Composite subsurface flow constructed wetland for low-temperature low-carbon-nitrogen-ratio sewage

Abstract

The invention relates to the technical field of ecological advanced treatment of low-pollution water, in particular to a composite subsurface flow constructed wetland for low-temperature 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-charcoal synergistic purification area and a water collection area along the water flowing direction, a water inlet pipe is connected with one side of the water distribution area, one side of the water collection area is connected with a water outlet pipe, and the areas are separated by perforated partition plates; aiming at low-carbon-nitrogen-ratio sewage which is represented by tail water of a sewage plant and is low in pollutant concentration, large in discharge amount and high in load, long-acting and stable release of ferrous ions, hydrogen and organic carbon is realized through iron anode low-voltage electrolysis and coupling of plant biomass; according to the present invention, ferrous ion and hydrogen driven autotrophic denitrification, plant biomass mediated heterotrophic denitrification, cathode electrochemical denitrification, electric flocculation phosphorus removal, cathode adsorption and electro-Fenton micro-plastic degradation are reinforced, and the long-term nitrogen-phosphorus-micro-plastic removal efficiency of the subsurface flow constructed wetland is improved under the low temperature condition.

Description

TECHNICAL FIELD

[0001] The present application relates to the 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. BACKGROUND

[0002] With the continuous population growth and accelerated urbanization, the tail water of sewage treatment plant has become the main point source of nitrogen and phosphorus pollution in surface water. The nitrogen and phosphorus concentrations in the discharge standard of the tail water are still much higher than the standard limit of surface water class V, and if directly discharged into natural receiving water, it will cause water body anoxia and large-scale algal blooms, and accelerate water body eutrophication. In addition, the tail water of sewage treatment plant is also considered to be an important source of microplastics in the environment, with a daily discharge of microplastics as high as 2 × 10 6 items d -1 , which poses a potential threat to aquatic ecosystems and human health. Therefore, it is urgent to remove nitrogen, phosphorus and microplastics from low-carbon-nitrogen-ratio tail water of sewage treatment plant, which has high nitrogen and phosphorus content, is dominated by nitrate, and lacks biodegradable organic matter.

[0003] The ecological treatment technology of constructed wetland is environmentally friendly and low-cost, and relies on substrate adsorption, aquatic plant absorption and microbial degradation to achieve denitrification and phosphorus removal of low-pollution water. At the same time, a large amount of microplastics is removed through substrate retention, biofilm adsorption and aquatic plant interception. However, in the advanced treatment of tail water in traditional constructed wetlands, the availability of denitrification electron donors is insufficient, the phosphorus removal effect of conventional substrate adsorption is poor, the growth of plants and the activity of microorganisms are stressed by low temperature, and the accumulation of microplastics limits the long-term nitrogen and phosphorus removal efficiency of constructed wetlands. In recent years, the coupling technology of electrochemical-constructed wetland has greatly expanded the application boundary and treatment capacity of constructed wetland.

[0004] In the existing combination system of subsurface flow constructed wetland and microbial fuel cell, organic matter is degraded by microorganisms to convert chemical energy into electrical energy. However, under low-temperature and low-carbon-nitrogen-ratio conditions, microbial activity is inhibited and available carbon sources are insufficient, resulting in a significant decrease in pollutant removal efficiency and power generation efficiency. In the existing electrolysis-assisted constructed wetland low-temperature denitrification and phosphorus removal system, nitrate reduction by electrocatalysis, hydrogen autotrophic denitrification and iron-phosphorus precipitation are used to achieve simultaneous removal of nitrogen and phosphorus. However, after long-term operation, the surface of the iron anode is easily passivated, which reduces the efficiency of electron transfer and makes the denitrification and phosphorus removal performance unstable or even decreased. Therefore, there is an urgent need for a strengthened constructed wetland that can stably provide energy and electron donors and effectively cope with low-temperature inhibition and microplastic stress. Coupling of low-voltage electrolysis of iron anode and plant biomass can reduce the distance between electrodes and optimize the arrangement of electrodes, which helps to stably improve the nitrogen, phosphorus and microplastic removal efficiency of constructed wetlands under low-temperature conditions for low-carbon-nitrogen-ratio sewage on the basis of reducing energy consumption and cost.

[0005] The prior art either utilizes microorganisms to degrade organic matter in raw water to produce electricity, but has poor applicability under low-temperature and low-carbon-nitrogen ratio conditions; or the iron anode surface is prone to hardening and passivation in the later stage of operation, so that the denitrification and phosphorus removal performance is unstable or even decreases; and the negative effects of continuous accumulation of microplastics on long-term treatment efficiency of the system are not considered. SUMMARY

[0006] In view of the deficiencies of the prior art, the present application aims to provide a composite subsurface constructed wetland for low-temperature and low-carbon-nitrogen ratio sewage, which realizes long-term stable, low-cost and high-efficiency simultaneous removal of nitrogen, phosphorus and microplastics in low-carbon-nitrogen ratio sewage under low-temperature conditions.

[0007] To solve the above problems, the present application adopts the following technical solution: The present application provides a composite subsurface constructed wetland for low-temperature and low-carbon-nitrogen ratio sewage, which is provided with a water distribution area, an electrode-biomass composite module area, a plant-biochar synergistic purification area and a water collection area in sequence along the water flow direction, a water inlet pipe is connected to one side of the water distribution area, a water outlet pipe is connected to one side of the water collection area, and each area is separated by a perforated partition plate.

[0008] Further, a perforated anode iron plate is placed in the center of the electrode-biomass composite module area, and one perforated cathode carbon felt is placed on each side of the electrode-biomass composite module area, the anode and the cathode are both connected to a solar power supply system with an insulating copper wire, a biomass support plate is arranged between the perforated anode iron plate and the perforated cathode carbon felt, and waste plant biomass is laid flat on the biomass support plate.

[0009] Further, the plant-biochar synergistic purification area is filled with substrate gravel and plant-derived biochar balls in the lower part and planted with aquatic plants in the upper part.

[0010] Further, the hole diameter of the perforated partition plate is 5-10 mm.

[0011] Further, the thickness of the perforated anode iron plate and the perforated cathode carbon felt is 3-5 mm, the spacing between the perforated anode iron plate and the perforated cathode carbon felt is 5-7 cm, and the hole diameter of the perforated anode iron plate and the perforated cathode carbon felt is 5-10 mm.

[0012] Further, the solar power supply system includes a solar cell panel, a charging controller, a storage battery, an inverter and a stabilized power supply, and the voltage of the solar power supply system is 3-5 V.

[0013] Further, the waste plant biomass is treated by washing, drying, crushing and screening, the particle size is 5-10 mm, and the dosage is 1.0-2.0 g / L -1 The width of the biomass support plate is 3-5 cm.

[0014] Further, the matrix gravel and plant-derived biochar ball filling ratio is 3-4:1, the particle size is 2-3 cm, and the porosity is 0.4-0.5; the plant-derived biochar ball is obtained by washing, drying and grinding the waste plant biomass, and then mixing the carbonized waste plant biomass at 300-400 DEG C for 2-3 h under a nitrogen atmosphere with clay and bentonite to form a ball and drying.

[0015] Further, the aquatic plant is yellow flag iris, and the planting density is not less than 25 plants / m -2 .

[0016] Further, the hydraulic retention time of the composite subsurface constructed wetland is 2-3 d, and the water depth is 30-50 cm.

[0017] The beneficial effects of the present application are: 1. For low carbon-nitrogen ratio sewage represented by tail water of sewage plant, long-term and stable release of ferrous ions, hydrogen and organic carbon is realized by coupling of iron anode low-voltage electrolysis and plant biomass, autotrophic denitrification driven by ferrous ions and hydrogen, heterotrophic denitrification mediated by plant biomass, cathodic electrochemical denitrification, electric flocculation phosphorus removal, cathodic adsorption and electric Fenton degradation of microplastics are strengthened, and long-term nitrogen-phosphorus-microplastic removal efficiency of subsurface constructed wetland is improved under low temperature conditions.

[0018] 2. Solar power supply and iron anode low-voltage electrolysis can effectively reduce energy consumption and control the loss rate of iron anode.

[0019] 3. Filling of plant-derived biochar balls can avoid secondary pollution of effluent iron, and promote biological utilization of iron (hydrogen) oxide and iron autotrophic denitrification.

[0020] 4. The waste plant biomass is widely available and low in price, which is conducive to realizing "waste pollution control" and promoting solid waste resource recycling.

[0021] 5. The electrode-biomass composite module configuration facilitates regular cleaning and replacement of electrodes and plant biomass, and reduces the difficulty and cost of maintenance of the constructed wetland. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a kind of composite subsurface constructed wetland for low-temperature low-carbon-nitrogen ratio sewage; Wherein, 1 is the water inlet pipe; 2 is the water distribution area; 3 is the perforated partition plate; 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 plant, 12 is the plant-biochar synergistic purification area, 13 is the matrix gravel, 14 is the plant-derived biochar ball, 15 is the water collecting area, and 16 is the water outlet pipe. DETAILED DESCRIPTION

[0023] The application will be further described in detail below with reference to specific examples.

[0024] It should be noted that the examples are only used to illustrate the application, but not to limit the application, and simple improvements of the method under the concept of the application are within the scope of the application.

[0025] Reference Figure 1 The application discloses a composite subsurface constructed wetland for low-temperature and low-carbon-nitrogen-ratio sewage, which comprises a water distribution area 2, an electrode-biomass composite module area 4, a plant-biochar synergistic purification area 9 and a water collecting area 15 arranged in sequence along the water flow direction, wherein a water inlet pipe 1 is connected to one side of the water distribution area 2, one side of the water collecting area 15 is connected to a water outlet pipe 16, and each area is separated by a perforated partition plate 3.

[0026] The electrode-biomass composite module area 4 is centrally provided with a perforated anode iron plate 7, and one perforated cathode carbon felt 5 is arranged on each side of the electrode-biomass composite module area 4; the anode and the cathode are both connected to a solar power supply system 10 through an insulating copper wire 9; a biomass supporting plate 6 is arranged between the perforated anode iron plate 7 and the perforated cathode carbon felt 5; and the biomass supporting plate 6 is paved with waste plant biomass 8.

[0027] The plant-biochar synergistic purification area 12 is filled with substrate gravel 13 and plant-derived biochar balls 14 in the lower part and planted with aquatic plants 11 in the upper part.

[0028] The perforated partition plate 3 has a hole diameter of 5-10 mm.

[0029] The perforated anode iron plate 7 and the perforated cathode carbon felt 5 have a thickness of 3-5 mm, a spacing of 5-7 cm and a hole diameter of 5-10 mm.

[0030] The iron anode electrolysis coupled with plant biomass can consume dissolved oxygen in the system through aerobic degradation of organic matter, delay the passivation of zero-valent iron surface, ensure the continuous and stable release of ferrous ions, further improve the degradation rate of plant cellulose, enhance the long-acting carbon release performance and biodegradability, and simultaneously strengthen the iron autotrophic denitrification and heterotrophic denitrification; and the iron ions and their hydrates produced can remove phosphate through adsorption, direct precipitation and co-precipitation. The carbon felt cathode can directly electrocatalyze the reduction of nitrate and produce hydrogen gas through electrolysis of water to strengthen the hydrogen autotrophic denitrification; and the huge surface area and rich pore structure can effectively adsorb most of the microplastics. The small electrode spacing is conducive to the in-situ and efficient use of hydrogen peroxide produced by oxygen reduction of the cathode and ferrous ions produced by iron oxidation of the anode, and the hydroxyl radicals produced through the electro-Fenton reaction can accelerate the aging degradation of microplastics and relieve the biological toxicity inhibition.

[0031] The solar power supply system 10 includes a solar panel, a charge controller, a storage battery, an inverter and a stabilized power supply, and the voltage of the solar power supply system 10 is 3-5 V.

[0032] The waste plant biomass 8 is washed, dried, crushed and screened to have a particle size of 5-10 mm, and a dosage of 1.0-2.0 g / L -1 The width of the biomass tray 6 is 3-5 cm.

[0033] The aquatic plant biomass has high carbon content and a large carbon-nitrogen ratio, and is a suitable natural cellulose-based solid-phase slow-release carbon source, which can effectively strengthen microbial denitrification.

[0034] The filling ratio of the matrix gravel 13 and the plant-derived biochar ball 14 is 3-4:1, the particle size is 2-3 cm, and the porosity is 0.4-0.5; the plant-derived biochar ball 14 is obtained by washing, drying and grinding waste plant biomass, carbonizing at 300-400 DEG C for 2-3 h under a nitrogen atmosphere, mixing with clay and bentonite to form a ball, and drying.

[0035] The plant-derived biochar ball can effectively remove residual iron ions in the water body through chemical adsorption, and the surface oxygen-containing functional groups can promote the activation and reduction of iron(hydro)oxides through charge and discharge cycles, further strengthening iron autotrophic denitrification. The mixture of gravel and plant-derived biochar ball is beneficial to the attachment growth of nitrifying bacteria and denitrifying bacteria.

[0036] The aquatic plant 11 is Acorus gramineus Soland, and the planting density is not less than 25 plants / m -2 .

[0037] The Acorus gramineus Soland has a reticular root system that can directly absorb nitrogen and phosphorus in the water body, and provide an attachment surface, photosynthetic oxygen and root exudates for nitrifying bacteria and denitrifying bacteria to strengthen microbial denitrification.

[0038] The hydraulic retention time of the composite subsurface constructed wetland is 2-3 d, and the water depth is 30-50 cm.

[0039] Example 1 A laboratory-scale horizontal subsurface flow constructed wetland device was constructed in the botanical garden of Shanghai Jiaotong University, which was composed of 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 a 10-mm-diameter hole. The water distribution area was 10 cm x 20 cm in size, the electrode-biomass composite module area was 10 cm x 20 cm in size, the plant-biochar synergistic purification area was 40 cm x 20 cm in size, and the water collection area was 10 cm x 20 cm in size, with a water depth of 30 cm. A perforated anode iron plate was placed in the center of the electrode-biomass composite module area, and one perforated cathode carbon felt was placed on each side. Each electrode was connected to a direct current stabilized power supply with an insulating copper wire, and the withered Acorus calamus stems and leaves were laid flat on the biomass support plate between the anode and cathode plates. The perforated anode iron plate and the perforated cathode carbon felt were 32 cm x 18 cm x 0.3 cm in size, with a 5-cm electrode spacing and a 10-mm hole diameter; the control voltage was constant at 5 V; the Acorus calamus residue was 1.5 g L -1 , with a size of 8 mm x 8 mm; and the biomass support plate was 18 cm x 3 cm x 0.5 cm in size. The plant-biochar synergistic purification area was filled with gravel and Acorus calamus-derived biochar balls with a particle size of 2-3 cm as a mixed substrate, with a filling ratio of 3:1 and a filling height of 32 cm, and four adult Acorus calamus plants were evenly planted.

[0040] The water to be treated was simulated sewage plant effluent, which was pumped into the water distribution area of the constructed wetland device by a peristaltic pump. The total nitrogen concentration was 14-15 mg L -1 , the nitrate nitrogen concentration was 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 , and the microplastic concentration was 500-1000 μg L -1 , with a size of 50-100 μm. The nominal retention time was 2 d, and the hydraulic loading rate was 150 L m -2 d -1 .

[0041] A total of four groups of horizontal subsurface flow constructed wetland systems were constructed, namely, a control group, an electrolysis group, a biomass group, and an electrolysis-biomass coupling group. As shown in Table 1, after continuous operation for 90 d under low temperature conditions of 5-10 ℃, the total nitrogen concentration of the effluent of the electrolysis-biomass coupling group was basically maintained at 6-8 mg L -1 , and the total phosphorus concentration was basically maintained at 0.00-0.02 mg L -1, microplastic removal rate was up to 99%. Compared with the control group, electrolysis group and biomass group, the nitrogen removal efficiency was increased by 46%, 19% and 42% respectively, the phosphorus removal efficiency was increased by 81%, 3% and 80% respectively, and the microplastic removal rate was increased by 14%, 4% and 9% respectively, which indicated that the iron anode low-voltage electrolysis coupled with plant biomass could effectively enhance the nitrogen-phosphorus-microplastic removal of subsurface flow constructed wetland at low temperature.

[0042] Table 1 Nitrogen and phosphorus removal efficiency of each system running continuously for 90 days

[0043] Finally, it should be pointed out that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described by referring to the preferred embodiments of the present application, it should be understood by those skilled in the art that various changes can be made in form and details without departing from the spirit and scope of the present application as defined by 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.

2. The composite subsurface flow constructed wetland according to claim 1, characterized in that, 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.

3. The composite subsurface flow constructed wetland according to claim 2, characterized in that, 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.

4. The composite subsurface flow constructed wetland according to claim 3, characterized in that, The perforated partition has a hole diameter of 5-10 mm.

5. The composite subsurface flow constructed wetland according to claim 4, characterized in that, 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.

6. The composite subsurface flow constructed wetland according to claim 5, characterized in that, 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.

7. The composite subsurface flow constructed wetland according to claim 6, characterized in that, 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.

8. The composite subsurface flow constructed wetland according to claim 7, characterized in that, 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.

9. The composite subsurface flow constructed wetland according to claim 8, characterized in that, The aquatic plant is yellow iris, and the planting density is no less than 25 plants per square meter. -2 .

10. The composite subsurface flow constructed wetland according to claim 9, 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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