Method for enhancing anaerobic digestion of sludge to efficiently produce methane

Abstract

A method for enhancing the anaerobic digestion of sludge to efficiently produce methane, which method aims to solve the existing technical problems of low anaerobic fermentation efficiency of sludge, a low utilization value of non-grain biomass, poor efficiency of a carbon material, etc. The present method comprises the main steps of: 1, a sludge pretreatment; 2, preparation of a carbon-based material; 3, an electrochemical pretreatment; 4, anaerobic fermentation; and 5, a subsequent treatment. In the present invention, a porous multi-pole carbon-based material is prepared by blending sludge and straw. Therefore, the non-grain biomass is fully utilized, making the high added value thereof increased; moreover, the produced carbon-based material can also enhance the electrochemical pretreatment and increase methane produced by anaerobic fermentation.

Description

Method for enhancing high-efficiency methane production of sludge anaerobic digestion TECHNICAL FIELD

[0001] The present application belongs to the technical field of municipal sludge resource utilization, and particularly relates to a method for enhancing high-efficiency methane production of sludge anaerobic digestion based on pretreatment and biochar modification. BACKGROUND

[0002] With the increase of sewage treatment rate in China, the amount of sludge produced is growing, and the sludge produced by industrial wastewater treatment is more harmful due to containing a large amount of heavy metals and other toxic substances. In addition, the cost of sludge treatment and disposal in sewage treatment plants is high. The basic status of sludge can be summarized as huge volume, complex composition and high water content, which reflects the difficulty of sludge treatment in technology and cost. At present, the main sludge disposal methods in China include drying, composting, incineration and resource utilization, but all have more or less shortcomings. Anaerobic digestion is a relatively mature technology in sludge resource disposal, which can well realize the reduction, stabilization, harmlessness and resource utilization of excess sludge, but anaerobic digestion also has problems such as large reactor, complex management and operation, strict fermentation conditions and long fermentation period. Sludge anaerobic fermentation has the characteristics of pollution reduction and resource recovery, and is considered as a good sludge harmless and resource treatment technology, so it is of great significance to improve the performance of anaerobic digestion.

[0003] In the process of municipal sludge anaerobic fermentation, the rate-limiting stage is often the hydrolysis stage. The cells, colloids and extracellular polymeric substances (EPS) of excess sludge can form zooglea, which are stable in nature and can hinder the dissolution of organic matter in excess sludge, limiting the progress of excess sludge anaerobic fermentation. Through appropriate pretreatment, the stability of zooglea can be destroyed, the dissolution of organic matter in excess sludge can be promoted, and the anaerobic fermentation of excess sludge can be improved.

[0004] There are various pretreatment methods for sludge, among which low-frequency (20-40 kHz) ultrasonic treatment can produce more obvious cavitation effect, thereby producing stronger hydrodynamic shear force and better pretreatment effect. At the same time, ultrasonic waves in water can produce ·OH, ·HO2, ·H and other free radicals. These free radicals can further destroy the structure of sludge flocs. Ultrasonic resonance can also cause the temperature of part of the sludge to rise, producing a thermal effect.

[0005] In recent years, electrochemical technology for sludge treatment is considered as one of the most promising technologies, and the electrochemical treatment is used to treat sewage plant sludge, wherein the electrochemical treatment is mainly achieved by the migration of pollutants through the mechanisms of electromigration, electrodialysis, electrophoresis and the like. Different ways of different electric field action, different electrode materials and different electric field intensity will affect various microorganisms in different ways. The electric field can stimulate or enhance the activity of certain microorganisms, promote enzyme activity, promote microbial growth and reproduction, and the microorganisms stimulated by the electric field intensity can convert more organic substrates into methane and CO2. As a kind of advanced oxidation technology, the electrochemical pretreatment is a method for treating sludge by using the oxidizing substances such as ·OH, O3, Fe 3+ , HClO generated by direct electrochemical oxidation or indirect oxidation. Compared with other methods, the electrochemical pretreatment has the advantages of small occupation area, short treatment time and strong controllability.

[0006] Meanwhile, China is the world's largest straw producer, but the production of excess straw is high, and the environmental pollution and resource waste problems caused by straw burning are becoming increasingly serious. As a non-food biomass, straw has great recycling value, and with the use of straw as carbon material in recent years, this cheap and effective resource is increasingly valued, realizing the double harvest of economy and environment. However, there are still problems such as low utilization value of non-food biomass and poor carbon material efficiency. In view of the low utilization value of non-food biomass, sludge and non-food biomass are combined to prepare carbon-based electrode materials for supercapacitors, solving the problems of low utilization degree of non-food biomass and sludge disposal, and the generated carbon-based electrode materials can also be subjected to subsequent electrochemical pretreatment.

[0007] In view of the above content analysis, the present application proposes an ultrasonic-sludge and straw doped porous multi-polar carbon-based material assisted-electrochemical coupling treatment method to improve the method of sludge anaerobic digestion. SUMMARY

[0008] The main purpose of the present application is to solve the problems of the prior art and treatment process, and to provide a method for increasing the efficiency of sludge treatment and resource utilization. The present application provides a better treatment and disposal of sludge for sewage plants, increases the production of biogas, and also promotes the resource utilization and harmlessness of sludge, and reduces the treatment cost of sewage treatment plants.

[0009] To achieve the above purpose, the present application provides a method for improving sludge anaerobic digestion and efficiently producing methane based on biochar modification, which comprises the following steps:

[0010] S1: ultrasonic pretreatment of the secondary sedimentation tank sludge to obtain the sludge after the first treatment.

[0011] S2: doping straw and part of the sludge in the secondary sedimentation tank according to a proportion to prepare a porous multi-polar carbon-based material.

[0012] S3: performing electrochemical pretreatment on the primary treated sludge with the aid of the prepared carbon-based material to obtain secondary treated sludge.

[0013] S4: performing anaerobic digestion fermentation treatment on the secondary treated sludge to obtain digestion gas and concentrated sludge.

[0014] S5: transporting the concentrated sludge for subsequent disposal; and converting the digestion gas into electric energy and / or heat.

[0015] In the present application, the method for preparing electrode material is used to assist subsequent electrochemical pretreatment and anaerobic digestion while using non-food biomass. The energy conversion of organic carbon by anaerobic digestion technology benefits the operation of the entire water plant, realizes effective recycling of carbon, and reduces carbon emissions.

[0016] According to the present application, preferably, the ultrasonic pretreatment is low-frequency ultrasonic treatment.

[0017] According to the present application, preferably, the straw is discarded wheat and / or corn straw waste.

[0018] According to the present application, preferably, the carbon-based material preparation method is high-temperature low-speed pyrolysis.

[0019] According to the present application, preferably, the electrochemical pretreatment method is sodium hypochlorite method.

[0020] According to the present application, preferably, the temperature of the anaerobic digestion fermentation treatment is 20-40°C, and the pH is 6.5-7.5.

[0021] According to the present application, preferably, the digestion gas is converted into electric energy and / or heat energy for supplementing the operation of the sewage plant.

[0022] The technical method of the present application has the following beneficial effects:

[0023] The present application takes electrochemical pretreatment and anaerobic digestion fermentation as the core, reduces the complex process of traditional water plants, improves the sludge treatment and disposal effect, and reduces carbon emissions by utilizing carbon cycle.

[0024] The present application adopts sludge and straw doping to prepare porous and multi-polar carbon-based material, fully utilizes non-food biomass such as straw, increases the high value-added, and the generated carbon-based material can enhance electrochemical pretreatment and enhance biogas (digestion gas) generated by anaerobic fermentation, supply energy for power generation of thermal power plants, and the generated electric energy and heat energy can be used for operation of the sewage plant.

[0025] Other features and advantages of the present application are described in detail in the following specific embodiments.

[0026] The present application is a kind of enhanced sludge anaerobic digestion high-efficiency methanogenesis method, digestion substrate comes from the excess sludge of municipal wastewater treatment plant and the inoculation sludge of laboratory, the excess sludge is pretreated and temporarily stored, corn straw is used as raw material to prepare biochar, and the inoculation sludge is taken from the long-term domesticated anaerobic sludge in the laboratory, anaerobic digestion is carried out in the anaerobic reactor. Biochar modification enhancement, compared with traditional sludge anaerobic digestion technology, significantly increases the yield of methane gas, and at the same time realizes the reduction and resource utilization of sludge. BRIEF DESCRIPTION OF DRAWINGS

[0027] The accompanying drawings show a high-efficiency sludge anaerobic fermentation methanogenesis method, which includes the preparation of carbon-based materials by mixing the excess sludge from the secondary sedimentation tank of a wastewater treatment plant with straw in a certain proportion, followed by the addition of the excess sludge pretreated by ultrasonic, and then electrochemical pretreatment, followed by anaerobic fermentation to produce methane. The produced biogas can be used by a heat and power plant, and the concentrated excess sludge can be transported for disposal.

[0028] Figure 1 is a flowchart of a method for enhanced sludge anaerobic digestion and high-efficiency methanogenesis according to an embodiment of the present application. DETAILED DESCRIPTION

[0029] The present application will be further described below in conjunction with the drawings and examples.

[0030] As shown in the figure, a high-efficiency sludge anaerobic fermentation methanogenesis method includes the preparation of carbon-based materials by mixing the excess sludge from the secondary sedimentation tank of a wastewater treatment plant with straw in a certain proportion, followed by the addition of the excess sludge pretreated by ultrasonic, and then electrochemical pretreatment, followed by anaerobic fermentation to produce methane. The produced biogas can be used by a heat and power plant, and the concentrated excess sludge can be transported for disposal.

[0031] EMBODIMENT

[0032] As shown in Figure 1, the present embodiment provides a method for enhanced sludge anaerobic digestion and high-efficiency methanogenesis, which includes the following steps:

[0033] S1. The sludge comes from the secondary sedimentation tank of a municipal wastewater treatment plant, the supernatant is discarded, and the sludge is filtered through a steel sieve with a pore size of 1 mm and then placed in a 4℃ refrigerator for 28h for storage;

[0034] S2. The sludge to be treated is taken out of the refrigerator and placed at room temperature, and then diluted to TS of 8% with deionized water, stirred uniformly, and then pretreated by low-frequency ultrasonic in a probe-type ultrasonic reactor to obtain the sludge after the first treatment;

[0035] S3. The corn / wheat straw is dried, crushed and sieved, and the particles with a particle size of 0.1-0.5 mm are selected;

[0036] S4. The sludge pretreated by low-frequency ultrasound and selected straw particles are mixed and doped, and then converted into carbonaceous materials after high-temperature pyrolysis. After removing inorganic mineral impurities through hydrofluoric acid pickling process, the pore structure is expanded and optimized through the regulation of the chemical activation process, and finally the multi-level porous carbon material is obtained;

[0037] S5. Add the required amount of NaClO solution to the primary treated sludge sample, and then fully stir it with a magnetic stirrer (about 10 min). Then, the multi-level porous carbon material prepared is used for electrochemical pretreatment, and the secondary treated sludge is obtained;

[0038] S6. The secondary treated sludge is put into an anaerobic digestion reactor, and the reaction system temperature is set to 20-40°C and the pH is set to 6.5-7.5. The anaerobic digestion reaction is carried out, and the digestion gas and concentrated sludge produced in the reaction are collected;

[0039] S7. The digestion gas produced in the anaerobic digestion is measured and used to convert into electrical energy and / or heat. The concentrated sludge is transported out for subsequent disposal.

[0040] Further, in step S2, the low-frequency ultrasonic pretreatment of the sludge to be treated further includes the following steps:

[0041] 1. The ultrasonic sludge treatment equipment is composed of an ultrasonic generator, a converter and a vibrator. The alternating current is converted into electromagnetic oscillation by the generator and transmitted to the converter at the front end of the vibrator. The converter converts the electromagnetic oscillation into ultrasonic vibration of the vibrator;

[0042] 2. The sludge enters the reactor from the sludge inlet located at the lower part and flows out after passing through the ultrasonic field composed of the vibrator. The vibration energy is input into the sludge by the vibrator, so that compression and expansion are formed in the sludge medium, and finally many micro air pockets are generated;

[0043] 3. The parameters of the sludge ultrasonic reactor are set as follows: ultrasonic specific energy consumption 2.0 W / mL, frequency 25 KHz, and action time 40 min. At this time, the drying rate of the sludge is good, the pretreatment effect is good, and the anaerobic digestion effect is also best.

[0044] Further, in step S4, during the preparation of the multi-level porous carbon material, the following detailed steps are included:

[0045] 1. Pre-carbonization. 10 g of dried sludge / straw mixture is ground in a mortar, placed in a crucible and sent into a horizontal tube furnace. 50 mL min -1 of N2 is introduced, and the temperature is raised to 700°C at a rate of 15 °C min -1 . The burning is continued for 2 h. Then N2 is continuously introduced, and the furnace is cooled to room temperature. The pyrolysis residue of the oil-containing sludge is repeatedly ground through a 200 mesh sieve;

[0046] 2. Acid washing to remove impurities. The obtained carbonaceous material was immersed in 100 mL of hydrofluoric acid solution with a concentration of 1 M, and then it was placed in a fume hood to digest at 100 °C. The solid product after washing with hot acid to remove impurities was repeatedly washed with deionized water until the pH of the filtrate was about 7, and the product obtained after drying in a 105 °C oven for 6 h was an ash-removed carbon material;

[0047] 3. KOH activation. The ash-removed carbon material was ultrasonically dispersed in a 1 M KOH solution, and the mass ratio of carbonaceous material / KOH was 1:3, and then it was dried at 80 °C and ground into a powder. The mixture was placed in a tube furnace, heated to 700 °C at a temperature rising rate of 10 °C / min and kept for 2 h. After continuing to cool with the furnace to room temperature, the obtained solid product was repeatedly washed with 1 M HCl solution and a large amount of deionized water, and dried at 105 °C for 6 h to obtain the final product, a hierarchical porous carbon material. -1

[0048] Further, in step S5, in the electrochemical pretreatment, the following detailed steps are further included:

[0049] 1. The electrochemical pretreatment was carried out in a 1 L organic glass reactor, and the length, width and height of the reactor were all 10 cm. The electrode plates used for the cathode and the anode were both Ti / RuO2 mesh electrode plates with a size of 10×12 cm2 (width×height), and the effective area was 40 cm 2 . The power supply was a stabilized direct current power supply. The amount of sludge for electrochemical treatment was 400 mL;

[0050] 2. During the electrochemical treatment, gas stirring (aeration amount was 0.78~0.80 m 3 / h•m 3 ) was adopted to prevent the sludge from stratifying during the electrochemical treatment, affecting the treatment effect;

[0051] 3. The required amount of NaClO solution was added to the sludge sample, and the effective chlorine concentration was not less than 0.8 mol / L;

[0052] 4. Adjust the pH value, and adjust with 5 mol / L sulfuric acid solution and sodium hydroxide solution before electrochemical treatment, and then stir evenly and conduct the pretreatment experiment after power on;

[0053] 5. After sufficient stirring (about 10 min) with a magnetic stirrer, electrochemical treatment was carried out, and the electrode material for electrochemical treatment was the hierarchical porous carbon material prepared in Example 3;

[0054] ​6. After the electrochemical pretreatment, some oxidizing substances will remain in the sludge, which is not conducive to the subsequent anaerobic digestion process. Therefore, the electrochemically treated sludge is pre-stirred for 30 min and then left to stand for 24 h before entering the anaerobic digestion stage.

[0055] Further, in step S6, the sludge anaerobic digestion reaction also includes the following detailed steps:

[0056] 1. A 500 mL serum bottle is used as the reactor, with an effective volume of 400 mL. The anaerobic digestion substrate is the sludge obtained after multiple pretreatments in the example. The pH of the anaerobic digestion system is adjusted to 6.5-7.5.

[0057] 2. After the anaerobic digestion system is filled with materials, it is sealed with a rubber plug, and then connected to a high-purity nitrogen cylinder for nitrogen blowing to remove oxygen for 5 min.

[0058] 3. The reactor is placed in a 35 ℃ constant temperature water bath for reaction, with a stirring / shaking speed of 120 rpm.

[0059] 4. The gas sample produced during the reaction is collected using the gas bag at the top of the reactor. After sampling, the methane concentration is determined. The biogas volume is determined by the saturated brine method and converted into the volume at standard atmospheric pressure for data calculation. The gas production is recorded once every 24 h. When the daily gas volume is less than 1% of the cumulative gas volume, the recording is stopped.

[0060] Further, in addition to containing a large amount of methane, the biogas produced in step S6 also contains a small amount of hydrogen sulfide, carbon dioxide, water vapor and some other impurity gases. Because the combustion of biogas is needed for power generation in the subsequent process, carbon dioxide and water vapor will hinder the combustion of biogas, and water vapor will condense during the biogas transportation process, which may cause pipe blockage. Hydrogen sulfide has strong corrosiveness and will strongly corrode the pipes, generator sets and instruments during transportation, thereby affecting the service life of the equipment. Therefore, the biogas needs to be purified before being burned for power generation.

[0061] In this embodiment, the desulfurization of biogas uses the Fe2O3 dry desulfurization method. H2S gas in the biogas can react with Fe2O3 to absorb H2S to generate Fe2S3 and elemental sulfur;

[0062] In this embodiment, the dehydration of biogas uses the cold drying and moisture absorption method, equipped with a fan and a water tank. The selected device for biogas dehydration is a refrigeration dryer. After desulfurization, the biogas is cooled by heat exchange with the refrigerant to reduce the temperature to the dew point temperature, so that it condenses into liquid droplets. The condensed water vapor becomes small droplets and flows out through the drainage pipeline.

[0063] In this embodiment, the waste heat collection system includes waste heat and tail gas waste heat. The waste heat is mainly recovered by a plate heat exchanger. The tail gas waste heat is divided into waste heat and secondary waste heat, which are recovered by a double-pipe heat exchanger and a shell-and-tube heat exchanger, respectively.

Claims

1. A method for enhancing the high-efficiency methanogenesis of anaerobic digestion of sludge, characterized by, The method comprises the following steps: Step one, sludge pretreatment: the sludge in the secondary sedimentation tank is ultrasonically pretreated to obtain the sludge after primary treatment. The sludge is from the secondary sedimentation tank of a municipal sewage treatment plant, the supernatant is discarded, and the sludge is stirred uniformly after sieving, and the sludge is pretreated by a probe-type ultrasonic reactor under low-frequency ultrasonic to obtain the sludge after primary treatment; Step two, preparation of carbon-based material: the straw and part of the sludge in the secondary sedimentation tank are doped in proportion to prepare a multi-porous multi-polar carbon-based material. The sludge after low-frequency ultrasonic pretreatment and selected straw particles are mixed and doped, and are converted into carbonaceous material after high-temperature pyrolysis, and inorganic mineral impurities are removed after acid washing by hydrofluoric acid; The pore structure is expanded and optimized through the regulation of the chemical activation process, and finally a multi-stage porous carbon material is obtained; Step three, electrochemical pretreatment: the sludge after primary treatment is electrochemically pretreated with the aid of the prepared carbon-based material to obtain the sludge after secondary treatment. A required amount of NaClO solution is added to the sludge sample after primary treatment, and the mixture is stirred thoroughly (about 10 min) by a magnetic stirrer, and then the prepared multi-stage porous carbon material is used for electrochemical pretreatment to obtain the sludge after secondary treatment; Step four, anaerobic fermentation: the sludge after secondary treatment is subjected to anaerobic digestion and fermentation treatment to obtain digestion gas and concentrated sludge; Step five, subsequent treatment: the concentrated sludge is transported out for subsequent disposal; and the digestion gas is converted into electric energy and / or heat.

2. The method for enhancing the high-efficiency methanogenesis of sludge anaerobic digestion according to claim 1, characterized in that, The sludge in step one is filtered by a steel sieve with a pore size of 1 mm and is placed in a 4°C refrigerator for 28-30 hours. The sludge to be treated is taken out of the refrigerator and placed at room temperature, and is diluted to a TS of 8%-10% by deionized water.

3. The method for enhancing the high-efficiency methanogenesis of sludge anaerobic digestion according to claim 1, characterized in that, The parameters of the ultrasonic reactor for the sludge in step one are an ultrasonic specific energy consumption of 2.0 W / mL, a frequency of 25 KHz, and an action time of 40 min.

4. The method for enhancing the high-efficiency methanogenesis of sludge anaerobic digestion according to claim 1, characterized in that, In the process of preparing the carbon-based material in step two, the high-temperature pyrolysis temperature is 600-700°C, and the burning time is 1.5-2 hours.

5. The method for enhancing the high-efficiency methanogenesis of sludge anaerobic digestion according to claim 1, characterized in that, In the process of preparing the carbon-based material in step two, the obtained carbonaceous material is immersed in a 100 mL hydrofluoric acid solution with a concentration of 1 M, and then is placed in a fume hood and heated at 100°C for digestion.

6. The method for enhancing the high-efficiency methanogenesis of sludge anaerobic digestion according to claim 1, characterized in that, In the process of preparing carbon-based materials described in Step two, the ash-free carbon material is ultrasonically dispersed in 1 M KOH solution with a carbon material / KOH mass ratio of 1:3, then dried at 80 °C and ground into powder. The mixture is placed in a tube furnace and heated to 700 °C at a temperature rising rate of 10 °C min -1 and kept for 2 h. After continuing to cool down with the furnace to room temperature, the obtained solid product is repeatedly washed with 1 M HC1 solution and a large amount of deionized water, and dried at 105 °C for 6 h to obtain the final product, a multi-stage porous carbon material.

7. The method according to claim 1, wherein the method is characterized by, In the electrochemical pretreatment in step three, the reaction is carried out in an organic glass reactor with a volume of 1 L, and the length, width and height of the reactor are all 10 cm. The electrode plates used for the cathode and the anode are both Ti / RuO2 mesh electrode plates with a size of 10×12 cm2 (width×height), and the power supply is a stabilized direct current power supply.

8. The method for enhancing the high-efficiency methanogenesis of sludge anaerobic digestion according to claim 1, characterized in that, In the electrochemical pretreatment of step three, gas stirring (aeration amount of 0.78~0.80 m 3 / h•m 3 ) is used in the treatment process to prevent the sludge from stratification during the electrochemical treatment, which affects the treatment effect.

9. The method according to claim 1, wherein the method is characterized by, In the anaerobic fermentation in step four, the sludge after secondary treatment is placed in an anaerobic digestion reactor, the temperature of the reaction system is set to 20-40°C, the pH is set to 6.5-7.5, and the anaerobic digestion reaction is carried out, and the digestion gas and concentrated sludge generated in the reaction are collected.

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