Methods of using aquatic plants for water purification

Abstract

The present invention relates to the application of aquatic plants in water purification and a method thereof, wherein the aquatic plants are composed of emergent plants, floating leaf plants and submerged plants to form a plant restoration ecosystem, wherein the emergent plants are selected from at least one of canna, loosestrife and lily; the floating leaf plants are water lilies; and the submerged plants are Vallisneria. Compared with the prior art, the present invention conducts experimental research on dozens of aquatic plants, determines the water purification capacity of each, and then forms a plant restoration ecosystem with high purification efficiency for water bodies by combining various types of aquatic plants.

Description

Technical Field

[0001] The present invention relates to the field of water ecological management, and in particular to the application of aquatic plants in water purification and a method thereof. Background Art

[0002] Water is the source of life and an indispensable material basis for human survival. It is also an important part of the ecosystem and plays an important role in urban water supply and climate regulation. In recent years, industrial wastewater and domestic sewage containing nutrients such as nitrogen and phosphorus have been discharged into rivers, lakes and other water bodies in large quantities, exacerbating the eutrophication of the water environment. When eutrophication occurs in water bodies, on the one hand, due to the decrease in the transparency of the water body, it is difficult for sunlight to penetrate the water layer, which leads to the inhibition of photosynthesis of aquatic plants, insufficient oxygen in the water, and the death of fish and other aquatic organisms due to lack of oxygen, which exacerbates the odor of the water body and forms a vicious cycle of ecology. On the other hand, the surface of eutrophic waters will be covered with a large number of plankton and algae, which will greatly affect the ecological stability of the water body, cause water quality deterioration and other problems, destroy the balance and stability of the aquatic ecosystem, and the series of toxic components released by the massive reproduction of algae will even pose a serious threat to human survival and public health. Therefore, choosing an effective and appropriate water remediation technology to purify pollutants in water bodies is crucial to protecting people's health.

[0003] In order to effectively solve the problem of water pollution, domestic and foreign researchers are actively exploring water remediation technology. According to the principles involved, water remediation technology is mainly divided into three categories: physical technology, chemical technology and biotechnology. Among them, biotechnology has the advantages of significant purification effect, low cost, long-lasting environmental protection, and not easy to cause secondary pollution. It has become a commonly used method for purifying water pollutants in recent years. Aquatic plants are important purification materials in biotechnology and are important structural and functional components in aquatic ecosystems. They not only have high water purification capabilities, but also have high ornamental value. They can beautify the environment and create a good water landscape effect.

[0004] Aquatic plants refer to a type of plants that can grow normally in water or moist soil for a long time and have developed aeration tissue. According to different living habits and morphological characteristics, aquatic plants are divided into four types: emergent plants, floating leaf plants, floating plants and submerged plants. Among them, emergent plants refer to plants whose roots grow in the soil, and whose stems and leaves extend out of the water, such as canna, lily and loosestrife; floating leaf plants refer to plants whose roots are rooted in the soil, without obvious stems and cannot remain upright, and whose leaves or plants float on the water, such as water lilies and duckweed; floating plants refer to plants whose roots do not grow in the soil, and the whole plant floats on the water. Such as big floating and water hyacinth; submerged plants refer to plants whose rhizomes are rooted in the bottom mud, and the whole plant sinks underwater. Such aquatic plants have good aeration tissue and can effectively absorb nutrients in the water, such as Vallisneria, Potamogeton and Hydrilla.

[0005] By combining different types of aquatic plants, a more stable plant restoration ecosystem can be constructed to achieve efficient purification of polluted water bodies. For example, patent CN106561238A discloses a method for constructing aquatic biological functional groups, which stabilizes the biological functional groups in the aquatic system by constructing emergent plant functional groups, submerged plant functional groups, fish functional groups, and benthic animal functional groups. Patent CN105858898A discloses a method for ecological restoration of water bodies, which purifies water bodies by planting submerged plants, emergent plants, and floating leaf plants. However, different aquatic plants have different effects on water purification. How to match different types of aquatic plants according to the water purification characteristics of different plants to further improve the removal rate of TN, TP, and COD in polluted water bodies and achieve efficient purification of polluted water bodies still needs further research. Summary of the invention

[0006] The purpose of the present invention is to provide an application of aquatic plants in water purification and a method thereof in order to solve the above problems. The present invention conducts experimental research on dozens of aquatic plants, determines their respective water purification capabilities, and then reasonably combines various types of aquatic plants to form a plant restoration ecosystem with high purification efficiency for water bodies.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] The application of aquatic plants in water purification, wherein the aquatic plants are composed of emergent plants, floating-leaf plants and submerged plants to form a phytoremediation ecosystem, which is used to purify Class V or even Class V-lower water bodies.

[0009] Wherein, the emergent plant is selected from at least one of Canna indica, Lythrum salicina, and Zygophyllum;

[0010] The floating leaf plant is a water lily;

[0011] The submerged plant is Vallisneria.

[0012] As a preferred technical solution of the present invention, according to the number of plants planted, the ratio of the emergent plants, floating leaf plants and submerged plants is 1-3:0.1-0.4:2-5.

[0013] As a further preferred technical solution of the present invention, the ratio of the emergent plants, floating-leaf plants and submerged plants is 1-3:0.2-0.3:3-4 according to the number of plants planted.

[0014] As a preferred technical solution of the present invention, the number of emergent plants planted is 8 to 18 plants per square meter of polluted water, preferably 10 to 15 plants;

[0015] The number of floating leaf plants planted is 1 to 5 plants per square meter of polluted water, preferably 1 to 2 plants;

[0016] The number of submerged plants planted is 20 to 30 plants per square meter of polluted water, preferably 22 to 25 plants.

[0017] As a preferred technical solution of the present invention, the aquatic plants are a combination of the following plants:

[0018] Lythrum salicina, lily of the valley, Vallisneria sinensis, and Nymphaea truncatula, the plant number ratio is 1-2:1-2:2-4:0.1-0.3;

[0019] Or, for Canna, Zygophyllum, Vallisneria, and Nymphaea, the plant number ratio is 0.5-1.5:1-2:3-5:0.1-0.3;

[0020] Or, for Canna, Lythrum salicina, Vallisneria sinensis, and Nymphaea salsa, the plant number ratio is 0.5-1.5:1-2:3-5:0.1-0.3;

[0021] Alternatively, the ratio of canna, Vallisneria, and water lily plants is 2-3:3-5:0.1-0.3.

[0022] A method for using aquatic plants for water purification, wherein the aquatic plants described above are planted in the water to form a plant restoration ecosystem.

[0023] As a preferred technical solution of the present invention, a soil planting layer is arranged in the water body, and the emergent plants, floating leaf plants and submerged plants are planted in the soil planting layer.

[0024] As a preferred technical solution of the present invention, the thickness of the soil planting layer is 0.1-0.7m, and the soil planting layer is located at a water depth of 0.3-1.2m. Preferably, the thickness of the soil planting layer is 0.2-0.5m, and the soil planting layer is located at a water depth of 0.3-1m.

[0025] As a preferred technical solution of the present invention, the soil in the soil planting layer is natural soil around sewage, which is crushed and mixed evenly and then spread on the bottom or in a supporting device. By planting plants with natural soil around sewage, the adaptability of plants can be improved.

[0026] As a preferred technical solution of the present invention, a degradable capsule containing a treatment agent is added into the soil planting layer, and the treatment agent consists of a microbial preparation, sodium nitrate and sodium acetate.

[0027] Furthermore, the microbial preparation includes one or more of Bacillus, nitrifying bacteria, and yeast.

[0028] The degradable capsule is made of a water-soluble resin selected from polyvinyl alcohol polymers, which can be polymerized from vinyl esters, such as vinyl acetate or other fatty acid vinyl esters (such as vinyl propionate). The water-soluble resin is selected from polyvinyl alcohol with a degree of alcoholysis of 92-95%.

[0029] The degradable capsule begins to decompose after 30-55 days, releasing the internal substances. Among them, sodium acetate is added to the bottom mud as an organic carbon source to promote the growth and metabolic activities of microorganisms. Sodium nitrate, as an oxidant, can provide oxygen to supply microorganisms for oxidative degradation. The microbial preparation decomposes the organic matter, organic phosphorus, organic nitrogen and other components in the mud, providing sufficient absorbable substances for the later growth of aquatic plants, improving the later growth and reproduction ability of aquatic plants, and helping to improve the water purification capacity.

[0030] The mechanism of the present invention is as follows: Nutrient elements such as N and P in polluted water are directly absorbed and utilized by various aquatic plants and converted into substances required by themselves. Submerged plants and floating leaf plants with developed root systems have an important influence on the growth and metabolism of microorganisms. The plant root system and microorganisms work together to improve the regional water environment. On the one hand, the root system of aquatic plants provides a suitable attachment carrier and habitat for microorganisms, which is conducive to the reproduction and growth of microorganisms and improves their ability to mineralize and decompose pollutants. On the other hand, photosynthesis transmits oxygen to the root system through aquatic plants, increases the oxygen content in the water body, promotes the formation of an aerobic environment in the root system area, and can ensure the metabolic activities of aerobic microorganisms; at the same time, the anaerobic zone formed outside the root system area can meet the metabolic activities of anaerobic microorganisms. The entire root system area constitutes a habitat where aerobic and anaerobic zones coexist, making the abundance and types of microorganisms more perfect, which is conducive to the nitrification and denitrification, and finally achieving nitrogen removal and phosphorus removal. The developed root system can form a filter layer when in contact with the water body, and reduce the content of suspended solids and insoluble colloids in the water by adsorbing and sticking them, thereby improving the transparency of the water body. In addition, the presence of emergent plants and floating-leaf plants with developed leaves and branches reduces the wind speed on the water surface near the plants, stabilizes the water flow speed, reduces the disturbance of the bottom mud, and is conducive to the sedimentation of suspended solids in the water body, effectively avoiding the possibility of resuspension and preventing the release of N and P in the bottom mud.

[0031] The present invention combines emergent plants, floating-leaf plants and submerged plants to form a plant restoration ecosystem, giving full play to the high absorption capacity of emergent plants for TN and TP in water bodies, the submerged plants Vallisneria and Potamogeton sphaerocephala, and the floating-leaf plant Water Lily for TP in water bodies, comprehensively considering the space utilization rate of aquatic plants in water bodies and the planting arrangement, combining different types of aquatic plants to construct an efficient plant restoration ecosystem, achieving efficient purification of polluted water bodies, having a synergistic compound effect, and being able to further improve the removal effect of TN, TP and COD in polluted water bodies compared with a single aquatic plant planting scheme. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1-4 is the removal rate of TN by different aquatic plants at 75 days;

[0033] Figure 5-8 is the removal rate of TP by different aquatic plants at 75 days;

[0034] Figure 9-12 is the COD removal rate of different aquatic plants at 75 days;

[0035] Figure 13-15 is the removal rate of TN by different aquatic plant combinations at 75 days;

[0036] Figure 16-18is the removal rate of TP by different aquatic plant combinations at 75 days;

[0037] Figure 19-21 COD removal rates of different aquatic plant combinations at 75 days. DETAILED DESCRIPTION

[0038]

Study on the water purification efficiency of different aquatic plants

[0039] The present invention studies the purification efficiency of TN, TP and COD in simulated eutrophic water by 28 different aquatic plants under natural light and rain shelter conditions.

[0040] The 28 different aquatic plants studied in the present invention include emergent plants, floating leaf plants, floating plants and submerged plants, among which the emergent plants include: lily, fish grass, calamus, calamus, yellow calamus, cattail, reed, variegated reed, canna, loosestrife, water plantain, Cyperus, lotus, iris; floating leaf plants include: water lily, Euryale ferox, water chestnut, water chestnut; floating plants include: Salvia salviae, water hyacinth, giant calamus, azolla; submerged plants include: Vallisneria, black algae, water chestnut, Malaysian pondweed, hornwort, elodea, Table 1 shows the basic information of the 28 aquatic plants.

[0041] Table 1 Basic information of aquatic plants

[0042]

[0043]

[0044] The experimental water was taken from a park in Shanghai. By adding ammonium nitrate, potassium dihydrogen phosphate and magnesium acetate reagents to the raw water, the initial concentrations of TN, TP and COD in the water were adjusted to 2.58 mg / L, 0.56 mg / L and 55.67 mg / L, respectively, meeting the Class V standard of the "Surface Water Environmental Quality Standard" (GB 3838-2002).

[0045] Healthy plants with good growth condition and consistent growth were selected and planted with soil in plastic boxes of length × width × height = 0.9m × 0.6m × 0.9m. The bottom of the box was placed with sediment of about 0.3m thickness and the water depth of the upper sediment was 0.5m. Floating plants were arranged in the water body through limiting devices. A total of 28 plant groups and 1 blank control group (without plants) were set up, and 3 replicates were set for each group.

[0046] To ensure consistent sampling conditions and avoid the influence of other factors, the sampling time was set at 8:00-9:00 in the morning. In order to replenish the water consumed by evaporation, plant absorption and sampling, pure water was added to the box after each sampling to keep the water level constant. The test lasted for 75 days, and samples were taken every 7 days to detect the concentrations of TN, TP and COD in the water body and calculate their removal rates.

[0047] The test lasted for 75 days (d). After the 75th day, the removal rates of TN, TP and COD in the water by different aquatic plants were tested, where removal rate = (initial concentration - final concentration) / initial concentration × %, and the blank test CK was used for comparison. The test results are detailed in Figures 1 to 12 And Table 2.

[0048] Table 2

[0049]

[0050]

[0051] It can be seen from Table 2 that, considering the average removal rates of TN, TP and COD in water bodies, the aquatic plants with better water purification efficiency are: canna, cattail, reed, variegated reed, loosestrife, calamus, calamus, Vallisneria, Cyperus, water plantain, yellow calamus, pickerel grass, lotus, water lily, etc.

[0052] Sort by the removal rates of TN, TP and COD respectively to select aquatic plants with better effects. See Table 3 for details.

[0053] Table 3

[0054]

[0055] As shown in Table 3, for the removal of TN in water bodies, emergent plants have excellent removal effects on TN, which are generally better than floating leaf plants, floating plants and submerged plants. Among them, the emergent plant cattail showed the highest removal efficiency, with a TN removal rate of up to 61.24% in water bodies. This is mainly because the emergent plants are large and grow fast. The developed root system can not only absorb and enrich nitrogen in the water body, but also provide a suitable growth environment for microorganisms, promote nitrification and denitrification, and improve the purification effect of TN. In addition, the floating leaf plant water lily also showed good TN removal ability, with a TN removal rate of 55.43%.

[0056] Regarding the removal of TP in water bodies, the removal effect of emergent plants is also excellent overall, including reeds, cannas, cattails, variegated reeds, loosestrife, etc., with a TP removal rate of about 70%. Among floating leaf plants, water lilies have maintained excellent TP removal capabilities. Among submerged plants, Vallisneria has a better TP removal rate. Phosphorus plays an important role in plants and can participate in the main metabolic processes in plants. The removal of phosphorus by aquatic plants mainly depends on the absorption and sedimentation of plants. It reduces the concentration of TP in water bodies by directly absorbing soluble phosphorus in the bottom mud and promoting the sedimentation of suspended matter.

[0057] Regarding the removal rate of COD in water bodies, the floating plant water hyacinth showed the best COD removal rate. The main reason is that the root system of water hyacinth can release anti-algae substances, which further inhibit the growth of algae through shading and competition for nutrients, thereby purifying water quality and eliminating COD. The rest are emergent plants such as canna, variegated reed, cattail, etc., and submerged plants such as Vallisneria. The removal of COD by aquatic plants can be mainly attributed to the microbial action of plants. Through the photosynthesis of plants, oxygen is transported to the root system, forming an aerobic and anaerobic environment suitable for the survival of microorganisms, promoting the decomposition of organic matter by microorganisms, and thus reducing the organic matter content in the water body.

[0058] Combined with the above test results, the plants with actual planting depth and good landscape effect are: canna, loosestrife, lily, water lily, and Vallisneria. Although the floating plant Eichhornia crassipes showed the best COD removal rate, considering that Eichhornia crassipes is an alien invasive species, it is easy to endanger the ecology of local species and is not suitable for introduction.

[0059]

Study on the water purification efficiency of different aquatic plant combinations

[0060] Taking into account the spatial utilization and planting arrangement of aquatic plants in water bodies, combined with the above experimental results, the present invention further combines different types of aquatic plants to construct a more efficient plant restoration ecosystem. According to the number of plants planted, the ratio of emergent plants, floating leaf plants, and submerged plants is controlled at 1-3: 0.1-0.4: 2-5. The different aquatic plant combination schemes are detailed in Table 4.

[0061] Table 4 Different types of aquatic plant combinations

[0062]

[0063]

[0064] The test results are detailed in Figures 13 to 21 And Table 5.

[0065] Table 5 Removal rate of different aquatic plant combinations

[0066]

[0067] From the above experimental results, it can be seen that compared with the other two combinations, the removal effect of the emergent-submerged-floating leaf combination on TN is slightly better. The removal rates of the emergent-submerged combination (C1), the emergent-floating leaf combination (F1) and the emergent-submerged-floating leaf combination (T4) on TN are 65.5%, 61.62%, and 89.86%, respectively, which are significantly different from the control. Through comparative analysis, it was found that canna and loosestrife were planted in all three treatment groups, indicating that the combination of canna and loosestrife has a better purification effect on TN in water. However, when the water lily and water lily were added, the removal rate of the treatment group T7 decreased by 29.98%. This may be due to the influence of the planting space. The plants compete fiercely for nutrients, which is not conducive to their own growth; at the same time, the presence of water lilies will block part of the light and affect the metabolism of the submerged plants in the lower layer, thereby affecting the absorption of nitrogen by submerged plants.

[0068] The removal rates of TP by the emergent-submerged combination, the emergent-floating leaf combination, and the emergent-submerged-floating leaf combination were slightly higher than those of single aquatic plants. Among them, the average removal rates of TP by the emergent-submerged combination and the emergent-submerged-floating leaf combination were 4.54% and 8.61% higher than those by the emergent-floating leaf combination, respectively, indicating that the combination with submerged plants had a better effect in degrading TP in water bodies. Treatment group T4 showed the highest removal efficiency, indicating that emergent plants have a large biomass, grow fast, and can absorb and accumulate more phosphorus. The blank control group can also remove a certain amount of phosphorus, probably because phosphorus in water bodies can exist in the form of phosphates, which combine with the matrix to form substances that are difficult to remove.

[0069] The COD removal rate of the emergent-submerged-floating leaf combination was significantly different from that of the emergent-submerged combination. The former performed best with a removal rate of 89.64%, and the latter performed worst with 55.09%. Overall, the emergent-submerged-floating leaf combination had better COD removal effect than the other two combinations. This was mainly because COD removal depended on microbial activity, and plant activity only accounted for 15% to 19% of the total COD removal. The combination of three different types of aquatic plants can provide more diverse habitats for microorganisms, which is beneficial to the degradation of organic matter.

[0070] Combining the average removal rates of TN, TP and COD, the emergent-submerged-floating leaf combination had the highest water purification efficiency, namely T4 (Canna + Lythrum salicaria + Vallisneria sinensis + Water lily), T5 (Canna + Lythrum salicaria + Vallisneria sinensis + Water lily), T6 (Lythrum salicaria + Lythrum salicaria + Vallisneria sinensis + Water lily), and T1 (Canna + Vallisneria sinensis + Water lily). The average removal rates were all above 76%, especially the combination of T4 (Canna + Lythrum salicaria + Vallisneria sinensis + Water lily), which reached more than 89%, which had obvious advantages over other groups.

[0071] In a specific implementation, as a more preferred technical solution, when aquatic plants are planted in a soil planting layer, the soil in the soil planting layer is the natural soil around the sewage, which is evenly spread on the bottom or in a carrying device after being crushed and mixed. By planting plants with the natural soil around the sewage, the adaptability of the plants can be improved. Wherein, a degradable capsule containing a treatment agent is added to the soil planting layer, and the doping amount of the degradable capsule is 0.1-1wt% of the soil. The treatment agent is composed of a microbial preparation, sodium nitrate and sodium acetate (configured in a weight ratio of 1:2-3:3-4), and the microbial preparation includes one or more of bacillus, nitrifying bacteria, and yeast. The degradable capsule is made of a water-soluble resin, selected from a polyvinyl alcohol polymer, and can be polymerized from vinyl esters, which can be vinyl acetate or other fatty acid vinyl esters (for example, vinyl propionate, etc.), and the water-soluble resin is selected from polyvinyl alcohol with a degree of alcoholysis of 92-95%.

[0072] In the T4 (canna + Lythrum salicaria + Vallisneria sinensis + Water lily) scheme, 0.5wt% of degradable capsules were added to the soil planting layer. The treatment agent in the degradable capsules consisted of a microbial preparation, sodium nitrate and sodium acetate in a weight ratio of 1:2:3. The microbial preparation included Bacillus, nitrifying bacteria and yeast. The total bacterial count was 7×10 10 ~8×10 10 cfu / ml, the degradable capsules are prepared from polyvinyl alcohol polymers and tested in the same manner as above. The tests were lasted for 35 days (d), 45 days (d), 55 days (d), 65 days (d) and 75 days (d). The removal rates of TN, TP and COD in water by aquatic plants are shown in Table 3. It can be seen from the experimental results in Table 3 that after adding the degradable capsules containing the treatment agent into the soil planting layer, the water body can be purified in a shorter time. In 45-55 days, the pollutant removal rate can reach about 90%.

[0073] Table 3

[0074] Processing time TN removal rate (%) TP removal rate (%) COD removal rate (%) Average removal rate (%) 35 days 82.67 83.32 82.17 82.72 45 days 88.45 89.61 88.74 88.93 55 days 89.26 90.23 88.79 89.43 65 days 90.24 90.48 89.45 90.06 75 days 90.68 90.88 90.14 90.57

[0075] Implementation Cases

[0076] The above T4 (canna + Lythrum salicaria + Vallisneria sinensis + Water lily) combination scheme was used for water treatment in a park in Shanghai, with a water area of ​​about 34,000m 2The initial water quality data are as follows: the average COD content in the main lake area is between Class IV and Class V of the surface water quality standard, the average total nitrogen content is between Class III and Class IV of the surface water, and the average total phosphorus content is between Class III and Class IV of the surface water; the average COD content in the South Lake area is higher than the Class V standard of surface water, the average total nitrogen content is higher than the Class V standard of surface water, and the average total phosphorus content is higher than the Class V standard of surface water. The water color is yellowish, the transparency is low, the pollutants are mainly nitrogen and phosphorus, and the eutrophication problem is prominent.

[0077] After adopting this treatment plan, the average COD concentration in the main lake area was reduced from 38mg / L to 9.6mg / L, and the average COD removal rate was 75%; the average COD concentration in the south lake area was reduced from 73mg / L to 11mg / L, and the average COD removal rate was 85%. The average TP concentration in the main lake area dropped from 0.250mg / L to 0.086mg / L, and the average TP removal rate was 66%; the average TP concentration in the south lake area dropped from 0.583mg / L to 0.093mg / L, and the average TP removal rate was 84%. The average TN concentration in the main lake area was always below 1.5mg / L, which met the Class IV standard of surface water quality standards; the average TN concentration in the south lake area dropped from 3.3mg / L to 1.29mg / L, and the average TN removal rate was 61%.

[0078] 75 days after the plants were planted, the transparency of the main lake area and the southern lake area was greatly improved, and the main water quality indicators (total nitrogen, total phosphorus, COD) reached the National Surface Water Environmental Quality Standard (GB 3838-2002) Class IV and above. The water ecosystem was relatively complete and had continuous self-purification capabilities.

[0079] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for using aquatic plants to purify water, characterized in that: Aquatic plants are planted in water bodies to form a phytoremediation ecosystem. The aquatic plants are composed of emergent plants, floating leaf plants and submerged plants to form a phytoremediation ecosystem. Wherein, the emergent plant is selected from Canna and Lythrum salicaria, the floating leaf plant is Water Lily, and the submerged plant is Vallisneria; Among them, the ratio of canna, loosestrife, vallisneria, and water lily is 1:1.5:4:0.2; Arranging a soil planting layer in the water body, wherein the emergent plants, floating leaf plants, and submerged plants are planted in the soil planting layer; The soil in the soil planting layer is natural soil around the sewage, which is crushed and mixed evenly before being spread on the bottom or in the supporting device. The degradable capsules containing treatment agents were added to the soil planting layer. The doping amount of the degradable capsules was 0.5wt% of the soil. The treatment agents in the degradable capsules were composed of microbial preparations, sodium nitrate and sodium acetate in a weight ratio of 1:2:

3. The microbial preparations included Bacillus, nitrifying bacteria and yeast. The total number of bacteria was 7×10 10 ~8×10 10 cfu / ml; The degradable capsule is prepared from polyvinyl alcohol with an alcoholysis degree of 92-95%. The degradable capsule begins to decompose after 30-55 days and releases the internal substances.

2. The method for using aquatic plants for water purification according to claim 1, characterized in that: The number of emergent plants planted is 8 to 18 plants per square meter of polluted water; The number of floating leaf plants planted is 1 to 5 plants per square meter of polluted water; The number of submerged plants planted is 20 to 30 per square meter of polluted water.

3. The method for using aquatic plants for water purification according to claim 1, characterized in that: The number of emergent plants planted is 10 to 15 plants per square meter of polluted water; The number of floating leaf plants planted is 1 to 2 plants per square meter of polluted water; The number of submerged plants planted is 22 to 25 per square meter of polluted water.

4. The method for using aquatic plants for water purification according to claim 1, characterized in that: The thickness of the soil planting layer is 0.1-0.7 m, and the soil planting layer is located at a water depth of 0.3-1.2 m.

5. The method for using aquatic plants for water purification according to claim 1, characterized in that: The thickness of the soil planting layer is 0.2-0.5 m, and the soil planting layer is located at a water depth of 0.3-1 m.

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