Downward-flowing subsurface constructed wetland for enhanced denitrification and phosphorus removal

Abstract

The application relates to a down-flow type subsurface constructed wetland for enhanced denitrification and dephosphorization, which is sequentially arranged with a planting layer, a water distribution main pipe, a filler layer and a drainage pipe, a plurality of water distribution branch pipes are branched out on the water distribution main pipe, and an enhanced adsorption and anaerobic biochemical layer is wrapped outside the water distribution branch pipes; the enhanced adsorption and anaerobic biochemical layer comprises: a hollow pipe, an anaerobic filler layer and a hard pipe which are sequentially wrapped outside the water distribution branch pipes from inside to outside, and the total opening area per unit length of the hard pipe is smaller than that of the water distribution branch pipe; a gas distribution pipe is arranged in the lower region of the filler layer, the pipe wall of the gas distribution pipe is provided with gas distribution holes, and the gas distribution pipe is connected with a gas pump. The beneficial effects are as follows: the anaerobic-anoxic-aerobic reaction layer is constructed in the down-flow type subsurface constructed wetland, the phosphorus accumulating bacteria release a small amount of phosphorus under the anaerobic environment, then store more energy, so as to absorb more phosphorus in the aerobic stage; denitrification and anaerobic ammonia oxidation are carried out in the anaerobic-anoxic stage, limited carbon source is used for effective denitrification, organic matters are decomposed in the aerobic stage, and the total nitrogen concentration is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of constructed wetlands, in particular to a down-flow subsurface flow constructed wetland for enhanced denitrification and phosphorus removal. BACKGROUND

[0002] Constructed wetlands mainly utilize the combined action of substrates, plants and microorganisms to achieve the purification effect of wastewater, and subsurface flow constructed wetlands are widely used in tail water treatment of wastewater treatment plants, reclaimed water reuse and other fields due to their small land occupation and relatively good decontamination effect.

[0003] Due to the low carbon-nitrogen ratio of effluent from wastewater treatment plants in China, traditional subsurface flow constructed wetlands cannot meet the requirements of efficient denitrification and phosphorus removal. Subsurface flow constructed wetlands are divided into horizontal subsurface flow and vertical subsurface flow, and vertical subsurface flow constructed wetlands are divided into two types, namely up-flow subsurface flow constructed wetlands and down-flow subsurface flow constructed wetlands. Horizontal subsurface flow and up-flow subsurface flow wetlands are both in a submerged state, and the only difference is the direction of water flow, one is horizontal flow and the other is vertical flow.

[0004] Up-flow subsurface flow constructed wetlands and horizontal subsurface flow constructed wetlands maintain an anaerobic environment for a long time, which is beneficial to denitrification by denitrifying bacteria, but it can cause the release of phosphorus by phosphorus accumulating organisms in wastewater and the excessive concentration of ammonia nitrogen.

[0005] Down-flow subsurface flow wetlands have relatively good oxygen-rich conditions, but there are still many problems:

[0006] 1) The oxygen competition between the aerobic bacteria in the upper layer of the down-flow subsurface flow constructed wetland and the nitrifying bacteria affects the removal of ammonia nitrogen by the nitrifying bacteria.

[0007] 2) The process of first aerobic and then anaerobic causes the carbon source to be consumed by aerobic bacteria too early, resulting in a lack of carbon source for denitrifying bacteria in the deep layer of the down-flow subsurface flow constructed wetland, which affects the removal of nitrate nitrogen.

[0008] 3) The aerobic environment in the upper layer of the down-flow subsurface flow constructed wetland is conducive to the absorption of phosphorus in wastewater by phosphorus accumulating organisms, but the anoxic and anaerobic environment in the lower layer causes the release of phosphorus by phosphorus accumulating organisms into wastewater, which affects the removal of phosphorus in the down-flow subsurface flow constructed wetland. SUMMARY

[0009] The technical problem to be solved by the present application is to provide a down-flow subsurface flow constructed wetland for enhanced denitrification and phosphorus removal to overcome the deficiencies in the prior art.

[0010] The technical solution for solving the above technical problem is as follows: a down-flow subsurface flow constructed wetland for enhanced denitrification and phosphorus removal, comprising: a planting layer, a water distribution main pipe, a filler layer and a drainage pipe arranged in sequence from top to bottom, a plurality of water distribution branch pipes with water distribution holes on the pipe wall branch out from the water distribution main pipe, and the water distribution branch pipes are wrapped with a strengthened adsorption and anaerobic biochemical layer.

[0011] The reinforced adsorption and anaerobic biochemical layer comprises: a hollow tube, an anaerobic filler layer and a hard tube which are sequentially wrapped outside the water distribution branch pipe from inside to outside, the lower half circle of the hard tube is provided with a hole, and the upper half circle is not provided with a hole, and the total opening area per unit length of the hard tube is less than the opening area of the water distribution branch pipe;

[0012] The gas distribution pipe is arranged in the lower region of the filler layer, the pipe wall of the gas distribution pipe is provided with a gas distribution hole, and the gas distribution pipe is connected with a gas pump.

[0013] The present application has the following beneficial effects:

[0014] The water flows from the water distribution main pipe into the water distribution branch pipe, and then flows into the anaerobic filler layer through the evenly distributed water distribution holes on the water distribution branch pipe, is adsorbed by the filler, and after the anaerobic microbial biochemical reaction, the sewage flows out from the hole on the lower half circle of the hard tube wrapped with the anaerobic filler layer, and the total opening area per unit length of the hard tube is less than the opening area of the water distribution branch pipe, so that the anaerobic filler layer forms a water accumulation, and an anaerobic state is formed, thereby creating an anaerobic environment for the upper layer of the wetland.

[0015] The gas pump can blow air into the gas distribution pipe, and the air entering the gas distribution pipe flows to the filler layer through the gas distribution hole on the gas distribution pipe to enrich oxygen in the lower region of the filler layer, which can provide an aerobic environment for the deep layer of the wetland, promote the growth and reproduction of aerobic microorganisms such as nitrifying bacteria, improve the removal efficiency of ammonia nitrogen and organic pollutants, and mineralize the clogging materials such as extracellular polymers to alleviate the clogging.

[0016] The whole artificial wetland creates an anaerobic environment in the upper layer along the flow direction of the sewage, an anoxic layer in the middle, and an aerobic layer in the lower part by oxygenation, thereby constructing an anaerobic-anoxic-aerobic reaction layer in the down-flowing subsurface flow artificial wetland, the polyphosphorus bacteria release a small amount of phosphorus in the anaerobic environment, then store more energy, so as to absorb more phosphorus in the aerobic stage, perform denitrification and anaerobic ammonia oxidation in the anaerobic-anoxic stage, effectively remove nitrogen by using limited carbon source, and decompose organic matter in the aerobic stage to reduce the total nitrogen concentration.

[0017] On the basis of the above technical solution, the present application can also be improved as follows.

[0018] Further, the water distribution holes are arranged on the periphery of the water distribution branch pipe, and the diameter of the water distribution holes is 8mm-10mm, and the diameter of the holes on the hard tube is 10mm-15mm.

[0019] Further, the lower part of the hard tube is arranged with a shell wrapping the lower half circle thereof and a sludge collection layer, the shell is in the shape of a hollow arc, the sludge collection layer is located in the lowest point region of the hard tube, and the hard tube is connected with the shell and the sludge collection layer through support and connection structures; the shell is arranged with rope-shaped artificial water grasses with large specific surface area and good hydrophilicity, and the shell side is provided with holes.

[0020] The above further beneficial effects are as follows:

[0021] The effluent from the anaerobic filler layer flows out of the hole in the lower half of the rigid pipe and into the shell, and then the sewage is further reduced in pollution by the rope-shaped artificial water grass, and finally flows out through the holes in the side of the shell, while the sludge produced in the anaerobic biochemical reaction is deposited in the sludge collection layer at the bottom under the action of gravity, realizing the collection of sludge, and the sludge collection layer is located in the lowest point area of the rigid pipe, which is beneficial to the collection of sludge to prevent the wetland from being blocked by the sludge flowing into the filler layer, thereby avoiding further pollution of the wetland caused by the sludge.

[0022] Further, the hole diameter of the shell is slightly smaller than the hole diameter of the rigid pipe, and the number of holes per unit length is the same.

[0023] The above further beneficial effects are that the design forms a water accumulation between the sludge collection layer, the shell and the rigid pipe, which is also in an anaerobic state, which is beneficial to the further biological strengthening of the microorganisms attached to the rope-shaped artificial water grass to remove pollutants, and at the same time, the hole diameter is reduced to prevent the sludge from being discharged, which is beneficial to the sludge produced by anaerobic biochemical reaction to enter the sludge collection layer.

[0024] Further, the hole diameter of the shell is 8mm-13mm.

[0025] Further, the outer diameter of the water distribution branch pipe is 50mm-70mm, the outer diameter of the anaerobic filler layer is 130mm-180mm, the outer diameter of the shell is 200mm-210mm, and the height of the lowermost part of the sludge collection layer from the shell is 150mm-200mm.

[0026] Further, the length of a single reinforced adsorption and anaerobic biochemical layer wrapped around the water distribution branch pipe is 0.8m-1m, multiple reinforced adsorption and anaerobic biochemical layers are wrapped around the water distribution branch pipe in sequence, and the water distribution branch pipe is arranged obliquely.

[0027] The above further beneficial effects are that the sludge collected by the sludge collection layer can flow towards the low end along the slope, which is convenient for subsequent treatment, and the use of modular reinforced adsorption and anaerobic biochemical layers greatly reduces the construction difficulty and improves the construction efficiency.

[0028] Further, the sludge in the sludge collection layer is pumped out of the artificial wetland.

[0029] The above further beneficial effects are that the sludge is pumped out in time to avoid negative effects such as pollution and blockage of the wetland.

[0030] Further, the anaerobic filler layer uses zeolite with a particle size of 5mm-15mm, volcanic rock with a particle size of 5mm-15mm, and activated carbon with a particle size of 5mm-10mm as filler.

[0031] The further beneficial effects are that the zeolite has good adsorption effect on ammonia nitrogen, the volcanic rock has good adsorption effect on phosphorus, the activated carbon has good adsorption effect on new pollutants and heavy metals, and the water distribution branch pipe flows into the anaerobic filler layer, so that the pollutants with the maximum concentration are concentrated in the anaerobic filler layer, and therefore, the material with good adsorption effect on conventional pollutants, heavy metals and new pollutants is arranged in the anaerobic filler layer, so that the concentration of the pollutants can be effectively reduced, and the pressure of subsequent biochemical reaction can be reduced.

[0032] Further, the air distribution pipe comprises: an air distribution main pipe, a plurality of air distribution branch pipes are branched out from the air distribution main pipe, the air distribution branch pipes have air distribution holes on the pipe walls, and the air distribution main pipe is connected with an air pump. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The figure is a structural diagram of the down-flowing subsurface constructed wetland for strengthening denitrification and phosphorus removal in the application;

[0034] Figure 2 The figure is a partial enlarged view of Figure 1 ;

[0035] Figure 3 The figure is a side view of Figure 2 .

[0036] In the drawings, the components represented by the respective reference numerals are listed as follows:

[0037] 1, planting layer, 2, filler layer, 3, water distribution main pipe, 310, water distribution branch pipe, 4, adsorption and anaerobic biochemical layer, 410, hollow pipe, 420, hard pipe, 430, anaerobic filler layer, 440, shell, 450, sludge collection layer, 460, rope-shaped artificial water grass, 470, support and connection structure, 5, drainage pipe, 6, air distribution main pipe, 610, air distribution branch pipe. DETAILED DESCRIPTION

[0038] The principles and characteristics of the application are described below in combination with the drawings, and the examples are only used to explain the application and are not used to limit the scope of the application.

[0039] Example 1

[0040] As shown in Figure 1 , Figure 2 , Figure 3 , a down-flowing subsurface constructed wetland for strengthening denitrification and phosphorus removal comprises: a planting layer 1, a water distribution main pipe 3, a filler layer 2 and a drainage pipe 5 arranged in sequence from top to bottom, a plurality of water distribution branch pipes 310 having water distribution holes on the pipe walls are branched out from the water distribution main pipe 3, and a fine sand cushion layer is laid under the drainage pipe 5.

[0041] The water distribution branch pipe 310 is wrapped with a strengthened adsorption and anaerobic biochemical layer 4, wherein the strengthened adsorption and anaerobic biochemical layer 4 comprises, from inside to outside, a hollow pipe 410, an anaerobic filler layer 430 and a hard pipe 420 wrapped outside the water distribution branch pipe 310; the hollow pipe 410 has larger holes and does not block water; the water flowing out of the water distribution branch pipe 310 can enter the anaerobic filler layer 430 through the hollow pipe 410; the hard pipe 420 is provided with holes on the lower semicircle and is not provided with holes on the upper semicircle, that is, the hard pipe 420 only discharges water from the lower semicircle, which is beneficial to the discharge of sludge generated in the anaerobic filler layer 430; the water flows into the water distribution branch pipe 310 from the water distribution main pipe 3, then flows into the anaerobic filler layer 430 through the water distribution holes uniformly distributed on the water distribution branch pipe 310, is adsorbed by the filler, and is subjected to biochemical reaction of anaerobic microorganisms, and then the sewage flows out of the holes on the lower semicircle of the hard pipe 420 wrapping the anaerobic filler layer 430; the total hole area per unit length of the hard pipe 420 is smaller than the hole area of the water distribution branch pipe 310, so that water accumulates in the anaerobic filler layer 430, and an anaerobic state is formed, thereby creating an anaerobic environment for the upper layer of the wetland;

[0042] The gas distribution pipe is arranged in the lower region of the filler layer 2, the pipe wall of the gas distribution pipe is provided with gas distribution holes, the gas distribution pipe is connected with a gas pump, the gas pump can blow air into the gas distribution pipe, and the air entering the gas distribution pipe flows to the filler layer through the gas distribution holes on the gas distribution pipe to charge the lower region of the filler layer with air and oxygen, which can provide an aerobic environment for the deep layer of the wetland, promote the growth and reproduction of aerobic microorganisms such as nitrifying bacteria, and improve the removal efficiency of ammonia nitrogen and organic pollutants, and can also mineralize clogging substances such as extracellular polymers to alleviate clogging.

[0043] The whole constructed wetland creates an anaerobic environment for the upper layer, an anoxic layer for the middle layer, and an aerobic layer for the lower layer by charging oxygen, thereby constructing an anaerobic-anoxic-aerobic reaction layer in the down-flowing subsurface flow constructed wetland; the polyphosphorus bacteria release a small amount of phosphorus in the anaerobic environment, and then store more energy so as to absorb more phosphorus in the aerobic stage; denitrification and anaerobic ammonia oxidation are carried out in the anaerobic and anoxic stages, and effective denitrification is achieved by using limited carbon source; and in the aerobic stage, organic matter is decomposed to reduce the total nitrogen concentration.

[0044] Embodiment 2

[0045] As shown in Figure 2 , the present embodiment is a further improvement on the basis of embodiment 1, and the specific improvements are as follows:

[0046] Water distribution holes are opened around the water distribution branch pipe 310, and the diameter of the water distribution holes is 8mm to 10mm. In common downflow constructed wetlands, the water distribution branch pipe 310 only has holes staggered at 60° on the lower semicircle, and the diameter of the holes is generally 10mm. However, in this invention, the position and size of the holes have been adjusted to adapt to the actual needs of this invention. The diameter of the holes opened on the rigid pipe 420 is 10mm to 15mm. The large diameter holes on the rigid pipe 420 are conducive to the discharge of sludge from the anaerobic packing layer 430.

[0047] Example 3

[0048] like Figure 2 As shown, this embodiment is a further improvement on embodiment 1 or 2, as detailed below:

[0049] Below the rigid pipe 420, there is an outer shell 440 that wraps around its lower semicircle and a sludge collection layer 450. The outer shell 440 is a hollow arc shape. A large amount of sludge will be generated during the anaerobic biological reaction. Therefore, the sludge collection layer 450 is designed and located at the lowest point of the rigid pipe 420. This is conducive to collecting sludge and prevents sludge from entering the filler layer 2 with the water flow and causing wetland blockage, thereby avoiding further pollution of the wetland due to sludge.

[0050] The rigid tube 420 is connected to the outer shell 440 via the support and connection structure 470, and also to the sediment collection layer 450 via the support and connection structure 470. The support and connection structure 470 integrates the entire structure, ensuring its stability and robustness, and facilitating modular installation. Rope-shaped artificial aquatic plants 460 with a large specific surface area and good hydrophilicity are arranged inside the outer shell 440. It is also best to arrange rope-shaped artificial aquatic plants 460 around the support and connection structure 470. Holes are provided on the side of the outer shell 440, and the rope-shaped artificial aquatic plants 460 are fixed to the rigid tube 420 and the outer shell via the support and connection structure 470. Between 440 and 430, the effluent from the anaerobic packing layer 430 flows out through the holes in the lower semicircle of the rigid pipe 420 and enters the outer shell 440. Then, it passes through the rope-shaped artificial aquatic plants 460 to further reduce pollution. Finally, the sewage flows into the packing layer 2 through the holes on the side of the outer shell 440. The sludge produced during the anaerobic biochemical reaction settles in the bottom sludge collection layer 450 under the action of gravity. The rope-shaped artificial aquatic plants 460 are made of polyester and vinylon materials with good hydrophilicity and biocompatibility. They have a high specific surface area and can enrich a large number of anaerobic microorganisms, greatly improving biological efficiency and enhancing the anaerobic decontamination effect.

[0051] Further, the diameter of the holes in the shell 440 is slightly smaller than the diameter of the holes in the rigid pipe 420, and the number of holes per unit length is the same. This design causes water to accumulate between the sludge collection layer 450, the shell 440, and the rigid pipe 420, and the water is in an anaerobic state, which is conducive to further biological strengthening of the microorganisms attached to the rope-like artificial water grass 460 to remove pollutants, and the small diameter prevents sludge from being discharged, which is conducive to allowing the sludge produced by anaerobic biodegradation to enter the sludge collection layer 450. In this embodiment, the diameter of the holes in the shell 440 is 8mm-13mm.

[0052] Embodiment 4

[0053] As shown in Figure 2 , this embodiment is a further improvement based on Embodiment 3, and the specific improvements are as follows:

[0054] The outer diameter of the water distribution branch pipe 310 is 50mm-70mm, and the outer diameter of the anaerobic filler layer 430 is 130mm-180mm. The thickness of the anaerobic filler layer 430 can ensure a certain anaerobic reaction time. The outer diameter of the shell 440 is 200mm-210mm, and the height of the lowermost part of the sludge collection layer 450 from the shell 440 is 150mm-200mm.

[0055] Embodiment 5

[0056] As shown in Figure 2 , this embodiment is a further improvement based on Embodiment 3 or 4, and the specific improvements are as follows:

[0057] The length of a single reinforced adsorption and anaerobic biodegradation layer 4 wrapped around the water distribution branch pipe 310 is 0.8m-1m. Multiple reinforced adsorption and anaerobic biodegradation layers 4 are wrapped around the water distribution branch pipe 310 in sequence. The use of modular reinforced adsorption and anaerobic biodegradation layers 4 greatly reduces the construction difficulty and improves the construction efficiency. During construction, the water distribution branch pipe 310 is placed at a certain slope, i.e., inclined arrangement, so that the sludge collection layer 450 as a whole presents a slope in the direction of the water flow. With the flowability of the sludge, the sludge collected by the sludge collection layer 450 can flow to the low end along the slope. The sludge is regularly pumped out at the low end to avoid negative effects such as pollution and blockage of the wetland.

[0058] Embodiment 6

[0059] As shown in Figure 2 , this embodiment is a further improvement based on any one of Embodiments 1-5, and the specific improvements are as follows:

[0060] The anaerobic packing layer 430 uses zeolite with a particle size of 5mm to 15mm, volcanic rock with a particle size of 5mm to 15mm, and activated carbon with a particle size of 5mm to 10mm as packing materials. Zeolite has a good adsorption effect on ammonia nitrogen, volcanic rock has a good adsorption effect on phosphorus, and activated carbon has a good adsorption effect on new pollutants and heavy metals. The highest concentration of pollution in the subsurface flow constructed wetland flowing into the water distribution branch pipe 310 is concentrated in the anaerobic packing layer 430. Therefore, arranging materials with good adsorption effects on conventional pollutants, heavy metals and new pollutants in the anaerobic packing layer 430 can effectively reduce the concentration of pollutants and reduce the pressure of subsequent biochemical reactions.

[0061] Example 7

[0062] like Figure 1 As shown, this embodiment is a further improvement on any one of embodiments 1 to 6, as detailed below:

[0063] The air distribution pipe includes: an air distribution main pipe 6, from which multiple air distribution branch pipes 610 branch out. Each air distribution branch pipe 610 has an air distribution hole on its wall. The air distribution main pipe 6 is connected to an air pump, which can be connected to mains power or to a solar panel for power supply. The air pump can blow air into the air distribution main pipe 6, and the air entering the air distribution main pipe 6 then flows through the air distribution branch pipes 610 to the packing layer 2 to fill and enrich the lower part of the packing layer 2 with oxygen.

[0064] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A down-flow subsurface constructed wetland for enhanced denitrification and phosphorus removal, comprising: The planting layer (1), water distribution main pipe (3), filler layer (2) and drainage pipe (5) are arranged from top to bottom, a plurality of water distribution branch pipes (310) with water distribution holes on the pipe wall are branched from the water distribution main pipe (3), characterized in that the water distribution branch pipe (310) is wrapped with a reinforced adsorption and anaerobic biochemical layer (4); the reinforced adsorption and anaerobic biochemical layer (4) comprises a hollow pipe (410), an anaerobic filler layer (430) and a hard pipe (420) wrapped from inside to outside on the water distribution branch pipe (310), the hard pipe (420) is provided with holes on the lower semicircle and is not provided with holes on the upper semicircle, and the total hole area per unit length of the hard pipe (420) is smaller than that of the water distribution branch pipe (310); The air distribution pipe is arranged in the lower region of the filler layer (2), the pipe wall of the air distribution pipe is provided with air distribution holes, and the air distribution pipe is connected with an air pump. The hard pipe (420) is wrapped with a shell (440) and a sludge collection layer (450) below, the shell (440) is in the shape of a hollow arc, the sludge collection layer (450) is located at the lowest point region of the hard pipe (420), the hard pipe (420) is connected with the shell (440) and the sludge collection layer (450) through a support and connection structure (470), the shell (440) is arranged with rope-shaped artificial water grass (460) with a large specific surface area and good hydrophilicity, and holes are formed in the side edge of the shell (440).

2. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 1, characterized in that, The water distribution holes are formed on the water distribution branch pipe (310), and the diameter of the water distribution holes is 8-10 mm; the diameter of the holes formed on the hard pipe (420) is 10-15 mm.

3. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 1 or 2, characterized in that, The diameter of the holes formed on the shell (440) is slightly smaller than that of the holes formed on the hard pipe (420), and the number of holes per unit length is the same.

4. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 3, characterized in that, The diameter of the holes formed on the shell (440) is 8-13 mm.

5. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 1, characterized in that, The outer diameter of the water distribution branch pipe (310) is 50-70 mm, the outer diameter of the anaerobic filler layer (430) is 130-180 mm, the outer diameter of the shell (440) is 200-210 mm, and the height of the lowermost part of the sludge collection layer (450) from the shell (440) is 150-200 mm.

6. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 1, characterized in that, The length of the reinforced adsorption and anaerobic biochemical layer (4) wrapped around the water distribution branch pipe (310) is 0.8-1 m, a plurality of reinforced adsorption and anaerobic biochemical layers (4) are wrapped around the water distribution branch pipe (310) in sequence, and the water distribution branch pipe (310) is arranged obliquely.

7. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 6, characterized in that, The sludge in the sludge collection layer (450) is pumped out of the artificial wetland.

8. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 1, characterized in that, The anaerobic filler layer (430) uses zeolite with a particle size of 5-15 mm, volcanic rock with a particle size of 5-15 mm and activated carbon with a particle size of 5-10 mm as fillers.

9. The enhanced denitrification and dephosphorization down-flow constructed wetland according to claim 1, characterized in that, The air distribution pipe comprises an air distribution main pipe (6), a plurality of air distribution branch pipes (610) are branched from the air distribution main pipe (6), the pipe wall of the air distribution branch pipe (610) is provided with air distribution holes, and the air distribution main pipe (6) is connected with an air pump.

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