A system and method for denitrifying and purifying water using coupled denitrification of plants, bacteria, and algae

Abstract

The present invention belongs to the technical field of water denitrification and ecological restoration, and discloses a system and method for denitrifying and purifying water bodies by coupling plants, bacteria, and algae. The system includes an inlet pipe, a microalgae growth pool, an algae-bacteria liquid pool, an inlet plant pool, a main reaction pool, and an outlet pool, which are connected in sequence; the inlet plant pool, the main reaction pool, and the outlet pool are each independently provided with plants carrying denitrifying bacteria; the main reaction pool includes an oxygen elimination pool, a facultative anaerobic pool, and an anoxic pool, which are connected in sequence; the main reaction pool is also provided with a bacterial liquid return pipe, which is connected to the inlet of the algae-bacteria liquid pool. The present invention realizes a natural and sustainable supply of carbon sources. The system and method of the present invention are stable in operation, highly reliable, and have strong resistance to shock loads.

Description

Technical Field

[0001] The present invention belongs to the technical field of water denitrification and ecological restoration, and more specifically, relates to a system and method for denitrifying and purifying water bodies by coupling plants, bacteria and algae. Background Art

[0002] Amidst increasing water scarcity in cities, recycled water has become a crucial source of fresh water supply. However, there is a significant gap between the nitrogen and phosphorus standards for sewage treatment plant effluent and the water quality requirements of surface water environments, making it difficult to maintain a consistently high quality water environment in urban rivers and lakes. For example, the total nitrogen content in water bodies is 20 mg / L for Class B effluent under the Pollutant Discharge Standard for Urban Wastewater Treatment Plants (GB18918-2022), while the total nitrogen content for Class IV water under the Surface Water Environmental Quality Standard (GB3838-2002) is 1.5 mg / L, a 13-fold difference. This difference is also 10-fold lower than the 15 mg / L for Class B effluent under GB11890-2012. Furthermore, this nitrogen content is primarily nitrate nitrogen, and recycled water has poor biodegradability, making it difficult to provide a viable carbon source for further nitrate nitrogen removal. Consequently, when recycled water is used as replenishment water, it is prone to algae growth and excessive nitrogen and phosphorus levels, making it difficult to maintain a sustainable balance in the ecosystem.

[0003] The current technology uses external carbon sources such as low molecular carbon sources such as sodium acetate, ethanol, methanol, etc. The denitrification effect of liquid carbon sources is generally good, but the control of effluent carbon source content is unstable and easily causes secondary pollution; and high molecular sugar substances such as glucose, sucrose and other sugar carbon sources are easily restricted by dissolved oxygen and easily produce nitrites, and sugar substances easily lead to excessive growth of microorganisms, thereby causing fouling; solid carbon sources such as straw plant fiber carbon sources can release monosaccharides and other nutrients as denitrification external carbon sources after treatment. This type of carbon source is economical and practical. As a carbon source, it can also provide a microbial fixation carrier, thereby improving the denitrification efficiency. However, the utilization cycle of this type of carbon source is relatively short, and it is difficult to solve the problem of nitrogen removal in large-scale and long-term applications of recycled water.

[0004] Generally speaking, traditional denitrification systems suffer from two major problems: 1) Insufficient carbon source: Traditional denitrification systems typically rely on externally added organic matter as a carbon source, but this organic matter is expensive and can be gradually depleted over long periods of operation, resulting in reduced denitrification effectiveness. 2) Unsustainable treatment effects: In traditional systems, denitrification performance is easily affected by environmental conditions such as temperature and humidity. Denitrification systems often perform well under certain conditions, but perform poorly under other conditions, leading to the risk of secondary contamination.

[0005] At present, the technologies related to "plants, bacteria, algae, denitrification" or "bacteria, algae, denitrification" for purifying water bodies include: Technology (1), a bacteria-algae collaborative carbon fixation and denitrification system and treatment method (application number CN202211591478.7), which is about improving the denitrification performance of denitrifying bacteria after microalgae pass through a water purification device, a denitrification and denitrification device, and a phosphorus removal device, thereby improving the denitrification performance; Technology (2), an artificial wetland device for denitrification of low-carbon and nitrogen water bodies and its treatment method (CN201410217395 .0), this invention uses external denitrification microbial liquid and nutrient solution to achieve denitrification treatment in the wetland, thereby achieving the purpose of strengthening the low denitrification of artificial wetlands; the above technologies all focus on the application of algae, bacteria and plants in denitrification, but on the one hand, the above technologies fail to clarify that although the algae liquid entering the system can provide a carbon source to further purify the water body, the nitrogen released by the algae after classification in the system can bring secondary risks to the system. Technology (2) requires the addition of external bacterial liquid and nutrient solution, which is difficult to solve the problem of self-sufficiency of the system's denitrification carbon source.

[0006] In summary, in view of the current problems in water environment applications, deep denitrification requires external carbon sources, pollution and blockage in the application of external carbon sources, and poor sustainability of carbon source utilization, it is urgent to explore a method that can rely on the environment itself to produce a continuous carbon source to solve the problem of lack of carbon sources for denitrification in low carbon-nitrogen ratio water bodies. Summary of the Invention

[0007] The present invention aims to overcome the shortcomings of existing carbon sources, such as the lack of sustainability and the risk of secondary nutrient introduction, by providing a system and method for water purification using a coupled system of plants, bacteria, and algae for denitrification. This system and method achieve a sustainable, natural carbon source supply, and are characterized by stable operation, high reliability, and strong resistance to shock loads.

[0008] To achieve the above objectives, the present invention provides a system for denitrification and purification of water bodies by coupling plants, bacteria and algae, the system comprising an inlet pipe, a microalgae growth pool, an algae-bacteria liquid pool, an inlet plant pool, a main reaction pool and an outlet pool connected in sequence;

[0009] The inlet plant pool, main reaction pool and outlet pool are each independently provided with plants and microorganisms with denitrifying bacteria; the main reaction pool includes an oxygen elimination pool, a facultative anaerobic pool and an anoxic pool connected in sequence;

[0010] The main reaction tank is further provided with a bacterial liquid reflux pipe, and the bacterial liquid reflux pipe is connected to the inlet of the algae-bacteria liquid tank.

[0011] According to the present invention, preferably, the depth of the microalgae growth pond is set within 60 cm.

[0012] According to the present invention, preferably, the plant is at least one of an emergent plant, a submerged plant and a floating plant.

[0013] In the present invention, the plant growth, metabolism, and decomposition, as well as the nutrients and biological growth and metabolism in the water, provide a growth environment for algae and denitrifying bacteria. The plant roots provide more space for the system's microbial growth and serve as a carrier for microbial attachment, promoting the colonization of denitrifying bacteria within the system and forming a more stable ecosystem. The fallen plants serve as a surface insulation layer in winter and also provide a partial carbon source, enhancing denitrification. The nutrients and denitrifying bacteria in the water of the bacterial liquid return pipe can be introduced as bacterial strains. The biological growth and metabolism also provide the nutrients, living conditions, and a source of strains required by the denitrifying bacteria. Furthermore, the main reaction tank is also equipped with plants carrying denitrifying bacteria and their attachment carriers. Thus, the main reaction tank integrates plants, bacteria, and algae. The main reaction tank comprises a sequentially connected deoxidation tank, a facultative anaerobic tank, and an anoxic tank. The microorganisms in the system are coupled and synergistic, achieving optimal denitrification performance and improving denitrification stability. Furthermore, because the microalgae growth tank removes some nitrogen and phosphorus, the system avoids secondary pollution. Afterwards, it passes through the outlet pool, which is also equipped with plants to further purify the water quality.

[0014] According to the present invention, preferably, the algae liquid pool is further provided with an algae liquid reflux pipe, and the algae liquid reflux pipe is connected to the inlet of the microalgae growth pool.

[0015] According to the present invention, preferably, the outlet of the microalgae growth pool is further connected to an algae liquid recovery pool.

[0016] According to the present invention, preferably, the microalgae growth pond is provided with a monitoring, sensing and lighting subsystem; the monitoring, sensing and lighting subsystem includes a regulating unit, a central control device and a sensor monitoring unit;

[0017] The sensing monitoring unit includes a light intensity sensor, an absorbance value sensor, a temperature sensor and a pH sensor; the regulating unit includes a light regulating device, a temperature regulating device and a pH regulating device;

[0018] The light intensity sensor is used to monitor the light intensity of the microalgae growth pond and feed back the light intensity data to the central control device; the absorbance value sensor is used to monitor the absorbance value of the algae and feed back the absorbance value data to the central control device; the temperature sensor is used to monitor the water temperature in the microalgae growth pond and feed back the temperature data to the central control device; the pH sensor is used to monitor the pH of the water in the microalgae growth pond and feed back the pH data to the central control device;

[0019] The central control device is used to adjust the light intensity, water temperature and water pH in the microalgae growth pond through a light adjustment device, a temperature adjustment device and a pH adjustment device.

[0020] In the present invention, as a preferred embodiment, the surface of the microalgae growth pond is unobstructed, and part or all of the illumination used for photosynthesis is natural illumination.

[0021] Another aspect of the present invention provides a method for denitrifying and purifying water using plants, bacteria, and algae. The method uses the system described above and includes the following steps:

[0022] S1: sending low-carbon-nitrogen ratio wastewater and algae into the microalgae growth pond, allowing the algae to grow, reproduce, and aggregate in the microalgae growth pond using the nitrogen and phosphorus in the low-carbon-nitrogen ratio wastewater. Then, by controlling the illumination time and illumination intensity, which are key aggregation factors for the algae, the extracellular secretion adhesive of the cells is changed to aggregate into a larger density volume, and the buoyancy characteristics of the algae are adjusted to achieve natural sedimentation (performed in the dark). The sedimentation rate can reach 600 μm / s. The illumination intensity is set in the range of 1000 lx-20000 lx according to the target requirements. By controlling the illumination and darkness time, that is, the aggregation and sedimentation of the algae cells under dark conditions can be adjusted, and the dark conditions are controlled to separate the supernatant algae separation liquid and the aggregated algae bodies settled at the bottom of the microalgae growth pond; at the same time, the algae fix carbon dioxide in the air through photosynthesis to synthesize their own nutrients, and then release extracellular secreted organic matter into the supernatant algae separation liquid through metabolism;

[0023] S2: feeding the bacterial liquid from the main reaction tank and part of the supernatant algae separation liquid from the microalgae growth tank into the algae-bacteria liquid tank to obtain a bacterial-algae mixed liquid; feeding the bacterial-algae mixed liquid into the influent plant tank and the main reaction tank in sequence, using the extracellular secreted organic matter in the bacterial-algae mixed liquid as a carbon source for denitrification, and under the action of the denitrifying bacteria in the influent plant tank and the main reaction tank, the nitrogen in the low carbon-nitrogen ratio wastewater is removed through the coupling of plants, bacteria and algae to obtain purified water;

[0024] S3: Sending the purified water out of the system through the outlet tank.

[0025] According to the present invention, preferably, the algae is at least one of Chlorella, Chlamydomonas, Scenedesmus, Haematococcus pluvialis and Chrysophyte.

[0026] According to the present invention, preferably, the time of the natural sedimentation separation (performed in the dark) is set according to the concentration of chlorophyll a in the microalgae growth pond and the particle size of the algae after growth, reproduction and condensation; preferably, when the concentration of chlorophyll a in the microalgae growth pond is 3000-8000ug / L and the particle size of the algae after growth, reproduction and condensation is 100-200μm, the time of the natural sedimentation separation is set to 9-13h.

[0027] In the present invention, the selection of control parameters for the effective separation of algae liquid is very important. Experiments have found that the size of Chlorella varies greatly in different states. As shown in Figure 1, the size of the coagulated algae is more than 10 times that of the single-cell individual, and thus its sedimentation rate is also different. By utilizing the coagulated state of algae, the natural sedimentation of algae can be effectively achieved, thereby achieving effective separation of the algae liquid.

[0028] In the present invention, the concentration change of chlorophyll a in the water body of the microalgae growth pool is used to judge the growth characteristics of the algae, thereby obtaining the sedimentation rate range of the algae, further determining the sedimentation time of the microalgae growth pool, and providing parameters for collecting the supernatant separation liquid of the algae in the algae liquid pool area.

[0029] According to the present invention, preferably, the method further comprises returning part of the bacteria-algae mixed liquid in the algae-bacteria liquid pool to the microalgae growth pool, thereby ensuring that the concentration of chlorophyll a in the bacteria-algae mixed liquid is 10-100 ug / L; preferably, the ratio of the bacteria-algae mixed liquid in the algae-bacteria liquid pool to the reflux liquid returned to the microalgae growth pool is (1-3):1.

[0030] According to the present invention, preferably, the concentration of chlorophyll a is calculated according to a linear equation between chlorophyll a and the absorbance at 680 nm;

[0031] The linear equation is y=6411x+27.71, wherein x is the absorbance value at 680nm, which is monitored by the absorbance value sensor; y is the concentration of chlorophyll a; the correlation coefficient R of the linear equation is 2 =0.992.

[0032] In the present invention, the growth and reproduction of algae directly impacts the carbon source supply, algal liquid separation efficiency, and the system's denitrification performance. Therefore, the present invention incorporates a monitoring, sensing, and illumination subsystem, comprising a light control device, a temperature control device, a pH control device, a light intensity sensor, an absorbance sensor, a temperature sensor, and a pH sensor. Specifically, the present invention utilizes the OD680 (680 nm) measured by the absorbance sensor to calculate the chlorophyll a concentration using the aforementioned linear equation, and then determines the settling time range based on the chlorophyll a concentration range.

[0033] The light intensity sensor, absorbance sensor, temperature sensor, and pH sensor are evenly distributed in the microalgae growth pond to monitor water conditions in real time. The sensors transmit data to a central control device, which automatically adjusts the light, temperature, and pH value based on the sensor data.

[0034] According to the present invention, preferably, the light intensity for performing the photosynthesis is determined based on the absorbance value monitored by the absorbance sensor; preferably, when the absorbance value monitored by the absorbance sensor is lower than the absorbance threshold value set by the central control device, the central control device adjusts the light intensity for performing the photosynthesis through the light adjustment device to promote the photosynthesis of algae and increase the organic carbon production; preferably, the light intensity for performing the photosynthesis is above 1000 lx.

[0035] In the present invention, the system can generate carbon sources independently through the photosynthesis of algae, and no longer rely on external addition, thus solving the problem of insufficient carbon sources in traditional systems. Moreover, the higher the algae content in the microalgae growth pool, the higher the COD value of the algae-bacteria mixed liquid in the algae-bacteria liquid pool. Figure 2 In the coupled synergistic denitrification system of the present invention, a synergistic relationship is formed between the algae, denitrifying bacteria and vegetation, making the entire system more stable and capable of self-regulation under different environmental conditions, thereby improving the sustainability of the treatment effect.

[0036] According to the present invention, preferably, the part of the supernatant algae separation liquid in the microalgae growth pond is the supernatant algae separation liquid in the microalgae growth pond from the uppermost surface layer to the lower 10-20 cm portion.

[0037] In the present invention, the main reaction tank includes an oxygen removal tank, a facultative anaerobic tank and an anoxic tank connected in sequence, wherein the oxygen-consuming microorganisms in the oxygen removal tank can consume the dissolved oxygen in the water from the inlet plant tank, and then the water in the oxygen removal tank enters the facultative anaerobic tank and the anoxic tank to achieve deep removal of ammonia nitrogen and nitrate nitrogen.

[0038] The beneficial effects of the technical solution of the present invention are as follows:

[0039] This method separates algae from liquid by leveraging the algae's settling characteristics. It utilizes extracellular organic matter released by natural algae in the supernatant algae fraction after carbon fixation as a carbon source. This method also avoids introducing nutrients such as nitrogen and phosphorus into the system. The coupled effects of plants, the supernatant algae fraction, and the bacteria in the system create a plant-bacteria-algae coupled denitrification system in the main reaction tank. Furthermore, the present invention leverages the photosynthetic properties of algae to achieve a sustainable carbon source, thereby enabling continuous and efficient nitrogen removal. This effectively addresses the difficulty in denitrifying water bodies with low carbon-nitrogen ratios due to a lack of carbon sources.

[0040] The present invention realizes the natural sustainable supply of carbon sources, and the system and method of the present invention are stable in operation, highly reliable, and have strong anti-shock load capability.

[0041] The system of the present invention fully utilizes the carbon source fixed by algae, avoiding the economic losses of adding external carbon sources and manual operation processes. The system of the present invention forms different functional zones, providing scenarios for the full utilization of algae. The system of the present invention can achieve the self-circulation of ecological materials and energy, solving ecological problems with ecological thinking.

[0042] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention with reference to the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0044] Figure 1 shows the situation before and after algae growth, reproduction and aggregation in a method for denitrification and purification of water bodies by coupling plants, bacteria and algae provided in Example 1 of the present invention (Figure 1 (b) shows the algae growth, reproduction and aggregation before, Figure 1 (b): (1) length 6.23 μm; (2) length 6.27 μm; Figure 1 (a) shows the algae growth, reproduction and aggregation after, Figure 1 (a): (1) longitudinal length 118.89 μm; (2) transverse length 106.72 μm; (3) transverse length 96.46 μm; (4) longitudinal length 118.89 μm).

[0045] Figure 2 The figure shows the relationship between the COD content in the algae liquid pool and the algae biomass in the microalgae growth pool in a method for denitrification of water by coupling plants, bacteria and algae provided in Example 1 of the present invention (the horizontal axis "absorbance value" represents the algae biomass in the microalgae growth pool, and the vertical axis represents the COD content in the algae liquid pool, Figure 2 The linear equation on is y=464.67x-61.284, and the correlation coefficient R 2 =0.9042).

[0046] Figure 3 A schematic diagram of a system for denitrification and purification of water using coupled plants, bacteria, and algae, provided in Example 1 of the present invention, is shown.

[0047] Figure 4 The relationship between the content of bacteria and algae and the nitrogen removal rate in the method for denitrification and purification of water by coupling plants, bacteria and algae provided in Example 1 of the present invention is shown ( Figure 4 The linear equation on is y = 2.8697x-0.0408, and the correlation coefficient R 2=0.8249, "N%" is the nitrogen removal rate)

[0048] Figure 5 A graph showing the relationship between chlorophyll a and absorbance at 680 nm in a method for denitrification and purification of water using coupled denitrification of plants, bacteria and algae provided in Example 1 of the present invention is shown.

[0049] The following are the descriptions of the reference numerals:

[0050] 1-water inlet pipe, 2-microalgae growth pool, 3-sensor monitoring unit, 4-regulation unit, 5-central control equipment,

[0051] 6-algae and bacteria liquid pool, 7-algae liquid return pipe, 8-water inlet plant pool, 9-plants, 10-oxygen elimination pool, 11-facultative anaerobic pool, 12-anoxic pool, 13-bacteria liquid return pipe, 14-outlet pool, 15-algae liquid recovery pool. DETAILED DESCRIPTION

[0052] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. Instead, these embodiments are provided to make the present invention more thorough and complete and to fully convey the scope of the present invention to those skilled in the art.

[0053] Example 1

[0054] This embodiment provides a system for denitrification and water purification using plants, bacteria and algae. Figure 3 As shown, the system includes an inlet pipe 1, a microalgae growth pool 2, an algae liquid pool 6, an inlet plant pool 8, a main reaction pool and an outlet pool 14 connected in sequence;

[0055] The inlet plant pool 8, the main reaction pool and the outlet pool 14 are each independently provided with plants 9 with denitrifying bacteria; the main reaction pool includes an oxygen elimination pool 10, a facultative anaerobic pool 11 and an anoxic pool 12 connected in sequence;

[0056] The main reaction tank is further provided with a bacterial liquid reflux pipe 13 , and the bacterial liquid reflux pipe 13 is connected to the inlet of the algae-bacteria liquid tank 6 .

[0057] The algae liquid pool 6 is further provided with an algae liquid return pipe 7, which is connected to the inlet of the microalgae growth pool 2. The depth of the microalgae growth pool 2 is set within 60 cm.

[0058] The outlet of the microalgae growth pool 2 is also connected to an algae liquid recovery pool 15 .

[0059] The plant 9 is at least one of an emergent plant, a submerged plant and a floating plant.

[0060] The microalgae growth pool 2 is provided with a monitoring, sensing and lighting subsystem; the monitoring, sensing and lighting subsystem includes an adjustment unit 4, a central control device 5 and a sensor monitoring unit 3;

[0061] The sensing and monitoring unit 3 includes a light intensity sensor, an absorbance value sensor, a temperature sensor and a pH sensor; the regulating unit 4 includes a light regulating device, a temperature regulating device and a pH regulating device;

[0062] The light intensity sensor is used to monitor the light intensity of the microalgae growth pond and feed back the light intensity data to the central control device 5; the absorbance value sensor is used to monitor the absorbance value of the algae and feed back the absorbance value data to the central control device 5; the temperature sensor is used to monitor the water temperature in the microalgae growth pond and feed back the temperature data to the central control device 5; the pH sensor is used to monitor the pH of the water in the microalgae growth pond and feed back the pH data to the central control device 5;

[0063] The central control device 5 is used to adjust the light intensity, water temperature and water pH in the microalgae growth pond 2 through a light adjustment device, a temperature adjustment device and a pH adjustment device.

[0064] This embodiment also provides a method for denitrifying and purifying water using plants, bacteria, and algae. The method uses the above-mentioned system and includes the following steps:

[0065] S1: sending low-carbon-nitrogen ratio wastewater and Chlorella vulgaris into the microalgae growth pool 2, allowing the algae to grow, reproduce, and aggregate in the microalgae growth pool 2 using the nitrogen and phosphorus in the low-carbon-nitrogen ratio wastewater, and then subjecting the algae to natural sedimentation separation (performed in the dark) to obtain a supernatant algae separation liquid and aggregated algae bodies settled at the bottom of the microalgae growth pool; at the same time, allowing the algae to fix carbon dioxide in the air through photosynthesis and release extracellular organic matter into the supernatant algae separation liquid;

[0066] In this embodiment, when according to Figure 5 When the chlorophyll a concentration in the microalgae growth pond 2 is calculated to be 3000-5000 ug / L and the algae particle size after growth, reproduction and aggregation is as shown in Figure 1, the natural sedimentation separation time is set to 10-12 hours in this embodiment. After the natural sedimentation separation is completed, the supernatant algae separation liquid and the aggregated algae settled at the bottom of the microalgae growth pond are obtained.

[0067] The light intensity for performing the photosynthesis is determined according to the absorbance value monitored by the absorbance value sensor; when the absorbance value monitored by the absorbance value sensor is lower than 680nm set by the central control device 5, the central control device 5 adjusts the light intensity for performing the photosynthesis through the light adjustment device;

[0068] S2: The bacterial liquid from the main reaction tank and part of the supernatant algae separation liquid from the microalgae growth tank are sent together into the algae-bacteria liquid pool 6 to obtain a bacterial-algae mixed liquid; the bacterial-algae mixed liquid is sent into the influent plant pool 8 and the main reaction tank in sequence, and the extracellular secreted organic matter in the bacterial-algae mixed liquid is used as a carbon source for denitrification. Under the action of the denitrifying bacteria in the influent plant pool 8 and the main reaction tank, the nitrogen in the low carbon-nitrogen ratio sewage is removed by coupling plants, bacteria and algae to obtain a purified water body. Figure 4 It can be seen that when the bacteria and algae content in the bacteria and algae mixed solution is 0.3 g / L, the nitrogen removal rate in the system reaches more than 90%;

[0069] In step S2, part of the bacteria-algae mixed solution in the algae-bacteria liquid pool 6 is returned to the microalgae growth pool 2, thereby ensuring that the concentration of chlorophyll a in the bacteria-algae mixed solution is 10-50 ug / L;

[0070] The part of the supernatant algae separation liquid in the microalgae growth pool 2 is the supernatant algae separation liquid from the uppermost surface layer of the supernatant algae separation liquid in the microalgae growth pool 2 to the lower 10-20 cm portion.

[0071] S3: Sending the purified water out of the system through the outlet pool 14.

[0072] While various embodiments of the present invention have been described above, the above description is intended to be illustrative, not exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for denitrification and purification of water bodies by coupling plants, bacteria and algae, characterized in that: The system used in the method includes an inlet pipe, a microalgae growth pool, an algae liquid pool, an inlet plant pool, a main reaction pool and an outlet pool connected in sequence; The inlet plant pool, main reaction pool and outlet pool are each independently provided with plants and microorganisms with denitrifying bacteria; the main reaction pool includes an oxygen elimination pool, a facultative anaerobic pool and an anoxic pool connected in sequence; The main reaction tank is further provided with a bacterial liquid reflux pipe, which is connected to the inlet of the algae-bacteria liquid tank; The algae liquid pool is further provided with an algae liquid reflux pipe, which is connected to the inlet of the microalgae growth pool; The microalgae growth pond is provided with a monitoring, sensing and lighting subsystem; the monitoring, sensing and lighting subsystem includes an adjustment unit, a central control device and a sensor monitoring unit; The sensing monitoring unit includes a light intensity sensor, an absorbance value sensor, a temperature sensor and a pH sensor; the regulating unit includes a light regulating device, a temperature regulating device and a pH regulating device; The light intensity sensor is used to monitor the light intensity of the microalgae growth pond and feed back the light intensity data to the central control device; the absorbance value sensor is used to monitor the absorbance value of the algae and feed back the absorbance value data to the central control device; the temperature sensor is used to monitor the water temperature in the microalgae growth pond and feed back the temperature data to the central control device; the pH sensor is used to monitor the pH of the water in the microalgae growth pond and feed back the pH data to the central control device; The central control device is used to adjust the light intensity, water temperature and water pH in the microalgae growth pond through a light adjustment device, a temperature adjustment device and a pH adjustment device; The method comprises the following steps: S1: sending low-carbon-nitrogen ratio wastewater and algae into the microalgae growth pond, allowing the algae to grow, multiply, and aggregate in the microalgae growth pond using nitrogen and phosphorus in the low-carbon-nitrogen ratio wastewater, and then separating through natural sedimentation to obtain a supernatant algae separation liquid and aggregated algae settled at the bottom of the microalgae growth pond; at the same time, the algae fix carbon dioxide in the air through photosynthesis and release extracellular secreted organic matter into the supernatant algae separation liquid; The time of natural sedimentation separation is set according to the concentration of chlorophyll a in the microalgae growth pool and the particle size of the algae after growth, reproduction and aggregation; S2: feeding the bacterial liquid from the main reaction tank and part of the supernatant algae separation liquid from the microalgae growth tank into the algae-bacteria liquid tank to obtain a bacterial-algae mixed liquid; feeding the bacterial-algae mixed liquid into the influent plant tank and the main reaction tank in sequence, using the extracellular secreted organic matter in the bacterial-algae mixed liquid as a carbon source for denitrification, and under the action of the denitrifying bacteria in the influent plant tank and the main reaction tank, the nitrogen in the low carbon-nitrogen ratio wastewater is removed through the coupling of plants, bacteria and algae to obtain purified water; The method further includes returning a portion of the bacteria-algae mixed solution in the algae-bacteria liquid pool to the microalgae growth pool, thereby ensuring that the concentration of chlorophyll a in the bacteria-algae mixed solution is 10-100 ug / L; S3: sending the purified water out of the system through the outlet tank; The concentration of chlorophyll a is calculated based on the linear equation of chlorophyll a and the absorbance at 680 nm; The linear equation is y=6411x+27.71, wherein x is the absorbance value at 680nm, which is monitored by the absorbance value sensor; y is the concentration of chlorophyll a; the correlation coefficient R of the linear equation is 2 =0.

992.

2. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 1, wherein: The depth of the microalgae growth pool is set within 60 cm; The plant is at least one of an emergent plant, a submerged plant and a floating plant; The outlet of the microalgae growth pool is also connected to an algae liquid recovery pool.

3. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 1, wherein: The algae is at least one of Chlorella, Chlamydomonas, Scenedesmus, Haematococcus pluvialis and Chrysophyte.

4. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 1, wherein: When the chlorophyll a concentration in the microalgae growth pool is 3000-8000 ug / L and the algae particle size after growth, reproduction and aggregation is 100-200 μm, the natural sedimentation separation time is set to 9-13 hours.

5. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 1, wherein: The ratio of the bacteria-algae mixed liquid in the algae-bacteria liquid pool to the reflux liquid returned to the microalgae growth pool is (1-3):

1.

6. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 1, wherein: The light intensity for performing the photosynthesis is determined according to the absorbance value monitored by the absorbance sensor.

7. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 6, wherein: When the absorbance value monitored by the absorbance sensor is lower than the absorbance threshold value set by the central control device, the central control device adjusts the light intensity for the photosynthesis through the light adjustment device.

8. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 6, wherein: The light intensity for performing the photosynthesis is 1000 lx or more.

9. The method for denitrification and purification of water bodies by coupling denitrification with plants, bacteria and algae according to claim 1, wherein: The part of the supernatant algae separation liquid in the microalgae growth pond is the supernatant algae separation liquid from the uppermost surface layer of the supernatant algae separation liquid in the microalgae growth pond to the lower 10-20 cm portion.

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