A method for reducing greenhouse gas emissions from constructed wetland systems

Abstract

This invention discloses a method for reducing greenhouse gas emissions from an constructed wetland system. The constructed wetland system includes: a vegetated area, a substrate filler layer located below the vegetated area, and a permeable baffle disposed below the substrate filler layer, and a liquid-sealed area located below the permeable baffle. The filler in the substrate filler layer is inoculated with intertidal sediments from coastal mangroves. By setting up the bottom liquid-sealed layer, the dissolved oxygen (DO) and pH conditions in the constructed wetland environment are improved, thereby reducing the emissions of greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a method for reducing greenhouse gas emissions from constructed wetland systems. Background Technology

[0002] Constructed wetlands are engineered systems that remove pollutants through physical, chemical, and biological (using microorganisms) processes. Compared to traditional wastewater treatment, constructed wetlands have lower greenhouse gas emissions and provide more ecosystem services, such as carbon sequestration, biodiversity conservation, and aesthetics. While constructed wetlands offer significant ecological benefits, as a wastewater treatment process, they inevitably involve the biotransformation of carbon and nitrogen, unavoidably releasing greenhouse gases such as CO2, CH4, and N2O. The greenhouse gas emissions per unit area of ​​constructed wetlands exceed those of natural wetlands by 2 to 10 times. However, greenhouse gas emissions are subject to many uncertainties, such as plant species, operational strategies, groundwater levels, temperature, pH, redox conditions, and operating parameters (water depth, hydraulic retention time, load, etc.). These factors all influence the removal of pollutants and the emission of greenhouse gases, placing constructed wetlands at the crossroads of greenhouse gas sources and sinks.

[0003] Therefore, how to reduce greenhouse gas emissions from constructed wetlands is an urgent problem to be solved. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for reducing greenhouse gas emissions from constructed wetland systems, in order to solve the problem of increased greenhouse gas emissions caused by high nitrogen and high salt pollution in existing constructed wetlands.

[0005] The technical solution provided by this invention is as follows:

[0006] A method for reducing greenhouse gas emissions from an constructed wetland system, wherein a liquid seal zone is provided at the outlet of the constructed wetland system.

[0007] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0008] As a preferred technical solution, the method for reducing greenhouse gas emissions from an artificial wetland system further includes: a vegetation zone, a substrate filler layer located below the vegetation zone, and a permeable baffle disposed below the substrate filler layer, wherein the liquid-sealed zone is located below the permeable baffle; the filler in the substrate filler layer is inoculated with intertidal sediments from coastal mangroves.

[0009] As a preferred technical solution, the method for reducing greenhouse gas emissions from constructed wetland systems, wherein, along the water flow direction of the constructed wetland system, the substrate filler layer sequentially comprises an alkaline material layer providing alkalinity, a sea sand layer, and a ceramsite layer.

[0010] As a preferred technical solution, in the method for reducing greenhouse gas emissions from constructed wetland systems, the volume ratio of the matrix filler material contained in the matrix filler layer to the intertidal sediments of coastal mangroves is 3-4:1.

[0011] As a preferred technical solution, the method for reducing greenhouse gas emissions from constructed wetland systems includes a sampling port in the liquid-sealed area for controlling the liquid level height within the liquid-sealed area.

[0012] As a preferred technical solution, in the method for reducing greenhouse gas emissions from constructed wetland systems, the particle size of the sea sand in the sea sand layer is 1-2 mm, and the particle size of the ceramsite in the ceramsite layer is 10-30 mm.

[0013] As a preferred technical solution, in the method for reducing greenhouse gas emissions from constructed wetland systems, the thickness of the alkaline material layer is 1-5 cm, the thickness of the sea sand layer is 10-30 cm, and the thickness of the ceramsite layer is 3-10 cm.

[0014] As a preferred technical solution, in the method for reducing greenhouse gas emissions from constructed wetland systems, the alkaline material in the alkaline material layer is selected from limestone, fly ash, volcanic rock, gravel, and oyster shells, etc.

[0015] As a preferred technical solution, in the method for reducing greenhouse gas emissions from constructed wetland systems, the planted plants in the vegetation area are Kandelia candel.

[0016] Beneficial effects: This invention provides a novel method for constructing artificial wetlands to reduce greenhouse gas emissions. By setting a bottom liquid seal layer, the dissolved oxygen (DO) and pH conditions in the artificial wetland environment are improved, thereby reducing the emissions of greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the mangrove artificial wetland pilot system of Embodiment 1 of the present invention;

[0018] Figure 2 The pH content of the influent and effluent of the mangrove artificial wetland pilot system in Embodiment 1 of the present invention;

[0019] Figure 3The DO content of the influent and effluent of the mangrove artificial wetland pilot system in Embodiment 1 of the present invention;

[0020] Figure 4 This describes the CO2 emission status of the mangrove artificial wetland pilot system under different nitrogen inputs in Example 1 of the present invention.

[0021] Figure 5 This describes the CH4 emission status of the mangrove artificial wetland pilot system under different nitrogen inputs in Example 1 of the present invention.

[0022] Figure 6 This describes the N2O emission status of the mangrove artificial wetland pilot system under different nitrogen inputs in Example 1 of this invention. Detailed Implementation

[0023] This invention provides a novel method for constructing artificial wetlands to reduce greenhouse gas emissions. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0024] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. It should be understood that the term “and / or” as used herein includes all or any units and all combinations of one or more associated listed items.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The constructed wetland referred to herein is a mangrove constructed wetland.

[0026] The inventors' research revealed that the continuous development of coastal areas and urban development has led to the continuous deterioration of the nearshore ecological environment. Among these issues, the large amount of nitrogen-containing pollutants carried by rivers flowing into the sea and mariculture is one of the main sources of inorganic nitrogen in coastal areas. The increase in nitrogen input to coastal areas will lead to an increase in the nitrogen load of natural and artificial wetlands in mangroves. Although an increase in nitrogen input within a certain range will promote the efficiency of nitrogen absorption and utilization by wetland plants, thereby enhancing the carbon sequestration capacity of the ecosystem, an excessively high nitrogen load will cause the plant roots to gradually shrink, resulting in wetland degradation, thereby reducing the carbon sequestration capacity of wetlands and promoting the emission of greenhouse gases (CO2, CH4 and N2O). Meanwhile, constructed wetlands, whose main ecological functions include water purification of polluted water bodies and the transformation of carbon and nitrogen elements in the environment, will lead to a significant increase in greenhouse gas emissions from constructed wetlands as the organic and nitrogen loads in wastewater increase. Therefore, by constructing a new type of constructed wetland to reduce greenhouse gas emissions, it is possible to achieve efficient purification of pollution and reduction of greenhouse gas emissions at the same time. This provides a theoretical basis and scientific guidance for coastal cities to formulate carbon neutrality policies and corresponding coastal environmental pollution control.

[0027] To address the above problems, this invention discloses a novel method for constructing artificial wetlands to reduce greenhouse gas emissions. This method can effectively improve the pH and dissolved oxygen environment in the wetland, thereby inhibiting greenhouse gas emissions, enhancing the absorption capacity of artificial wetlands for greenhouse gases, and effectively reducing global warming potential. It also enables artificial wetlands to achieve efficient pollution purification and greenhouse gas emission reduction capabilities, and mitigate the greenhouse effect.

[0028] Specifically, an artificial wetland with dimensions of 20cm × 20cm × 65cm was constructed. A 1.5cm thick permeable baffle was installed 45cm from the top of the device to divide the artificial wetland system, with the upper part being the substrate filling space and the lower part being the liquid seal zone. A sampling port, which also serves as the wetland's drainage outlet, was installed at the bottom 3.5cm of the artificial wetland system to control the liquid level of the lower liquid seal layer. A PVB pipe was installed at the top of the device, connected to a peristaltic pump to inject artificial wastewater into the system. Two outlets were installed on the inner side of the pipe to ensure that each small unit receives sufficient and uniform water intake.

[0029] The nitrification process in wetlands consumes a certain amount of alkalinity, and insufficient alkalinity will limit the completion of the nitrification reaction. Therefore, in this simulated wetland, the top 5cm layer was filled with limestone with a particle size of 1cm to provide sufficient alkalinity; the middle 30cm layer was filled with sea sand with a particle size of 1mm; and the bottom 5cm layer was filled with expanded clay pellets with a particle size of 10-30mm to improve the dissolved oxygen environment in the wetland. Before filling, each filler was mixed with intertidal sediments of coastal mangroves (collected from Shenzhen Xiwang Mangrove Park) at a volume ratio of 3.3:1 for microbial inoculation.

[0030] A sampling port was set at the bottom 3.5cm of the simulated wetland system, and one 55-65cm tall Kandelia obovata seedling was planted in each small unit. According to the trend of the change of the porosity of the filler along the direction of water flow from small to large, the denitrification capacity of the mangrove simulated wetland can be improved, and finally a vertical downflow mangrove artificial wetland is constructed.

[0031] The experiment simulates the current nitrogen pollution situation in the nearshore waters of Shenzhen and sets up three nitrogen input levels: low nitrogen (LN-CW), medium nitrogen (MN-CW), and high nitrogen (HN-CW), with two replicates for each group. Referring to the inorganic nitrogen concentration in the seawater of the Futian Mangrove Nature Reserve and the water quality of the Shenzhen nearshore area, a certain amount of artificial sea salt, ammonium chloride, sodium nitrate, and trisodium citrate were added to the storage tank, and then dissolved in tap water to form the three groups of influent with different inorganic nitrogen and organic carbon concentrations shown in Table 1. Simulating natural semi-diurnal tidal flow, water was introduced at 2:00 and 14:00 daily, with each inundation lasting 6 hours; water was discharged at 8:00 and 20:00 daily, with each discharge lasting 6 hours. Tables 1 and 2 show the physicochemical indicators of the influent and the operating parameters of the device, respectively.

[0032] Table 1 Physicochemical indicators of water quality entering the simulated mangrove wetland

[0033]

[0034] Table 2. Operational parameters of the mangrove simulated wetland

[0035]

[0036] The testing and analysis methods used a portable water quality analyzer (Orion Star A, Thermo Fisher Scientific, USA) to determine DO and pH. To accurately obtain the greenhouse gas fluxes of the mangrove simulated wetland, CO2, CH4, and N2O fluxes were measured using the closed static chamber method. Before measurement, a transparent cylindrical chamber (20×20cm inner diameter, 50cm height and 50×81cm inner diameter, 50cm height) was placed over the small-scale experimental setup, covering the upper surface of the setup and the mangrove plants to form a sealed space. A polytetrafluoroethylene (PTFE) sampling tube was connected to the top of the chamber to collect gas samples. A TD600-SH-B-M3 portable greenhouse gas analyzer (Beijing Tiandi Shouhe Technology Development Co., Ltd.) was used for continuous measurement for 48 hours, recording greenhouse gas data every half hour to explore the hourly and diurnal variations of greenhouse gas emissions.

[0037] Results and Analysis: Changes in pH values ​​in the influent and effluent of a simulated wetland under different nitrogen inputs. Figure 1It can be seen that the pH changes from influent to effluent in the LN-CW, MN-CW and HN-CW groups were 8.46±0.15 to 7.18±0.08, 8.53±0.17 to 7.28±0.21 and 8.70±0.07 to 7.27±0.10, respectively, indicating that the substrate environment of the three simulated wetlands was in a neutral to slightly alkaline state. Figure 2 The figure shows the changes in DO concentration in the influent and effluent of the simulated wetland under different nitrogen inputs. As shown in the figure, the changes in DO concentration from influent to effluent in the LN-CW, MN-CW, and HN-CW groups were 5.27±0.75 to 2.36±0.32 mg / L, 5.6±0.89 to 1.62±0.82 mg / L, and 5.78±0.44 to 2.06±0.62 mg / L, respectively. This indicates that the substrate of the three simulated wetlands was an aerobic environment, which fundamentally improved the dissolved oxygen environment of the bottom layer of the constructed wetland, making the entire constructed wetland an aerobic environment.

[0038] Depend on Figure 3 It can be seen that the CO2 concentration in the three simulated wetlands was significantly higher at night than during the day, reaching its maximum between 6 and 7 a.m. each day, then rapidly decreasing to 0 ppm, and gradually increasing again after 5 p.m. each day. The average emission concentration of the three simulated wetlands over 48 hours was: LN-CW group (437 ppm) > MN-CW group (404 ppm) > HN-CW group (330 ppm). Only the average CO2 emission concentration of the LN-CW group on sunny days was higher than the domestic atmospheric background value of 419.3 ppm in 2022, indicating that mangrove artificial wetlands can still maintain a strong CO2 absorption capacity under different nitrogen input backgrounds.

[0039] Depend on Figure 4 As shown, unlike CO2, CH4 exhibits the opposite diurnal variation trend. The daytime CH4 concentration in the three simulated wetland groups is significantly higher than that at night. Only during the daytime when temperatures are higher is there a small amount of emission, with the concentration remaining at 0 ppm at other times. The 48-hour average emission concentrations for the three simulated wetland groups are: LN-CW group (0.09 ppm) > MN-CW group (0 ppm) = HN-CW group (0 ppm). The average CH4 emission concentrations of the three simulated wetland groups are all lower than the 2022 domestic atmospheric background value of 1.979 ppm, indicating that mangrove simulated wetlands can achieve low CH4 emissions under different nitrogen load gradients.

[0040] The changes in nitrous oxide (N2O) emission concentration in simulated wetlands under different nitrogen inputs are as follows: Figure 5As shown, the diurnal variation of N2O exhibits a similar trend to that of CH4. The daytime N2O concentration in all three constructed wetlands is significantly higher than at night, with only low daytime emissions and zero ppb at other times. The 48-hour average emission concentrations of the three simulated wetlands are: LN-CW group (72.85 ppb) > HN-CW group (43.64 ppb) > MN-CW group (23.02 ppb). The average N2O emission concentrations of all three simulated wetlands are significantly lower than the 2022 atmospheric background value of 336.5 ppb in China. The average emission concentrations of different types of greenhouse gases in these three simulated wetlands are all lower than those in similar constructed wetlands and are generally lower than the atmospheric background values.

[0041] Subsequently, the corresponding greenhouse gas emission fluxes and global warming potential were calculated using formulas (Table 3). It can be seen that only the LN-CW group shows a positive CO2 emission flux, while the emission fluxes of the other groups are all negative. This indicates that under different nitrogen input levels, the greenhouse gas emissions from constructed wetlands are mostly greenhouse gas sinks, effectively reducing greenhouse gas emissions. According to the global warming potential values ​​shown in Table 3, the value gradually decreases with increasing nitrogen input levels, indicating that the novel constructed wetland construction method of this invention can maintain a low global warming potential even at higher nitrogen input levels, effectively suppressing the greenhouse effect.

[0042] Table 3 Greenhouse gas emission fluxes and global warming potential of mangrove artificial wetlands.

[0043]

[0044] In summary, this invention provides a novel method for constructing artificial wetlands to reduce greenhouse gas emissions. The method includes: constructing an artificial wetland and, by setting permeable baffles, a special combination of fillers, and a semi-diurnal tidal flow operation mode, simulating inorganic nitrogen pollution near the coast of Shenzhen, setting up artificial wetlands with different nitrogen input levels, and continuously monitoring the artificial wetlands for 48 hours. It was found that the artificial wetland significantly improved the pH and DO environment, making the entire artificial wetland an aerobic environment. At the same time, except for CO2 emissions in the LN-CW group, the emissions of CO2, CH4, and N2O at other nitrogen input levels were all far below the background values ​​of atmospheric greenhouse gases, proving that the novel artificial wetland is a greenhouse gas sink. Furthermore, by calculating the global warming potential, it was found that the global warming effect can be suppressed at different nitrogen input levels.

[0045] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for reducing greenhouse gas emissions from constructed wetland systems, characterized in that, A liquid seal area is provided at the outlet of the constructed wetland system; The constructed wetland system further includes: a vegetation zone, a substrate filler layer located below the vegetation zone, and a permeable baffle disposed below the substrate filler layer, with the liquid seal zone located below the permeable baffle; the filler in the substrate filler layer is inoculated with intertidal sediments from coastal mangroves; a pipe and a peristaltic pump connected to the pipe, the pipe being used to inject wastewater into the constructed wetland system; Along the direction of the water flow into the constructed wetland system, the substrate filler layer sequentially includes an alkaline material layer that provides alkalinity, a sea sand layer, and a ceramsite layer; The volume ratio of the matrix filler layer to the intertidal sediments of the coastal mangrove forest is 3-4:1; The alkaline material in the alkaline material layer is limestone.

2. The method for reducing greenhouse gas emissions from constructed wetland systems according to claim 1, characterized in that, The liquid-sealed area is equipped with a sampling port for controlling the liquid level height within the liquid-sealed area.

3. The method for reducing greenhouse gas emissions from constructed wetland systems according to claim 1, characterized in that, The sea sand in the sea sand layer has a particle size of 1-2 mm, and the ceramic particles in the ceramic particle layer have a particle size of 10-30 mm.

4. The method for reducing greenhouse gas emissions from constructed wetland systems according to claim 1, characterized in that, The thickness of the alkaline material layer is 1-5cm, the thickness of the sea sand layer is 10-30cm, and the thickness of the ceramsite layer is 3-10cm.

5. The method for reducing greenhouse gas emissions from constructed wetland systems according to claim 1, characterized in that, The vegetation in the area is planted with Kandelia candel.

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