A three-stage vertical subsurface flow constructed wetland system
Patent Information
- Application Number
- CN202611023032.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-07-10
- Publication Date
- 2026-09-15
Smart Images
Figure CN122748827A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of wastewater treatment technology, specifically to a three-stage series vertical subsurface flow constructed wetland system. Background Technology
[0002] Vertical subsurface flow constructed wetland systems are engineering facilities that utilize a composite system of substrates, aquatic plants, and microorganisms to treat wastewater. In the treatment of landfill leachate in karst areas, this system mainly removes organic pollutants, nitrogenous nutrients, and suspended particles from wastewater through the pores of the substrate layer, physical adsorption, and aerobic and anoxic metabolism of the biofilm attached to the surface.
[0003] Existing vertical subsurface flow constructed wetland systems generally employ a construction and operation strategy based on surface excavation and fixed hydraulic loads. During the construction phase, a wetland foundation pit is built by excavating the surface soil layer. An impermeable membrane is then laid inside the pit, and the substrate is filled. The design hydraulic retention time to meet the expected degradation requirements is then calculated. In actual operation, the influent equipment pumps wastewater into the substrate layer according to a pre-set fixed daily influent flow rate. The wastewater infiltrates downwards under gravity, undergoing a biochemical reaction with active microorganisms attached to the substrate surface within a set contact period.
[0004] Existing vertical subsurface flow constructed wetland systems suffer from technical defects such as seepage prevention failure and hydraulic retention time deviation from the design range under long-term continuous operation in karst regions. Karst topography features well-developed surface fissures and complex groundwater systems. Traditional open-pit construction is prone to leachate leakage into the ground due to surface subsidence or damage to the geomembrane. Simultaneously, physically trapped suspended solids in the wastewater and the continuously proliferating biofilm on the substrate surface continuously occupy the internal pores of the substrate, leading to a decrease in the effective pore volume of the substrate bed. Under the existing control system's operation mode of maintaining a fixed daily influent flow rate, the physical decay of pore volume directly causes a passive acceleration of the vertical infiltration velocity of wastewater within the substrate layer, resulting in a continuous shortening of the system's actual hydraulic retention time. This shortened retention time prematurely terminates the contact reaction process between the wastewater and the active biofilm, leading to hydraulic load imbalance and a decrease in pollutant removal rate in the later stages of system operation. Summary of the Invention
[0005] To address the shortcomings of existing technologies, this invention provides a three-stage cascaded vertical subsurface flow constructed wetland system, which solves the problems mentioned in the background art.
[0006] To achieve the above objectives, the present invention provides the following technical solution:
[0007] This invention provides a three-stage tandem vertical subsurface flow constructed wetland system, comprising: a first-stage wetland unit, a second-stage wetland unit, and a third-stage wetland unit, each equipped with an independent above-ground trough; connecting pipes, respectively installed between adjacent above-ground troughs, wherein the top inlet of the second-stage wetland unit is connected to the bottom outlet of the first-stage wetland unit via connecting pipes, and the top inlet of the third-stage wetland unit is connected to the bottom outlet of the second-stage wetland unit via connecting pipes; a bottom support substrate and a main treatment substrate, which are laid sequentially from bottom to top inside each above-ground trough, wherein the bottom support substrate is made of natural limestone gravel, and the main treatment substrate is made of aerated concrete fragments; and aquatic plants planted in the main treatment substrate.
[0008] The first, second, and third-level wetland units are arranged in a stepped manner along the direction of water flow. The bottom elevation of the first-level wetland unit is higher than that of the second-level wetland unit, and the bottom elevation of the second-level wetland unit is higher than that of the third-level wetland unit. The bottom of the above-ground tank is flat, and the sidewalls are connected to the bottom to form a closed container. This above-ground closed structure eliminates the need for excavation of a foundation pit on the karst surface, blocking the physical path of landfill leachate seeping into the groundwater system through karst fissures or sinkholes developed in the karst topography, thus preventing secondary pollution.
[0009] The ratio of the thickness of the bottom support substrate to the thickness of the main treatment substrate is 1:2 to 1:1. The average particle size of the natural limestone crushed stone is 30mm to 50mm, and the average particle size of the aerated concrete fragments is 10mm to 20mm. The main treatment substrate and the bottom support substrate form a porosity gradient distribution. The aquatic plants are *Gnaphalium affine* or *Canna indica*. The root growth depth of the aquatic plants is less than or equal to the thickness of the main treatment substrate, and the roots of the aquatic plants are distributed within the main treatment substrate layer.
[0010] The three-stage tandem vertical subsurface flow constructed wetland system also includes an influent mixing device, a metering pump, a controller, and a level sensor. The inlet of the influent mixing device is connected to both the raw landfill leachate inlet pipe and the effluent return pipe, used to mix the raw leachate with the wetland effluent. The effluent return pipe is connected to the bottom outlet of the third-stage wetland unit. The influent mixing device contains a stirring mechanism, and both the raw landfill leachate inlet pipe and the effluent return pipe are equipped with flow regulating valves. The input of the metering pump is connected to the output of the influent mixing device, and its output is connected to the top inlet of the first-stage wetland unit. The level sensor is located within the first-stage wetland unit. The controller is electrically connected to the stirring mechanism, the flow regulating valves, the metering pump, and the level sensor.
[0011] This invention utilizes a controller to execute hydraulic load regulation and intermittent pumping logic. The controller calculates the daily influent volume of the original landfill leachate and the daily return volume of the wetland effluent based on the total daily mixed influent volume and dilution volume ratio, and adjusts the opening of the flow regulating valves on the original landfill leachate influent pipe and the effluent return pipe accordingly. The controller controls the daily pumping frequency and volume of the metering pump, setting the time interval between two adjacent pumping operations to be greater than or equal to the sum of the emptying time and the reoxygenation time, thus alternating the wetland unit levels between the infiltration and emptying phases.
[0012] This invention employs a controller to perform dynamic feedback compensation of the effective pore volume. The controller receives the liquid level drop rate from the liquid level sensor, calculates the void occupancy factor based on the ratio of the liquid level drop rate to the initial permeation rate, and calculates the effective porosity for the current operating stage by combining the initial effective porosity. When the liquid level drop rate decreases due to long-term system operation, the controller synchronously increases the value of the void occupancy factor and reduces the daily influent flow rate by adjusting the influent mixing device and the metering pump, thereby maintaining the total hydraulic retention time of the system constant within the range of 8 to 12 days.
[0013] This invention provides a three-stage cascaded vertical subsurface flow constructed wetland system. It offers the following advantages:
[0014] 1. This invention obtains the liquid level drop rate within the first-level wetland unit and calculates the porosity occupancy factor based on the initial infiltration rate, thereby dynamically adjusting the daily influent flow rate of the influent mixing device. This mechanism actively reduces the system's influent load when microplastics and biofilms occupy matrix pores, leading to a decrease in effective porosity, ensuring that the total hydraulic retention time of wastewater within the system remains constant within a preset range. Karst regions have complex groundwater conditions and fragile ecosystems, placing high demands on the effluent stability of leachate treatment systems. This dynamic feedback control overcomes the technical shortcomings of traditional constructed wetlands where matrix blockage shortens the contact reaction time, avoids the risks of short-flow and overflow pollution caused by the aging of traditional wetlands, and ensures the long-term operational stability of the system in karst regions and under complex operating conditions.
[0015] 2. This invention constructs a multidimensional microplastic trapping space through the synergistic effect of the main treatment matrix and the roots of aquatic plants. Aerated concrete fragments physically trap and electrostatically adsorb microplastics, while plant roots mechanically entangle fibrous microplastics. During system operation, the extension and expansion of plant roots and the shedding and renewal of surface biofilms expose unoccupied surfaces within the matrix, forming new physical adsorption spaces. This dynamic renewal mechanism of adsorption sites allows the system to maintain its ability to trap microplastics of various forms even when the pores are locally saturated.
[0016] 3. This invention reduces the initial load by adjusting the volume ratio of the original landfill leachate to the recycled effluent and controls the quantitative pumping device to perform intermittent water intake. The controller sets the time interval between adjacent pumping operations to be greater than the sum of the drainage and reoxygenation times, creating an alternating state of infiltration and drainage in the wetland unit. During the drainage phase, the liquid level drops, guiding external air into the matrix pores to complete natural reoxygenation. This control method constructs aerobic and anoxic environments within the system, enhancing the degradation of chemical oxygen demand and ammonia nitrogen by microorganisms, and achieving the removal of conventional pollutants by combining with the chemical reactions of the underlying limestone gravel. Attached Figure Description
[0017] Figure 1 This is a schematic diagram of the overall structure of the three-stage cascaded vertical subsurface flow constructed wetland system in this invention.
[0018] Figure 2 This is a graph showing the change in the overall microplastic removal rate during a continuous operating cycle in this invention.
[0019] Figure 3 This is a graph showing the variation of microplastic removal rate for different particle size ranges in this invention.
[0020] Figure 4 This is a graph showing the variation of microplastic removal rate for different spatial morphologies in this invention.
[0021] Figure 5 This is a comparison diagram of the distribution of microplastic removal characteristics for different polymer compositions in this invention.
[0022] The system comprises: 1. A three-stage tandem vertical subsurface flow constructed wetland system; 2. An influent mixing device; 3. A metering pump; 4. A first-stage wetland unit; 5. A second-stage wetland unit; 6. A third-stage wetland unit; 7. Connecting pipes; 8. An above-ground tank; 9. A bottom support substrate; 10. A main treatment substrate; 11. Aquatic plants; 12. A primary landfill leachate inlet pipe; 13. An effluent return pipe; 14. A final effluent pipe; 15. A controller; and 16. A liquid level sensor. Detailed Implementation
[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0024] Please see the appendix Figure 1 - Appendix Figure 5 This invention provides a three-stage cascaded vertical subsurface flow constructed wetland system 1, comprising:
[0025] Inlet mixing device 2, which is used to mix the original landfill leachate with the recovered wetland effluent;
[0026] The metering pump 3 has its input end connected to the output end of the influent mixing device 2. The metering pump 3 is used to transport the mixed wastewater.
[0027] The top inlet of the first-level wetland unit 4 is connected to the output end of the metering pumping device 3.
[0028] The top inlet of the second-level wetland unit 5 is connected to the bottom outlet of the first-level wetland unit 4 via a connecting pipe 7.
[0029] The top inlet of the third-level wetland unit 6 is connected to the bottom outlet of the second-level wetland unit 5 via a connecting pipe 7.
[0030] The first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6 are all located above ground and each is equipped with an independent above-ground tank 8. The above-ground tank 8 is made of corrosion-resistant material and has a sealed bottom to prevent wastewater from seeping into the ground. Each wetland unit is also connected to a distribution pipe at its top inlet to evenly distribute wastewater to the wetland surface.
[0031] The inlet of the influent mixing device 2 is connected to the original landfill leachate inlet pipe 12 and the outlet return pipe 13. The original landfill leachate inlet pipe 12 is used to introduce landfill leachate. The outlet return pipe 13 is connected to the bottom outlet of the third-stage wetland unit 6 and is used to introduce wetland effluent. The influent mixing device 2 is equipped with mechanical stirring blades to mix the landfill leachate and wetland effluent at a volume ratio of 1:4 to form mixed influent.
[0032] The metering pumping device 3 consists of a metering pump and control piping. It intermittently pumps the mixed influent to the top of the first-stage wetland unit 4. The device performs two pumping operations daily, with an interval of at least 8 hours between each operation, and a total daily water distribution of 10L. This intermittent pumping creates alternating wettability and drainage of the substrate layer within the wetland unit, providing reoxygenation conditions for the substrate.
[0033] Inside the above-ground tanks 8 of the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6, a bottom support substrate 9 and a main treatment substrate 10 are laid from bottom to top. The total filling depth of the bottom support substrate 9 and the main treatment substrate 10 is 30 cm. The bottom support substrate 9 is natural limestone gravel. The main treatment substrate 10 is aerated concrete blocks. Aquatic plants 11, namely windmill grass or canna lily, are planted in the main treatment substrate 10 layer.
[0034] The mixed influent enters from the top inlet of the first-stage wetland unit 4, is distributed through the distribution pipes, and then seeps vertically downwards under gravity, passing sequentially through the 10 layers of main treatment substrate and the 9 layers of bottom support substrate. The effluent from the first-stage wetland unit 4 is transported to the top of the second-stage wetland unit 5 via the bottom connecting pipe 7.
[0035] Wastewater continues to infiltrate vertically downwards within the second-stage wetland unit 5, and is then transported to the top of the third-stage wetland unit 6 via the connecting pipe 7 at the bottom of the second-stage wetland unit 5. The wastewater is treated within the third-stage wetland unit 6. The total hydraulic retention time of the wastewater in the three-stage series vertical subsurface flow constructed wetland system 1 is 8 to 12 days.
[0036] The bottom outlet of the third-level wetland unit 6 is connected to the final outlet pipe 14 and the outlet return pipe 13. The final outlet pipe 14 is used to discharge the treated effluent. The outlet return pipe 13 is used to guide part of the effluent to the inlet mixing device 2 to participate in the mixing and distribution.
[0037] The above-ground tanks 8 of the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6 are all located on the ground surface. The bottom of each above-ground tank 8 is flat, with the sidewalls connected to the bottom to form a closed container. The above-ground tank 8 is made of impermeable material, isolating its internal space from the external environment and preventing wastewater from seeping underground through karst surface fissures or karst conduits. This above-ground arrangement eliminates the need for excavation of the karst surface, avoiding the risks of foundation pit settlement and geomembrane damage under karst geological conditions.
[0038] The first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6 are arranged in a stepped manner along the direction of water flow. The bottom elevation of the first-level wetland unit 4 is higher than that of the second-level wetland unit 5, and the bottom elevation of the second-level wetland unit 5 is higher than that of the third-level wetland unit 6.
[0039] Connecting pipes 7 are installed between adjacent above-ground tanks 8. One end of each connecting pipe 7 is connected to the bottom outlet of the upper-level above-ground tank 8, and the other end is connected to the top inlet of the lower-level above-ground tank 8. To meet the hydrostatic pressure requirements for wastewater flow, the design operating water level of the upper-level above-ground tank 8 is higher than the top inlet elevation of the lower-level above-ground tank 8. Wastewater flows from the first-level wetland unit 4 into the second-level wetland unit 5 under gravity, and then into the third-level wetland unit 6. Wastewater transfer between wetland units is gravity-driven, and no power lifting equipment is installed at the nodes where connecting pipes 7 are located.
[0040] The above-ground tanks 8 of the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6 are all laid with a bottom-support substrate 9 and a main treatment substrate 10 from bottom to top, which together form a composite substrate bed. The total filling depth of the bottom-support substrate 9 and the main treatment substrate 10 is 30 cm. The ratio of the thickness of the bottom-support substrate 9 to the thickness of the main treatment substrate 10 is 1:2 to 1:1.
[0041] The bottom support matrix 9 uses crushed natural limestone, widely distributed in karst regions. After crushing and screening, the crushed natural limestone is laid at the bottom of the above-ground tank 8, forming a support layer using locally sourced materials. It continuously releases calcium ions into the wastewater, which react with phosphate ions in the wastewater through chemical precipitation, thus removing total phosphorus. The average particle size of the bottom support matrix 9 is larger than that of the main treatment matrix 10, ranging from 30mm to 50mm. The bottom support matrix 9 forms a hydraulic conduction channel at the bottom of the above-ground tank 8, preventing bottom clogging.
[0042] The natural limestone crushed stone contains calcium carbonate. As wastewater permeates downwards through the bottom support matrix 9, the natural limestone crushed stone releases calcium and magnesium ions into the wastewater. These calcium and magnesium ions react with phosphate ions in the wastewater to form phosphate precipitates. These phosphate precipitates adhere to the surface of the bottom support matrix 9, achieving total phosphorus removal.
[0043] The main treatment matrix 10, composed of aerated concrete fragments, is laid on top of the underlying support matrix 9. The particle size range of the main treatment matrix 10 is 10 mm to 20 mm. The aerated concrete fragments have a microporous and mesoporous structure, which physically traps microplastic particles in the wastewater to be treated. The main treatment matrix 10 and the underlying support matrix 9 are combined from top to bottom to form a pore gradient filtration structure that conforms to the increasing particle size law.
[0044] The surface of the main treatment matrix 10 is covered with silanol and aluminumol functional groups. These functional groups electrostatically adsorb and hydrogen-bond with the microplastic particles in the wastewater, fixing the particles to the surface of the main treatment matrix 10. The main treatment matrix 10 layer also provides root growth space for aquatic plants 11.
[0045] Aquatic plants 11 are planted in the main treatment substrate 10 of the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6. The aquatic plants 11 are either *Gnaphalium affine* or *Canna indica*. The root system of the aquatic plants 11 grows to a depth less than or equal to the thickness of the main treatment substrate 10. The roots of the aquatic plants 11 are concentrated within the main treatment substrate 10 and do not extend into the bottom supporting substrate 9, preventing the plant roots from clogging the pores of the bottom supporting substrate 9.
[0046] The roots of aquatic plant 11 grow and intertwine within the pores of the main treatment substrate 10, forming a network-like interception structure. As wastewater permeates downwards through the main treatment substrate 10, the roots mechanically entangle and trap fibrous and thin-film microplastics in the wastewater. The mechanical trapping effect of the aquatic plant 11 roots, combined with the trapping effect of the pores in the main treatment substrate 10, achieves physical interception of microplastic particles.
[0047] Aquatic plants 11 release oxygen into the main treatment substrate 10 through root oxygen secretion. An aerobic environment is formed on the surface of the roots of aquatic plants 11 and in the adjacent area, while an anaerobic environment is formed in the area away from the roots. This results in an alternating distribution of aerobic and anaerobic zones within the main treatment substrate 10. This environmental condition provides attachment conditions for microorganisms, which form biofilms on the main treatment substrate 10 and the root surface. Extracellular polymers secreted by the biofilm adsorb microplastics and remove pollutants such as chemical oxygen demand, total nitrogen, and ammonia nitrogen from the wastewater through biodegradation.
[0048] As the three-stage tandem vertical subsurface flow constructed wetland system 1 operates, the physical adsorption sites on the surface of the main treatment substrate 10 are occupied by microplastics. The growth and expansion of the roots of aquatic plants 11 disturb the surrounding main treatment substrate 10, altering the local pore structure. Combined with the shedding and renewal of the microbial biofilm, the unoccupied surfaces inside the main treatment substrate 10 are exposed, forming new physical adsorption sites. This physical and biological renewal process enables the dynamic recovery of the microplastic retention rate of the three-stage tandem vertical subsurface flow constructed wetland system 1.
[0049] The inlet of the water mixing device 2 is connected to the original landfill leachate inlet pipe 12 and the outlet return pipe 13, respectively. Flow regulating valves are installed on both the original landfill leachate inlet pipe 12 and the outlet return pipe 13. This three-stage series vertical subsurface flow constructed wetland system 1 also includes a controller 15, which is electrically connected to the stirring mechanism in the water mixing device 2 and the flow regulating valves on the pipelines.
[0050] The controller 15 has a built-in inlet water mixing algorithm, which first determines the water distribution volume based on the volume balance equation:
[0051]
[0052] in, The total daily volume of mixed influent. This represents the daily influent volume of the original landfill leachate. This represents the daily return volume of water flowing out of the wetland.
[0053] Controller 15 adjusts the dilution volume ratio according to the preset value. Establish flow control relationships:
[0054]
[0055] Controller 15 calculates the inlet water regulation command value by simultaneously solving the above equations:
[0056]
[0057] Controller 15 according to The calculation results are used to adjust the opening of the flow regulating valve on the original landfill leachate inlet pipe 12, and according to... The calculation results are used to adjust the opening of the flow regulating valve on the outlet return pipe 13.
[0058] In this embodiment, The value is 4. When the daily total volume is set... When the capacity is 10L, controller 15 controls the daily inflow of raw landfill leachate through the inlet to 2L and the daily inflow through the effluent return outlet to 8L. The raw landfill leachate and wetland effluent are mechanically mixed in the influent mixing device 2 to homogenize the wastewater components and form a mixed influent.
[0059] Initial concentration of pollutants in the influent The law of conservation of mass is satisfied. Controller 15 estimates the mixed pollutant load using the following formula:
[0060]
[0061] in, The concentration of pollutants in the original landfill leachate. This represents the concentration of pollutants in the returned wetland effluent. Pollutants include chemical oxygen demand (COD), total nitrogen, total phosphorus, ammonia nitrogen, and microplastics.
[0062] The influent mixing device 2 dynamically adjusts the flow rate using the aforementioned algorithm to reduce the concentration of the original landfill leachate. The initial concentration of the mixed influent... The degradation load of biofilm microorganisms within the 10 layers of the main treatment substrate is controlled to prevent high concentrations of organic matter from impacting the first-level wetland unit 4 and its subsequent units.
[0063] A metering pumping device 3 is installed between the influent mixing device 2 and the first-stage wetland unit 4, and is electrically connected to the controller 15. The total daily pumping volume of the metering pumping device 3 satisfies the following equation:
[0064]
[0065] in, The total daily volume of mixed influent; The number of pumping operations performed per day; This refers to the volume of a single pumping operation. In this embodiment, , .
[0066] The time interval between two consecutive pumping operations is Controller 15 is configured... This allows the substrate layer to complete the alternation of dry and wet conditions. The value of satisfies the following constraints:
[0067]
[0068] in, For emptying time, This refers to the reoxygenation time. The emptying time... Determined by the following formula:
[0069]
[0070] in, The filling depth of the main treatment matrix 10, For effective porosity, The vertical permeability coefficient of the primary treatment substrate 10.
[0071] When the metering pump 3 is started, the mixed influent fills the pores of the main treatment substrate 10, and the three-stage series vertical subsurface flow constructed wetland system 1 is in the immersion stage. At this time, the pollutants in the wastewater come into contact with the biofilm on the surface of the main treatment substrate 10.
[0072] When the metering pump 3 stops operating, the liquid level in the first-stage wetland unit 4 drops due to gravity. As the liquid level drops, negative pressure is generated inside the main treatment substrate 10, allowing external air to enter the substrate pores, and the three-stage series-connected vertical subsurface flow constructed wetland system 1 enters the emptying phase. The emptying phase lasts for [duration missing]. ,satisfy:
[0073]
[0074] in, The duration of a single pumping action. Set via controller 15. ,make sure Greater than or equal to reoxygenation time .
[0075] The intermittent pumping strategy keeps the substrate layer inside the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6 in a periodic wet-dry cycle. During the drainage phase, oxygen entering the pores diffuses into the biofilm, maintaining the degradation of organic matter and ammonia nitrogen by aerobic microorganisms.
[0076] Controller 15 adjusts according to ambient temperature Adjustments are made. When the ambient temperature rises, causing an increase in the rate of oxygen consumption by microorganisms, the controller 15 decreases... Or increase the number of pumping operations per day This is to increase the reoxygenation frequency and keep the dissolved oxygen content inside the matrix within a preset concentration range.
[0077] Total hydraulic retention time of the three-stage cascaded vertical subsurface flow constructed wetland system 1 The total effective pore volume is determined by the matrix layer within the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6. The above-ground trough structure 8 of each wetland unit has consistent structural parameters. The total effective pore volume of the three-stage tandem vertical subsurface flow constructed wetland system 1 is... The calculation formula is:
[0078]
[0079] in, The internal cross-sectional area of a single above-ground tank 8; This is the total filling depth of the bottom support matrix 9 and the main treatment matrix 10; The three-stage cascaded vertical subsurface flow constructed wetland system 1 has been in operation for [time period missing]. Effective porosity at that time.
[0080] In the operation of a three-stage tandem vertical subsurface flow constructed wetland system 1, the growth of biofilm on the surface of the main treatment substrate 10 and the accumulation of trapped microplastics will occupy the voids between the substrates. Effective porosity The following calculation equations are satisfied:
[0081]
[0082] in, This represents the initial effective porosity. This is the gap occupancy factor.
[0083] The controller 15 is connected to a liquid level sensor 16, which is located within the first-stage wetland unit 4. The controller 15 obtains the rate of liquid level drop after a single pumping operation through the liquid level sensor 16. And based on the rate of drop in liquid level With initial permeation rate The ratio calibrates the void occupancy factor. When the liquid level drops at a certain rate When decreasing, controller 15 increases. The value.
[0084] The overall hydraulic retention time of a three-stage cascaded vertical subsurface flow constructed wetland system. The governing equations are:
[0085]
[0086] in, The daily inflow rate is the value of a three-stage cascaded vertical subsurface flow constructed wetland system 1.
[0087] Controller 15 executes flow regulation commands. When Enlargement leads to When the flow rate is reduced, the controller 15 adjusts the influent mixing device 2 and the metering pump 3 to reduce the daily influent flow rate. This increases the total hydraulic retention time of the three-stage cascaded vertical subsurface flow constructed wetland system. Keep it within the range of 8 to 12 days.
[0088] During the intermittent pumping cycle, the single pumping volume delivered by the metering pumping device 3 is... Total height of the above-ground trough 8 Matrix depth and effective porosity The following inequalities must be satisfied:
[0089]
[0090] Controller 15 calculates in real time Correction The set value is used to prevent liquid from overflowing from the tank.
[0091] Through the aforementioned feedback adjustment process, the three-stage cascaded vertical subsurface flow constructed wetland system 1 maintains a constant hydraulic load during different operating phases. Controller 15 monitors... Numerical changes and corrections When the mixed influent passes through the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6, the contact time between it and the main treatment substrate 10, the bottom support substrate 9, and the roots of the aquatic plants 11 is controlled within a preset time range to treat conventional pollutants and microplastics.
[0092] The first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6 form a tiered interception space through a series structure. When the mixed influent flows vertically downward through the main treatment matrix 10, the micropores and mesopores inside the aerated concrete fragments physically trap the microplastic particles in the wastewater to be treated. The main treatment matrix 10 traps particulate microplastics with a particle size of less than 20 μm, while the roots of the aquatic plants 11 form a spatial network structure that mechanically entangles the fibrous and film-like microplastics in the wastewater.
[0093] The silanol and aluminol functional groups on the surface of the main treatment matrix 10 are positively charged, and they electrostatically adsorb onto the negatively charged microplastic particles. The bottom support matrix 9 utilizes the macropores formed by natural limestone gravel to guide the water flow downwards. The porosity gradient distribution formed by the main treatment matrix 10 and the bottom support matrix 9 allows suspended solids in the mixed influent to settle to the bottom of the tank step by step with the water flow, reducing the risk of clogging of the upper matrix.
[0094] The removal of conventional pollutants is achieved through matrix adsorption and biofilm metabolism. A quantitative pumping device 3 performs intermittent pumping, keeping the main treatment matrix 10 in a state of alternating dry and wet conditions. During the evacuation phase, oxygen entering the pores supports the biofilm on the surface of the main treatment matrix 10 for aerobic metabolism, degrading the chemical oxygen demand (COD) in the wastewater. Nitrifying bacteria within the biofilm convert ammonia nitrogen to nitrate nitrogen under aerobic conditions. As the wastewater flows through the anoxic areas of the deep layers of the main treatment matrix 10 and the bottom supporting matrix 9, denitrification is achieved.
[0095] The bottom support matrix 9 removes total phosphorus through a chemical reaction. Natural limestone gravel releases calcium ions, which react with phosphate ions in the wastewater to form phosphate precipitates. The precipitates adhere to the surface of the bottom support matrix 9 or are deposited at the bottom of the tank along with biofilm metabolic products.
[0096] During the operation of the three-stage tandem vertical subsurface flow constructed wetland system 1, the removal rate of microplastics exhibits a non-linear variation over time. During the saturation phase of operation, the physical adsorption sites on the surface of the main treatment substrate 10 are covered, leading to a decrease in the removal rate. As the three-stage tandem vertical subsurface flow constructed wetland system 1 continues to operate, the biofilm undergoes periodic shedding and renewal, and the root systems of the aquatic plants 11 extend and compress the substrate, exposing unoccupied surface areas within the main treatment substrate 10 and creating new physical adsorption spaces, thus restoring the microplastic removal rate. This dynamic equilibrium process enables the three-stage tandem vertical subsurface flow constructed wetland system 1 to maintain its microplastic retention capacity during long-term operation.
[0097] Extracellular polymers secreted by the biofilm encapsulate fine-particle microplastics, fixing them onto the surface of the main treatment substrate 10. Alkaline components in the wastewater neutralize acidic substances produced during biofilm nitrification and acidic substances secreted by the roots of aquatic plants 11. After the mixed influent flows through the three-stage unit, the pH is adjusted from alkaline to neutral, and the pH of the product water is maintained within the range of 7.5 to 7.6.
[0098] Example 1:
[0099] This embodiment provides the specific construction and operation mode of a three-stage tandem vertical subsurface flow constructed wetland system 1. (Refer to...) Figure 1The above-ground tank 8 is made of stainless steel. The internal dimensions of a single above-ground tank 8 are 0.55m long and 0.4m wide, with an internal cross-sectional area of 0.22m². To accommodate the substrate and allow space for the water head above, the total height is set at 0.4m. The three sets of above-ground tanks 8 are arranged in a stepped pattern along the water flow direction, with a bottom elevation difference of 0.1m between adjacent stages.
[0100] Substrate was laid in the above-ground tanks 8 of the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6. A 10cm thick bottom support substrate 9, composed of natural limestone crushed stone with a particle size of 30mm to 50mm, was laid at the bottom of the above-ground tank 8. A 20cm thick main treatment substrate 10, composed of aerated concrete fragments with a particle size of 10mm to 20mm, was laid on top of the bottom support substrate 9. The total filling depth of the bottom support substrate 9 and the main treatment substrate 10 was 30cm. The initial effective porosity of the mixture of the bottom support substrate 9 and the main treatment substrate 10 was determined to be 0.50.
[0101] Aquatic plants 11 were planted within the 10 layers of the main treatment substrate at a density of 16 plants per square meter. Windmill grass was planted in the first-level wetland unit 4, and canna lilies were planted in the second-level wetland unit 5 and the third-level wetland unit 6. The root growth depth of the aquatic plants 11 was controlled to within 20 cm, and the roots were distributed within the 10 layers of the main treatment substrate.
[0102] The operation process of the three-stage tandem vertical subsurface flow constructed wetland system 1 is as follows: The influent mixing device 2 introduces landfill leachate through the original landfill leachate influent pipe 12, and introduces wetland effluent through the effluent return pipe 13. The controller 15 is set to a dilution volume ratio coefficient of 4. The controller 15 adjusts the flow regulating valves on the corresponding pipelines to make the daily inflow volume of the original leachate 2L and the daily inflow volume of the return effluent 8L, mixing to form a mixed influent with a total daily influent volume of 10L.
[0103] The metering pump 3, controlled by the controller 15, performs intermittent pumping. The total number of pumping operations per day is 2, with a pumping interval of 12 hours, and a single pumping volume of 5L. The controller 15 calculates the void occupancy factor based on the liquid level drop rate fed back by the liquid level sensor 16. In the initial stage of operation in this embodiment, the void occupancy factor is 0.05. The calculated total effective pore volume of system 1 is 94.05L. Combined with a daily influent flow rate of 10L / d, the total hydraulic retention time of system 1 is 9.4 days.
[0104] The mixed influent enters through the top distribution pipe of the first-stage wetland unit 4. Under gravity, the wastewater sequentially permeates through the main treatment substrate 10 and the bottom support substrate 9. Wastewater flow between each unit is accomplished through connecting pipes 7. The wastewater undergoes an alternating process of infiltration and drainage within each unit, with the reoxygenation time during the drainage phase set at 4 hours.
[0105] In a three-stage tandem vertical subsurface flow constructed wetland system 1, under continuous operation, operators collect water samples at the final outlet pipe 14 and perform pollutant concentration and microplastic abundance detection. Controller 15 receives the liquid level signal from the first-stage wetland unit 4. When the effective pore volume decreases, the system reduces the daily inflow rate to maintain the total hydraulic retention time within the range of 8 to 12 days.
[0106] Test Example 1
[0107] A continuous operation test was conducted on the three-stage tandem vertical subsurface flow constructed wetland system 1. After the system reached a stable operating state, raw wastewater samples were collected at the sampling port of the original landfill leachate inlet pipe 12, and treated effluent samples were collected at the sampling port of the final outlet pipe 14.
[0108] COD (mg / L) 2146.25±201.25 283.5±28.070 86.79 TN (mg / L) 625.72±99.51 34.33±2.540 94.51 TP (mg / L) 6.29±0.11 0.53±0.064 94.67 <![CDATA[NH4 + -N (mg / L)]]> 416.77±33.64 22.22±1.360 91.62 pH 8.36±0.19 7.55±0.090 - EC (mS / cm) 8.60±0.15 4.05±0.540 -
[0109] The pH value of 12 water samples from the original landfill leachate inlet pipe was 8.36. The pH value of 14 effluent samples from the final outlet pipe was 7.55. The three-stage series vertical subsurface flow constructed wetland system 1 adjusted the pH value of the wastewater to the neutral range.
[0110] The total phosphorus concentration of 12 water samples from the original landfill leachate inlet pipe was 15.60 mg / L. The total phosphorus concentration of 14 effluent samples from the final outlet pipe was 0.83 mg / L. Based on the influent and effluent concentrations, the average total phosphorus removal rate of the three-stage tandem vertical subsurface flow constructed wetland system 1 was 94.67%. The natural limestone gravel in the bottom support matrix 9 reacted with the wastewater to produce a chemical precipitation reaction, reducing the total phosphorus concentration in the effluent.
[0111] The chemical oxygen demand (COD) concentration of 12 water samples from the original landfill leachate inlet pipe was 850.0 mg / L, the total nitrogen (TNI) concentration was 120.0 mg / L, and the ammonia nitrogen concentration was 85.0 mg / L. The COD concentration of 14 effluent samples from the final outlet pipe was 136.0 mg / L, the TNI concentration was 36.0 mg / L, and the ammonia nitrogen concentration was 12.7 mg / L.
[0112] Calculations show that the three-stage tandem vertical subsurface flow constructed wetland system 1 achieved a chemical oxygen demand (COD) removal rate of 84.0%, a total nitrogen removal rate of 70.0%, and ammonia nitrogen removal rate of 85.0%. Microorganisms attached to the surface of the main treatment substrate 10 within the first-stage wetland unit 4, the second-stage wetland unit 5, and the third-stage wetland unit 6, as well as the roots of aquatic plants 11, degraded organic matter and nitrogenous pollutants in the wastewater.
[0113] Test Example 2:
[0114] The three-stage tandem vertical subsurface flow constructed wetland system 1 was continuously monitored for 120 days. After the system reached a stable operating state, mixed influent samples were collected every 7 days from the sampling port of the original landfill leachate inlet pipe 12 and effluent samples were collected from the sampling port of the final outlet pipe 14.
[0115] The collected mixed influent and effluent samples were digested with hydrogen peroxide to remove organic interferences, and then filtered through a glass fiber membrane. The abundance of microplastics on the filter membrane after filtration was quantitatively analyzed using a microscopic counting method. The system's microplastic removal rate was [not specified]. Calculate according to the following formula:
[0116]
[0117] in, The abundance of microplastics in the mixed influent from 12 points in the original landfill leachate inlet pipe is expressed in items / L. The abundance of microplastics in the water effluent from the 14 outlet pipes is expressed in items / L.
[0118] Test results showed that the three-stage tandem vertical subsurface flow constructed wetland system 1 achieved an average microplastic removal rate of 84.78%. Wastewater flowed sequentially through the first-stage wetland unit 4, the second-stage wetland unit 5, and the third-stage wetland unit 6. Microplastic particles were physically trapped within the micropores and mesopores of the main treatment matrix 10. The silanol and aluminol functional groups on the surface of the main treatment matrix 10 fixed negatively charged microplastic particles through electrostatic adsorption. The root network of the aquatic plants 11 mechanically entangled the fibrous microplastics.
[0119] During continuous operation, the removal rate of microplastics exhibited a V-shaped change over time. In the first 30 days of system operation, the main treatment matrix 10 had unoccupied physical adsorption sites, and the microplastic removal rate remained between 85.0% and 88.0%. From day 30 to day 60 of system operation, some active sites on the surface of the main treatment matrix 10 were occupied by microplastics and suspended solids from the leachate, and the removal rate decreased to between 75.0% and 78.0%.
[0120] After 60 days of system operation, the microplastic removal rate rebounded and stabilized above 82.0%. During this stage, the roots of aquatic plants 11 extended and expanded within the main treatment substrate 10 layer, disturbing the substrate particles and creating new mechanical interception spaces. The biofilm on the surface of the main treatment substrate 10 detached, exposing unoccupied surfaces within the substrate.
[0121] During the aforementioned operation, the controller 15 obtains the rate of liquid level decline within the first-stage wetland unit 4 via the liquid level sensor 16. When microplastics accumulate in the main treatment substrate 10, causing a decrease in the permeability coefficient, the controller 15 adjusts the influent mixing device 2 to reduce the total daily influent volume, maintaining the hydraulic retention time within the range of 8 to 12 days. Through the renewal of physical adsorption sites in the substrate and the feedback adjustment of the hydraulic retention time, the three-stage tandem vertical subsurface flow constructed wetland system 1 maintains the microplastic removal effect.
[0122] The system exhibits different retention characteristics for microplastics of different particle sizes. Particulate microplastics with a particle size of less than 20 μm aggregate in the internal pores of the main treatment matrix 10. Fibrous microplastics are enriched in the root zone of aquatic plants 11. The three-stage tandem vertical subsurface flow constructed wetland system 1 extends the contact path between wastewater and the composite matrix bed, reducing the probability of microplastics being lost with the effluent.
[0123] Test Example 3:
[0124] Refer to the instruction manual appendix Figure 3 Particle size classification tests were conducted based on the geometry of the microplastics. Microplastics were divided into three size ranges: 500 μm to 5 mm, 100 μm to 500 μm, and less than 100 μm. Test results showed that the three-stage tandem vertical subsurface flow constructed wetland system achieved a microplastic removal rate of 92.4% for the 500 μm to 5 mm particle size range; 86.2% for the 100 μm to 500 μm particle size range; and 78.5% for the less than 100 μm particle size range.
[0125] Microplastics with a particle size greater than 100 μm are intercepted by the interparticle pores of the main treatment matrix 10 layer as they flow through the first-level wetland unit 4, the second-level wetland unit 5, and the third-level wetland unit 6. Microplastics with a particle size less than 100 μm enter the pores on the surface of the main treatment matrix 10 and undergo physical adsorption with the silanol functional groups on the surface of the aerated concrete fragments.
[0126] Refer to the instruction manual appendix Figure 4 Shape classification tests were conducted based on the spatial morphology of microplastics. Microplastics were categorized into fibrous, film-like, and fragment-like forms. Test data showed that the three-stage tandem vertical subsurface flow constructed wetland system 1 achieved a removal rate of 88.9% for fibrous microplastics, 85.3% for film-like microplastics, and 81.5% for fragment-like microplastics.
[0127] During infiltration, fibrous microplastics are mechanically entangled by the root network structure of aquatic plants 11. Thin-film microplastics adhere to the biofilm on the surface of the main treatment matrix 10. Fragmented microplastics are retained by the pore gradient formed by the main treatment matrix 10 and the underlying support matrix 9.
[0128] Refer to the instruction manual appendix Figure 5 Polymer composition testing was conducted based on the chemical composition of the microplastics. The tested materials included polyethylene, polypropylene, and polystyrene. The three-stage tandem vertical subsurface flow constructed wetland system achieved a removal rate of 86.1% for polyethylene microplastics, 83.4% for polypropylene microplastics, and 85.2% for polystyrene microplastics.
[0129] The microplastics with different chemical compositions carry negative charges on their surfaces. The aluminum hydroxyl and silanol functional groups on the surface of the main treatment matrix 10 are electrostatically attracted to the microplastic surface. In the pathway formed by the tertiary unit series, the microplastics are fixed inside the system through the charge adsorption of the main treatment matrix 10 and the encapsulation of the biofilm.
[0130] The test results above demonstrate that the three-stage tandem vertical subsurface flow constructed wetland system 1 has the capacity to treat microplastic components in landfill leachate. The tandem layout of the three stages increases the number of contacts between wastewater and the main treatment substrate 10 and the roots of aquatic plants 11, thus achieving the retention of multiple types of microplastics.
[0131] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.
Claims
1. A three-stage cascaded vertical subsurface flow constructed wetland system, characterized in that, include: The first-level wetland unit (4), the second-level wetland unit (5), and the third-level wetland unit (6) are each equipped with an independent above-ground trough (8). Connecting pipes (7) are respectively set between adjacent above-ground tanks (8); the top inlet of the second-level wetland unit (5) is connected to the bottom outlet of the first-level wetland unit (4) through the connecting pipes (7); the top inlet of the third-level wetland unit (6) is connected to the bottom outlet of the second-level wetland unit (5) through the connecting pipes (7); The bottom support matrix (9) and the main treatment matrix (10) are laid sequentially from bottom to top inside each of the above-ground troughs (8); the bottom support matrix (9) is made of natural limestone crushed stone; the main treatment matrix (10) is made of aerated concrete crushed blocks; Aquatic plants (11) are planted in the main treatment substrate (10).
2. The three-stage tandem vertical subsurface flow constructed wetland system according to claim 1, characterized in that, The first-level wetland unit (4), the second-level wetland unit (5), and the third-level wetland unit (6) are arranged in a stepped descending manner along the water flow direction; the bottom elevation of the first-level wetland unit (4) is greater than the bottom elevation of the second-level wetland unit (5), and the bottom elevation of the second-level wetland unit (5) is greater than the bottom elevation of the third-level wetland unit (6); the bottom of the above-ground trough (8) is a flat-bottom structure, and the sidewalls of the above-ground trough (8) are connected to the bottom to form a closed container.
3. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 1, characterized in that, The ratio of the thickness of the bottom support matrix (9) to the thickness of the main treatment matrix (10) is 1:2 to 1:1; the average particle size of the natural limestone crushed stone is 30mm to 50mm, and the average particle size of the aerated concrete fragments is 10mm to 20mm; the main treatment matrix (10) and the bottom support matrix (9) form a porosity gradient distribution.
4. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 1, characterized in that, The aquatic plant (11) is a windmill grass or a canna; the root growth depth of the aquatic plant (11) is less than or equal to the laying thickness of the main treatment substrate (10), and the root system of the aquatic plant (11) is distributed within the main treatment substrate (10) layer.
5. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 1, characterized in that, The three-stage tandem vertical subsurface flow constructed wetland system also includes: The water inlet mixing device (2) is connected to the original landfill leachate inlet pipe (12) and the outlet return pipe (13) respectively; the water inlet mixing device (2) is used to mix the original landfill leachate with the wetland effluent; the outlet return pipe (13) is connected to the bottom outlet of the third-level wetland unit (6); A metering pumping device (3) is provided, the input end of which is connected to the output end of the water mixing device (2), and the output end of which is connected to the top water inlet of the first-stage wetland unit (4).
6. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 5, characterized in that, The three-stage tandem vertical subsurface flow constructed wetland system also includes: The controller (15) is electrically connected to the water mixing device (2) and the metering pump (3) respectively; A liquid level sensor (16) is installed in the first-level wetland unit (4) and is electrically connected to the controller (15).
7. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 6, characterized in that, Both the original landfill leachate inlet pipe (12) and the outlet return pipe (13) are equipped with flow regulating valves; the inlet mixing device (2) is equipped with a stirring mechanism; both the flow regulating valve and the stirring mechanism are electrically connected to the controller (15).
8. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 7, characterized in that, The controller (15) is used to calculate the daily influent volume of the original landfill leachate and the daily return volume of the wetland effluent based on the daily mixed influent volume and the dilution volume ratio coefficient; the controller (15) is used to adjust the opening of the flow regulating valve on the original landfill leachate influent pipe (12) according to the daily influent volume of the original landfill leachate, and to adjust the opening of the flow regulating valve on the effluent return pipe (13) according to the daily return volume of the wetland effluent.
9. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 8, characterized in that, The controller (15) is used to control the number of pumping operations per day and the volume of pumping operation per pumping operation of the metering pumping device (3); the controller (15) is used to set the time interval between two adjacent pumping operations, the time interval being greater than or equal to the sum of the emptying time and the reoxygenation time; the controller (15) is used to control the first-level wetland unit (4), the second-level wetland unit (5) and the third-level wetland unit (6) to alternately be in the infiltration stage and the emptying stage by starting and stopping the metering pumping device (3).
10. A three-stage tandem vertical subsurface flow constructed wetland system according to claim 9, characterized in that, The controller (15) is used to receive the liquid level drop rate obtained by the liquid level sensor (16); the controller (15) is used to calculate the void occupancy factor according to the ratio of the liquid level drop rate to the initial permeation rate, and to calculate the effective porosity according to the void occupancy factor and the initial effective porosity; the controller (15) is used to increase the value of the void occupancy factor when the liquid level drop rate decreases, and to reduce the daily inflow of the three-stage series vertical subsurface flow constructed wetland system by adjusting the inlet water mixing device (2) and the metering pumping device (3), so as to maintain the total hydraulic retention time within the range of 8 to 12 days.