CN117534199A - Biological retention filler, preparation method thereof and biological retention facility - Google Patents

Abstract

The application provides a biological retention filler, a preparation method thereof and a biological retention facility, which belong to the technical field of rainwater treatment, wherein the filler comprises: zeolite and clay inoculated with nitrifying bacteria. The preparation method comprises calcining zeolite; placing the roasted zeolite in NaCl for soaking, and then drying the zeolite to obtain modified zeolite; enriching whole nitrifying bacteria through sludge; inoculating nitrifying bacteria in the whole course of modified zeolite. The bioretention facility comprises a matrix layer comprising the bioretention filler described above; the gravel layer is paved below the matrix layer; a planting soil layer laid above the matrix layer; and the third water-permeable geotextile is arranged at the bottom of the gravel layer and coats the periphery of the gravel layer, the matrix layer and the planting soil layer. The invention can enable the biological detention facility to convert the ammonia nitrogen remained in the rainwater into nitrate nitrogen under the low-oxygen environment, thereby improving the problem of poor denitrification effect of the traditional biological detention facility.

Description

Bioretention filler and preparation method thereof, bioretention facilities

Technical field

The invention belongs to the technical field of rainwater treatment, and is particularly related to a bioretention filler, a preparation method thereof, and bioretention facilities.

Background technique

Nitrogen removal from urban rainfall runoff has gradually become an important research direction, and low nitrification efficiency is a difficult issue that restricts its treatment effect. With the innovation of urban sewage treatment facilities and the upgrading of management levels, point source pollution has been effectively controlled. Non-point source pollution control represented by rainfall runoff pollution has become an important direction in urban water environment pollution control. Most runoff treatment facilities, represented by bioretention facilities, have the function of removing suspended pollutants, but they are not effective in removing nitrogen. The nitrogen in rainwater runoff is mainly ammonia nitrogen (about 50%) and nitrate nitrogen. Improving nitrification efficiency is one of the difficult issues to further increase the total nitrogen removal rate. The main reason for this problem is: there is no oxygen supplementation measure in the bioretention facility. When the runoff enters the facility, the oxygen carried is continuously consumed due to the respiratory metabolism of aerobic bacteria, and the interior of the facility gradually enters a low dissolved oxygen state. Traditional two-step nitrifying bacteria The group is a strictly aerobic microorganism, and the nitrification pathway is blocked in a low dissolved oxygen environment. The amount of oxygen in the aerobic section cannot meet the needs of all ammonia nitrogen to be removed by nitrification, making it difficult to further improve the nitrification efficiency of the facility.

Contents of the invention

In order to solve the above-mentioned deficiencies in the prior art, this application provides a bioretention filler, a preparation method thereof, and a bioretention facility, so that the bioretention facility can still convert residual ammonia nitrogen in rainwater into nitrate nitrogen in a low-oxygen environment, thereby improving The problem of poor nitrogen removal in traditional bioretention facilities.

In order to achieve the above objects, the present invention adopts the following technologies:

Bioretention filler is characterized by including:

Zeolite and clay. When used, the clay is laid on top of the gravel layer of the bioretention facility, and the clay is mixed with organic matter. When used, the zeolite is laid on top of the clay. The zeolite and clay are inoculated with full-scale nitrifying bacteria for use in deficiencies. To remove ammonia nitrogen from rainwater in an oxygen environment, the zeolite is modified zeolite that has undergone heat treatment and Na ion exchange.

Bioretention filler preparation method, used to prepare the above-mentioned bioretention filler, includes the steps:

S100. Roast the zeolite;

S200. After soaking the roasted zeolite in a NaCl solution, the zeolite is dried to obtain modified zeolite;

S300, enrich the whole process of nitrifying bacteria through sludge;

S400, inoculate the modified zeolite with nitrifying bacteria throughout the entire process;

S500. Mix clay and organic matter and inoculate the entire process with nitrifying bacteria.

Further, the calcination temperature in step S200 is 500°C-550°C.

Further, in steps S400 and S500, when inoculating full-stage nitrifying bacteria, first dilute the sludge enriched with full-stage nitrifying bacteria with clean water to more than 3000 mg/L, and then pour it into zeolite and clay in multiple batches.

A bioretention facility consisting of:

The matrix layer, including the bioretention facility fill mentioned above;

A gravel layer is laid below the matrix layer, and the gravel layer and the matrix layer are separated by a first permeable geotextile;

The planting soil layer is laid above the matrix layer, and the planting soil layer and the matrix layer are separated by a second permeable geotextile;

The third permeable geotextile is located at the bottom of the gravel layer and covers the gravel layer, matrix layer, and planting soil layer.

Further, the facility also includes an L-shaped shell, which has a receiving cavity. Multiple L-shaped supporting ribs are provided at intervals in the receiving cavity. The horizontal section of the L-shaped shell is provided with multiple vertical passages. hole, and the horizontal section is located between the clay and zeolite, the vertical section of the L-shaped shell is located in the third permeable geotextile, and is located on one side of the planting soil layer and zeolite, and the top of the vertical section has an opening , the vertical section extends out of the top of the planting soil layer.

Further, the diameter of the through hole is set smaller than the particle size of the zeolite to prevent the zeolite from falling into the accommodation cavity.

Further, a section of track is provided on both sides of the top of the planting soil layer, and an inoculation vehicle is provided above the track. The inoculation vehicle includes two pairs of rail wheels and a water tank. Both pairs of rail wheels are rotatably connected to the bottom of the water tank. The rail wheels are fixed on The water tank is driven by a driving mechanism, and two pairs of rail wheels are clamped on the track. A first water outlet pipe and a second water outlet pipe are provided at the bottom of the water tank. The water outlet of the first water outlet pipe is set toward the opening of the vertical section, and the second water outlet pipe is provided at the bottom of the water tank. The length direction of the water outlet pipe is the same as the axial direction of the rail wheel. There are multiple water outlet holes at the bottom of the second water outlet pipe. The water inlets of the first water outlet pipe and the second water outlet pipe are connected to the water tank and connected to an electric valve. .

Further, it also includes an overflow pipe, the top of the overflow pipe is higher than a predetermined height of the planting soil layer, and the bottom of the overflow pipe extends into the gravel layer and is connected to the sewer pipe.

The beneficial effects of the present invention are:

1. The ammonia nitrogen adsorption capacity of the zeolite selected for the filler layer can reach more than 13.4 mg/g, which can effectively absorb ammonia nitrogen in rainwater flowing down from the planting soil layer, and the full-process nitrifying bacteria inoculated can operate in both hypoxic and oxygen-rich environments. The ammonia nitrogen in the rainwater flowing down from the planting soil layer and the ammonia nitrogen adsorbed in the zeolite are converted into nitrate nitrogen, which is decomposed and utilized by microorganisms to prevent the ammonia nitrogen adsorbed in the zeolite from reaching saturation. The filler layer is modified by heat treatment and Na ion exchange. It can further improve the adsorption and removal of ammonia nitrogen by zeolite and improve the denitrification effect of rainwater.

2. By setting up an L-shaped shell, after the construction of the bioretention facility is completed, the full-process nitrifying bacteria enrichment solution can be injected from the top of the L-shaped shell, so that the full-process nitrifying bacteria enrichment solution reaches between the zeolite and clay, which facilitates subsequent biological processes. When the nitrifying bacteria in the retention facility are reduced throughout the process, the nitrifying bacteria enrichment solution is directly added to the bioretention facility through the L-shaped shell to improve the inoculation effect of the clay layer.

3. The track on the top of the planting soil layer and the setting of the inoculation vehicle, combined with the L-shaped shell, can simultaneously inject the full-process nitrifying bacteria enrichment solution from above the planting soil layer and above the clay of the matrix layer, improving the inoculation efficiency and uniformity of inoculation. .

Description of drawings

Figure 1 is a perspective view of the overall structure of the bioretention facility according to the embodiment of the present application.

Figure 2 is a cross-sectional view of a bioretention facility according to an embodiment of the present application.

Figure 3 is an enlarged view of part A in Figure 2.

Figure 4 is a top view of the bioretention facility according to the embodiment of the present application.

Figure 5 is an enlarged view of part C in Figure 4.

Figure 6 is a structural perspective view of the vaccination vehicle in the bioretention facility according to the application embodiment.

Figure 7 is an enlarged view of part B in Figure 6.

Reference signs: planting soil layer-1, matrix layer-2, gravel layer-3, first permeable geotextile-4, second permeable geotextile-5, third permeable geotextile-6, L-shaped shell-7 , track-8, track wheel-9, water tank-10, overflow pipe-11, zeolite-201, clay-202, L-shaped support rib-701, horizontal section-702, vertical section-703, opening-704, The first water outlet pipe-1001, the second water outlet pipe-1002, the water outlet hole-1003.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. However, the described embodiments of the present invention are part of the embodiments of the present invention, not all of them. .

The embodiment of the present application provides a bioretention filler, including zeolite 201 and clay 202. When used, the clay 202 is laid above the gravel layer 3 of the bioretention facility, and organic matter is mixed in the clay 202 as nutrients for the growth of microorganisms. Application At this time, zeolite 201 is laid on top of clay 202, and both zeolite 201 and clay 202 are inoculated with full-process nitrifying bacteria to remove ammonia nitrogen in rainwater in an anoxic environment.

Because zeolite 201 has a polyregular pore structure, its pores are about the same size as the molecules of ordinary substances, with diameters ranging from about 0.3nm to 1nm, and its specific surface area is as high as 400m ² /g ~ 800m ² /g. The special physical structure gives zeolite Highly efficient adsorption, it can effectively adsorb ammonia nitrogen in flowing rainwater. At the same time, in anoxic and oxygen-rich environments, the adsorbed ammonia nitrogen will be converted into nitrate nitrogen by the full-process nitrifying bacteria inoculated on the zeolite, and then be converted into nitrate nitrogen by microorganisms. Decomposition and utilization can prevent the ammonia nitrogen adsorbed in zeolite 201 from reaching saturation.

Preferably, the selected zeolite 201 is a modified zeolite that has undergone heat treatment and Na ion exchange. After zeolite 201 is heat treated, the water in zeolite 201 escapes to form a loose and porous sponge, which can effectively improve the adsorption performance. After zeolite 201 undergoes Na ion exchange, more Na ions are obtained, which can further improve the ammonia nitrogen adsorption effect.

The filler is prepared using a bioretention filler preparation method. The specific steps are: roasting zeolite 201; soaking the roasted zeolite 201 in a NaCl solution, and then drying the zeolite 201 to obtain modified zeolite ; Enrich nitrifying bacteria through sludge; inoculate modified zeolite with nitrifying bacteria; mix clay 202 with organic matter and inoculate nitrifying bacteria throughout the process.

Specifically, the method for enriching nitrifying bacteria throughout the process is: first inoculate 1L of sludge from the aerobic section of the sewage treatment plant in a 5L continuous flow SBR reactor, and continuously and slowly input no more than 50 mgN/L ammonia nitrogen without drainage. concentration of the water sample to be treated. Among them, the typical feed water composition of the reactor is as follows: each liter contains 26.745mg NH ₄ Cl, 50mg KH ₂ PO ₄ , 75mg KCl, 50mgMgSO ₄ ·7H ₂ O, 584mg NaCl, 50mg CaCl ₂ and 1mL trace element solution. The composition of trace element solution per liter is as follows: 34.4mg MnSO ₄ ·1H ₂ O, 50mg H ₃ BO ₃ , 70mg ZnCl ₂ , 72.6mg Na ₂ MoO ₄ ·2H ₂ O, 2mg CuCl ₂ ·2H ₂ O, 24mg NiCl ₂ · 6H ₂ O, 80 mg CoCl ₂ ·6H ₂ O, 1gFeSO ₄ ·6H ₂ O. Then control the dissolved oxygen concentration to be lower than 1.0mg/L, inject water for 8 hours in each cycle, and the water inlet rate is 1.45mL/min; precipitate for 45min, discharge water at a rate of 29mL/min, and then idle for a certain period of time. Finally, the ammonia nitrogen concentration in the reactor is regularly monitored. When the ammonia nitrogen concentration of the effluent is lower than 0.05mgN/L, the enrichment is completed.

Specifically, when calcining zeolite 201, set the calcining temperature to 500°C-550°C. Zeolite 201 has high temperature resistance, but if the temperature is too high, its structure will be destroyed and it will lose its ion exchange function. In this embodiment, zeolite 201 is roasted at a temperature of 500°C to 550°C, which can improve the mechanical strength of zeolite 201, increase the pore volume, increase the specific surface area, and increase the movement activity of cations, so that subsequent Na ion exchange can be carried out more fully. .

Specifically, when inoculating full-stage nitrifying bacteria, first dilute the sludge enriched with full-stage nitrifying bacteria with clean water to above 3000 mg/L, and then pour it into zeolite 201 and clay 202 in multiple batches, so that the full-stage nitrifying bacteria can follow the inoculation. The clean water gradually penetrates into the zeolite 201 and clay 202.

On the other hand, as shown in Figures 1 to 7, this application provides a bioretention facility, including a matrix layer 2, a gravel layer 3, a planting soil layer 1, etc.

Specifically, referring to Figures 1 and 2, the matrix layer 2 includes the above-mentioned bioretention filler, in which clay 202 is laid above the gravel layer 3 of the bioretention facility, zeolite 201 is laid above the clay 202 and organic matter, and the gravel layer 3 is laid Below the matrix layer 2, and the gravel layer 3 and the matrix layer 2 are separated by the first permeable geotextile 4, the planting soil layer 1 is laid above the matrix layer 2, and the planting soil layer 1 and the matrix layer 2 are separated by the second permeable geotextile. Separated by cloth 5 , a third permeable geotextile 6 is provided at the bottom of the gravel layer 3 , and the third permeable geotextile 6 covers the gravel layer 3 , the matrix layer 2 , and the planting soil layer 1 .

When rainwater flows into the bioretention facility, it first passes through the topmost soil layer 1 for preliminary filtration, and then penetrates downward into the matrix layer 2. Zeolite 201 and clay 202 inoculated with full-scale nitrifying bacteria in the matrix layer 2 will remove the rainwater. The ammonia nitrogen adsorbs, transforms and decomposes to achieve denitrification of rainwater, and subsequent rainwater continues to flow downward into the gravel layer 3 and then is discharged into the sewer pipe.

Specifically, referring to Figures 1 and 4, the bioretention facility also includes an overflow pipe 11. The top of the overflow pipe 11 is higher than a predetermined height of the planting soil layer 1, and the bottom of the overflow pipe 11 extends into the gravel layer 3 and the sewer pipe. connection, when there is too much rainfall and the maximum water storage height is exceeded in the bioretention facility, the excess rainwater can directly flow into the sewer pipe from the overflow pipe 11 to prevent the road surface from being flooded by rainwater.

Preferably, referring to Figures 1-2 and 4-5, the bioretention facility also includes an L-shaped housing 7. The L-shaped housing 7 has a receiving chamber, and multiple L-shaped chambers are spaced in the receiving chamber. Support ribs 701, L-shaped support ribs 701 can be used to support the L-shaped shell 7 to prevent the zeolite 201 above from deforming the L-shaped shell 7 due to downward pressure. The top and bottom of the horizontal section 702 are provided with multiple through holes. Specifically, , the diameter of the through hole is set to be smaller than the particle size of the zeolite 201 to prevent the zeolite 201 from falling into the accommodation cavity, and the horizontal section 702 is provided between the clay 202 and the zeolite 201 of the matrix layer 2, and the L-shaped shell 7 The vertical section 703 is provided in the third water-permeable geotextile 6 and is provided on one side of the planting soil layer 1 and the zeolite 201. The top of the vertical section 703 has an opening 704, and the vertical section extends out of the top of the planting soil layer 1. Because the L-shaped support ribs 701 can divide the accommodation cavity into multiple L-shaped accommodation spaces, after injecting the full-process nitrifying bacteria enrichment solution from the top of the L-shaped housing 7, the inoculum solution will be divided into multiple L-shaped accommodation spaces. The accommodation space forms a water column with a certain height, which slowly flows downward into the clay 202 from the through hole.

After the filler layer inoculated with full-stage nitrifying bacteria is used for a period of time, due to environmental reasons (such as the reduction of nutrients in the bioretention facility, or the decrease in ammonia nitrogen flowing into the bioretention facility due to less rainfall), the reproduction of full-stage nitrifying bacteria in it slows down. As a result, the total nitrifying bacteria are reduced. At this time, it is necessary to inoculate the full-process nitrifying bacteria again to improve the ammonia nitrogen removal effect of rainwater. The more direct way is to directly pour the enriched solution of the full-process nitrifying bacteria from above the planting soil layer 1. This method will make most of the The sludge with full-stage nitrifying bacteria adheres to the planting soil layer 1 and zeolite 201, while the clay 202 is inoculated with less full-stage nitrifying bacteria. After the L-shaped shell 7 is installed, the enriched liquid of the whole-process nitrifying bacteria can flow directly into the clay 202 evenly through the cavity of the L-shaped shell 7, thereby improving the uniformity of inoculation and improving the removal effect of ammonia nitrogen in rainwater.

Preferably, referring to Figures 1, 2, 6, and 7, a section of track 8 is provided on both sides of the top of the planting soil layer 1, and an inoculation vehicle is provided above the track 8. The inoculation vehicle includes two pairs of rail wheels 9 and a water tank 10. , both pairs of rail wheels 9 are rotatably connected to the bottom of the water tank 10, and the rail wheels 9 are driven by a driving mechanism fixed to the water tank 10. Specifically, the driving mechanism can select a motor, and both pairs of rail wheels 9 are stuck on the track 8. A first water outlet pipe 1001 and a second water outlet pipe 1002 are provided at the bottom of the water tank 10. The water outlet of the first water outlet pipe 1001 is arranged toward the opening 704 of the vertical section 703. The length direction of the second water outlet pipe 1002 is the same as the axial direction of the rail wheel 9. , a plurality of water outlets 1003 are provided at the bottom of the second water outlet pipe 1002, and the water inlet ends of the first water outlet pipe 1001 and the second water outlet pipe 1002 are both connected to the water tank 10 and connected to an electric valve. When it is necessary to supplement the inoculation of full-stage nitrifying bacteria in the bioretention facility, inject the diluted full-stage nitrifying bacteria enrichment solution into the water tank 10, then open the electric valve, and control the driving mechanism to drive the rail wheel 9 to rotate to enrich the full-stage nitrifying bacteria. The collected liquid is injected above the planting soil layer 1 and into the L-shaped housing 7 at the same time, which can effectively improve the inoculation efficiency. After the inoculation is completed, the top of the L-shaped housing 7 can be covered with materials such as waterproof cloth and waterproof tape to prevent rainwater. Just flow into it from the top.

After testing, after the bioretention facility operated stably for 20 days, the following table was obtained regarding the average removal rate of pollutants in the bioretention facility.

Inlet water concentration Effluent concentration removal rate Ammonia nitrogen 28mg/L 0.52±0.25mg/L 85.1±0.9%

After testing, after the bioretention facility operated stably for 50 days, the following table was obtained regarding the average removal rate of pollutants in the bioretention facility.

Inlet water concentration Effluent concentration removal rate Ammonia nitrogen 28mg/L 0.48±0.51mg/L 85.1±1.8%

Specifically, the test method is to first configure simulated rainwater: add 180L tap water to a 200L PE dosing barrel and let it sit for at least 24 hours to reduce the impact of residual chlorine. Stir and aerate before the start of each test to simulate the high dissolved oxygen characteristics of actual rainwater. Then add the prepared pollutant stock solution, adjust the volume to 200L and stir clockwise for 15 minutes. The main components of pollutants come from the following substances, nitric acid (KNO ₃ ), ammonium chloride (NH ₄ Cl), potassium dihydrogen phosphate (KH ₂ PO ₄ ), glycine (C ₂ H ₅ NO ₂ ), sodium acetate (CH ₃ COONa). The specific concentration of pollutants was selected with reference to the concentration of dissolved nutrients in urban rainwater runoff measured in the field. The higher value in these studies was taken and the influence of free nitrogen carried by the tap water in the laboratory was taken into consideration. The pollutant concentration for this experimental method was selected as the ammonia nitrogen concentration of 28 mg/L when the full load of enriched nitrifying bacteria (Commammox) is fully loaded.

Finally, the configured simulated rainwater is slowly transported to the 15-hole water distribution nozzle through a peristaltic pump and evenly distributed over the bioretention facility. The effluent is collected regularly and the water quality is monitored.

Obviously, compared with the ammonia nitrogen removal rate of traditional bioretention facilities, which is less than 70%, this bioretention facility can significantly increase the ammonia nitrogen removal rate of bioretention facilities in rainwater. If these modifications and variations of the invention fall within the scope of the claims of the present invention and equivalent technologies, the present invention is also intended to include these modifications and variations. The above-mentioned examples or implementations are only examples of the present invention, and the present invention can also be implemented in other specific ways or other specific forms without departing from the gist or essential characteristics of the present invention.

Claims (10)

1. The biological detention filler is characterized by comprising zeolite (201) and clay (202), wherein the clay (202) is paved above a gravel layer (3) of a biological detention facility in application, organic matters are mixed in the clay (202), the zeolite (201) is paved above the clay (202) in application, and the zeolite (201) and the clay (202) are inoculated with nitrifying bacteria in the whole process for removing ammonia nitrogen in rainwater in an anoxic environment.

2. The bioretention filler according to claim 1 wherein the zeolite (201) is a modified zeolite that has been heat treated and Na ion exchanged.

3. A method of preparing a bioretention filler for preparing the bioretention filler of claim 2, comprising the steps of:

s100, roasting zeolite (201);

s200, placing the roasted zeolite (201) in NaCl solution for soaking, and then drying the zeolite (201) to obtain modified zeolite;

s300, enriching whole nitrifying bacteria through sludge;

s400, inoculating nitrifying bacteria in the whole course of the modified zeolite;

s500, mixing clay (202) and organic matters, and then inoculating nitrifying bacteria in the whole process.

4. A method of preparing a bioretention filler according to claim 3, wherein the firing temperature in step S200 is 500-550 ℃.

5. The method for preparing a bio-retentive filler of claim 3, wherein in the steps S400 and S500, when whole nitrifying bacteria are inoculated, the sludge enriched with whole nitrifying bacteria is diluted to more than 3000mg/L by clean water, and then the sludge is sprayed into the modified zeolite and clay (202) for a plurality of times.

6. A bioretention facility, comprising:

a matrix layer (2) comprising the bioretention filler according to any one of claims 1 or 2;

the gravel layer (3) is paved below the matrix layer (2), and the gravel layer (3) is separated from the matrix layer (2) through the first permeable geotextile (4);

the soil planting layer (1) is paved above the matrix layer (2), and the soil planting layer (1) and the matrix layer (2) are separated by a second permeable geotextile (5);

and the third water-permeable geotextile (6) is arranged at the bottom of the gravel layer (3) and coats the periphery of the gravel layer (3), the matrix layer (2) and the planting soil layer (1).

7. The biological retention facility according to claim 6, further comprising an L-shaped shell (7), wherein a containing cavity is formed in the L-shaped shell (7), a plurality of L-shaped supporting ribs (701) are arranged in the containing cavity at intervals, a plurality of vertical through holes are formed in a horizontal section (702) of the L-shaped shell (7), the horizontal section (702) is arranged between clay (202) and zeolite (201) of the matrix layer (2), a vertical section (703) of the L-shaped shell (7) is arranged in the third water-permeable geotechnical cloth (6) and is arranged on one side of the soil planting layer (1) and the zeolite (201), an opening (704) is formed in the top of the vertical section (703), and the vertical section extends out of the top of the soil planting layer (1).

8. A bio-detention device according to claim 7, characterized in that the diameter of the through-holes is set smaller than the particle size of the zeolite (201).

9. The biological retention facility according to claim 7, wherein one section of track (8) is arranged on two sides of the top of the soil planting layer (1), an inoculation vehicle is arranged above the track (8), the inoculation vehicle comprises two pairs of track wheels (9) and a water tank (10), the two pairs of track wheels (9) are all rotationally connected to the bottom of the water tank (10), the track wheels (9) are driven by a driving mechanism fixed to the water tank (10), the two pairs of track wheels (9) are all clamped on the track (8), a first water outlet pipe (1001) and a second water outlet pipe (1002) are arranged at the bottom of the water tank (10), a water outlet of the first water outlet pipe (1001) is arranged towards the opening (704), the length direction of the second water outlet pipe (1002) is the same as the axial direction of the track wheels (9), a plurality of water outlet holes (1003) are formed in the bottom of the second water outlet pipe (1002), and the water inlet ends of the first water outlet pipe (1001) and the second water outlet pipe (1002) are all connected to the water tank (10) and are connected with an electric valve.

10. The bioretention facility according to claim 6, further comprising an overflow pipe (11), wherein the top of the overflow pipe (11) is higher than the planting soil layer (1) by a predetermined height, and the bottom of the overflow pipe (11) extends into the gravel layer (3) to be connected with a sewer pipe.