A method for low-carbon co-disposal of landfill leachate and chromium-containing wastewater

Abstract

The application provides a green and low-carbon synchronous treatment method for chromium-containing wastewater and landfill leachate, which comprises the following steps: firstly, carrying out anaerobic biochemical reaction on the mixed solution of the chromium-containing wastewater and the landfill leachate, introducing short-term micro-aeration technology in the process, accelerating the reduction of Cr(VI), and consuming COD and sulfate; then, entering the intermittent aeration process, completely reducing Cr(VI), basically consuming small-molecule organic matter, and greatly reducing ammonia nitrogen; then, carrying out nanofiltration, intercepting most of the high-molecular organic matter, filtering out the filtrate to carry out electrochemistry, collecting chlorine, and removing ammonia nitrogen through the breakpoint chlorination method formed in the electrolysis process.

Description

Technical Field

[0001] This invention belongs to the field of water treatment, and more specifically, relates to a green and low-carbon treatment technology for organic and heavy metal wastewater. Background Technology

[0002] Chromium is a vital metal essential to national welfare and people's livelihoods, widely used in military, electroplating, and tanning industries. China is the world's largest producer of chromium salts, and the production process generates large amounts of chromium-containing wastewater. Furthermore, the electroplating industry, a downstream sector of chromium-related industries, also discharges significant amounts of wastewater. During waste disposal, a large amount of leachate is generated, some from landfills and others from the bottom leachate accumulated during the pre-incineration settling process. Leachate contains a large amount of organic matter, as well as sulfates, ammonia nitrogen, and chloride ions (Cl).

[0003] The treatment approach for chromium-containing wastewater involves reducing the highly toxic Cr(VI) in the wastewater to Cr(III). The ferrous sulfate method is currently widely used, but it consumes excessive amounts of the chemical reagent ferrous sulfate, subsequently producing a large amount of flocculent precipitate. This precipitate is hazardous waste, significantly increasing the hazardous waste content. Furthermore, this method releases a large amount of sulfate into the water. Biological reduction methods have gained some attention due to their eco-friendly and low-carbon nature. Patent No. 201610508413.X describes a method for the co-treatment of Cr(VI) wastewater using sponge iron and microorganisms. This method utilizes sponge iron and sulfate-reducing bacteria to co-prepare a bacterial agent, which is then mixed with the chromium-containing wastewater under static conditions. This method requires a large dosage of bacterial agent. Patent No. 201510005873.6 describes a method for the co-treatment of chromium-containing wastewater using α-Fe₂O₃ and sulfate-reducing bacteria activated sludge. This method requires continuous stirring, consumes a large amount of energy, has a slow reaction time, and requires a large dosage of bacterial agent. Additionally, the supernatant ultimately produces a large amount of sulfides, which require further treatment. In fact, the literature "Study on the Synergistic Effect of Ferrous Sulfate and Biogas Sludge Co-treatment of Chromium-Containing Soil" found that stirring is not suitable for the microbial reduction of Cr(VI). While stirring creates a thorough mixture, it also exacerbates the toxicity of Cr(VI) to the biogas sludge inoculant, resulting in a poorer effect. At the same time, stirring can easily introduce air, which is not conducive to the formation of an anaerobic environment for microbial reduction.

[0004] Regarding landfill leachate, current technology primarily involves biological treatment followed by ultrafiltration and nanofiltration, which is costly and results in leachate concentrate that cannot be disposed of. Furthermore, while biological treatment significantly reduces organic matter, it leaves extremely high levels of ammonia nitrogen and chloride, making it difficult to treat. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a green and low-carbon method for the simultaneous treatment of landfill leachate and chromium-containing wastewater. Based on traditional leachate biochemical treatment, Cr(VI) biochemical treatment is introduced to rapidly treat organic pollution while further treating sulfate and ammonia nitrogen. Finally, Cl is removed through electrochemical action.

[0006] This application provides a method for the low-carbon co-treatment of landfill leachate and chromium-containing wastewater, comprising the following steps:

[0007] (1) Three wastewater reaction tanks are set up, namely reaction tank A, reaction tank B and reaction tank C; reaction tank A is an anaerobic biological reaction tank, reaction tank B is an aerobic biological reaction tank and reaction tank C is an electrochemical reaction tank.

[0008] (2) Add biogas residue to biochemical reaction tank A. The biogas residue is required to settle freely to form a biogas residue sedimentation layer of a certain thickness. A certain number of micro-aeration systems are set up in the biochemical reaction tank. Each micro-aeration system is connected by a hose. One end of the hose is connected to the air outlet of the blower, and the other end is connected to the bottom of the biogas residue sedimentation layer. A small amount of air can be introduced into the bottom of the sedimentation layer through the blower.

[0009] (3) An aeration system is installed at the bottom of reaction tank B, and a certain proportion of facultative and aerobic sludge is added.

[0010] (4) Mix the landfill leachate and chromium-containing wastewater in a certain proportion, and then let them enter the reaction tank A;

[0011] (5) When Cr(VI) is reduced to a certain concentration, the supernatant above the sludge sedimentation layer is fed into the reaction tank B. After the sludge sedimentation layer is left to stand for a period of time for biochemical reaction, short-term micro-aeration is required to react the sulfites and sulfides formed at the bottom with Cr(VI) and consume each other.

[0012] (6) After the supernatant enters the reaction tank B, ensure that the initial Cr(VI) is within a certain range, then aeration is carried out. After aeration for a certain period of time, aeration is stopped. After a certain period of time, aeration is carried out again, forming an alternation of aeration, settling and sedimentation. After the last settling, the supernatant is discharged.

[0013] (7) The supernatant is passed through ultrafiltration and nanofiltration in sequence and then enters reaction tank C for electrochemical treatment. During the process, Cl2 is collected at the anode and ammonia nitrogen is removed by breakpoint chlorination.

[0014] (8) After the above system has been running for a certain period of time, the sludge at the bottom of reaction tank A will thicken to a certain extent. The surface sediment can be cleaned out and fresh sludge can be added to ensure that the bottom sediment layer has a certain thickness before proceeding to the next stage of reaction. At the same time, the sludge in reaction tank B will be discharged appropriately.

[0015] The biogas residue mentioned in step (2) can be replaced by sludge, by a microbial agent mainly prepared from biogas residue, or by chromium-reducing bacteria such as sulfate-reducing bacteria.

[0016] In step (3), the mass ratio of facultative sludge to aerobic sludge is (1-2):(1-2), ensuring that the sludge accumulation volume after sedimentation accounts for 1 / 4 to 1 / 2 of the entire pool.

[0017] In step (4), after the landfill leachate is mixed with chromium-containing wastewater, the mass ratio of Cr(VI) to COD in the mixture is required to be in the range of 1:15 to 1:30.

[0018] In step (5), the biogas residue sedimentation layer is left to stand for 8-24 hours for biochemical treatment. Short-term micro-aeration is required, and the short-term micro-aeration time should be controlled within 10 minutes.

[0019] In step (6), the initial Cr(VI) concentration in reaction tank B is in the range of 10-25 mg / L. If it is insufficient, chromium-containing wastewater can be directly added. The mass ratio of Cr(VI) to COD is <1:10. During the alternating interval between aeration and settling, the aeration time is controlled at 1-12 h, the settling time is controlled at 1-16 h, and the total residence time in reaction tank B is controlled at 24-96 h.

[0020] In step (6), the nanofiltration membrane pore size is controlled at 10-20 nm to ensure that ammonia nitrogen, Cl and a small amount of small molecule organic matter can pass through, while large molecule organic matter is retained. The concentrated liquid formed by the nanofiltration process can partially replace the landfill leachate in step (4).

[0021] The thickness of the new biogas residue sediment layer formed after the sediment is discharged in step (8) shall not be less than 30cm.

[0022] Traditional theory holds that stirring facilitates mass transfer, leading to closer contact between microorganisms and Cr(VI) in biogas residue, thus enhancing toxicity. Furthermore, stirring is energy-intensive. Therefore, it is generally believed that stirring should be avoided during the microbial reduction of Cr(VI), maintaining a certain degree of heterogeneity. This invention combines micro-aeration mixing with static settling, alternating between the two. Experiments have shown that this method is significantly more effective than static settling alone, and even more effective than continuous stirring.

[0023] The core technology of this process is the formation of a thick biogas sludge sediment layer. After the addition of chromium-containing wastewater, the sediment layer gradually transforms from an oxidizing to a reducing environment from top to bottom. The bottom of the biogas sludge sediment layer is inherently a strongly reducing environment, making it difficult for Cr(VI) liquid to penetrate. Simultaneously, sulfates are reduced to sulfites or sulfides, leading to their accumulation. When the accumulation of sulfites or sulfides reaches a certain level, they inhibit the reduction of chromium at the bottom. At this point, micro-aeration is implemented, allowing the Cr(VI)-containing liquid to penetrate to the bottom, reacting with and consuming the sulfites or sulfides. This eliminates the microbial inhibition effect, reduces Cr(VI), and ensures that Cr(VI) does not become toxic to the microorganisms at the bottom. During the rising aeration gas process, some sulfites and sulfides are also carried upwards, further consuming Cr(VI). This creates a continuous, stable, and highly efficient Cr(VI) reduction effect.

[0024] In addition, a certain amount of active sulfur will be formed in the biogas residue sedimentation layer in reaction tank A, as shown in the following reaction formula:

[0025] C2H4O (organic carbon source) + SO4 2- →SO3 2- +CO2 +H2O (Reaction at rest)

[0026] C2H4O (organic carbon source) + SO3 2- →S 2- +CO2 +H2O (Reaction at rest)

[0027] CrO4 2- +S 2- +H + →Cr 3+ After micro-aeration with +S+H2O

[0028] The formation of reactive sulfur can further catalyze microbial reduction, as shown in the following reaction formula:

[0029] C2H4O (organic carbon source) + S (active) → S 2- +CO2+H2O

[0030] CrO4 2- +S 2- +H + →Cr 3+ +S (active) +H2O

[0031] The presence of a certain initial low concentration of Cr(VI) in reaction tank B is beneficial to the oxidation activity of the sludge in the tank. The reduction to Cr(III) is also a good trace element for microorganisms. At the same time, there is no need for reaction tank A to completely reduce Cr(VI), thus saving reaction time.

[0032] After intermittent aeration in the reaction tank, Cr(VI) is completely reduced, COD is significantly consumed, and small-molecule organic matter is basically consumed, leaving some ammonia nitrogen and Cl. The Cl concentration is too low to be worth recovering, making it suitable for discharge, but it needs to meet standards. After ultrafiltration and nanofiltration, only ammonia nitrogen and Cl remain in the wastewater. Cl2 is collected and ammonia nitrogen is removed simultaneously by electrochemical methods. Because large-molecule organic matter is retained by the nanofiltration membrane, the generation of toxic organochlorines during electrochemical electrolysis is avoided. Small-molecule organic matter is basically consumed during the intermittent aeration reaction in reaction tank B, and even if there is an electrolysis process, it is easily and completely oxidized, making it difficult to form organochlorines.

[0033] This technology is a co-treatment technology for landfill leachate and chromium-containing wastewater. It achieves a synergistic effect through biochemical and electrochemical processes, and has the following advantages compared to previous technologies:

[0034] (1) The reaction rate is higher than that of continuous stirring or continuous standing;

[0035] (2) By micro-aeration, the generated sulfites and sulfides react with Cr(VI) in time, which accelerates the reduction of Cr(VI) and avoids the accumulation of sulfides to produce hydrogen sulfide and form a foul odor.

[0036] (3) The alternating operation of short-term micro-aeration and static setting consumes less energy and is green and low-carbon.

[0037] (4) The alternating operation of short-term micro-aeration and static setting, the supernatant formed after static setting does not carry away the sludge, avoiding the sludge return process, saving process and saving cost.

[0038] (5) The presence of an initial low concentration of Cr(VI) in reaction tank B reduces the residence time in reaction tank A and promotes the microbial activity in reaction tank B.

[0039] (6) The alternating operation of aeration and settling in reaction tank B basically consumes small-molecule organic matter, while large-molecule organic matter is retained by nanofiltration and enters the concentrate. This makes it less likely for toxic and harmful organochlorides to be formed in the subsequent electrolytic dechlorination process.

[0040] (7) The presence of appropriate Cr(VI) itself is conducive to the decomposition of high molecular weight organic matter in landfill leachate into low molecular weight organic matter;

[0041] (8) Regarding the treatment of Cr(VI), most of the Cr(VI) is first reduced by anaerobic process, and the remaining low concentration of Cr(VI) is used as an active promoter of aerobic process. Anaerobic and aerobic processes are carried out in tandem to promote the co-treatment of Cr(VI) and organic pollution.

[0042] (9) In terms of organic pollution removal, the biodegradability of organic pollution in landfill leachate is improved by the oxidative destruction of Cr(VI) in conjunction with anaerobic hydrolysis, providing favorable conditions for subsequent aerobic treatment. Attached Figure Description

[0043] Figure 1 This is a process diagram of the present invention. Detailed Implementation

[0044] The present application will be further described below with reference to specific embodiments.

[0045] Example 1

[0046] The chromium-containing wastewater has a Cr(VI) concentration of 1000 mg / L, the landfill leachate has a COD content of 80000 mg / L, a sulfate content of 4000 mg / L, a Cl content of 3000 mg / L, and a Kjeldahl nitrogen content of 4000 mg / L.

[0047] (1) Three wastewater reaction tanks are set up, namely reaction tank A, reaction tank B and reaction tank C; reaction tank A is an anaerobic biological reaction tank, reaction tank B is an aerobic biological reaction tank and reaction tank C is an electrochemical reaction tank.

[0048] (2) Add biogas residue to biochemical reaction tank A. The biogas residue is required to settle freely to form a biogas residue sedimentation layer of a certain thickness. The thickness of the biogas residue sedimentation tank is >80cm. The accumulation volume accounts for one-third of the reaction tank A. A certain number of micro-aeration systems are set up in the biochemical reaction tank. Each micro-aeration system is connected by a hose. One end of the hose is connected to the air outlet of the blower, and the other end is connected to the bottom of the biogas residue sedimentation layer. A small amount of air can be introduced into the bottom of the sedimentation layer through the blower.

[0049] (3) An aeration system is installed at the bottom of reaction tank B, and facultative and aerobic sludge is added in a mass ratio of 1:1. The total volume of the free-settling sludge sedimentation layer accounts for 1 / 3 of the tank volume, and the total height of the sedimentation tank is 4 meters.

[0050] (4) The landfill leachate and chromium-containing wastewater are mixed at a volume ratio of 1:4 and then introduced into reaction tank A. During the process, COD, sulfate, Kjeldahl nitrogen and other substances in the leachate are diluted.

[0051] (5) After staying in reaction tank A for 36 hours, the supernatant from the top of the biogas residue sedimentation layer is fed into reaction tank B. After the biogas residue sedimentation layer is left to stand for 8 hours for biochemical reaction, short-term micro-aeration is required. The short-term micro-aeration time is 3 minutes. The sulfites and sulfides formed at the bottom react with Cr(VI) and consume each other, generating active S that remains in the biogas residue sedimentation layer, further reducing the sulfate content. The sulfate removal rate of the supernatant is 50%, and the COD removal rate is 60%.

[0052] (6) After the supernatant enters the reaction tank B, chromium-containing wastewater is added to make the initial Cr(VI) 50mg / L. Then aeration is carried out. After aeration for 2 hours, aeration is stopped. After standing for 2 hours, aeration is carried out again for 2 hours. The aeration, standing and sedimentation are alternated for 2 hours each. After a total retention time of 48 hours, the supernatant is discharged.

[0053] (7) The supernatant is passed through ultrafiltration and nanofiltration in sequence and then enters reaction tank C for electrochemical treatment. During the process, Cl2 is collected at the anode and ammonia nitrogen is removed by breakpoint chlorination.

[0054] (8) After the above system has been running continuously for 30 days, when the sludge at the bottom of reaction tank A has thickened to a certain extent, the surface sediment can be cleaned out and fresh sludge can be added. After ensuring that the bottom sediment layer is >80cm, the next stage of reaction can be carried out. At the same time, the sludge in reaction tank B is discharged appropriately.

[0055] The effluent contained less than 300 mg / L Cl, less than 500 mg / L sulfate, less than 50 mg / L ammonia nitrogen, less than 100 mg / L COD, and no Cr(VI) was detected, thus fully meeting the effluent standards.

[0056] Example 2

[0057] The chromium-containing wastewater has a Cr(VI) concentration of 500 mg / L, the landfill leachate has a COD content of 80,000 mg / L, a sulfate content of 4,000 mg / L, a Cl content of 3,000 mg / L, and a Kjeldahl nitrogen content of 4,000 mg / L.

[0058] (1) Three wastewater reaction tanks are set up, namely reaction tank A, reaction tank B and reaction tank C; reaction tank A is an anaerobic biological reaction tank, reaction tank B is an aerobic biological reaction tank and reaction tank C is an electrochemical reaction tank.

[0059] (2) Add biogas residue to biochemical reaction tank A. The biogas residue is required to settle freely to form a biogas residue sedimentation layer of a certain thickness. The thickness of the biogas residue sedimentation tank is >80cm, and the accumulation volume accounts for one-quarter of the reaction tank A. A certain number of micro-aeration systems are set up in the biochemical reaction tank. Each micro-aeration system is connected by a hose. One end of the hose is connected to the air outlet of the blower, and the other end is connected to the bottom of the biogas residue sedimentation layer. A small amount of air can be introduced into the bottom of the sedimentation layer through the blower.

[0060] (3) An aeration system is installed at the bottom of reaction tank B, and facultative and aerobic sludge is added in a mass ratio of 1:1. The total volume of the free-settling sludge sedimentation layer accounts for 1 / 4 of the tank volume, and the total height of the sedimentation tank is 4 meters.

[0061] (4) The landfill leachate and chromium-containing wastewater are mixed at a volume ratio of 1:9 and then introduced into reaction tank A. During the process, COD, sulfate, Kjeldahl nitrogen and other substances in the leachate are diluted. After one reaction cycle, 10% of the landfill leachate is replaced by the subsequent nanofiltration concentrate.

[0062] (5) After 36 hours in reaction tank A, Cr(VI) was reduced to 10 mg / L. The supernatant from the top of the biogas residue sedimentation layer was then introduced into reaction tank B. After 12 hours of static biochemical reaction in the biogas residue sedimentation layer, short-term micro-aeration was required. The short-term micro-aeration time was 2 minutes. The sulfites and sulfides formed at the bottom reacted with Cr(VI) and consumed each other, generating active S which remained in the biogas residue sedimentation layer, further reducing the sulfate content. The sulfate removal rate of the supernatant was 50%, and the COD removal rate was 60%.

[0063] (6) After the supernatant enters the reaction tank B, chromium-containing wastewater is added to make the initial Cr(VI) 50mg / L. Then aeration is carried out. After aeration for 4 hours, aeration is stopped, and after standing for 2 hours, aeration is carried out again for 4 hours. The aeration, standing and sedimentation are alternated. During the process, aeration is carried out for 4 hours and standing for 2 hours. After a total retention time of 36 hours, the supernatant is discharged.

[0064] (7) The supernatant is passed through ultrafiltration and nanofiltration in sequence and then enters the reaction tank C for electrochemical treatment. During the process, Cl2 is collected at the anode and ammonia nitrogen is removed by breakpoint chlorination.

[0065] (8) After the above system has been running continuously for 30 days, when the sludge at the bottom of reaction tank A has thickened to a certain extent, the surface sediment can be cleaned out and fresh sludge can be added. After ensuring that the bottom sediment layer is >80cm, the next stage of reaction can be carried out. At the same time, the sludge in reaction tank B is discharged appropriately.

[0066] The effluent contained less than 200 mg / L Cl, less than 400 mg / L sulfate, less than 30 mg / L ammonia nitrogen, less than 100 mg / L COD, and no Cr(VI) was detected, thus fully meeting the effluent standards.

[0067] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions claimed by the present invention.

Claims

1. A method for low-carbon co-treatment of landfill leachate and chromium-containing wastewater, characterized in that, Comprising the following steps: (1) Set up three sewage reaction pools, respectively, reaction pool A, reaction pool B and reaction pool C; reaction pool A is an anaerobic biochemical reaction pool, reaction pool B is an aerobic biochemical reaction pool, and reaction pool C is an electrochemical reaction pool; (2) The biogas residue is added to the biochemical reaction pool A, and the biogas residue is required to be freely settled to form a certain thickness of biogas residue sediment layer, and a certain number of micro-aeration systems are arranged in the biochemical reaction pool, each micro-aeration system is connected by a hose, one end of the hose is connected to the air outlet of the air blower, and the other end is connected to the bottom of the biogas residue sediment layer, and a small amount of air is introduced into the bottom of the sediment layer by the air blower; (3) An aeration system is arranged at the bottom of the reaction pool B, and a certain proportion of facultative and aerobic sludge is added; (4) The landfill leachate and chromium-containing wastewater are mixed in a certain proportion, and then enter the reaction pool A; (5) When Cr(VI) is reduced to a certain concentration, the supernatant at the upper part of the biogas residue sediment layer is input into the reaction pool B, and after the biogas residue sediment layer is statically biochemically reacted for 8-24h, short-term micro-aeration is required, the sulfite and sulfide formed at the bottom react with Cr(VI) to consume each other, and the short-term micro-aeration time is controlled to be less than 10min; (6) After the supernatant enters the reaction pool B, the initial Cr(VI) is ensured to be in the range of 10-25mg / L, and if it is insufficient, the chromium-containing wastewater is directly supplemented, the mass ratio of Cr(VI) to COD is less than 1:10, and then aeration is carried out, after a certain time of aeration, the aeration is stopped, after a certain time, the aeration is carried out again, and the aeration and static precipitation are alternated; during the interval of aeration and static precipitation, the aeration time is controlled to be 1-12h, the static precipitation time is controlled to be 1-16h, and the total residence time of the reaction pool B is controlled to be 24-96h; after the last static precipitation, the supernatant is discharged; (7) The supernatant is sequentially subjected to ultrafiltration and nanofiltration, and then enters the reaction pool C for electrochemical treatment, in the process, Cl2 is collected by the anode, and ammonia nitrogen is removed by breakpoint chlorination method; (8) After the above system is continuously operated for a certain period, the sludge at the bottom of the reaction pool A is thickened to a certain extent, the surface layer of the biogas residue is cleaned out, fresh biogas residue is supplemented, and after the biogas residue sediment layer at the bottom is ensured to have a certain thickness, the next stage reaction is carried out, and at the same time, the sludge in the reaction pool B is appropriately discharged.

2. The method according to claim 1, characterized in that, The biogas residue in step (2) is replaced by sludge, sulfate-reducing bacteria or a bacteria agent mainly prepared from biogas residue.

3. The method according to claim 1, characterized in that, In step (3), the mass ratio of facultative sludge to aerobic sludge is (1-2):(1-2), and the volume of the sludge accumulation after sedimentation accounts for 1 / 4-1 / 2 of the entire pool.

4. The method for low-carbon co-treatment of landfill leachate and chromium-containing wastewater according to claim 1, characterized in that, In step (4), after the landfill leachate and chromium-containing wastewater are mixed, the mass ratio of Cr(VI) to COD in the mixed solution is in the range of 1:15 to 1:

30.

5. The method according to claim 1, characterized in that, The nanofiltration membrane pore size is controlled to be 10-20nm, so as to ensure that ammonia nitrogen, Cl and a small amount of small molecule organic matter can penetrate, and large molecule organic matter is intercepted, and the concentrated liquid formed by the nanofiltration process partially replaces the landfill leachate in step (4).

6. The method for low-carbon co-treatment of landfill leachate and chromium-containing wastewater according to claim 1, characterized in that, The thickness of the new biogas residue sediment layer formed after the sedimentation in step (8) is not less than 30cm.

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