A method for removing heavy metals from dehydrated activated sludge based on density difference centrifugation

Abstract

The present invention discloses a method for removing heavy metals from dehydrated activated sludge by density differential centrifugation, comprising the following steps: Step 1: adding a heavy metal adsorbent to the dehydrated sludge, stirring the mixture thoroughly, and allowing the mixture to stand to obtain conditioned dehydrated sludge; Step 2: preparing centrifuges of different densities using a gradient centrifugation agent to obtain a gradient centrifuge; Step 3: adding the gradient centrifuge and the dehydrated activated sludge to a centrifuge, collecting the adsorbent in the lower layer enriched with heavy metals, collecting the gradient centrifuge in the middle layer containing dissolved heavy metal ions, and finally collecting the floating sludge in the upper layer free of heavy metals; Step 4: removing heavy metals from the heavy metal adsorbent in the lower layer by acid leaching and drying; Step 5: recycling the gradient centrifuge in the middle layer; and Step 6: adjusting the pH of the sludge in the upper layer to neutral and conditioning the sludge by aerobic or anaerobic composting to make it suitable for use as fertilizer. The present invention improves the removal rate of heavy metals and is time-efficient.

Description

Technical Field

[0001] The invention belongs to the technical field of sludge treatment, and in particular is a method for removing heavy metals from dehydrated activated sludge by centrifugation based on density difference. Background Art

[0002] In recent years, with the depletion of fossil energy, the intensification of the greenhouse effect, increasingly stringent national environmental protection policies and regulations, and the international community's focus on reducing carbon emissions, the academic community in environmental engineering and science has embraced the extraction and recovery of organic matter and nutrients from wastewater and biomass generated by daily life and production, and the environmental protection community has gradually followed suit. Municipal sludge and some industrial organic wastewater treatment contain significant amounts of nitrogen, phosphorus, and potassium, as well as trace metals and organic matter essential for plant and soil ecosystems. Proper stabilization of this sludge, such as aerobic or anaerobic composting, followed by sterilization and removal of harmful substances, ultimately produces organic fertilizer for use as high-quality fertilizer in municipal green belts and agriculture, a promising path for the future of sludge / biomass resource utilization. However, the treatment and disposal of municipal sludge remains a global challenge, primarily due to high sludge treatment costs and the inability to effectively remove some organic and inorganic contaminants, which hinders the sludge's potential for fertilizer utilization.

[0003] The "Technical Guidelines for Treatment and Disposal of Sludge from Urban Wastewater Treatment Plants (Trial)" jointly issued by my country's Ministry of Housing and Urban-Rural Development and the National Development and Reform Commission in 2022 mentioned that from the perspective of carbon emissions, anaerobic digestion and land utilization are the only ways among many sludge treatment methods that can achieve negative carbon emissions. However, the sludge produced in my country's sewage treatment plants and some industrial organic wastewater treatment processes has long been characterized by low organic components and high sediment content, making anaerobic digestion impossible to apply on a large scale. In addition, due to the high mineral content in the sewage received by a large number of sewage treatment plants and the fact that some sewage treatment plants accept industrial and corporate drainage, a considerable proportion of heavy metals are present in the sludge produced after sewage treatment. Therefore, although the sludge produced by biochemical processes has the potential to be used as high-quality organic fertilizer, the heavy metals in the sludge have become an important constraint hindering the resource utilization and land utilization of sludge.

[0004] Activated sludge is an aggregate of bacteria, protozoa, and fungi, as well as macromolecular polymers secreted outside the cells by microorganisms. It exists in the form of particles / flocculation in the reaction tanks that rely on the activated sludge process. Due to the special composition of activated sludge, the bacterial cells and these activated sludge particles often carry negative charges on their outer surfaces under biochemical treatment conditions (the deprotonation of carboxyl groups, hydroxyl groups, etc. results in a negative surface zeta potential), and can capture dissolved heavy metal ions in water through ion exchange, chelation, surface micro-area deposition, etc. Therefore, except for a small amount of heavy metals that enter the cells, most of the heavy metals in the sewage are transferred from the aqueous phase to the solid phase by the activated sludge through biological adsorption.

[0005] At present, the common methods for reducing heavy metals in sludge in domestic and foreign literature include electrokinetics, supercritical fluid extraction, chemical reagents, plant extraction, ion exchange technology, advanced oxidation and bioleaching. However, only the electrokinetic method has entered the industrial pilot stage, and most of the other methods are still in the laboratory research stage. Now we briefly introduce the above methods for reducing heavy metals in sludge as follows: ① Chemical reagent method, generally refers to chemical leaching method, which generally uses a pH of 1.5 to 3 eluent to seep through heavy metal-contaminated sludge under gravity or head pressure, and replaces metal cations by protons to remove various forms of heavy metals and collect sludge. In addition to using nitric acid, sulfuric acid, phosphoric acid and citric acid as eluents, metal compounds such as iron sulfate can also be used to achieve the effect of reducing sludge pH. Studies have shown that adding 1.5g Fe / L to dewatered sludge (98% water content) can reduce the sludge pH to 2.5, and after 120 hours at room temperature, 81%, 89% and 86% of Cu, Zn and Cd are removed, respectively. See [1] for details. Because eluents (such as EDTA, citric acid, dilute sulfuric acid, and dilute phosphoric acid) are generally not selective for cations, some Na, Ca, and K ions may be removed during the elution process. Furthermore, the elution process can easily lead to uneven heavy metal removal rates in sludge of different depths, and the addition of large amounts of eluent can easily increase the salt content of the sludge. Therefore, finding a suitable, selective, and biodegradable eluent and adjusting and optimizing the elution method are key challenges in this method. ② Plant extraction methods are not discussed in detail here due to their long treatment cycle and low efficiency. ③ Ion exchange methods are difficult to use alone and generally need to be combined with elution methods, using strong cation exchange resins to recover heavy metals from the eluent. When ion exchange resins are directly added to the sludge, they often only recover free, adsorbed, and exchangeable heavy metal ions, which is costly. Directly added ion exchange resins are difficult to recover, and there is a risk of heavy metal ion desorption.④ Biological leaching is a relatively mild and effective method for removing heavy metals from sludge. It uses the acid produced by the oxidation of iron and sulfur by acidophilic and chemoautotrophic microorganisms to elute heavy metals from sludge. On the one hand, in direct sulfur-based biological leaching, microorganisms can oxidize sulfur in sulfides, converting insoluble sulfides into dissolved states, thereby removing heavy metal cations; on the other hand, in indirect sulfur-based biological leaching, reduced sulfides need to be added. After sulfur-oxidizing microorganisms convert them into sulfuric acid, they dissolve and remove heavy metals from the sludge. This step is similar to chemical leaching. Furthermore, iron-based biological leaching involves a series of complex chemical reactions. Generally, ammonium ferrous sulfate or ferrous sulfate needs to be added as nutrients for microorganisms. Under aerobic conditions, the ferrous reagent is oxidized by microorganisms to ferric compounds. The ferric compounds can oxidize insoluble metal sulfides in the sludge and thus be reduced to ferrous compounds. The generated sulfuric acid can oxidize part of the metal sulfides under aerobic conditions, and the metal cations in the metal sulfides dissolved by these two reactions can enter the aqueous phase and be leached and removed. Although bioleaching can remove many heavy metal ions in sludge, the general removal rate ranges from 2 to 65% for Pb, 58 to 78% for Cd, 22 to 91% for Cu, 60 to 95% for Ni, 64 to 99% for Zn, and 10 to 84% for Cr. For details, see [2], [3], [4], and [5]. Compared with chemical leaching, the biggest disadvantage of bioleaching is that it takes a long time, because there are usually residual fatty acids in the sludge, such as acetic acid and propionic acid, which will inhibit the oxidation of iron and sulfur by autotrophic microorganisms. The general leaching cycle is at least 5 to 14 days. In addition, bioleaching is relatively limited in removing residual heavy metals, and mainly removes heavy metals in oxidizable states (organically bound and sulfide) and reducible states (iron and manganese oxide bound and parts that are easily reduced under anoxic conditions). ⑤ Electrokinetic method, which generally uses a small direct current to cause heavy metal ions (usually cations) in the sludge to migrate to the cathode to remove heavy metals in the sludge. Because the electrode process will cause the pH in the cathode area to rise, resulting in the precipitation of metal ions, this method is often combined with biological leaching, or a chelating agent is added to the system to promote the migration and removal of heavy metal cations. Pei Dongdong et al. found that when the amount of EDTA solution added was 0.06 times the volume of the dewatered sludge to be treated and the applied voltage was 18V, after 120 hours of electrokinetic method, 87% of Zn, 68% of Cu and 58% of Pb could be removed. See [6] for details; Wang Jingyuan et al. used the electrokinetic method to treat dewatered sludge after anaerobic digestion. After adjusting the sludge pH with dilute nitric acid, 95% of Zn, 90% of Ni, 68% of Cr, 96% of Cu and 19% of Pb were removed at a voltage gradient of 1.25V / cm. See [7] for details.Domestic research on electrokinetics started early, with some patents generated at the beginning of this century. Yang Changming et al. found that when polyepoxysuccinic acid was used as a chelating agent, the voltage gradient was 1.2-1.3 V / cm, and the electrolyte pH was maintained at 5-8, the removal rates of Pb, Cd, Cu, Ni, Zn, and Cr could reach 59.3%, 43.7%, 38.7%, 47.8%, 55.4%, and 41.7% for sludge (water content 80-85%) that had been acidified with dilute nitric acid for 7-14 days. See [8] for details. In newer electrokinetic studies, cation exchange membranes are often used to isolate sludge and cathode. Heavy metal ions can be selectively removed through the ion exchange membrane, reducing the difficulty of cathode treatment. Existing pilot results show that the removal rate of carbonate-bound and exchangeable heavy metals in sludge by electrokinetics is higher than 70%, but the removal rate of organic-bound and sulfide heavy metals is lower than 35%, and the overall effect is not ideal. See [9] for details.

[0006] The heavy metal removal technologies mentioned above will inevitably cause the loss of nitrogen, phosphorus and potassium nutrients in the process of removing heavy metals. Studies have shown that in the biological leaching with the participation of ferrous sulfate, the loss of nitrogen and phosphorus is as high as 39% and 45% in 16 days of treatment, while the biological leaching with the participation of ferrous disulfide can effectively slow down the loss of the two elements, with only 15% of nitrogen and 6% of phosphorus lost in the same period of time, see

[10] ; Wang Jing et al. used sulfur-oxidizing bacteria to conduct biological leaching tests on air-dried dewatered sludge and found that after 14 days of leaching, the losses of nitrogen, phosphorus and potassium were 7.02%, 40.14% and 12.78% respectively, see [5]; He Miaomiao et al. applied iron-based and sulfur-based biological leaching to the pre-digested sludge of Hangzhou Sibao Wastewater Treatment Plant, and the heavy metals Zn and Cu had a relatively good removal rate, but about 25% of nitrogen was lost in this process, and the phosphorus loss in the iron-based leaching process exceeded 60%, and the phosphorus loss in the sulfur-based leaching process was about 50%, see

[11] for details.

[0007] Early studies have shown that freshly dewatered activated sludge contains fewer heavy metals in oxidizable forms (organically bound, sulfide, etc.) than sludge after anaerobic or aerobic digestion. Based on the literature review above, a large proportion of heavy metals in dewatered activated sludge exist in exchangeable, carbonate-bound, or simply adsorbed forms, suggesting the potential for selective heavy metal adsorption using adsorbents.

[0008] In my country, representative patents for the treatment of heavy metals in sludge include: "CN201921479642.9 A system for treating heavy metals in sludge", "CN201810098308.2 Sludge heavy metal, fertilizer and energy recovery system and method", "CN201310703822.1 Preparation method and application of sludge heavy metal extractant", "CN201811257504.6 A preparation method of sewage treatment sludge heavy metal stabilizer" and "CN202010759288.6 A sludge However, the technical solutions involved have the following problems: ① The method of using microbial fuel cells is still in the experimental stage. Because the anode microorganisms are easily disturbed, the potential difference between the cathode and anode is unstable, and the migration of heavy metal cations in the medium is difficult to control. Because it is based on the principle of electrodynamics, it is time-consuming and inefficient. Microbial fuel cells cannot provide sufficient voltage to accelerate ion migration. In addition, actual electrodynamic investigations have shown that the migration between the anode and anode is mostly macro-anions and cations, and the cations are mainly sodium and potassium. ② The method of using sulfate-reducing bacteria, although well-conceived, is difficult to operate in practice because the organic matter content in sludge is high, but it takes days for sulfate-reducing bacteria to utilize the sludge organic matter. In addition, the generated sulfur ions are mostly combined with macro-heavy metals such as iron in the sludge and precipitated, and cannot completely combine with trace amounts of toxic heavy metals. Furthermore, this process also requires the recovery of methane produced by anaerobic digestion. Therefore, the amount of sulfate added to the front-end sulfate-reducing bacteria process is limited. Both sulfate and sulfur ions inhibit the activity of methanogenic archaea, requiring a certain number of methanogenic archaea to be maintained in the anaerobic digestion unit. Iron depletion in the front-end also inhibits the activity of some enzymes involved in methanogenesis. Furthermore, the process involves equipment such as ejectors for sludge fragmentation, hydrolyzers or ultrasonic crushers for thermal hydrolysis, and three-phase separators for gas-liquid-solid separation, all of which are energy-intensive or difficult to operate. This complicates the process and makes costs difficult to control. (3) The extractant method requires the introduction of large amounts of chemicals, including polyaluminum chloride, polyacrylamide, ethylenediaminetetraacetic acid, and sodium dodecylsulfonate, into the sludge, increasing costs and hindering its subsequent use as fertilizer. (4) Based on the formation of insoluble metal phosphides, this method involves the addition of numerous organic and inorganic chemicals and natural stone materials. It represents a heavy metal solidification or stabilization process, rather than a heavy metal removal process. ⑤ The core heavy metal removal steps are flocculation and adsorption, which rely on inert anode electrolysis coupled with flocculation and reaction with quicklime to remove heavy metals. The technical operability and principle are questionable.

[0009] In summary, the existing sludge heavy metal treatment methods have the following three obvious defects: (1) The use of reagents to react with heavy metals in the sludge to precipitate, or the addition of materials with stronger adsorption capacity for heavy metals to achieve stabilization of heavy metals, cannot separate heavy metals from sludge; (2) It involves drastic adjustments to the physical and chemical properties of the sludge, especially the acidity and alkalinity, which requires the input of large amounts of acid and alkali, or the application of voltage at both ends of the sludge, which requires the introduction of electrolyte to increase the heavy metal removal rate. These operations increase the cost of treatment and the risk of secondary pollution; (3) Although the steps of inorganic acid leaching and electrokinetic methods for removing heavy metals can be shortened to within one day, the more economical and feasible biological process, common inorganic acid leaching, and electrokinetic method take many days, mostly 5-14 days. Therefore, sludge and other biomass need to stay in the treatment device or structure for a long time, which increases the difficulty of transportation and storage during the sludge disposal process, restricting the efficiency of downstream product generation and final sludge disposal.

[0010] [1]Ito A,Umita T,Aizawa J,Takachi T,Morinaga K.Removal of heavymetals from anaerobically digested sewage sludge by a new chemical methodusing ferric sulfate[J].Water Research,2000,34(3):751-758.

[0011] [2]Geng H,Xu Y,Zheng L,Gong H,Dai L,Dai X.An overview of removing heavy metals from sewage sludge:Achievements and perspectives[J].Environmental Pollution,2020,266(115375.

[0012] [3]Pathak A,Dastidar MG,Sreekrishnan T R.Bioleaching of heavy metals from sewage sludge by indigenous iron-oxidizing microorganisms using ammoniumferrous sulfate and ferrous sulfate as energy sources:A comparative study[J].Journal of Hazardous Materials,2009,171(1):273-278.

[0013] [4]Gu

[0014] [5] Wang Jing, Ran Quan, Fan Bailing, Zhang Mingzan, Chu Wang. Study on the removal of heavy metals from municipal sludge by bioleaching[J]. Jiangsu Agricultural Sciences, 2018, 46(24): 363-365.

[0015] [6]Pei D, Xiao C, Hu Q, Tang J. Electrokinetic Gathering and Removal of Heavy Metals from Sewage Sludge by Ethylenediamine Chelation[J]. Procedia Environmental Sciences, 2016, 31, 725-734.

[0016] [7]Wang JY, Zhang DS, Stabnikova O, Tay JH. Evaluation of electrokinetic removal of heavy metals from sewage sludge[J]. Journal of Hazardous Materials, 2005, 124(1):139-146.

[0017] [8] Yang Changming, Li Jianhua. A method for removing heavy metals from municipal sludge using electrokinetic remediation technology: China, CN101265007A[P / OL]. 2008

[0018] [9]Kim SO,Moon SH,Kim KW,Yun ST.Pilot scale study on the ex situelectrokinetic removal of heavy metals from municipal wastewater sludges[J].Water Research,2002,36(19):4765-4774.

[0019]

[10] Wong JWC, Xiang L, Gu XY, Zhou L

[0020]

[11] He Miaomiao, Yu Yitong, Hua Yumei, Zhou Gendi, Xu Jun, Tian Guangming. Study on the removal effect of Zn and Cu in sludge by bioleaching and sludge nutrient loss[J]. Journal of Agro-Environment Science, 2006, 25(5): 1359-1364. Summary of the Invention

[0021] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method for centrifugal removal of heavy metals in dewatered activated sludge based on density difference, which improves the removal rate of heavy metals in dewatered activated sludge, significantly shortens the time spent on heavy metal removal, and reduces the amount of reagents used.

[0022] In order to achieve the above object, the present invention adopts the following technical solutions:

[0023] A method for removing heavy metals from dehydrated activated sludge by centrifugation based on density difference, comprising the following steps:

[0024] Step 1: Add heavy metal adsorbent to dehydrated activated sludge in a mass ratio of 1: (100-1000) and stir thoroughly. Then, let it stand at a temperature not higher than 25°C until the density after conditioning is 0.9-1.1 g / cm 3 Dewatered activated sludge;

[0025] Step 2: Gradient centrifugation was used to prepare a solution with a density of 1.3 to 2.0 g / cm 3 The gradient centrifuge liquid is used, and the density of the gradient centrifuge liquid is 1.2 to 1.9 times the density of the dehydrated activated sludge, and the pH value of the gradient centrifuge liquid is adjusted to be 0.5 units higher than the pH value of the zero charge point of the heavy metal adsorbent;

[0026] Step 3, at a temperature not higher than 25° C., the dehydrated activated sludge and the gradient centrifuge selected in step 2 are first added to a blender and stirred according to a volume ratio of dehydrated activated sludge to centrifuge of 1: (3 to 10), and then transferred to a centrifuge for centrifugation. After centrifugation, the adsorbent enriched in heavy metals in the lower layer is first collected, and then the gradient centrifuge in which heavy metal ions are dissolved is collected in the middle layer, and then the sludge floating on the upper layer from which heavy metals have been removed is collected;

[0027] Step 4: removing heavy metals from the lower layer of heavy metal-enriched adsorbent by drying and acid elution to obtain a metal adsorbent with restored heavy metal adsorption capacity, which is used in the next cycle to continue removing heavy metals;

[0028] Step 5: The middle layer of the gradient centrifuge containing dissolved heavy metal ions is continuously added to the centrifuge of step 2 until the collected middle layer of the gradient centrifuge containing dissolved heavy metal ions becomes turbid due to the contamination of the sludge. The distilled water and gradient centrifugation reagent are recovered by evaporation and dehydration. After acid elution, neutralization adjustment, and drying, the gradient centrifugation reagent for preparing the gradient centrifuge is recovered.

[0029] Step 6: Adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and condition the sludge by aerobic or anaerobic composting to make it suitable for use as fertilizer.

[0030] Furthermore, the dehydrated activated sludge is obtained by the following method: using aerobic sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing moisture by centrifugation or plate and frame filtration, and obtaining a semi-solid dehydrated activated sludge with a moisture content of no more than 90%.

[0031] Furthermore, the heavy metal adsorbent in step 1 is iron oxide or Si / Al>100, and has a density of 2-3 g / cm 3 of artificial zeolite.

[0032] Furthermore, the iron oxide is at least one of ferric oxide and ferroferric oxide, or natural iron ore powder.

[0033] Furthermore, the stirring in step 1 is carried out at a rotation speed of 200 to 500 rpm for 0.5 to 1 hour.

[0034] Furthermore, the standing time in step 1 is 1 to 2 hours.

[0035] Furthermore, the gradient centrifugation agent in step 2 is silica nanoparticles, sodium silicate or sodium dihydrogen phosphate.

[0036] Furthermore, the operation of step 3 is to firstly stir the mixed dewatered activated sludge and the centrifuge liquid at a rotation speed of 50 to 100 rpm to make the mixture uniform, and then centrifuge at a relative centrifugal force of 2000 to 6000 g for 10 to 20 minutes.

[0037] Furthermore, the eluate produced by the acid solution washing in step 4 is purified by precipitation to generate insoluble substances.

[0038] Furthermore, the step 5 also includes heating the recovered gradient centrifugation reagent to 550° C. to remove activated sludge mixed during use.

[0039] Compared with the prior art, the present invention has the following technical effects:

[0040] 1) The gradient centrifugation method used in the present invention can tailor centrifuge liquid for activated sludge with different organic matter contents. The process parameters can be adjusted flexibly without changing the process equipment. Only the equipment parameters need to be changed based on the original experiment to be applicable to the removal of heavy metals from new sludge raw materials. This not only improves the effect of heavy metal removal in sludge, but also reduces the time consumed for actual sludge heavy metal removal to about 30 minutes, which is 5 to 14 days compared to the prior art. This greatly shortens the sludge treatment cycle. In addition, compared to the incineration method, the present invention can return the nutrients and humus in the sludge to the land while reducing carbon emissions, reducing my country's dependence on industrial fertilizers and imported fertilizers, enhancing my country's autonomy in agricultural production, and improving the country's strategic competitive advantage.

[0041] 2) The present invention does not involve extreme acidification of the initial activated sludge. It only requires maintaining the system pH 0.5 higher than the zero charge point of the adsorbent (the zero charge point of iron oxide is 7-9) during the adsorption and heavy metal removal process, so that the surface of the centrifugal agent carries a negative charge, reducing its adsorption of phosphate in the sludge, avoiding the loss of sludge phosphorus, and adding a small amount of agent to adjust the sludge to neutrality after heavy metal removal; although a small amount of solid-liquid mixture containing heavy metals at the bottom requires further leaching treatment, compared with the prior art of leaching a large amount of sludge, the amount of leaching liquid required in the acid washing process is very small, thereby reducing the amount of wastewater produced during the heavy metal removal process and reducing the possibility of secondary pollution to the environment.

[0042] 3) The centrifugal agent and heavy metal adsorbent relied upon by the present invention can be regenerated after a use cycle through simple steps such as dehydration, pickling, drying, and baking, making them easy to reuse, with the advantages of low one-time investment and low subsequent maintenance costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 : A process flow chart for removing heavy metals from dehydrated activated sludge according to the present invention;

[0044] Figure 2-1 :The present invention is based on the mass concentration of heavy metals in dehydrated activated sludge characterized by the heavy metal step-by-step extraction method proposed by the European Community Bureau of Standards (BCR);

[0045] Figure 2-2 :The present invention is based on the mass concentration ratio of heavy metals in dehydrated activated sludge characterized by the heavy metal step-by-step extraction method proposed by the European Community Bureau of Standards (BCR);

[0046] Figure 3 : Photos of some centrifugation results in a small-scale test of removing dewatered activated sludge according to the present invention;

[0047] Figure 4-1 : Bar graph showing the removal rates of common heavy metals in dehydrated activated sludge under different conditions;

[0048] Figure 4-2 : Bar chart of the removal rate of common heavy metals in dewatered activated sludge with heavy metal addition under different conditions. DETAILED DESCRIPTION

[0049] The specific contents of the present invention are further explained in detail below with reference to the embodiments.

[0050] The process flow of removing heavy metals from dewatered activated sludge of municipal sewage treatment plants according to Examples 1 to 6 of the present invention is as follows: Figure 1 shown.

[0051] Example 1

[0052] Step 1: using aerobic activated sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing water by centrifugation to obtain activated sludge with a moisture content of 80%, and crushing it into a semi-solid loose dehydrated activated sludge;

[0053] Step 2: Add artificial zeolite with a particle size of 100-300 μm and a zero charge point of 6-7 to dehydrated activated sludge at a mass ratio of 1:100, stir at 200 rpm for 0.5 h, and then let it stand at 25°C for 2 h to obtain a density of 0.9-1.1 g / cm after conditioning. 3 Dewatered activated sludge;

[0054] The artificial zeolite has a Si / Al ratio greater than 100 and a density of 2 to 3 g / cm 3 ;

[0055] Step 3: Use particle size of 20-50nm and density of 2.6g / cm3 The silica nanoparticles were formulated with a density of 1.3 g / cm 3 The pH of the gradient centrifugation fluid is 0.5 units higher than the zero charge point of the artificial zeolite;

[0056] Step 4: at 25° C., the dehydrated activated sludge and the gradient centrifuge selected in step 3 are added to a blender at a volume ratio of 1:3 between the dehydrated activated sludge and the centrifuge, the dehydrated activated sludge and the centrifuge are first stirred and mixed at a speed of 50 rpm by the blender to make them uniform, and then transferred into a centrifuge and centrifuged at a relative centrifugal force of 2000 g for 10 minutes. Subsequently, the adsorbent enriched in heavy metals in the lower layer is first discharged through the discharge hole and collected, the gradient centrifuge in which heavy metal ions are dissolved in the middle layer is collected, and then the sludge floating on the upper layer from which heavy metals have been removed is collected;

[0057] Step 5: Remove heavy metals from the heavy metal-enriched adsorbent in the lower layer by acid elution, neutralization, and drying. The eluate produced by the acid elution is purified by precipitation to generate insoluble substances (such as metal phosphates or hydroxides). The adsorbent obtained by heavy metal removal enters the next cycle and serves as the initial raw material for step 1 to continue to remove heavy metals.

[0058] Step 6: The gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer enters the next cycle and is continuously added to the centrifuge in step 2 until the collected gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer becomes turbid. The distilled water and gradient centrifuge reagent are recovered by evaporation and dehydration, and the recovered gradient centrifuge reagent is heated to 550° C. to remove the activated sludge mixed during use. The distilled water and gradient centrifuge reagent finally obtained are used to prepare the centrifuge liquid in step 2.

[0059] Step 7: Using acid or alkali solution to adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and subjecting the sludge to conditioning treatment through aerobic or anaerobic composting and other sludge stabilization treatment processes to make its composition more suitable for use as fertilizer.

[0060] The artificial zeolite selected in Example 1 can withstand a high temperature of 700°C. In the process of recycling heavy metal adsorbents, it can withstand the temperature of drying, dehydration and removal of organic impurities, so it will not lose its original properties due to structural changes and is convenient for recycling.

[0061] Example 2

[0062] Step 1: using aerobic activated sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing water by plate and frame filter pressing to obtain activated sludge with a moisture content of 80%, and crushing it to prepare semi-solid dehydrated activated sludge;

[0063] Step 2: Add ferric oxide particles with a particle size of 50 nm to 300 μm and a zero charge point of 7 to 9 to the dehydrated activated sludge at a mass ratio of 1:1000, stir at 500 rpm for 1 hour, and then let it stand at 20°C for 1 hour to obtain a density of 0.9 to 1.1 g / cm after conditioning. 3 Dewatered activated sludge;

[0064] Step 3: Use sodium silicate to prepare a solution with a density of 1.6 g / cm 3 The pH value of the gradient centrifugation fluid was adjusted to 0.5 units higher than the zero charge point of ferric oxide;

[0065] Step 4: at 20° C., the dehydrated activated sludge and the gradient centrifuge selected in step 3 are added to a blender at a volume ratio of 1:6 between the dehydrated activated sludge and the centrifuge, and the dehydrated activated sludge and the centrifuge are first stirred and mixed at a speed of 100 rpm by the blender to make them uniformly mixed. The mixture is then transferred to a centrifuge and centrifuged at a relative centrifugal force of 3000 g for 15 minutes. Subsequently, the adsorbent enriched in heavy metals in the lower layer is first discharged through the discharge hole and collected, and then the gradient centrifuge in which heavy metal ions are dissolved in the middle layer is collected, and then the heavy metal-removed sludge floating on the upper layer is collected;

[0066] Step 5: Remove heavy metals from the heavy metal-enriched adsorbent in the lower layer by acid elution, neutralization, and drying. The eluate produced by the acid elution is purified by precipitation to generate insoluble substances (such as metal phosphates or hydroxides). The adsorbent obtained by heavy metal removal enters the next cycle and serves as the initial raw material for step 1 to continue to remove heavy metals.

[0067] Step 6: The gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer enters the next cycle and is continuously added to the centrifuge in step 2 until the collected gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer becomes turbid. The distilled water and gradient centrifuge reagent are recovered by evaporation and dehydration, and the recovered gradient centrifuge reagent is heated to 550° C. to remove the activated sludge mixed during use. The distilled water and gradient centrifuge reagent finally obtained are used to prepare the centrifuge liquid in step 2.

[0068] Step 7: Using acid or alkali solution to adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and subjecting the sludge to conditioning treatment through aerobic or anaerobic composting and other sludge stabilization treatment processes to make its composition more suitable for use as fertilizer.

[0069] Example 3

[0070] Step 1: using aerobic activated sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing water by plate and frame filter pressing to obtain activated sludge with a moisture content of 75%, and crushing the activated sludge into loose semi-solid dehydrated sludge;

[0071] Step 2: Add ferroferric oxide with a particle size of 50 nm to 300 μm to dehydrated activated sludge at a mass ratio of 1:550, stir at 300 rpm for 0.5 h, and then let it stand at 25 ° C for 2 h to obtain a density of 0.9 to 1.1 g / cm after conditioning. 3 Dewatered activated sludge;

[0072] Step 3: Use sodium dihydrogen phosphate to prepare a solution with a density of 2.0 g / cm 3 The pH value of the gradient centrifugation medium is adjusted to 0.5 units higher than the zero charge point of ferroferric oxide;

[0073] Step 4: at 20° C., the dehydrated activated sludge and the gradient centrifuge selected in step 3 are added to a blender at a volume ratio of 1:10 between the dehydrated activated sludge and the centrifuge, the dehydrated activated sludge and the centrifuge are first stirred and mixed at a speed of 80 rpm using the blender, and then transferred into a centrifuge. The mixture is centrifuged at a relative centrifugal force of 4000 g for 20 minutes, and then the adsorbent enriched in heavy metals in the lower layer is first discharged through the discharge hole and collected, the gradient centrifuge in which heavy metal ions are dissolved in the middle layer is collected, and then the heavy metal-removed sludge floating on the upper layer is collected;

[0074] Step 5: Remove heavy metals from the heavy metal-enriched adsorbent in the lower layer by acid elution, neutralization, and drying. The eluate produced by the acid elution is purified by precipitation to generate insoluble substances (such as metal phosphates or hydroxides). The adsorbent obtained by heavy metal removal enters the next cycle and serves as the initial raw material for step 1 to continue to remove heavy metals.

[0075] Step 6: The gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer enters the next cycle and is continuously added to the centrifuge in step 2 until the collected gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer becomes turbid. The distilled water and gradient centrifuge reagent are recovered by evaporation and dehydration, and the recovered gradient centrifuge reagent is heated to 550° C. to remove the activated sludge mixed during use. The distilled water and gradient centrifuge reagent finally obtained are used to prepare the centrifuge liquid in step 2.

[0076] Step 7: Using acid or alkali solution to adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and subjecting the sludge to conditioning treatment through aerobic or anaerobic composting and other sludge stabilization treatment processes to make its composition more suitable for use as fertilizer.

[0077] Example 4

[0078] Step 1: using aerobic activated sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing water by centrifugation to obtain activated sludge with a moisture content of 85%, and crushing the sludge to prepare loose semi-solid dehydrated activated sludge;

[0079] Step 2: Add iron oxide to dehydrated activated sludge at a mass ratio of 1:300, stir at 400 rpm for 1 hour, and then let it stand at 20°C for 2 hours to obtain a density of 0.9-1.1 g / cm after conditioning. 3 Dewatered activated sludge;

[0080] The iron oxide is a mixture of ferroferric oxide and ferrous oxide;

[0081] Step 3: Use particle size of 20-50nm and density of 2.6g / cm 3 The density of the silica nanoparticles is 1.8 g / cm 3 The pH value of the gradient centrifugation medium is adjusted to be 0.5 units higher than the zero charge point of the iron oxide mixture;

[0082] Step 4: At 20°C, add the dehydrated activated sludge and the gradient centrifuge selected in step 3 to a blender at a volume ratio of 1:5. The blender is first used to stir and mix the dehydrated activated sludge and centrifuge at 75 rpm, and then transferred to a centrifuge for centrifugation at a relative centrifugal force of 5000 g for 10 minutes. Next, the lower layer of adsorbent enriched in heavy metals is discharged through the discharge hole and collected, followed by the collection of the middle layer of gradient centrifuge containing dissolved heavy metal ions, and finally, the upper layer of floating sludge free of heavy metals.

[0083] Step 5: Remove heavy metals from the heavy metal-enriched adsorbent in the lower layer by acid elution, neutralization, and drying. The eluate produced by the acid elution is purified by precipitation to generate insoluble substances (such as metal phosphates or hydroxides). The adsorbent obtained by heavy metal removal enters the next cycle and serves as the initial raw material for step 1 to continue to remove heavy metals.

[0084] Step 6: The gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer enters the next cycle and is continuously added to the centrifuge in step 2 until the collected gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer becomes turbid. The distilled water and gradient centrifuge reagent are recovered by evaporation and dehydration, and the recovered gradient centrifuge reagent is heated to 550° C. to remove the activated sludge mixed during use. The distilled water and gradient centrifuge reagent finally obtained are used to prepare the centrifuge liquid in step 2.

[0085] Step 7: Using acid or alkali solution to adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and subjecting the sludge to conditioning treatment through aerobic or anaerobic composting and other sludge stabilization treatment processes to make its composition more suitable for use as fertilizer.

[0086] Example 5

[0087] Step 1: using aerobic activated sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing water by centrifugation to obtain activated sludge with a moisture content of 80%, and crushing it into loose semi-solid dehydrated activated sludge;

[0088] Step 2: Add magnetite powder to dehydrated activated sludge at a mass ratio of 1:800, stir at 350 rpm for 1 hour, and then let it stand at 25°C for 1 hour to obtain a density of 0.9-1.1 g / cm after conditioning. 3 Dewatered activated sludge;

[0089] Step 3: Use sodium silicate to prepare a solution with a density of 1.7 g / cm 3 The pH value of the gradient centrifugation fluid is adjusted to 0.5 units higher than the zero charge point of the magnetite powder;

[0090] Step 4: at 25° C., taking the dehydrated activated sludge and the gradient centrifuge selected in step 3 into a blender at a volume ratio of 1:8 between the dehydrated activated sludge and the centrifuge, first using the blender to stir the dehydrated activated sludge and the centrifuge at a speed of 100 rpm to mix them evenly, then transferring them into a centrifuge, centrifuging them at a relative centrifugal force of 6000 g for 15 minutes, then first discharging the lower layer of the adsorbent enriched in heavy metals through the discharge hole and collecting it, then collecting the middle layer of the gradient centrifuge in which heavy metal ions are dissolved, and then collecting the upper layer of the sludge from which heavy metals have been removed;

[0091] Step 5: Remove heavy metals from the heavy metal-enriched adsorbent in the lower layer by acid elution, neutralization, and drying. The eluate produced by the acid elution is purified by precipitation to generate insoluble substances (such as metal phosphates or hydroxides). The adsorbent obtained by heavy metal removal enters the next cycle and serves as the initial raw material for step 1 to continue to remove heavy metals.

[0092] Step 6: The gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer enters the next cycle and is continuously added to the centrifuge in step 2 until the collected gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer becomes turbid. The distilled water and gradient centrifuge reagent are recovered by evaporation and dehydration, and the recovered gradient centrifuge reagent is heated to 550° C. to remove the activated sludge mixed during use. The distilled water and gradient centrifuge reagent finally obtained are used to prepare the centrifuge liquid in step 2.

[0093] Step 7: Using acid or alkali solution to adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and subjecting the sludge to conditioning treatment through aerobic or anaerobic composting and other sludge stabilization treatment processes to make its composition more suitable for use as fertilizer.

[0094] Example 6

[0095] Step 1: using aerobic activated sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing water by centrifugation to obtain activated sludge with a moisture content of 75%, and crushing it into loose semi-solid dehydrated activated sludge;

[0096] Step 2: Add dehydrated activated sludge to the water-extracted iron ore powder at a mass ratio of 1:700, stir at 250 rpm for 0.5 h, and then let it stand at 20 ° C for 2 h to obtain a density of 0.9-1.1 g / cm after conditioning. 3 Dewatered activated sludge;

[0097] Step 3: Sodium dihydrogen phosphate was used to prepare a solution with a density of 1.6 g / cm 3 The pH value of the gradient centrifugation solution was adjusted to 0.5 units higher than the zero charge point of ferrihydrite powder;

[0098] Step 4: at 25° C., the dehydrated activated sludge and the gradient centrifuge selected in step 3 are added to a blender at a volume ratio of 1:7 between the dehydrated activated sludge and the centrifuge, the dehydrated activated sludge and the centrifuge are first stirred and mixed at a speed of 50 rpm using the blender, and then transferred into a centrifuge for centrifugation at a relative centrifugal force of 6000 g for 20 minutes. Subsequently, the adsorbent enriched in heavy metals in the lower layer is first discharged through the discharge hole and collected, the gradient centrifuge in which heavy metal ions are dissolved in the middle layer is collected, and then the heavy metal-removed sludge floating on the upper layer is collected;

[0099] Step 5: Remove heavy metals from the lower layer of the heavy metal-enriched adsorbent by acid elution, neutralization, and drying. The eluate produced by the acid elution is purified by precipitation to generate insoluble substances (such as metal phosphates or hydroxides). The resulting metal-removed adsorbent enters the next cycle and serves as the initial raw material for step 1 to continue removing heavy metals.

[0100] Step 6: The gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer enters the next cycle and is continuously added to the centrifuge in step 2 until the collected gradient centrifuge liquid containing dissolved heavy metal ions in the middle layer becomes turbid. The distilled water and gradient centrifuge reagent are recovered by evaporation and dehydration, and the recovered gradient centrifuge reagent is heated to 550° C. to remove the activated sludge mixed during use. The distilled water and gradient centrifuge reagent finally obtained are used to prepare the centrifuge liquid in step 2.

[0101] Step 7: Using acid or alkali solution to adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and subjecting the sludge to conditioning treatment through aerobic or anaerobic composting and other sludge stabilization treatment processes to make its composition more suitable for use as fertilizer.

[0102] See also Figure 2-1 and Figure 2-2 It can be seen that the contents of heavy metals Zn and Cu in activated sludge are the highest. Most heavy metals contain more organic matter-bound states (which are oxidizable components, because the sulfide components in dehydrated activated sludge can be ignored), exchangeable states and carbonate-bound states, which have better conditions for adsorption and removal.

[0103] See also Figure 3 The gradient centrifuge fluid of group 1 has a density of 1.6 g / cm 3 Sodium silicate aqueous solution, the gradient centrifugation solution of group 2 has a density of 1.7 g / cm 3 Sodium silicate aqueous solution, the gradient centrifugation solution of group 3 has a density of 1.8g / cm 3 Sodium silicate aqueous solution, the test conditions of the two samples in the same group are the same. It can be seen that when the density of the centrifuge liquid is relatively large, the density of the sludge and the heavy metal adsorbent is relatively small, and the sludge and the adsorbent still float on it after centrifugation; when the density of the centrifuge liquid is lower than the density of the heavy metal adsorbent but higher than the density of the sludge, the sludge still floats on it after centrifugation, while the heavy metal adsorbent tends to migrate downward.

[0104] See also Figure 4-1 , OS marked on the figure represents dewatered activated sludge, SLT represents gradient centrifuge liquid, the percentage in front represents the mass ratio of centrifuge reagent, MD represents heavy metal adsorbent, HCl represents hydrochloric acid, AoL6 represents the average removal rate of 6 heavy metals (Cd, Ni, Pb, Cr, Cu, Zn); and the black column filled with slashes in the figure represents the use of traditional methods, that is, only using hydrochloric acid to acidify the dewatered activated sludge to pH = 3, oscillating at 300rpm for 4 hours, and then centrifuging at a relative centrifugal force of 6000g. The measured heavy metal removal rate of the sedimented sludge is compared with the original dewatered sludge. The removal rate of various heavy metals is less than 10% or basically no removal. If the removal rate needs to be improved, it will take more days; the solid column represents the use of a heavy metal adsorbent with a mass concentration of 1% and a density of 1.6g / cm 3 or 1.7 g / cm 3 The removal rate of various heavy metals during the gradient centrifugation of sodium silicate aqueous solution; the hollow column means that no heavy metal adsorbent is used, only the density of 1.6g / cm 3 or 1.7 g / cm 3The removal rates of various heavy metals were compared when the gradient centrifugation solution was used. It can be seen that only when the heavy metal adsorbent and the gradient centrifugation solution were used at the same time could a good heavy metal removal rate be achieved, especially when the mass concentration of the heavy metal adsorbent was 1% and the density of the gradient centrifugation solution was 1.6 g / cm in Example 6. 3 The removal rate of heavy metals was the highest when the centrifugal separation was carried out. Among them, the removal rate of Cd, Ni, Pb, Cr, Cu and Zn reached 68-78%, and the removal rate of As was 97%. This proves that the method for removing heavy metals from dehydrated activated sludge based on density difference centrifugation in Examples 1 to 6 of the present invention has a high removal rate for heavy metals in sludge and shortens the time consumption. Furthermore, the density of the gradient centrifuge solution can be no more than 1.7 g / cm 3 .

[0105] See also Figure 4-2 HMOS represents sludge with added heavy metals, that is, heavy metals are added to the original activated sludge stage without dehydration. That is, under the working conditions of the activated sludge tank with dissolved COD of 400-600 mg / L, continuous aeration is carried out and chlorides or nitrates of various heavy metals are added to make the final concentration of each heavy metal ion three times the original. After stable operation for 5 hours, dehydration is carried out to obtain dehydrated sludge for testing. Hollow columns represent that no heavy metal adsorbent is used, and only heavy metal adsorbent with a density of 1.6 g / cm is used. 3 or 1.7 g / cm 3 The removal rate of various heavy metals when the gradient centrifugation liquid is used; the solid column represents the use of a heavy metal adsorbent with a mass concentration of 1% and a density of 1.6g / cm 3 or 1.7 g / cm 3 The removal rates of various heavy metals when the gradient centrifuge was used were as follows. It can be seen that only when the heavy metal adsorbent and the gradient centrifuge were used at the same time could a good heavy metal removal rate be achieved, especially when the mass concentration of the heavy metal adsorbent was 1% and the density of the gradient centrifuge was 1.6 g / cm 3 The heavy metal removal rate is the highest when the density is 1.7g / cm 3 of the centrifuge fluid.

Claims

1. A method for removing heavy metals from dehydrated activated sludge based on density difference centrifugation, characterized in that: The steps include: Step 1: Add heavy metal adsorbent to dehydrated sludge in a mass ratio of 1: (100-1000) and stir thoroughly. Then, let it stand at a temperature not higher than 25°C until the density after conditioning is 0.9-1.1 g / cm 3 Dehydrated activated sludge, wherein the heavy metal adsorbent is iron oxide or Si / Al>100, and the density is 2-3g / cm 3 artificial zeolite; Step 2: Gradient centrifugation was used to prepare a solution with a density of 1.3 to 2.0 g / cm 3 The gradient centrifuge liquid has a density of 1.2 to 1.9 times that of the dehydrated activated sludge, and the pH value of the gradient centrifuge liquid is adjusted to be 0.5 units higher than the zero charge point of the heavy metal adsorbent, wherein the gradient centrifuge agent is silica nanoparticles, sodium silicate or sodium dihydrogen phosphate; Step 3, at a temperature not higher than 25° C., the dehydrated activated sludge and the gradient centrifuge selected in step 2 are first added to a blender and stirred according to a volume ratio of dehydrated activated sludge to centrifuge of 1: (3 to 10), and then transferred to a centrifuge for centrifugation. After centrifugation, the adsorbent enriched in heavy metals in the lower layer is first collected, and then the gradient centrifuge in which heavy metal ions are dissolved is collected in the middle layer, and then the sludge floating on the upper layer from which heavy metals have been removed is collected; Step 4: removing heavy metals from the heavy metal-enriched adsorbent in the lower layer by drying and eluting with acid solution to obtain a heavy metal adsorbent with restored heavy metal adsorption capacity, which is used in the next cycle to continue removing heavy metals; Step 5: The middle layer of the gradient centrifuge containing dissolved heavy metal ions is continuously added to the centrifuge of step 2 until the collected middle layer of the gradient centrifuge containing dissolved heavy metal ions becomes turbid due to the contamination of the sludge. The distilled water and gradient centrifugation reagent are recovered by evaporation and dehydration. After acid elution, neutralization adjustment, and drying, the gradient centrifugation reagent for preparing the gradient centrifuge is recovered. Step 6: Adjust the pH value of the upper floating sludge from which heavy metals have been removed to neutral, and condition the sludge by aerobic or anaerobic composting to make it suitable for use as fertilizer.

2. The method for removing heavy metals from dewatered sludge by centrifugation based on density difference according to claim 1, characterized in that: The dehydrated activated sludge is obtained by the following method: using aerobic sludge produced by a municipal sewage treatment plant or an industrial organic wastewater biochemical reaction unit as the initial raw material, removing moisture by centrifugation or plate and frame filtration to obtain semi-solid dehydrated sludge with a moisture content of no more than 90%.

3. The method for removing heavy metals from dehydrated activated sludge based on density difference centrifugation according to claim 1, characterized in that: The iron oxide is at least one of ferric oxide and ferroferric oxide, or is natural iron ore powder.

4. The method for removing heavy metals from dehydrated activated sludge by centrifugation based on density difference according to claim 1, characterized in that: The stirring in step 1 is carried out at a rotation speed of 200 to 500 rpm for 0.5 to 1 hour.

5. The method for removing heavy metals from dewatered sludge by centrifugation based on density difference according to claim 1, characterized in that: The standing time of step 1 is 1 to 2 hours.

6. The method for removing heavy metals from dehydrated activated sludge based on density difference centrifugation according to claim 1, characterized in that: The operation of step 3 is to firstly stir the mixed dewatered sludge and the centrifuge liquid at a rotation speed of 50 to 100 rpm to make the mixture uniform, and then centrifuge at a relative centrifugal force of 2000 to 6000 g for 10 to 20 minutes.

7. The method for removing heavy metals from dehydrated activated sludge based on density difference centrifugation according to claim 1, characterized in that: The eluate produced by the acid solution elution in step 4 is purified by precipitation to generate insoluble substances.

8. The method for removing heavy metals from dehydrated activated sludge based on density difference centrifugation according to claim 1, characterized in that: The step 5 further includes heating the recovered gradient centrifugation reagent to 550° C. to remove sludge mixed during use.

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