A composite subsurface constructed wetland for low-temperature low-carbon-nitrogen-ratio sewage

Abstract

The present application relates to the technical field of low-pollution water ecological advanced treatment, and particularly relates to a composite subsurface flow constructed wetland for low-temperature and low-carbon-nitrogen-ratio sewage; the composite subsurface flow constructed wetland is sequentially provided with a water distribution area, an electrode-biomass composite module area, a plant-biochar collaborative purification area and a water collecting area along the water flow direction, a water inlet pipe is connected to one side of the water distribution area, one side of the water collecting area is connected to a water outlet pipe, and the areas are separated by perforated partitions; the present application is aimed at low-carbon-nitrogen-ratio sewage represented by tail water of a sewage treatment plant, and realizes long-term and stable release of ferrous ions, hydrogen and organic carbon by coupling of low-voltage electrolysis of iron anodes and plants biomass, so as to strengthen autotrophic denitrification driven by ferrous ions and hydrogen, heterotrophic denitrification mediated by plant biomass, cathode electrochemical denitrification, electric flocculation and phosphorus removal, cathode adsorption and electric Fenton degradation of microplastics, and improve long-term nitrogen-phosphorus-microplastic removal efficiency of the subsurface flow constructed wetland under low-temperature conditions.

Description

Technical Field

[0001] This invention relates to the field of deep ecological treatment technology for low-pollution water, specifically to a composite subsurface flow constructed wetland for low-temperature, low-carbon-nitrogen ratio wastewater. Background Technology

[0002] With continued population growth and accelerated urbanization, wastewater treatment plant effluent has become the primary source of nitrogen and phosphorus pollutants in surface water. The nitrogen and phosphorus concentrations in this effluent, even under the Class A standard, are still far higher than the limits for Class V surface water. Direct discharge into natural receiving water bodies would cause oxygen depletion and large-scale algal blooms, accelerating eutrophication. Furthermore, wastewater treatment plant effluent is also considered a significant source of microplastics in the environment, with daily microplastic emissions reaching as high as 2 × 10⁻⁶. 6 items d -1 This poses a potential threat to aquatic ecosystems and human health. Therefore, it is urgent to simultaneously remove nitrogen, phosphorus, and microplastics from wastewater treatment plant effluent with high nitrogen and phosphorus content, mainly nitrates, and lacking biodegradable organic matter.

[0003] Constructed wetland ecological treatment technology is environmentally friendly and cost-effective. It achieves nitrogen and phosphorus removal from low-pollution water through substrate adsorption, aquatic plant absorption, and microbial degradation, while simultaneously removing large amounts of microplastics through substrate retention, biofilm adsorption, and aquatic plant interception. However, in traditional constructed wetland effluent advanced treatment, insufficient availability of denitrification electron donors, poor phosphorus removal efficiency from conventional substrate adsorption, plant growth and microbial activity limited by low-temperature stress, and the biotoxicity of accumulated microplastics limit the long-term nitrogen and phosphorus removal efficiency of constructed wetlands. In recent years, electrochemical-constructed wetland coupling technology has greatly expanded the application boundaries and treatment capacity of constructed wetlands.

[0004] In existing combined systems of subsurface flow constructed wetlands and microbial fuel cells, chemical energy is converted into electrical energy by microbial degradation of organic matter. However, under low temperature and low C / N ratio conditions, microbial activity is inhibited and available carbon sources are insufficient, resulting in a significant decrease in both pollutant removal efficiency and power generation efficiency. In existing electrolysis-assisted constructed wetland systems for low-temperature nitrogen and phosphorus removal, simultaneous nitrogen and phosphorus removal is achieved through electrocatalytic nitrate reduction, hydrogen autotrophic denitrification, and iron-phosphorus precipitation. However, after long-term operation, the iron anode surface is prone to caking and passivation, leading to reduced electron transfer efficiency and unstable or even decreased nitrogen and phosphorus removal performance. Therefore, there is an urgent need for an enhanced constructed wetland that can stably provide energy and electron donors and effectively cope with low-temperature inhibition and microplastic stress. Coupled with low-voltage electrolysis of iron anodes and plant biomass, reducing electrode spacing and optimizing electrode arrangement can help stabilize and improve the nitrogen-phosphorus-microplastic removal efficiency of constructed wetlands for low C / N ratio wastewater under low-temperature conditions, while reducing energy consumption and cost.

[0005] Existing technologies may utilize microorganisms to degrade organic matter in raw water for wastewater treatment and power generation, but their applicability is poor under low temperature and low carbon-to-nitrogen ratio conditions; or the iron anode surface is prone to caking and passivation in the later stages of operation, making the denitrification and phosphorus removal performance unstable or even declining; and none of them have considered the negative impact of continuous accumulation of microplastics on the long-term treatment efficiency of the system. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention aims to provide a composite subsurface flow constructed wetland for low-temperature, low-carbon-nitrogen ratio wastewater, achieving long-term, stable, low-cost, and high-efficiency simultaneous removal of nitrogen, phosphorus, and microplastics from low-carbon-nitrogen ratio wastewater under low-temperature conditions.

[0007] To solve the above problems, the present invention adopts the following technical solution:

[0008] This invention provides a composite subsurface flow constructed wetland for low-temperature, low-carbon-nitrogen-ratio wastewater. The composite subsurface flow constructed wetland is provided with a water distribution zone, an electrode-biomass composite module zone, a plant-biochar synergistic purification zone, and a water collection zone in sequence along the water flow direction. The inlet pipe is connected to one side of the water distribution zone, and the outlet pipe is connected to one side of the water collection zone. Each zone is separated by a perforated partition.

[0009] Furthermore, a perforated anode iron plate is placed in the center of the electrode-biomass composite module area, and a perforated cathode carbon felt is placed on each side of the electrode-biomass composite module area. Both the anode and cathode are connected to the solar power supply system by insulated copper wires. A biomass tray is set in the area between the perforated anode iron plate and the perforated cathode carbon felt, and waste plant biomass is laid flat on the biomass tray.

[0010] Furthermore, the lower part of the plant-biochar synergistic purification zone is filled with matrix gravel and plant-derived biochar balls, while the upper part is planted with aquatic plants.

[0011] Furthermore, the diameter of the holes in the perforated partition is 5-10 mm.

[0012] Furthermore, the thickness of the perforated anode iron plate and the perforated cathode carbon felt is 3-5 mm, the distance between the perforated anode iron plate and the perforated cathode carbon felt is 5-7 cm, and the diameter of the holes in the perforated anode iron plate and the perforated cathode carbon felt is 5-10 mm.

[0013] Furthermore, the solar power supply system includes solar panels, a charge controller, a battery, an inverter, and a voltage regulator, and the voltage of the solar power supply system is 3-5 V.

[0014] Furthermore, the waste plant biomass is treated by washing, drying, crushing, and screening to achieve a particle size of 5-10 mm, with an addition amount of 1.0-2.0 g / L. -1 The width of the biomass tray is 3-5 cm.

[0015] Furthermore, the matrix gravel and plant-derived biochar balls have a filling ratio of 3-4:1, a particle size of 2-3 cm, and a porosity of 0.4-0.5. The plant-derived biochar balls are obtained by washing, drying, and grinding waste plant biomass, carbonizing it at 300-400 ℃ for 2-3 h in a nitrogen atmosphere, and then mixing it with clay and bentonite to form balls and drying them.

[0016] Furthermore, the aquatic plant is yellow iris, and the planting density is no less than 25 plants per square meter. -2 .

[0017] Furthermore, the hydraulic retention time of the composite subsurface flow constructed wetland is 2-3 days, and the water depth is 30-50 cm.

[0018] The beneficial effects of this invention are as follows:

[0019] 1. For low-carbon-nitrogen ratio wastewater, represented by wastewater treatment plant effluent, characterized by low pollutant concentration, large discharge volume, and high load, a long-term and stable release of ferrous ions, hydrogen, and organic carbon is achieved through low-voltage electrolytic coupling of plant biomass with iron anodes. This enhances ferrous ion and hydrogen-driven autotrophic denitrification, plant biomass-mediated heterotrophic denitrification, cathodic electrochemical denitrification, electrocoagulation phosphorus removal, cathodic adsorption, and electro-Fenton degradation of microplastics, thereby improving the long-term nitrogen-phosphorus-microplastic removal efficiency of subsurface flow constructed wetlands under low-temperature conditions.

[0020] 2. Using solar power and low-voltage electrolysis of iron anodes can effectively reduce energy consumption and control the iron anode loss rate.

[0021] 3. Filling with plant-derived biochar balls can avoid secondary iron pollution in the effluent, while promoting the bioavailability of iron (hydrogen) oxides and iron autotrophic denitrification.

[0022] 4. Waste plant biomass is widely available and inexpensive, which is conducive to "treating pollution with waste" and promoting the recycling of solid waste resources.

[0023] 5. The electrode-biomass composite module configuration facilitates the regular cleaning and replacement of electrodes and plant biomass, reducing the difficulty and cost of constructive wetland maintenance. Attached Figure Description

[0024] Figure 1 A schematic diagram of a composite subsurface flow constructed wetland for low-temperature, low-carbon-nitrogen-ratio wastewater.

[0025] Among them, 1 is the water inlet pipe; 2 is the water distribution area; 3 is the perforated partition; 4 is the electrode-biomass composite module area; 5 is the perforated cathode carbon felt; 6 is the biomass support plate; 7 is the perforated anode iron plate; 8 is the waste plant biomass; 9 is the insulated copper wire; 10 is the solar power supply system; 11 is the aquatic plants; 12 is the plant-biochar synergistic purification area; 13 is the matrix gravel; 14 is the plant-derived biochar ball; 15 is the water collection area; and 16 is the water outlet pipe. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to specific embodiments.

[0027] It should be noted that these embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Simple improvements to the method under the premise of the present invention are all within the scope of protection claimed by the present invention.

[0028] See Figure 1 This is a composite subsurface flow constructed wetland for low-temperature, low-carbon-nitrogen-ratio wastewater. The composite subsurface flow constructed wetland is arranged in sequence along the water flow direction as follows: water distribution zone 2, electrode-biomass composite module zone 4, plant-biochar synergistic purification zone 9, and water collection zone 15. The inlet pipe 1 is connected to one side of the water distribution zone 2, and the outlet pipe 16 is connected to one side of the water collection zone 15. Each zone is separated by a perforated partition 3.

[0029] A perforated anode iron plate 7 is placed in the center of the electrode-biomass composite module area 4, and a perforated cathode carbon felt 5 is placed on each side of the electrode-biomass composite module area 4. Both the anode and cathode are connected to the solar power supply system 10 by insulated copper wires 9. A biomass tray 6 is set in the area between the perforated anode iron plate 7 and the perforated cathode carbon felt 5, and waste plant biomass 8 is laid flat on the biomass tray 6.

[0030] The plant-biochar synergistic purification zone 12 is filled with substrate gravel 13 and plant-derived biochar balls 14 at the bottom, and aquatic plants 11 are planted on the top.

[0031] The perforated partition 3 has a hole diameter of 5-10 mm.

[0032] The thickness of the perforated anode iron plate 7 and the perforated cathode carbon felt 5 is 3-5 mm, the distance between the perforated anode iron plate 7 and the perforated cathode carbon felt 5 is 5-7 cm, and the diameter of the holes in the perforated anode iron plate 7 and the perforated cathode carbon felt 5 is 5-10 mm.

[0033] The iron anode electrolytic coupling with plant biomass can degrade organic matter aerobically, consuming dissolved oxygen in the system, delaying the passivation and hardening of zero-valent iron surfaces, and ensuring a continuous and stable release of ferrous ions. The released ferrous ions can further enhance the degradation rate of plant cellulose, improving its long-term carbon release performance and biodegradability, while simultaneously strengthening both ferrotrophic and heterotrophic denitrification. It can also generate iron ions and their hydrates to remove phosphates through adsorption, direct sedimentation, and co-precipitation. The carbon felt cathode can directly electrocatalytically reduce nitrates and can also electrolyze water to generate hydrogen, enhancing hydrogen autotrophic denitrification. Its large surface area and abundant pore structure can effectively adsorb most microplastics. The smaller electrode spacing facilitates the in-situ efficient utilization of hydrogen peroxide (a cathode oxygen reduction product) and ferrous ions (anode iron oxidation product), generating hydroxyl radicals through an electro-Fenton reaction to accelerate the aging and degradation of microplastics and alleviate biotoxicity inhibition.

[0034] The solar power supply system 10 includes solar panels, a charge controller, a battery, an inverter, and a voltage regulator. The voltage of the solar power supply system 10 is 3-5 V.

[0035] Among them, the waste plant biomass 8 is washed, dried, crushed and screened, with a particle size of 5-10 mm, and the dosage is 1.0-2.0 g / L. -1 The width of the biomass tray 6 is 3-5 cm.

[0036] Aquatic plant biomass has a high carbon content and a large carbon-to-nitrogen ratio, making it a suitable natural cellulose-based solid-phase slow-release carbon source that can effectively enhance microbial denitrification.

[0037] The matrix gravel 13 and plant-derived biochar balls 14 have a filling ratio of 3-4:1, a particle size of 2-3 cm, and a porosity of 0.4-0.5. The plant-derived biochar balls 14 are obtained by washing, drying and grinding waste plant biomass, carbonizing it at 300-400 ℃ for 2-3 h in a nitrogen atmosphere, and then mixing it with clay and bentonite to form balls and drying them.

[0038] Plant-derived biochar balls can effectively remove residual iron ions from water through chemical adsorption. Their surface oxygen-containing functional groups, through charge-discharge cycles, can promote the activation and reduction of iron (hydrogen) oxides, further enhancing iron autotrophic denitrification. A mixture of gravel and plant-derived biochar balls is beneficial for the attachment and growth of nitrifying and denitrifying bacteria.

[0039] Among them, 11 aquatic plants are yellow iris, with a planting density of no less than 25 plants per square meter. -2 .

[0040] The reticulate root system of yellow iris can directly absorb nitrogen and phosphorus from the water and provide attachment surfaces, photosynthetic oxygen, and root exudates for nitrifying and denitrifying bacteria to enhance microbial denitrification.

[0041] Among them, the hydraulic retention time of the composite subsurface flow constructed wetland is 2-3 days, and the water depth is 30-50 cm.

[0042] Example 1

[0043] A laboratory-scale horizontal subsurface flow constructed wetland device was constructed within the Botanical Garden of Shanghai Jiao Tong University. It consists of four parts: a water distribution area, an electrode-biomass composite module area, a plant-biochar synergistic purification area, and a water collection area, separated by a 5 mm thick perforated partition with 10 mm diameter holes. The dimensions of the water distribution area are 10 cm × 20 cm (length × width), the electrode-biomass composite module area is 10 cm × 20 cm (length × width), the plant-biochar synergistic purification area is 40 cm × 20 cm (length × width), and the water collection area is 10 cm × 20 cm (length × width), with a water depth of 30 cm. A perforated anode iron plate is placed in the center of the electrode-biomass composite module area, with a perforated cathode carbon felt on each side. Each electrode is connected to a DC regulated power supply via insulated copper wire. Withered stems and leaves of *Iris tectorum* are laid flat on a biomass tray between the anode and cathode plates. The dimensions of the perforated anode iron plate and the perforated cathode carbon felt are 32 cm × 18 cm × 0.3 cm (length × width × thickness), with an electrode spacing of 5 cm and a hole diameter of 10 mm. The control voltage is constant at 5 V. The dosage of *Iris tectorum* residue is 1.5 g / L. -1 The dimensions are 8 mm × 8 mm; the biomass tray dimensions are length × width × thickness = 18 cm × 3 cm × 0.5 cm. The plant-biochar synergistic purification zone is laid with gravel with a particle size of 2-3 cm and yellow iris-derived biochar balls as a mixed substrate, with a filling ratio of 3:1 and a filling height of 32 cm, and 4 mature yellow iris plants are evenly planted.

[0044] The wastewater to be treated uses simulated wastewater effluent from a wastewater treatment plant, pumped into the water distribution area of ​​the constructed wetland device via a peristaltic pump. The total nitrogen concentration is 14-15 mg / L. -1 The nitrate nitrogen concentration is 10-11 mg / L. -1 The ammonia nitrogen concentration was 4-5 mg / L. -1 The total phosphorus concentration was 0.4-0.5 mg / L. -1 The concentration of microplastics was 500-1000 μg / L. -1 Size range: 50-100 μm. Nominal residence time: 2 days. Hydraulic loading rate: 150 L / m. -2 d -1 .

[0045] Four horizontal subsurface flow constructed wetland systems were constructed: a control group, an electrolysis group, a biomass group, and an electrolysis-biomass coupled group. As shown in Table 1, after 90 days of continuous operation at a low temperature of 5-10 ℃, the total nitrogen concentration in the effluent of the electrolysis-biomass coupled group remained at approximately 6-8 mg / L. -1 The total phosphorus concentration remained basically between 0.00-0.02 mg / L. -1 The microplastic removal rate reached approximately 99%. Compared to the control group, electrolysis group, and biomass group, the nitrogen removal efficiency increased by 46%, 19%, and 42%, respectively; the phosphorus removal efficiency increased by 81%, 3%, and 80%, respectively; and the microplastic removal rate increased by 14%, 4%, and 9%, respectively. This indicates that low-voltage electrolysis coupled with plant biomass at iron anodes can effectively enhance the removal of nitrogen, phosphorus, and microplastics in low-temperature subsurface flow constructed wetlands.

[0046] Table 1. Nitrogen and phosphorus removal efficiency of each system after 90 days of continuous operation

[0047]

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described with reference to preferred embodiments, those skilled in the art should understand that various changes in form and detail can be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A composite subsurface flow constructed wetland for low-temperature, low-carbon-nitrogen ratio wastewater, characterized in that, The composite subsurface flow constructed wetland is arranged in sequence along the water flow direction as a water distribution area, an electrode-biomass composite module area, a plant-biochar synergistic purification area, and a water collection area. The inlet pipe is connected to one side of the water distribution area, and the outlet pipe is connected to one side of the water collection area. Each area is separated by a perforated partition. The low temperature is 5-10 ℃. A perforated anode iron plate is placed in the center of the electrode-biomass composite module area, and a perforated cathode carbon felt is placed on each side of the electrode-biomass composite module area. Both the anode and cathode are connected to the solar power supply system by insulated copper wires. A biomass tray is set in the area between the perforated anode iron plate and the perforated cathode carbon felt, and waste plant biomass is laid flat on the biomass tray. The lower part of the plant-biochar synergistic purification zone is filled with matrix gravel and plant-derived biochar balls, while the upper part is planted with aquatic plants. The perforated partition has a hole diameter of 5-10 mm; The thickness of the perforated anode iron plate and the perforated cathode carbon felt is 3-5 mm, the distance between the perforated anode iron plate and the perforated cathode carbon felt is 5-7 cm, and the diameter of the holes in the perforated anode iron plate and the perforated cathode carbon felt is 5-10 mm. The solar power supply system includes solar panels, a charge controller, a battery, an inverter, and a voltage regulator. The voltage of the solar power supply system is 3-5 V. The waste plant biomass is treated by washing, drying, crushing and screening, with a particle size of 5-10 mm, and the dosage is 1.0-2.0 g / L. -1 The width of the biomass tray is 3-5 cm; The matrix gravel and plant-derived biochar balls have a filling ratio of 3-4:1, a particle size of 2-3 cm, and a porosity of 0.4-0.

5. The plant-derived biochar balls are made by washing, drying, and grinding waste plant biomass, carbonizing it at 300-400 ℃ for 2-3 h in a nitrogen atmosphere, and then mixing it with clay and bentonite to form balls and drying them.

2. The composite subsurface flow constructed wetland according to claim 1, characterized in that, The aquatic plant is yellow iris, and the planting density is no less than 25 plants per square meter. -2 .

3. The composite subsurface flow constructed wetland according to claim 2, characterized in that, The hydraulic retention time of the composite subsurface flow constructed wetland is 2-3 days, and the water depth is 30-50 cm.

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