Multifunctional bioretention pond for enhancing nitrogen and phosphorus removal

Abstract

The invention belongs to the technical field of rainwater treatment, and relates to a multifunctional bioretention pond for enhancing nitrogen and phosphorus removal, a water accumulation layer, a covering layer, a planting layer, a medium layer, a submerging layer and a drainage layer are sequentially distributed in the pond from top to bottom, the drainage layer is provided with a drainage pipe, and a water outlet of the drainage pipe is flush with the top of the submerging layer; the submerged layer contains modified wood chips and scrap iron which are filled in a layered manner, the modified wood chips are obtained by grafting reaction of wood chips through a functional copolymer, and the use amount of the functional copolymer is 6-10 wt% of the wood chips; the functional copolymer is obtained by carrying out a copolymerization reaction on a pyridyl-containing acrylic acid monomer and a double-bond-containing acyl halide monomer according to a molar ratio of 1: (0.1-0.3). Through wood chip modification and filling structure improvement, a submerged layer of the bioretention pond is filled with modified wood chips and scrap iron in a layered manner, so that the bioretention pond has the effect of enhancing nitrogen and phosphorus removal, and meanwhile, the removal effect on heavy metal pollutants is also greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of rainwater treatment, and in particular relates to a multifunctional biological retention pond for enhancing nitrogen and phosphorus removal. Background Art

[0002] Due to global climate change and the intensification of urbanization, the problem of urban rainwater runoff pollution is becoming increasingly serious. The large amount of nitrogen, phosphorus and other nutrients and heavy metal pollution in urban rainwater runoff can easily lead to eutrophication of water bodies and ecosystem degradation. Bioretention ponds are the most commonly used low-impact development (LID) technology in sponge city construction. Through the joint action of plants, fillers and microorganisms, they intercept, adsorb and degrade pollutants in urban rainwater runoff, integrating important functions such as storage and purification. However, the structure of general bioretention ponds is unreasonable, the filler function is single, and the denitrification and phosphorus removal efficiency is low, that is, they cannot effectively remove ammonia nitrogen (NH4 + -N), nitrate (NO3-N), phosphate (PO4 3- -P) and other soluble pollutants.

[0003] In existing research, the denitrification and phosphorus removal capabilities of bioretention ponds are generally improved through structural improvements or filler improvements. Structural improvements usually achieve functional zoning of aerobic and anoxic zones within the facility by raising the height of the outlet pipe, providing a suitable growth environment for nitrifying and denitrifying bacteria, thereby optimizing the removal of dissolved pollutants. Filler improvements are generally achieved by adding functional fillers, such as vermiculite, zeolite, volcanic rock, and other highly adsorbent media with abundant pores to achieve rapid adsorption of organic matter and ammonia nitrogen, or by adding organic or inorganic electron donors such as sawdust and sulfur to promote heterotrophic or autotrophic denitrification processes in bioretention ponds to remove nitrate and nitrogen. Some studies have also promoted the flocculation and precipitation of dissolved phosphorus by adding aluminum-based or iron-based water treatment residues. However, most fillers can only achieve single pollutant removal and are generally mixed filling structures. For example, the high performance of bioretention columns filled with biochar and pyrite has been confirmed (Kong Z et al., Water Research, 2021, 206:117737). However, when the bioretention column is affected by changes in the rainwater environment (for example, pollutant concentration, previous dry period, rainfall intensity, rainfall duration, etc.), its performance cannot be guaranteed. In addition, pyrite is prone to leaching of sulfate and iron ions in large quantities during the dry period before rainfall, becoming new pollutants. In addition, the addition of solid carbon sources poses the risk of organic matter leakage and facility blockage, which can easily cause secondary pollution of water bodies and increase facility operation and maintenance costs. Summary of the Invention

[0004] In view of the fact that the pollutant removal effect, especially the denitrification and phosphorus removal effect, of the biological retention pond in the prior art is not ideal and needs to be further improved, the present invention provides a multifunctional biological retention pond with enhanced denitrification and phosphorus removal. It utilizes simple and easily available materials such as sawdust, iron filings, gravel, zeolite and sand, and through sawdust modification and improved filling structure, that is, modified sawdust and iron filings are layered and filled in the submerged layer of the biological retention pond, so that the biological retention pond has the effect of enhanced denitrification and phosphorus removal, and at the same time, the removal effect of heavy metal pollutants is greatly improved, thereby realizing the control of rainwater runoff pollution from the source, which is of great significance to the future sponge city construction.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A multifunctional bioretention pond for enhanced nitrogen and phosphorus removal comprises a water accumulation layer, a covering layer, a planting layer, a medium layer, a submerged layer and a drainage layer arranged in sequence from top to bottom in the pond. The drainage layer is provided with a drainage pipe, and the outlet of the drainage pipe is flush with the top of the submerged layer. The submerged layer contains modified sawdust and iron chips filled in layers. The modified sawdust is obtained by grafting a functional copolymer onto sawdust, and the amount of the functional copolymer is 6-10 wt% of the sawdust. The functional copolymer is obtained by copolymerizing a pyridine-containing acrylic monomer and a double-bond-containing acyl halide monomer in a molar ratio of 1:(0.1-0.3).

[0007] The present invention obtains a functional copolymer rich in pyridine groups, acyl halide groups, and carboxylic acid groups by copolymerizing a pyridine-containing acrylic monomer and a double-bond acyl halide monomer in a specific ratio. The functional copolymer is then used to graft-modify wood chips. During the graft modification, the active acyl halide groups in the functional copolymer react with the hydroxyl groups in the wood chips to form ester groups, thereby introducing the copolymer containing pyridine groups and rich carboxylic acid groups into the side chains of the wood chips. When the modified wood chips are used as fillers in bioretention ponds, the rich carboxylic acid groups in the wood chip graft chains have a complexing effect on heavy metals in rainwater, thereby "adsorbing" and "retaining" heavy metal pollutants. At the same time, the pyridine groups on the graft chains are large π-bonded heterocyclic structures that can interact with the benzene rings in the wood chips, weakening the hydrogen bonding between or within the cellulose molecules in the wood chips and increasing the spatial volume of the cellulose, thereby increasing the looseness of the wood chips, avoiding agglomeration and clogging, and further improving the water permeability of the wood chips, extending the hydraulic retention time, and allowing more time for pollutants to be adsorbed. The present invention then fills the above modified sawdust and iron chips in layers in the submerged layer. The iron chips are the iron source required by the autotrophic denitrifying bacteria, and the modified sawdust is the organic carbon source required by the heterotrophic denitrifying microorganisms. The two play a coupling role to achieve efficient denitrification and denitrification, effectively treating the nitrate nitrogen (NO3 - -N) and nitrite nitrogen (NO2 - -N); while the by-product of iron filings Fe 3+ / Fe 2+It further adsorbs the remaining phosphates to further remove phosphorus. In addition, the modified sawdust can not only promote the rapid proliferation and microbial activity of auto-oxygen denitrifying bacteria, but also reduce the by-product Fe 3+ / Fe 2+ Through research, the inventors found that layering modified sawdust and iron chips in the submerged layer was more effective than mixing them together. This is likely due to the synergistic effect of layered filling and the continuous reaction. The aforementioned sawdust modification and layered loading of modified sawdust and iron chips in the submerged layer, combined with the other layers in the bioretention tank, achieved highly efficient pollutant removal.

[0008] Preferably, the functional copolymer is used in an amount of 6-8 wt % of the wood chips. If the functional copolymer is used too little, the wood chip modification effect is limited; if the functional copolymer is used too much, an overly cross-linked network structure is formed, which in turn reduces the looseness of the wood chips. Furthermore, excessive use of the functional copolymer increases costs.

[0009] Preferably, the molar ratio of the pyridyl-containing acrylic monomer to the double-bond-containing acyl halide monomer is 1:(0.2-0.3). If the amount of the double-bond-containing acyl halide monomer is too low, the functional copolymer will contain a small amount of acyl halide groups, resulting in fewer grafting sites during the grafting modification of sawdust, and the grafting modification effect will be limited. If the amount of the double-bond-containing acyl halide monomer is too high, the amount of acrylic monomer is relatively low, and the pyridyl and carboxylic acid groups in the functional copolymer will be limited, resulting in limited improvement in the looseness and water permeability of the sawdust, and insufficient ability to "adsorb" and "retain" heavy metal pollutants. Therefore, the molar ratio of the pyridyl-containing acrylic monomer to the double-bond-containing acyl halide monomer must be controlled within the aforementioned range.

[0010] Furthermore, the pyridyl-containing acrylic monomer is selected from at least one of 3-(2-pyridyl)acrylic acid, 3-(3-pyridyl)acrylic acid, and 3-(4-pyridyl)acrylic acid; and the double-bond-containing acyl halide monomer is selected from at least one of acryloyl chloride, methacryloyl chloride, β-phenylacryloyl chloride, and 2-butenoic acid chloride.

[0011] Furthermore, the functional copolymer is prepared by the following preparation method: dissolving a pyridine-containing acrylic monomer, a double-bond-containing acyl halide monomer, and an initiator in a solvent inert to the acyl halide, then reacting at 60-80° C. for 3-5 hours, cooling to obtain a suspension, and then filtering and drying the suspension to obtain the functional copolymer.

[0012] Furthermore, the initiator is at least one of azobisisobutyronitrile and benzoyl peroxide, and the amount of the initiator used is 2-3% of the total weight of the monomers. The amount of the initiator used affects the molecular weight of the functional copolymer. In the present invention, the amount of the initiator is controlled within the above range. If the molecular weight is too small, the graft chains introduced onto the sawdust during subsequent graft modification are too short, resulting in limited grafting effect. If the molecular weight is too large, there are many entanglement points between the molecular chains, which affects the mobility of the molecular chains and ultimately affects the adsorption capacity of the modified sawdust.

[0013] Furthermore, the solvent inert to acyl halides is tetrahydrofuran (THF). The acyl halide monomer containing a double bond has a greater reaction activity due to the presence of the acyl halide group. In order to reduce side reactions, a solvent inert to acyl halides needs to be selected; the drying is vacuum drying at 60-80° C. for 12-24 hours.

[0014] Furthermore, the modified sawdust is prepared by the following steps: mixing sawdust and a functional copolymer and subjecting them to a grafting reaction at 75-100°C under a vacuum environment for 2-4 hours. The acyl halide groups in the functional copolymer react with hydroxyl groups in the sawdust to form ester groups, thereby introducing the polycarboxylic acid polymer into the side chains of the sawdust.

[0015] Furthermore, the moisture content of the sawdust is ≤5%; the sawdust is pine sawdust with an average particle size of 2mm to 10mm. The pine sawdust has a certain toughness and can provide organic nutrients for a long time without leaching excessive dissolved organic carbon (DOC); the vacuum environment has a vacuum degree of 0.05 to 0.1Mpa; the grafting reaction is carried out in a high-speed mixer, and the rotation speed of the high-speed mixer is 150 to 250rpm.

[0016] Furthermore, the submerged layer is filled in layers as follows: the upper layer is filled with a mixture of modified sawdust, sand and gravel, and the lower layer is filled with a mixture of iron chips, sand and gravel; or the upper layer is filled with a mixture of iron chips, sand and gravel, and the lower layer is filled with a mixture of modified sawdust, sand and gravel; the upper and lower layers are filled so that the volume ratio of modified sawdust to iron chips is 1:(0.8-1.2), such as 1:0.8, 1:1 or 1:1.2; preferably, the submerged layer is filled in layers as follows: the upper layer is filled with a mixture of modified sawdust, sand and gravel, and the lower layer is filled with a mixture of iron chips, sand and gravel. The inventors have found through research that the denitrification and phosphorus removal effect of modified sawdust and iron chips in the submerged layer is better than that of a mixed filling of the two. The possible reason is that the layered filling has a synergistic effect between the upper and lower layers, and the reaction is continuous. For example, when the upper layer is filled with modified sawdust and the lower layer is filled with iron filings, heterotrophic denitrification mainly occurs in the upper layer, while autotrophic denitrification mainly occurs in the lower layer. At the same time, the nutrients produced by the decomposition of sawdust continuously flowing down from the upper layer promote the autotrophic denitrification and the heterotrophic reduction of nitrate to ammonium (DNRA) and anaerobic ammonia oxidation (Anammox) in the lower layer, while the autotrophic denitrification and DNRA in the lower layer further remove NO3 - Anammox can also remove the byproduct NO2 produced in the upper layer during heavy rainfall. - , while controlling NH4 + The reaction in the entire flooded area is continuous, so that the concentration of nitrogen pollutants in the effluent is very low. For example, when the upper layer is filled with iron chips and the lower layer is filled with modified sawdust, the upper layer mainly undergoes autotrophic denitrification and zero-valent iron / Fe 2+ Mediated redox reaction, heterotrophic denitrification mainly occurs in the lower layer; Fe 2+ Acting as an electron donor, it promotes heterotrophic denitrification in the lower layer and can significantly remove NO3 - The reaction in the entire submerged area is continuous. However, when the modified sawdust and iron chips are mixed and filled in the submerged layer, the sawdust and zero-valent iron are widely distributed in the submerged area, lacking the reaction continuity and synergy between the upper and lower layers.

[0017] Furthermore, the average particle size of the iron filings is 2-8 mm, the average particle size of the sand is 0.4-0.8 mm, and the average particle size of the gravel is 5-10 mm.

[0018] Furthermore, in the layered filling, the volume proportions of modified wood chips, sand, and gravel are 5-25%, 25-35%, and 45-55%, respectively, and the sum of the volume proportions of the three is 100%; the volume proportions of iron chips, sand, and gravel are 15-25%, 25-35%, and 45-55%, respectively, and the sum of the volume proportions of the three is 100%.

[0019] Furthermore, the height of the water accumulation layer is 100 to 150 mm, and the water accumulation layer provides a certain storage space for rainwater that does not have time to seep in during short-term heavy rainfall.

[0020] Furthermore, the height of the covering layer is 50-60 mm, and the covering layer is filled with gravel, and the average particle size of the gravel is 4-8 mm. The covering layer provides protection for the lower system, blocks larger suspended matter in the runoff, and reduces water evaporation from the planting layer.

[0021] Furthermore, the planting layer is 100-120 mm high and is filled with a mixture of loam and sand. The average loam particle size is 0.002-0.2 mm, and the average sand particle size is 0.4-0.8 mm. The volume proportions of loam and sand are 35-45% and 55-65%, respectively, for a total volume proportion of 100%. Perennial grasses, such as ryegrass, bermudagrass, bahiagrass, and sedge of paradise, are planted in the planting layer. These plants are not only aesthetically pleasing but also serve to purify water. The addition of loam to the planting layer reduces macropores, optimizes pore uniformity and pore structure, and prolongs hydraulic retention time. The planting layer also provides a habitat for plant root growth and absorbs and intercepts particulate organic nutrients.

[0022] Furthermore, the height of the medium layer is 150-180 mm, and the medium layer is filled with a mixture of sand and zeolite, with an average particle size of 0.4-0.8 mm for the sand and 2-6 mm for the zeolite; the volume proportions of the sand and zeolite are 15-20% and 75-85% respectively, and the sum of the volume proportions of the two is 100%. The medium layer is rich in pores and can absorb dissolved nutrients, such as ammonia nitrogen (NH4 + -N), phosphate (PO4 3- -P) etc.

[0023] Furthermore, the height of the submerged layer is 300-400 mm. The outlet of the drainage pipe in the lower drainage layer is flush with the top of the submerged layer, so that the pores of the submerged layer are in a saturated state for a long time, thereby creating anaerobic conditions. The addition of modified sawdust and iron chips to the submerged layer provides conditions for the rapid growth of microbial communities. The combination of the two promotes reactions such as denitrification, anaerobic ammonia oxidation, and nitrate reduction to ammonium, as well as the byproduct of iron chips, Fe 3+ / Fe 2+ It can further intercept the remaining phosphates, that is, the coupling effect of modified sawdust and iron chips improves the denitrification and phosphorus removal rate, while reducing the by-product Fe 3+ / Fe 2+ The outflow.

[0024] Furthermore, the drainage layer is 50-60 mm high and filled with gravel, with an average particle size of 5-20 mm. Rainwater that percolates through the aforementioned layers is ultimately discharged through a drainage pipe in the drainage layer. The height of the drainage pipe outlet in the drainage layer is critical. In the present invention, the water outlet of the drainage pipe is flush with the top of the submerged layer, thereby creating anaerobic conditions.

[0025] In a second aspect, the present invention also provides a method for constructing the above-mentioned multifunctional bioretention pond, comprising the following steps: laying a drainage layer, a flooding layer, a medium layer, a planting layer, and a covering layer of fillers in the bioretention pond in sequence, and then planting plants on the planting layer.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. The present invention constructs an improved bioretention pond filled with modified wood chips and iron chips in layers, which significantly improves the removal efficiency of nitrogen and phosphorus. The preferred embodiment has a good effect on the removal of NH4 + -N and NO3 - -N removal rates can reach 89% and 90% respectively, phosphorus (PO4 3- -P) removal rate is as high as 95%; the removal rate of heavy metals is also improved. Even under high pollutant concentrations and during the previous drought period, the removal rate of nitrogen and phosphorus is still high.

[0028] 2. Compared with traditional bioretention ponds, the multifunctional bioretention pond of the present invention can handle higher concentrations of pollution, cope with more complex hydrological conditions (preliminary drought period, rainfall, rainfall intensity, rainfall duration, etc.), and has more stable performance during operation.

[0029] 3. The wood chips and iron chips used in the present invention are agricultural or industrial wastes, which can be turned into treasures to give full play to the surplus value of the materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the structural device of the multifunctional biological retention pond of Example 1, Example 5 and Comparative Example 4. DETAILED DESCRIPTION

[0031] The present invention will be described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0032] The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials are commercially available unless otherwise specified.

[0033] Preparation of modified sawdust

[0034] Preparation Example 1

[0035] 1) A mixture of 3-(2-pyridyl)acrylic acid and acryloyl chloride in a molar ratio of 1:0.1 was prepared to a total mass of 1000 g, and the mixture and 25 g of benzoyl peroxide (BPO) were dissolved in tetrahydrofuran (THF), followed by stirring at 70° C. for 4 h. After cooling, a suspension was obtained, which was filtered and vacuum-dried at 60° C. for 24 h to obtain a functional copolymer;

[0036] 2) Dry pine sawdust in a dryer (scraper speed 80 rpm) at 80°C for 24 h (moisture content is about 4.2%). Then, 10 kg of dried pine sawdust (particle size about 4-6 mm) and 600 g of the functional copolymer are placed in a vacuum high mixer with a vacuum degree set to 0.08 MPa. The mixture is reacted at 85°C and a speed of 180 rpm for 3 h. After cooling, the modified sawdust is obtained.

[0037] Preparation Example 2

[0038] The rest is the same as Preparation Example 1, except that: in step 1), β-phenylacryloyl chloride is used instead of acryloyl chloride, and the molar ratio of 3-(2-pyridyl)acrylic acid and β-phenylacryloyl chloride is 1:0.2; specifically:

[0039] 1) A mixture of 3-(2-pyridyl)acrylic acid and β-phenylacryloyl chloride in a molar ratio of 1:0.2 was prepared to a total mass of 1000 g, and the mixture and 30 g of BPO were dissolved in THF, followed by stirring at 75° C. for 4 hours; after cooling, a suspension was obtained, which was filtered and vacuum dried at 70° C. for 24 hours to obtain a functional copolymer;

[0040] 2) Same as Example 1.

[0041] Preparation Example 3

[0042] The rest is the same as Preparation Example 1, except that: in step 1), the molar ratio of 3-(2-pyridyl)acrylic acid and acryloyl chloride is 1:0.3; in step 2), the amount of the functional copolymer is 8 wt% of the sawdust; specifically:

[0043] 1) A mixture of 3-(2-pyridyl)acrylic acid and acryloyl chloride in a molar ratio of 1:0.3 was prepared to a total mass of 1000 g, and the mixture and 30 g of BPO were dissolved in THF, followed by stirring at 70° C. for 4 hours. After cooling, a suspension was obtained, which was filtered and vacuum-dried at 60° C. for 24 hours to obtain a functional copolymer;

[0044] 2) Dry the pine sawdust at 80°C for 24 h in a dryer (copy board speed 80 rpm), then place 10 kg of the dried pine sawdust and 800 g of the functional copolymer in a vacuum high mixer with a vacuum degree set to 0.08 MPa, and react at 85°C and a speed of 180 rpm for 3.5 h. After cooling, the modified sawdust is obtained.

[0045] Preparation Example 4

[0046] The rest is the same as Preparation Example 1, except that: in step 1), the molar ratio of 3-(2-pyridyl)acrylic acid to acryloyl chloride is 1:0.3; in step 2), the mass of the functional copolymer is 10 wt% of the sawdust; specifically:

[0047] 1) A mixture of 3-(2-pyridyl)acrylic acid and acryloyl chloride in a molar ratio of 1:0.3 was prepared to a total mass of 1000 g, and the mixture and 30 g of BPO were dissolved in THF, followed by stirring at 70° C. for 4 hours. After cooling, a suspension was obtained, which was filtered and vacuum-dried at 60° C. for 24 hours to obtain a functional copolymer;

[0048] 2) Dry the pine sawdust at 80°C for 24 h in a dryer (copy board speed 80 rpm), then place 10 kg of the dried pine sawdust and 800 g of the functional copolymer in a vacuum high mixer with a vacuum degree set to 0.08 MPa, and react at 85°C and a speed of 180 rpm for 3.5 h. After cooling, the modified sawdust is obtained.

[0049] Comparative Preparation Example 1

[0050] The rest is the same as Preparation Example 1, except that: in step 1), the molar ratio of 3-(2-pyridyl)acrylic acid and acryloyl chloride is 1:0.4.

[0051] Comparative Preparation Example 2

[0052] The rest is the same as Preparation Example 1, except that in step 1), acrylic acid is used instead of 3-(2-pyridyl)acrylic acid.

[0053] Example 1

[0054] A multifunctional bioretention pond for enhanced nitrogen and phosphorus removal was constructed using organic glass columns with an inner diameter of 10 cm and a height of 80 cm. Figure 1 As shown, the pond is distributed from top to bottom with a 100mm high water accumulation layer, a 50mm high covering layer, a 100mm high planting layer, a 150mm high medium layer, a 300mm high submerged layer (upper layer 150mm, lower layer 150mm) and a 50mm high drainage layer. A drainage pipe is set in the middle of the drainage layer, and the outlet of the drainage pipe is flush with the top of the submerged layer.

[0055] In terms of volume ratio, the covering layer is filled with gravel (average particle size of about 5mm to 6mm), the planting layer is filled with a uniform mixture of 40% loam (average particle size of about 0.002mm to 0.2mm) and 60% sand (average particle size of about 0.4 to 0.8mm), the medium layer is filled with a uniform mixture of 20% sand (average particle size of about 0.6 to 0.8mm) and 80% zeolite (average particle size of about 4 to 5mm); the upper layer of the submerged layer is filled with a uniform mixture of 20% modified sawdust (prepared in Preparation Example 1), 30% sand (average particle size of about 0.4 to 0.8mm) and 50% gravel (average particle size of 8 to 9mm), the lower layer of the submerged layer is filled with a uniform mixture of 20% iron filings (average particle size of 5 to 6mm), 30% sand (average particle size of about 0.4 to 0.8mm) and 50% gravel (average particle size of about 8mm to 9mm); and the drainage layer is filled with gravel (average particle size of about 12mm to 15mm).

[0056] Examples 2-4

[0057] The rest is the same as Example 1, except that the modified wood chips used to fill the submerged layer are prepared from Preparation Examples 2-4, respectively.

[0058] Example 5

[0059] The rest is the same as in Example 1, except that the upper and lower layers of the submerged layer are opposite to those in Example 1, as shown in FIG. Figure 1 As shown, the upper layer of the submerged layer is filled with a uniform mixture of 20% iron filings (average particle size 5-6 mm), 30% sand (average particle size of about 0.4-0.8 mm), and 50% gravel (average particle size of about 8 mm-9 mm), and the lower layer of the submerged layer is filled with a uniform mixture of 20% modified wood chips (prepared in Preparation Example 1), 30% sand (average particle size of about 0.4-0.8 mm), and 50% gravel (average particle size of 8-9 mm).

[0060] Comparative Example 1-2

[0061] The rest is the same as Example 1, except that the modified wood chips used to fill the submerged layer are prepared from Comparative Preparation Examples 1-2, respectively.

[0062] Comparative Example 3

[0063] The rest is the same as Example 1, except that pine sawdust is used in the flooded layer instead of modified sawdust, that is, unmodified sawdust is used.

[0064] Comparative Example 4

[0065] The rest is the same as the embodiment 1, except that the submerged layer is not filled in layers, that is, the modified wood chips and iron chips are mixed and filled, such as Figure 1 As shown, specifically:

[0066] A bioretention pond for enhanced nitrogen and phosphorus removal, wherein the pond comprises, from top to bottom, a 100mm high water accumulation layer, a 50mm high covering layer, a 100mm high planting layer, a 150mm high medium layer, a 300mm high submerged layer, and a 50mm high drainage layer. A drainage pipe is provided in the middle of the drainage layer, and the outlet of the drainage pipe is flush with the top of the submerged layer.

[0067] In terms of volume ratio, the covering layer is filled with gravel (average particle size of about 5mm to 6mm), the planting layer is filled with a uniform mixture of 40% loam (average particle size of about 0.002mm to 0.2mm) and 60% sand (average particle size of about 0.4 to 0.8mm), the medium layer is filled with a uniform mixture of 20% sand (average particle size of about 0.4 to 0.8mm) and 80% zeolite (average particle size of about 4 to 5mm), the submerged layer is filled with a uniform mixture of 10% modified sawdust (prepared in Preparation Example 1), 10% iron filings (average particle size of 5 to 6mm), 30% sand (average particle size of about 0.4 to 0.8mm) and 50% gravel (average particle size of 8 to 9mm), and the drainage layer is filled with gravel (average particle size of about 12mm to 15mm).

[0068] Application and Analysis

[0069] The bioretention ponds prepared in the above embodiments and comparative examples were subjected to nitrogen and phosphorus removal performance tests under a simulated rainfall experiment.

[0070] The experimental process is as follows:

[0071] The rainwater used to simulate rainfall was artificially distributed. 0.5 hours before each experiment, about 10L of tap water was added to a 25L plastic barrel and allowed to stand for dechlorination. After the standing period, pollutants were added to the barrel, that is, KNO3 was added to simulate NO3 in natural rainfall. - N, adding NH4Cl to simulate NH4 in natural rainfall + -N, adding KH2PO4 to simulate PO4 in natural rainfall 3- -P, adding ZnSO4 to simulate Zn in natural rainfall 2+ , adding CuCl2 to simulate the Cu in natural rainfall 2+ , and the water volume in the bucket was adjusted to 12L, and shaken for 5 minutes to mix the pollutants thoroughly. Two types of simulated rainwater were prepared, and the formula is shown in Table 1 below. Simulated rainwater A is close to the average concentration of pollutants in normal rainwater, and simulated rainwater B is rainwater with high concentrations of pollutants. The tap water in the laboratory area contains about 1.1 to 2.1 mg / L of NO3 - -N, this part is not included in the configuration NO3 - -N.

[0072] Table 1 Formulation of simulated rainwater

[0073]

[0074] The simulated rainfall is mainly determined by the control area (catchment area), runoff coefficient and rainfall intensity (rainfall depth). Referring to the calculation method in the "Sponge City Rainwater Control and Utilization Engineering Design Code (DB11 / 685-2021)", the total runoff W is calculated using the following formula:

[0075] W=10ψ zc h y F

[0076] Where, ψ zc (0.85) is the comprehensive runoff coefficient; h y is the rainfall depth. Considering the actual situation in the study area, the rainfall adopts the rainstorm level "Precipitation Level (GBT 28592-2012)", with a continuous rainfall depth of 50 mm for 120 minutes; F is the catchment area, which is 10 times the surface area of ​​the bioretention pond (0.785×10- 5 hm 2 ; The calculated total runoff volume W is 3.34L.

[0077] During the experiment, the simulated rainwater was transported to the top of the bioretention tanks of the embodiment and the comparative example by a peristaltic pump to simulate rainfall. According to the total runoff W, the rainfall depth of 50 mm, and the rainfall duration of 120 minutes, the peristaltic pump flow rate can be calculated to be 27.84 mL / min. At the same time, different pre-dry periods were designed, and a total of 20 simulated rainfalls were conducted. The 1st to 10th times were experiments to evaluate the removal effect of pollutants in rainwater with two different pollutant concentrations in the lower bioretention tank. Five simulated rainfalls were conducted at each concentration, with one rainfall every two days. The 11th to 20th times used simulated rainwater B to study the effects of different pre-dry periods on the removal effect of pollutants. The pre-dry periods were set to 3 and 5 days respectively. The specific working conditions of the simulated rainfall are shown in Table 2.

[0078] Table 2 Simulated rainfall conditions

[0079]

[0080]

[0081] Denitrification and dephosphorization performance test:

[0082] A water bucket was connected to the drainage outlet of the bioretention tank. The experiment was considered to be over after 3 hours of cessation of simulated rainfall. The water in the bucket was taken for pollutant concentration testing. Specifically, the water quality test adopted the national standard test method of "Water and Wastewater Monitoring and Analysis Method" (4th edition), and a UV-visible spectrophotometer (UV-3100PC, MAPADA, China) was used to measure NH4 + -N, NO3 - -N、PO4 3- The content of -P was determined by atomic absorption spectrophotometry according to GB 7475-87. Zn and Cu were measured using atomic absorption spectrophotometry. The average removal rates of nitrogen, phosphorus, and heavy metals were calculated based on the concentrations of each pollutant sampled from each outlet. After the 1st to 10th simulated rainfall experiments, the results of the nitrogen, phosphorus, and heavy metal removal rates of the bioretention ponds in each embodiment and comparative example for the two simulated rainwaters are shown in Tables 3 and 4. Table 3 shows the nitrogen and phosphorus removal rates, and Table 4 shows the heavy metal Zn and Cu removal rates.

[0083] Table 3 Nitrogen and phosphorus removal rates under different pollution concentrations

[0084]

[0085]

[0086] Table 4 Removal rates of heavy metals Zn and Cu at different pollutant concentrations

[0087]

[0088] The data in Table 3 demonstrate that the bioretention ponds of the present invention exhibit high nitrogen and phosphorus removal capabilities at varying pollutant concentrations. In particular, Examples 2 and 3 achieved ammonia nitrogen removal rates of up to 89%, nitrate nitrogen removal rates of up to 90%, and phosphorus removal rates of up to 99% under normal rainwater pollutant concentrations (i.e., Simulated Rainwater A). Even under high pollutant concentrations (i.e., Simulated Rainwater B), ammonia nitrogen removal rates reached 85%, nitrate nitrogen removal rates reached 75%, and phosphorus removal rates reached 95%. In contrast, the bioretention ponds of Comparative Examples 1-4 exhibited significantly lower nitrogen and phosphorus removal capabilities.

[0089] The data in Table 4 show that the bioretention tanks of the present invention also have good removal rates for the heavy metals Zn and Cu. The bioretention tanks of Comparative Examples 1 and 3 have relatively low removal rates for these heavy metals. While the bioretention tank of Comparative Example 2 has relatively high removal rates for these heavy metals, Table 3 shows that its denitrification and phosphorus removal capabilities are relatively low. In Comparative Example 4, a mixture of modified sawdust and iron filings is packed into the retention layer, showing relatively high removal rates for Zn and Cu. In other words, whether or not the layered filling system has a significant effect on the removal of these heavy metals, however, Table 3 shows that its removal rate for nitrogen and phosphorus is relatively low.

[0090] Table 5 shows the test results of nitrogen and phosphorus removal rates of the bioretention ponds of the embodiments and comparative examples after the 11th to 20th simulated rainfall experiments (i.e., simulated rainwater B with high pollutant concentrations during different previous drought periods).

[0091] Table 5 Nitrogen and phosphorus removal rates under different initial drought periods

[0092]

[0093] The data in Table 5 demonstrate that, even under pre-dry conditions, the bioretention ponds of the present invention maintain high denitrification and phosphorus removal capabilities. Comparing the data in Table 3 for simulated rainwater B, it can be seen that the bioretention ponds of Examples 1-4 (with modified sawdust in the upper layer and iron filings in the lower layer of the retention layer) exhibited improved ammonia and nitrate nitrogen removal rates as the pre-dry period prolonged. However, the bioretention pond of Example 5 (with modified sawdust in the lower layer and iron filings in the upper layer of the retention layer) exhibited slightly lower ammonia and nitrate nitrogen removal rates as the pre-dry period prolonged. In other words, even with a pre-dry period, the bioretention ponds of Examples 1-4 (with modified sawdust in the upper layer and iron filings in the lower layer of the retention layer) exhibited superior denitrification performance.

[0094] In summary, the present invention has constructed an improved bioretention tank filled with modified sawdust and iron filings in layers, resulting in a multifunctional bioretention tank that enhances nitrogen and phosphorus removal. This significantly improves the removal efficiency of nitrogen and phosphorus, while also increasing the removal rate of heavy metals. Even under high pollutant concentrations and prolonged initial drought periods, this system maintains high nitrogen and phosphorus removal rates, thereby controlling stormwater runoff pollution at its source and having important implications for future sponge city development.

Claims

1. A multifunctional bioretention pond for enhanced nitrogen and phosphorus removal, characterized in that: The pool is provided with a water accumulation layer, a covering layer, a planting layer, a medium layer, a submerged layer and a drainage layer from top to bottom. The drainage layer is provided with a drainage pipe, and the outlet of the drainage pipe is flush with the top of the submerged layer. The submerged layer contains modified sawdust and iron chips filled in layers. The modified sawdust is obtained by grafting a functional copolymer on sawdust, and the amount of the functional copolymer is 6 to 10 wt% of the sawdust. The functional copolymer is obtained by copolymerizing a pyridine-containing acrylic monomer and a double-bond acyl halide monomer in a molar ratio of 1:(0.1 to 0.3).

2. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that: The amount of the functional copolymer used is 6-8 wt% of the wood chips.

3. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that: The molar ratio of the pyridyl-containing acrylic monomer to the double-bond-containing acyl halide monomer is 1:(0.2-0.3).

4. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that: The pyridyl-containing acrylic monomer is selected from at least one of 3-(2-pyridyl)acrylic acid, 3-(3-pyridyl)acrylic acid, and 3-(4-pyridyl)acrylic acid; the double-bond-containing acyl halide monomer is selected from at least one of acryloyl chloride, methacryloyl chloride, β-phenylacryloyl chloride, and 2-butenoic acid chloride.

5. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that: The functional copolymer is prepared by the following preparation method: dissolving a pyridine-containing acrylic monomer, a double-bond-containing acyl halide monomer, and an initiator in a solvent inert to the acyl halide, reacting at 60-80° C. for 3-5 hours, cooling to obtain a suspension, and filtering and drying the suspension to obtain the functional copolymer; Preferably, the initiator is at least one of azobisisobutyronitrile and benzoyl peroxide, and the amount of the initiator is 2-3% of the total weight of the monomers; Preferably, the solvent inert to the acyl halide is tetrahydrofuran (THF); and the drying is vacuum drying at 60-80° C. for 12-24 hours.

6. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that: The modified sawdust is prepared by the following steps: mixing sawdust and functional copolymer and then grafting them under vacuum at 75-100° C. for 2-4 hours; Preferably, the moisture content of the sawdust is ≤5%; the sawdust is pine sawdust with an average particle size of 2mm to 10mm; the vacuum environment has a vacuum degree of 0.05 to 0.1Mpa; the grafting reaction is carried out in a high-speed mixer with a rotation speed of 150 to 250rpm.

7. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that: The submerged layer is filled in layers as follows: the upper layer is filled with a mixture of modified sawdust, sand and gravel, and the lower layer is filled with a mixture of iron chips, sand and gravel; or the upper layer is filled with a mixture of iron chips, sand and gravel, and the lower layer is filled with a mixture of modified sawdust, sand and gravel; the filling of the upper and lower layers satisfies the volume ratio of modified sawdust to iron chips of 1:(0.8-1.2).

8. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 7, characterized in that: The submerged layer is filled in layers: the upper layer is filled with a mixture of modified wood chips, sand and gravel, and the lower layer is filled with a mixture of iron chips, sand and gravel.

9. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 7, characterized in that: The average particle size of the iron filings is 2 to 8 mm, the average particle size of the sand is 0.4 to 0.8 mm, and the average particle size of the gravel is 5 to 10 mm; and / or In the layered filling, the volume proportions of modified wood chips, sand, and gravel are 5-25%, 25-35%, and 45-55%, respectively, and the sum of the volume proportions of the three is 100%; the volume proportions of iron chips, sand, and gravel are 15-25%, 25-35%, and 45-55%, respectively, and the sum of the volume proportions of the three is 100%.

10. The multifunctional bioretention pond for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that: The height of the water accumulation layer is 100 to 150 mm; and / or The height of the covering layer is 50 to 60 mm, the covering layer is filled with gravel, and the average particle size of the gravel is 4 to 8 mm; and / or The height of the planting layer is 100-120 mm, and the planting layer is filled with a mixture of loam and sand, the average particle size of the loam is 0.002 mm-0.2 mm, the average particle size of the sand is 0.4-0.8 mm, the volume proportions of the loam and sand are 35-45% and 55-65% respectively, and the sum of the volume proportions of the two is 100%; and / or The height of the medium layer is 150-180 mm, and the medium layer is filled with a mixture of sand and zeolite, the average particle size of the sand is 0.4-0.8 mm, and the average particle size of the zeolite is 2-6 mm; the volume proportions of the sand and the zeolite are 15-20% and 75-85% respectively, and the sum of the volume proportions of the two is 100%; and / or The height of the submerged layer is 300 to 400 mm; and / or The height of the drainage layer is 50-60 mm, and the drainage layer is filled with gravel, and the average particle size of the gravel is 5-20 mm.

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