Multifunctional biological retention tank for enhanced denitrification and phosphorus removal

Abstract

This invention belongs to the field of rainwater treatment technology and relates to a multifunctional bioretention tank for enhanced nitrogen and phosphorus removal. The tank comprises, from top to bottom, a water accumulation layer, a covering layer, a planting layer, a media layer, a submerged layer, and a drainage layer. The drainage layer is equipped with a drainage pipe, the outlet of which is flush with the top of the submerged layer. The submerged layer contains layered modified wood chips and iron filings. The modified wood chips are obtained by grafting functional copolymers onto the wood chips, with the amount of the functional copolymers being 6-10 wt% of the wood chips. The functional copolymers are obtained through copolymerization of pyridine-containing acrylic monomers and double-bonded acyl halide monomers at a molar ratio of 1:(0.1-0.3). This invention, through wood chip modification and improved filling structure, layers modified wood chips and iron filings in the submerged layer of the bioretention tank, thereby enhancing the nitrogen and phosphorus removal effect and significantly improving the removal efficiency of heavy metal pollutants.

Description

Technical Field

[0001] This invention belongs to the field of rainwater treatment technology, specifically relating to a multifunctional biological retention pond for enhanced nitrogen and phosphorus removal. Background Technology

[0002] Due to global climate change and intensified urbanization, urban stormwater runoff pollution is becoming increasingly serious. The large amounts of nitrogen, phosphorus, and other nutrients, as well as heavy metals present in urban stormwater runoff, easily lead to eutrophication and ecosystem degradation. Bioretention ponds are the most commonly used low-impact development (LID) technology in sponge city construction. Through the combined action of plants, packing materials, and microorganisms, they intercept, adsorb, and degrade pollutants in urban stormwater runoff, integrating important functions such as regulation and purification. However, typical bioretention ponds often have unreasonable structures, single-function packing materials, and low nitrogen and phosphorus removal efficiency, meaning they cannot effectively remove ammonia nitrogen (NH4) from stormwater runoff. + -N), nitrate (NO3-N), phosphate (PO4-) 3- Dissolved pollutants such as P (-P).

[0003] Existing research generally improves the nitrogen and phosphorus removal capacity of bioretention tanks through structural modifications or packing improvements. Structural modifications typically involve raising the height of the effluent pipe to achieve functional zoning of aerobic and anoxic zones within the facility, providing a suitable growth environment for nitrifying and denitrifying bacteria, thereby optimizing the removal of dissolved pollutants. Packing improvements generally involve adding functional packing materials, such as vermiculite, zeolite, and volcanic rock, which have abundant pores and high adsorption capacity, to achieve rapid adsorption of organic matter and ammonia nitrogen. Alternatively, adding organic or inorganic electron donors such as sawdust and sulfur can promote heterotrophic or autotrophic denitrification processes within the bioretention tank to remove nitrate nitrogen. Some studies have also explored adding aluminum-based or iron-based water treatment residues to promote the flocculation and sedimentation of dissolved phosphorus. However, most packing materials can only achieve single-pollutant removal and are generally mixed packing structures. For example, the high performance of bioretention columns filled with a mixture of biochar and pyrite has been proven (Kong Z et al., Water Research, 2021, 206:117737). However, the performance of such bioretention columns cannot be guaranteed when faced with changes in the rainwater environment (e.g., pollutant concentration, pre-drought period, rainfall intensity, rainfall duration, etc.). Furthermore, pyrite is prone to causing large amounts of sulfate and iron ions to leach out during the pre-rainfall drought period, becoming new pollutants. In addition, adding solid carbon sources poses risks of organic matter leakage and facility blockage, which can easily lead to secondary water pollution and increased facility operation and maintenance costs. Summary of the Invention

[0004] Given that the existing bioretention ponds are not ideal in removing pollutants, especially nitrogen and phosphorus, and need further improvement, this invention provides a multifunctional bioretention pond with enhanced nitrogen and phosphorus removal. Utilizing readily available materials such as sawdust, iron filings, gravel, zeolite, and sand, the invention modifies the sawdust and improves the filling structure by layering modified sawdust and iron filings in the submerged layer of the bioretention pond. This enhances the nitrogen and phosphorus removal effect while significantly improving the removal of heavy metal pollutants, thereby controlling rainwater runoff pollution at its source and having significant implications for future sponge city construction.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A multifunctional biological retention tank for enhanced nitrogen and phosphorus removal comprises, from top to bottom, a water accumulation layer, a covering layer, a planting layer, a media layer, a submerged layer, and a drainage layer. The drainage layer is equipped with a drainage pipe, the outlet of which is flush with the top of the submerged layer. The submerged layer contains layered modified wood chips and iron filings. The modified wood chips are obtained by grafting wood chips with functional copolymers, and the amount of the functional copolymers is 6-10 wt% of the wood chips. The functional copolymers are obtained by copolymerization of pyridine-containing acrylic monomers and double-bonded acyl halide monomers at a molar ratio of 1:(0.1-0.3).

[0007] This invention involves copolymerizing pyridine-containing acrylic monomers and double-bonded acyl halide monomers in a specific ratio to obtain a functional copolymer rich in pyridine, acyl halide, and carboxylic acid groups. This functional copolymer is then used to graft-modify wood chips. During grafting modification, the active acyl halide groups in the functional copolymer undergo acylation with the hydroxyl groups in the wood chips to generate ester groups, thereby introducing the copolymer containing pyridine and abundant carboxylic acid groups into the branched chains of the wood chips. When the modified wood chips are used as filler in bioretention ponds, the abundant carboxylic acid groups in the grafted chains of the wood chips have a complexing effect on heavy metals in rainwater, thus "adsorbing" and "retaining" heavy metal pollutants. Simultaneously, the pyridine groups on the grafted chains have a large π-bonded heterocyclic structure, which can interact with the benzene rings in the wood chips, weakening the hydrogen bonding between or within cellulose molecules and increasing the spatial volume of cellulose. This improves the looseness of the wood chips, preventing agglomeration and clogging, thereby increasing the permeability of the wood chips, prolonging the hydraulic retention time, and allowing more time for pollutants to be adsorbed. In this invention, the modified sawdust and iron filings are layered and filled into the submerged layer. The iron filings are the iron source required by autotrophic denitrifying bacteria, while the modified sawdust is the organic carbon source required by heterotrophic denitrifying microorganisms. The two work together to achieve efficient denitrification and nitrogen removal, effectively treating the nitrate nitrogen (NO3) that has permeated down. - -N) and nitrite nitrogen (NO2) - -N); at the same time, the byproduct of iron filings, Fe 3+ / Fe 2+It further adsorbs residual phosphates, thus achieving further phosphorus removal. In addition, modified sawdust not only promotes the rapid proliferation and microbial activity of autoaerobic denitrifying bacteria, but also reduces the byproduct Fe. 3+ / Fe 2+ The inventors discovered through research that layering modified wood chips and iron filings in the submerged layer is more effective than mixing them together. This is likely because layering creates a synergistic effect and allows for a continuous reaction. Through the aforementioned wood chip modification and layering of modified wood chips and iron filings in the submerged layer, combined with the interaction with other layers in the bioretention tank, a highly efficient ability to remove pollutants is achieved.

[0008] Preferably, the amount of the functional copolymer is 6-8 wt% of the wood chips. If the amount of functional copolymer is too low, the modification effect on the wood chips will be limited; if the amount of functional copolymer is too high, an over-crosslinked network structure will be generated, which will reduce the looseness of the wood chips, and the cost will also increase if the amount of functional copolymer is too high.

[0009] Preferably, the molar ratio of the pyridine-containing acrylic monomer to the double-bonded acyl halide monomer is 1:(0.2-0.3). If the amount of the double-bonded acyl halide monomer is too low, the content of acyl halide groups in the functional copolymer will be low, resulting in fewer grafting sites during wood chip grafting modification and a limited grafting modification effect. If the amount of the double-bonded acyl halide monomer is too high, the amount of acrylic monomer will be relatively low, resulting in a limited number of pyridine and carboxylic acid groups in the functional copolymer. This will limit the improvement in the looseness and permeability of the wood chips and the ability to "adsorb" and "retain" heavy metal pollutants. Therefore, the molar ratio of the pyridine-containing acrylic monomer to the double-bonded acyl halide monomer needs to be controlled within the aforementioned range.

[0010] Further, the pyridyl-containing acrylic monomer is selected from at least one of 3-(2-pyridyl)acrylic acid, 3-(3-pyridine)acrylic acid, and 3-(4-pyridyl)acrylic acid; the double-bonded 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 steps: dissolving an acrylic monomer containing pyridinyl groups, an acyl halide monomer containing double bonds, and an initiator together in a solvent inert to acyl halides, 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 (AIBN) and benzoyl peroxide, and the amount of initiator used is 2-3% of the total mass of the monomers. The amount of initiator affects the molecular weight of the functional copolymer. In this invention, the amount of initiator is controlled within the above range. If the molecular weight is too small, the grafted chains introduced onto the wood chips during subsequent grafting modification will be too short, resulting in limited grafting effect. If the molecular weight is too large, there will be many entanglement points between its molecular chains, affecting the mobility of its molecular chains, and ultimately affecting the adsorption capacity of the modified wood chips.

[0013] Furthermore, the solvent inert to acyl halides is tetrahydrofuran (THF). Acyl halide monomers containing double bonds have high reactivity due to the presence of acyl halide groups. To reduce side reactions, a solvent inert to acyl halides must be selected. The drying process is vacuum drying at 60–80°C for 12–24 hours.

[0014] Furthermore, the modified wood chips are prepared by the following steps: wood chips and functional copolymers are mixed and grafted under vacuum at 75–100°C for 2–4 hours. The acyl halide groups in the functional copolymer react with the hydroxyl groups in the wood chips to form ester groups, thereby introducing the polycarboxylic acid polymer into the branched chains of the wood chips.

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

[0016] Further, the layered filling of the submerged layer is as follows: 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 filings, sand, and gravel; or the upper layer is filled with a mixture of iron filings, sand, and gravel, and the lower layer is filled with a mixture of modified wood chips, sand, and gravel; the filling of the upper and lower layers satisfies the volume ratio of modified wood chips to iron filings of 1:(0.8~1.2), for example, 1:0.8, 1:1, or 1:1.2; preferably, the layered filling of the submerged layer is as follows: 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 filings, sand, and gravel. The inventors have found through research that the denitrification and phosphorus removal effect of layered filling of modified wood chips and iron filings in the submerged layer is better than that of a mixture of the two, possibly because layered filling has a synergistic effect between the upper and lower layers, and the reaction is continuous. When the upper layer is filled with modified sawdust and the lower layer with iron filings, heterotrophic denitrification mainly occurs in the upper layer, while autotrophic denitrification mainly occurs in the lower layer. Simultaneously, the nutrients produced by the continuous flow of sawdust from the upper layer promote autotrophic denitrification, as well as the dissimilatory reduction of nitrate to ammonium (DNRA) and anaerobic ammonium oxidation (Anammox) in the lower layer. Furthermore, the autotrophic denitrification and DNRA in the lower layer further remove NO3. - Anammox can also remove NO2, a byproduct generated in the upper layer during heavy rainfall. - Simultaneously control NH4 + The reactions throughout the flooded zone are continuous, resulting in very low concentrations of nitrogenous pollutants in the effluent. For example, when the upper layer is filled with iron filings and the lower layer with modified wood chips, autotrophic denitrification and zero-valent iron / Fe2+ mainly occur in the upper layer. 2+ The redox reaction is mediated, with heterotrophic denitrification mainly occurring in the lower layer; Fe flows down from the upper layer. 2+ As an electron donor, it promotes heterotrophic denitrification in the lower layer and can significantly remove NO3. - The reaction throughout the flooded zone is continuous. However, when modified wood chips are mixed with iron chips and filled into the flooded layer, the wood chips and zero-valent iron are widely distributed throughout the flooded zone, lacking the continuity and synergistic effect of the reaction 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 percentages of modified wood chips, sand, and gravel are 5-25%, 25-35%, and 45-55%, respectively, with the sum of their volume percentages being 100%; the volume percentages of iron filings, sand, and gravel are 15-25%, 25-35%, and 45-55%, respectively, with the sum of their volume percentages being 100%.

[0019] Furthermore, the height of the water accumulation layer is 100-150mm, and the water accumulation layer provides a certain storage space for rainwater that cannot infiltrate during short-term heavy rainfall.

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

[0021] Furthermore, the planting layer has a height of 100–120 mm and is filled with a mixture of loam and sand. The average particle size of the loam is 0.002 mm–0.2 mm, and the average particle size of the sand is 0.4–0.8 mm. The volume percentages of loam and sand are 35–45% and 55–65%, respectively, with a total volume percentage of 100%. Perennial grasses, such as ryegrass, bermudagrass, bahiagrass, and celestial grass, are planted on the planting layer. These plants are not only aesthetically pleasing but also help purify water. The addition of loam to the planting layer reduces large pores, optimizes pore uniformity and structure, and extends hydraulic retention time. Simultaneously, the planting layer provides a space for plant root growth and can adsorb and intercept 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. 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 percentages of sand and zeolite are 15–20% and 75–85%, respectively, with the sum of their volume percentages being 100%. The medium layer is highly porous and can adsorb dissolved nutrients, such as ammonia nitrogen (NH4). + -N), phosphate (PO4) 3- -P) etc.

[0023] Furthermore, the height of the flooding layer is 300–400 mm. The outlet of the drainage pipe in the lower drainage layer is flush with the top of the flooding layer, ensuring that the pores of the flooding layer remain saturated for a long period, thus creating anaerobic conditions. The addition of modified sawdust and iron filings to the flooding layer provides conditions for the rapid growth of microbial communities. The combination of these two promotes reactions such as denitrification, anaerobic ammonium oxidation, and the dissimilatory reduction of nitrate to ammonium, as well as the production of Fe byproducts from the iron filings. 3+ / Fe 2+ It can further intercept residual phosphates; that is, the coupling effect of modified wood chips and iron chips improves the denitrification and phosphorus removal rate, while reducing the by-product Fe. 3+ / Fe 2+ The water dripped out.

[0024] Furthermore, the height of the drainage layer is 50-60 mm, and the drainage layer is filled with gravel with an average particle size of 5 mm-20 mm. The rainwater that has been filtered through the above layers is finally discharged through the drainage pipes in the drainage layer. The height of the outlet of the drainage pipe in the drainage layer is crucial. In this invention, the water outlet of the drainage pipe is flush with the top of the flooded layer, thereby creating anaerobic conditions.

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

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

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

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

[0029] 3. The wood chips and iron filings used in this invention are waste products from agricultural or industrial production, which can turn waste into treasure and make full use of the residual value of the materials. Attached Figure Description

[0030] Figure 1 The diagram shows the structural device of the multifunctional biological retention tank in Examples 1, 5, and 4. Detailed Implementation

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

[0033] Preparation of modified wood chips

[0034] Preparation Example 1

[0035] 1) Prepare a mixture of 3-(2-pyridyl)acrylic acid and acryloyl chloride in a molar ratio of 1:0.1 with a total mass of 1000g. Then, dissolve the mixture and 25g of benzoyl peroxide (BPO) in tetrahydrofuran (THF) and stir the mixture at 70℃ for 4h. After cooling, a suspension is obtained. The suspension is filtered and dried under vacuum at 60℃ for 24h to obtain the functional copolymer.

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

[0037] Preparation Example 2

[0038] The rest is the same as in 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 to β-phenylacryloyl chloride is 1:0.2; specifically:

[0039] 1) Prepare a mixture of 3-(2-pyridyl)acrylic acid and β-phenylacryloyl chloride in a molar ratio of 1:0.2 with a total mass of 1000g. Then, dissolve the mixture and 30g of BPO in THF and stir at 75°C for 4h. After cooling, a suspension is obtained. The suspension is filtered and dried under vacuum at 70°C for 24h to obtain the functional copolymer.

[0040] 2) Same as Example 1.

[0041] Preparation Example 3

[0042] The rest is the same as in 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 amount of the functional copolymer is 8 wt% of the wood chips; specifically:

[0043] 1) Prepare a mixture of 3-(2-pyridyl)acrylic acid and acryloyl chloride in a molar ratio of 1:0.3 with a total mass of 1000g. Then, dissolve the mixture and 30g of BPO in THF and stir at 70℃ for 4h. After cooling, a suspension is obtained. The suspension is filtered and dried under vacuum at 60℃ for 24h to obtain the functional copolymer.

[0044] 2) Dry the pine wood chips at 80°C for 24 hours in a dryer (scraper speed 80 rpm). Then, place 10 kg of dried pine wood chips and 800 g of functional copolymer in a vacuum high mixer with a vacuum degree of 0.08 MPa. React at 85°C and 180 rpm for 3.5 hours. After cooling, the modified wood chips are obtained.

[0045] Preparation Example 4

[0046] The rest is the same as in 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 wood chips; specifically:

[0047] 1) Prepare a mixture of 3-(2-pyridyl)acrylic acid and acryloyl chloride in a molar ratio of 1:0.3 with a total mass of 1000g. Then, dissolve the mixture and 30g of BPO in THF and stir at 70℃ for 4h. After cooling, a suspension is obtained. The suspension is filtered and dried under vacuum at 60℃ for 24h to obtain the functional copolymer.

[0048] 2) Dry the pine wood chips at 80°C for 24 hours in a dryer (scraper speed 80 rpm). Then, place 10 kg of dried pine wood chips and 800 g of functional copolymer in a vacuum high mixer with a vacuum degree of 0.08 MPa. React at 85°C and 180 rpm for 3.5 hours. After cooling, the modified wood chips are obtained.

[0049] Comparative Preparation Example 1

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

[0051] Comparative Preparation Example 2

[0052] The rest is the same as in 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 biological retention tank for enhanced nitrogen and phosphorus removal is constructed using plexiglass columns with an inner diameter of 10cm and a height of 80cm. Figure 1 As shown, the pool contains, from top to bottom, a water layer with a height of 100mm, a covering layer with a height of 50mm, a planting layer with a height of 100mm, a medium layer with a height of 150mm, a flooding layer with a height of 300mm (150mm upper and 150mm lower), and a drainage layer with a height of 50mm. A drainage pipe is installed in the middle of the drainage layer, and the outlet of the drainage pipe is flush with the top of the flooding layer.

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

[0056] Examples 2-4

[0057] The rest is the same as in Example 1, except that the modified wood chips used to fill the flooding layer were prepared in Examples 2-4.

[0058] Example 5

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

[0060] Comparative Examples 1-2

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

[0062] Comparative Example 3

[0063] The rest is the same as in Example 1, except that pine wood chips are used instead of modified wood chips in the flooding layer, that is, unmodified wood chips are used.

[0064] Comparative Example 4

[0065] The remaining differences from Embodiment 1 are that the flooding layer is not filled in layers, but rather a mixture of modified wood chips and iron filings, such as... Figure 1 As shown, specifically:

[0066] A biological retention pond for enhanced nitrogen and phosphorus removal, in which there are successively distributed from top to bottom a water accumulation layer with a height of 100 mm, a covering layer with a height of 50 mm, a planting layer with a height of 100 mm, a medium layer with a height of 150 mm, a submerged layer with a height of 300 mm, and a drainage layer with a height of 50 mm. A drain pipe is provided in the middle of the drainage layer, and the outlet of the drain pipe is flush with the top of the submerged layer.

[0067] By volume, the covering layer is filled with gravel (average particle size about 5 mm - 6 mm), the planting layer is filled with 40% loam (average particle size about 0.002 mm - 0.2 mm) and 60% sand (average particle size about 0.4 - 0.8 mm) mixed evenly, the medium layer is filled with 20% sand (average particle size about 0.4 - 0.8 mm) and 80% zeolite (average particle size about 4 - 5 mm) mixed evenly, the submerged layer is filled with 10% modified wood chips (prepared in Preparation Example 1), 10% iron filings (average particle size 5 - 6 mm), 30% sand (average particle size about 0.4 - 0.8 mm), and 50% gravel (average particle size 8 - 9 mm), and the drainage layer is filled with gravel (average particle size about 12 mm - 15 mm).

[0068] Application and analysis

[0069] The biological retention ponds prepared in the above-mentioned various examples and comparative examples were tested for nitrogen and phosphorus removal performance under a simulated rainfall experiment.

[0070] The experimental process is as follows:

[0071] The rainwater used in the simulated rainfall is artificial water. About 10 L of tap water is added to a 25 L plastic bucket 0.5 hours before each experiment and left to stand to remove chlorine; after standing, pollutants are added, that is, KNO3 is added to simulate NO3 - -N in natural rainfall, NH4Cl is added to simulate NH4 + -N in natural rainfall, KH2PO4 is added to simulate PO4 3- -P in natural rainfall, ZnSO4 is added to simulate Zn 2+ in natural rainfall, CuCl2 is added to simulate Cu 2+ in natural rainfall, and the water volume in the bucket is made up to 12 L, and shaken for 5 minutes to mix the pollutants thoroughly. A total of two kinds of simulated rainwaters are configured, and the formulas are 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 containing high concentrations of pollutants. There is about 1.1 - 2. .1 mg / L of NO3 - -N in the tap water in the area where the laboratory is located, and this part is not included in the configured NO3 - -N.

[0072] Table 1 Formulas of simulated rainwaters

[0073]

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

[0075] W = 10ψ zc h y F

[0076] In the formula, ψ zc (0.85) represents the comprehensive runoff coefficient; h y For rainfall depth, considering the actual conditions of the study area, the rainfall amount was determined using the heavy rain level (Precipitation Grades (GBT 28592-2012)), with a continuous rainfall depth of 50 mm over 120 minutes; F represents the catchment area, taken as 10 times the surface area of ​​the biological retention pond, which is 0.785 × 10⁻⁶. 5 hm 2 The total runoff W was calculated to be 3.34 L.

[0077] During the experiment, the simulated rainwater was pumped to the top of the bioretention tanks in the examples and comparative cases using a peristaltic pump to simulate rainfall. Based on the total runoff W, rainfall depth of 50 mm, and rainfall duration of 120 minutes, the peristaltic pump flow rate was calculated to be 27.84 mL / min. Simultaneously, different pre-drought periods were designed, resulting in 20 simulated rainfall events. Events 1-10 evaluated the removal efficiency of pollutants from rainwater with two different pollutant concentrations in the lower bioretention tank, with 5 simulated rainfall events at each concentration, occurring once every 2 days. Events 11-20 used simulated rainwater B to study the impact of different pre-drought periods on pollutant removal efficiency, with pre-drought periods set to 3 and 5 days respectively. The specific operating conditions of the simulated rainfall are shown in Table 2.

[0078] Table 2 Simulated Rainfall Conditions

[0079]

[0080]

[0081] Denitrification and phosphorus removal performance test:

[0082] A bucket was connected to the drain outlet of the bioretention tank. The experiment was considered complete 3 hours after each simulated rainfall ceased, and water from the bucket was collected for pollutant concentration testing. Specifically, water quality testing followed the national standard testing methods outlined in "Methods for Monitoring and Analysis of Water and Wastewater" (Fourth Edition). An ultraviolet-visible spectrophotometer (UV-3100PC, MAPADA, China) was used to determine NH4 according to standards HJ535-2009, HJ / T 346-2007, and HJ670-2013. + -N, NO3 - -N,PO4 3- The content of -P was determined by atomic absorption spectrophotometry according to GB 7475-87 standard for Zn and Cu. The average removal rates of nitrogen, phosphorus, and heavy metals were calculated based on the concentrations of each pollutant obtained from each drainage outlet sampling test. After the 1st to 10th simulated rainfall experiments, the removal rate test results of nitrogen, phosphorus, and heavy metals in the bioretention tanks of each embodiment and comparative example for the two simulated rainwater are shown in Tables 3 and 4. Table 3 shows the removal rate test results of nitrogen and phosphorus, and Table 4 shows the removal rate test results of heavy metals Zn and Cu.

[0083] Table 3. Removal rates of nitrogen and phosphorus at different pollution concentrations

[0084]

[0085]

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

[0087]

[0088] As can be seen from the data in Table 3, the bioretention tanks of this invention exhibit high nitrogen and phosphorus removal capabilities under different pollutant concentrations. In particular, Examples 2 and 3 show ammonia nitrogen removal rates as high as 89%, nitrate nitrogen removal rates as high as 90%, and phosphorus removal rates as high as 99% under normal rainwater pollutant concentrations (i.e., simulated rainwater A). Even under high pollutant concentrations (i.e., simulated rainwater B), the ammonia nitrogen removal rate reaches 85%, the nitrate nitrogen removal rate reaches 75%, and the phosphorus removal rate reaches 95%. In contrast, the nitrogen and phosphorus removal capabilities of the bioretention tanks in Comparative Examples 1-4 are significantly lower.

[0089] As can be seen from the data in Table 4, the bioretention tanks of the present invention all exhibit good removal rates for heavy metals Zn and Cu. The bioretention tanks of Comparative Examples 1 and 3 show relatively low removal rates for Zn and Cu. Although the bioretention tank of Comparative Example 2 shows a higher removal rate for Zn and Cu, Table 3 indicates that its nitrogen and phosphorus removal capabilities are relatively low. In Comparative Example 4, the modified wood chips and iron filings are mixed and filled in the retention layer, resulting in a higher removal rate for Zn and Cu. This suggests that whether or not the filling is layered has little impact on the removal of Zn and Cu, but Table 3 shows that its nitrogen and phosphorus removal rates are relatively low.

[0090] The results of nitrogen and phosphorus removal rates of the bioretention ponds of each embodiment and comparative example after the 11th-20th simulated rainfall experiments (i.e., simulated rainwater B with high pollutant concentrations during different pre-drought periods) are shown in Table 5.

[0091] Table 5. Nitrogen and phosphorus removal rates under different pre-drought periods.

[0092]

[0093] As can be seen from the data in Table 5, the bioretention pond of this invention still exhibits high nitrogen and phosphorus removal capabilities even under conditions of initial drought. Comparing the data in Table 3 under simulated rainwater B, it can be seen that the bioretention ponds of Examples 1-4 (with modified wood chips in the upper layer and iron filings in the lower layer) showed increased removal rates of ammonia and nitrate nitrogen with prolonged initial drought; however, the bioretention pond of Example 5 (with modified wood chips in the lower layer and iron filings in the upper layer) showed a slight decrease in removal rates of ammonia and nitrate nitrogen with prolonged initial drought. In other words, under conditions of initial drought, the bioretention ponds of Examples 1-4 (with modified wood chips in the upper layer and iron filings in the lower layer) demonstrated superior nitrogen removal performance.

[0094] In summary, this invention constructs an improved bioretention pond filled with modified wood chips and iron filings in layers, namely a multifunctional bioretention pond with enhanced nitrogen and phosphorus removal, significantly improving the removal efficiency of nitrogen and phosphorus, while also increasing the removal rate of heavy metals. Even under high pollutant concentrations and prolonged pre-drought conditions, it maintains a high removal rate of nitrogen and phosphorus; thus achieving source control of stormwater runoff pollution, which is of great significance for the future construction of sponge cities.

Claims

1. A multifunctional biological retention tank for enhanced nitrogen and phosphorus removal, characterized in that, The pool contains, from top to bottom, a water accumulation layer, a covering layer, a planting layer, a medium layer, a submerged layer, and a drainage layer. The drainage layer is equipped with a drainage pipe, the outlet of which is flush with the top of the submerged layer. The submerged layer contains layered modified wood chips and iron filings. The modified wood chips are obtained by grafting wood chips with functional copolymers, and the amount of functional copolymers is 6-10 wt% of the wood chips. The functional copolymers are obtained by copolymerizing pyridine-containing acrylic monomers and double-bonded acyl halide monomers in a molar ratio of 1:(0.1-0.3).

2. The multifunctional bioretention tank 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 biological retention tank for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that, The molar ratio of the pyridinyl-containing acrylic monomer and the double-bonded acyl halide monomer is 1:(0.2~0.3).

4. The multifunctional bioretention tank 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-pyridine)acrylic acid, and 3-(4-pyridyl)acrylic acid; the double-bonded 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 biological retention tank for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that, The functional copolymer is prepared by the following steps: dissolving an acrylic monomer containing pyridinyl groups, an acyl halide monomer containing double bonds, and an initiator together in a solvent inert to acyl halides, 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.

6. The multifunctional bioretention tank for enhanced nitrogen and phosphorus removal according to claim 5, characterized in that, The initiator is at least one of azobisisobutyronitrile and benzoyl peroxide, and the amount of initiator is 2-3% of the total mass of the monomer; the solvent that is inert to acyl halides is tetrahydrofuran (THF); the drying is vacuum drying at 60-80°C for 12-24 hours.

7. The multifunctional biological retention tank for enhanced nitrogen and phosphorus removal according to claim 1, characterized in that, The modified wood chips are prepared by the following steps: wood chips and functional copolymers are mixed and then grafted at 75-100°C for 2-4 hours under vacuum.

8. The multifunctional biological retention tank 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 wood chips, sand and gravel, and the lower layer is filled with a mixture of iron filings, sand and gravel; or the upper layer is filled with a mixture of iron filings, sand and gravel, and the lower layer is filled with a mixture of modified wood chips, sand and gravel; the filling of the upper and lower layers satisfies the volume ratio of modified wood chips to iron filings of 1:(0.8~1.2).

9. The multifunctional biological retention tank for enhanced nitrogen and phosphorus removal according to claim 7, characterized in that, The iron filings have an average particle size of 2-8 mm, the sand has an average particle size of 0.4-0.8 mm, and the gravel has an average particle size of 5-10 mm; and / or In the layered filling, the volume percentages of modified wood chips, sand, and gravel are 5-25%, 25-35%, and 45-55%, respectively, with the sum of their volume percentages being 100%; the volume percentages of iron filings, sand, and gravel are 15-25%, 25-35%, and 45-55%, respectively, with the sum of their volume percentages being 100%.

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

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