Combined treatment system and method for tail water and initial rainwater of domestic sewage treatment plant

Abstract

According to the combined treatment system and method for the tail water and the initial rainwater of the domestic sewage treatment plant, through the arrangement of the rainwater overflow pond, the bioretention pond, the nitrogen and phosphorus removal constructed wetland pond and the post-treatment constructed wetland pond, deep nitrogen and phosphorus removal of the tail water of the domestic sewage treatment plant in a dry season mode and purification of the initial rainwater in a rainy season mode are achieved; the advanced treatment requirement of the domestic sewage tail water can be met, efficient utilization of rainwater can be achieved, obtained effluent can be used as miscellaneous water for greening, toilet flushing, car washing and the like, the utilization rate of non-traditional water sources is increased, and dependence on traditional water resources is reduced. The system is efficient in treatment capacity, high in flexibility, capable of switching modes in dry and rainy seasons, capable of effectively treating tail water and initial rainwater of a domestic sewage treatment plant, capable of recycling the initial rainwater, capable of improving the utilization rate of water resources and capable of reducing energy consumption, simple in operation, convenient to maintain, low in operation and maintenance cost and high in impact load resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of water treatment and relates to a combined treatment system and method. Background Art

[0002] Domestic sewage treatment plant tailwater and initial rainwater are usually discharged directly without sufficient treatment, resulting in both resource waste and environmental pollution. For these scenarios, existing sewage treatment systems mostly operate in a single mode, making it difficult to simultaneously take into account the needs of domestic sewage tailwater deep treatment and rainwater resource utilization, resulting in widespread problems such as domestic sewage treatment plant tailwater not meeting discharge standards and low rainwater utilization.

[0003] At the same time, the country's requirements for saving water resources and promoting the use of non-traditional water sources are constantly increasing. In the process of developing infrastructure such as sewage treatment plants in a green and sustainable direction, how to make full use of domestic sewage and rainwater resources has become an urgent problem to be solved. However, for domestic sewage and rainwater resources, traditional artificial wetland technologies are mostly limited to a single function, lack flexibility, and cannot effectively treat the mixed discharge of seasonal rainwater and domestic sewage tailwater, and fail to optimize the design in combination with the recycling goal of non-traditional water sources. Summary of the invention

[0004] In order to solve the problems that the traditional artificial wetland technologies described in the background technology are mostly limited to a single function, lack flexibility, cannot effectively treat the mixed discharge of seasonal rainwater and domestic sewage treatment plant tail water, and fail to optimize the design in combination with the recycling goal of non-traditional water sources, the present invention provides a combined treatment system and method for domestic sewage treatment plant tail water and initial rain water.

[0005] The combined treatment system of the present invention comprises a rainwater overflow tank, a biological retention tank, a denitrification and phosphorus removal artificial wetland tank, a post-treatment artificial wetland tank and a water storage tank; a rainwater overflow port is arranged on the top of the rainwater overflow tank, and a rainwater discharge port is arranged at the bottom of the rainwater overflow tank; the biological retention tank is arranged on the side of the rainwater overflow tank and is connected with the rainwater overflow port, the biological retention tank comprises an impermeable layer, a gravel layer, a sand filter layer and a planting soil layer arranged in sequence from the bottom to the top, a tailwater inlet of domestic sewage tailwater is arranged on the upper side of the planting soil layer, the upper side of the planting soil layer is connected with the rainwater overflow inlet, and a percolation outlet is arranged on the lower side of the gravel layer; the denitrification and phosphorus removal artificial wetland The ground pool is arranged on the side of the biological retention pool and is connected with the infiltration outlet. The denitrification and phosphorus removal artificial wetland pool uses siderite and pyrrhotite as the matrix, wetland plants are planted on the surface of the denitrification and phosphorus removal artificial wetland pool, and a denitrification and phosphorus removal outlet is arranged on the lower side of the denitrification and phosphorus removal artificial wetland pool; the post-treatment artificial wetland pool is arranged on the side of the denitrification and phosphorus removal artificial wetland pool and is connected with the denitrification and phosphorus removal outlet, an aeration mechanism is arranged at the bottom of the post-treatment artificial wetland pool, the post-treatment artificial wetland pool uses zeolite as the matrix, wetland plants are planted on the surface of the post-treatment artificial wetland pool, and a post-treatment outlet is arranged on the upper side of the zeolite matrix of the post-treatment artificial wetland pool; the water storage tank is connected with the post-treatment outlet.

[0006] Furthermore, the thickness of the gravel layer of the biological retention pond is 300 mm, and the gravel particle size of the gravel layer is 12-15 mm.

[0007] Furthermore, the thickness of the sand filter layer of the biological retention pond is 100 mm, the volume ratio of fine sand to medium-coarse sand in the sand filter layer is 1:1, the particle size of the medium-coarse sand is 3-5 mm, and the particle size of the fine sand is 0.5-1.5 mm.

[0008] Furthermore, the thickness of the planting soil layer of the bioretention pond is 400-1000 mm.

[0009] Furthermore, the rainwater overflow inlet of the rainwater overflow pool is more than 20 cm higher than the surface of the planting soil layer.

[0010] Furthermore, the artificial wetland pool for denitrification and phosphorus removal includes an artificial wetland pool I and an artificial wetland pool II which are arranged in parallel and connected at the top. The artificial wetland pool I and the artificial wetland pool II are both based on siderite and pyrrhotite. The volume ratio of the siderite and pyrrhotite is 1:1. The particle size of the siderite is 4-8 mm, and the particle size of the pyrrhotite is 4-8 mm.

[0011] Furthermore, the zeolite particle size of the post-treatment artificial wetland pool is 4-8 mm.

[0012] Furthermore, the surface load of the denitrification and phosphorus removal artificial wetland pool and the post-treatment artificial wetland pool is 0.5-0.6m3 / (m 2 ·d), the effective height is 1.2m.

[0013] Furthermore, the wetland plants planted on the surface of the denitrification and phosphorus removal artificial wetland pool and the post-treatment artificial wetland pool include calamus, and the planting density of the wetland plants is 10-12 plants / m 2 .

[0014] Based on the above-mentioned combined treatment system, the present invention also proposes a combined treatment method for tail water and initial rainwater from a domestic sewage treatment plant, comprising the following steps:

[0015] In the dry season, the tailwater of domestic sewage flows into the top of the biological retention pond through the tailwater inlet, and then filters through the planting soil layer, sand filter layer, and gravel layer in turn. The water after the infiltration treatment enters the artificial wetland pool for nitrogen and phosphorus removal through the infiltration outlet.

[0016] During the rainy season, the tailwater inlet of the domestic sewage treatment plant is closed. The initial rainwater is filtered through the planting soil layer, sand filter layer, and gravel layer of the biological retention pond in turn. The water after the infiltration treatment enters the denitrification and phosphorus removal artificial wetland pool through the infiltration outlet. When the rainwater height above the planting soil layer exceeds the height of the rainwater overflow inlet, the rainwater flows into the rainwater overflow pool through the rainwater overflow inlet and is discharged through the rainwater discharge outlet.

[0017] The water after the filtration treatment enters the artificial wetland pool for nitrogen removal and phosphorus removal, and the nitrate nitrogen and phosphate in the water are removed by siderite and pyrrhotite. The water after the nitrogen removal and phosphorus removal treatment is collected into the post-treatment artificial wetland pool through the nitrogen removal and phosphorus removal outlet.

[0018] The water after denitrification and dephosphorization treatment enters the post-treatment artificial wetland pool, where ammonia nitrogen in the water is further removed through zeolite. The post-treated water is then collected into the water storage tank through the post-treatment outlet. At the same time, the post-treatment artificial wetland pool biologically regenerates the zeolite by regularly starting the aeration mechanism at the bottom.

[0019] Compared with the prior art, the present invention realizes deep denitrification and phosphorus removal of tail water of domestic sewage treatment plant in dry season mode and purification of initial rainwater in rainy season mode through the setting of rainwater overflow pool, biological retention pool, artificial wetland pool for denitrification and phosphorus removal, and post-treatment artificial wetland pool, which can not only meet the deep treatment requirements of tail water of domestic sewage treatment plant, but also realize efficient utilization of rainwater. The effluent obtained can be used for miscellaneous water such as greening, flushing toilets and washing cars, improve the utilization rate of non-traditional water sources, and reduce dependence on traditional water resources. The present invention integrates biological retention facilities and artificial wetlands, has high denitrification and phosphorus removal efficiency, high processing capacity, strong flexibility, can switch between dry and rainy season modes, can realize effective treatment of tail water and initial rainwater of domestic sewage treatment plant, realize rainwater reuse, and improve water resource utilization. The present invention adopts a treatment method mainly based on natural purification, reduces energy consumption, is simple to operate, convenient to maintain, has low operation and maintenance costs, and has strong impact load resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A partial schematic diagram of the combined processing system.

[0021] Figure 2 A cross-sectional schematic diagram of the combined treatment system. DETAILED DESCRIPTION

[0022] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0023] Combined treatment system for tailwater and initial rainwater from domestic sewage treatment plants, such as Figure 1 and Figure 2 As shown, it is specifically composed of a rainwater overflow tank, a biological retention tank, a denitrification and phosphorus removal artificial wetland tank, a post-treatment artificial wetland tank and a water storage tank; the rainwater overflow tank, the biological retention tank, the denitrification and phosphorus removal artificial wetland tank, the post-treatment artificial wetland tank and the water storage tank can be arranged in sequence.

[0024] Specifically, a rainwater overflow port is arranged at the top of the rainwater overflow pool, and a rainwater discharge port is arranged at the bottom of the rainwater overflow pool. The rainwater discharge port can be connected to a nearby rainwater well through a rainwater pipe.

[0025] Specifically, the biological retention tank is arranged on the side of the rainwater overflow tank and connected to the rainwater overflow port. The biological retention tank is provided with an anti-seepage layer, a gravel layer, a sand filter layer, and a planting soil layer from the bottom to the top. More specifically, the thickness of the gravel layer of the biological retention tank is 300mm, and the gravel particle size of the gravel layer is 12-15mm; the thickness of the sand filter layer of the biological retention tank is 100mm, and the volume ratio of fine sand to medium coarse sand in the sand filter layer is 1:1, the medium coarse sand particle size is 3-5mm, and the fine sand particle size is 0.5-1.5mm; the thickness of the planting soil layer of the biological retention tank is 400-1000mm. The upper side of the planting soil layer is provided with a tailwater inlet for domestic sewage tailwater; the upper side of the planting soil layer is connected to the rainwater overflow inlet. More specifically, the rainwater overflow port of the rainwater overflow tank is more than 20cm higher than the surface of the planting soil layer, that is, the highest water level of the biological retention tank = the overflow water level. The lower side of the gravel layer is provided with a percolation outlet. Figure 2 As shown, a geotextile can be set under the planting soil layer, and a sand filter layer and a gravel layer can be laid in sequence under the geotextile. A puncture-proof permeability membrane can be set under the gravel layer as an anti-seepage layer, and the bottom of the anti-seepage layer is compacted by tamping the bare soil.

[0026] Specifically, the artificial wetland pool for denitrification and phosphorus removal is arranged on the side of the biological retention pool and is connected to the infiltration outlet. More specifically, the artificial wetland pool for denitrification and phosphorus removal includes an artificial wetland pool I and an artificial wetland pool II which are arranged in parallel and connected at the top. Both the artificial wetland pool I and the artificial wetland pool II use siderite and pyrrhotite as the matrix. More specifically, the volume ratio of siderite and pyrrhotite is 1:1, the particle size of siderite is 4-8mm, and the particle size of pyrrhotite is 4-8mm. Wetland plants are planted on the surface of the artificial wetland pool for denitrification and phosphorus removal. The lower side of the artificial wetland pool for denitrification and phosphorus removal, i.e., the lower side of the artificial wetland pool II, is provided with a denitrification and phosphorus removal outlet. Among them, the planted wetland plants can be calamus, and the planting density is 10-12 plants / m 2 ; The surface load of the artificial wetland pool for nitrogen and phosphorus removal is 0.5-0.6m 3 / (m 2 ·d), the effective height is 1.2m. Figure 2 As shown, the bottom of the artificial wetland pool for nitrogen and phosphorus removal, that is, the bottom of the siderite and pyrrhotite matrix, is first backfilled with 20 cm of coarse sand, and then a composite anti-seepage geotextile is set underneath, and the bottom of the geotextile is compacted with plain soil.

[0027] Specifically, the post-treatment artificial wetland pool is arranged on the side of the denitrification and phosphorus removal artificial wetland pool and is connected to the denitrification and phosphorus removal outlet. An aeration mechanism is arranged at the bottom of the post-treatment artificial wetland pool. The post-treatment artificial wetland pool uses zeolite as a matrix. More specifically, the zeolite particle size of the post-treatment artificial wetland pool is 4-8 mm. Wetland plants are planted on the surface of the post-treatment artificial wetland pool, and a post-treatment outlet is arranged on the upper side of the zeolite matrix of the post-treatment artificial wetland pool. Among them, the planted wetland plants can be calamus, and the planting density is 10-12 plants / m 2 ; The surface load of the post-treatment artificial wetland pool is 0.5-0.6m 3 / (m 2 ·d), the effective height is 1.2m. Figure 2 As shown, the bottom of the post-treatment artificial wetland pool, i.e. the bottom of the aeration mechanism, is first backfilled with 20 cm of coarse sand, and then a composite anti-seepage geotextile is set underneath, and plain soil is used to compact under the geotextile.

[0028] Specifically, the water storage tank is connected to the post-treatment water outlet. After the water from the post-treatment artificial wetland pool is disinfected and tested in the water storage tank and passes the inspection, it can be used for miscellaneous water such as greening, flushing toilets and washing cars.

[0029] Based on the above-mentioned combined treatment system, a combined treatment method for tail water and initial rainwater from a domestic sewage treatment plant is provided, and the specific steps are as follows.

[0030] In the dry season, the tailwater of the domestic sewage treatment plant flows into the top of the biological retention pond through the water inlet, and then filters through the planting soil layer, sand filter layer, and gravel layer in turn. The water after the infiltration treatment enters the artificial wetland pool for nitrogen and phosphorus removal through the infiltration outlet. In this process, the suspended solid matter in the water is effectively removed through infiltration.

[0031] During the rainy season, the water inlet of the tailwater of the domestic sewage treatment plant is closed. The initial rainwater is filtered through the planting soil layer, sand filter layer, and gravel layer of the biological retention pond in turn. The water after the infiltration treatment enters the artificial wetland pool for nitrogen removal and phosphorus removal through the infiltration outlet. When the rainwater height above the planting soil layer exceeds the height of the rainwater overflow port, the rainwater flows into the rainwater overflow pool through the rainwater overflow inlet and is discharged through the rainwater discharge port. In this process, the suspended solid matter in the water is effectively removed through infiltration. The initial rainwater enters the subsequent artificial wetland pool after infiltration in the biological retention pond. The mid- and late-stage rainwater is generally cleaner. After the mid- and late-stage rainwater enters the biological retention pond, due to the limited infiltration coefficient of the biological retention pond, the mid- and late-stage rainwater height exceeds the height of the rainwater overflow inlet. The mid- and late-stage rainwater is discharged through the overflow pool and the rainwater discharge port to realize the resource utilization of rainwater.

[0032] The water after the filtration treatment enters the artificial wetland pool for nitrogen removal and phosphorus removal, and the nitrate nitrogen and phosphate in the water are removed by siderite and pyrrhotite. The water after nitrogen removal and phosphorus removal is collected into the post-treatment artificial wetland pool through the outlet of nitrogen removal and phosphorus removal. In this process, nitrate nitrogen is removed by iron autotrophic denitrifying bacteria and sulfur autotrophic denitrifying bacteria with natural pyrite minerals as electrons, and phosphate is removed by substances such as hydroxyl iron produced on the surface of natural pyrite minerals through adsorption, coprecipitation and other effects.

[0033] The water after denitrification and dephosphorization treatment enters the post-treatment artificial wetland pool, where ammonia nitrogen in the water is further removed by zeolite to ensure that the effluent is stable and meets the standards. The post-treated water is then collected into the water storage tank through the post-treatment outlet. At the same time, the post-treatment artificial wetland pool biologically regenerates the zeolite by regularly starting the aeration mechanism at the bottom, eliminating the need to replace the zeolite filler.

[0034] The effluent from the post-processed artificial wetland pool can be used for landscaping, flushing toilets, car washing and other miscellaneous water after being disinfected and tested in the storage tank.

[0035] Through the above process, the present invention is mainly used to treat tail water of domestic sewage treatment plants in the dry season, especially the tail water of domestic sewage treatment plants after multi-stage biological enhanced contact oxidation. Through the present invention, further nitrogen and phosphorus removal is carried out, so that the effluent water quality meets the Class A standard of the Pollutant Discharge Standard for Urban Sewage Treatment Plants (GB 18918-2002), and some indicators reach the quasi-Class IV standard, so that the effluent can be used for miscellaneous water such as greening, flushing toilets and washing cars. In the rainy season, the present invention can also collect and treat initial rainwater, so that the effluent water quality meets the reuse water standard, and realizes the efficient use of initial rainwater.

[0036] In practical operation, when the COD and NH + - N, NO3 - When the concentrations of -N, TN, and TP were 19.61±6.93, 0.29±0.36, 10.83±2.32, 11.68±2.15, and 0.56±0.63 mg / L, respectively, COD, NH4 + -N、NO3 - -N, TN, TP concentrations were reduced to 16.75±6.98, 0.09±0.23, 2.85±1.39, 3.15±1.32, 0.27±0.15 mg / L, respectively. The average water quality of the reservoir was better than the Class IV water standard (non-lake reservoir) of the Environmental Quality Standard for Surface Water (GB3838-2002). When the initial rainwater turbidity, BOD5, NH4 +When the -N concentration was 52.78±7.49NTU, 18.76±7.38mg / L, and 2.74±3.49mg / L, respectively, after being treated in the biological retention tank, the artificial wetland pool for nitrogen and phosphorus removal, and the post-treatment artificial wetland pool, the turbidity, BOD5, and NH4 + -N concentrations were 1.26±0.93NTU, 6.47±2.35mg / L, and 0.29±0.37mg / L, respectively. The effluent quality was better than the "Water Quality Standard for Urban Miscellaneous Water for Urban Wastewater Recycling" (GB / T 18920-2020).

[0037] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant, characterized by: Including rainwater overflow pond, biological retention pond, artificial wetland pool for nitrogen and phosphorus removal, post-treatment artificial wetland pool and water storage tank; The top of the rainwater overflow pool is provided with a rainwater overflow port, and the lower part of the rainwater overflow pool is provided with a rainwater discharge port; The biological retention tank is arranged on the side of the rainwater overflow tank and is connected to the rainwater overflow port. The biological retention tank includes an anti-seepage layer, a gravel layer, a sand filter layer, and a planting soil layer arranged in sequence from the bottom to the top. The upper side of the planting soil layer is provided with a water inlet for tail water of a domestic sewage treatment plant. The upper side of the planting soil layer is connected to the rainwater overflow port. The lower side of the gravel layer is provided with a percolation outlet. The denitrification and phosphorus removal artificial wetland pool is arranged on the side of the biological retention pool and is connected to the infiltration outlet. The denitrification and phosphorus removal artificial wetland pool uses siderite and pyrrhotite as a matrix, wetland plants are planted on the surface of the denitrification and phosphorus removal artificial wetland pool, and a denitrification and phosphorus removal outlet is arranged on the lower side of the denitrification and phosphorus removal artificial wetland pool; The post-treatment artificial wetland pool is arranged on the side of the denitrification and phosphorus removal artificial wetland pool and is connected to the denitrification and phosphorus removal water outlet, an aeration mechanism is arranged at the bottom of the post-treatment artificial wetland pool, the post-treatment artificial wetland pool uses zeolite as a matrix, wetland plants are planted on the surface of the post-treatment artificial wetland pool, and a post-treatment water outlet is arranged on the upper side of the zeolite matrix of the post-treatment artificial wetland pool; The water storage tank is communicated with the post-processing water outlet.

2. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 1 is characterized by: The thickness of the gravel layer of the biological retention pond is 300 mm, and the gravel particle size of the gravel layer is 12-15 mm.

3. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 2 is characterized by: The thickness of the sand filter layer of the biological retention pond is 100 mm, the volume ratio of fine sand to medium-coarse sand in the sand filter layer is 1:1, the particle size of the medium-coarse sand is 3-5 mm, and the particle size of the fine sand is 0.5-1.5 mm.

4. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 3 is characterized by: The thickness of the planting soil layer of the bioretention pond is 400-1000 mm.

5. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 4 is characterized by: The rainwater overflow port of the rainwater overflow pool is more than 20 cm higher than the surface of the planting soil layer.

6. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 1, characterized in that: The artificial wetland pool for denitrification and phosphorus removal comprises an artificial wetland pool I and an artificial wetland pool II which are arranged in parallel and connected at the top. The artificial wetland pool I and the artificial wetland pool II are both based on siderite and pyrrhotite. The volume ratio of the siderite and pyrrhotite is 1:

1. The particle size of the siderite is 4-8 mm, and the particle size of the pyrrhotite is 4-8 mm.

7. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 6, characterized in that: The zeolite particle size of the post-treatment artificial wetland pool is 4-8 mm.

8. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 7, characterized in that: The surface load of the denitrification and phosphorus removal artificial wetland pool and the post-treatment artificial wetland pool is 0.5-0.6m 3 / (m 2 ·d), the effective height is 1.2m.

9. The combined treatment system for tail water and initial rainwater from a domestic sewage treatment plant according to claim 1, characterized in that: The wetland plants planted on the surface of the denitrification and phosphorus removal artificial wetland pool and the post-treatment artificial wetland pool include calamus, and the planting density of the wetland plants is 10-12 plants / m 2 .

10. A method for combined treatment of tail water and initial rainwater from a domestic sewage treatment plant using the system according to any one of claims 1 to 9, characterized in that: The following steps are involved: In the dry season, the tailwater from the domestic sewage treatment plant flows into the top of the biological retention pond through the tailwater inlet. The tailwater from the domestic sewage is filtered through the planting soil layer, sand filter layer, and gravel layer in turn. The water after the infiltration treatment enters the artificial wetland pool for nitrogen and phosphorus removal through the infiltration outlet. During the rainy season, the tailwater inlet of the domestic sewage treatment plant is closed. The initial rainwater is filtered through the planting soil layer, sand filter layer, and gravel layer of the biological retention pond in turn. The water after the infiltration treatment enters the denitrification and phosphorus removal artificial wetland pool through the infiltration outlet. When the rainwater height above the planting soil layer exceeds the height of the rainwater overflow inlet, the rainwater flows into the rainwater overflow pool through the rainwater overflow outlet and is discharged through the rainwater discharge outlet. The water after the filtration treatment enters the artificial wetland pool for nitrogen removal and phosphorus removal, and the nitrate nitrogen and phosphate in the water are removed by siderite and pyrrhotite. The water after the nitrogen removal and phosphorus removal treatment is collected into the post-treatment artificial wetland pool through the nitrogen removal and phosphorus removal outlet. The water after denitrification and dephosphorization treatment enters the post-treatment artificial wetland pool, where ammonia nitrogen in the water is further removed through zeolite. The post-treated water is then collected into the water storage tank through the post-treatment outlet. At the same time, the post-treatment artificial wetland pool biologically regenerates the zeolite by regularly starting the aeration mechanism at the bottom.

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