Green low-carbon treatment method for chromium-containing wastewater

Abstract

The application provides a green, low-carbon and rapid chromium-containing wastewater treatment method, which is improved on the basis of traditional biogas residue and ferrous sulfate treatment of chromium-containing wastewater, introduces short-term micro-aeration technology, and produces unexpected results, which not only treats the chromium-containing wastewater in a green and low-carbon way, but also greatly speeds up the reaction progress. In the process, a certain thickness of biogas residue sediment layer is formed in the biochemical reaction tank, then ferrous sulfate is added, then chromium-containing wastewater and biogas liquid are added, and after standing for a period of time, micro-aeration is carried out, which accelerates the biochemical reduction of Cr(VI) in the chromium-containing wastewater, promotes the activity of microorganisms, and saves resources.

Description

Technical Field

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

[0002] Chromium is an important metal related to national economy and people's livelihood, and is widely used in military industry, electroplating, tanning and other industries. China is the world's largest producer of chromium salts. The production of chromium salts will discharge a large amount of chromium-containing waste liquid. In addition, the electroplating industry is a downstream industry of chromium-related industries and will also discharge a large amount of waste liquid.

[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. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a green, low-carbon, and rapid method for treating chromium-containing wastewater. It improves upon the traditional microbial treatment of chromium-containing wastewater by employing a short-time aeration method, which produces unexpected results. This method not only treats chromium-containing wastewater in a green and low-carbon manner but also significantly accelerates the reaction process while reducing the emission of sulfates and sulfides.

[0005] This application provides a green and low-carbon treatment technology for chromium-containing wastewater, comprising the following steps:

[0006] (1) Add biogas residue to the biochemical reaction tank. The biogas residue is required to settle freely to form a biogas residue sedimentation layer of a certain thickness. The thickness of the bottom biogas residue sedimentation layer is not less than 30cm. 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.

[0007] (2) Add an appropriate amount of ferrous sulfate granules to the biochemical reaction tank, dissolve them, and then add biogas slurry and chromium-containing wastewater. The ratio of Fe to Cr(VI) should be less than 3:1 to ensure that ferrous sulfate is completely oxidized to ferric iron by Cr(VI) and that there is Cr(VI) remaining. After mixing, the ratio of Cr(VI) to COD should be within a certain range. Then the mixture will undergo a biochemical reaction.

[0008] (3) After a period of biochemical reaction, a small amount of air is injected into the bottom of the biogas residue sedimentation layer through a blower and a hose to form short-term micro-aeration, which makes the bottom sedimentation biogas residue suspended for a short time and then settle down. The upper mixed liquid enters the bottom. During the air introduction process, after short-term micro-aeration, aeration is stopped and biochemical reaction is carried out again.

[0009] (4) Repeat step (3) several times until Cr(VI) reaches the standard, then discharge the supernatant;

[0010] (5) Then enter the next biochemical reaction cycle, add ferrous sulfate, fresh biogas slurry and chromium-containing wastewater, and repeat steps (2)-(4); (6) After running for a certain period of time, the bottom sludge will thicken to a certain extent. After the qualified supernatant is discharged, the surface sediment can be cleaned out, fresh biogas slurry can be added, and the bottom biogas slurry sediment layer can be made to ensure a certain thickness before the next cycle of reaction.

[0011] The biogas residue mentioned in step (1) can be replaced by sludge, by a microbial agent mainly prepared from biogas residue, or by chromium-reducing bacteria such as sulfate-reducing bacteria.

[0012] In step (2), Cr(VI):COD < 1:5.

[0013] In step (3), the biochemical reaction time before micro-aeration needs to be >8 hours to ensure sufficient biochemical reaction time to form sulfites and sulfides, and the micro-aeration time should not exceed 10 minutes.

[0014] In step (2), the biogas slurry can be replaced by other organic waste liquids.

[0015] In step (5), ferrous sulfate is replaced by sodium sulfite or sodium sulfate. After one cycle of ferrous sulfate addition, it is not necessary to add it again to reduce the amount of sediment. A certain amount of active sulfur will accumulate in the biogas residue sedimentation layer in each cycle. After a certain number of cycles, sulfate or sulfite can be added or only a small amount can be added.

[0016] The thickness of the new biogas residue sediment layer formed in step (6) shall not be less than 30 cm.

[0017] 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.

[0018] 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. In addition, the sludge sedimentation layer formed by this technology is relatively thick, creating an alternating oxidation-reduction environment. While treating Cr(VI), it also rapidly treats COD. Furthermore, due to the stable growth effect of microorganisms, Cr(VI) liquid can be continuously reduced without the need to add new biogas residue.

[0019] Furthermore, this invention first adds ferrous sulfate to the biochemical reaction tank, followed by the addition of a chromium-containing solution. The ferrous sulfate reacts chemically with Cr(VI) to form ferric-chromium oxide flocs that precipitate to the surface of the biogas sludge sediment layer, covering and protecting the reducing microorganisms within the sediment layer. Simultaneously, the formed ferric oxide flocs exhibit extremely high activity, accelerating the catalytic reduction of Cr(VI) by microorganisms. This avoids the need for expensive ferric oxide materials in process 201510005873.6, and creatively utilizes the waste generated after the chemical reaction of ferrous sulfate and Cr(VI).

[0020] In addition to active iron, the biogas residue sediment layer also forms a certain amount of active sulfur, as shown in the following reaction formula:

[0021] C2H4O (organic carbon source) + SO4 2- →SO3 2- +CO2 +H2O (Reaction at rest)

[0022] C2H4O (organic carbon source) + SO3 2- →S 2- +CO2 +H2O (Reaction at rest)

[0023] CrO4 2- +S 2- +H + →Cr 3+ After micro-aeration with +S+H2O

[0024] The formation of reactive sulfur can further catalyze microbial reduction, as shown in the following reaction formula:

[0025] C2H4O (organic carbon source) + S (active) → S 2- +CO2+H2O

[0026] CrO4 2- +S 2- +H + →Cr 3+ +S (active) +H2O

[0027] The accumulation of active sulfur can prevent the loss of system sulfates with the supernatant, thereby reducing sulfate addition and resource consumption. In addition, chromium oxides (chromium trioxide) formed after Cr(VI) reduction will accumulate on the surface of the sludge sediment layer. After several operating cycles, the surface sediment will be cleaned away, and this part of the solid waste will have a high chromium content, which is beneficial for resource recovery.

[0028] This technology is a novel microbial-biochemical synergistic treatment technology for chromium-containing wastewater, which has the following advantages compared to previous technologies:

[0029] (1) The reaction rate is higher than that of continuous stirring or continuous standing;

[0030] (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.

[0031] (3) After the reaction of ferrous sulfate, active iron and active sulfur are formed, which accelerates the biochemical reaction of microorganisms. At the same time, the active substances come from the waste after the reaction of ferrous sulfate, avoiding the addition of active substances and saving resources.

[0032] (4) The alternating operation of short-term micro-aeration and static setting consumes less energy and is green and low-carbon.

[0033] (5) 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.

[0034] (6) The sludge sedimentation layer formed by this technology is relatively thick, forming an alternating oxidation-reduction environment. While treating Cr(VI), it can also quickly treat COD. In addition, due to the stable growth effect of microorganisms, Cr(VI) liquid can be continuously reduced without the need to add new biogas residue.

[0035] (7) Through the anaerobic reaction of biogas residue and its reaction with Cr(VI), the high-cyclic organic matter in biogas slurry is broken down, which enhances the biodegradability of organic pollution and provides support for the next step of biogas slurry treatment. Detailed Implementation

[0036] The present application will be further described below with reference to specific embodiments.

[0037] Example 1

[0038] The Cr(VI) concentration in the chromium-containing wastewater is 1000 mg / L.

[0039] (1) Add biogas residue to the biochemical reaction tank. The biochemical reaction tank is 5 meters high. The biogas residue settles freely to form an 80cm biogas residue sedimentation layer. A small-power blower is set up outside. Multiple hoses are set up. One end of each hose is connected to the airflow outlet of the blower, and the other end is connected to different positions at the bottom of the biogas residue sedimentation layer. A small amount of air can be introduced into different positions at the bottom of the sedimentation layer through the blower.

[0040] (2) Add an appropriate amount of ferrous sulfate granules to the biochemical reaction tank, dissolve them, and then add biogas slurry and chromium-containing wastewater. The ratio of Fe to Cr(VI) should be 2:1. After mixing, the ratio of Cr(VI) to COD should be 1:6. Then the mixture will undergo a biochemical reaction.

[0041] (3) After 12 hours of biochemical reaction, a small amount of air is injected into the bottom of the biogas residue sedimentation layer through a blower and a hose to form short-term micro-aeration. The short-term micro-aeration time does not exceed 2 minutes, so that the bottom sediment biogas residue is suspended for a short time and then settles down. The upper mixed liquid enters the bottom. During the air introduction process, after short-term micro-aeration, aeration is stopped and biochemical reaction is carried out again.

[0042] (4) Repeat step (3) twice. After 8 hours of biochemical reaction, Cr(VI) was not detected, and then the supernatant was discharged.

[0043] (5) Then proceed to the next biochemical reaction cycle, add ferrous sulfate, fresh biogas slurry and chromium-containing wastewater, and repeat steps (2)-(4); (6) After running for 8 cycles, the bottom sludge will thicken to 100cm. After discharging the qualified supernatant, the 40cm sediment on the surface of the biogas residue sedimentation layer can be cleaned out and fresh biogas residue can be added to ensure that the bottom biogas residue sedimentation layer is not less than 60cm for the next cycle reaction.

[0044] Cr(VI) in chromium-containing wastewater is continuously reduced, while the biodegradability B / C ratio in the biogas slurry increases from 0.35 to 0.5.

[0045] Example 2

[0046] The Cr(VI) concentration in the chromium-containing wastewater is 200 mg / L.

[0047] (1) Add biogas residue to the biochemical reaction tank. The biochemical reaction tank is 2 meters high. The biogas residue settles freely to form a biogas residue sedimentation layer of not less than 30 cm. A small power blower is set up outside and multiple hoses are set up. One end of each hose is connected to the airflow outlet of the blower, and the other end is connected to different positions at the bottom of the biogas residue sedimentation layer. A small amount of air can be introduced into different positions at the bottom of the sedimentation layer through the blower.

[0048] (2) Add an appropriate amount of ferrous sulfate granules to the biochemical reaction tank, dissolve them, and then add biogas slurry and chromium-containing wastewater. The ratio of Fe to Cr(VI) should be 1:1. After mixing, the ratio of Cr(VI) to COD should be 1:15. Then the mixture will undergo a biochemical reaction.

[0049] (3) After 12 hours of biochemical reaction, a small amount of air is injected into the bottom of the biogas residue sedimentation layer through a blower and a hose to form short-term micro-aeration. The short-term micro-aeration time does not exceed 1 minute, so that the bottom sediment biogas residue is suspended for a short time and then settles down. The upper mixed liquid enters the bottom. During the air introduction process, after short-term micro-aeration, the aeration is stopped and the biochemical reaction is carried out again.

[0050] (4) After 8 hours of biochemical reaction, Cr(VI) was not detected, and the supernatant was then discharged.

[0051] (5) Then proceed to the next biochemical reaction cycle, add sodium sulfate, fresh biogas slurry and chromium-containing wastewater, and repeat steps (2)-(4);

[0052] (6) After 8 cycles of continuous operation, the bottom sludge will thicken to more than 50cm. After the qualified supernatant is discharged, the top 20cm of sediment in the biogas residue sedimentation layer can be cleaned out and fresh biogas residue can be added to ensure that the bottom biogas residue sedimentation layer is not less than 30cm for the next cycle of reaction.

[0053] Example 3

[0054] The Cr(VI) concentration in the chromium-containing wastewater is 200 mg / L.

[0055] (1) Add biogas residue to the biochemical reaction tank. The biochemical reaction tank is 2 meters high. The biogas residue settles freely to form a biogas residue sedimentation layer of not less than 30 cm. A small power blower is set up outside and multiple hoses are set up. One end of each hose is connected to the airflow outlet of the blower, and the other end is connected to different positions at the bottom of the biogas residue sedimentation layer. A small amount of air can be introduced into different positions at the bottom of the sedimentation layer through the blower.

[0056] (2) Add an appropriate amount of ferrous sulfate granules to the biochemical reaction tank, dissolve them, and then add biogas slurry and chromium-containing wastewater. The ratio of Fe to Cr(VI) should be 1:1. After mixing, the ratio of Cr(VI) to COD should be 1:10. Then the mixture will undergo a biochemical reaction.

[0057] (3) After 12 hours of biochemical reaction, a small amount of air is injected into the bottom of the biogas residue sedimentation layer through a blower and a hose to form short-term micro-aeration. The short-term micro-aeration time does not exceed 1 minute, so that the bottom sediment biogas residue is suspended for a short time and then settles down. The upper mixed liquid enters the bottom. During the air introduction process, after short-term micro-aeration, the aeration is stopped and the biochemical reaction is carried out again.

[0058] (4) Repeat step (3) once. After 8 hours of biochemical reaction, Cr(VI) was not detected. Then the supernatant was discharged.

[0059] (5) Then proceed to the next biochemical reaction cycle, add sodium sulfite, fresh biogas slurry and chromium-containing wastewater, and repeat steps (2)-(4); (6) After running for 12 cycles, the bottom sludge will thicken to more than 50cm. After discharging the qualified supernatant, the top 20cm of sediment on the surface of the biogas residue sedimentation layer can be cleaned out and fresh biogas residue can be added to ensure that the bottom biogas residue sedimentation layer is not less than 30cm for the next cycle reaction.

[0060] 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 green and low-carbon treatment method for chromium-containing wastewater, characterized in that, Includes the following steps: (1) Add biogas residue to the biochemical reaction tank. The biogas residue is required to settle freely to form a biogas residue sedimentation layer of not less than 30cm. The biochemical reaction tank is equipped with multiple micro-aeration systems. Each system is connected to the bottom of the biogas residue sedimentation layer through a hose. One end of the hose is a small blower, and the other end is connected to the bottom of the biogas residue sedimentation layer. A small amount of air is introduced into the bottom of the sedimentation layer through the blower. (2) Add an appropriate amount of ferrous sulfate granules to the biochemical reaction tank, dissolve them, and then add biogas slurry and chromium-containing wastewater. The ratio of Fe:Cr(VI) < 3:1 is required to ensure that ferrous sulfate is completely oxidized to ferric iron by Cr(VI) and that there is Cr(VI) remaining. After mixing, the mass ratio of Cr(VI) to COD is < 1:

5. Then the mixture is subjected to biochemical reaction. (3) After the biochemical reaction is >8 hours, a small amount of air is injected into the bottom of the biogas residue sedimentation layer through a blower and a hose to form short-term micro-aeration, so that the bottom sediment biogas residue is suspended for a short time and then settles down. The upper mixed liquid enters the bottom. During the air introduction process, after short-term micro-aeration, the aeration is stopped and the biochemical reaction is carried out again. The micro-aeration time does not exceed 10 minutes. (4) Repeat step (3) a certain number of times until Cr(VI) reaches the standard, and then discharge the supernatant; (5) Then proceed to the next biochemical reaction cycle, add ferrous sulfate, fresh biogas slurry and chromium-containing wastewater, and repeat steps (2)-(4). (6) After a certain period of continuous operation, when the bottom sludge thickens to a certain extent, the qualified supernatant is discharged, the surface sediment is cleaned out, fresh sludge is added, and the bottom sludge sedimentation layer is ensured to be no less than 30cm before the next cycle of reaction is carried out.

2. The green and low-carbon treatment method for chromium-containing wastewater according to claim 1, characterized in that, In step (5), ferrous sulfate is replaced by sodium sulfite or sodium sulfate. After one cycle of ferrous sulfate addition, it is not necessary to add more or add less to reduce the amount of sediment. Each cycle of biogas residue sedimentation layer will accumulate a certain amount of active sulfur. After a certain cycle of operation, sulfate or sulfite will no longer be added or will be added in small amounts.