A method for preparing clean biomethane gas from duckweed residue

Abstract

This invention relates to the field of bio-fermentation engineering technology and discloses a method for preparing clean biomethane gas from duckweed residue. The method includes using anaerobic granular sludge as inoculum and duckweed residue as fermentation substrate, fermenting in an anaerobic fermenter to produce methane. The anaerobic fermenter includes a tank body, a cover, and a control unit. The control unit includes a controller, a display screen, a cylindrical container, a telescopic device for driving the cylindrical container's lifting and lowering, and a pH sensor, a conductivity sensor, and an acid / alkali solution dosing pipe fixed inside the cylindrical container. The inlet end of the acid / alkali solution dosing pipe is connected to an acid solution pipe, an alkali solution pipe, and a rinsing solution pipe via a four-way valve. The telescopic device, pH sensor, conductivity sensor, four-way valve, and display screen are all electrically connected to the controller. This solution, by adding a control unit, effectively and precisely controls the pH of the fermentation broth in real time, ensuring that the pH of the fermentation broth is within the optimal activity pH range of the methanogens, thereby increasing the activity of the methanogens and the amount of methane produced, and improving production efficiency.

Description

Technical Field

[0001] This invention relates to the field of bio-fermentation engineering technology, specifically to a method for preparing clean biomethane gas from duckweed residue. Background Technology

[0002] Energy shortages and the environmental problems caused by traditional energy sources have attracted global attention. Duckweed, with its advantages of easy cultivation, rapid growth, high starch content leading to rapid accumulation, low lignin content, no need for arable land, and direct cultivation in wastewater, is not only a green and efficient plant for remediating livestock, industrial, and domestic wastewater, but is also considered a high-quality new biomass raw material, primarily used to produce clean energy such as bioethanol and biomethane. However, existing duckweed cultivated under high pollution loads such as livestock wastewater is rich in heavy metals such as zinc and copper, which not only limits the fermentation of duckweed extract to produce bioenergy, but also severely inhibits the anaerobic fermentation of duckweed residue to produce bioenergy, especially the ability to produce biomethane. Even after acid pyrolysis, the duckweed residue from high-concentration heavy metal cultivation still contains a significant amount of carbohydrates. This residue is also an important bioenergy resource; its disposal not only pollutes the environment but also wastes resources. Therefore, how to improve the ability of green plant by-products such as duckweed residue from wastewater rich in heavy metals to produce biomethane, and to achieve the goal of deep resource utilization, energy conversion, and volume reduction of green plant by-products, has become an urgent problem to be solved.

[0003] Anaerobic granular sludge, as a highly efficient anaerobic activated sludge strain, is rich in various anaerobic microorganisms such as methanogens, acetogens, and hydrolytic fermentation bacteria. These microbial populations work together to produce biomethane through the co-fermentation of 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 process for producing biomethane from duckweed residue still has the following technical problems: (1) During the process, the pH of the fermentation broth gradually decreases as fermentation progresses, even falling 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 broth during fermentation, if the adjustment is not timely, the fermentation effect will be reduced; moreover, when preparing clean biomethane gas, different fermentation substrates require different degrees of acid and alkali control, and the pH during fermentation must be precisely controlled. (3) Especially for bioenergy-type 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] Therefore, developing a method for preparing clean biomethane gas from duckweed residue by timely pH adjustment not only effectively compensates for the shortcomings of existing technologies, but also allows for targeted pH adjustment of the fermentation broth without compromising the fermentation seal. This is of great significance for 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, in order to solve the technical problem of reduced gas production caused by unstable pH control during the fermentation process of duckweed residue.

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

[0007] The pH sensor is used to detect the pH of the fermentation broth in the anaerobic fermenter, generate a pH signal, and transmit it to the controller. The conductivity sensor is used to detect the conductivity and temperature of the fermentation broth, generate conductivity and temperature signals respectively, and transmit them to the controller. The controller receives the pH, conductivity, and temperature signals, and while transmitting the pH, conductivity, and temperature signals to the display screen, it controls the four-way valve to allow liquid to enter based on the pH and conductivity signals.

[0008] The principle and advantages of this scheme are:

[0009] 1. Compared with the existing acid-treated duckweed residue fermentation process, which suffers from unstable pH control leading to reduced gas production, this solution effectively controls the pH of the fermentation broth in real time and with precision by adding a control unit. This ensures that the pH of the fermentation broth is within the optimal pH range for methanogen activity, thereby increasing the activity of methanogens and the amount of methanogens produced, and improving production efficiency.

[0010] 2. Compared to existing methods where pH adjustment without opening the lid during duckweed fermentation can slow down the fermentation process and reduce methane production, this solution adds a control unit to facilitate the detection and control of the pH of the fermentation liquid in the anaerobic fermenter without opening the lid or sealing it. This effectively avoids the impact of unsealed 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 / alkali solution dosing tubing inside a cylindrical container and adds a telescopic device to drive the container's lifting and lowering. This facilitates the switching between "non-detection" and "detection" states for the pH and conductivity sensors. Specifically, when the telescopic device lowers, the container and its internal detection elements are submerged in the fermentation broth, entering the "detection state." In this state, both the pH and conductivity sensors are in contact with the broth to detect pH and conductivity. Conversely, when detection and control cease, the telescopic device rises, and the container and its detection elements are removed from the broth, entering the "non-detection state." In this state, both the pH and conductivity sensors are removed from the broth to prevent corrosion caused by prolonged contact with the detection elements. Therefore, by combining the container and the telescopic device, not only can the simultaneous lifting and lowering of the pH sensor, conductivity sensor, and acid / alkali solution dosing tubing be achieved, but the three components are also effectively protected from damage caused by physical impact and prolonged chemical corrosion.

[0012] 4. This solution incorporates acid and alkali solution dosing pipes, with the inlet end of these pipes connected to an acid pipe, an alkali pipe, and a rinsing solution pipe via a four-way valve. This allows for easy control of acid or alkali addition to adjust the pH of the fermentation broth based on test results. After addition, rinsing solution is added to flush the detection elements that have come into contact with the fermentation broth, preventing damage from prolonged contact. Furthermore, if the fermentation broth is too concentrated, rinsing solution can be added to dilute it, 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 corresponding to the conductivity sensor. The detection elements of the pH sensor and the conductivity sensor are fixed at the lifting end of the telescopic device. Both the first telescopic device and the second telescopic device are electrically connected to the controller.

[0014] Technical Effect: This solution, employing the above-described configuration, facilitates the separate raising and lowering of the detection elements of the pH sensor and conductivity sensor according to testing needs. Specifically, taking a lifting cylinder as an example, the connection and lifting method are explained: the detection elements of the pH sensor and conductivity sensor are respectively fixed on the piston of a lifting cylinder. When it is necessary to detect the physicochemical properties of the fermentation broth, the controller controls the upper chambers of the first and second telescopic devices to inflate, causing the piston to move downwards, thus moving the detection elements of the pH sensor and conductivity sensor downwards to contact the fermentation broth, achieving the detection of the physicochemical properties of the fermentation broth. After detection and adjustment are completed, the controller controls the lower chambers of the first and second telescopic devices to inflate, causing the piston to move upwards, thus moving the detection elements of the pH sensor and conductivity sensor upwards to leave the fermentation broth, preventing the fermentation broth from prolonged contact with the detection elements and corroding or damaging them.

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

[0016] Technical benefits: The above-mentioned setup facilitates equipment maintenance and replacement.

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

[0018] Technical benefits: The above-mentioned setup facilitates fixation, observation, and possible intervention and control.

[0019] Preferably, as an improvement, the side wall of the cylinder has an opening that divides the longitudinal space inside the cylinder into a measuring cavity and a protective cavity.

[0020] Technical Effects: With the above-mentioned setup, when the cylindrical barrel descends into the fermentation broth, the fermentation broth enters the measuring chamber through the opening, facilitating contact between the pH sensor and the conductivity sensor's detection elements as they descend to complete the detection. After the detection is completed, the detection elements are rinsed with rinsing solution, and then the detection elements are controlled to rise into the protective chamber. The rinsing solution then falls into the measuring chamber to flush out the fermentation broth and acid / alkali solutions, effectively preventing high-concentration acid and alkali solutions from remaining in the measuring chamber for extended periods and affecting the overall pH of the fermentation broth.

[0021] Preferably, as an improvement, the bottom of the barrel is provided with multiple liquid inlets, the diameter of which gradually decreases from top to bottom.

[0022] Technical Effects: This solution employs the above-described configuration. When the entire cylindrical tank descends, the fermentation broth enters through the inlet, facilitating contact between the pH and conductivity sensors as they descend and the broth for detection. After detection, as the tank rises, the fermentation broth automatically exits through the inlet, emptying the tank. Furthermore, by gradually decreasing the diameter of the inlet from top to bottom, the diameter of the broth gradually increases as it enters the tank during descent. This gradually reduces the pressure of the broth, slows the flow rate, and prevents splashing onto the inner wall or telescopic device, thus avoiding damage to the equipment.

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

[0024] Technical benefits: The above-mentioned setup facilitates the stirring and homogenization of the fermentation broth in the fermenter, which accelerates the fermentation process. Furthermore, the rapid stirring and homogenization after the addition of acid / alkali solutions allows for accurate testing of the physicochemical properties of the fermentation broth.

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

[0026] Preferably, as an improvement, before sealing and fermenting, the zinc ion content in the fermentation broth is adjusted to be below 0.42 mg / L.

[0027] Technical benefits: The above-mentioned setup facilitates the full fermentation of duckweed residue and ensures sufficient space for biomethane production.

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

[0029] Technical Effects: This scheme, employing the above-mentioned settings, facilitates rapid fermentation. Specifically, methanogenic microorganisms are highly sensitive to pH, with optimal conditions at temperatures between 35 and 40°C and pH levels between 6.1 and 8.0. Acid-producing bacteria, on the other hand, have a wider pH tolerance range, generally maintaining high activity between 4.5 and 8.0. Considering the comprehensive fermentation effects of various microorganisms on duckweed residue, and limiting the above fermentation conditions, effective and continuous methane production is achieved, thereby increasing methane yield and production efficiency.

[0030] Preferably, as an improvement, the pH change of the fermentation broth is observed. When the pH of the fermentation broth is lower than 5.5 or the alarm sounds, alkali solution is added to regulate the fermentation broth. When the pH of the fermentation broth is higher than 8.0 or the alarm sounds, acid solution is added to regulate the pH of the fermentation broth. When the pH is regulated to 6.5-7.5, the regulation unit is turned off and anaerobic fermentation continues. The pH of the fermentation broth is monitored and regulated every 3-8 hours until the fermentation is completed.

[0031] Technical Effects: This scheme, employing the aforementioned settings, facilitates precise control of the fermentation broth's pH, thereby improving fermentation efficiency and methanogenesis. Through long-term experiments, the applicant discovered that the anaerobic fermentation of duckweed residue mainly consists of three stages: hydrolysis fermentation, acid production, and methanogenesis. In the first stage, proteins are primarily broken down into amino acids, etc. In the second stage, simple organic matter is further decomposed by acid-producing bacteria, breaking down large organic molecules into smaller organic acids and alcohols, providing substrates for the subsequent methanogenesis stage. This also allows for the initial degradation of complex organic matter, facilitating further utilization by methanogens. In the final stage, methanogens ultimately convert small organic acids and alcohols into methane and carbon dioxide, transforming the carbon in the organic matter into usable energy substances in the form of methane, thus completing the transformation from complex organic matter to simple inorganic matter and energy gases. When acid-producing bacteria continuously produce acid, causing the fermentation broth pH to drop below 5.5, without intervention, the methanogens in the fermentation broth will be in an overly acidic environment for an extended period, significantly reducing their ability to produce methanogens (partly because the bacterial population cannot increase rapidly, and partly because the excessively low pH reduces their activity), thus decreasing biogenic methane production. However, premature intervention will result in a lower degree of fermentation and decomposition of organic matter by the acid-producing bacteria, reducing the amount of substrate available to the methanogens, and still reducing the biogenic methane production. The applicant also investigated the relationship between the minimum pH of anaerobic fermentation and biogenic methane production under uninterrupted conditions, finding that the lower the minimum pH, the lower the biogenic methane production, thus validating the above analysis. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the fermenter in an embodiment of the present invention.

[0033] Figure 2 This is a schematic diagram of the structure of the cylindrical tank, acid / alkali solution dosing pipe, pH sensor, and conductivity sensor in the fermenter in an embodiment of the present invention.

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

[0035] Figure 4 This is a structural diagram showing the location of the liquid permeation hole at the bottom of the cylindrical tank in an embodiment of the present invention.

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

[0037] Figure 6 The pH change curves of the fermentation broth in the control group and the experimental group in the method for preparing clean biomethane gas from duckweed residue according to an embodiment of the present invention are shown.

[0038] Figure 7 This is a curve showing the change in zinc content during fermentation of fermentation broths with different zinc contents in Experiment Example 2 of this invention.

[0039] Figure 8 This is the biomethane production curve of fermentation broth with different zinc contents in Experiment Example 2 of the present invention. Detailed Implementation

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

[0041] The reference numerals in the accompanying drawings include: tank body 1, cover body 2, feed inlet 21, air inlet 22, air outlet 23, stirrer 24, controller 31, display screen 32, cylinder 41, third cylinder 42, opening 43, measuring chamber 431, protective chamber 432, liquid outlet 44, acid and alkali solution dosing pipe 5, four-way valve 51, pH sensor 6, first cylinder 61, conductivity sensor 7, and second cylinder 71.

[0042] Overview of the Plan

[0043] This solution provides a fermenter for preparing clean biomethane gas from duckweed residue, as shown in 1-4. It includes a tank body 1, a cover body 2, and a control unit. The control unit includes a controller 31, a display screen 32, a cylindrical tank 41, a telescopic device for driving the cylindrical tank 41 to rise and fall, and a pH sensor 6, a conductivity sensor 7, and an acid / alkali solution dosing pipe 5 fixed inside the cylindrical tank 41. The inlet end of the acid / alkali solution dosing pipe 5 is connected to an acid solution pipe, an alkali solution pipe, and a flushing solution pipe via a four-way valve 51. The telescopic device, pH sensor 6, conductivity sensor 7, four-way valve 51, and display screen 32 are all electrically connected to the controller 31.

[0044] pH sensor 6 is used to detect the pH of the fermentation liquid in the anaerobic fermenter, generate a pH signal, and transmit it to controller 31. Conductivity sensor 7 is used to generate conductivity and temperature signals for the fermentation liquid, respectively, and transmit them to controller 31. Controller 31 is used to receive pH, conductivity, and temperature signals. While transmitting the pH and conductivity signals to the display screen 32 for display, it controls the four-way valve 51 to allow liquid to enter based on the pH and conductivity signals.

[0045] Specifically, tank 1 and cover 2 are threaded together, and cover 2 is equipped with a sealing ring. After the two are threaded together, a sealed space is formed inside tank 1, which facilitates anaerobic fermentation. Figure 1As shown, the cover 2 is also equipped with conventional openings 43 required for an anaerobic fermenter, such as a feed inlet 21, an air inlet 22, and an air outlet 23. The feed inlet 21 is used for inoculation and feeding, and is later used for connection. The air inlet 22 is used to introduce nitrogen gas to create an oxygen-deficient environment inside the tank 1. The air outlet 23 is used to discharge the air inside the tank 1, and is also used to collect the clean methane gas produced by anaerobic fermentation after connecting to the collection equipment. The other openings 43 or instruments required for anaerobic fermentation are existing technologies and will not be described in detail here.

[0046] The cover 2 is detachably connected to a stirring device that extends into the tank 1. For reference, the stirring device is a multi-bladed agitator 24. The cover 2 also has an adjustment port for inserting and securing the telescopic device into the cylindrical barrel 41. A sealing ring is provided at the adjustment port to ensure a tight seal. For reference, the telescopic device driving the cylindrical barrel 41 can be a cylinder (also a third cylinder 42). The cylinder body of the third cylinder 42 is fixed at the adjustment port, and the cylindrical barrel 41 is fixed to the end of the piston rod of the cylinder. The end walls and piston of the third cylinder 42 have sealing ports for the acid / alkali solution dosing tube 5 and internal instrument wires to pass through. Sufficient lengths of acid / alkali solution dosing tube 5 and internal instrument wires are reserved in each chamber of the third cylinder 42 to protect these components while ensuring the airtightness and normal lifting and lowering of the third cylinder 42.

[0047] like Figure 2 As shown, the inner top wall of the cylindrical container 41 has a first telescopic device corresponding to the pH sensor 6 and a second telescopic device corresponding to the conductivity sensor 7. A protective tube is provided outside the acid / alkali solution dosing tube 5. A fixing component for securing the first telescopic device, the second telescopic device, and the protective tube is located in the middle of the cylindrical container 41. For reference, the first telescopic device is a first cylinder 61, and the second telescopic device is a second cylinder 71. The detection elements of the pH sensor 6 and the conductivity sensor 7 are respectively fixed to the lifting ends of the first cylinder 61 and the second cylinder 71, facilitating the control of the lifting and lowering of the detection elements of the pH sensor 6 and the conductivity sensor 7 according to detection needs.

[0048] The end of the acid / alkali solution dosing tube 5 is equipped with a nozzle for quick dispersion or spraying of the liquid added by the dosing tube 5. Especially after adding alkali (or acid), the cleaning solution is sprayed to rinse and clean the detection elements of pH sensor 6 and conductivity sensor 7, preventing them from being severely corroded by prolonged contact with acid / alkali solutions, and effectively extending the service life of the detection elements of pH sensor 6 and conductivity sensor 7.

[0049] As an improvement, the side wall of the cylinder 41 has an opening 43, which divides the longitudinal space inside the cylinder 41 into a measuring cavity 431 and a protective cavity 432 (e.g., Figure 3(As shown). When the third cylinder 42 descends to the point where the barrel 41 extends into the fermentation liquid, the fermentation liquid enters the measuring chamber 431 through the opening 43, which facilitates the first cylinder 61 and the second cylinder 71 to drive the detection elements of the pH sensor 6 and the conductivity sensor 7 to descend and come into contact with the fermentation liquid to complete the measurement.

[0050] In another embodiment of this solution, the bottom of the cylindrical barrel 41 is provided with multiple liquid inlets 44, the diameter of which gradually decreases from top to bottom. This facilitates the entry of the fermentation liquid into the cylindrical barrel 41 through the liquid inlets 44 after the third cylinder 42 descends to the point where the fermentation liquid is inserted into the barrel 41, thus creating a measurement environment.

[0051] In another embodiment of this solution, such as Figure 4 As shown, the cylindrical barrel 41 has openings 43 on its side wall and multiple liquid inlets 44 at its bottom. The combination of the side wall openings 43 and the bottom liquid inlets 44 further facilitates the rapid entry and exit of the fermentation broth into and out of the cylindrical barrel 41.

[0052] The electrical equipment mentioned in this solution, including the first cylinder 61, the second cylinder 71, the third cylinder 42, the four-way valve 51, the display screen 32, and the agitator motor, 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 1, and the controller 31 is also electrically connected to an alarm. All electrical equipment mentioned in this document is existing technology; its structure and model can be selected as needed, and will not be elaborated further here.

[0053] This solution provides a method for preparing clean biomethane gas from duckweed residue, comprising using anaerobic granular sludge as inoculum and duckweed residue as fermentation substrate, and fermenting to prepare methane in an anaerobic fermenter; including the following steps:

[0054] Step 1: Check the airtightness of the equipment: After the equipment is installed, apply soapy water to all connection points and purge the tank with nitrogen for 0.2 to 0.5 hours. Once the airtightness of the equipment is verified, proceed to the next steps.

[0055] Step 2: Adding and Mixing to Prepare Fermentation Broth: Add inoculum, duckweed residue, and sterilized deionized water to the fermenter and mix thoroughly to prepare the fermentation broth. The mass ratio of inoculum to duckweed residue (volatile organic compound content) is 1–3:1. Use deionized water to adjust the working volume of the fermentation broth to the total volume of the anaerobic apparatus to 3:5–4:5. The solid content in the fermentation broth is 15%–20%. This also includes adjusting the zinc ion content in the fermentation broth to be below 0.42 mg / L. After sealing, re-test the airtightness to prevent leakage.

[0056] The preparation method of duckweed residue is as follows: fresh duckweed and acidic water (1% dilute sulfuric acid can be selected) are in a mass ratio of 1:25. After being heated at 100℃ for 1 hour for hydrolysis, the solid part is duckweed residue after solid-liquid separation.

[0057] Step 3: Fermentation and monitoring of fermentation broth pH and gas production: Set the stirring speed to 110-130 rpm, the fermentation temperature to 30-38℃, and the initial pH to 7.0-7.8 to start fermentation. Collect the clean methane gas produced during fermentation and measure its volume.

[0058] During the initial 0-10 days of cultivation, the controller lowers the cylindrical tank to 1 / 3 to 3 / 4 of the depth of the fermentation broth. The pH of the fermentation broth is then observed. When the pH falls below 5.5 or the alarm sounds (pre-set to activate when pH ≤ 5.5), alkali solution is added to adjust the pH. When the pH rises above 8.0 or the alarm sounds (pre-set to activate when pH ≥ 8), acid solution is added to adjust the pH. The agitator is used for rapid mixing. Once the pH reaches 6.5-7.5, alkali addition is stopped, and the anaerobic fermentation and pH monitoring continue as described above. This method allows for pH adjustment of the fermentation broth under sealed conditions, avoiding the need for re-purging nitrogen when the fermentation device is opened for pH adjustment. This saves on nitrogen purging and fermentation time, effectively conserving energy, improving fermentation efficiency, and increasing gas production.

[0059] Step 4: End fermentation: When the pH level is basically stable, fermentation is considered complete.

[0060] Example 1 (Experimental Group)

[0061] Duckweed cultured in zinc-rich wastewater (zinc content 5 mg / L) was used. The mass ratio of fresh duckweed to acidic water (1% dilute sulfuric acid) was 1:25. Hydrolysis was performed by heating at 100℃ for 1 hour. After solid-liquid separation, duckweed residue was formed and used as the fermentation substrate. A fermenter equipped with a control unit was prepared. Anaerobic granular activated sludge was added to the fermenter as inoculum at a volatile solids (VS) ratio of 2:1 to the duckweed residue. The working volume of the fermentation broth was 4:5 of the total fermentation broth volume. Specifically, in this embodiment, the fermentation device volume was 250 ml, the duckweed residue dosage was 21.19 g, the inoculum dosage was 9.59 g, and the working volume of the fermentation broth was 200 ml (i.e., the solid inoculum content was 15%). The zinc ion content in the fermentation broth was adjusted to be below 0.42 mg / L.

[0062] The initial pH of the fermentation broth was approximately 7.0. Nitrogen was purged for 20 minutes. The seal was checked using detergent or soapy water. The stirrer speed was 110 rpm, and the temperature was 37°C. During the cultivation process, the controller was used daily to lower the cylindrical tank at fixed times to monitor the pH and temperature of the fermentation broth. In actual fermentation, the stirrer could be turned off after mixing the materials or kept on continuously. In this scheme, the stirrer was kept on continuously after mixing the materials, and restarted when adjusting the pH to quickly mix the alkaline / acidic solution and the materials.

[0063] After 4 days of cultivation, the system alarm indicates that the pH is below 5.1. The controller then connects the alkali solution pipe in the four-way valve to the acid / alkali solution feeding pipe, adding alkali solution (optional phosphate buffer) to the fermentation broth to adjust its pH. While stirring, the amount of acid / alkali solution added is observed and adjusted. Once the pH is adjusted to 7.0–7.8, the addition of phosphate buffer is stopped, the stirrer is turned off, and the controller raises the fermentation tank to continue fermentation and produce 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 and biomethane production curves of the fermentation broth in Example 1 and Comparative Example 1 are shown below. Figure 5 and Figure 6 As shown.

[0067] After 32 days 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. This method effectively increased methane production by sealing and adjusting the pH of the fermentation broth.

[0068] Specifically, this scheme uses an improved Gompertz model to perform nonlinear fitting to evaluate biological methane production potentials (BMPs), and the calculation formula is as follows:

[0069]

[0070] In equation (1): M(t) is the cumulative biomethane yield (mL / g COD) over time t (days, d). M0 is the maximum biomethane yield (mL / g COD), R m λ represents the maximum methane yield (mL / (g COD·d)), λ represents the lag phase time (d), and e represents the Euler's totient function, which has a value of 2.7183.

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

[0072] In Example 1, the fermentation lag time λ was 2 days earlier than in Comparative Example 1, and the fermentation time to achieve the maximum biomethane yield was 11.89 days, approximately 3.5 days earlier than in Comparative Example 1. The maximum biomethane production was 149.22 mL / gCOD, 10.4 times higher than in Comparative Example 1, and the biomethane production rate was 19.42 mL / (gCOD·d), approximately 15 times higher than in Comparative Example 1. A detailed comparison of the data from Example 1 and Comparative Example 1 is shown in Table 1. Therefore, precise control of the fermentation broth pH not only saves fermentation time and improves biomethane production efficiency but also significantly increases biomethane yield, achieving the goal of resource and energy utilization of duckweed residue and anaerobic granular sludge.

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

[0074] 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: T max T is the fermentation time at which biomethane production reaches its maximum. max =[(M0 / R m ) / e]+λ,d;R2 is the squared value of R for nonlinear fitting.

[0076] Experimental Example 1: The effect of the lowest pH value on the amount of methanogen produced.

[0077] Duckweed cultured in zinc-rich wastewater (zinc content 5 mg / L) was used. The mass ratio of fresh duckweed to acidic water (1% dilute sulfuric acid) was 1:25. Hydrolysis was performed by heating at 100℃ for 1 hour. After solid-liquid separation, duckweed residue was formed and used as the fermentation substrate. Fermentation tanks with control units were prepared. Inoculum to duckweed residue was added to the fermentation tank at a mass ratio of 2:1. The working volume of the fermentation broth was 4:5 of the total volume of the fermentation device. The solid content in the fermentation broth varied from 15% to 22%, with a total of 6 experimental groups. The initial pH of the fermentation broth was adjusted to approximately 7.0. Nitrogen gas was purged for 20 minutes, and the seal was checked with detergent. The stirrer speed was 110 rpm, and the temperature was 37℃. After 4 days of cultivation, the pH value was recorded. No treatment was performed (i.e., no pH adjustment), and cultivation continued for another 32 days. Methane gas was collected. The relationship between the lowest pH value and the amount of methane gas produced in different batches of experimental groups is detailed in Table 2.

[0078] Table 2. Relationship between the lowest pH value and the amount of methane produced in different batches of experimental groups.

[0079] 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] Experimental data show that, without intervention, in experimental groups with higher minimum pH values ​​during fermentation, the pH of the fermentation broth had less impact on the population size and activity of methanogens, and the amount of methanogens produced essentially stopped increasing after 32 days of fermentation. Conversely, in experimental groups with lower minimum pH values, the methanogen production capacity was more significantly affected by pH stress (including a greater impact on the population size and activity of methanogens), leading to a significant decrease in methanogen production. The applicant's analysis suggests that in experimental groups with lower minimum pH values ​​(such as experimental group 1), prolonged exposure to a low pH environment (without intervention or regulation) may cause a large number of fermenting microorganisms (such as methanogens) to die off. Only a few mutant, resistant microorganisms may survive and continue to reproduce and ferment, resulting in a significant decrease in methanogen production and affecting the full fermentation capacity of the duckweed residue. Therefore, this scheme, by intervening at lower pH levels, such as adding buffer alkali to regulate the pH of the fermentation broth, not only relieves the stress of low pH on methanogens, accelerates fermentation, and effectively increases methane production, but also fully utilizes the organic matter in the duckweed residue.

[0081] Furthermore, premature pH intervention (e.g., adjusting pH to 5.8 or above 6.0) can reduce microbial resistance. Additionally, the organic matter in the duckweed residue may not be fully decomposed into usable substrate for methanogenic bacteria, resulting in minimal increase in methane production. This also increases alkali usage, raising both costs and salinity in the fermentation broth, thus impacting clean biomethane production. This indirectly confirms the viewpoint that "adjusting the fermentation broth pH when it is below 5.5 in this scheme can effectively increase methane production."

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

[0083] To address the varying zinc content in duckweed residue obtained from different batches of acid hydrolysis treatment, anaerobic fermentation was conducted on different batches of duckweed residue with different zinc contents, following the method described in Example 1. A total of six treatments were performed, and the zinc content variation curves in the fermentation broth are detailed below. Figure 7 For details of the biomethane gas emission curves obtained from each fermentation treatment, please refer to [link / reference needed]. Figure 8 .

[0084] Experimental data show that after the duckweed residue obtained from the cultivation of duckweed in zinc-containing wastewater was prepared into fermentation broth, the initial zinc concentration in the fermentation broth of treatment group 3 was approximately 0.70 mg / L. Treatment group 3 produced the lowest amount of biogenic methane after 30 days of fermentation. This indicates that a zinc content higher than or equal to 0.70 mg / L in the fermentation broth significantly inhibits the ability of microorganisms in the fermentation broth to produce methane, thereby reducing biogenic methane production. In contrast, the initial zinc concentration in the fermentation broth of the other treatment groups was all below 0.42 mg / L, and this concentration had a relatively small impact on methane production.

[0085] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described 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 method involves using anaerobic granular sludge as inoculum and duckweed residue as fermentation substrate to produce methane through fermentation in an anaerobic fermenter. The anaerobic fermenter includes a tank body, a cover, and a control unit. The control unit includes a controller, a display screen, a cylindrical tank, a telescopic device for driving the cylindrical tank to rise and fall, and a pH sensor, a conductivity sensor, and an acid / alkali solution dosing pipe fixed inside the cylindrical tank. The inlet end of the acid / alkali solution dosing pipe is connected to an acid solution pipe, an alkali solution pipe, and a rinsing solution pipe via a four-way valve. The telescopic device, pH sensor, conductivity sensor, four-way valve, and 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 fermenter, generate a pH signal, and transmit it to the controller. The conductivity sensor is used to detect the conductivity and temperature of the fermentation liquid, generate conductivity and temperature signals respectively, and transmit them to the controller. The controller receives the pH signal, conductivity signal, and temperature signal. While transmitting the pH signal and conductivity signal to the display screen, it controls the four-way valve to allow liquid to enter based on the pH signal and conductivity signal. The inner top wall of the cylinder 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 end of the telescopic device. Both the first and second telescopic devices are electrically connected to the controller; the side wall of the cylinder has an opening that divides the longitudinal space inside the cylinder into a measuring chamber and a protective chamber; the bottom of the cylinder is provided with multiple liquid inlets, the diameter of which gradually decreases from top to bottom; When the cylindrical barrel descends into the fermentation broth, the fermentation broth enters the measuring chamber through the opening. The detection elements of the pH sensor and conductivity sensor descend to contact the fermentation broth to complete the detection. After the detection is completed, the detection elements are rinsed with rinsing liquid, and then the detection elements are raised to the protective chamber, while the rinsing liquid falls into the measuring chamber and automatically exits from the liquid outlet.

2. The method for preparing clean biomethane gas from duckweed residue according to claim 1, characterized in that: The cover is provided with an adjustment port for inserting a cylindrical barrel and fixing the telescopic device.

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

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

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

5.

6. A method for preparing clean biomethane gas from duckweed residue according to claim 5, characterized in that: The stirring speed during fermentation was 110~130 rpm, the fermentation temperature was 30~38℃, and the initial pH was 7.0~7.

8.

7. The method for preparing clean biomethane gas from duckweed residue according to claim 6, characterized in that: Observe the pH changes of the fermentation broth. When the pH of the fermentation broth is lower than 5.5 or the alarm sounds, add alkali solution to adjust the pH of the fermentation broth. When the pH of the fermentation broth is higher than 8.0 or the alarm sounds, add acid solution to adjust the pH of the fermentation broth. Once the pH is adjusted to 6.5-7.5, turn off the control unit and continue anaerobic fermentation; The pH of the fermentation broth was monitored and adjusted every 3 to 8 hours until fermentation was completed.

Images