Biological detention pond operation maintenance method

Abstract

The invention relates to the technical field of biochemical engineering, and discloses a bioretention pond operation maintenance method which comprises the following steps: S1, collecting soil samples of a newly built bioretention pond and each layer of the bioretention pond after operation, carrying out particle size analysis, and respectively measuring the proportions of sand grains, powder grains and clay grains in each soil sample; s2, according to the particle size data of each soil sample detected in S1, respectively calculating the saturated permeability coefficient Ks by using a natural soil parameter calculation function in Hydrus-1d software; and S3, comparing the saturated permeability coefficients Ks of the newly-built bioretention pond and the bioretention pond after operation, and replacing the soil with the obviously reduced permeability coefficient, so that the soil is restored to the saturated permeability coefficient of the soil sample of the newly-built bioretention pond. The permeability change part of the bioretention facility is identified from the angle of particle size change analysis, so that the operation maintenance of the bioretention pond is carried out, the operation efficiency of the bioretention facility is effectively improved, and the service life of the bioretention facility is effectively prolonged.

Description

Technical Field

[0001] The present invention relates to the field of water treatment technology, and specifically to a method for operating and maintaining a bioretention pond. It focuses on studying effective strategies for restoring the permeability of bioretention facilities, aiming to address the key issue of decreased permeability of bioretention facilities in actual projects and ensure their continued and stable functioning in scenarios such as sponge city construction. Background Art

[0002] Bioretention facilities, one of the most widely used measures in sponge city construction, maintain or restore an area's pre-development hydrological state by enhancing infiltration and evaporation. They effectively control stormwater runoff at its source and leverage natural features to improve runoff quality. Bioretention facilities can be flexibly integrated into landscaping and offer advantages in intercepting surface runoff, reducing peak flows, improving runoff quality, and replenishing groundwater resources. To ensure effective water control and avoid overflows, their infiltration capacity must be maintained within an appropriate range.

[0003] However, in actual applications, bioretention facilities are plagued by reduced infiltration capacity. Physically, high-intensity and long-duration rainfall can significantly increase the depth of surface water accumulation and the peak outflow from the perforated pipes. The filler has poor stability and disintegrates after runoff rainwater flows in. The separated particles then settle into the original pores, changing the pore distribution and reducing the permeability of the device. Additives with larger particle sizes can more effectively increase the tensile strength of the soil. The height of the filler in bioretention facilities can also affect the facility's infiltration performance. Rainwater erosion causes soil particles to displace and pores to clog. Chemically, dissolved substances in the water react with the soil, changing its structure. In terms of biological factors, plant growth also plays a role in controlling rainwater runoff, and plant roots and microbial activity also interfere with soil pores. These factors cause the infiltration capacity of many facilities to drop sharply or even fail within a few years of construction, making the search for effective infiltration remediation methods extremely critical. Summary of the Invention

[0004] The present invention provides a bioretention pond operation and maintenance method, which can evaluate the permeability changes during the operation of the bioretention facility, identify the parts of the bioretention facility where the soil permeability changes, and provide a basis for optimizing the operation and maintenance of the bioretention facility.

[0005] The technical solution of the present invention is to provide a method for operating and maintaining a biological retention pond, comprising the following steps:

[0006] S1. Collect soil samples from each layer of the newly constructed and operational bioretention ponds and perform particle size analysis to determine the proportion of sand, silt, and clay in each soil sample.

[0007] S2. Calculate the saturated permeability coefficient Ks of each soil sample using the natural soil parameter calculation function in the Hydrus-1D software based on the particle size data of each soil sample detected in S1;

[0008] S3. Compare the saturated permeability coefficients Ks of the newly built bioretention pond and the bioretention pond after operation, and replace the soil where the permeability coefficient has been significantly reduced to restore it to the saturated permeability coefficient of the soil sample of the newly built bioretention pond.

[0009] Optionally, if a newly constructed bioretention pond exists in S1, soil samples from the newly constructed bioretention pond can be directly collected for particle size analysis. The soil surface (0-10 cm) is affected by particulate matter, pollutants, and fallen leaves, resulting in an increased concentration of fine particles, which can often lead to clogging. Meanwhile, the soil at a depth of 10-20 cm, influenced by animals and microorganisms, maintains good permeability, close to its original state. If no soil samples from the newly constructed bioretention pond exist, soil samples from the target bioretention pond (15-20 cm) can be collected as the soil samples for the newly constructed bioretention pond for particle size analysis.

[0010] Optionally, when the collected soil samples are not analyzed, they can be sealed in sealed bags and stored at low temperature.

[0011] Optionally, before performing particle size analysis on soil samples, the samples are first dried to a loose state, and then sieved or hammered to disperse the soil particles to ensure that soil particle aggregates are broken into individual particles. The sample is sieved using a 100-mesh sieve and the sieve residue is taken for particle size analysis.

[0012] Optionally, the following formula is used to calculate the proportion of sand, silt, and clay in each soil sample:

[0013]

[0014] Where: r a is the proportion of sieved soil samples, %; r b is the proportion of unscreened soil samples, %; W a is the mass of the sieved soil sample, g; W b is the mass of the sieved soil sample, g;

[0015] R sand =θ sand ×r a +r b

[0016] R silt =θ silt ×r a

[0017] R clay =θ clay ×ra

[0018] Where: R sand is the proportion of sand particles in the soil sample, %; R silt is the proportion of fine particles in the soil sample, %; R clay is the proportion of clay in the soil sample, %; θ sand is the proportion of sand particles in the sieved soil sample, %; θ silt is the proportion of fine particles in the sieved soil sample, %; θ clay is the proportion of clay in the sieved soil samples, %.

[0019] Optionally, the sand particle size is 0.02 to 2 mm; the powder particle size is 0.002 to 0.02 mm; and the clay particle size is less than 0.002 mm.

[0020] Optionally, when analyzing the particle size of soil samples from each layer of the bioretention pond after operation, collect surface soil samples from 0 to 10 cm, with one sample collected every 1 cm, and calculate the permeability coefficient of each layer of soil after operation through particle size analysis and model prediction.

[0021] Optionally, the relationship between the saturated permeability coefficient Ks and the particle size of each soil sample is derived from the Rosetta model in the Hydrus-1d software.

[0022] Optionally, a decrease of 20-50% in Ks is considered a significant decrease, and a decrease greater than 50% requires replacement as soon as possible.

[0023] The present invention also relates to the application of the method in the fields of sponge city construction and rainwater treatment.

[0024] The present invention has the following beneficial effects:

[0025] The present invention provides a bioretention operation and maintenance method based on particle size detection. It starts from the key perspective of soil particle size change, accurately identifies the location of permeability changes, provides highly precise guidance for the operation and maintenance of bioretention ponds, significantly reduces maintenance costs, and effectively improves operating efficiency and service life. It has important promotion and application value in the fields of sponge city construction and rainwater treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A flow chart of the method of the present invention is provided.

[0027] Figure 2 1 is a structural diagram of a bioretention column in an embodiment. DETAILED DESCRIPTION

[0028] The experimental methods in the following examples are conventional methods unless otherwise specified. The materials used in the following examples are commercially available products unless otherwise specified. The embodiments of the present invention will be described in detail below with reference to the examples. However, it will be understood by those skilled in the art that the following examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention.

[0029] Example 1

[0030] This study uses Wuhan as a typical case study to deeply explore the practical application of operation and maintenance methods for bioretention facilities. As one of the first pilot cities for sponge cities, Wuhan has abundant annual rainfall, which is concentrated in the summer, often causing urban waterlogging, highlighting the importance of bioretention facilities. At the same time, its soil texture includes a variety of types such as clay, loam, and sand. The soil characteristics in different regions vary significantly, and the impact on the performance of bioretention facilities is complex and diverse, making the research results more universal and applicable. The study combines the local soil and meteorological conditions of Wuhan City, and through experimental methods, screens out the optimal formulation of the planting layer filler for bioretention facilities that meets the local conditions of Wuhan City.

[0031] The bioretention facility in this embodiment is a bioretention column, such as Figure 2 As shown, it is constructed by splicing five sections of 300mm long, 110mm diameter PVC pipe and four sections of 110mm diameter extended double-threaded expansion joints. This modular design facilitates assembly, disassembly, and internal structure observation, facilitating data monitoring and sample collection during experiments. Functionally, the bioretention column consists of four main layers from top to bottom: a gravel layer (300mm), a filler layer (600mm), a planting layer (300mm), and an aquifer layer (300mm). A perforated blind pipe is installed 100mm from the bottom of the gravel layer as a drainage pipe. The perforated blind pipe is made of 20mm diameter PVC pipe, perforated on one side, and wrapped with a permeable geotextile to prevent soil particles from entering the perforated blind pipe and causing blockage. The filler layer consists of a uniform mixture of 97% sand and 3% corn straw biochar. The planting layer consists of a mixture of sand and silt loam, with the proportions of the planting layer media varying in each experiment. As shown in Table 1, the medium of A1 is a uniform mixture of 80% sand and 20% silt loam; the medium of A2 is a uniform mixture of 60% sand and 40% silt loam; and the medium of A3 is a uniform mixture of 40% sand and 60% silt loam.

[0032] Table 1 Bioretention column planting layer filler ratio

[0033]

[0034] 1. Rainfall simulation

[0035] The experiment simulated a 60-minute rainfall with a recurrence period of two years in Wuhan. The rainfall intensity and depth were determined according to the "Wuhan Rainstorm Intensity Formula and Design Rainstorm Rain Pattern". This formula was derived based on statistical analysis of a large amount of historical rainfall data in the Wuhan area and hydrological model derivation. It fully considered the impact of local climate, topography and other factors on rainfall intensity, and can accurately reflect the rainfall characteristics under a specific recurrence period in Wuhan.

[0036]

[0037] h=qt

[0038] Where: q is the design rainstorm intensity (mm / min); P(a) is the rainfall return period; t is the rainfall duration (min); h is the rainfall depth (mm).

[0039] Calculation method of inflow (V):

[0040]

[0041] Where: F is the catchment area (m 2 ); is the runoff coefficient, that is, the ratio of runoff to rainfall; other symbols have the same meanings as before.

[0042] The runoff coefficient is taken as 0.9 according to the "Technical Guidelines for Sponge City Planning in Wuhan". This coefficient is determined through field monitoring and model simulation based on multiple factors such as the urban underlying surface type, vegetation cover, and soil infiltration in Wuhan. It can accurately characterize the proportional relationship between rainfall and runoff in the Wuhan area). The confluence ratio is set to 10:1, which is determined based on factors such as the experimental site and the surrounding topography, drainage system layout, etc. The total rainfall is estimated to be 3987ml based on the horizontal surface area of the bioretention column. In order to facilitate the setting of inflow intensity and improve the accuracy of water volume control, the total inflow in the test is 4000ml. In the test, the inflow is in the form of uniform rainfall, and the calculated inflow rate is 66.67ml / min. In this test, an inflow point is set 20cm above the bottom of the bioretention column aquifer to ensure uniform inflow.

[0043] Table 2 Inflow flow design table

[0044]

[0045] 2. Sample Collection

[0046] Before each experiment, pre-inflow fill samples were collected from the bioretention column planting layer using a multi-point stratified sampling method. This involves selecting multiple sampling points at different depths (e.g., 0-10cm, 10-20cm, 20-30cm, etc.) and horizontal locations (e.g., center, edge, and quartering points) within the planting layer to ensure that the collected samples fully represent the original fill state of the newly constructed bioretention column planting layer. Immediately after collection, the samples were sealed and quickly stored in a low-temperature freezer. This low temperature prevented any changes in soil composition prior to the experiment, preserving the samples' original properties to the greatest extent possible.

[0047] Fifteen days after the experiment was completed, professional sampling tools were used to collect samples from the 0-8 cm area at the top of the planting layer. The sampling point was located at the center of the horizontal cross-section of the bioretention column, and filler samples were collected at a depth interval of 1 cm. Each sample was individually packaged in a sealed bag and marked with the sampling depth and location information to ensure that the sample was not contaminated or confused, providing reliable materials for subsequent accurate analysis.

[0048] 3. Sample Processing

[0049] After collecting all the filler samples, first let the filler dry naturally until it is dry and loose, then put it into a sealed bag, and use a rubber hammer to knock until the soil particles are dispersed to ensure that the soil particle agglomerates are broken into separate particles. Before the particle size test, use a 100-mesh soil sieve (0.15mm) to sieve the filler sample, and then use an analytical balance to measure and record the mass of the two parts of filler with a particle size greater than 0.15mm and less than 0.15mm. Calculate the weight ratio of the two parts of filler and approximately convert it to a volume ratio. Collect all fillers smaller than 0.15mm and store them in a sealed bag for soil particle size analysis. The calculation formula is as follows:

[0050]

[0051] Where: r a is the proportion of sieved soil samples, %; r b is the proportion of unscreened soil samples, %; W a is the mass of the sieved soil sample, g; W b is the mass of the sieved soil sample, g.

[0052] 4. Soil Particle Size Analysis

[0053] Soil particle size analysis was conducted at the Environmental Chemistry Laboratory of Wuhan University using a BT-9300ST laser particle size analyzer (Batek, Dandong) to measure the proportion of sand, silt, and clay in the sieved soil samples. The particle size analysis calculation method is as follows:

[0054] R sand =θ sand ×r a +rb

[0055] R silt =θ silt ×r a

[0056] R clay =θ clay ×r a

[0057] Where: R sand is the proportion of sand particles in the soil sample, %; R silt is the proportion of fine particles in the soil sample, %; R clay is the proportion of clay in the soil sample, %; θ sand is the proportion of sand particles in the sieved soil sample, %; θ silt is the proportion of fine particles in the sieved soil sample, %; θ clay is the proportion of clay in the sieved soil samples, %.

[0058] 5. After analyzing the soil particle size, use the natural soil parameter calculation function in Hydrus-1d to calculate its saturated permeability coefficient (K s This software is a professional soil water movement simulation software. It has a large number of built-in verified soil hydraulic parameter models and algorithms. It can accurately calculate the soil permeability coefficient based on input parameters such as soil particle size distribution, porosity, and bulk density. The results of the analysis of fill materials at different depths in different experimental groups (A1, A2, and A3) are as follows:

[0059] Table 3 Analysis of filler particle size and permeability coefficient at different depths in bioretention columns

[0060]

[0061] After the experiment, the proportions of clay and silt in the top 1 cm of the soil layer in the A1, A2, and A3 bioretention columns were significantly higher than before the experiment, while the proportion of sand was significantly lower. The proportion of clay in the 0-8 cm depth of the soil layer was generally higher in A1, A2, and A3 than before the experiment, with a decreasing trend with increasing depth. The proportion of silt in A1 and A2 was also generally higher than before the experiment after the experiment, and gradually decreased with increasing depth. In contrast, the proportion of sand in A1, A2, and A3 was generally lower than before the experiment within the 0-8 cm depth, with a decreasing trend with increasing depth.

[0062] The saturated permeability coefficients of the soil at various depths were calculated based on soil particle size analysis. Changes in the saturated permeability coefficients of A1, A2, and A3 after the experiment corresponded to changes in soil particle size composition. After the inflow experiment, the saturated permeability coefficients of the bioretention column planting soil layers (0-8 cm) in A1, A2, and A3 were generally lower than before the experiment. The permeability coefficient of A3 below 3 cm showed minimal fluctuations, remaining close to pre-experimental levels. The permeability coefficients of A1 and A2 increased with depth and stabilized below 7 cm. These patterns provide important insights for subsequent permeability remediation experiments.

[0063] 6. Bioretention column permeability remediation experiment

[0064] After a 15-day work period, particle size analysis at different depths in the planting layer revealed that the primary changes in soil particle size structure were concentrated within the top 8 cm of the planting layer. To restore the permeability coefficient attenuation caused by these changes in soil structure, this experiment repaired the bioretention column by removing the top 8 cm of filler from the planting layer and backfilling it with the same filler as the original planting layer to restore it to its original height. After the bioretention column was repaired, its permeability coefficient was tested, and the results are shown in Table 4:

[0065] Table 4 Experimental results of permeability recovery of planting soil with different proportions

[0066]

[0067] After replacing the top 8 cm of filler in the planting layer, the permeability coefficient of the A1 bioretention column recovered from 0.19 cm / min to 0.21 cm / min, a recovery rate of 97%. The permeability coefficient of the A2 bioretention column recovered from 0.11 cm / min to 0.19 cm / min, a recovery rate of 103%. The permeability coefficient of the A3 bioretention column recovered from 0.06 cm / min to 0.13 cm / min, a recovery rate of 91.8%. These results strongly demonstrate that replacing the top 8 cm of filler in the planting layer can effectively restore the permeability coefficient of the bioretention columns, verifying the effectiveness and feasibility of the proposed method and providing a reliable practical solution for the operation and maintenance of bioretention facilities.

[0068] Through systematic detection and analysis of the soil particle size and permeability coefficient of bioretention columns, this study accurately revealed the changing patterns of soil particle size composition, and innovatively proposed a remediation plan for replacing the surface soil of the planting soil. The experiment fully verified that this plan can effectively restore the permeability of the bioretention columns, significantly improve the operating efficiency of the facilities, and extend their service life. This provides important technical support and practical guidance for the widespread application of bioretention facilities in sponge city construction and rainwater treatment.

[0069] The above embodiments describe preferred embodiments of the present invention, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent of the present invention shall be based on the appended claims.

Claims

1. A method for operating and maintaining a biological retention pond, characterized in that: The following steps are involved: S1. Collect soil samples from each layer of the newly constructed and operational bioretention ponds and perform particle size analysis to determine the proportion of sand, silt, and clay in each soil sample. S2. Calculate the saturated permeability coefficient Ks of each soil sample using the natural soil parameter calculation function in the Hydrus-1D software based on the particle size data of each soil sample detected in S1; S3. Compare the saturated permeability coefficients Ks of the newly built bioretention pond and the bioretention pond after operation, and replace the soil where the permeability coefficient has been significantly reduced to restore it to the saturated permeability coefficient of the soil sample of the newly built bioretention pond.

2. The method according to claim 1, wherein: When there is a newly built bioretention pond in S1, soil samples from the newly built bioretention pond are directly collected for particle size analysis; if there is no soil sample from the newly built bioretention pond, soil samples of 15 to 20 cm in the target bioretention pond are collected as soil samples for the newly built bioretention pond for particle size analysis.

3. The method according to claim 2, wherein: When the collected soil samples are not analyzed, they are sealed in sealed bags and stored in low temperature.

4. The method according to claim 2, wherein: Before particle size analysis of soil samples, the samples were dried to a loose state, then sieved or hammered until the soil particles were dispersed to ensure that the soil particle aggregates were broken into individual particles. The samples were sieved with a 100-mesh sieve and the sieve material was taken for particle size analysis.

5. The method according to claim 4, characterized in that The following formula is used to calculate the proportion of sand, silt and clay in each soil sample: Where: r a is the proportion of sieved soil samples, %; r b is the proportion of unscreened soil samples, %; W a is the mass of the sieved soil sample, g; W b is the mass of the sieved soil sample, g; R sand =θ sand ×r a +r b ; R silt =θ silt ×r a ; R clay =θ clay ×r a ; Where: R sand is the proportion of sand particles in the soil sample, %; R silt is the proportion of fine particles in the soil sample, %; R clay is the proportion of clay in the soil sample, %; θ sand is the proportion of sand particles in the sieved soil sample, %; θ silt is the proportion of fine particles in the sieved soil sample, %; θ clay is the proportion of clay in the sieved soil samples, %.

6. The method according to any one of claims 1 to 5, characterized in that: The particle size of sand is 0.02~2mm; the particle size of silt is 0.002~0.02mm; the particle size of clay is <0.002mm.

7. The method according to claim 1, wherein: When analyzing the particle size of soil samples from each layer of the bioretention pond after operation, collect surface soil samples from 0 to 10 cm, with one sample collected every 1 cm. The permeability coefficient of each layer of soil after operation is calculated through particle size analysis and model prediction.

8. The method according to claim 1, wherein: The relationship between the saturated permeability coefficient Ks and the particle size of each soil sample is derived from the Rosetta model in the Hydrus-1d software.

9. The method according to claim 1, wherein: A 20-50% decrease in Ks is considered a significant decrease.

10. Application of the method according to any one of claims 1 to 9 in the fields of sponge city construction and rainwater treatment.

Images