Method for preparing clean biological methane gas from duckweed residues

Abstract

The invention relates to the technical field of biological fermentation engineering, and discloses a method for preparing clean biological methane gas from duckweed residues, which comprises the following steps: fermenting anaerobic granular sludge as an inoculant and duckweed residues as a fermentation substrate in an anaerobic fermentation tank to prepare methane; the anaerobic fermentation tank comprises a tank body, a cover body and a regulation and control unit, wherein the regulation and control unit comprises a controller, a display screen, a barrel, a telescopic device for driving the barrel to lift, and a pH sensor, a conductivity sensor and an acid and alkali liquor adding pipe which are fixed in the barrel; the liquid inlet end of the acid and alkali liquid adding pipe is communicated with an acid liquid pipe, an alkali liquid pipe and a flushing liquid pipe through a four-way valve; the telescopic device, the pH sensor, the conductivity sensor, the four-way valve and the display screen are all electrically connected with the controller. According to the scheme, the regulation and control unit is additionally arranged, so that the pH of the fermentation liquor is effectively and accurately regulated and controlled in real time, the pH of the fermentation liquor is ensured to be within the optimal active pH range of methanogens, the activity of the methanogens and the methane yield are further improved, and the production benefits are improved.

Description

Technical Field

[0001] The invention relates to the technical field of biological fermentation engineering, and in particular to a method for preparing clean biomethane gas from duckweed residue. Background Art

[0002] Energy shortage and environmental problems caused by traditional energy have attracted great attention from the world. Duckweed is not only a green and efficient plant for repairing livestock and poultry sewage, industrial sewage and domestic sewage, but also considered as a high-quality new biomass raw material, mainly used to produce clean energy such as bioethanol and biomethane, because it is easy to cultivate, grows fast, has high starch content and fast accumulation, has low lignin content, does not occupy arable land, and can be cultivated directly in sewage. However, the existing duckweed cultivated with high pollution loads such as livestock and poultry sewage, because it is rich in heavy metals such as zinc and copper, not only limits the ability of duckweed green plant extract to ferment and produce bioenergy, but also seriously inhibits the ability of duckweed residue to prepare bioenergy by anaerobic fermentation, especially the ability of duckweed to prepare biomethane. For the duckweed residue after hydrolysis of duckweed cultivated with high concentration of heavy metals, the duckweed residue after acid pyrolysis still contains more carbohydrates. This kind of duckweed residue is also an important resource for bioenergy. Abandonment not only pollutes the environment but also causes waste of resources. Therefore, how to improve the ability of duckweed residue and other by-products of green plants cultivated in heavy metal-rich wastewater to produce biomethane and achieve the goal of deep resource utilization, energy utilization and volume reduction of green plant by-products has become an urgent problem to be solved.

[0003] Anaerobic granular sludge is a highly efficient anaerobic activated sludge strain, rich in various anaerobic microorganisms such as methanogens, acetogens and hydrolytic fermentation bacteria. These microbial populations work together to produce biomethane by fermenting duckweed residue and anaerobic granular sludge, realizing the energy conversion of pollutants and the reduction, resource utilization and harmlessness of sludge. However, the existing anaerobic sludge fermentation duckweed residue methanogen production process still has the following technical problems: (1) During the process, the pH of the fermentation liquid gradually decreases as the fermentation progresses, and even falls below the active pH of methanogens, resulting in reduced methanogen activity and reduced fermentation gas production. (2) In order to adjust the pH of the fermentation liquid during the fermentation process, if the adjustment is not timely, the fermentation effect will be reduced; and when preparing clean biomethane gas, the degree of acid and alkali solution control required for different fermentation substrates is different, and the pH during the fermentation process must be accurately controlled. (3) Especially for bioenergy wastes such as duckweed residue after acid treatment, it is not only affected by the pH but also by the metal ions it contains, which limits the anaerobic fermentation process, prolongs the fermentation time, and also affects the fermentation gas production.

[0004] In view of this, developing a method for preparing clean biomethane gas from duckweed residue by timely adjusting pH not only effectively makes up for the shortcomings of the existing technology, but also can specifically adjust the pH of the fermentation liquid without destroying the fermentation sealing, which is of great significance to improving the ability of duckweed residue to prepare clean biomethane gas and increasing the gas production of duckweed residue. Summary of the invention

[0005] The present invention aims to provide a method for preparing clean biomethane gas from duckweed residue, so as to solve the technical problem of reduced gas production caused by unstable pH control during the existing duckweed residue fermentation process.

[0006] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a method for preparing clean biomethane gas from duckweed residue, comprising fermenting and preparing clean methane in an anaerobic fermentation tank using anaerobic granular sludge as an inoculum and duckweed residue as a fermentation substrate; the anaerobic fermentation tank comprises a tank body, a cover body and a control unit, the control unit comprises a controller, a display screen, a barrel, a telescopic device for driving the barrel to rise and fall, and a pH sensor, a conductivity sensor and an acid and alkali solution dosing pipe fixed in the barrel; the liquid inlet end of the acid and alkali solution dosing pipe is connected to an acid pipe, an alkali pipe and a flushing liquid pipe through a four-way valve; the telescopic device, the pH sensor, the conductivity sensor, the four-way valve and the display screen are all electrically connected to the controller;

[0007] The pH sensor is used to detect the pH of the fermentation liquid in the anaerobic fermentation tank to form a pH signal and transmit it to the controller. The conductivity sensor is used to detect the conductivity and temperature of the fermentation liquid to form a conductivity signal and a temperature signal respectively and transmit them to the controller. The controller is used to receive the pH signal, the conductivity signal and the temperature signal, and while transmitting the pH signal, the conductivity signal and the temperature signal to the display screen for display, the four-way valve is controlled to allow liquid to flow in according to the pH signal and the conductivity signal.

[0008] The principles and advantages of this solution are:

[0009] 1. Compared with the reduced gas production caused by unstable pH control during the fermentation of duckweed residue treated with acid, this scheme effectively and accurately controls the pH of the fermentation liquid in real time by adding a control unit to ensure that the pH of the fermentation liquid is within the optimal active pH range of methanogens, thereby improving the activity of methanogens and methane production, and improving production efficiency.

[0010] 2. Compared with the existing duckweed residue fermentation process in which the pH is not adjusted or the pH is adjusted by opening the lid, which can easily delay the fermentation process and reduce the methane production, this solution adds a control unit to facilitate the detection and control of the pH of the fermentation liquid in the anaerobic fermentation tank without opening the lid or sealing, effectively avoiding the impact of non-sealed pH adjustment on methane production, thereby effectively increasing methane production and improving production efficiency.

[0011] 3. This solution fixes the pH sensor, conductivity sensor and acid and alkali solution dosing tube in the barrel, and adds a telescopic device to drive the barrel to rise and fall, so as to facilitate the control of the pH sensor and conductivity sensor to switch between the "non-detection state" and the "detection state". Specifically, when the telescopic device is driven to descend, the barrel and the detection element in the barrel are both extended into the fermentation liquid and are in the "detection state". At this time, the pH sensor and the conductivity sensor are in contact with the fermentation liquid to detect the pH and conductivity in the fermentation liquid; and when the detection and regulation are completed, the telescopic device is driven to rise, and the barrel and the detection element in the barrel leave the fermentation liquid and are in the "non-detection state". At this time, the pH sensor and the conductivity sensor leave the fermentation liquid to avoid corrosion caused by long-term contact between the fermentation liquid and the detection element. It can be seen that by combining the barrel and the telescopic device, not only can the pH sensor, the conductivity sensor and the acid and alkali solution dosing tube be lifted and lowered at the same time, but the three can also be effectively protected from damage caused by physical impact and long-term chemical contact corrosion.

[0012] 4. This scheme sets up an acid and alkali solution dosing pipe, and connects the liquid inlet end of the acid and alkali solution dosing pipe to an acid pipe, an alkali pipe and a flushing liquid pipe through a four-way valve, so as to control the addition of acid or alkali to adjust the pH of the fermentation liquid according to the test results. After the addition, the detection element that has been in contact with the fermentation liquid can be flushed by adding flushing liquid to avoid damage due to long-term contact with the detection element; and when the fermentation liquid is too concentrated, the fermentation liquid can be diluted by adding flushing liquid, thereby ensuring high fermentation efficiency and increasing gas production.

[0013] Preferably, as an improvement, the inner top wall of the barrel is provided with a first telescopic device corresponding to the pH sensor, and a second telescopic device is provided corresponding to the conductivity sensor, and the detection elements of the pH sensor and the conductivity sensor are fixed at the lifting ends of the telescopic devices; the first telescopic device and the second telescopic device are both electrically connected to the controller.

[0014] Technical effect: This scheme adopts the above-mentioned setting, which is convenient for driving the detection elements of the pH sensor and the conductivity sensor to rise and fall respectively according to the detection needs. Specifically, the connection and lifting method are explained by taking the lifting cylinder as an example: the detection elements of the pH sensor and the conductivity sensor are respectively fixed on the piston of a lifting cylinder. When it is necessary to detect the physical and chemical properties in the fermentation liquid, the controller controls the upper chamber of the first telescopic device and the second telescopic device to be inflated, so that the piston moves downward, driving the detection elements of the pH sensor and the conductivity sensor to move downward to contact the fermentation liquid, thereby realizing the detection of the physical and chemical properties of the fermentation liquid. After the detection and regulation are completed, the controller controls the lower chamber of the first telescopic device and the second telescopic device to be inflated, so that the piston moves upward, driving the detection elements of the pH sensor and the conductivity sensor to move upward to leave the fermentation liquid, thereby preventing the fermentation liquid from contacting the detection elements for a long time and corroding and damaging the detection elements.

[0015] Preferably, as an improvement, the cover body is provided with an adjustment opening for inserting the barrel and fixing the telescopic device.

[0016] Technical effect: This scheme adopts the above settings to facilitate equipment maintenance and replacement.

[0017] Preferably, as an improvement, the controller and the display screen are fixed to the side wall of the tank body, and the controller is also electrically connected to an alarm.

[0018] Technical effect: This scheme adopts the above settings, which is convenient for fixation, observation and possible intervention and regulation.

[0019] Preferably, as an improvement, the side wall of the barrel is provided with an opening, and the opening divides the longitudinal space in the barrel into a measuring chamber and a protection chamber.

[0020] Technical effect: This scheme adopts the above-mentioned setting. When the drum descends to extend into the fermentation liquid, the fermentation liquid enters the measuring cavity from the opening, which makes it convenient for the detection elements of the pH sensor and the conductivity sensor to contact the fermentation liquid when they descend to complete the detection; when the detection is completed, the detection element is rinsed with a rinse liquid, and the detection element is controlled to rise into the protection cavity, and the rinse liquid falls into the measuring cavity to flush out the fermentation liquid and acid / alkali solution, effectively avoiding high-concentration acid and alkali solution from remaining in the measuring cavity for a long time and affecting the overall pH of the fermentation liquid.

[0021] Preferably, as an improvement, a plurality of liquid permeable openings are provided at the bottom of the barrel, and the diameters of the liquid permeable openings gradually decrease from top to bottom.

[0022] Technical effect: This scheme adopts the above-mentioned setting. When the drum as a whole descends, the fermentation liquid enters the drum from the liquid permeable port, which makes it easier for the detection elements of the pH sensor and the conductivity sensor to contact the fermentation liquid when they descend to complete the detection; and when the detection is completed, the fermentation liquid automatically permeates out from the liquid permeable port when the drum rises, so as to empty the fermentation liquid in the drum. By setting the diameter of the liquid permeable port to gradually decrease from top to bottom, when the drum descends, the diameter of the fermentation liquid gradually increases when it enters the drum from the liquid permeable port, so that the pressure of the fermentation liquid gradually decreases, slowing down the liquid flow speed, and avoiding splashing onto the inner wall or the telescopic device when the fermentation liquid rapidly enters the drum and damaging the equipment.

[0023] Preferably, as an improvement, the cover body is detachably connected with a stirring device, and the stirring device extends into the tank body.

[0024] Technical effect: This scheme adopts the above-mentioned setting, which is convenient for stirring and homogenizing the fermentation liquid in the fermentation tank, on the one hand, accelerating the fermentation process; on the other hand, when the acid / alkali solution is added, it is quickly stirred and homogenized to facilitate the real detection of the physicochemical properties of the fermentation liquid.

[0025] Preferably, as an improvement, the mass ratio of the inoculum to duckweed residue (mass ratio of volatile solids) is 1 to 3:1, and deionized water is used to adjust the ratio of the working volume of the fermentation liquid to the total volume of the anaerobic device to 3:5 to 4:5.

[0026] Preferably, as an improvement, before sealed fermentation, the method further includes adjusting the zinc ion content in the fermentation liquid to less than 0.42 mg / L.

[0027] Technical effect: This scheme adopts the above-mentioned setting to facilitate the full fermentation of duckweed residue and ensure the production gasification space for preparing biomethane.

[0028] Preferably, as an improvement, the fermentation stirring speed is 110-130 rpm, the fermentation temperature is 30-38° C., and the initial pH is 7.0-7.8.

[0029] Technical effect: This scheme adopts the above settings to facilitate rapid fermentation. Specifically, methanogenic microorganisms of the genus Methanogen are more sensitive to pH values, and are more suitable at temperatures between 35 and 40°C and pH between 6.1 and 8.0; acid-producing bacteria have a wide range of adaptability to pH values, and can generally maintain high activity between 4.5 and 8.0. Taking into account the comprehensive fermentation effects of various microorganisms on duckweed residue, the above fermentation conditions are limited to effectively and continuously ferment and produce methane, thereby increasing methane production and production efficiency.

[0030] Preferably, as an improvement, the pH change of the fermentation liquid is observed, and when the pH of the fermentation liquid is lower than 5.5 or the alarm sounds, alkaline solution is added to regulate the fermentation liquid; when the pH of the fermentation liquid is higher than 8.0 or the alarm sounds, acid solution is added to regulate the pH of the fermentation liquid; when the pH is regulated to 6.5-7.5, the regulation unit is closed and anaerobic fermentation is continued; and the pH of the fermentation liquid is detected and regulated every 3-8 hours until the fermentation is completed.

[0031] Technical effect: This scheme adopts the above-mentioned setting, which is convenient for accurately controlling the pH of the fermentation liquid, thereby improving the fermentation efficiency and methane production. Through long-term experiments, the applicant found that the anaerobic fermentation process of duckweed residue is mainly divided into three stages: hydrolysis and fermentation stage, acid production stage, and methane production stage. The first stage is mainly the decomposition of protein into amino acids, etc. In the second stage, simple organic matter is further decomposed by the action of acid-producing bacteria, and large-molecule organic matter is decomposed into small-molecule organic acids and alcohols, providing substrates for the subsequent methane production stage. At the same time, it can also make complex organic matter initially degraded, which is convenient for methanogens to further utilize. In the latter stage, methanogens finally convert small-molecule organic acids, alcohols and other substances into methane and carbon dioxide, and convert the carbon in organic matter into usable energy substances in the form of methane, completing the transformation from complex organic matter to simple inorganic matter and energy gas. When the acid-producing bacteria continue to produce acid and the pH of the fermentation liquid drops below 5.5, if no intervention is made, the methanogens in the fermentation liquid will be in an overly acidic environment for a long time, which will significantly reduce their ability to produce methane by fermentation (on the one hand, the number of bacteria cannot be increased rapidly, and on the other hand, the low pH reduces their activity), thereby reducing the production of biomethane. If intervention is made too early, the degree of fermentation and decomposition of organic matter by acid-producing bacteria will be low, reducing the amount of substrate that can be used by methanogens, and still reducing the production of biomethane by methanogens. The applicant also explored the relationship between the minimum pH of anaerobic fermentation and the biomethane production without intervention and found that the lower the minimum pH, the lower the biomethane production obtained by fermentation, which also verifies the above analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the structure of a fermentation tank in an embodiment of the present invention.

[0033] Figure 2 It is a schematic diagram of the structure of the barrel, acid and alkali solution dosing tube, pH sensor and conductivity sensor in the fermentation tank in the embodiment of the present invention.

[0034] Figure 3 This is a reference diagram of the detection status of the pH sensor and the conductivity sensor in the fermentation tank in the embodiment of the present invention.

[0035] Figure 4 This is a structural diagram of the position of the liquid permeable holes at the bottom of the barrel in the fermentation tank in an embodiment of the present invention.

[0036] Figure 5 The biomethane production curves before and after pH adjustment (i.e., control group and test group) in the method for preparing clean biomethane gas from duckweed residue in an embodiment of the present invention are shown.

[0037] Figure 6 This is the pH change curve of the fermentation liquid of the control group and the test group in the method for preparing clean biomethane gas from duckweed residue in the embodiment of the present invention.

[0038] Figure 7 This is the change curve of zinc content in fermentation broth with different zinc contents during the fermentation process in Experimental Example 2 of the present invention.

[0039] Figure 8 This is the biomethane production curve of fermentation broth with different zinc contents during the fermentation process in Experimental Example 2 of the present invention. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the following examples and experimental examples are conventional means well known to those skilled in the art, and the materials, reagents, etc. used can be obtained from commercial sources.

[0041] The figure marks in the drawings of the specification include: tank body 1, cover body 2, feed port 21, air inlet 22, air outlet 23, agitator 24, controller 31, display screen 32, barrel 41, third cylinder 42, opening 43, measuring cavity 431, protection cavity 432, liquid permeable port 44, acid and alkali solution dosing pipe 5, four-way valve 51, pH sensor 6, first cylinder 61, conductivity sensor 7, second cylinder 71.

[0042] Program Overview

[0043] The present invention provides a fermentation tank for preparing clean biomethane gas from duckweed residue, as shown in 1 to 4, comprising a tank body 1, a cover body 2 and a control unit, wherein the control unit comprises a controller 31, a display screen 32, a barrel 41, a telescopic device for driving the barrel 41 to rise and fall, and a pH sensor 6, a conductivity sensor 7 and an acid and alkali solution dosing pipe 5 fixed in the barrel 41; the liquid inlet end of the acid and alkali solution dosing pipe 5 is connected to an acid pipe, an alkali pipe and a flushing liquid pipe through a four-way valve 51; the telescopic device, the pH sensor 6, the conductivity sensor 7, the four-way valve 51 and the display screen 32 are all electrically connected to the controller 31;

[0044] The pH sensor 6 is used to detect the pH of the fermentation liquid in the anaerobic fermentation tank to form a pH signal and transmit it to the controller 31. The conductivity sensor 7 is used to detect the conductivity and temperature of the fermentation liquid to form a conductivity signal and a temperature signal respectively and transmit them to the controller 31. The controller 31 is used to receive the pH signal, the conductivity signal and the temperature signal, and while transmitting the pH signal and the conductivity signal to the display screen 32 for display, the four-way valve 51 is controlled to allow liquid to enter according to the pH signal and the conductivity signal.

[0045] Specifically, the tank body 1 and the cover body 2 are threadedly connected, and a sealing ring is provided on the cover body 2. After the two are threadedly connected, a sealed space is formed in the tank body 1, which is convenient for anaerobic fermentation. Figure 1As shown, the cover body 2 is also provided with a feed port 21, an air inlet 22, an air outlet 23 and other conventional openings 43 required for an anaerobic fermentation tank. The feed port 21 is used for inoculation and feeding, and is used for connection later. The air inlet 22 is used to introduce nitrogen to create an oxygen-deficient environment in the tank body 1. The air outlet 23 is used to discharge the air inside the tank body 1, and is also used to collect clean methane gas produced by anaerobic fermentation after connecting to a collection device. The remaining openings 43 or instruments required for anaerobic fermentation are prior art and will not be described here.

[0046] The cover body 2 is detachably connected with a stirring device, and the stirring device extends into the tank body 1. As a reference, the stirring device is a stirrer 24 with multiple blades. The cover body 2 is also provided with an adjustment port for the barrel 41 to insert and fix the telescopic device, and a sealing ring is provided at the adjustment port to ensure the sealing effect. As a reference, the telescopic device for driving the barrel 41 can be an optional cylinder (also the third cylinder 42), the cylinder body part of the third cylinder 42 is fixed at the adjustment port, the barrel 41 is fixed at the end of the piston rod of the cylinder, and the two end walls and the piston of the third cylinder 42 are provided with sealing ports for the acid and alkali solution dosing tube 5 and the internal instrument wires to pass through, and each chamber of the third cylinder 42 is reserved with a sufficient length of the acid and alkali solution dosing tube 5 and the internal instrument wires, while protecting the acid and alkali solution dosing tube 5 and the internal instrument wires, ensuring the air tightness and normal lifting of the third cylinder 42.

[0047] like Figure 2 As shown, the top wall of the inner part of the drum 41 is provided with a first telescopic device corresponding to the pH sensor 6, and a second telescopic device is provided corresponding to the conductivity sensor 7. A protective tube is provided outside the acid and alkali solution dosing tube 5, and a fixing piece for fixing the first telescopic device, the second telescopic device and the protective tube is provided in the middle of the drum 41. For reference, the first telescopic device is the first cylinder 61, and the second telescopic device is the second cylinder 71. The detection elements of the pH sensor 6 and the conductivity sensor 7 are respectively fixed at the lifting ends of the first cylinder 61 and the second cylinder 71, so as to facilitate the control of the lifting of the detection elements of the pH sensor 6 and the conductivity sensor 7 according to the detection needs.

[0048] A nozzle is provided at the end of the acid and alkali solution dosing tube 5 to facilitate rapid dispersion or spraying of the liquid added by the acid and alkali solution dosing tube 5. In particular, after the addition of the alkali solution (or acid solution), the cleaning liquid is continued to be sprayed to flush and clean the detection elements of the pH sensor 6 and the conductivity sensor 7, so as to avoid long-term contact with the acid and alkali solution and serious corrosion, thereby effectively extending the service life of the detection elements of the pH sensor 6 and the conductivity sensor 7.

[0049] As an improvement, the side wall of the barrel 41 is provided with an opening 43, and the opening 43 divides the longitudinal space inside the barrel 41 into a measuring chamber 431 and a protection chamber 432 (such as Figure 3When the third cylinder 42 descends until the barrel 41 extends into the fermentation liquid, the fermentation liquid enters the measuring chamber 431 along the opening 43, so that the first cylinder 61 and the second cylinder 71 drive the detection elements of the pH sensor 6 and the conductivity sensor 7 to descend and contact with the fermentation liquid to complete the measurement.

[0050] In another embodiment of the present invention, a plurality of liquid permeable openings 44 are provided at the bottom of the barrel 41, and the diameter of the liquid permeable openings 44 gradually decreases from top to bottom, so that after the third cylinder 42 descends to the barrel 41 and extends into the fermentation liquid, the fermentation liquid enters the barrel 41 along the liquid permeable openings 44, thereby constructing a measurement environment.

[0051] In another embodiment of this solution, Figure 4 As shown, the side wall of the drum 41 is provided with an opening 43, and the bottom is provided with a plurality of liquid permeable openings 44. The combination of the side wall openings 43 and the bottom liquid permeable openings 44 further facilitates the fermentation liquid to quickly enter and exit the drum 41.

[0052] The first cylinder 61, the second cylinder 71, the third cylinder 42, the four-way valve 51, the display screen 32, the agitator motor and other electrical equipment mentioned in this solution are all electrically connected to the controller 31. The controller 31 and the display screen 32 are fixed to the side wall of the tank body 1, and the controller 31 is also electrically connected to an alarm. The electrical equipment mentioned in this article are all prior art, and their structures and models can be selected according to needs, and will not be repeated here.

[0053] The present invention provides a method for preparing clean biomethane gas from duckweed residue, which comprises using anaerobic granular sludge as an inoculum and duckweed residue as a fermentation substrate to ferment and prepare methane in an anaerobic fermentation tank; the method comprises the following steps:

[0054] Step 1: Check the air tightness of the equipment: After the equipment is installed, apply soapy water to each connection position, and introduce nitrogen into the tank for 0.2 to 0.5 hours. After the equipment is tested for air tightness, you can proceed to the subsequent steps;

[0055] Step 2, add sample and mix to form fermentation liquid: add inoculum, duckweed residue and sterilized deionized water into the fermentation tank, stir and mix to form fermentation liquid. Among them, the mass ratio of inoculum and duckweed residue (volatile organic matter solid content) is 1 to 3:1, and deionized water is used to adjust the working volume of fermentation liquid to the total volume of anaerobic device to 3:5 to 4:5, wherein the solid content in the fermentation liquid is 15% to 20%; and also includes adjusting the zinc ion content in the fermentation liquid to be lower than 0.42 mg / L. After sealing, check the air tightness again to prevent leakage.

[0056] The preparation method of duckweed residue is as follows: the mass ratio of fresh duckweed to acid water (optionally 1% dilute sulfuric acid) is 1:25, and the duckweed is heated at 100°C for 1 hour for hydrolysis. After solid-liquid separation, the solid part is the duckweed residue.

[0057] Step 3: Fermentation and monitoring of the pH and gas production of the fermentation liquid: Set the stirring speed to 110-130 rpm, the fermentation temperature to 30-38° C., and the initial pH to 7.0-7.8 to start the fermentation, collect the clean methane gas produced by the fermentation and measure the volume.

[0058] During the first 0 to 10 days of cultivation, the controller lowers the barrel to 1 / 3 to 3 / 4 of the depth of the fermentation liquid, and then observes the pH change of the fermentation liquid. When the pH is lower than 5.5 or the alarm sounds (the alarm is activated when the detection pH is set to ≤5.5 in advance), add alkali solution to adjust the pH of the fermentation liquid; when the pH of the fermentation liquid is higher than 8.0 or the alarm sounds (the alarm is activated when the detection pH is set to ≥8 in advance), add acid solution to adjust the pH of the fermentation liquid; the agitator quickly mixes, and when the pH reaches 6.5 to 7.5, stop adding alkali solution, and continue the above anaerobic fermentation and pH monitoring. This solution is to adjust the pH of the fermentation liquid in a sealed state, avoiding the process of refilling nitrogen due to opening the fermentation device to adjust the pH, saving nitrogen filling and fermentation time, effectively saving energy, improving fermentation efficiency, and increasing gas production.

[0059] Step 4: End the fermentation: When the pH value becomes stable, the fermentation is over.

[0060] Example 1 (Test Group)

[0061] Duckweed cultured in zinc-rich sewage (zinc content 5 mg / L) is used, and the mass ratio of fresh duckweed to acid water (1% dilute sulfuric acid) is 1:25. After heating at 100°C for 1h for hydrolysis, duckweed residue is formed after solid-liquid separation, which is used as a fermentation substrate for standby. A fermentation tank equipped with a control unit is prepared, and anaerobic granular activated sludge is used as an inoculum and the volatile solids (VS) in the duckweed residue are added to the fermentation tank in a ratio of 2:1, and the working volume of the fermentation liquid is 4:5 to the total volume of the fermentation tank. Specifically, in this embodiment, the volume of the fermentation device is 250ml, the amount of duckweed residue added is 21.19g, the amount of inoculum added is 9.59g, and the working volume of the fermentation liquid is 200ml (i.e., the solid inoculum amount is 15%). And the zinc ion content in the fermentation liquid is adjusted to be lower than 0.42mg / L.

[0062] The initial pH of the fermentation liquid is about 7.0, and nitrogen is filled for 20 minutes. The sealing is checked with detergent or soapy water. The agitator speed is 110rpm and the temperature is 37°C. During the cultivation process, the controller lowers the barrel at a fixed time every day to detect the pH and temperature of the fermentation liquid. In the actual fermentation process, the agitator can be turned off or always on after stirring the material. In this solution, the agitator is always on after the material is stirred, and it is started again when the pH is adjusted to quickly mix the alkali / acid and the material.

[0063] After 4 days of cultivation, when the system alarm shows that the pH is lower than 5.1, the controller controls the alkali liquid pipe in the four-way valve to connect to the acid and alkali liquid feeding pipe, and adds alkali liquid (optionally phosphate buffer) to the fermentation liquid to adjust the pH of the fermentation liquid. While stirring, observe and adjust the amount of acid and alkali liquid added. After adjusting the pH to 7.0-7.8, stop adding phosphate buffer, turn off the agitator, and the controller controls the drum to rise, and continue fermentation to prepare clean biomethane gas.

[0064] Comparative Example 1 (control group)

[0065] This comparative example is basically the same as Example 1, except that no pH adjustment treatment is performed after pH detection.

[0066] The pH change curves of the fermentation broth and the biomethane production curves in Example 1 and Comparative Example 1 are as follows: Figure 5 and Figure 6 shown.

[0067] By the end of the 32nd day of fermentation, the methane production in Comparative Example 1 was 14.35 (±0.95) ml / gCOD, and the methane production in Example 1 was 153.70 (±12.11) ml / gCOD. The sealed adjustment of the pH of the fermentation liquid in this solution effectively increased the methane production.

[0068] Specifically, this scheme evaluates the biological methane production potentials (BMPs) by performing nonlinear fitting through an improved Gompertz model, and the calculation formula is as follows:

[0069]

[0070] In formula (1), M(t) is the cumulative biomethane yield (mL / gCOD) at time t (day, d). M0 is the maximum biomethane yield (mL / g COD), R m is the maximum methane yield (mL / (g COD·d)), λ is the lag phase time (d), and e is the Euler function, whose value is 2.7183.

[0071] By improving the Gompertz model fitting coefficient R 2 Let's see, R 2 The values ​​of are all above 0.99, indicating that the fitting effect is good and consistent with the law of anaerobic fermentation.

[0072] Among them, the fermentation lag time λ in Example 1 is 2 days earlier than that in Comparative Example 1, and the maximum biomethane fermentation time is 11.89 days, which is about 3.5 days earlier than that in Comparative Example 1; the maximum biomethane production is 149.22 mL / gCOD, which is 10.4 times higher than that in Comparative Example 1, and the biomethane production rate is 19.42 mL / (g COD·d), which is about 15 times higher than that in Comparative Example 1. The data comparison between Example 1 and Comparative Example 1 is shown in Table 1. It can be seen that accurate control of the pH of the fermentation liquid can not only save fermentation time and improve the efficiency of biomethane production, but also greatly increase the biomethane production, and realize the resource and energy utilization of duckweed residue and anaerobic granular sludge.

[0073] Table 1 Comparison of data in Example 1 and Comparative Example 1

[0074] Treatment Group <![CDATA[M0(mL / gCOD)]]> <![CDATA[R m (mL / (g COD·d))]]> λ(d) <![CDATA[T max (d)]]> <![CDATA[R 2 ]]> Example 1 149.22±5.15 19.42±2.86 9.06±0.58 11.89 0.990 Comparative Example 1 14.32±0.54 1.29±0.16 11.33±0.63 15.41 0.993

[0075] Note: max is the fermentation time when biomethane production is maximum, T max =[(M0 / R m ) / e]+λ, d; R2 is the square value of the nonlinear fitting R.

[0076] Experimental Example 1: Effect of the lowest pH value on the amount of methane produced

[0077] Duckweed was cultivated in zinc-rich sewage (zinc content 5 mg / L), with a mass ratio of fresh duckweed to acid water (1% dilute sulfuric acid) of 1:25. After heating at 100℃ for 1h, it was hydrolyzed. After solid-liquid separation, duckweed residue was formed and used as a fermentation substrate. A fermenter with a control unit was prepared, and the inoculum and duckweed residue were added to the fermenter at a mass ratio of 2:1. The working volume of the fermentation liquid and the total volume of the fermentation device were 4:5. The solid content in the fermentation liquid varied from 15% to 22%, with a total of 6 experimental groups. The initial pH of the fermentation liquid was adjusted to about 7.0, nitrogen was filled for 20 minutes, and detergent was used to check the sealing. The agitator speed was 110rpm, the temperature was 37℃, and the pH value displayed by the recording system was recorded after 4 days of cultivation. No treatment was done (i.e., no pH adjustment was made), and the cultivation was continued for 32 days to collect methane gas. The relationship between the lowest pH value and the amount of methane gas produced in different batches of experimental groups is shown in Table 2.

[0078] Table 2 Relationship between the lowest pH value and methane production in different batches of test groups

[0079] Treatment Group pH after 4 days of culture <![CDATA[M0(mL / gCOD)]]> Experimental Group 1 5.01±0.14 14.35±0.95 Experimental Group 2 5.26±0.03 27.85±2.91 Experimental Group 3 5.40±0.12 40.05±2.51 Experimental Group 4 5.48±0.41 48.23±3.29 Experimental Group 5 5.60±0.44 61.45±0.14 Experimental Group 6 5.81±0.22 78.57±1.69

[0080] The experimental data show that, without intervention, the experimental group with a higher minimum pH value during the fermentation process has a smaller effect on the population and activity of methanogens in the fermentation broth pH, ​​and the amount of methane gas produced basically no longer increases after 32 days of fermentation; while the experimental group with a lower minimum pH value has a greater impact on the methane production capacity of methanogens by pH stress (including a greater impact on the population and active bacteria of methanogens), resulting in a significant decrease in the amount of methane gas produced. The applicant analyzed the reason as follows: the experimental group with a lower minimum pH value (such as experimental group 1) is in a low pH environment for a long time (without intervention or regulation), and fermentation microorganisms (such as methanogens) may die in large numbers, and only a few mutant resistant microorganisms survive and continue to reproduce and ferment, resulting in a significant decrease in the amount of methane gas produced by fermentation, affecting the full fermentation capacity of duckweed residue. Therefore, this scheme can not only relieve the stress of low pH on methanogens, accelerate the fermentation speed, and effectively increase methane production by intervening at a lower pH, such as adding buffered alkali solution to regulate the pH of the fermentation broth; it can also make full use of the organic matter in the duckweed residue.

[0081] In addition, if the pH is intervened too early (such as pH 5.8 or above 6.0), on the one hand, it will reduce the resistance of microorganisms, and on the other hand, the organic matter in the duckweed residue is not fully decomposed into substrates that can be used by methanogens for fermentation. On the contrary, the increase in methane gas production is not obvious, and the amount of alkali solution will increase, which not only increases the cost of addition, but also increases the salinity of the fermentation liquid, affecting the production capacity of clean biomethane. This also indirectly confirms the view that "in this scheme, adjusting the pH of the fermentation liquid when the pH is lower than 5.5 can effectively increase methane production."

[0082] Experimental Example 2: Effect of different zinc ion contents in fermentation broth on methane production

[0083] For the duckweed residues obtained from different batches of acid hydrolysis treatment, the zinc content was different. The duckweed residues with different zinc contents obtained from different batches were subjected to anaerobic fermentation according to the method in Example 1. A total of 6 treatments were performed. The change curve of the zinc content in the fermentation liquid is shown in detail. Figure 7 The variation curve of biomethane gas obtained by fermentation in each treatment is shown in Figure 8 .

[0084] Experimental data show that after the duckweed residue obtained by preparing duckweed cultured in zinc-containing sewage was prepared into fermentation liquid, the initial zinc content concentration of the fermentation liquid in treatment group 3 was about 0.70 mg / L, and the amount of biomethane produced in treatment group 3 after 30 days of fermentation was the lowest, which indicates that the zinc content in the fermentation liquid higher than or equal to 0.70 mg / L will significantly inhibit the ability of microorganisms in the fermentation liquid to produce methane, thereby reducing the biomethane production; while the initial zinc content concentrations of the fermentation liquids of other treatment groups were all lower than 0.42 mg / L, and this concentration had relatively little impact on methane production.

[0085] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the technical solution of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing clean biomethane gas from duckweed residue, characterized in that: The invention comprises fermenting and preparing methane in an anaerobic fermentation tank by using anaerobic granular sludge as an inoculum and duckweed residue as a fermentation substrate; the anaerobic fermentation tank comprises a tank body, a cover body and a control unit, the control unit comprises a controller, a display screen, a barrel, a telescopic device for driving the barrel to rise and fall, and a pH sensor, a conductivity sensor and an acid and alkali solution dosing pipe fixed in the barrel; the liquid inlet end of the acid and alkali solution dosing pipe is connected to an acid pipe, an alkali pipe and a flushing liquid pipe through a four-way valve; the telescopic device, the pH sensor, the conductivity sensor, the four-way valve and the display screen are all electrically connected to the controller; The pH sensor is used to detect the pH of the fermentation liquid in the anaerobic fermentation tank to form a pH signal and transmit it to the controller. The conductivity sensor is used to detect the conductivity and temperature of the fermentation liquid to form a conductivity signal and a temperature signal respectively and transmit them to the controller. The controller is used to receive the pH signal, the conductivity signal and the temperature signal, and while transmitting the pH signal and the conductivity signal to the display screen for display, it controls the liquid inlet of the four-way valve according to the pH signal and the conductivity signal.

2. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: The top wall inside the drum is provided with a first telescopic device corresponding to the pH sensor, and a second telescopic device corresponding to the conductivity sensor. The detection elements of the pH sensor and the conductivity sensor are fixed at the lifting ends of the telescopic devices; the first telescopic device and the second telescopic device are both electrically connected to the controller.

3. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: The cover body is provided with an adjusting opening for inserting the barrel and fixing the telescopic device.

4. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: The controller and the display screen are fixed on the side wall of the tank body, and the controller is also electrically connected to an alarm.

5. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: The side wall of the barrel is provided with an opening, and the opening divides the longitudinal space inside the barrel into a measuring cavity and a protection cavity.

6. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: A plurality of liquid permeable openings are arranged at the bottom of the barrel, and the diameters of the liquid permeable openings gradually decrease from top to bottom.

7. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: The cover body is detachably connected with a stirring device, and the stirring device extends into the tank body.

8. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: The mass ratio of the inoculum to the duckweed residue is 1 to 3:1, and deionized water is used to adjust the ratio of the working volume of the fermentation liquid to the total volume of the anaerobic device to 3:5 to 4:

5.

9. The method for preparing clean biomethane gas from duckweed residue according to claim 8, characterized in that: The fermentation stirring speed is 110-130 rpm, the fermentation temperature is 30-38°C, and the initial pH is 7.0-7.

8.

10. The method for preparing clean biomethane gas from duckweed residue according to claim 9, characterized in that: Observe the pH changes of the fermentation liquid. When the pH of the fermentation liquid is lower than 5.5 or the alarm sounds, add alkali solution to adjust the pH of the fermentation liquid; when the pH of the fermentation liquid is higher than 8.0 or the alarm sounds, add acid solution to adjust the pH of the fermentation liquid; When the pH is adjusted to 6.5-7.5, the control unit is closed and anaerobic fermentation is continued; The pH of the fermentation liquid is tested and adjusted every 3 to 8 hours until the fermentation is completed.

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