CN108911155A - A kind of hybrid constructed wetland system and its method for handling low-concentration heavy metal sewage - Google Patents

Abstract

The invention discloses a combined constructed wetland system. The system sequentially comprises a regulating water distribution pool, a vertical flow constructed wetland, a horizontal flow constructed wetland and a surface flow constructed wetland. The vertical flow constructed wetland is provided with two groups, and the two groups operate alternately. The invention also discloses a method for treating low-concentration heavy metal sewage by using the combined artificial wetland system. The invention realizes the synergistic removal of conventional pollutants and heavy metals, the treatment effect is stable and efficient, and the removal rates of COD, ammonia nitrogen, total nitrogen, total phosphorus, hexavalent chromium and total nickel reach 60%, 80%, 70%, 65%, 50%, respectively. % and over 50%, COD _Cr ≤20mg/L, NH ₃ ‑N≤1.0mg/L, TN≤1.0mg/L, TP≤0.2mg/L, hexavalent chromium≤0.05mg/L, total nickel≤0.02 mg/L. The operating cost is less than 0.2 yuan/ton, with flexible operation, convenient management, no secondary pollution, and remarkable environmental and social benefits.

Description

A combined constructed wetland system and its method for treating low-concentration heavy metal sewage

technical field

The invention belongs to the field of sewage treatment, and relates to a combined constructed wetland system and a method for treating low-concentration heavy metal sewage, in particular to a method for using a combined constructed wetland to treat tail water from a sewage station in an electroplating concentration area, which can efficiently reduce nitrogen and phosphorus in the tail water The low concentration of pollutants such as organic matter and heavy metals enables it to meet the Class III water standard of the "Surface Water Environmental Quality Standard" (GB3838-2002), especially suitable for industrial concentration areas with serious heavy metal pollution.

Background technique

The development of the industrial industry has promoted the rapid rise of my country's economic situation and also brought serious pollution problems. With the continuous increase of industrial sewage discharge, many cities in my country have serious water pollution, which has affected the daily life of urban residents. Restricting the further development of urban economy. Most of the sewage in industrial concentration areas contains a large amount of heavy metals. If it is not treated and discharged arbitrarily, it will definitely cause serious harm to the environment and human beings. However, conventional pollutants such as heavy metals and nitrogen and phosphorus in these sewages are more harmful to co-pollution, and if they are not handled properly, they will cause serious pollution problems. As the country pays more and more attention to environmental issues, the requirements for the water quality of the discharged water are becoming more and more stringent, and it is often necessary to meet the environmental quality standards of surface water before discharge. According to incomplete statistics, my country's annual sewage discharge amounts to tens of billions of tons, but only a very small part of industrial sewage can be effectively treated. Although many cities in my country are using traditional centralized sewage treatment systems to control sewage, traditional sewage treatment plants have high investment and construction costs, high energy consumption, and require a lot of manpower and material resources for operation and management. The constructed wetland method has gradually replaced some traditional sewage treatment technologies with its advantages of low energy consumption, low cost, and high efficiency. It is widely used in domestic sewage, agricultural wastewater, and ecological restoration of rivers and lakes.

Application No. 201610884361.6 discloses a constructed wetland system, including a plurality of vertical flow constructed wetland units; the vertical flow constructed wetland units include water inlet channels, water inlet distribution pipes, wetland beds, outlet bottom channels, outlet pipes and drainage channels; A sewage treatment method for a constructed wetland system, comprising collecting sewage in a water inlet channel, entering the wetland bed through the water inlet distribution pipe, and collecting the tail water purified by the wetland bed in the drainage channel, It is discharged through the outlet pipe; wherein, multiple vertical flow constructed wetland units adopt the method of alternating and intermittent water intake, and the cycle running time of each vertical flow constructed wetland unit is 4-6h, and the time between water intake and drying The duration ratio is less than 1:2. The constructed wetland system can only remove nitrogen and phosphorus pollutants in sewage, but cannot effectively remove heavy metals in industrially polluted areas, and is not suitable for industrially polluted areas in cities. Application No. 201710382037.9 discloses a method for efficiently removing heavy metals in farmland irrigation water by using plant ponds + constructed wetlands + adsorption ponds. It mainly selects fields that can use elevation differences to realize natural water inflow and outflow according to the distribution of farmland irrigation canals to form "plant ponds" +Constructed wetland+Absorptive pond" purification process, planting aquatic plants such as cattails and black algae in plant ponds and constructed wetlands, and filling the adsorption ponds with industrial and agricultural wastes such as straw and other strong adsorption capacity of heavy metals to purify heavy metal pollution in farmland irrigation water. The concentration standard of heavy metals treated by this method is not high, and the synergistic effect of the constructed wetland system itself to remove pollutants is not considered. Application No. 201711332295.2 relates to a method and device for preventing and controlling pollutants in the use of water resources in a watershed. The method includes setting up sequentially connected hydrolysis tanks, nano-aeration tanks, and vertical subsurface artificial wetlands, salvaging duckweed and algae in the watershed and crushing them in the hydrolysis tanks. , Acidification and digestion, the treated supernatant in the hydrolysis tank is input into the nano-aeration tank, the effluent of the nano-aeration tank is mixed with the basin water and input into the vertical subsurface flow artificial wetland, and the basin water that meets the irrigation requirements is obtained after treatment. In this method, operations such as salvaging duckweed, crushing, and aerating all need manpower to complete, and the investment cost and operating cost are high in the process of processing. Application number 201510178728.8 discloses a nitrogen and phosphorus removal artificial wetland method technology, which mainly includes stabilization ponds, subsurface flow wetlands, deep purification ponds and enhanced phosphorus removal units. The stabilization pond provides an anaerobic environment, and pre-treats the sewage to convert the macromolecular organic matter in the water body into a small molecular organic matter; the effluent from the stabilization pond enters the subsurface flow wetland, and the subsurface flow wetland mainly removes COD in the water body and performs nitrification. Ammonia nitrogen is converted into nitrate nitrogen; the effluent from the subsurface wetland enters the deep purification pond, which provides an anoxic environment, and the main function is to convert nitrate nitrogen into nitrogen gas to realize the removal of nitrogen in the water; The unit is equipped with calcite filler, and the sewage flows through the calcite, and the phosphate plasma in the water body reacts with calcium ions to form phosphate precipitation, which is finally removed. This constructed wetland method does not consider the size of the treatment capacity, and the waste of resources cannot be avoided by adjusting the operation of the wetland for changes in the treatment capacity. Application No. 201611093325.4 discloses a composite constructed wetland system for denitrification and phosphorus removal, including water inlet pipes, outlet pipes, and multi-stage composite constructed wetlands connected in series between the water inlet pipes and outlet pipes; each stage of composite artificial wetlands All wetlands include an aerobic biological pond located upstream and a constructed wetland located downstream, and each constructed wetland includes several constructed wetland units; in the constructed wetland of the first-level composite constructed wetland, each constructed wetland unit is connected in parallel, And they are all vertical subsurface flow constructed wetland units; in the constructed wetlands of composite constructed wetlands at other levels except the first level composite constructed wetland, the constructed wetland includes a group of constructed wetland units connected in series, and each group is connected in parallel ; The constructed wetland unit in each group includes both the horizontal subsurface flow constructed wetland unit and the surface flow constructed wetland unit. The constructed wetland method system is not equipped with backflow facilities, and the sewage that has not been treated up to the standard cannot be retreated, and the stable operation effect cannot be guaranteed.

Contents of the invention

Purpose of the invention: In order to solve the problem of synergistic and efficient removal of nitrogen, phosphorus and heavy metal pollutants in the tail water of the sewage treatment station in the electroplating concentration area, realize the standard treatment from Table 3 of the "Electroplating Pollutant Discharge Standard" (GB21900-2008) to the "Surface Water Environmental Quality Standard" (GB3838-2002) Class III water standard. The present invention obtains a constructed wetland system by combining different types of constructed wetlands to gradually process sewage, takes the control of dissolved oxygen and backflow as the core, strengthens the leading role of microbial degradation on the filter material, and assists in emergency physical and chemical treatment to finally achieve stable treatment, thereby avoiding damage to the sewage The surrounding water body is polluted and the water resource environment of our country is protected.

Technical solution: In order to achieve the above technical purpose, the technical solution of the present invention is as follows: a combined constructed wetland system, the combined constructed wetland system sequentially includes a combination of regulating water distribution pool, vertical flow constructed wetland, horizontal flow constructed wetland and surface flow constructed wetland As a result, the vertical flow constructed wetland is provided with two groups, and the two groups operate alternately.

Wherein, the combined constructed wetland system is provided with three return pumps, which are respectively installed in the regulating water distribution pool, in the horizontal flow constructed wetland and in the surface flow constructed wetland.

Wherein, a detection system and a physical and chemical emergency dosing device are installed in the regulating water distribution pool. Mainly detect COD _cr , NH ₃ -N, hexavalent chromium, total nickel, and TN in the water to verify whether the sewage water quality meets the designed influent and effluent indicators.

Adjust the detection system in the water distribution pool to detect the quality of influent water, and measure the dissolved oxygen of each wetland unit. When COD _Cr ≤ 50mg/L, NH ₃ -N ≤ 8mg/L, TN ≤ 6.0mg/L, TP ≤0.5mg/L, hexavalent chromium≤0.10mg/L, and total nickel≤0.10mg/L, the sewage enters the vertical flow artificial wetland through the water distribution system; The set return pump pumps the sewage into the sewage station for reprocessing.

The content of the present invention also includes a method for treating low-concentration heavy metal sewage using a combined constructed wetland system, including the following steps:

1) The low-concentration heavy metal sewage is lifted to the regulating water distribution pool through the conveying pipeline, and the detection system in the regulating water distribution pool is used to detect the quality of the influent water. When the pollutant discharge standard is exceeded, dosing and precipitation is carried out according to the concentration of heavy metals in the sewage; when the pollution is met When the waste discharge standard is met, the sewage enters the vertical flow artificial wetland through the water distribution system;

2) Under the condition of alternating aerobic and anaerobic conditions, the sewage in the vertical flow artificial wetland passes through the filtration, adsorption and absorption of the matrix and plant roots in the wetland, and the organic matter, suspended matter, nitrogen, phosphorus and heavy metals in the sewage The pollutants are initially degraded and adsorbed, and the sewage flows through the vertical flow artificial wetland and then enters the horizontal flow artificial wetland;

3) Under anoxic conditions, the sewage completes denitrification and adsorption of heavy metals in the horizontal flow constructed wetland, and then the sewage enters the surface flow constructed wetland;

4) Under aerobic conditions, the sewage pollutants are further purified in the surface flow constructed wetland. A detection system and a return pump are installed in the surface flow constructed wetland. Send it back to the regulating cloth pool for retreatment; when the water quality meets the design water quality, it will be discharged into the surrounding water body.

Among them, in order to enhance the water treatment effect and cooperate with the operation mode of the vertical flow constructed wetland, a return pump is installed in the horizontal flow constructed wetland. If the concentration discharge requirements of hexavalent chromium and total nickel are not met, the sewage is returned to the vertical flow constructed wetland.

Wherein, the calculation formula of the reflux flow of the reflux pump is: Q _reflux =α(C ₁ /C ₃ +C ₂ /C ₄ )*Q _{water inflow} , α is the reflux coefficient, C ₁ is the influent concentration of hexavalent chromium, C ₂ is the influent concentration of total nickel, C ₃ and C ₄ are the maximum concentrations of hexavalent chromium and total nickel, both of which are 0.10mg/L.

Among them, when the dissolved oxygen DO value in the sewage ranges from 2.0 to 3.5 mg/L, it belongs to an aerobic environment; when the vertical flow constructed wetland is continuously influent, the DO value is generally lower than 0 to 2.5 mg/L, which belongs to an aerobic and anaerobic alternating environment; Constructed wetlands with horizontal flow whose DO value ranges from 0.5 to 0.8 mg/L belong to anoxic environment; surface flow constructed wetlands whose DO value is higher than 2.5 to 3.5 mg/L belong to an aerobic environment.

Among them, in order to enhance the efficiency of the whole method in treating nitrogen, phosphorus and heavy metals and reduce energy consumption, the vertical flow artificial wetland is provided with two groups, and the two groups operate alternately, each group adopts intermittent water intake, and the vertical flow artificial wetland adopts PLC water distribution, The frequency of water distribution is 5 to 10 times per day, through the formation of aerobic and anaerobic alternate operations in the vertical flow artificial wetland, organic matter and ammonia nitrogen are removed under aerobic conditions; under anaerobic conditions, denitrification and heavy metals form highly insoluble Metal sulfides can remove organic matter and nitrogen and phosphorus first, and then remove heavy metals.

Wherein, the detection system in the regulating pool mainly detects COD _cr , NH ₃ -N, hexavalent chromium, total nickel, and TN in the water, and verifies whether the sewage water quality meets the designed inflow and outflow indicators.

Wherein, the influent water quality meets _COD Cr≤50mg/L, NH ₃ -N≤8mg/L, TN≤6.0mg/L, TP≤0.5mg/L, hexavalent chromium≤0.10mg/L, total nickel≤ 0.10mg/L; the effluent quality meets _COD Cr≤20mg/L, NH ₃ -N≤1.0mg/L, TN≤1.0mg/L, TN≤0.2mg/L, hexavalent chromium≤0.05mg/L, Total nickel≤0.02mg/L.

Among them, the low-concentration heavy metal sewage of the present invention can be, but not limited to, the tail water after the sewage is treated by the sewage station in the electroplating concentration area, and can be all sewage that meets the requirements of influent water quality. Specifically,

Step 1: The tail water is lifted to the regulating distribution pool through the tail water conveying pipeline of the sewage station in the electroplating concentration area. The detection system in the regulating distribution pool will detect the quality of the influent water. When it exceeds the "Electroplating Pollutant Discharge Standard" (GB21900-2008) When the tail water is the standard in Table 3, dosing and precipitation is carried out according to the concentration of heavy metals in the tail water; when the water quality meets the standard water quality in Table 3 of the "Electroplating Pollutant Discharge Standard" (GB21900-2008), then the sewage enters the vertical flow artificial wetland through the water distribution system. In addition, there is a physical and chemical emergency dosing device in the regulating pool, which can carry out emergency dosing treatment when the water quality fluctuates greatly and the treatment effect of the whole system is not good. The medicine added is nano MnO ₂ (particle size 40-70nm, relatively Surface area 20-40m ² /g, bulk density 0.6-0.8g/cm ³ , spherical) coagulation to remove trace heavy metals in surface water;

Step 2: Under the condition of alternating aerobic and anaerobic conditions, the sewage in the vertical flow artificial wetland, through the filtration, adsorption and absorption of the substrate and plant roots in the wetland, the organic matter, suspended matter, nitrogen, phosphorus and heavy metals in the sewage Pollutants are initially degraded and adsorbed. Sewage flows through the vertical flow artificial wetland and then enters the horizontal flow artificial wetland;

Step 3: Under anoxic conditions, denitrification and adsorption of heavy metals are completed in the horizontal flow artificial wetland. In order to enhance the treatment effect, in accordance with the operation mode of the vertical flow constructed wetland, a return pump is installed in the horizontal flow constructed wetland, and the return flow is according to the formula Q _{return flow} = α(C ₁ /C ₃ +C ₂ /C ₄ )*Q _influent ( α is the return coefficient, C ₁ is the influent concentration of hexavalent chromium; C ₂ is the influent concentration of total nickel; C ₃ and C ₄ are respectively the hexavalent chromium and total nickel in Table 3 of the "Electroplating Pollutant Discharge Standard" (GB21900-2008) The maximum concentration values are all 0.10mg/L. ) is determined, the return liquid flows back to the vertical flow constructed wetland, and the sewage enters the surface flow constructed wetland.

Step 4: Under aerobic conditions, the sewage is further purified and treated in the surface flow constructed wetland. A detection system and a return pump are installed in the surface flow artificial wetland. When the water quality exceeds the design water quality, the sewage is sent back to the regulating distribution pool for retreatment through the return pump; when the water quality meets the design water quality, it is discharged into the surrounding water body.

Specifically, in order to enhance the treatment effect, three backflow facilities are set up, namely in the regulating pool, in the horizontal flow constructed wetland and in the surface flow constructed wetland.

In the present invention, the water distribution pool is adjusted according to the concentration of heavy metals to carry out dosing and precipitation, vertical flow wetland anaerobic operation, horizontal flow wetland is mainly anoxic operation, surface flow wetland is mainly aerobic operation, and microorganisms and bacteria are strengthened under different dissolved oxygen conditions. The removal effect of plants on low-concentration heavy metals has formed a method of "physicochemical emergency + biology and ecology" to treat tail water.

Beneficial effect: compared with the prior art, the present invention has the following advantages:

(1) The control of dissolved oxygen in the sewage of the present invention is the core, realizes removing organic matter and nitrogen and phosphorus under aerobic conditions, and then realizes the removal of heavy metals under anaerobic conditions, and realizes the synergistic removal of organic matter, nitrogen, phosphorus and heavy metals. The removal rates of COD, ammonia nitrogen, total nitrogen, total phosphorus, hexavalent chromium and total nickel reach 60%, 80%, 70%, 65%, 50% and 50% respectively, COD _Cr ≤20mg/L, NH ₃ -N ≤1.0mg/L, TN≤1.0mg/L, TP≤0.2mg/L, hexavalent chromium≤0.05mg/L, total nickel≤0.02mg/L. Realize the standard treatment from Table 3 of "Electroplating Pollutant Discharge Standard" (GB21900-2008) to the Class III water standard of "Surface Water Environmental Quality Standard" (GB3838-2002).

(2) The present invention provides good environmental factors for the microorganisms on the filter material through precise control of the return flow rate, which is beneficial to the effective removal of pollutants by the combined constructed wetland.

(3) The operating cost of the present invention is lower than 0.2 yuan/ton, with flexible operation, convenient management, no secondary pollution, and remarkable environmental and social benefits.

Description of drawings

Fig. 1 is the combined constructed wetland system and its process flow chart of the present invention.

Detailed ways

The present invention is described in further detail below according to the accompanying drawings:

Example 1:

A combined constructed wetland treatment system, the combined constructed wetland system sequentially includes an adjustment pool, a vertical flow constructed wetland 1, a vertical flow constructed wetland 2, a horizontal flow constructed wetland and a surface flow constructed wetland, the vertical flow constructed wetland There are two groups, and the two groups run alternately. In order to enhance the treatment effect, three backflow pumps are set up. A return pump is provided in the regulating pool, a return pump is provided in the horizontal flow constructed wetland, and a return pump is provided in the surface flow constructed wetland. A physicochemical emergency dosing device is installed in the regulating distribution pool. A detection system is also installed in the surface flow artificial wetland to detect the water quality. A physicochemical emergency dosing device is installed in the adjusting distribution pool, and the medicine added is coagulation of nanometer MnO ₂ (particle size 40-70nm, specific surface area 20-40m ² /g, bulk density 0.6-0.8g/cm ³ , spherical) Remove trace heavy metals in surface water; the vertical flow artificial wetland adopts PLC water distribution, and the frequency of water distribution is 8 times per day. Reeds, cannas, calamus and other sewage-cleaning plants are planted on the filter beds of the above-mentioned constructed wetlands, and the planting density is 26 plants/m ² . And the bottom of the constructed wetland is laid with PE anti-seepage membrane, the thickness is not less than 1mm, and the weight of the geotextile is not less than 200g/m ² .

As shown in Figure 1, the combined constructed wetland treatment system proposed by the present invention is used to treat the tail water of the sewage station in the electroplating concentrated area, including the following steps:

Step 1: After the sewage is treated by the sewage station in the electroplating concentration area, the tail water is lifted to the regulating distribution pool through the transmission pipeline, and the detection system in the regulating distribution pool detects the quality of the influent water. -2008) Table 3 standard tail water, dosing and precipitation according to the heavy metal concentration in the tail water; when the water quality meets the standard water quality in Table 3 of the "Electroplating Pollutant Discharge Standard" (GB21900-2008), then the sewage enters the vertical flow through the water distribution system Constructed wetlands, in addition, the regulating pool is equipped with a physical and chemical emergency dosing device, which can be used for emergency dosing treatment when the water quality fluctuates greatly and the treatment effect of the whole system is not good;

Step 2: Under the condition of alternating aerobic and anaerobic conditions, the sewage in the vertical flow artificial wetland, through the filtration, adsorption and absorption of the substrate and plant roots in the wetland, the organic matter, suspended matter, nitrogen, phosphorus and heavy metals in the sewage Pollutants are initially degraded and adsorbed. Sewage flows through the vertical flow artificial wetland and then enters the horizontal flow artificial wetland;

Step 3: Under anoxic conditions, denitrification and adsorption of heavy metals are completed in the horizontal flow artificial wetland. In order to enhance the treatment effect, in accordance with the operation mode of the vertical flow constructed wetland, a return pump is installed in the horizontal flow constructed wetland, and the return flow is according to the formula Q _{return flow} = α(C ₁ /C ₃ +C ₂ /C ₄ )*Q _influent ( α is the return coefficient, C ₁ is the influent concentration of hexavalent chromium; C ₂ is the influent concentration of total nickel; C ₃ and C ₄ are respectively the hexavalent chromium and total nickel in Table 3 of the "Electroplating Pollutant Discharge Standard" (GB21900-2008) The maximum concentration values are all 0.10mg/L. ) is determined, the return liquid flows back to the vertical flow constructed wetland, and the sewage enters the surface flow constructed wetland.

Step 4: Under aerobic conditions, the sewage is further purified and treated in the surface flow constructed wetland. A detection system and a return pump are installed in the surface flow artificial wetland. When the water quality exceeds the design water quality, the sewage is sent back to the regulating distribution pool for retreatment through the return pump; when the water quality meets the design water quality, it is discharged into the surrounding water body.

Specifically, when in the actual sewage treatment process, the sewage treatment capacity is 1800m ³ /d, in order to save resources, the vertical flow constructed wetlands 1 and 2 that intermittently feed wastewater are operated alternately, and the vertical flow constructed wetland 1 is first opened and operated for 3 hours Close, the vertical flow constructed wetland 2 starts to run, and after 3 hours, close and then turn on the vertical flow constructed wetland 1, so as to alternately operate, realize the vertical flow wetland uses air to provide oxygen transfer during intermittent periods, and forms an aerobic and anaerobic alternating filter bed. After the sewage is treated by the treatment system of the sewage station, it is lifted by the pump to the regulating water distribution pool located in front of the wetland. The detection system in the regulating water distribution pool detects the quality of the influent water, and measures the dissolved oxygen of each wetland unit. When COD _Cr ≤50mg/L, NH ₃ -N≤8mg/L, TN≤6.0mg/L, TP≤0.5mg/L, hexavalent chromium≤0.10mg/L, total nickel≤0.10mg/L, the sewage passes through the water distribution The system enters the vertical flow constructed wetland. Otherwise, the sewage is transported back to the sewage station by the return pump for further treatment; the sewage flows through the vertical flow artificial wetland and then enters the horizontal _flow artificial wetland. (C ₁ /C ₃ +C ₂ /C ₄ )*Q _{Influent water} is transported back to the vertical flow constructed wetland for further treatment; the rest of the sewage enters the surface flow constructed wetland, and a detection system and a return pump are installed in the surface flow constructed wetland. When the water quality exceeds the "Surface Water Environmental Quality Standard" (GB3838-2002) Class III water standard, the sewage is sent back to the regulating cloth pool for retreatment through the return pump; when the water quality meets the "Surface Water Environmental Quality Standard" (GB3838-2002) Class III When the water standard is met, it is discharged into the surrounding water bodies. The treatment effects of the specific programs are shown in Table 1.

Table 1 Treatment effect of sewage through combined wetland

Example 2: The difference between this example and Example 1 is that the sewage treatment capacity is 3500m ³ /d, and the two sets of vertical flow artificial wetlands 1 and 2 operate alternately. Due to the increase in water volume, the alternating time is changed to 4 hours to increase the treatment Speed, in order to ensure a stable treatment effect, the return pump at the horizontal flow constructed _wetland transports the sewage back to the vertical flow constructed wetland according to Q _{return flow} = α(C ₁ /C ₃ +C ₂ /C ₄ )*Q deal with. Other steps and parameters are the same as in Example 1. The specific treatment effect is shown in Table 2.

Table 2 Treatment effect of sewage through combined wetland

In Examples 1 and 2 of the present invention, different types of wetlands are arranged reasonably, and aerobic, anaerobic, and anoxic environments are effectively formed, and the removal effect on heavy metals is remarkable. During the whole treatment process, there is no aeration device, and the pollutants are reasonably removed through the natural dissolved oxygen state of the sewage at different stages. The DO of the tail water of the sewage station: 2-3.5mg/L, belongs to an aerobic environment, and the COD and ammonia nitrogen in the sewage are removed first ; When the vertical flow constructed wetland is intermittently fed with water, DO: 0-2.5mg/L belongs to the alternate environment of aerobic and anaerobic, and denitrification is carried out in an anaerobic environment to remove phosphorus and adsorb heavy metals in the sewage; horizontal flow constructed wetland DO : 0.5-0.8mg/L belongs to anoxic environment, in this environment removes TN, TP and adsorbs heavy metals; surface flow artificial wetland DO: 2.5-3.5mg/L belongs to aerobic environment, further removes pollutants. The sewage containing low-concentration heavy metals is treated by the present invention, and the water quality is raised to the surface water Class III water standard, and can be directly discharged into the surrounding water body, and the pollution degree of the water body is reduced to an extremely low level. And in the whole process, according to the different concentrations of heavy metals, the amount of sewage that needs to be refluxed for retreatment can be precisely controlled and determined according to the formula, thus ensuring the stability of sewage treatment. The whole process is automated, the operation is simple and convenient, the operating cost is less than 0.2 yuan/ton, and the environmental and economic benefits are remarkable.

Claims (10)

1. A combined constructed wetland system, characterized in that, the combined constructed wetland system comprises in turn a regulating water distribution pool, a vertical flow constructed wetland, a horizontal flow constructed wetland and a surface flow constructed wetland, and the vertical flow constructed wetland is set There are two groups, and the two groups run alternately. 2. The combined constructed wetland system according to claim 1, characterized in that, the combined constructed wetland system is provided with three return pumps, which are respectively installed in the regulating pool, the horizontal flow constructed wetland and the surface flow constructed wetland. 3. The combined constructed wetland system according to claim 1 or 2, characterized in that a detection system and a physical and chemical emergency dosing device are installed in the regulating water distribution pool. 4. The method for treating low-concentration heavy metal sewage by using a combined constructed wetland system, is characterized in that it comprises the following steps: 1) The low-concentration heavy metal sewage is lifted to the regulating water distribution pool through the conveying pipeline, and the detection system in the regulating water distribution pool is used to detect the quality of the influent water. When the pollutant discharge standard is exceeded, dosing and precipitation is carried out according to the concentration of heavy metals in the sewage; when the pollution is met When the waste discharge standard is met, the sewage enters the vertical flow artificial wetland through the water distribution system; 2) Under the condition of alternating aerobic and anaerobic conditions, the sewage in the vertical flow artificial wetland passes through the filtration, adsorption and absorption of the matrix and plant roots in the wetland, and the organic matter, suspended matter, nitrogen, phosphorus and heavy metals in the sewage The pollutants are initially degraded and adsorbed, and the sewage flows through the vertical flow artificial wetland and then enters the horizontal flow artificial wetland; 3) Under anoxic conditions, the sewage completes denitrification and adsorption of heavy metals in the horizontal flow constructed wetland, and then the sewage enters the surface flow constructed wetland; 4) Under aerobic conditions, the sewage pollutants are further purified in the surface flow constructed wetland. A detection system and a return pump are installed in the surface flow constructed wetland. Send it back to the regulating cloth pool for retreatment; when the water quality meets the designed water quality, it will be discharged into the surrounding water body. 5. The method for treating low-concentration heavy metal sewage by using a combined constructed wetland system according to claim 4, characterized in that, in order to strengthen the water treatment effect, in conjunction with the operation mode of the vertical flow constructed wetland, a return pump is provided in the horizontal flow constructed wetland , if the concentration discharge requirements of hexavalent chromium and total nickel are not met, the sewage is returned to the vertical flow artificial wetland. 6. The method for treating low-concentration heavy metal sewage using a combined constructed wetland system according to claim 5, wherein the formula for calculating the return flow of the return pump is: Q _Return = α(C ₁ /C ₃ +C ₂ /C ₄ )*Q _{water intake} , α is the reflux coefficient, C ₁ is the influent concentration of hexavalent chromium, C ₂ is the influent concentration of total nickel, C ₃ and C ₄ are the maximum concentrations of hexavalent chromium and total nickel, both of which are 0.10mg/L. 7. The method for treating low-concentration heavy metal sewage using a combined constructed wetland system according to claim 4, characterized in that the entire treatment process does not require aeration treatment, when the dissolved oxygen DO value in the sewage ranges from 2.0 to 3.5mg/L It belongs to an aerobic environment, and removes COD and ammonia nitrogen in the sewage first; when the vertical flow artificial wetland is intermittently fed, the DO value is generally lower than 0-2.5mg/L. Remove phosphorus and adsorb heavy metals in sewage; the DO value of horizontal flow constructed wetlands ranges from 0.5 to 0.8mg/L, which belongs to anoxic environment, and remove TN, TP and adsorb heavy metals in this environment; the DO value of surface flow constructed wetlands is higher than 2.5 ~3.5mg/L is an aerobic environment, and finally further removes pollutants to make the sewage meet the discharge standard. 8. the method for utilizing combined constructed wetland system to process low-concentration heavy metal sewage according to claim 4, characterized in that, in order to strengthen the efficiency of the whole method in handling nitrogen, phosphorus and heavy metals and reduce energy consumption, the vertical flow constructed wetland is provided with Two groups, the two groups operate alternately, each group adopts intermittent water intake, and the vertical flow constructed wetland adopts PLC water distribution, the frequency of water distribution is 5 to 10 times per day, through the formation of aerobic and anaerobic alternate operation in the vertical flow constructed wetland, in Organic matter and ammonia nitrogen are removed under aerobic conditions; under anaerobic conditions, denitrification and heavy metals form highly insoluble metal sulfides, which realizes the removal of organic matter and nitrogen and phosphorus first, and then removes heavy metals. 9. The method for treating low-concentration heavy metal sewage by using a combined constructed wetland system according to claim 4, characterized in that, the detection system in the adjusting water distribution pool mainly detects COD _cr , NH ₃ -N, hexavalent chromium in water , total nickel, and TN are tested to verify whether the sewage water quality meets the designed influent and effluent indicators. 10. The method for treating low-concentration heavy metal sewage by using a combined constructed wetland system according to claim 9, characterized in that the influent water quality meets COD _Cr ≤ 50mg/L, NH ₃ -N ≤ 8mg/L, TN ≤ 6.0mg/L, TP≤0.5mg/L, hexavalent chromium≤0.10mg/L, total nickel≤0.10mg/L; the effluent quality meets _COD Cr≤20mg/L, NH ₃ -N≤1.0mg/L , TN≤1.0mg/L, TN≤0.2mg/L, hexavalent chromium≤0.05mg/L, total nickel≤0.02mg/L.