A method for ecological management of tail water in aquaculture ponds

Abstract

The present invention relates to an off-site ecological treatment method for tail water of aquaculture ponds, comprising the following steps: S1: constructing a treatment unit based on three ponds; S2: constructing a purification area in the second pond in the treatment unit; S3: intermittently adding microbial strains in the purification area; S4: the purification area includes an ecological floating island restoration area, an imitation aquatic plant filler restoration area, an ecological floating frame restoration area and a fence; S5: constructing a composite matrix filter dam between the second pond and the drainage and irrigation river; S6: performing water quality testing on the first pond and the second pond, and if the water quality of the first pond and the second pond does not meet the standard, repeating steps S2-S5 until the water quality meets the standard. The treatment method of the present invention occupies a small area, realizes the recycling of pond tail water, and belongs to the technical field of sewage treatment.

Description

Technical Field

[0001] The invention relates to the technical field of sewage treatment, and in particular to an ex situ ecological treatment method for tail water of aquaculture ponds. Background Art

[0002] At present, the main methods for treating pond aquaculture effluent are physical, chemical, and ecological. Among them, physical and chemical treatment methods (physical treatment methods: mechanical filtration, foam separation, membrane separation, etc.; chemical treatment methods: ozone treatment, electrochemistry, chemical preparations, etc.; Wang Yanni, Wang Jinle: Pond aquaculture effluent purification methods and technical models [J]. Agricultural Technology Service, 2021, 38(04): 55-56+59) have problems such as high cost, complex operation, and secondary pollution, and are greatly restricted in application. A variety of new pond aquaculture systems have emerged in China, including pond recirculating water aquaculture systems based on artificial wetlands, “three ponds and two dams” pond aquaculture water purification systems ([1] Li Muhua, Li Muliang. Technical points of “three ponds and two dams” for freshwater aquaculture pond tailwater treatment [J]. Southern Agriculture, 2022, 16(12): 204-206. Patent application number: CN202220999664.3 Authorization publication number: CN217103254U), “container + ecological pond” aquaculture systems, green and efficient enclosure systems, plant floating bed-pond composite aquaculture systems, water trough-pond internal circulation aquaculture systems, and multi-nutrient level integrated aquaculture systems. Compared with traditional aquaculture ponds, these aquaculture systems have made certain progress in aquaculture object management and water quality purification. The above-mentioned pond aquaculture tailwater purification systems will occupy more than 60% of the pond aquaculture area to a certain extent. Therefore, there is still no effective solution with less space required for the treatment of aquaculture pond tailwater. Especially during the flood season, due to the high water level of the outer river, the gate cannot be opened to change the water, and the water quality of the aquaculture pond is getting worse and worse. Therefore, it is urgent to solve the problem of too large a footprint of aquaculture pond tailwater treatment technology and the problem that the aquaculture pond tailwater cannot be recycled. Summary of the invention

[0003] In view of the technical problems existing in the prior art, the purpose of the present invention is to provide an ex situ ecological management method for aquaculture pond tail water, which occupies a small area and realizes the recycling of pond tail water.

[0004] In order to achieve the above object, the present invention adopts the following technical solution:

[0005] A method for ectopic ecological management of tail water in aquaculture ponds comprises the following steps:

[0006] S1: A governance unit is constructed based on three ponds. The three ponds are the first pond, the second pond and the third pond. The first pond, the second pond and the third pond are arranged in sequence. The first pond and the third pond are connected to the second pond through a water diversion pipe.

[0007] S2: constructing a purification area in the second pond in the treatment unit, and directing the water from the second pond to the purification area for purification through a water flow guide device;

[0008] S3: intermittent addition of microbial strains in the purification area;

[0009] S4: The purification area includes the ecological floating island restoration area, the imitation aquatic filler restoration area, the ecological floating frame restoration area and the fence. The ecological floating island restoration area, the imitation aquatic filler restoration area and the ecological floating frame restoration area are all located within the fence. The ecological floating island restoration area, the imitation aquatic filler restoration area and the ecological floating frame restoration area are connected in sequence;

[0010] S5: construct a composite matrix filter dam between the second pond and the drainage and irrigation river; both ends of the composite matrix filter dam are connected to the second pond and the drainage and irrigation river respectively;

[0011] S6: Perform water quality testing on the first pond and the second pond. If the water quality of the first pond and the second pond does not meet the standard, repeat steps S2-S5 until the water quality meets the standard.

[0012] As a preferred embodiment, the matrix filtration dam includes two side walls and a purification area. The two side walls are spaced apart to form a water flow channel. One end of the water flow channel is connected to the second pond, and one end of the water flow channel is arranged corresponding to the ecological floating frame restoration area; the other end of the water flow channel is connected to the drainage and irrigation river; the purification area is arranged in the water flow channel.

[0013] As a preferred embodiment, the purification area includes a first gravel layer, a first mixed layer, a second mixed layer, a third mixed layer and a second gravel layer. The first gravel layer, the first mixed layer, the second mixed layer, the third mixed layer and the second gravel layer are connected in sequence. The first gravel layer is connected to the second pond, and the second gravel layer is connected to the drainage and irrigation river.

[0014] As a preferred embodiment, the mixed materials of the first mixed layer and the third mixed layer are zeolite and vermiculite, and the mixed material of the second mixed layer is ceramsite and bamboo charcoal.

[0015] As a preferred embodiment, the walls on both sides are constructed with hollow bricks, and the gaps in the hollow bricks are in the same direction as the water flow channel.

[0016] As a preferred embodiment, emergent plants are planted on the matrix filter dam.

[0017] As a preferred embodiment, the water flow guide device includes three four-blade waterwheel aerators, one four-blade waterwheel aerator is located at the outer end of the ecological floating island restoration area, one four-blade waterwheel aerator is located at the outer end of the imitation aquatic plant filler, and one four-blade waterwheel aerator is located at the outer end of the ecological floating frame restoration area.

[0018] As a preference, the outer surface of the purification zone is wrapped with nylon cloth.

[0019] As a preferred embodiment, the microbial strains include nitrifying bacteria, Bacillus, complex bacteria and photosynthetic bacteria.

[0020] As a preferred embodiment, the fence is a U-shaped structure and is surrounded by canvas.

[0021] In general, the present invention has the following advantages:

[0022] 1. The treatment method of the present invention is to build a purification area inside a pond or to build a tailwater treatment facility of a biochemical ecological comprehensive treatment pool integrating a sedimentation tank, an aerated biochemical pool and an ecological pool in an entire pond, thereby forming a "one-pool" ecological treatment system. The "one pool" is to concentrate the tailwater that needs to be treated from multiple ponds into one pond for treatment, and then build a composite matrix filter dam, i.e., "one dam", after the "one pool", to filter the treated water of the "one pool" to remove suspended matter (referring to solid matter suspended in water, including inorganic matter, organic matter and mud, clay, microorganisms, etc. that are insoluble in water), thereby forming a "one pool and one dam" ectopic ecological treatment system. This method can quickly purify the water quality of the pond tailwater, and compared with the original "three pools and two dams", it saves more than 60% of the pond breeding area, and can also save a lot of initial investment and construction costs.

[0023] 2. Compared with the existing "three ponds and two dams" process, the treatment method of the present invention is that the sedimentation pond, the aeration biochemical pond and the ecological pond are all independent single ponds, and each pond is only designed to treat the tail water of the breeding pond, so it occupies a large area and has low treatment efficiency. The present invention plants aquatic plants in the aeration biochemical pond, and merges the ecological pond into the aeration biochemical pond, saving 30% of the land area required for the construction of the ecological pond; in addition, a sedimentation area is set at the front end of the aeration biochemical pond, and the sedimentation pond can be merged into the aeration biochemical pond, which not only saves 40% of the land required for the construction of the sedimentation pond, but also can remove part of the bottom mud in the sedimentation area through artificial aeration and microbial aerobic decomposition, achieving the effect of killing two birds with one stone.

[0024] 3. The "one dam" in the present invention is a composite matrix filtration dam, which is placed behind the ecological floating frame restoration area, plays a role in filtering out water and is not easy to be blocked. The "two dams" in the "three pools and two dams" are respectively located between the sedimentation pool and the biochemical pool and between the biochemical pool and the ecological pool. The former is most prone to blockage, and the latter is also relatively easy to be blocked.

[0025] 4. The present invention can solve the problem that the water level of the outer river is too high during the summer flood season, which makes it impossible for the breeding pond to open the gate to change water through the flood tide of the drainage and irrigation river and discharge the breeding tail water through the ebb tide of the drainage and irrigation river. The tail water discharged from the three ponds is concentrated into one pond or a purification area by a pump, and the four-leaf waterwheel aerator, the addition of microbial strains, the repair area of ​​the imitation water grass filler, and the plant absorption effect of the plant of the emergent plants and floating plants planted on the ecological floating island and the ecological floating frame are used. After the aquaculture tail water of the second pond is treated to meet the standards, it is pumped to the first pond and the third pond by a pump, and then flows to the second pond through the water diversion pipe, and so on, forming an internal cycle, so that the breeding pond does not need to change the drainage and irrigation river water during the summer flood season, and does not discharge tail water to the drainage and irrigation river, so as to realize the internal circulation treatment and utilization of the three ponds. If other physical and chemical methods are used to solve the problem, it will cost a lot of money, so it also saves the management cost to a certain extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a distribution map of the treatment unit, composite matrix filter dam and drainage and irrigation river in an ex situ ecological treatment method for tailwater of aquaculture ponds.

[0027] Figure 2 This is a schematic diagram of the second pond.

[0028] Figure 3 Schematic diagram of the structure of the composite matrix filter dam.

[0029] Figure 4 Schematic diagram of the composite matrix filter dam.

[0030] Figure 5 This is a graph showing the dynamic changes in various water quality indicators in the first pond.

[0031] Figure 6 This is a graph showing the dynamic changes in various water quality indicators in the second pond.

[0032] Among them, 1 is the first pond, 2 is the second pond, 3 is the third pond, 4 is the drainage and irrigation river, 5 is the composite matrix filtration dam, 6 is the ecological floating island restoration area, 7 is the imitation aquatic plant filler restoration area, 8 is the ecological floating frame restoration area, 9 is the four-blade waterwheel aerator, 10 is the microbial strain, 11 is nylon cloth, 12 is the hollow brick, 13 is the first gravel layer, 14 is the first mixed layer, 15 is the second mixed layer, 16 is the third mixed layer, 17 is the second gravel layer, and 18 is the dam. DETAILED DESCRIPTION

[0033] The present invention will be further described in detail below in conjunction with specific implementation methods.

[0034] A method for ectopic ecological management of tail water in aquaculture ponds comprises the following steps:

[0035] S1: A governance unit is constructed based on three ponds. The three ponds are the first pond, the second pond and the third pond. The first pond, the second pond and the third pond are arranged in sequence. The first pond and the third pond are connected to the second pond through a water diversion pipe.

[0036] S2: constructing a purification area in the second pond in the treatment unit, and directing the water from the second pond to the purification area for purification through a water flow guide device;

[0037] S3: intermittent addition of microbial strains in the purification area;

[0038] S4: The purification area includes the ecological floating island restoration area, the imitation aquatic filler restoration area, the ecological floating frame restoration area and the fence. The ecological floating island restoration area, the imitation aquatic filler restoration area and the ecological floating frame restoration area are all located within the fence. The ecological floating island restoration area, the imitation aquatic filler restoration area and the ecological floating frame restoration area are connected in sequence;

[0039] The repair area of ​​the simulated water grass filler in the purification area, a single set of simulated water grass filler is 1m wide, consisting of 5 biological ropes with a length of 150cm and a diameter of 6cm and 6 buoys per meter. Multiple sets of simulated water grass fillers are installed at right angles to the water flow to form a row. In order to avoid the simulated water grass fillers from getting entangled, the spacing between rows is 1.5m. A total of 40 rows are constructed, and each row has 2 simulated water grass fillers with a length of 5m. They are fixed in the pond bottom mud with bamboo poles and nylon cable ties, floating underwater, and the total simulated water grass filler area is 600m 2 The ecological floating islands in the purification area are restored with aquatic plants. The length is about 60m and the width is 10m. Six rows of ecological floating islands are built along the length of the wall and three rows of ecological floating islands are built along the width of the wall. Each ecological floating island is 8m long and 2m wide. The area of ​​a single ecological floating island is 16m 2 The ecological floating frame in the purification area is designed to be spliced ​​by a single ecological floating frame with a length of 4m and a width of 2m, covering an area of ​​8m 2 , 12 ecological floating frames are set along the length of the enclosure, and 3 ecological floating frames are set along the width of the enclosure, a total of 36 ecological floating frames, with a total area of ​​288m 2 , plant foxtail algae and aquatic vegetables on the ecological floating frame (arrange the planting according to the seasons, such as in the Pearl River Delta region: plant aquatic vegetables from April to October, and plant water celery and watercress from November to March).

[0040] S5: construct a composite matrix filter dam between the second pond and the drainage and irrigation river; both ends of the composite matrix filter dam are connected to the second pond and the drainage and irrigation river respectively;

[0041] S6: Perform water quality testing on the first pond and the second pond. If the water quality of the first pond and the second pond does not meet the standard, repeat steps S2-S5 until the water quality meets the standard.

[0042] Microbial enhanced purification is carried out in the purification area. Microbial species (including nitrifying bacteria, Bacillus, composite bacteria and photosynthetic bacteria, etc.) are intermittently added in this purification area to make them multiply in large numbers in the eutrophic area and attach to the imitation water grass fillers and the roots of aquatic plants to slowly decompose organic matter such as COD, ammonia nitrogen and total phosphorus in the pond. During the period of ecological management, bacteria were added once every 5 days, and each time nitrifying bacteria, Bacillus, compound bacteria and photosynthetic bacteria were added at dosages of 10mg / L, 10mg / L, 10mg / L and 8mg / L respectively. The addition was done 6 times a month, and a total of 12 times were added during the two-month ecological management period. The amount of nitrifying bacteria, Bacillus and compound bacteria was 720.4Kg=6mu×667m2×1.5m×10mg / L×12 times, and the amount of photosynthetic bacteria was 576.3Kg=6mu×667m2×1.5m×8mg / L×12 times; a total of 2737.5Kg=720.4+576.3Kg.

[0043] During the ecological management period, samples were collected from the first and second ponds. Samples were collected 6 times a month, for a total of 12 times. Figure 5-6 It was concluded that the contents of COD, NH3-N, TP and TN in the tail water of the pond treated by this system were reduced. During the sixth and seventh sampling, the quality of the tail water discharged could basically meet the expected water quality indicators (COD < 40 mg / L, NH3-N < 1.5 mg / L, TP < 0.3 mg / L, TN < 3.0 mg / L).

[0044] In this embodiment, a purification area is set in the rectangular second pond, which consists of a 60m*10m ecological floating island restoration area, a 53m*10m imitation water grass filler restoration area, a 60m*10m ecological floating frame, and a U-shaped canvas fence inside the pond. There are three ponds, namely the first pond, the second pond, and the third pond. The tail water of the three ponds is concentrated into the second pond by using a water diversion pipe. After the tail water passes through the ecological floating island restoration area, the imitation water grass filler restoration area, and the ecological floating frame restoration area, please refer to Figure 1-4, and introduce it into the composite matrix filter dam. The composite matrix filter dam uses two rows of hollow bricks to build the walls on both sides. The original dam of the pond is 8m wide. The designed composite matrix filter dam is 6m long and 4m wide. The spacing between the two rows of hollow bricks in the composite matrix filter dam is 6m. The gap in the direction of the hollow bricks is consistent with the water flow, which is convenient for water to flow in both directions. Nylon cloth is used to wrap the inner side of the composite matrix filter dam to form a nylon mesh wall to prevent gravel collapse. First, fill it with gravel with larger particle size, and then fill it with zeolite and vermiculite with smaller particle size. Fill the middle with expanded clay and bamboo charcoal, and then fill it with zeolite and vermiculite. Finally, fill the area close to the nylon mesh wall with larger particle size gravel, thus forming a composite matrix filter dam that can filter in both directions, and plant emergent plants such as canna, windmill grass, lily of the valley, pickerel grass, iris, etc. on the filter dam to form a horizontal subsurface flow artificial wetland treatment project.

[0045] The matrix filtration dam includes two side walls and a purification area. The two side walls are spaced apart to form a water flow channel. One end of the water flow channel is connected to the second pond, and one end of the water flow channel is set corresponding to the ecological floating frame restoration area; the other end of the water flow channel is connected to the drainage and irrigation river; the purification area is set in the water flow channel.

[0046] The purification area includes the first gravel layer, the first mixed layer, the second mixed layer, the third mixed layer and the second gravel layer. The first gravel layer, the first mixed layer, the second mixed layer, the third mixed layer and the second gravel layer are connected in sequence. The first gravel layer is connected to the second pond, and the second gravel layer is connected to the drainage and irrigation river.

[0047] The mixed materials of the first and third mixed layers are zeolite and vermiculite, and the mixed materials of the second mixed layer are ceramsite and bamboo charcoal. The material data used in the purification area are as follows:

[0048] Serial number name model unit quantity 1 gravel 30-50mm cube 88 2 Zeolite, Vermiculite 20-30mm cube 88 3 Clay granules, bamboo charcoal 5-10mm cube 44 4 Hollow Brick 390*390*190mm indivual

[0049] The walls on both sides are constructed with hollow bricks, and the gaps inside the hollow bricks are consistent with the direction of the water flow channel.

[0050] Emergent plants are planted on the substrate filter dam.

[0051] The water flow guide device includes three four-blade waterwheel aerators, one of which is located at the outer end of the ecological floating island restoration area, one of which is located at the outer end of the imitation water grass filler, and one of which is located at the outer end of the ecological floating frame restoration area. The four-blade waterwheel aerator is configured as 1.5KW three-phase 380V. It increases the dissolved oxygen in the water body, improves the redox potential, and degrades COD and ammonia nitrogen under the decomposition of aerobic microorganisms.

[0052] The outer surface of the clean area is wrapped with nylon cloth.

[0053] Microbial species include nitrifying bacteria, Bacillus, complex bacteria and photosynthetic bacteria.

[0054] The fence is a U-shaped structure and is made of canvas.

[0055] Specific process:

[0056] 1. In order to solve the problem that the tailwater treatment needs to occupy the pond breeding area, a tailwater ecological management method of "one pond and one dam" is proposed, in which the tailwaters of several ponds are concentrated into one tailwater treatment pond, and then a U-shaped wall or the entire tailwater treatment pond is enclosed by canvas in the tailwater treatment pond (no canvas is needed for enclosure), and a four-leaf waterwheel aerator, a waterweed filler restoration area, an ecological floating island restoration area and an ecological floating frame restoration area are installed in the tailwater treatment area to cultivate emergent plants and floating plants, so as to form a tailwater treatment purification area or a pond, in which the tailwater can be precipitated, aerated, oxygenated and biochemically treated, and ecologically purified, and then the tailwater is discharged after being treated by a composite matrix filter dam, so that the treated effluent water quality is stable and meets the discharge standards, thus forming a site for the implementation of the "one pond and one dam" ex situ ecological management method, and the management method has a small footprint and low cost compared with the existing treatment methods.

[0057] 2. In order to reduce the floor area of ​​the "three pools and two dams" treatment system, the method of this embodiment combines the "three pools" of the "three pools and two dams", namely the sedimentation pool, the aeration pool, and the ecological pool into a "treatment unit", in which a four-leaf waterwheel aerator, biological contact oxidation imitation waterweed filler, microbial strains are installed, and ecological floating islands and ecological floating frames are constructed to cultivate emergent plants and floating plants. This is a treatment unit that integrates the functions of sedimentation, biochemical pools, and ecological pools. It can not only realize the treatment functions of sedimentation pools, aeration pools, and ecological pools, but also can degrade the sludge in the tail water of the aquaculture pond and reduce the floor area of ​​the "three pools" by more than 60%.

[0058] 3. In order to reduce the floor space occupied by the "two dams" in the "three pools and two dams" treatment system, the "two dams" in the "three pools and two dams" are changed to "one dam" and placed on one side of the "one pool" to form a "one pool and one dam" system in series. The composite matrix filter dam is composed of multi-stage media. In the direction of horizontal undercurrent, it is first filled with gravel with larger particle size, then filled with zeolite and vermiculite with smaller particle size, filled with ceramsite and bamboo charcoal in the middle, then filled with zeolite and vermiculite, and finally filled with gravel. Canna, windmill grass, lily, pickerel grass, iris, etc. can be planted on the composite matrix filter dam to form a horizontal undercurrent artificial wetland treatment system to remove suspended matter in the tailwater.

[0059] 4. In order to solve the problem that the water level of the outer river is too high during the summer flood season, making it impossible for the aquaculture ponds to open the gates to change water at high tide of the drainage and irrigation river and to discharge the aquaculture tail water at low tide of the drainage and irrigation river, the tail water of the three ponds is concentrated into the first pond or a purification area by using a pump. By using the four-leaf waterwheel aerator, the addition of microbial strains, imitation aquatic grass fillers, and the plant absorption of emergent plants and floating plants planted on ecological floating islands and ecological floating frames, after the aquaculture tail water of the second pond is treated to meet the standards, it is pumped to the first pond and the third pond by using a pump, and then flows to the second pond through the water diversion pipe, and so on, forming an internal circulation, so that the aquaculture ponds do not need to replace the drainage and irrigation river water during the summer flood season, and the tail water is not discharged into the drainage and irrigation river, so as to realize the internal circulation treatment and utilization of the three ponds.

[0060] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for ecological management of tail water in an aquaculture pond, characterized in that: The following steps are involved: S1: A governance unit is constructed based on three ponds. The three ponds are the first pond, the second pond and the third pond. The first pond, the second pond and the third pond are arranged in sequence. The first pond and the third pond are connected to the second pond through a water diversion pipe. S2: constructing a purification area in the second pond in the treatment unit, and directing the water from the second pond to the purification area for purification through a water flow guide device; S3: intermittent addition of microbial strains in the purification area; S4: The purification area includes the ecological floating island restoration area, the imitation aquatic filler restoration area, the ecological floating frame restoration area and the fence. The ecological floating island restoration area, the imitation aquatic filler restoration area and the ecological floating frame restoration area are all located within the fence. The ecological floating island restoration area, the imitation aquatic filler restoration area and the ecological floating frame restoration area are connected in sequence; S5: construct a composite matrix filter dam between the second pond and the drainage and irrigation river; both ends of the composite matrix filter dam are connected to the second pond and the drainage and irrigation river respectively; S6: Testing the water quality of the first pond and the second pond. If the water quality of the first pond and the second pond does not meet the standard, repeating steps S2-S5 until the water quality meets the standard; The matrix filtration dam includes two side walls and a purification area. The two side walls are spaced apart to form a water flow channel. One end of the water flow channel is connected to the second pond. One end of the water flow channel is set corresponding to the ecological floating frame restoration area. The other end of the water flow channel is connected to the drainage and irrigation river. The purification area is set in the water flow channel. The purification area includes a first gravel layer, a first mixed layer, a second mixed layer, a third mixed layer and a second gravel layer, the first gravel layer, the first mixed layer, the second mixed layer, the third mixed layer and the second gravel layer are connected in sequence, the first gravel layer is connected to the second pond, and the second gravel layer is connected to the drainage and irrigation river; The mixed materials of the first and third mixed layers are zeolite and vermiculite, and the mixed materials of the second mixed layer are ceramsite and bamboo charcoal; The walls on both sides are constructed with hollow bricks, and the gaps inside the hollow bricks are consistent with the direction of the water flow channel.

2. The method for ex situ ecological management of tail water in aquaculture ponds according to claim 1, characterized in that: Emergent plants are planted on the substrate filter dam.

3. The method for ecological management of tail water of aquaculture ponds according to claim 1, characterized in that: The water flow guiding device includes three four-blade waterwheel aerators, one four-blade waterwheel aerator is located at the outer end of the ecological floating island restoration area, one four-blade waterwheel aerator is located at the outer end of the imitation aquatic plant filler, and one four-blade waterwheel aerator is located at the outer end of the ecological floating frame restoration area.

4. The method for ex situ ecological management of tail water of aquaculture ponds according to claim 1, characterized in that: The outer surface of the clean area is wrapped with nylon cloth.

5. The method for ex situ ecological management of tail water of aquaculture ponds according to claim 1, characterized in that: Microbial species include nitrifying bacteria, Bacillus, complex bacteria and photosynthetic bacteria.

6. The method for ex situ ecological management of tail water of aquaculture ponds according to claim 1, characterized in that: The fence is a U-shaped structure and is made of canvas.

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