Process and apparatus for the production of methane from sludge anaerobic fermentation based on free ammonia pretreatment

Abstract

The application provides a sludge anaerobic fermentation methane production in-situ biogas upgrading method and device based on free ammonia pretreatment, and the method comprises the following steps: adding a free ammonia conditioning solution to sludge to be pretreated in a pretreatment unit, so that the free ammonia concentration reaches 500-1500 mg / L, and the sludge is treated for 12-36 hours to obtain a fermentation substrate with a pH value of about 6.5-8.3; and the fermentation substrate pretreated by free ammonia is input into an anaerobic biological reactor for fermentation to produce biological natural gas mainly composed of methane. The application is implemented based on free ammonia to destroy the intracellular and extracellular proton and potassium ion concentration balance of the fermentation substrate, accelerate the dissolution of intracellular organic matter of the sludge in the pretreatment and hydrolysis stage, increase the release amount of soluble organic matter and the release amount of total organic matter, and further utilize the hydrogenotrophic methanogenesis pathway principle specifically regulated by free ammonia to synergistically improve the methane concentration and yield of the biogas.

Description

Technical Field

[0001] This invention relates to the technical field of solid waste resource utilization and new energy, and in particular to an in-situ biogas upgrading method and apparatus based on free ammonia pretreatment for anaerobic fermentation of sludge to produce methane. Background Technology

[0002] In urban water environment treatment processes, the harmless disposal rate of urban sewage sludge reaches 90%. With the continuous upgrading of treatment processes and the development of new energy sources, the resource utilization of urban sewage sludge has gradually become a new focus of attention.

[0003] Biogas is a renewable energy source produced through the anaerobic fermentation and degradation of organic matter in sewage sludge, and it can partially replace natural gas and fossil fuels. Biomethane offers various applications in heating, power generation, and as vehicle fuel. Currently, anaerobic fermentation is the preferred technology for producing methane from urban sewage sludge resources. However, because the organic matter in the sludge is encapsulated by extracellular polymers and bound by cell walls, it is difficult to utilize and convert into methane. Low methane yield and concentration limit the resource utilization and high-value utilization of sewage sludge.

[0004] Among existing sludge resource utilization technologies, accelerating organic matter dissolution and enhancing methanogenesis through sludge pretreatment remains a key technology for improving methane yield and realizing sludge resource utilization. Currently, methods to improve methane yield mainly focus on sludge pretreatment technology research. High-temperature hot water hydrolysis pretreatment of sludge can effectively increase methane yield, but it requires advanced equipment and processes and involves significant initial investment. Pretreatment can effectively break down extracellular polymers and cell walls in sludge, promoting cell lysis and the release of intracellular and extracellular organic matter, increasing the hydrolysis rate and biodegradability of sludge, thereby accelerating the reaction and anaerobic digestion process, ultimately increasing methane yield. However, most pretreatment methods still require a large input of energy or chemicals, reducing the efficiency of sludge recycling and hindering large-scale application. Summary of the Invention

[0005] To at least partially address at least one of the aforementioned technical deficiencies, embodiments of the present invention primarily provide an in-situ biogas upgrading method and apparatus for anaerobic fermentation of sludge to produce methanates based on free ammonia pretreatment. During the pretreatment process, free ammonia disrupts the intracellular and extracellular proton and potassium ion concentration balance of the extracellular polymers of organic matter in the fermentation substrate, thereby promoting the release of intracellular small molecule organic matter from the extracellular polymers through the cell wall under the action of ammonia osmotic pressure, and accelerating the dissolution of organic matter during the hydrolysis stage.

[0006] To achieve the above objectives, as an embodiment of one aspect of the present invention, an in-situ biogas upgrading method for anaerobic fermentation of sludge based on free ammonia pretreatment is provided, comprising: adding a free ammonia conditioning solution to the sludge to be pretreated in a pretreatment unit, and after the free ammonia concentration reaches 200-1500 mg / L, allowing it to stand for 12-36 hours to obtain a fermentation substrate with a pH of 6.5-8.3 and a free ammonia concentration of 380-580 mg / L; and inputting the above fermentation substrate into an anaerobic bioreactor inoculated with anaerobic digested sludge for fermentation to produce biogas mainly composed of methane (occupying more than 80-85% of the volume).

[0007] As another embodiment of the present invention, an apparatus is provided for performing the above-described in-situ biogas upgrading method for anaerobic fermentation of sludge based on free ammonia pretreatment to produce methane, comprising: a pretreatment unit, adapted to add a free ammonia conditioning solution to the sludge to be pretreated, and after the free ammonia concentration reaches 200~1500 mg / L, perform a static treatment for 12~36 h to obtain a fermentation substrate with a pH value of 6.5~8.3 and a free ammonia concentration of 380~580 mg / L; and an anaerobic bioreactor, the inlet of which is connected to the outlet of the pretreatment unit via a first inlet pump, the anaerobic bioreactor being adapted to receive the fermentation substrate from the pretreatment unit and inoculate it with anaerobic digested sludge to ferment the fermentation substrate and produce biogas mainly composed of methane (occupying more than 80~85% of the volume).

[0008] The in-situ biogas upgrading method and apparatus for methane production based on sludge anaerobic fermentation provided in the above embodiments of the present invention, through treatment with an alkaline solution containing free ammonia, disrupts the extracellular polymers encapsulating organic matter in the fermentation substrate and the balance of proton and potassium ion concentrations inside and outside the cell wall, promoting the release of intracellular small molecule organic matter through the cell wall under the action of ammonia osmotic pressure, thus obtaining the fermentation substrate. The free ammonia is also suitable for weakening the acetic acid-based methane production pathway and simultaneously regulating the hydrogen-nutritive methane production pathway to convert it into an accumulation-consumption process of acetic acid. Based on the fixation effect of free ammonia on carbon dioxide produced during fermentation in the fermentation system, the buffering effect of the formed carbonate buffer system on pH value is improved, establishing a stable fermentation system. Simultaneously, since carbon dioxide is fixed in the slightly alkaline fermentation system, it can also promote the hydrogen-nutritive methane production process that utilizes carbon dioxide and hydrogen to ferment and produce methane, thereby greatly improving the efficiency and concentration of methane production based on sludge anaerobic fermentation. Attached Figure Description

[0009] Figure 1 A simplified process flow diagram of the in-situ biogas upgrading method for anaerobic fermentation of sludge to produce methanates based on free ammonia pretreatment, according to an embodiment of the present invention.

[0010] Figure 2 This is a schematic diagram of an apparatus for performing an in-situ biogas upgrading method for anaerobic fermentation of sludge to produce methanates based on free ammonia pretreatment, according to an embodiment of the present invention.

[0011] Figure 3 Line graph showing data on methane production from anaerobic fermentation of sludge before and after pretreatment in a continuous experiment according to an exemplary embodiment of the present invention.

[0012] Figure label:

[0013] 1-Preprocessing unit;

[0014] 1-1 The outlet of the pretreatment unit;

[0015] 2-Anaerobic bioreactor;

[0016] 2-1 Inlet of the anaerobic bioreactor;

[0017] 2-2 Gas circulation inlet of the anaerobic bioreactor;

[0018] 2-3 Water bath circulation insulation layer of anaerobic bioreactor;

[0019] 2-4 First-stage three-phase separator;

[0020] 2-5 Gas circulation outlet of anaerobic bioreactor;

[0021] 2-6 Second-stage three-phase separator;

[0022] 2-7 exhaust ports;

[0023] 2-8 The effluent outlet of the anaerobic bioreactor;

[0024] 3-Biofilm tank;

[0025] 3-1 Gas circulation inlet of the biofilm tank;

[0026] 3-2 Inlet of the biofilm tank;

[0027] 3-3 Gas circulation outlet of the biofilm tank;

[0028] 3-4. The outlet of the biofilm tank;

[0029] 3-5 ceramic membrane;

[0030] 4-Biogas meter;

[0031] 5-Methane;

[0032] 6-Programmable Logic Controller (PLC). Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0034] Figure 1 This is a simplified process flow diagram of an in-situ biogas upgrading method based on anaerobic fermentation of sludge to produce methane, according to an embodiment of the present invention.

[0035] In some embodiments of the present invention, see Figure 1 The outlet of anaerobic bioreactor 2 includes a gas circulation outlet and a effluent outlet. The gas circulation outlet of anaerobic bioreactor 2 is connected to the membrane module inside biofilm tank 3 via a pipeline equipped with a first biogas circulation pump. The effluent outlet of anaerobic bioreactor 2 is connected to the inlet of biofilm tank 3 via a pipeline equipped with an effluent pump. The outlet of biofilm tank 3 includes a gas circulation outlet and a effluent outlet. The gas circulation outlet of biofilm tank 3 is connected to the gas circulation inlet of anaerobic bioreactor 2 via a pipeline equipped with a second biogas circulation pump. The effluent outlet of biofilm tank 3 is connected to the pipeline equipped with an effluent pump.

[0036] Reference Figure 1 The sludge to be pretreated is treated with a free ammonia-modified solution in pretreatment unit 1 and then allowed to stand to obtain a fermentation substrate with a pH of 6.5-8.3 and a free ammonia concentration of 380-580 mg / L. The fermentation substrate pretreated with free ammonia is then fed into anaerobic bioreactor 2 for fermentation to produce biogas, which is mainly composed of methane (occupying more than 80-85% of the volume). The biogas from anaerobic bioreactor 2 enters biofilm tank 3 through pipelines, and the biogas from biofilm tank 3 is transported back to anaerobic bioreactor 2 through pipelines, thus forming a biogas cycle and the recycling and retention of carbon dioxide in the biogas.

[0037] According to one aspect of the present invention, an in-situ biogas upgrading method for anaerobic fermentation of sludge to produce methane based on free ammonia pretreatment is provided, comprising: step 1): adding a free ammonia conditioning solution to the sludge to be pretreated in the pretreatment unit, and after the free ammonia concentration reaches 200~1500 mg / L, allowing it to stand for 12~36 h to obtain a fermentation substrate with a pH value of 6.5~8.3 (e.g. 6.98, 7.16, 7.23, 7.32, 7.41, 7.48, 7.58) and a free ammonia concentration of 380~580 mg / L (e.g. 380 mg / L, 400 mg / L, 460 mg / L, 580 mg / L); and step 2): inputting the fermentation substrate from the pretreatment unit into an anaerobic bioreactor inoculated with anaerobic digested sludge for fermentation to produce biogas mainly composed of methane (occupying more than 80~85% of the volume).

[0038] The in-situ biogas upgrading method for anaerobic fermentation of sludge to produce methane based on free ammonia pretreatment provided in the above embodiments of the present invention, through treatment with an alkaline solution containing free ammonia, disrupts the extracellular polymers encapsulating organic matter in the fermentation substrate and the balance of proton and potassium ion concentrations inside and outside the cell wall, promoting the release of intracellular small molecule organic matter through the cell wall under the action of ammonia osmotic pressure, thus obtaining pretreated sludge; wherein, free ammonia is also suitable for weakening the acetic acid-based methane production pathway, and synchronously regulating the hydrogen nutrient-based methane production pathway to convert it into the accumulation-consumption process of acetic acid; based on the fixation effect of free ammonia in the fermentation system on the carbon dioxide produced by fermentation, the buffering effect of the carbonate buffer system on the pH value is improved, and a stable fermentation system is established.

[0039] According to an embodiment of the present invention, the sludge to be pretreated in step 1) is concentrated sludge from a high-efficiency sedimentation tank in wastewater treatment, with a total solids (TS) of 29.51±2.95 mg / L and volatile solids (VS) of 19.08±1.79 mg / L, and has not undergone any other treatment, so as to facilitate direct engineering application in the future; the pH value of the fermentation substrate that allows for direct fermentation in the anaerobic bioreactor is preferably 7.5~7.8. When the pretreated fermentation substrate is input into the anaerobic bioreactor for reaction, the liquid level of the pretreatment unit in the conveying unit is not lower than the center position.

[0040] According to an embodiment of the invention, step 1) specifically includes: step 1A) adding a free ammonia conditioning solution to the sludge to be pretreated in the pretreatment unit to obtain a pretreatment substrate with a pH of 7.5~8.6 and a free ammonia concentration of 200~1500 mg / L; and step 1B) allowing the above pretreatment substrate to stand at 20℃~40℃ for 12~36 days to obtain the above fermentation substrate with a pH of 6.5~8.3 and a free ammonia concentration of 380~580 mg / L.

[0041] According to an embodiment of the present invention, the free ammonia preparation solution in step 1A) comprises free ammonia and at least one base selected from sodium hydroxide and potassium hydroxide. According to an embodiment of the present invention, the free ammonia is ammonium (NH4) + The non-protonated form of NH3-N can be obtained directly from the anaerobic digestion liquid (30–680 mg NH3-N / L), which not only reduces initial input but also greatly improves methanogenesis efficiency. However, it is not limited to this; ammonium salts such as ammonium chloride can also be used. That is, the solution prepared with free ammonia can be at least one of alkaline ammonium chloride solution or anaerobic digestion liquid.

[0042] According to embodiments of the present invention, the pH value of the alkaline ammonium chloride solution is 8.05–8.35 (e.g., 8.12, 8.16, 8.23, 8.27, 8.29, 8.34), preferably 8.27, and the concentration of free ammonia is 200–1500 mg / L (e.g., 230 mg / L, 290 mg / L, 370 mg / L, 480 mg / L, 595 mg / L, 1000 mg / L). According to embodiments of the present invention, the pH value of the pretreated matrix obtained after adding the free ammonia conditioning solution in step 1A) is 7.5–8.6, for example, 7.6, 7.8, 8.0, 8.4, etc., and the concentration of free ammonia is 200–1500 mg / L, for example, 400 mg / L, 420 mg / L, 500 mg / L, 800 mg / L, 1000 mg / L, etc., preferably 400 mg / L.

[0043] According to an embodiment of the present invention, in step 1B), a settling treatment is performed for 12 to 36 hours (e.g., 3.6 hours, 8.6 hours, 12.8 hours, 16.9 hours, 23.8 hours) at a temperature of 20°C to 40°C (e.g., 23.5°C, 28.5°C, 32.5°C, 36.5°C, 39.5°C). After 24 hours of settling treatment, the organic matter in the liquid phase can reach a high release level. Preferably, the fermentation substrate with a pH of approximately 7.5 to 7.8 can be directly used for anaerobic fermentation.

[0044] According to an embodiment of the present invention, before the above-mentioned fermentation substrate is fed into an anaerobic bioreactor inoculated with anaerobic digested sludge for fermentation, an anaerobic state adjustment operation is also included: after settling treatment, an inert gas is introduced into the anaerobic bioreactor to keep the fermentation substrate in an anaerobic state.

[0045] According to embodiments of this disclosure, the in-situ biogas upgrading method for anaerobic fermentation of sludge to produce methane based on free ammonia pretreatment further includes: transporting the effluent from the anaerobic bioreactor to a biofilm tank for continued fermentation, and separating the effluent from the sludge in the biofilm tank to achieve sludge resource utilization and biogas production; transporting the biogas from the anaerobic bioreactor to the biofilm tank for further absorption of carbon dioxide in the biogas, and aerating the membrane modules of the biofilm tank to control membrane fouling; and returning the biogas from the biofilm tank to the anaerobic bioreactor to form a biogas cycle and a cycle of carbon dioxide retention and absorption in the biogas.

[0046] According to an embodiment of the present invention, since the effluent from the anaerobic bioreactor to the biofilm tank still retains some organic matter, some carbon dioxide, and free ammonia, anaerobic fermentation to produce methanogens can continue in the biofilm tank, further reducing the concentration of organic matter or sludge in the effluent. Furthermore, the biogas, through circulation, can not only aerate the membrane modules in the biofilm tank to reduce membrane fouling, but also retain more carbon dioxide in the anaerobic bioreactor, thereby purifying the biogas and enhancing the hydrogen-nutritive methanogenesis pathway in the anaerobic bioreactor that utilizes hydrogen decomposed from carbon dioxide, acetic acid, organic matter, etc., to produce methanogens.

[0047] According to an embodiment of the present invention, the biofilm tank is suitable for receiving effluent from the anaerobic bioreactor to continue fermentation and to separate the effluent from the sludge, thereby realizing resource-based sludge production; receiving biogas from the anaerobic bioreactor to absorb carbon dioxide in the biogas and aerate the membrane module; and returning the biogas from the biofilm tank to the anaerobic bioreactor to form a biogas cycle and a cycle of carbon dioxide retention and absorption in the biogas.

[0048] According to an embodiment of the present invention, the membrane module in the biofilm tank can be a plurality of ceramic membranes spaced apart in a direction perpendicular to the bottom of the biofilm tank. The ceramic membranes are suitable for buffering the impact of biogas circulation on the reactor fermentation system and for intercepting organic matter and sludge in biogas, so as to reduce the chemical oxygen demand of the effluent from the biofilm tank.

[0049] According to an embodiment of the present invention, an apparatus is also provided for performing the above-mentioned in-situ biogas upgrading method for anaerobic fermentation of sludge based on free ammonia pretreatment to produce methane, comprising: a pretreatment unit 1, adapted to add a free ammonia conditioning solution to the sludge to be pretreated, and after the free ammonia concentration reaches 200~1500 mg / L, perform a static treatment for 12~36 h to obtain a fermentation substrate with a pH value of 6.5~8.3 and a free ammonia concentration of 380~580 mg / L; and an anaerobic bioreactor 2, wherein the inlet 2-1 of the anaerobic bioreactor is connected to the outlet 1-1 of the pretreatment unit 1 through a first inlet pump, and the anaerobic bioreactor 2 is adapted to receive the fermentation substrate of the pretreatment unit 1 and inoculate it with anaerobic digested sludge to ferment the fermentation substrate to produce biogas mainly composed of methane (volume occupying more than 80~85%).

[0050] Reference Figure 2 The diagram shown is a schematic representation of an apparatus for implementing the above-described in-situ biogas upgrading method based on anaerobic fermentation of sludge to produce methane, according to an embodiment of the present invention.

[0051] According to an embodiment of the present invention, see Figure 2The aforementioned apparatus includes a pretreatment unit 1 and an anaerobic bioreactor 2. Specifically, the pretreatment unit 1 is used to add a free ammonia-modified solution to the sludge to be pretreated, obtaining a pretreatment substrate with a pH of 7.5–8.6 and a free ammonia concentration of 200–1500 mg / L. This pretreatment substrate is then subjected to static treatment at 20°C–40°C for 12–36 hours to obtain an accelerated hydrolysis fermentation substrate.

[0052] According to an embodiment of the present invention, the anaerobic bioreactor further separates water, large particulate matter, and gas in the sludge multiple times through the action of a first-stage three-phase separator 2-4 and a second-stage three-phase separator 2-6, so that the biogas produced by the anaerobic fermentation process is discharged from the exhaust port 2-7 of the anaerobic bioreactor 2. The discharged gas is treated with an aqueous solution and then reaches a biogas meter 4 that can measure the volume of biogas. After being treated with a sodium hydroxide aqueous solution, the gas is then transported to a methane meter 5 that can measure the volume of methane gas.

[0053] According to an embodiment of the present invention, the above-mentioned device further includes a biofilm tank 3. The inlet 3-2 of the biofilm tank is connected to the outlet 2-8 of the anaerobic bioreactor via a second inlet pump. The membrane module of the biofilm tank 3 is connected to the gas circulation outlet 2-5 of the anaerobic bioreactor via a first biogas circulation pump. The gas circulation outlet 3-3 of the biofilm tank is connected to the gas circulation inlet 2-2 of the anaerobic bioreactor via a second biogas circulation pump. The biogas in the anaerobic bioreactor 2 first passes through the gas circulation outlet 2-5 of the anaerobic bioreactor, then passes through the pipeline equipped with the first biogas circulation pump to the gas circulation inlet 3-1 of the biofilm tank, and enters the membrane module in the biofilm tank 3. After the membrane module in the biofilm tank 3 removes organic matter and sludge from the biogas, it flows out through the gas circulation outlet 3-3 of the biofilm tank and passes through the pipeline equipped with the second biogas circulation pump to the gas circulation inlet 2-2 of the anaerobic bioreactor, forming a complete biogas circuit. On the other hand, the effluent from the anaerobic bioreactor outlet 2-8 flows through a pipeline equipped with a second inlet pump to the inlet 3-2 of the biofilm tank. After passing through the membrane module inside the biofilm tank 3, it reaches the outlet 3-4 of the biofilm tank and flows out through a pipeline equipped with an outlet pump, for example, into a storage tank for later use. A water bath circulation insulation layer 2-3 for the anaerobic bioreactor is installed around the anaerobic bioreactor 2.

[0054] According to an embodiment of the present invention, see Figure 2 The membrane components in the biofilm tank 3 consist of multiple ceramic membranes 3-5 spaced apart in a direction perpendicular to the bottom of the biofilm tank. The ceramic membranes 3-5 are suitable for buffering the impact of biogas circulation on the reactor fermentation system and for intercepting organic matter and sludge in biogas, so as to reduce the chemical oxygen demand of the effluent from the biofilm tank 3.

[0055] According to an embodiment of the present invention, the methane generating apparatus further includes a programmable logic controller (PLC) data control and metering device; the PLC data control and metering device may include a programmable logic controller 6, a first pH sensor disposed in the pretreatment unit 1, a second pH sensor, an oxidation-reduction potential (ORP) sensor, and a temperature sensor disposed between the first-stage three-phase separator 2-4 and the second-stage three-phase separator 2-6 of the anaerobic bioreactor 2; a third pH sensor, an oxidation-reduction potential (ORP) sensor, and a liquid phase conductivity (EC) sensor disposed below the first-stage three-phase separator 2-4 of the anaerobic bioreactor 2; a fourth pH sensor disposed in the biofilm tank 3; and a portable metering device and a micro gas flow meter disposed in the pipeline. Based on the measurements from these sensors, some operational characteristics of the pretreatment unit, the fermentation system of the reactor, and the biofilm tank can be determined.

[0056] According to embodiments of the present invention, the PLC data control metering equipment is suitable for metering the influent rate, effluent rate, and biogas circulation rate of the pretreatment unit, anaerobic bioreactor, and biofilm tank, as well as controlling the first influent pump, the second influent pump, the first biogas circulation pump, and the second biogas circulation pump. This is achieved by using a water-coordinated mode to regulate the influent or effluent of the pretreatment unit 1, the anaerobic bioreactor 2, and the biofilm tank 3, and to regulate the gas phase circulation between the anaerobic bioreactor 2 and the biofilm tank 3. Specifically, the water-coordinated mode refers to simultaneously regulating the influent and effluent rates of the anaerobic bioreactor to meet the circulating water requirements of the anaerobic bioreactor and the biofilm tank, based on the need for coordination between the influent rate of the pretreatment unit and the effluent rate of the biofilm tank.

[0057] In some embodiments of the present invention, the portable meter and the micro gas flow meter are connected to the programmable logic controller (PLC) data control metering device via a data transmission line; the PLC data control metering device is connected to the first water inlet pump, the second water inlet pump, the first biogas circulation pump, and the second biogas circulation pump via control signal transmission lines respectively.

[0058] The present invention will be further illustrated below by way of comparative examples and embodiments. In the following detailed description, numerous specific details are set forth for ease of explanation in order to provide a comprehensive explanation of the embodiments of the invention. However, it will be apparent that one or more embodiments may be practiced without these specific details. Moreover, the details in the following embodiments may be arbitrarily combined to form other feasible embodiments without conflict.

[0059] Example 1

[0060] Parallel experiments on free ammonia pretreatment methods for methanogenesis from anaerobic fermentation of sludge.

[0061] Five parallel experiments were conducted, with different concentration gradients of free ammonia (FA) designed as follows: 0, 200, 400, 600, and 800 mg / L. The volumes of the pretreatment unit and the anaerobic bioreactor were 1500 mL each. The specific steps are as follows:

[0062] S1: Select the sludge to be pretreated

[0063] The concentrated sludge from the high-efficiency sedimentation tank with characteristic indicators of [total solids (TS) = 23.3 g / L, volatile solids (VS) = 16.1 g / L, pH = 6.52, free ammonia (FA) = 0.42 mg / L] is directly used as the sludge to be pretreated without any other treatment process;

[0064] S2: Preparation of pretreatment matrix

[0065] 1000 mL of sludge to be pretreated was measured into the reactors of five parallel pretreatment units. Different volumes of NaOH and NH4Cl stock solutions (3 mol / L) were added as free ammonia conditioning solutions. The pH of the pretreatment substrate was controlled at 8.27 ± 0.1, and the corresponding free ammonia (FA) concentration gradients were 0, 200, 400, 600, and 800 mg / L, respectively. The total ammonia nitrogen level ranged from 96 to 3896.5 mg / L. Ultrapure water was used to supplement the substrate to an equal volume of 1200 mL to ensure consistent sludge concentration. The pretreatment substrate was obtained. Since the pretreatment units were not inoculated, the release rate of organic matter could be studied. No further condition adjustments were made afterward, and the experiment was conducted at an ambient temperature of 20℃ to 25℃.

[0066] S3: Preparation of fermentation substrate through static treatment

[0067] Nitrogen gas was introduced into the reactor containing the pretreated substrate in S2 for 2 minutes. The settling time during pretreatment was 1 to 7 days. 50 mL samples were taken every 24 hours to determine the release rate of organic matter in the fermentation substrate.

[0068] Experimental results showed that when the treatment time was 1 day, the release of organic matter reached its peak and essentially stagnated from day 5 to day 7. Compared with the control group [free ammonia (FA) concentration of 0 mg / L], the optimized free ammonia treatment increased the concentrations of TCOD (total chemical oxygen demand) and SCOD (dissolved chemical oxygen demand) by 134.64%–250.34% and 154.70%–274.55%, respectively. This indicates that the optimized free ammonia level effectively enhanced the sludge pretreatment effect. Free ammonia pretreatment accelerated the release of organic matter in the sludge during the hydrolysis stage, effectively increasing the concentration of dissolved organic matter in the sludge liquid phase. The optimized settling time was 1 day.

[0069] Example 2

[0070] Parallel Experiments of an In-situ Biogas Upgrade Method Based on Free Ammonia Pretreatment for Anaerobic Fermentation of Sludge to Produce Methanum

[0071] Five parallel experiments (15 sets) were conducted, with different free ammonia (FA) concentration gradients designed as follows: 0, 200, 400, 600, and 800 mg / L. The reactor volumes used were 600 mL. Free ammonia pretreatment and anaerobic fermentation were carried out in batches in the reactors. The free ammonia level conditions of the pretreatment unit were designed in the same manner as in Example 1. The specific steps are as follows:

[0072] S1: Select the sludge to be pretreated and the inoculated sludge.

[0073] The concentrated sludge from the high-efficiency sedimentation tank with characteristic indicators [Total solids (TS) = 11.5 g / L, volatile solids (VS) = 7.0 g / L, pH = 7.47, free ammonia (FA) = 20.64 mg / L] was used directly as the sludge to be treated without any other treatment process; the inoculated sludge with characteristic indicators [Total solids (TS) = 105.8 g / L, volatile solids (VS) = 77.8 g / L, pH = 7.51] was used, with an inoculation volume ratio of 2:1.

[0074] S2: Preparation of fermentation substrate

[0075] Referring to Example 1, a free ammonia concentration gradient was designed. The same volume of sludge to be pretreated was added to each reactor in the pretreatment unit of 5 parallel experiments. NaOH and NH4Cl stock solutions (3 mol / L) were added as free ammonia conditioning solutions. The pH value of the pretreatment substrate was adjusted to 8.27±0.1 and the corresponding free ammonia (FA) concentration gradients were 0, 200, 400, 600, and 800 mg / L, respectively. Ultrapure water was used to make up to an equal volume of 400 mL to ensure that the sludge concentration was consistent. The mixture was then kept in a constant temperature (37℃) water bath for 24 h to obtain the fermentation substrate.

[0076] S3: Anaerobic fermentation process

[0077] Nitrogen gas was introduced into the reactor containing the fermentation substrate obtained from S2 for 2 minutes, and the reactor was then used as an anaerobic bioreactor for anaerobic fermentation. Before connecting to the testing system, high-purity nitrogen gas was purged for 2 minutes to ensure that the anaerobic bioreactor was operating under anaerobic conditions. The stirring speed was 120 rpm / min, and the intermittent frequency was 5 minutes for running time and 10 minutes for stopping time. During this period, the water bath temperature was kept constant at 37℃. The fermentation cycle was 28 days, and samples were taken on days 0, 7, 17, and 28 of reactor operation (based on the periodic decrease in methane production and the daily methane production being less than 5 mL). The fermentation process was evaluated using a fully automated biochemical methane potential (BMP) and testing system (AMPTSⅡ, Bioprocess Control).

[0078] The results showed that, compared with the control group (FA concentration of 0 mg / L), the rates of the first gas production peak (D0-D5) and the second gas production peak (D9-D12) of the fermentation substrate pretreated with free ammonia were increased by 21.04% and 120.39%, respectively; the cumulative methane production and methane production potential in the anaerobic bioreactor were increased by 34.6% and 23.3%, respectively, and the optimized concentration of free ammonia in the pretreated substrate was 400 mg / L.

[0079] Example 3

[0080] An apparatus for producing methane through anaerobic fermentation of sludge based on free ammonia pretreatment includes a pretreatment unit 1, an anaerobic bioreactor 2, and a biofilm tank 3. The anaerobic bioreactor is a cylindrical reactor with a diameter (R×H) of 0.15m × 1.8m and a working volume of 27L. Figure 2 As shown, the specific structure of the device is as follows:

[0081] (1) The inlet 2-1 of the anaerobic bioreactor is connected to the outlet 1-1 of the pretreatment unit through the first inlet pump installed on the pipeline;

[0082] (2) The outlet of the anaerobic bioreactor 2 is divided into a gas circulation outlet and a water outlet. The gas circulation outlet 2-5 of the anaerobic bioreactor is connected to the ceramic membrane 3-5 inside the biofilm tank 3 through the first biogas circulation pump installed on the pipeline. The water outlet 2-8 of the anaerobic bioreactor is connected to the water inlet 3-2 of the biofilm tank through the second water inlet pump installed on the pipeline.

[0083] (3) The outlet of the biofilm tank 3 is divided into a gas circulation outlet and a water outlet. The gas circulation outlet 3-3 of the biofilm tank is connected to the gas circulation inlet 2-2 of the anaerobic bioreactor through a second biogas circulation pump installed on the pipeline. A water outlet pump is installed on the pipeline at the water outlet 3-7 of the biofilm tank.

[0084] (4) The gas circulation outlet of the anaerobic bioreactor is connected to a micro gas flow meter, which is connected to PLC 6 through a data transmission line; PLC 6 is connected to the first water inlet pump, the second water inlet pump, the first biogas circulation pump and the second biogas circulation pump through a control signal transmission line.

[0085] An in-situ biogas upgrading method based on anaerobic fermentation of sludge pretreated with free ammonia to produce methanates is a continuous methanogenesis method, and the specific steps are as follows:

[0086] S1: Continuous Experiment - Selection of Sludge and Inoculum to be Pretreated

[0087] The concentrated sludge from the high-efficiency sedimentation tank with characteristic indicators [Total solids (TS) = 29.51 ± 2.95 g / L, Volatile solids (VS) = 19.08 ± 1.79 g / L, pH = 7.35 ± 0.11, Free ammonia (FA) = 2.84 ± 0.10 g / L] was used directly as the pre-treated sludge without any other treatment process; the inoculated sludge had characteristic indicators [Total solids (TS) = 105.8 g / L, Volatile solids (VS) = 77.8 g / L, pH = 7.51], and the inoculation volume ratio was 2:1.

[0088] S2: Continuous Experiment - Control Anaerobic Fermentation Process

[0089] The sludge to be pretreated obtained from S1 was directly fed into the anaerobic bioreactor as the fermentation substrate. The biogas recirculation system was controlled by the first and second biogas circulation pumps at 30 rpm / min to perform partial gas-liquid recirculation. The operating temperature was set at 37℃, and the fermentation cycle was 47 days. During this period, a micro biogas meter and a PLC data control metering device recorded the biogas, methane yield, and cumulative production in real time. The experimental results are shown in... Figure 3 middle.

[0090] S3: Continuous experiment - pretreatment to obtain fermentation substrate

[0091] Add sludge to be pretreated at a flow rate of 2000 mL / d to the pretreatment unit, and add NaOH (3 mol / L) and NH4Cl stock solution (5 mol / L) to control the pH value of the pretreatment substrate at 8.27±0.1 and the free ammonia (FA) = 400±10 mg / L. Let it stand for 24 h at an ambient temperature of 18℃~25℃ to complete the pretreatment and obtain the fermentation substrate.

[0092] S4: Continuous Experiment - Experimental Anaerobic Fermentation Process

[0093] After nitrogen gas was introduced into anaerobic bioreactor 2 for 2 minutes, the fermentation substrate obtained in step S3 was transferred into the anaerobic bioreactor for reaction. The biogas recirculation system was controlled by a biogas circulation pump at 30 rpm / min for partial gas-liquid recirculation. The operating temperature was set at 37℃, and the fermentation cycle was 98 days. During this period, a micro-biogas meter and PLC 6 recorded the biogas and methane yields and cumulative production in real time. The experimental results are shown in... Figure 3 middle.

[0094] like Figure 3 As shown, compared with the control anaerobic fermentation process (fermentation cycle of 1-47 days), the methane concentration in the anaerobic fermentation process after free ammonia pretreatment increased from 42.97±4.48% to 84.7±2.69%, an increase of 97.28%, and the methane yield increased from 0.59±0.30L / d to 5.34±1.01L / d, an increase of 9.05 times. The method of producing methane through anaerobic fermentation of sludge with free ammonia pretreatment greatly improves the problems of low hydrolysis yield of organic matter in sludge and low methane production efficiency, thereby increasing the methane concentration in the biogas obtained from anaerobic fermentation, accelerating the start-up of the reaction, and shortening the anaerobic digestion methanogenesis cycle, showing the potential for a new type of in-situ biogas upgrade.

[0095] The in-situ biogas upgrading method and apparatus for anaerobic fermentation of sludge to produce methane based on free ammonia pretreatment provided in this invention disrupts the intracellular and extracellular proton and potassium ion concentration balance of the extracellular polymers of organic matter in the fermentation substrate through treatment with an alkaline solution containing free ammonia. This promotes the release of intracellular small-molecule organic matter from the extracellular polymers under the action of ammonia osmotic pressure, resulting in pretreated sludge. Furthermore, free ammonia is also suitable for weakening the acetic acid-based methane production pathway and simultaneously regulating the hydrogen-nutritive methane production pathway to convert it into an acetic acid accumulation-consumption process. Based on the fixation effect of free ammonia on carbon dioxide produced during fermentation, the pH buffering effect of the formed carbonate buffer system is improved, establishing a stable fermentation system. Simultaneously, since carbon dioxide is fixed in the slightly alkaline fermentation system, it also promotes the hydrogen-nutritive methane production process utilizing carbon dioxide and hydrogen, greatly improving the efficiency and concentration of methane production based on anaerobic fermentation of sludge. Furthermore, by recycling biogas, carbon dioxide is retained in the alkaline fermentation system, thereby further increasing the methane concentration in the biogas to 80%–90% and reducing the carbon dioxide concentration to 10%–20%. In addition, free ammonia (FA) is ammonium (NH4) + The nonprotonated form of the anaerobic digestion solution can be obtained directly from the anaerobic digestion liquid without increasing additional consumption, thus greatly reducing the cost of pretreatment.

[0096] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for upgrading biogas production from sludge anaerobic fermentation based on free ammonia pretreatment, characterized in that, include: A free ammonia conditioning solution is added to the sludge to be pretreated in the pretreatment unit. The free ammonia conditioning solution has a pH of 8.05–8.35 and a free ammonia concentration of 200–1500 mg / L. After the free ammonia concentration reaches 200–1500 mg / L, the mixture is allowed to stand for 12–36 hours to obtain a fermentation substrate with a pH of 6.5–8.3 and a free ammonia concentration of 380–580 mg / L. as well as The fermentation substrate is fed into an anaerobic bioreactor inoculated with anaerobic digested sludge for fermentation to produce biogas, mainly methane. The effluent from the anaerobic bioreactor is transported to the biofilm tank for continued fermentation, and the effluent from the biofilm tank is separated from the sludge to realize sludge resource utilization and biogas production. The biogas from the anaerobic bioreactor is transported to the biofilm tank for further absorption of carbon dioxide from the biogas, and the membrane modules of the biofilm tank are aerated to control membrane fouling; and The biogas in the biofilm tank is returned to the anaerobic bioreactor through the gas circulation inlet of the anaerobic bioreactor to form a biogas circulation and carbon dioxide circulation and absorption in the biogas. The gas circulation inlet is located at the bottom of the anaerobic bioreactor.

2. The method according to claim 1, characterized in that, The free ammonia-modified solution includes free ammonia and at least one base selected from sodium hydroxide and potassium hydroxide.

3. The method according to claim 2, characterized in that, The free ammonia originates from ammonium chloride or anaerobic digestion liquid from wastewater treatment.

4. The method according to claim 2, characterized in that, Pretreatment was performed for 12 to 36 hours at a temperature of 20℃ to 40℃.

5. The method according to claim 1, characterized in that, The free ammonia-modified solution was added to the sludge to be pretreated in the pretreatment unit and allowed to stand for 12–36 hours to obtain a fermentation substrate with a pH of 6.5–8.3 and a free ammonia concentration of 380–580 mg / L, including: The free ammonia-modified solution was added to the sludge to be pretreated in the pretreatment unit to obtain a pretreatment matrix with a pH of 7.5-8.6 and a free ammonia concentration of 200-1500 mg / L. The pretreated substrate was allowed to stand at 20℃~40℃ for 12~36h to obtain the fermentation substrate with a pH of 6.5~8.3 and a free ammonia concentration of 380~580 mg / L.

6. The method according to claim 1, characterized in that, Before the fermentation substrate is fed into an anaerobic bioreactor inoculated with anaerobic digested sludge for fermentation, the process further includes: After settling, an inert gas is introduced into the anaerobic bioreactor to bring the fermentation substrate into an anaerobic state.

7. The method according to claim 1, characterized in that, The anaerobic digested sludge is granular sludge that has been acclimated to inhibit ammonia nitrogen; the sludge to be pretreated is concentrated sludge from sedimentation tanks in wastewater treatment.

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