A method for electrochemical treatment of sludge

Abstract

The present invention discloses a method for electrochemically treating sludge, comprising the steps of adding an additive to the sludge, mixing and allowing the mixture to stand before electrolysis, and then filtering to obtain dehydrated sludge; wherein the additive is a citric acid solution; during electrolysis, oxygen generated at the anode is mixed with air and then transported to the cathode for continuous electrolysis. The present invention utilizes the additive to chelate heavy metal ions and adjusts the pH of the sludge to maintain it at a weakly acidic state. Electrolysis is then used to simultaneously dehydrate the sludge and effectively remove heavy metals. The removal rates of copper, zinc, and chromium reached 65.83%, 92.83%, and 90.40%, respectively, improving sludge treatment efficiency and avoiding secondary pollution to the environment. The prepared dehydrated sludge meets the standards for landscaping mud and demonstrates good fertility in grass planting experiments.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge treatment, and particularly 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 production of sludge, a byproduct of sewage treatment, is also increasing accordingly. Sludge has a complex composition, a very high water content, and is difficult to dehydrate. It contains a large number of recalcitrant organic pollutants, harmful heavy metals, and pathogenic microorganisms, posing a serious threat to human survival, health, and development. Practice has proven that sludge resource utilization is an inevitable solution for sludge treatment. However, more than half of the heavy metals produced during sewage treatment are transferred to the sludge, and this heavy metal content in the sludge seriously hinders its resource utilization. When sludge with high heavy metal content is used in agriculture, it not only increases the heavy metal content in crops but also causes heavy metal contamination in the soil. When precipitation occurs, some heavy metals in the sludge and soil enter surface runoff and groundwater seepage, migrating with the water and causing secondary contamination of groundwater. Therefore, how to effectively treat heavy metals has become a crucial issue in sludge treatment and disposal.

[0003] Currently, heavy metal removal technologies are primarily categorized into chemical, adsorption, and leaching methods. Chemical methods are effective in treating electroplating wastewater, but they can lead to nutrient loss in the sludge. Adsorption methods are efficient, convenient, and economical, but they face challenges with heat resistance, stability, and selectivity. Furthermore, industrial wastewater may contain complex organic pollutants, which can interfere with the adsorbent's absorption of metal ions. Leaching technology effectively removes heavy metals from sludge and is effective in eliminating pathogens and controlling sludge odor. Leaching also has a positive effect on sludge dewaterability, offering broad application prospects. However, this method suffers from long retention times and can easily produce large amounts of highly concentrated heavy metal leachate. Electrochemical methods, a relatively mature subcategory of chemical methods, offer high selectivity for heavy metal ions, excellent treatment results, no secondary pollution, and the recyclable heavy metal precipitate formed. These technologies are increasingly popular due to their mature process, simple equipment, ease of operation, small footprint, and rapid processing speed.

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

[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 and stirring to obtain sludge to be treated; adding the sludge to be treated to a sludge tank, and adding citric acid solution to the cathode electrolyte storage tank, the cathode tank, and the anode tank; connecting the anode to the positive electrode of a DC power supply and the cathode to the negative electrode 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, thereby destroying sludge flocs and desorbing 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, when existing technologies are used to treat heavy metals in sludge, multiple reagents need to be added, and only the removal of heavy metals in the sludge can be achieved, and the removal method is relatively cumbersome. In addition, sludge generally has a high water content. If heavy metal removal and dehydration 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 response to the above technical problems, the present invention provides a method for electrochemical treatment of 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:

[0009] (1) Add additives to the sludge, mix well and let it stand to obtain mixed sludge;

[0010] (2) transporting the mixed sludge to an electrolyzer for electrolysis and filtration to obtain electrolytic sludge and dewater;

[0011] (3) Repeat step (2) until the water content of the dewatered sludge is less than 60%, thereby obtaining dewatered sludge.

[0012] 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.

[0013] Preferably, the additive in step (1) is a heavy metal chelating agent.

[0014] More preferably, the heavy metal chelating agent is citric acid.

[0015] Further preferably, the additive further includes an electrolyte; the electrolyte is sodium chloride.

[0016] Preferably, the pH of the mixed sludge in step (1) is 5.5-6.5, and the conductivity is 1000-5000 μS / cm.

[0017] 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.

[0018] 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 the mixture. Polytetrafluoroethylene is added at a mass ratio of 1:0.4-0.8 to the carbon powder, and ultrasonically vibrating the mixture until the carbon powder agglomerates. The obtained agglomerates are pressed on a 0.5 mm titanium mesh by hot pressing to obtain the cathode.

[0019] More preferably, the volume ratio of oxygen to air is 10-0.1:1.

[0020] More preferably, the porous carbon material has nano-scale pores and contains polytetrafluoroethylene, wherein the mass ratio of polytetrafluoroethylene to carbon powder is 0.1-0.8:1.

[0021] More preferably, the distance between the anode and cathode is 2-10 cm.

[0022] 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.

[0023] 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.

[0024] The beneficial effects of the present invention are:

[0025] 1. Adding citric acid to the sludge can not only effectively form stable chelates with heavy metal ions in the sludge, reduce their solubility, and promote the precipitation and separation of heavy metals, but also serve 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 their charged properties, preventing heavy metal ions from existing in the sludge in a free state and easily combining with alkaline substances to form precipitation, which is not conducive to their 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, affecting its working life.

[0026] 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 also provides chloride ions, thereby generating a strong oxidant, sodium hypochlorite, in the subsequent electrolysis process, which helps to oxidize and decompose extracellular polymers in the sludge and promote the migration and release of heavy metal ions.

[0027] 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 applied, 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) in 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.

[0028] 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. This simplifies the dehydration step and shortens the drying time. At the same time, since the sludge has undergone heavy metal removal treatment, secondary pollution caused by heavy metals is avoided during the drying process, which is environmentally friendly.

[0029] 5. The present invention simultaneously achieves sludge dehydration during the process of heavy metal removal without the need to add 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, reduces operational complexity, and provides a solid foundation for subsequent sludge treatment and resource recovery.

[0030] 6. Continuous treatment of sludge is achieved, 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 causing the sludge water content to be too low and thus block the pipeline, and also avoid too high flow rate causing the sludge to have insufficient residence time and reduce the treatment effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the electrolysis equipment used in the present invention. In the figure, 1 is a gas collection shell, 2 is an anode, 3 is sludge, 4 is a cathode, and 5 is a gas storage tank.

[0032] Figure 2 The aboveground growth of ryegrass in different substrates.

[0033] Figure 3 The above-ground growth of lawn grass in different substrates. DETAILED DESCRIPTION

[0034] The technical solutions of the present invention are further explained below with reference to 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 construed as limiting the present invention. The scope of protection of the present invention shall be determined by the claims. Any modifications or substitutions made to the technical solutions of the present invention by those skilled in the art without creative effort shall fall within the scope of protection of the present invention.

[0035] In the following examples, the municipal sludge is derived from a sewage treatment plant, has a pH of 7.68, a water content of 78.6%, a copper content of approximately 920 mg / kg, a zinc content of approximately 2300 mg / kg, a chromium content of approximately 840 mg / kg, and an electrical conductivity of 1200 μS / cm;

[0036] 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 approximately 970 mg / kg, a zinc content of approximately 2600 mg / kg, a chromium content of approximately 900 mg / kg, and an electrical conductivity of 2600 μS / cm.

[0037] Example 1

[0038] (1) Take 1 portion of municipal sludge, add sodium chloride solution to make its conductivity ≥2000μS / cm, then add citric acid solution (0.1mol / L) until the pH value of the municipal sludge is 6.0, stir and mix, and let it stand for 2h to obtain mixed sludge;

[0039] (2) The mixed sludge is transported to the electrolysis equipment at a feed rate of 5 m / h. 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 being passed 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 due to the introduction of gas), and the volume ratio of oxygen to air is 1:1.

[0040] (3) After 2 h 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%;

[0041] (4) The sludge is discharged from the electrolysis equipment and initially dried using the waste heat generated by 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.

[0042] Example 2

[0043] The method and steps are the same as those in Example 1, except that the dosage of the citric acid solution in step (1) is adjusted to make the pH value of the municipal sludge 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.

[0044] Example 3

[0045] The method and steps were the same as those in Example 1. The dosage of the citric acid solution in step (1) was adjusted to make the pH value of the municipal sludge 6.5. The feed rate in step (2) was changed to 10 m / h, and the voltage gradient was changed to 4 V / cm to prepare dehydrated sludge.

[0046] Example 4

[0047] 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.

[0048] Example 5

[0049] 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 dewatered sludge.

[0050] Example 6

[0051] The method and steps are the same as those in Example 1, except that the ventilation volume in step (2) is changed to self-breathing, that is, no gas is actively introduced, and oxygen in the air is consumed by the electrodes themselves to prepare dehydrated sludge.

[0052] Example 7

[0053] 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.

[0054] Example 8

[0055] (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 sludge conductivity to 3000 μS / cm, mix well and let it stand for 4 h to obtain mixed sludge;

[0056] (2)-(4): Same as Example 1, wherein the voltage gradient is 2 V / cm and the flow rate of the mixed gas is 3 L / min, and dewatered sludge is prepared.

[0057] Comparative Example 1

[0058] The method and steps are the same as those in Example 1, except that step (1) is omitted, i.e., the pH value is not adjusted using a citric acid solution, and electrolysis is performed directly to prepare dehydrated sludge.

[0059] Comparative Example 2

[0060] 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.

[0061] Comparative Example 3

[0062] 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.

[0063] Comparative Example 4

[0064] 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.

[0065] Comparative Example 5

[0066] 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.

[0067] Comparative Example 6

[0068] 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.

[0069] Comparative Example 7

[0070] 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.

[0071] Comparative Example 8

[0072] 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.

[0073] Comparative Example 9

[0074] 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.

[0075] Comparative Example 10

[0076] 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 the dewatered sludge.

[0077] Comparative Example 11

[0078] 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.

[0079] Results: The sludge prepared in the above examples and comparative examples was tested for its 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.

[0080] The dewatered sludge prepared in Example 1 and the sludge prepared in Comparative Example 9 were selectively mixed with yellow loam at a volume ratio of 1:1 as a matrix. Then, equal amounts of ryegrass and bermudagrass seeds were sown in the soil. The average length of the aboveground part within 20 days was determined by grouping as shown in Table 1:

[0081] Table 1 Proportions of grass seed planting substrates

[0082]

[0083] The above grouping can only illustrate the effects of different treatment methods on grass seed growth, but it does not explain the effects of changes in treatment conditions on grass seed growth. Therefore, it is recommended to supplement the effects of dewatered sludge prepared under different parameter conditions on grass seed growth. The supplementary content is as follows:

[0084] The dewatered sludge prepared in the above examples and comparative examples was mixed with yellow loam at a volume ratio of 1:1 as a matrix. Then, equal amounts of ryegrass and bermudagrass seeds were sown in the matrix. The germination rates after 7 days were calculated. The results are shown in Table 2.

[0085] Table 2 Physical and chemical properties of dewatered sludge and grass seed germination

[0086]

[0087] Results show that within the required operating conditions (voltage gradient, air intake volume, air and oxygen ratio), copper, zinc, and chromium can be effectively removed from sludge, while simultaneously dehydrating and drying the sludge. In grass planting experiments, well-treated sludge significantly promoted grass seed germination, demonstrating that this method minimizes the damage to the sludge's inherent nutrients during treatment. Once the sludge's heavy metal content and moisture content are within the specified ranges, the sludge can be used for non-agricultural applications (forestland, gardening, 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 it stand to obtain mixed sludge; (2) Electrolyzing and filtering the mixed sludge to obtain electrolytic sludge and dewatering; (3) Repeat step (2) until the water content of the dewatered sludge is less than 60%, thereby obtaining dewatered sludge; The additive in step (1) is a heavy metal chelating agent; the heavy metal chelating agent is citric acid; the additive also includes an electrolyte; the electrolyte is sodium chloride; In the electrolysis of 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; The porous carbon material has nano-scale pores and contains polytetrafluoroethylene.

2. 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.

3. The method for electrochemical treatment of sludge according to claim 1, characterized in that: The volume ratio of oxygen to air is 10-0.1:

1.

4. The method for electrochemical treatment of sludge according to claim 1, characterized in that: The mass ratio of polytetrafluoroethylene to carbon powder is 0.1-0.8:

1.

5. The method for electrochemical treatment of sludge according to claim 1, characterized in that: The distance between the anode and the cathode is 2-10 cm.

6. 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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