Method for electrochemically treating sludge

Abstract

The invention discloses an electrochemical sludge treatment method which comprises the following steps: adding an additive into sludge, uniformly mixing, standing, electrolyzing, and filtering to obtain dewatered sludge; wherein the additive is a citric acid solution, and oxygen generated by the anode is mixed with air during electrolysis and then conveyed to the cathode for continuous electrolysis. According to the method, the additive is used for chelating heavy metal ions, the pH value of the sludge is adjusted so that the sludge can be maintained to be faintly acid, then dehydration of the sludge and effective removal of heavy metal are synchronously achieved through electrolysis, the removal rates of copper, zinc and chromium reach 65.83%, 92.83% and 90.40% respectively, the sludge treatment efficiency is improved, secondary pollution to the environment is avoided, and the method is suitable for industrial production. And the prepared dewatered sludge meets the standard of landscaping sludge, and shows good fertility in a grass planting experiment.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge treatment, and in particular relates to a method for electrochemically treating sludge. Background Art

[0002] With the acceleration of urbanization, the discharge of domestic and industrial wastewater is increasing, and the output of sludge as a by-product of sewage treatment is also increasing accordingly. Sludge has complex composition, extremely high water content and is not easy to dehydrate. It contains a large number of difficult-to-degrade organic pollutants, harmful heavy metals and pathogenic microorganisms, which seriously threaten human survival, health and development. Practice has proved that sludge resource utilization is an inevitable way out for sludge treatment. However, more than half of the heavy metals in the sewage treatment process will be transferred to the sludge, and the heavy metals in the sludge seriously hinder the resource utilization of sludge. When sludge containing high heavy metals is used for agricultural purposes, it not only increases the heavy metal content in crops, but also causes heavy metal pollution in the soil. When there is precipitation, part of the heavy metals in the sludge and soil enter the surface runoff and underground seepage and migrate with the water flow, thereby causing secondary pollution to the groundwater. Therefore, how to effectively treat heavy metals has become a problem that must be solved in sludge treatment and disposal.

[0003] At present, heavy metal removal technologies are mainly divided into: chemical method, adsorption method and leaching method. The chemical method has a good effect in treating electroplating wastewater, but it will cause the loss of nutrients in the sludge; the adsorption method has the characteristics of high efficiency, convenience and economy, but there are some problems in heat resistance, stability and selectivity. In addition, industrial wastewater may contain complex organic pollutants, which will interfere with the absorption of metal ions by the adsorbent; leaching technology can effectively remove heavy metals in sludge, and it also has good effects in eliminating pathogens and controlling sludge odor. In addition, the leaching method also has a positive effect on the dewaterability of sludge and has a wide range of application prospects, but this method has a long retention time and is prone to produce a large amount of high-concentration heavy metal leachate. The electrochemical method belongs to a small category of the chemical method. As a relatively mature cleaning technology, it has high selectivity for heavy metal ions, good treatment effect, no secondary pollution, and the heavy metal precipitates formed can be recycled. It has mature technology, simple equipment, easy operation, small footprint, and fast processing speed, so it is becoming more and more popular.

[0004] CN104986929B discloses an electrochemical method for removing heavy metals from residual sludge. In the dry sludge containing heavy metals, 0.1 mol / L EDTA, citric acid, tartaric acid and EDTA sodium iron are added as repair agents respectively, ultrasonic oscillation is performed for 7 hours, and the pH value is adjusted to 2-4 with hydrochloric acid. Then, the method is placed in an electrochemical device with graphite as an electrode, the voltage is controlled to 3.5V, the plate spacing is 14cm, the electrochemical treatment is performed for 12 hours, and the method is dried and boiled to achieve a heavy metal removal rate of more than 90%. The method has the advantages of low power consumption, short electrolysis time, time saving, heavy metals in the residual sludge are all less than the limit value in a wide pH range, stable electrode voltage, reliable performance, etc.

[0005] CN114620911B discloses a method for electrochemical treatment of heavy metals in sludge, comprising the following steps: adding citric acid and hydrogen peroxide solution to the sludge, stirring to obtain sludge to be treated; adding the sludge to be treated to a sludge pool, and adding citric acid solution to the cathode electrolyte storage tank, the cathode pool and the anode pool; connecting the anode to the positive pole of a DC power supply and the cathode to the negative pole of a DC power supply, while starting a first peristaltic pump to perform an electrolytic reaction; utilizing citric acid and hydrogen peroxide to generate a Fenton reaction, destroying sludge flocs to desorb metal ions from the sludge floc structure to form sludge to be treated, thereby improving the removal rate of heavy metals in the sludge.

[0006] However, the existing technology needs to add a variety of reagents to treat heavy metals in sludge, and can only remove heavy metals in sludge, and the removal method is relatively cumbersome; in addition, sludge generally has a high water content. If heavy metal removal and dehydration and drying cannot be achieved at the same time, secondary pollution problems are likely to occur. Existing electrochemical methods still have problems such as low efficiency and high energy consumption in the removal of heavy metals in sludge, which is not conducive to large-scale application. Although some technologies can achieve the stabilization and harmless treatment of heavy metals, how to effectively recover these valuable metal resources while reducing the negative impact on the environment is still a challenge that current technologies need to overcome. Summary of the invention

[0007] In view of the above technical problems, the present invention provides a method for electrochemically treating sludge, which simultaneously achieves heavy metal removal and dehydration and drying of sludge, and has the advantages of high heavy metal removal rate and high sludge treatment efficiency.

[0008] In order to achieve the above object, the present invention provides a method for electrochemically treating sludge, comprising the following steps: (1) Add additives to the sludge, mix well and let stand to obtain mixed sludge; (2) transporting the mixed sludge to an electrolyzer for electrolysis and filtration to obtain electrolyzed sludge and dewater; (3) Repeat step (2) until the water content of the dewatered sludge is less than 60%, thereby obtaining dewatered sludge.

[0009] Preferably, when the heavy metal content in the removed water is less than 1 mg / kg, the removed water is recycled as an additive solvent; when the heavy metal content is ≥1 mg / kg, the removed water is electrolyzed and then filtered, the heavy metal ions are reduced to metal elements and separated and collected, and the remaining liquid is added with additives to prepare an additive solution for recycling.

[0010] Preferably, the additive in step (1) is a heavy metal chelating agent.

[0011] More preferably, the heavy metal chelating agent is citric acid.

[0012] Further preferably, the additive also includes an electrolyte; the electrolyte is sodium chloride.

[0013] Preferably, the mixed sludge in step (1) has a pH of 5.5-6.5 and an electrical conductivity of 1000-5000 μS / cm.

[0014] Preferably, in the electrolysis described in step (2), a ruthenium-iridium-coated titanium-based electrode is used as the anode, and a porous carbon material is used as the cathode; during the electrolysis, oxygen generated at the anode is collected and mixed with air before being passed into the cathode.

[0015] Further preferably, the cathode is prepared by boiling carbon powder in 10% hydrogen peroxide for 30-60 minutes, filtering and drying the carbon powder, dispersing the carbon powder in n-butanol and mixing evenly, adding polytetrafluoroethylene in a mass ratio of 1:0.4-0.8 to the carbon powder, and ultrasonically vibrating until the carbon powder agglomerates, and pressing the obtained agglomerates on a 0.5 mm titanium mesh by hot pressing to obtain Further preferably, the volume ratio of oxygen to air is 10-0.1:1.

[0016] More preferably, the porous carbon material has nanoscale pores and contains polytetrafluoroethylene, wherein the mass ratio of polytetrafluoroethylene to carbon powder is 0.1-0.8:1.

[0017] More preferably, the distance between the anode and the cathode is 2-10 cm.

[0018] Preferably, the voltage gradient during the electrolysis in step (2) is 0.5-4 V / cm, and the sludge feed flow rate is 1-10 m / h.

[0019] Preferably, the dewatered sludge can be directly applied as soil material for forest land, or can be mixed with other types of soil in proportion after being dried and dehydrated and then applied.

[0020] The beneficial effects of the present invention are: 1. Adding citric acid to the sludge can not only effectively form stable chelates with heavy metal ions in the sludge, reduce its solubility, and promote the precipitation and separation of heavy metals, but also can be used as a weak electrolyte to maintain the sludge in a weakly acidic environment, which is beneficial to the subsequent electrochemical treatment process. Specifically, citric acid can chelate with heavy metal ions as a chelating agent and maintain its charged properties, avoiding the existence of heavy metal ions in the sludge in a free state, which is easy to combine with alkaline substances to form precipitation and is not conducive to its removal; secondly, citric acid, as a weak acid, has a certain buffering capacity. Due to the large differences in the properties of sludge, the addition of citric acid can effectively maintain the pH value within an appropriate range (5.5-6.5). Too high a pH will precipitate heavy metal ions, and too low a pH will damage the electrode and affect its working life.

[0021] 2. When the conductivity of the sludge is too low, sodium chloride can be selectively added as an electrolyte to increase the conductivity of the sludge. While enhancing the conductivity of the sludge, it provides chloride ions, thereby generating a strong oxidant, sodium hypochlorite, in the subsequent electrolysis process, which helps to oxidize and decompose the extracellular polymers in the sludge and promote the migration and release of heavy metal ions.

[0022] 3. A ruthenium-iridium-coated titanium-based electrode is used as the anode, and an oxygen-depolarized electrode made of a porous carbon material containing PDFE is used as the cathode. After power is turned on, the anode produces chlorine-based oxidants and oxygen, and the cathode produces hydrogen peroxide. The chlorine-based oxidants and hydrogen peroxide act synergistically on the sludge, efficiently oxidizing and decomposing the extracellular polymers (EPS) of the sludge, thereby releasing heavy metals and improving the dewatering performance of the sludge. In addition, the released heavy metals and water migrate to the cathode, and then are discharged from the pores in the cathode and separated from the sludge, thereby achieving efficient removal of heavy metals in the sludge.

[0023] 4. During electrolysis, water molecules migrate from the anode to the cathode along with heavy metal ions and are discharged through the pores of the cathode, achieving initial dehydration of the sludge. At the same time, electrolysis generates heat, causing water to evaporate and further achieve dehydration. Finally, sludge with low water content is directly obtained, and then dehydrated sludge is obtained by drying, which simplifies the dehydration steps and shortens the drying time. At the same time, since the heavy metals have been removed, secondary pollution caused by heavy metals is avoided during the drying process, which is environmentally friendly.

[0024] 5. The present invention realizes sludge dehydration simultaneously in the process of heavy metal removal without adding complex additives, thus avoiding the impact on sludge performance and secondary pollution and reducing costs. At the same time, it has good heavy metal removal efficiency and reduces operational complexity, thus providing a solid foundation for subsequent sludge treatment and resource recovery.

[0025] 6. The continuous treatment of sludge is realized, and the feed flow rate of sludge is controlled at 1-10m / h to ensure the treatment effect of sludge, avoid too slow flow rate to make the water content of sludge too low and thus block the pipeline, and also avoid too high flow rate to make the sludge stay time insufficient and reduce the treatment effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The electrolysis equipment used in the present invention, in which 1 is a gas collecting shell, 2 is an anode, 3 is sludge, 4 is a cathode, and 5 is a gas storage tank.

[0027] Figure 2 The aboveground growth of ryegrass in different substrates.

[0028] Figure 3 The above-ground growth of lawn grass in different substrates. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be understood as limiting the present invention. The protection scope of the present invention shall be based on the contents recorded in the claims. Any modification or replacement made by those skilled in the art to the technical solution of the present invention without creative work shall fall within the protection scope of the present invention.

[0030] In the following examples, the municipal sludge is derived from a sewage plant, has a pH value of 7.68, a water content of 78.6%, a copper content of about 920 mg / kg, a zinc content of about 2300 mg / kg, a chromium content of about 840 mg / kg, and an electrical conductivity of 1200 μS / cm; The sludge of the industrial wastewater treatment station comes from a certain wastewater treatment station, has a pH value of 7.42, a water content of 67.4%, a copper content of about 970 mg / kg, a zinc content of about 2600 mg / kg, a chromium content of about 900 mg / kg, and an electrical conductivity of 2600 μS / cm.

[0031] Example 1 (1) Take 1 portion of municipal sludge, add sodium chloride solution to make its conductivity ≥ 2000 μS / cm, then add citric acid solution (0.1 mol / L) until the pH value of the municipal sludge is 6.0, stir and mix well, and let it stand for 2 hours to obtain mixed sludge; (2) The mixed sludge is transported to the electrolysis equipment at a feed rate of 5 m / h, and a gradient voltage of 1 V / cm is applied through a DC power supply. At the same time, the oxygen generated at the anode is collected and mixed with air before passing into the cathode; the anode is a titanium-based ruthenium-iridium plated electrode, and the cathode is a porous carbon material electrode containing PDFE (polytetrafluoroethylene); the flow rate of the mixed gas is 2 L / min (at this time, the surface of the porous carbon material does not cause bubbles to appear on its surface due to the introduction of gas), and the volume ratio of oxygen to air is 1:1; (3) After 2 hours of electrolysis, collect the sludge with a moisture content higher than 60%, and repeat steps (1) and (2) until the moisture content of the sludge is less than 50%; (4) The sludge is output from the electrolysis equipment and initially dried using the waste heat generated by the electrolysis. The sludge is then dried in an outdoor drying yard. When the sludge moisture content is less than 40%, the treatment is completed to obtain dehydrated sludge.

[0032] Example 2 The method and steps are the same as those in Example 1. The dosage of the citric acid solution in step (1) is adjusted so that the pH value of the municipal sludge is 5.5. The feed rate in step (2) is changed to 1 m / h and the voltage gradient is changed to 0.5 V / cm to prepare dehydrated sludge.

[0033] Example 3 The method and steps are the same as those in Example 1. The dosage of the citric acid solution in step (1) is adjusted so that the pH value of the municipal sludge is 6.5. The feed rate in step (2) is changed to 10 m / h, and the voltage gradient is changed to 4 V / cm to prepare dehydrated sludge.

[0034] Example 4 The method and steps are the same as those in Example 1, except that the volume ratio of oxygen to air in step (2) is changed to 0.1:1 to prepare dewatered sludge.

[0035] Example 5 The method and steps are the same as those in Example 1, except that the volume ratio of oxygen to air in step (2) is changed to 10:1 to prepare dehydrated sludge.

[0036] Example 6 The method and steps are the same as those in Example 1, except that the ventilation volume in step (2) is changed to a self-breathing form, that is, the gas is not actively introduced, and the oxygen in the air is consumed by the electrodes to prepare the dehydrated sludge.

[0037] Example 7 The method and steps are the same as those in Example 1, except that the ventilation volume in step (2) is increased to 3 L / min, so that a large number of bubbles are generated on the electrode surface due to the introduction of gas, thereby preparing dehydrated sludge.

[0038] Example 8 (1) Take one portion of heavy metal sludge from an industrial wastewater treatment station, add citric acid solution (0.05 mol / L) until the pH value of the municipal sludge is 6.5, then add 0.1 mol / L sodium chloride solution to adjust the conductivity of the sludge to 3000 μS / cm, mix well and let stand for 4 h to obtain mixed sludge; (2)-(4): Same as Example 1, wherein the voltage gradient is 2 V / cm, the flow rate of the mixed gas is 3 L / min, and dewatered sludge is prepared.

[0039] Comparative Example 1 The method and steps are the same as those of Example 1, except that step (1) is omitted, i.e., the pH value is not adjusted using a citric acid solution, and electrolysis is directly performed to prepare dehydrated sludge.

[0040] Comparative Example 2 The method and steps are the same as those in Example 1, except that the pH value in step (1) is adjusted to 5.0 to prepare dewatered sludge.

[0041] Comparative Example 3 The method and steps are the same as those in Example 1, except that the pH value in step (1) is adjusted to 7.0 to prepare dewatered sludge.

[0042] Comparative Example 4 The method and steps are the same as those in Example 1, except that the volume ratio of oxygen to air in step (2) is changed to 0.05:1 to prepare dewatered sludge.

[0043] Comparative Example 5 The method and steps are the same as those in Example 1, except that the volume ratio of oxygen to air in step (2) is changed to 11:1 to prepare dewatered sludge.

[0044] Comparative Example 6 The method and steps are the same as those in Example 1, except that in step (2), no gas is introduced into the cathode and the ventilation pipe is closed to prepare dewatered sludge.

[0045] Comparative Example 7 The method and steps are the same as those in Example 1, except that the ventilation rate in step (2) is changed to 5 L / min. At this time, a large number of bubbles are generated on the electrode surface due to the introduction of gas, and dehydrated sludge is prepared.

[0046] Comparative Example 8 The method and steps are the same as those in Example 1, except that the sludge after electrolysis for 2 hours in step (3) is directly output without repeated electrolysis to obtain dewatered sludge.

[0047] Comparative Example 9 The method and steps are the same as those in Example 1, except that the drying process in step (4) is omitted to obtain sludge.

[0048] Comparative Example 10 The method and steps are the same as those in Example 1, except that the sludge feeding speed in step (2) is changed to 0.5 m / h to prepare dewatered sludge.

[0049] Comparative Example 11 The method and steps are the same as those in Example 1, except that the sludge feeding speed in step (2) is changed to 12 m / h to prepare dewatered sludge.

[0050] Results: The sludge prepared in the above examples and comparative examples was tested for physical and chemical properties with reference to "CJ / T 221-2005 Testing Methods for Sludge from Municipal Wastewater Treatment Plants". The results are shown in Table 2. The dewatered sludge prepared in Example 1 and the sludge prepared in Comparative Example 9 were selectively mixed with yellow loam in a volume ratio of 1:1 as a matrix, and then equal amounts of ryegrass and Bermuda grass seeds were sown therein, respectively. The average length of the upper part of the ground within 20 days was specifically grouped as shown in Table 1: Table 1 Proportions of grass seed planting substrate

[0051] The above grouping can only explain the effect of different treatment methods on the growth of grass seeds, but it does not explain the effect of changes in treatment conditions on the growth of grass seeds. Therefore, it is recommended to supplement the effect of dewatered sludge prepared under different parameter conditions on the growth of grass seeds. The supplementary content is as follows: The dewatered sludge prepared in the above examples and comparative examples was mixed with yellow soil at a volume ratio of 1:1 as a matrix, and then equal amounts of ryegrass and bermudagrass seeds were sown therein, and the germination rates were counted after 7 days. The results are shown in Table 2 Table 2 Physical and chemical properties of dewatered sludge and grass seed germination

[0052] The results show that within the required operating range (voltage gradient, air intake, air and oxygen ratio, etc.), copper, zinc and chromium in sludge can be effectively removed, and sludge dehydration and drying can be achieved simultaneously. In the grass planting experiment, sludge with good treatment conditions has a significant promoting effect on the germination rate of grass seeds, which shows that this method has less damage to the original nutrients in the sludge during the treatment process. After the heavy metal content and moisture content of the sludge are treated to the range required by the relevant specifications, the sludge can be used for non-agricultural crop soil application (forest, garden, soil improvement, etc.).

Claims

1. A method for electrochemical treatment of sludge, characterized in that: The following steps are involved: (1) Add additives to the sludge, mix well and let stand to obtain mixed sludge; (2) Electrolyzing and filtering the mixed sludge to obtain electrolyzed sludge and dewatered water; (3) Repeat step (2) until the water content of the dewatered sludge is less than 60%, thereby obtaining dewatered sludge.

2. The method for electrochemical treatment of sludge according to claim 1, characterized in that: The additive in step (1) is a heavy metal chelating agent.

3. The method for electrochemical treatment of sludge according to claim 2, characterized in that: The heavy metal chelating agent is citric acid.

4. The method for electrochemical treatment of sludge according to claim 2, characterized in that: The additive also includes an electrolyte; the electrolyte is sodium chloride.

5. The method for electrochemical treatment of sludge according to claim 1, characterized in that: The pH of the mixed sludge in step (1) is 5.5-6.5, and the conductivity is 1000-5000 μS / cm.

6. The method for electrochemical treatment of sludge according to claim 1, characterized in that: In the electrolysis described in step (2), a ruthenium-iridium-coated titanium-based electrode is used as an anode, and a porous carbon material is used as a cathode; during the electrolysis, oxygen generated at the anode is collected and mixed with air before being passed into the cathode.

7. The method for electrochemical treatment of sludge according to claim 6, characterized in that: The volume ratio of oxygen to air is 10-0.1:

1.

8. The method for electrochemical treatment of sludge according to claim 6, characterized in that: The porous carbon material has nanometer-scale pores and contains polytetrafluoroethylene; wherein the mass ratio of polytetrafluoroethylene to carbon powder is 0.1-0.8:

1.

9. The method for electrochemical treatment of sludge according to claim 6, characterized in that: The distance between the anode and the cathode is 2-10 cm.

10. The method for electrochemical treatment of sludge according to claim 1, characterized in that: During the electrolysis in step (2), the voltage gradient is 0.5-4 V / cm, and the sludge feed flow rate is 1-10 m / h.

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