Method for concentrating, filtering and recycling biogas slurry

Abstract

The invention belongs to the technical field of biogas slurry recycling, and particularly relates to a biogas slurry concentrating, filtering and recycling method which comprises the following steps: introducing biogas slurry in a biogas slurry tank into a plate filter, putting a filter aid into the plate filter to form a filter cake layer, and filtering out large particles in the biogas slurry through the filter cake layer to finish pretreatment; introducing the pretreated biogas slurry into an ultrafiltration device, carrying out ultrafiltration treatment, and further filtering macromolecular substances in the biogas slurry; introducing the clarified liquid subjected to ultrafiltration treatment into an electrodialysis device for filtering salt; the biogas slurry flowing through the electrodialysis device is introduced into a reverse osmosis membrane system, nutrient substances in the biogas slurry are concentrated, and the nutrient substances are used as organic fertilizer raw materials. Most suspended substances in the biogas slurry can be effectively removed through ultrafiltration, salt in the biogas slurry can be effectively reduced through electrodialysis, and nutrient components in the biogas slurry are concentrated and enriched to serve as organic fertilizer raw materials to achieve resource utilization of the biogas slurry.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of biogas slurry resource utilization, and particularly relates to a biogas slurry concentration and filtration recovery method. BACKGROUND

[0002] At present, the consumption of renewable energy is increasing rapidly in the world. In the face of energy security issues, the impact of traditional fuels on the environment, and the requirements of improving living standards and renewable technologies, bioenergy can play a core role in promoting renewable alternatives. Bioenergy is the fourth largest energy in the world, and due to its renewable and widely applicable characteristics and abundant resources, it is almost a neutral substitute for fossil fuels. Biogas engineering produces biogas and biogas fertilizer through microbial decomposition of bioenergy in a closed biogas tank through anaerobic fermentation technology, and is a promising means to meet global energy demand and provide various environmental benefits.

[0003] However, a large amount of biogas slurry produced by biogas engineering will greatly increase the operating cost if it is treated as waste water, and will cause secondary pollution if it is not properly treated. Therefore, it is necessary to treat biogas slurry in biogas engineering.

[0004] In addition, biogas slurry contains a large amount of elements such as nitrogen, phosphorus, potassium, trace elements such as Fe, Cu, Mn, Zn, and various amino acids, humic acid, vitamins, plant hormones, etc., which are easily absorbed by crops, so biogas slurry has high fertilizer efficiency. However, the biogas slurry around the biogas engineering has limited capacity for absorption by farmland, and excessive use of biogas slurry may cause crop seedling burn and soil eutrophication. If the absorption radius is expanded, the transportation cost is too high, and it lacks economic feasibility. Therefore, concentrating and reducing the amount of biogas slurry and then expanding the transportation and absorption radius is a more economically feasible treatment method.

[0005] In the process of treating biogas slurry by existing technology, reverse osmosis membrane concentration technology is commonly used. It uses a reverse osmosis membrane system to separate pure water from wastewater by applying high pressure. However, the salt content in biogas slurry is high, and as the concentration process proceeds, the osmotic pressure increases, resulting in a decrease in the driving force for reverse osmosis. It is difficult to achieve high concentration multiple of biogas slurry in the reverse osmosis membrane system. SUMMARY

[0006] The purpose of the present application is to provide a biogas slurry concentration and filtration recovery method, which can effectively remove most of the suspended matter in the biogas slurry by ultrafiltration, and effectively reduce the salt content in the biogas slurry by electrodialysis. After concentration and enrichment of the nutrient components in the biogas slurry as organic fertilizer raw materials, the biogas slurry is resourcefully utilized.

[0007] The technical solutions adopted by the present application are as follows: A biogas slurry concentration and filtration recovery method, comprising the following steps: The biogas slurry in the biogas slurry pool is introduced into the plate filter, and the filter aid is put into the plate filter to form a filter cake layer, and the large particles in the biogas slurry are filtered through the filter cake layer to complete the pretreatment; The biogas slurry after the pretreatment is introduced into the ultrafiltration device to perform the ultrafiltration treatment, and the macromolecular substances in the biogas slurry are further filtered. The clarified liquid after the ultrafiltration treatment is introduced into the electrodialysis device to filter the salt. The biogas slurry flowing through the electrodialysis device is introduced into the reverse osmosis membrane system to concentrate the nutrients in the biogas slurry, and the nutrients are used as raw materials for organic fertilizer.

[0008] As an optional solution, the filter aid includes one or more of diatomite, perlite and activated carbon.

[0009] As an optional solution, the effective membrane area of the ultrafiltration device is 1484.4 cm 2 , the filtering precision is 0.01 um-0.1 um, and the operating pressure is 1 bar-2 bar.

[0010] As an optional solution, the ultrafiltration device includes a filtering channel composed of 35 hollow fiber membranes, and the filtering channel ports are sequentially connected to the booster pump and the transfer tank.

[0011] As an optional solution, the ultrafiltration treatment specifically includes the following steps: The biogas slurry after the pretreatment is poured into the transfer tank; The booster pump is started to suck the biogas slurry in the transfer tank, the output pressure of the booster pump is adjusted to 1 bar, the biogas slurry flows through the filtering channel, and the ultrafiltration is started; The running time of one ultrafiltration is set to 1 h, and the obtained ultrafiltration biogas slurry is recorded every 10 h.

[0012] As an optional solution, the direct current voltage of the electrodialysis device is set to 12 V-24 V, and the current is 0 A-4 A.

[0013] As an optional solution, the salt filtering specifically includes the following steps: Preparation after starting: the electrodialysis device is checked to ensure that there is no residual impurities in the water tank, the pipeline connection is correct without leakage, and the circuit contact is normal, Operation control: when starting, the corresponding water samples are added to the water tanks in the concentration chamber, the dilution chamber and the electrode chamber of the electrodialysis device; After starting the water pump, the valves of the water tanks in the concentration chamber, the dilution chamber and the electrode chamber are sequentially opened, the water inlet pressure is increased to the rated flow, and after the electrodialysis device is stabilized, the direct current power supply is started, and the operating voltage is gradually adjusted to 12 V; Shutdown management: the power supply is turned off, the water pump is stopped, and the membrane stack is flushed with biogas slurry or fresh water for 10 min.

[0014] As an alternative, 1L volume of 3% sodium chloride solution is added to the water sample in the polar chamber water tank, and the three-way return water pipeline is adjusted until the water flow returns to the corresponding water tank and maintains the stable liquid level.

[0015] As an alternative, while the salt is filtered, the voltage, current, concentration and dilution water flow and conductivity data of the electrodialysis device are recorded every 10 minutes, and after the concentration and dilution water outlet meets the standard through water quality detection, the reverse osmosis membrane system is started.

[0016] As an alternative, the reverse osmosis membrane system includes a disc tube reverse osmosis membrane, and the operating pressure of the reverse osmosis membrane system is 5bar-25bar.

[0017] The technical effects obtained by the present application are: The ultrafiltration of the present application can effectively remove most of the suspended matter in the biogas slurry, and the removal rate of suspended matter reaches 97.9%, which greatly reduces the pollution and damage to the subsequent equipment, and the electrodialysis of the ultrafiltration can effectively reduce the salt content in the biogas slurry, and the desalination rate reaches 99.2%, which reduces the osmotic pressure during reverse osmosis and improves the concentration efficiency.

[0018] The reverse osmosis of the present application has good effect on concentrating biogas slurry, and the concentration effect of ammonia nitrogen in the biogas slurry water reaches 9 times, and the concentration effect of COD reaches 5 times, and the nutrient components enriched in the concentrated liquid can be used as organic fertilizer raw materials to realize resource utilization of biogas slurry, and the overall processing flow through the processes of reduction, harmless and nutrient enrichment, provides strong support for resource utilization of biogas slurry after concentration, such as fertilizer production and water resource recycling, which meets the target and demand of biogas slurry concentration resource utilization. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a flow chart of the biogas slurry concentration filtration and recovery method of the present application; Figure 2 is a statistical chart of the flux when the biogas slurry is ultrafiltered; Figure 3 is a statistical chart of the voltage and current changes when the electrodialysis is performed; Figure 4 is a statistical chart of the concentration water, dilution water and polar water flow changes when the electrodialysis is performed; Figure 5 is a statistical chart of the conductivity changes of the concentration water and dilution water when the electrodialysis is performed; Figure 6 is a statistical chart of the dilution water flux and interception of the reverse osmosis. DETAILED DESCRIPTION

[0020] In order to make the objects and advantages of the present application more clear, the following will specifically describe the present application in combination with examples. It should be understood that the following description is only used to describe one or several specific embodiments of the present application, and does not strictly limit the protection scope of the present application.

[0021] As shown in Figures 1-6 , a biogas slurry concentration filtration recovery method comprises the following steps: Pre-treatment coarse filtration: the biogas slurry in the biogas slurry pool is introduced into a plate filter, and a filter aid is put into the plate filter to form a filter cake layer, so that large particles in the biogas slurry are filtered out through the filter cake layer, and the pre-treatment is completed; Wherein, the biogas slurry can be the discharge of a biogas project using livestock and poultry manure and straw and other agricultural wastes as fermentation raw materials, and the filter aid can be one or more of diatomite, perlite and activated carbon; Ultrafiltration: a plurality of hollow fiber membranes are bundled side by side into an ultrafiltration device, and the ultrafiltration device is used to perform ultrafiltration treatment on the coarsely filtered biogas slurry, so as to further filter large molecular substances and the like in the biogas slurry; Wherein, the large molecular substances include bacteria, suspended solids, colloids and the like; As an optional embodiment, the ultrafiltration device comprises a filter channel composed of 35 hollow fiber membranes with an outer diameter size of 0.18 cm and a length size of 75 cm, and the filter channel ports are sequentially connected to a booster pump and a transfer tank. The effective membrane area of the entire ultrafiltration device is 1484.4 cm 2 , the filtration precision is 0.01 um-0.1 um, and the operating pressure of the booster pump is 1 bar-2 bar; During the ultrafiltration treatment, the coarsely filtered biogas slurry is poured into the transfer tank of the ultrafiltration device, the output pressure of the booster pump is adjusted to 1 bar, the biogas slurry flows through the filter channel, the ultrafiltration starts, and the running time of one set of experiments in the ultrafiltration is 1 h. The obtained ultrafiltration biogas slurry is recorded once every 10 h, and the permeation flux is calculated by using the flux formula as follows: Formula (1) Wherein, the permeation flux is Lm -2 h -1 bar -1 , is the volume of the permeated biogas slurry, m 3 , is the effective membrane area, is the filtration time, h, is the operating pressure, bar; As shown in Figure 2 , the records of the biogas slurry ultrafiltration data are shown in Table 1, and the biogas slurry flux is 0.145 Lm -2 h-1 bar -1 decreased to 0.095 Lm -2 h -1 bar -1 It can be seen that the biogas slurry flux gradually decreases, the hollow fiber membrane is rapidly contaminated in the initial stage, the flow channel of the hollow fiber membrane is narrow, and the fluid shear force is low during operation, so that a sludge layer or particle deposition is rapidly formed on the surface of the hollow fiber membrane, resulting in a decrease in the flux; Table 1: Data measured after biogas slurry filtration Type of biogas slurry SS (mg / L) Ammonia nitrogen (mg / L) COD (mg / L) Conductivity TP (mg / L) Stock solution 5130.4 1789.75 6865 14.30 30.40 Pre-treatment coarse filtration 3101.94 1711.01 5755 13.31 30.20 Ultrafiltration 105.22 1537.50 1930 13.28 30.00 As can be seen from Table 1, the biogas slurry flux as a whole shows a downward trend, but the decrease is not large and is relatively stable, which is suitable for small water treatment equipment, has strong anti-pollution stability, and is suitable for continuous operation; In terms of suspended matter removal effect, as shown in Table 1, the ultrafiltration has a very significant effect on the removal of suspended solids, with a removal rate of 97.9%; This indicates that the hollow fiber membrane in the ultrafiltration device can almost completely intercept the suspended particles, bacteria, colloids and other macromolecular substances in the biogas slurry; After ultrafiltration, the biogas slurry changes from turbid to very clear, which is the most core and direct effect of the ultrafiltration process, and the concentration of pollutants in the biogas slurry is significantly reduced, thereby providing strong technical support for subsequent biogas slurry treatment and resource recovery; At the same time, the decrease in COD indicates that a large amount of insoluble, colloidal macromolecular substances in the biogas slurry are intercepted by the ultrafiltration membrane, thereby causing a small decrease in the COD value, which indicates that ultrafiltration plays an outstanding role in reducing the organic pollution load in the biogas slurry water; At the same time, the ammonia nitrogen content also shows a small decrease, which indicates that the ultrafiltration can well remove the macromolecular pollutants in the biogas slurry while retaining most of the nutrients in the biogas slurry, thereby ensuring a good foundation for subsequent treatment; Electrodialysis: The clear liquid after ultrafiltration is introduced into an electrodialysis device for filtering salt, including three stages of starting preparation, operation control and shutdown management, as follows: Starting preparation: Before starting the electrodialysis device, device inspection should be completed to ensure that there are no foreign matter residues in the water tank, the electrodialysis device pipeline connection is correct without leakage, and the circuit system contact is normal, Operation control: When starting, the first step is to add corresponding water samples in the concentrated chamber, the dilute chamber and the electrode chamber water tank; Among them, about 1L of 3% sodium chloride solution needs to be added to the water sample in the electrode chamber water tank, the three-way return water pipeline is adjusted to ensure that the water flow returns to the corresponding water tank and maintains stable liquid level, and all pipelines follow the "down-in and up-out" flow direction; After starting the water pump, open the valves of the concentrated chamber, dilute chamber and polar chamber water tank in turn, gradually increase the water inlet pressure to the rated flow, the concentrated and dilute water flow is about 200 L / h, the polar water flow is 80 L / h, and the direct current power is turned on after the electrodialysis device is stable, and the operating voltage is gradually adjusted to 12V; During operation, record the voltage, current, concentrated and dilute water flow and conductivity data every 10 minutes, and confirm that the concentrated and dilute water outlets meet the standards before entering the next stage through water quality detection; Figure 3 Record the changes of voltage (12V) and current with time (every 10 minutes) during the electrodialysis (ED) process. The current is high at the beginning of operation, for example, the initial current is 4A, and it gradually stabilizes to 0.10A over time, indicating that the membrane stack has serious concentration polarization. When the voltage is constant at 12V, the current decreases, which is related to the membrane surface pollution or ion concentration gradient change; Figure 4 Record the flow changes of concentrated water 200L / h, dilute water 200L / h and polar water 80L / h; Referring to Figure 4 , the polar water flow decreases due to air lock effect and crystal precipitation, and the concentrated and dilute water flows also decrease slightly due to concentration polarization in the later stage. The flow stability directly affects the ion migration efficiency and membrane pollution rate. By electrodialysis to reduce the osmotic pressure of the feed liquid, stable concentrated and dilute water flow can ensure effective separation of salt ions and ensure the feasibility of the technical route; To obtain the salt ion rejection rate (%) calculated by the following formula (2): Formula (2) Wherein, is the concentration of salt in the original biogas slurry entering the electrodialysis device, is the concentration of salt in the permeate; Figure 5 Record the changes of concentrated and dilute water conductivity during electrodialysis. The conductivity of concentrated water increases from 13.28ms / cm to 21.10ms / cm. The significant increase in the conductivity of concentrated water indicates the enrichment effect of electrodialysis on salt ions. The conductivity of dilute water decreases to 106.01μs / cm, and the desalination rate reaches 99.2%, which verifies the excellent salt rejection effect of electrodialysis on biogas slurry; Shutdown management: turn off the power and stop the water pump, and flush the membrane stack with biogas slurry or dilute water for 10 minutes. This step is crucial because it replaces the high-concentration salt water in the membrane stack to prevent ion diffusion and scaling caused by concentration difference during shutdown. After shutdown, ensure that the membrane stack is filled with dilute water or softened water, and close all inlet and outlet valves to prevent water evaporation and air entry. If there is a risk of freezing in the ambient temperature, insulation or heating measures must be taken to prevent membrane stack cracking; After the electrodialysis, the effluent biogas slurry was measured; Table 2 Measurement data of biogas slurry after electrodialysis Item SS (mg / L) Ammonia nitrogen (mg / L) COD (mg / L) Conductivity TP (mg / L) Ultrafiltrated biogas slurry 105.22 1537.50 1930 13.28 30.00 Concentrate 37.94 1975.75 2225 21.10 40.00 Dilute water 9.62 50.49 1920 106.01 μs / cm 20.00 As shown in Table 2, the ammonia nitrogen content in the concentrated water after electrodialysis increased to 1975.75 mg / L, but in order to reduce the osmotic pressure of high-salt biogas slurry in reverse osmosis and reduce the energy consumption of reverse osmosis, the embodiment selects to use the fresh water after electrodialysis as subsequent concentration. After the electrodialysis treatment, the ammonia nitrogen in the fresh water of the biogas slurry showed a downward trend and decreased to 50.49 mg / L, which was due to the enrichment of ammonium ions to the concentrated water compartment under the action of the electric field, resulting in a substantial decrease, but the nutrients in the fresh water can be well concentrated by subsequent reverse osmosis; The core value of electrodialysis in this scenario is desalination, and very few neutral small-molecule organic matters are removed, so the changes of COD and TP contents in the biogas slurry are small; Reverse osmosis: the biogas slurry after electrodialysis is introduced into the reverse osmosis membrane system to concentrate the nutrients in the biogas slurry, and the nutrients are used as organic fertilizer raw materials. The specific steps are as follows: After the liquid is injected into 10 L, the flow rate is adjusted to 300 L / h-500 L / h by the frequency converter, and the equipment is kept running until the flow rate is stable to ensure that the residual air in the system is completely discharged. After the above steps are completed, the opening degree of the pressure regulating valve is gradually adjusted to slowly reach the preset experimental parameter 5 bar-10 bar, and the adjustment process is kept continuous and stable. When the experiment is terminated, the bypass valve is opened first to reduce the pressure to the minimum threshold, then the stop switch is pressed to cut off the power supply, the running indicator light is turned off, and finally all the valves are reset to the initial state to complete the operation process. To obtain the concentration multiple of the nutrients , the following formula (3) is used for calculation: Formula (3) Wherein, is the concentration of nutrients in the concentrated solution, is the concentration of nutrients in the biogas slurry entering the reverse osmosis membrane system; Figure 6 The flux and interception of the biogas slurry concentrated by the long-time running reverse osmosis device are recorded. The reverse osmosis has a good concentration effect on the biogas slurry, and the salt interception rate of the biogas slurry is more than 99%. The flux is changed from the initial 4.3 Lm -2 h -1 bar -1 to the later 3.7 Lm -2 h -1 bar -1And the flux remains stable, which shows that the reverse osmosis membrane has stability for the concentration effect of biogas slurry, and effectively concentrates the nutrients in the biogas slurry; And the fresh water produced by the reverse osmosis membrane system is measured, and the measurement data is shown in Table 3 as follows: Table 3 Reverse osmosis fresh water measurement data Item Ammonia nitrogen (mg / L) COD (mg / L) Conductivity TP (mg / L) Dilute water 50.49 1920 106.01 μs / cm 20 Produced water 3.01 60 21.5 μs / cm 15 Concentrate 456.26 9595 20.01 40 As shown in Table 3, the reverse osmosis membrane system has very good interception effect for concentrating the nutrients in the biogas slurry, because the pore size of 0.1nm-0.7nm of the reverse osmosis membrane can effectively intercept small molecular substances; The concentration effect of the reverse osmosis membrane system for ammonia nitrogen in the biogas slurry reaches 9 times, the concentration effect for COD reaches 5 times, and the concentration effect for phosphorus reaches 2 times, so that the nutrient components are concentrated in the concentrated liquid, which lays a foundation for subsequent preparation of high-nutrient organic fertilizer and other resource utilization.

[0022] The concentrated liquid rich in nutrient components can be used as organic fertilizer raw material to realize resource utilization of biogas slurry, and the overall treatment process provides strong support for resource utilization of biogas slurry after concentration, such as fertilizer production and water resource recycling, and meets the target and demand of biogas slurry concentration resource utilization.

[0023] The above is only an optional embodiment of the present application, and it should be pointed out that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which should be considered as the protection scope of the present application. The structures, devices and operation methods not specifically described and explained in the present application, such as without special description and limitation, are implemented according to the conventional means in the art.

Claims

1. A method for concentrating, filtering, and recovering biogas slurry, characterized in that, Includes the following steps: The biogas slurry in the biogas slurry pond is fed into a plate filter, and a filter aid is added to the plate filter to form a filter cake layer. Large particles in the biogas slurry are filtered out through the filter cake layer, thus completing the pretreatment. The pretreated biogas slurry is passed into an ultrafiltration device for ultrafiltration treatment to further filter out macromolecular substances in the biogas slurry; The clarified liquid after ultrafiltration is passed into an electrodialysis device to filter out salts. The biogas slurry flowing through the electrodialysis unit is passed into a reverse osmosis membrane system to concentrate the nutrients in the biogas slurry, which are then used as raw materials for organic fertilizer.

2. The method for concentrating, filtering, and recovering biogas slurry according to claim 1, characterized in that: The filter aid includes one or more of diatomaceous earth, perlite, and activated carbon.

3. The method for concentrating, filtering, and recovering biogas slurry according to claim 1, characterized in that: The effective membrane area of ​​the ultrafiltration device is 1484.4 cm². 2 The filtration accuracy is 0.01um to 0.1um, and the operating pressure is 1 bar to 2 bar.

4. The method for concentrating, filtering, and recovering biogas slurry according to claim 1, characterized in that: The ultrafiltration device includes a filtration channel composed of 35 hollow fiber membranes, and the ports of the filtration channel are sequentially connected to a booster pump and a transfer tank.

5. The method for concentrating, filtering, and recovering biogas slurry according to claim 4, characterized in that, The ultrafiltration process specifically includes the following steps: The pretreated biogas slurry is poured into the transfer tank; Start the booster pump to draw biogas slurry from the transfer tank, adjust the output pressure of the booster pump to 1 bar, and let the biogas slurry flow through the filter channel to start ultrafiltration; The running time for one ultrafiltration cycle is set to 1 hour, and the amount of ultrafiltration slurry obtained is recorded every 10 hours.

6. The method for concentrating, filtering, and recovering biogas slurry according to claim 1, characterized in that: The electrodialysis device is set with a DC voltage of 12V to 24V and a current of 0A to 4A.

7. The method for concentrating, filtering, and recovering biogas slurry according to claim 1, characterized in that, The process of filtering out salt specifically includes the following steps: Start-up preparation: Complete the inspection of the electrodialysis device to ensure that there is no debris remaining in the water tank, that the pipeline connections are correct and leak-free, and that the circuit contacts are normal. Operation control: When starting up, add the corresponding water samples to the concentrated chamber, dilute chamber and electrode chamber water tank of the electrodialysis device respectively; After starting the water pump, open the valves of the concentrate chamber, dilute chamber and polarization chamber water tank in sequence to increase the inlet water pressure to the rated flow. After the electrodialysis device stabilizes, turn on the DC power supply and gradually adjust the operating voltage to 12V. Shutdown management: Turn off the power, stop the water pump, and flush the membrane stack with biogas slurry or fresh water for 10 minutes.

8. The method for concentrating, filtering, and recovering biogas slurry according to claim 7, characterized in that: Add 1L of 3% sodium chloride solution to the water sample in the polar chamber water tank, and adjust the three return water pipelines until the water flows back to the corresponding water tank and maintains a stable liquid level.

9. The method for concentrating, filtering, and recovering biogas slurry according to claim 7, characterized in that: While filtering out salts, the voltage, current, concentrated and dilute water flow rates, and conductivity data of the electrodialysis device are recorded every 10 minutes. After confirming that the concentrated and dilute water outlets meet the standards through water quality testing, the reverse osmosis membrane system is turned on.

10. The method for concentrating, filtering, and recovering biogas slurry according to claim 1, characterized in that: The reverse osmosis membrane system includes a disc tube type reverse osmosis membrane, and the operating pressure of the reverse osmosis membrane system is 5 bar to 25 bar.

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