointly improving the denitrification efficiency and system adaptability. It not only has the advantages of high efficiency, low energy consumption, anti-blockage, and long service life, but also has good prospects for popularization and application.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Those of ordinary skill in the art should understand that any modifications or equivalent replacements made to the technical solutions of the present invention without departing from the purpose and scope of the present technical solution shall be covered by the scope of the claims of the present invention.

Claims

1. A bioretention pond for iron cycle synergistic enhanced denitrification, characterized in that, It includes a pool body (1). Inside the pool body, there are successively arranged a water storage layer (2), a gravel layer (3), an aerobic layer (4), an organic carbon source layer (5), a submerged layer (6), a quartz sand transition layer (7), and a pebble drainage layer (8) from top to bottom in the vertical direction; Among them, in the submerged layer, a mixture of quartz sand, biochar, and hematite is filled; at the same time, denitrifying sludge, iron-reducing sludge, and Feammox sludge are inoculated in the submerged layer; A perforated collecting pipe (9) is arranged in the pebble drainage layer. One end of the perforated collecting pipe is communicated with the pebble drainage layer, and the other end is communicated with the water inlet end of a raised outlet pipe (10); an overflow pool (11) is also arranged on one side of the pool body. The water outlet end of the raised outlet pipe is communicated with the overflow pool, and the water from the overflow pool flows into the municipal rainwater pipe network (12); an overflow pipe (13) is arranged at the top of the pool body, and the overflow pipe is communicated with the overflow pool.

2. The bioretention pool according to claim 1, wherein, The laying thickness of the gravel layer is 50 mm, and gravel with a particle size of 8 mm - 12 mm is laid inside.

3. The bioretention pond according to claim 1, wherein The laying thickness of the aerobic layer is 250 - 300 mm and is composed of quartz sand and zeolite; among them, calculated by volume percentage, quartz sand with a particle size of 1 mm - 2 mm is 30% - 50%, zeolite with a particle size of 0.5 mm - 1 mm is 30% - 50%, quartz sand with a particle size of 0.425 mm - 0.85 mm is 10% - 20%, and quartz sand with a particle size of 0.125 mm - 0.175 mm is 4% - 10%.

4. The bioretention pool according to claim 1, wherein The laying thickness of the organic carbon source layer is 150 - 250 mm and is composed of wood chips and quartz sand; among them, calculated by volume percentage, wood chips with a particle size of 2 mm - 6 mm are 5% - 15%, quartz sand with a particle size of 1 mm - 2 mm is 15% - 25%, quartz sand with a particle size of 0.425 mm - 0.85 mm is 40% - 50%, quartz sand with a particle size of 0.175 mm - 0.425 mm is 5% - 15%, and quartz sand with a particle size of 0.125 mm - 0.175 mm is 10% - 20%.

5. The bioretention pool according to claim 1, wherein, The laying thickness of the submerged layer is 250 - 350 mm and is composed of coconut shell biochar, quartz sand, and hematite; among them, calculated by volume percentage, coconut shell biochar with a particle size of 0.85 mm - 1.70 mm is 5% - 15%, quartz sand with a particle size of 1 mm - 2 mm is 20% - 30%, quartz sand with a particle size of 0.425 mm - 0.85 mm is 40% - 50%, quartz sand with a particle size of 0.175 mm - 0.425 mm is 5% - 10%, and quartz sand with a particle size of 0.125 mm - 0.175 mm is 10% - 15%; among them, after hematite and Feammox sludge are evenly mixed, they are laid flat every 5 cm in the submerged layer; iron-reducing sludge and denitrifying sludge are inoculated on the top layer of the submerged layer.

6. The bioretention pool according to claim 1, wherein The laying thickness of the quartz sand transition layer is 50 mm and it is composed of quartz sand with different particle sizes; among them, calculated by volume percentage, the quartz sand with a particle size of 1 mm to 2 mm is 25% to 35%, the quartz sand with a particle size of 0.425 mm to 0.85 mm is 35% to 45%, the quartz sand with a particle size of 0.175 mm to 0.425 mm is 5% to 15%, and the quartz sand with a particle size of 0.125 mm to 0.175 mm is 15% to 25%.

7. The bioretention pond according to claim 1, wherein, The laying thickness of the pebble drainage layer is 100 mm, and the pebbles filled therein have a particle size of 6 mm to 9 mm.

8. The bioretention pool according to claim 1, characterized in that, The water outlet end of the raised water outlet pipe is raised to half of the thickness of the organic carbon source layer and is flush with it.

9. The bioretention pond according to any one of claims 1 to 8, characterized in that, The permeability coefficient of the aerobic layer is 400 - 600 mm / h; the permeability coefficient of the organic carbon source layer is 300 - 500 mm / h; the permeability coefficient of the submerged layer is 300 - 500 mm / h; the permeability coefficient of the quartz sand transition layer is not less than 250 mm / h.

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