A method for enhancing high-organic-load two-phase anaerobic digestion by coupling biogas residue and biogas slurry reflux

Abstract

The present application belongs to the technical field of organic waste anaerobic digestion, and specifically discloses a method for enhancing high-organic-load two-phase anaerobic digestion by coupling and refluxing biogas residue and biogas slurry.The present application uses multi-source organic waste as raw material, and in a semi-continuous two-phase anaerobic reactor, biogas residue and biogas slurry separated from the digestion residues are respectively refluxed to the acid-producing phase and the methane-producing phase of the two-phase anaerobic digestion system.The reflux of biogas slurry significantly improves the stability and acidification capacity of the acid-producing phase, and the reflux of biogas residue improves the stability and gas production performance of the methane-producing system.The coupling and refluxing of biogas slurry and biogas residue can effectively reduce the accumulation of propionic acid in the methane-producing phase, effectively alleviate the acidification inhibition phenomenon caused by the accumulation of propionic acid, and improve the stability and high-load operation capacity of the system.The present application is an important way to achieve carbon reduction and resource utilization for rapidly growing urban organic solid waste and anaerobic sludge.

Description

Technical Field

[0001] This application belongs to the field of anaerobic digestion technology of organic waste, and more specifically, relates to a method for enhanced two-phase anaerobic digestion with high organic load through coupled reflux of biogas residue and biogas slurry. Background Technology

[0002] With the acceleration of urbanization, the output of urban organic solid waste is showing a rapid growth trend. Anaerobic digestion, as a relatively mature organic solid waste treatment technology, can achieve the reduction, harmlessness, and resource recovery of organic solid waste. Under anaerobic conditions, anaerobic and facultative microorganisms convert biodegradable organic components into methane, enabling stable treatment of organic solid waste while also achieving energy recovery. Existing organic solid waste treatment processes still face the dual challenges of low treatment efficiency and poor system stability. Compared with low organic load operation, high organic load has the advantages of higher gas production per unit volume, lower infrastructure costs, and larger treatment capacity. Maintaining the stability of the anaerobic digestion system under high organic load conditions has become an urgent problem to be solved.

[0003] High organic loading (HOC) operation technology is an important research direction in anaerobic digestion, aiming to improve the efficiency of anaerobic digestion by increasing the system's organic matter processing capacity, thereby achieving a higher volumetric gas production rate. However, high HOC operation also brings a series of challenges, such as system instability, acidification, ammonia nitrogen inhibition, and microbial community imbalance. These problems not only affect the stable operation of the system but may also lead to a decrease in gas production efficiency. Existing methods for improving the stability of high-load anaerobic digestion mostly rely on adding conductive materials and chemical agents, which result in secondary pollution and high treatment costs.

[0004] In terms of process design, current engineering applications are mostly limited to single-phase or two-phase anaerobic digestion. Single-phase processes attempt to complete the entire process of hydrolysis, acid production, and methanogenesis within the same reactor. However, due to the significant differences in growth kinetics and environmental requirements between acid-producing and methanogenic bacteria, the system's buffering capacity is extremely poor, easily leading to the accumulation of volatile fatty acids (VFAs) and pH imbalance. Two-phase anaerobic digestion technology places acid-producing bacteria and hydrogen- and acetic acid-producing bacteria in the acid-producing phase, and methanogenic bacteria in the methanogenic phase, providing suitable environments for each. This effectively avoids mutual inhibition between different microorganisms and their metabolites.

[0005] Anaerobic digestion residues refer to the waste products produced after anaerobic digestion, which have complex compositions and properties. The biogas slurry and biogas residue produced during anaerobic digestion are usually treated as waste and discharged or landfilled. Biogas slurry has an extremely complex composition, containing not only high concentrations of ammonia nitrogen and salt, but also humic substances and heavy metal ions that are difficult to biodegrade. Direct reinjection or discharge can easily cause soil salinization and eutrophication of water bodies. While biogas residue is rich in organic matter, its extremely high water content makes dehydration difficult and transportation expensive. Direct dumping or landfilling without proper treatment not only occupies a large amount of land, but the pathogenic microorganisms and incompletely stabilized organic matter it contains can also cause serious secondary pollution. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this application is to provide a method for enhancing high organic load two-phase anaerobic digestion by coupled reflux of biogas residue and biogas slurry, which aims to solve the technical problems of instability and reduced gas production efficiency in existing high organic load two-phase anaerobic digestion systems.

[0007] To achieve the above objectives, in a first aspect, this application provides a method for enhanced two-phase anaerobic digestion with high organic loading through coupled reflux of biogas residue and biogas slurry, comprising the following steps: (1) After mixing and stirring the kitchen waste and dewatered sludge, anaerobic fermentation is carried out in the acid-producing phase reactor to obtain hydrolyzed acidified liquid, which is used as the substrate of the methanogenic phase reactor. In the methanogenic phase reactor, under the action of methanogenic bacteria, the hydrolyzed acidified liquid is converted into gaseous products containing methane and carbon dioxide. (2) After the system reaches steady state, the digestion residue in the methanogenic reactor is subjected to solid-liquid separation to obtain biogas slurry and biogas residue; (3) The biogas slurry is recycled to the acid-producing phase reactor and the biogas residue is recycled to the methanogenic phase reactor. The biogas slurry recycling ratio (defined as the volume of biogas slurry recycled to the acid-producing phase reactor / the daily feed volume of the acid-producing phase reactor) is 0.15-0.30 and the biogas residue recycling ratio (defined as the volume of biogas residue recycled to the methanogenic phase reactor / the daily feed volume of the methanogenic phase reactor) is 0.15-0.25.

[0008] Preferably, the mass ratio of VS (volatile solids) in the kitchen waste and dewatered sludge is (3-5):1.

[0009] Preferably, the kitchen waste is kitchen waste that has undergone oil removal treatment, with a pH of 3-5, TS of 8%-10%, and VS of 7%-9%; the dewatered sludge has a water content of 80wt%-90wt%, contains polyaluminum chloride, and has a TS of 12wt%-16% and a VS of 7wt%-9wt%; the mixed matrix of kitchen waste and dewatered sludge has a pH of 4-5.

[0010] Preferably, the organic loading of the acid-producing phase reactor is 10-20 g VS / (L·d); and the organic loading of the methanogenic phase reactor is 3-10 g VS / (L·d).

[0011] Preferably, the solid-liquid separation in step (2) is centrifugal separation, natural sedimentation, or plate and frame filtration.

[0012] More preferably, the solid-liquid separation in step (2) is centrifugal separation, with a centrifugation speed of 800-1000 rpm and a centrifugation time of 5-10 min.

[0013] Preferably, the system is carried out in a semi-continuous flow two-phase anaerobic digester, with semi-continuous treatment performed once a day at regular intervals, while the biogas slurry and biogas residue are coupled and refluxed.

[0014] Preferably, the water content in the biogas residue is 90-95 wt%; The biogas residue contains functional microbial communities related to organic matter hydrolysis, fermentation, intermediate metabolite conversion, and methanogenesis; at the phylum level, the biogas residue contains... Firmicutes Relative abundance not less than 45%, Bacteroidota The relative abundance is not less than 20%; at the genus level, the biogas residue contains Fastidiosipila The relative abundance is not less than 10%. W5053 The relative abundance is not less than 8%. Christensenellaceae_R-7_group The relative abundance is not less than 10%. Lactobacillus The relative abundance of methanogenic archaea does not exceed 5%, and the total relative abundance of methanogenic archaea is not less than 20%.

[0015] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: (1) This invention successfully increases the organic load of the system by recirculating the biogas slurry from the methanogenic phase to the acidogenic phase and the biogas residue to the methanogenic phase. This strategy of strengthening the functional zoning and coupled recirculation of the acidogenic and methanogenic stages successfully enhances the organic load of the system. Compared with traditional single-phase or two-phase processes, this invention significantly increases the organic matter throughput and processing efficiency per unit volume reactor, significantly reduces the required reactor volume, and lowers infrastructure costs.

[0016] (2) This invention effectively solves the common inhibition problem in conventional reflux processes. Although traditional biogas slurry reflux can provide alkalinity, it easily leads to an increase in the pH of the acid-producing phase to 6.5-7, which exceeds the optimal activity range of acid-producing bacteria (pH 2-5), thus inhibiting the acid-producing process. At the same time, biogas slurry reflux is often accompanied by a significant increase in propionic acid concentration, which is not easily utilized by methanogenic bacteria, easily causing accumulation and inhibiting the system. This invention precisely controls the process through phase-coupled reflux experiments: the biogas slurry rich in ammonia nitrogen is refluxed to the acid-producing phase, specifically for neutralizing the acidity at the front end and maintaining a suitable acidification environment; at the same time, the biogas residue rich in microorganisms is refluxed to the methanogenic phase, directly supplementing the biomass and enhancing the long-term stability of the system.

[0017] (3) In traditional processes, biogas residue and biogas slurry are treated as end-of-pipe wastes, which are costly to treat and pose a risk of secondary pollution. This invention realizes the internal circulation of digestion residues within the system. Biogas slurry recirculation replaces fresh process water, reducing water costs and the load on subsequent biogas slurry treatment; biogas residue recirculation serves as a supplementary means of biomass carrier and microbial supplement. This achieves minimal external discharge of waste, regulates the stability of the anaerobic system through coupled recirculation, and also realizes the resource recycling of digestion residues, reducing the risk of secondary pollution and engineering treatment costs.

[0018] (4) The coupled reflux strategy proposed in this invention improves gas production performance. Recirculating the biogas slurry increases the production of volatile fatty acids (VFAs) in the acid-producing phase, providing a richer substrate for the subsequent methanogenic phase. Recirculating biogas residue to the methanogenic phase effectively promotes the secretion of tyrosine proteins and fulvic acids, increases the concentration of methanogenic bacteria, and promotes substrate degradation and transformation. Coupled reflux of biogas slurry and biogas residue effectively reduces propionic acid accumulation in the methanogenic phase, effectively alleviating acidification inhibition caused by propionic acid accumulation, and improving the system's stability and high-load operating capacity. Furthermore, coupled reflux increases the ETS activity of both the acid-producing and methanogenic phases, enhancing the metabolic capacity of microorganisms. Under high organic load conditions, it increases the concentration of anaerobic methanogenic microorganisms, enhancing the methanogenic phase's ability to utilize VFAs. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the semi-continuous flow two-phase anaerobic digester system used in the embodiments of this application; Figure 2 This describes the pH changes of the acid-producing and methanogenic phases in the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application. Figure 3 This describes the changes in ammonia nitrogen in the acid-producing and methanogenic phases during the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application. Figure 4 This describes the changes in gas production in the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application; Figure 5 This describes the changes in gas composition in the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application; Figure 6 The variations of the acid-producing phase VFA in the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application are shown. Figure 7 The variations in the methane-producing VFA in the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application are shown. Figure 8 This document shows the changes in the acid-producing microbial community at the phylum level (content (a)) and genus level (content (b)) in the three processes of Comparative Example 2 and Example 1 of this application; Figure 9 This document shows the changes in the phylum-level bacteria community of the methanogenic phase bacteria in the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application. Figure 10 This document shows the genus-level changes in the methanogenic bacterial community in the three processes of Comparative Example 1, Comparative Example 2, and Example 1 of this application. Figure 11 This document illustrates the genus-level changes in the methanogenic archaea community during the three processes described in Comparative Example 1, Comparative Example 2, and Example 1 of this application. Figure 12 This is the process flow and material balance of the two-phase anaerobic digestion treatment in this embodiment of the invention; In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1-Acid-producing phase reactor; 2-Methan-producing phase reactor; 3-Acid-producing phase reactor outlet; 4-Peristaltic pump; 5-Methan-producing phase reactor inlet; 6-Methan-producing phase reactor digestion residue outlet; 7-Digestion residue solid-liquid separation device; 8-Online monitoring system. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0021] Existing technologies often fail to clearly distinguish the functional differences between biogas residue and biogas slurry, employing a complete biogas slurry recirculation approach. This type of recirculation is not designed to address the different environmental requirements of the two-phase anaerobic "acid-producing-methan-producing" bacterial communities, leading to the circulation of ammonia nitrogen and salts in the biogas slurry throughout the system. This frequently results in excessively high pH in the acid-producing phase or ammonia inhibition in the methanogenic phase. If unsorted biogas residue or ordinary digested sludge is directly recirculated, inert solids and recalcitrant humic substances accumulate simultaneously, reducing effective volume and weakening mass transfer. This invention addresses this by physically separating biogas residue and biogas slurry and then recirculating them in a directional manner—the biogas slurry (rich in alkalinity and trace elements) is precisely recirculated to the acid-producing phase to neutralize acidification and optimize fermentation; the biogas residue (enriched with methanogenic bacteria and highly adsorbent) is precisely recirculated to the methanogenic phase to replenish biomass and inhibit propionic acid accumulation.

[0022] Existing reflux designs are mostly suitable for low to medium organic loads (OLR ≤ 6 g VS / (L·d)). When the load increases to 8 g VS / (L·d) or higher, excessive accumulation of volatile fatty acids (VFA) is common. Under high loads, the short hydraulic retention time (HRT) leads to the elution of methanogenic bacteria, causing system collapse. This invention designs a reflux mechanism for high organic loads (especially when the theoretical organic load of the methanogenic phase reaches 9.97 g VS / (L·d) or higher); it compensates for biomass loss under short HRTs through sludge reflux.

[0023] Existing technologies for treating biogas residue and biogas slurry are mostly end-of-pipe treatments. Biogas slurry requires additional solid-liquid separation and wastewater treatment, while biogas residue needs to be dried, landfilled, or incinerated. This invention transforms biogas residue and biogas slurry into resources within the system—biogas slurry replaces external alkalinity supplementation, and biogas residue replaces the addition of external microbial inoculants, reducing back-end treatment costs and minimizing secondary pollution.

[0024] The current mainstream technology is the single-phase anaerobic reactor, where the biogas slurry and biogas residue are usually recycled together into the single-phase reactor. For two-phase reactors, usually only the biogas slurry is recycled, possibly because some components of the biogas residue are harmful or useless (such as metal salts or recalcitrant substances), and recycling them would affect the mass transfer efficiency (e.g., more solids would reduce fluidity).

[0025] Existing anaerobic digestion methods neglect the potential value of the alkalinity and trace elements in the biogas slurry, as well as the active microbial communities enriched in the biogas residue, and may even cause secondary pollution due to improper treatment. Current technologies lack an effective coupling reflux mechanism, failing to transform end-products into a means of controlling the upstream process. Constructing a synergistic digestion method that leverages the advantages of two-phase anaerobic processes, tolerates high organic loads, and enhances the system's shock resistance and biomass retention through a phase-coupling reflux mechanism of biogas residue and biogas slurry is a pressing technical challenge in the field of organic solid waste resource utilization. Currently, there is no phased, controlled reflux strategy for the food waste-sludge co-process, and the regulatory effect of biogas residue and biogas slurry reflux on different organic load stages cannot be fully verified. This application accidentally discovered in experiments that not only is the biogas slurry rich in microorganisms and nutrients in the anaerobic digestion residue, but the microbial nutrients in the biogas residue also have the potential to regulate the fermentation environment when coupled with the biogas slurry for reflux. This can effectively solve the problems of acid inhibition, system collapse, and low gas production efficiency in current high-load organic phase anaerobic digestion.

[0026] This invention provides a method for enhanced two-phase anaerobic digestion with high organic load through coupled reflux of biogas residue and biogas slurry, comprising the following steps: (1) After mixing and stirring the kitchen waste and dewatered sludge, anaerobic fermentation is carried out in the acid-producing phase reactor to obtain hydrolyzed acidified liquid, which is used as the substrate of the methanogenic phase reactor. In the methanogenic phase reactor, under the action of methanogenic bacteria, the hydrolyzed acidified liquid is converted into gaseous products containing methane and carbon dioxide. (2) After the system reaches steady state, the digestion residue in the methanogenic reactor is subjected to solid-liquid separation to obtain biogas slurry and biogas residue; (3) The biogas slurry is recycled to the acid-producing phase reactor and the biogas residue is recycled to the methanogenic phase reactor. The biogas slurry recycling ratio (defined as the volume of biogas slurry recycled to the acid-producing phase reactor / the daily feed volume of the acid-producing phase reactor) is 0.15-0.30 (more preferably 0.23-0.27) and the biogas residue recycling ratio (defined as the volume of biogas residue recycled to the methanogenic phase reactor / the daily feed volume of the methanogenic phase reactor) is 0.15-0.25 (more preferably 0.16-0.20).

[0027] In some embodiments, the mass ratio of VS (volatile solids) in the food waste and dewatered sludge is (3-5):1.

[0028] In some embodiments, the food waste is food waste that has undergone oil removal treatment, with a pH of 3-5, TS (total solids) of 8wt%-10wt%, and VS of 7wt%-9wt%. The dewatered sludge has a moisture content of 80wt%-90wt%, contains polyaluminum chloride (PAC) at a mass percentage of 0.5wt%-5wt%, has a total saturation (TS) of 12wt%-16wt%, and a total saturation (VS) of 7wt%-9wt%. The pH of the mixed substrate of the kitchen waste and dewatered sludge is 4-5.

[0029] The method for enhanced high organic loading two-phase anaerobic digestion using coupled recirculation of biogas residue and biogas slurry of this invention is applicable to a wide range of organic loading, suitable not only for low organic loading two-phase anaerobic digestion processes but also for high organic loading processes. In some embodiments, the organic loading (actual organic loading after recirculation of biogas slurry) of the acidogenic phase reactor is 10-20 g VS / (L·d); the organic loading (actual organic loading after recirculation of biogas residue) of the methanogenic phase reactor is 3-10 g VS / (L·d), preferably 8-10 g VS / (L·d), and more preferably 8-9 g VS / (L·d).

[0030] In some embodiments of the present invention, the system reaches stability specifically when the daily VFA (volatile fatty acid) content produced by the system differs by less than 5%, and the daily biogas production differs by less than 5%.

[0031] In some embodiments, the temperature in the acid-producing phase reactor and the methanogenic phase reactor is 37±1°C.

[0032] In some embodiments, the solid-liquid separation in step (2) is centrifugal separation, natural sedimentation, or plate and frame filtration. Preferably, the solid-liquid separation is centrifugal separation, with a centrifugation speed of 800-1000 rpm and a centrifugation time of 5-10 min. Under these conditions, dense inert solid particles and bioflocs enriched with methanogens can settle rapidly, ensuring separation efficiency while ensuring that the cytoplasm of microorganisms is not damaged.

[0033] In some embodiments, the system is carried out in a semi-continuous flow two-phase anaerobic digester, with semi-continuous treatment performed once a day at set times, while the biogas slurry and biogas residue are coupled and refluxed.

[0034] In a preferred embodiment, the water content of the biogas residue is 90-95 wt%; and the biogas residue contains functional microbial communities related to organic matter hydrolysis, fermentation, intermediate metabolite conversion, and methanogenesis processes; at the phylum level, the biogas residue contains Firmicutes Relative abundance not less than 45%, Bacteroidota The relative abundance is not less than 20%; at the genus level, the biogas residue contains Fastidiosipila The relative abundance is not less than 10%. W5053 The relative abundance is not less than 8%. Christensenellaceae_R-7_groupThe relative abundance is not less than 10%. Lactobacillus The relative abundance of methanogenic archaea does not exceed 5%, including Methanosarcina , Methanosaeta and Methanobrevibacter The total relative abundance of methanogenic archaea (including methanogenic archaea) is not less than 20%, preferably not less than 30%, and more preferably not less than 40%.

[0035] This invention discloses a high-organic-load two-phase anaerobic co-digestion method based on coupled reflux of biogas residue and biogas slurry. Through steps of mixing, anaerobic digestion, gas collection, analytical sampling, solid-liquid separation, and phase-separated reflux, the biogas residue and biogas slurry after solid-liquid separation are directionally refluxed to the acidogenic and methanogenic phases of the two-phase anaerobic digestion system, respectively. Using multi-source organic waste as raw materials, this invention investigates the effects of coupled reflux on methane yield, liquid-solid phase fermentation characteristics, and system stability during long-term digestion in a semi-continuous two-phase anaerobic reactor with different organic loads. The coupled reflux of biogas residue and biogas slurry significantly improves the stability and acidification capacity of the acidogenic phase, while internal reflux of biogas residue enhances the stability and gas production performance of the methanogenic system. Coupled reflux of biogas slurry and biogas residue effectively reduces propionic acid accumulation in the methanogenic phase, effectively alleviating acidification inhibition caused by propionic acid accumulation and improving system stability and high-load operation capacity. This is an important approach for achieving carbon reduction and resource recovery from rapidly growing urban organic solid waste and anaerobic sludge.

[0036] The embodiments of the present invention are implemented based on the technical solution of the present invention, and detailed implementation methods and processes are given. However, the scope of protection of the present invention is not limited to the following embodiments. Process parameters in the following embodiments that do not specify specific conditions are generally performed under conventional conditions. Percentages appearing in the following embodiments, unless otherwise specified, generally represent mass percentages.

[0037] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention. The terms "not less than" and "not higher than" in the specification mean "higher than or equal to," and "not exceeding" or "not higher than" means "lower than or equal to."

[0038] Table 1 shows the main physicochemical properties of the kitchen waste, dewatered sludge, and the mixed matrix formed by the two used in this invention.

[0039] Table 1. Physicochemical properties of food waste, dewatered sludge, and mixed substrate.

[0040] Unless otherwise specified, the raw materials or processing techniques used in the following embodiments and comparative examples are all conventional commercially available raw materials or conventional processing techniques in the art.

[0041] In the following examples and comparative examples, the kitchen waste used was kitchen waste after removing plastics, bones, and other materials, and then undergoing three-phase separation and oil removal treatment. The dewatered sludge was taken from the dewatering workshop of a sewage treatment plant in Wuhan City. The sludge was conditioned with polyaluminum chloride (PAC) during dewatering, and the amount of PAC added was 2 wt%. The pH of the kitchen waste used was 3.98, which is weakly acidic; the pH of the mixed matrix of kitchen waste and sludge rose to 4.46; the overall pH was in the weakly acidic to near-neutral range; the TS of dewatered sludge was 14.36 wt%, kitchen waste was 9.29 wt%, and mixed matrix was 9.27 wt%; the VS of kitchen waste was 7.84 wt%, dewatered sludge was 7.36 wt%, and mixed matrix was 7.48 wt%. The water content of the dewatered sludge was 85.64%, and the mixture maintained high organic matter activity.

[0042] This invention employs a semi-continuous flow two-phase anaerobic digestion reactor system, such as... Figure 1 As shown, the acidogenic fermentation process and the methanogenic process are completely isolated in physical space to achieve targeted enrichment of the microbial population and independent control of environmental parameters. In the following examples and comparative examples, the acidogenic phase reactor 1 is a closed reactor with an effective volume of 10 L. The methanogenic phase reactor 2 is a closed reactor with an effective volume of 20 L. The outlet 3 of the acidogenic phase reactor is connected to the inlet 5 of the methanogenic phase reactor via a peristaltic pump 4, ensuring that the hydrolyzed acidified liquid produced by the acidogenic phase serves as the sole carbon source matrix for the methanogenic phase. The entire system uses a water bath to maintain a constant temperature and a mesophilic anaerobic environment. The system is fed and discharged once a day at set times to maintain steady-state operation. The organic load of the acidogenic phase is maintained at 18.70 g VS / (L·d), and the organic load of the methanogenic phase is forcibly increased by continuously shortening the hydraulic residence time (HRT), i.e., reducing the residence time of the matrix in the methanogenic phase, thus forcing more organic matter to be processed per unit time. To ensure the validity of experimental data, data acquisition and switching must only be carried out after the system reaches steady state; and the system running time must be no less than twice the HRT (hydraulic retention time) of the methanogenic phase to ensure that the microbial community adapts to the new load environment. In this embodiment of the invention, after the system reaches steady state, the digestion residue outlet 6 of the methanogenic phase reactor is discharged to the digestion residue solid-liquid separation device 7. After solid-liquid separation, biogas slurry and biogas residue are obtained. The biogas slurry is returned to the acidogenic phase reactor 1, and the biogas residue is recycled back to the methanogenic phase reactor 2. The system also includes an online monitoring system 8 for monitoring the temperature, pH, and gas production in the acidogenic and methanogenic phase reactors. This invention investigates the system phenomena under high load and the system improvement after reflux countermeasures under three different process conditions.

[0043] (1) Under low load conditions, the organic load of the acid-producing phase is maintained at 18.70 g VS / (L·d), the hydraulic retention time of the methanogenic phase is 12.5d, the organic load is 5.98 g VS / (L·d), the system does not reflux, the acid-producing phase is mainly acetic acid, the pH fluctuates in the range of 4-4.5, and the anaerobic digestion methanogenic reaction is running normally.

[0044] (2) Further shorten the hydraulic residence time of the methanogenic phase to 7.5 days, so that the theoretical organic loading of the methanogenic phase is increased to 9.97 g VS / (L·d) (the organic loading before reflux, also referred to as the theoretical organic loading in this invention), and then perform distilled water reflux: 1 L of distilled water is refluxed to the acidogenic phase, and the actual organic loading of the acidogenic phase is reduced to 14.96 g VS / (L·d). The product is mainly acetic acid, accounting for about 60%-70%. 0.6 L of distilled water is refluxed to the methanogenic phase, and the actual organic loading of the methanogenic phase is 8.47 g. With a VS / (L·d) ratio, the pH of the methanogenic phase rapidly decreases to between 4.5 and 5. The suitable pH range for methanogenesis is 6.5-8.2. When the pH is below 6.5, hydrolysis and acidification reactions dominate, methanogenic activity decreases, and the shortened HRT leads to the rapid loss of methanogenic bacteria with long generation cycles, resulting in the accumulation of VFAs, system acidification and collapse, reduced biogas production, and biogas composition dominated by carbon dioxide with an increased proportion of hydrogen.

[0045] (3) After the collapse, the system was re-acclimated under low-load conditions, i.e., the hydraulic retention time of the methanogenic phase was 12.5 days and the organic load was 5.98 g VS / (L·d), without recirculation. After re-inoculation, the system stabilized and all indicators returned to normal. The hydraulic retention time of the methanogenic phase was then shortened to 7.5 days, which increased the theoretical organic load of the methanogenic phase to 9.97 g VS / (L·d). At the same time, 1 L of biogas slurry was recirculated to the acidogenic phase. After the biogas slurry was recirculated, the actual organic load of the acidogenic phase decreased to 14.96 g VS / (L·d). The recirculation of biogas slurry increased the ammonia nitrogen concentration. Ammonia nitrogen can act as a buffer substance for the acidogenic phase and increase the pH of the acidogenic phase. 0.6 L of biogas residue was recirculated to the methanogenic phase. After the biogas residue was recirculated, the actual organic load of the methanogenic phase was 8.47 g VS / (L·d). The recirculation of biogas residue can retain more anaerobic microorganisms in the methanogenic phase, especially methanogenic bacteria with long generation cycles. By coupling and refluxing biogas slurry and biogas residue, the system's utilization of propionic acid is enhanced, effectively mitigating the impact of propionic acid accumulation on the system. At the same time, the utilization efficiency of organic matter is improved, enabling the two-phase anaerobic digestion system to operate efficiently and stably under high organic load conditions.

[0046] Biogas production was measured online using the water displacement method and recorded within the system. Methane, hydrogen, and carbon dioxide content were analyzed by gas chromatography. Daily, partial samples of matrix liquid from the acid-producing and methanogenic phases were analyzed. pH was measured directly using a Leici pH meter. After sampling, samples were centrifuged at 10,000 rpm for 20 minutes, and the supernatant was filtered through a 0.45 μm filter. Ammonia nitrogen concentration was determined spectrophotometrically using Nessler's reagent; SCOD concentration was determined using Lianhua Technology COD reagent; the concentration and composition of fatty acids (VFAs) were determined using a gas chromatograph equipped with a Wonda Cap Wax column (30m*0.25mm*0.25μm) and an FID detector. The parameters were set as follows: injection port temperature and flame ionization detector temperature were 200℃ and 230℃, respectively. The column temperature was first increased to 60℃, then increased to 85℃ at a rate of 5℃ / min, and finally held at 170℃ for 8.0 minutes. The concentration of VFAs was determined by injecting 1.0 μL of the supernatant filtered through a 0.45 μm filter and acidified with 2.5% (volume fraction) phosphoric acid, based on a calibration curve of a standard acid sample of known concentration.

[0047] The digestion residue produced daily from the methanogenic phase is centrifuged; the supernatant is biogas slurry, and the bottom sediment is biogas residue. Excessive rotation speed or excessive centrifugation time can cause cells to rupture due to shear force or drastic changes in osmotic pressure, releasing intracellular organic matter, which increases the system burden and induces inhibition; while excessively low rotation speed cannot effectively retain tiny methanogenic flocs.

[0048] The digestion residues in the methanogenic phase can be separated into slurry and residue by centrifugation. The slurry, the liquid portion with a high water content, is rich in nutrients, promoting microbial growth and metabolism. Simultaneously, the buffering substances in the slurry help regulate the acid-base balance of the fermentation system, preventing acid shock to the methanogenic system caused by excessively low pH. Compared to the slurry, analysis revealed that the residue contains more microorganisms; its reflux can increase the microbial biomass in the system. The slurry and residue are refluxed to the acidogenic and methanogenic phases of the two-phase anaerobic system, respectively, thus forming a coupled reflux of the digestion residues' slurry and residue.

[0049] Comparative Example 1 (Low Load Conditions) (1) Mixing: Mix kitchen waste and dewatered sludge at a VS mass ratio of 4:1. After mixing, pour the mixture into the acid-producing reactor and aerate with nitrogen for 30 minutes to ensure an anaerobic environment. (2) Anaerobic digestion: The experimental apparatus used is as follows Figure 1The semi-continuous two-phase anaerobic digester shown has one feed and one discharge per day. The hydraulic retention time (HRT) of the acid-producing phase is 4 days, and the organic loading is 18.70 g VS / (L·d). The outflowing hydrolyzed acidified liquid serves as the matrix for the methanogenic phase. The hydraulic retention time of the methanogenic phase is 12.5 days, and the organic loading concentration is 5.98 g VS / (L·d). The system is considered stable when the daily VFA content and the daily biogas production differ by less than 5%.

[0050] (3) Gas collection: Biogas production is measured online using the drainage method and recorded in the system; (4) Sample retention for analysis: Take a portion of the matrix liquid from the acid-producing phase and the methanogenic phase respectively, centrifuge and filter, and retain the supernatant for analysis; (5) Phase separation reflux: reflux has not yet started at this stage.

[0051] The reactor temperature was maintained at 37±1℃. The above fermentation method produces an acidic phase with a low pH, and ethanol and acetic acid accumulate significantly.

[0052] Comparative Example 2 (High Load Conditions) (1) Mixing: Mix kitchen waste and dewatered sludge at a VS mass ratio of 4:1. After mixing, pour the mixture into the acid-producing phase of the reactor and aerate with nitrogen for 30 minutes to ensure an anaerobic environment. (2) Anaerobic digestion: The experimental setup used a semi-continuous two-phase anaerobic digestion reactor, with feeding and discharging once a day; the hydraulic retention time of the acid-producing phase was HRT=4 days, and the theoretical organic load was 18.70 g VS / (L·d), with the outflowing hydrolyzed acidified liquid serving as the matrix for the methanogenic phase; by shortening the hydraulic retention time of the methanogenic phase to 7.5 days, the theoretical organic load of the methanogenic phase was increased to 9.97 g VS / (L·d).

[0053] (3) Gas collection: Biogas production is measured online using the drainage method and recorded in the system; (4) Sample retention for analysis: Take a portion of the matrix liquid from the acid-producing phase and the methanogenic phase respectively, centrifuge and filter, and retain the supernatant for analysis; (5) Phase separation reflux: Distilled water reflux is performed. 1L of distilled water is refluxed to the acid-producing phase, and the actual organic load of the acid-producing phase is reduced to 14.96 g VS / (L·d). The main product is acetic acid, which accounts for about 60%-70%. 0.6L of distilled water is refluxed to the methanogenic phase, and the actual organic load of the methanogenic phase is 8.47 g VS / (L·d).

[0054] The reactor temperature was maintained at 37±1℃. In the above fermentation method, the pH of the methanogenic phase rapidly decreased to between 4.5 and 5, while the biogas production of the system decreased, the biogas composition became mainly carbon dioxide with an increased proportion of hydrogen, and the methanogenic system gradually collapsed, with the methane yield per unit VS falling below 5 mL CH4 / (g·VS) / d.

[0055] Example 1 (High-load Coupled Return) (1) Mixing: Mix kitchen waste and dewatered sludge at a VS mass ratio of 4:1. After mixing, pour the mixture into the acid-producing phase of the reactor and aerate with nitrogen for 30 minutes to ensure an anaerobic environment. (2) Anaerobic digestion: The experimental setup used a semi-continuous two-phase anaerobic digester, with feeding and discharging once a day. After the collapse, the system was re-acclimated under low-load conditions, i.e., the hydraulic retention time of the methanogenic phase was 12.5 days, the theoretical organic load was 18.70 gVS / (L·d), and there was no reflux. After re-inoculation, the system stabilized and all indicators returned to normal, and the hydraulic retention time of the methanogenic phase was shortened to 7.5 days, increasing the theoretical organic load of the methanogenic phase to 9.97 gVS / (L·d). The hydrolyzed acidified liquid was used as the matrix for the methanogenic phase.

[0056] (3) Gas collection: Biogas production is measured online using the drainage method and recorded in the system; (4) Sample retention for analysis: Take a portion of the matrix liquid from the acid-producing phase and the methanogenic phase respectively, centrifuge and filter, and retain the supernatant for analysis; (5) Phase separation and reflux: After the system reaches steady state, the digestion residue in the methanogenic phase reactor is centrifuged at 1000 rpm for 5 min to obtain biogas slurry and biogas residue. 1 L of biogas slurry is refluxed to the acidogenic phase (biogas slurry reflux ratio is 0.25), and the actual organic load of the acidogenic phase is reduced to 14.96 g VS / (L·d). The biogas slurry reflux increases the ammonia nitrogen concentration, which can act as a buffer in the acidogenic phase and increase the pH of the acidogenic phase. 0.6 L of biogas residue is refluxed to the methanogenic phase (biogas residue reflux ratio is 0.18), and the actual organic load of the methanogenic phase is 8.47 g VS / (L·d).

[0057] In this embodiment, the fermentation method utilizes the buffer material introduced by the reflux of biogas slurry, which optimizes the acid-producing environment (pH increases from 4.5-5 to 5.0-5.5), transforming the fermentation from acetic acid fermentation to mixed acid fermentation, thereby increasing the concentration of VFAs. The reflux of biogas residue also increases the concentration of methanogenic bacteria, resulting in a methane yield per unit VS of 366.30 mL CH4 / (g·VS) / d.

[0058] Comparative Example 1, Comparative Example 2, and Example 1 correspond to stages one, two, and three in Table 1, respectively. The specific parameters and conditions for the acid-producing phase and methanogenic phase in different stages are shown in Table 2.

[0059] Table 2 Specific parameters and conditions for acid-producing and methanogenic phases at different stages

[0060] from Figure 2 and Figure 3 It can be seen that the two-phase anaerobic digestion system operates relatively stably under low organic load conditions (Comparative Example 1). The pH of the acid-producing phase is maintained between 4.0 and 4.3, which is consistent with the acidic reaction environment characteristics of the acid-producing stage; the pH of the methanogenic phase is stable between 7.6 and 7.9, which is within the suitable range for the growth and metabolism of methanogens. Simultaneously, the ammonia nitrogen concentration in the acid-producing phase is maintained at approximately 600–750 mg / L, and the ammonia nitrogen concentration in the methanogenic phase is maintained at approximately 2300–2500 mg / L, indicating that the system has a certain buffering capacity and can maintain the stable operation of the two-phase reactor. As the organic load gradually increases to the high organic load process of Comparative Example 2, it can be seen that the pH of the methanogenic phase decreases rapidly, and the ammonia nitrogen concentration in the acid-producing phase decreases, leading to system acidification and collapse. However, when the coupled reflux process of Example 1 is adopted, it is evident that the reflux of the digestate increases the ammonia nitrogen concentration in the acid-producing phase. Ammonia nitrogen can act as a buffer in the acid-producing phase, thereby increasing the pH of the acid-producing phase. The system became more stable; after the biogas residue was recycled, the pH of the methanogenic phase remained stable at around 7.7–8.0; its ammonia nitrogen concentration also increased from approximately 2300–2500 mg / L during the restart phase to approximately 2700–3000 mg / L, indicating that the methanogenic phase has stronger buffering capacity and shock resistance. This demonstrates that biogas residue recycling helps maintain a high biomass and a favorable reaction environment within the methanogenic phase, thereby improving the system's operational stability under high organic load conditions.

[0061] from Figure 4 It can be seen that under low organic load (Comparative Example 1), the daily biogas production of the system is relatively stable, basically maintained between 90 and 95 L, with small fluctuations. This indicates that the two-phase anaerobic digestion system is operating smoothly and the methanogenesis process is proceeding normally. Combined with... Figure 5 Further analysis revealed that methane comprised approximately 77% of the biogas composition at this stage, carbon dioxide approximately 23%, and hydrogen was almost undetectable. This indicates a good connection between acid and hydrogen production processes and methanogenesis, with intermediate metabolites being readily converted and utilized. The system exhibits good methanogenesis capacity and operational stability. Under high organic loading (Comparative Example 2), the increased organic matter in the system necessitates greater microbial degradation, leading to increased pressure for gas production and severe inhibition of methanogenesis. Furthermore, high organic loading reduces residence time, causing the rapid loss of certain long-generation-cycle methanogenic bacteria, further diminishing the system's methanogenesis capacity. Figure 5 As can be seen, in Comparative Example 2, the methane content was only 11.1%, and the hydrogen content was 7.1%. Due to acidification inhibition of the methanogenic phase, the amount of methane produced was small. In Example 1, which employed a coupled reflux process, the coupled reflux of the biogas slurry and biogas residue improved the stability of the methanogenic phase under high organic load conditions. Through the coupled reflux of biogas slurry and biogas residue, the biogas production did not decrease sharply under high load conditions, the methane content remained within the normal range, and hydrogen was fully utilized, indicating that the coupled reflux of biogas slurry and biogas residue effectively improved the operational stability of the two-phase anaerobic digestion system.

[0062] from Figure 6 and Figure 7 It can be seen that under low organic loading conditions (Comparative Example 1), the total TVFAs in the acid-producing phase remained at a relatively high and stable level, approximately 12,000–15,000 mg / L. Acetic acid was the dominant component, accounting for about 70%–80%, accompanied by small amounts of ethanol, propionic acid, butyric acid, and trace amounts of valeric acid and hexanoic acid. This indicates that the hydrolysis and acidification process at this stage operated smoothly and could continuously provide usable intermediate metabolites for the subsequent methanogenesis stage. Meanwhile, the concentration of TVFAs in the methanogenesis phase remained consistently low, only a few hundred mg / L, and no significant accumulation was observed, indicating that the VFAs entering the methanogenesis phase could be consumed promptly and further converted into methane, and the system operated stably overall. For Comparative Example 2 under high organic loading, the pH of the acid-producing phase still fluctuated within the range of 4.0–4.5, and the VFA composition in the acid-producing phase was still dominated by acetic acid, indicating that a large amount of acidic intermediates continued to be produced in the acid-producing phase under high loading conditions. After these intermediates entered the methanogenesis phase in large quantities, the methanogenesis phase's ability to consume VFAs significantly decreased. Figure 7 ( Figures 7 to 11 As can be seen from Example 1 (hereinafter referred to as "Example"), the concentration of TVFAs (Total Volatile Fatty Acids) in the methanogenic phase increased rapidly, from several hundred mg / L in the low-load stage to approximately 10,000–12,000 mg / L, indicating a significant accumulation of VFAs. Simultaneously, the proportion of propionic acid increased significantly, indicating a typical propionic acid accumulation inhibition in the system. The accumulation of propionic acid stressed the microorganisms, leading to the ineffective utilization of acetic acid and ultimately system collapse. In Example 1, which employed coupled reflux, the reflux of the biogas slurry provided a buffer, optimizing the acidogenic phase environment, reducing the proportion of acetic acid, and increasing the pH of the hydrolyzed acidified liquid, thus reducing the impact on the methanogenic phase. From a concentration perspective, the reflux of the biogas slurry promoted the growth of VFAs in the acidogenic phase. The increased concentration of VFAs indicates that methanogens can utilize more substrate, which is beneficial for the methanogenic reaction. The recirculation of biogas residue improves the utilization of methanogenic phases by VFAs, especially propionic acid, alleviates the stress caused by propionic acid accumulation, and enables the methanogenic phase to operate normally.

[0063] Figure 8 This document describes the changes in the acid-producing microbial community at the phylum level (content (a)) and genus level (content (b)) in the three processes of Comparative Example 2 and Example 1 of this application; as can be seen from content (a), the microbial community structure of Comparative Example 2 and Example 1 is dominated by Firmicutes (… Firmicutes Firmicutes were the dominant phylum. In Example 1, the relative abundance of Firmicutes increased from 61% to 75% compared to Comparative Example 2, indicating that coupled reflux had a certain enrichment effect on this type of microbial community. Firmicutes contain a large number of functional bacteria related to the hydrolysis, fermentation, and volatile fatty acid production of complex organic matter. The increased abundance of Firmicutes indicates that Example 1 has a stronger microbial basis in substrate decomposition and front-end hydrolysis and acidification.

[0064] from Figure 8 Content (b) shows that, in Comparative Example 2: Lactobacillus spp. ( Lactobacillus Lactobacillus was the dominant genus, with the highest relative abundance, accounting for 59%. Excessive enrichment of Lactobacillus usually indicates a system more prone to rapid acid production, easily leading to the accumulation of acidic metabolites and a decrease in pH. Coupled reflux reduced the abundance of this genus, alleviating system acidification. Example 1: Lactobacillus The relative abundance of [something] decreased significantly to 33%, while [something] also [something] Eubacterium_nodatum_group , Lachnospiraceae_NK3A20_group , Erysipelotrichaceae_UCG-007 The relative abundance of certain taxa has increased. Eubacterium_nodatum_group The metabolites are acetic acid and butyric acid. The increase in their relative abundance is beneficial to the production of butyric acid, and therefore their increase is beneficial to improving the generation and transformation of intermediate metabolites in the system. Erysipelotrichaceae_UCG-007 The increase in the abundance of certain taxa indicates that coupled reflux helps to enrich more functional microbial communities involved in organic matter transformation, thereby enhancing the system's substrate utilization capacity and operational stability under high load conditions.

[0065] Figure 9 Phylum level comparison: It shows that the bacterial community is dominated by Firmicutes under all three operating conditions ( Firmicutes () is the dominant phylum, but its relative abundance varies significantly.

[0066] Comparative Example 1: Firmicutes They hold an absolute advantage, accounting for approximately 80%, while Bacteroidetes ( Bacteroidota )and Cloacimonadota The proportions were relatively low, approximately 10% and 5% respectively, indicating that under low organic load conditions, the system's microbial community structure was relatively concentrated but generally stable. This suggests that the reactor was dominated by fermentation, hydrolysis, and acid-producing bacteria, and the system maintained normal anaerobic digestion.

[0067] Comparative Example 2: Firmicutes The relative abundance of [the substance] decreased significantly, dropping to approximately 60%. Actinobacteriota(Actinomycetes) increased to approximately 20%-21%, Proteobacteria (Proteobacteria) increased to approximately 12%-13%, Bacteroidota (Bacteroidetes) decreased to approximately 3%-5%, Cloacimonadota The decrease in candidate phyla to approximately 2%-3% indicates a significant restructuring of the system's microbial community under high organic load. The previously stable dominant microbial community was suppressed, and the community structure shifted towards a stress state, reflecting a decline in system stability under high load conditions.

[0068] Example 1: After reflux, Firmicutes The relative abundance is approximately 50%; Bacteroidetes ( Bacteroidota The abundance rose to 29%. Cloacimonadota The relative abundance rose to 14%. Cloacimonadota Rising to approximately 12%-14%, Actinobacteriota Down to 3%, Proteobacteria The percentage decreased to approximately 1%, indicating that under coupled reflux conditions, the system developed a more diverse and balanced microbial community structure. This type of microbial community change signifies a more coordinated process in the hydrolysis, fermentation, and transformation of intermediate metabolites within the system, which is beneficial for maintaining stable operation under high load conditions.

[0069] Figure 10 The comparison at the genus level showed that the differences in dominant bacterial genera were more obvious under different working conditions.

[0070] Comparative Example 1: Fastidiosipila The relative abundance of [a specific compound] was 15%, and the relative abundance of W5053 was 7%. Christensenellaceae_R-7_group The relative abundance was 7%. Candidatus_Cloacimonas The relative abundance was 13%. Rikenellaceae_RC9_gut_group The relative abundance was 17%. Lactobacillus The relative abundance of this species is only 1%, and its horizontal distribution is relatively dispersed. No single genus exhibits absolute dominance, and various genera coexist. This indicates that under low-load conditions, the system's microbial community composition is relatively balanced, with a reasonable division of functions, enabling them to work together to complete the degradation of organic matter and the transformation of intermediate products. Therefore, the system operates relatively stably. Comparative Example 2: Lactobacillus Its relative abundance increased significantly to 47%, becoming the dominant genus, accounting for about half. Fastidiosipila Reduced to 1%, W5053 Reduced to 2%, Christensenellaceae_R-7_ group The concentration dropped to 1%. Lactobacillus is typically a fast-fermenting microorganism; its over-enrichment often indicates a system biased towards rapid acid production, easily leading to the accumulation of acidic metabolites, a decrease in pH, and system acidification. Example: Lactobacillus Its relative abundance decreased from 47% to 2%, and it no longer holds a dominant position; Fastidiosipila Rising to 27%, W5053Rising to 18%, Christensenellaceae_R-7_group The percentage increased to approximately 15%, and the microbial community structure regained its diversified characteristics. Coupled reflux inhibited the over-accumulation of single rapid acid-producing bacteria and promoted the synergistic growth of multiple functional bacteria, thereby contributing to the graded degradation of organic matter and the further transformation of intermediate metabolites, reducing the risk of system acidification, and improving stability under high-load operating conditions.

[0071] Figure 11 The comparison at the Archaea level showed that the dominant archaea genera differed more significantly under different working conditions.

[0072] Comparative Example 1: During the low-load stable operation phase, the archaeal community in the methanogenic phase was dominated by unclassified archaea, with a relative abundance of approximately 85%. Methanosarcina The relative abundance is approximately 10%. Methanosaeta The result, approximately 1%, indicates that while the system can stably produce methanogens under low-load conditions, the enrichment of typical dominant methanogenic archaea is limited, and the methanogenic process mainly relies on the existing stable bacterial community for maintenance.

[0073] Comparative Example 2: After a high organic load shock, the relative abundance of unclassified archaea increased to approximately 92%-94%. Methanosarcina (Methanocytosporum) decreased to approximately 1%, Methanosaeta (Methanifers) also remained at extremely low levels. This change indicates that high-load acidification and VFA accumulation significantly inhibited the formation of typical methanogenic archaea, particularly weakening their formation. Methanosarcina The ability to utilize acetic acid and partially methylated substrates prevents the methanogenic phase from promptly consuming intermediate products such as acetic acid, hydrogen, and carbon dioxide input from the front end.

[0074] Example 1: After using a coupled reflux of biogas slurry and biogas residue, the relative abundance of unclassified archaea decreased to 50%. Methanosarcina It then increases significantly to about 45%. Methanosaeta Approximately 2%, Methanobrevibacter (Bacillus methanogenus) was approximately 2%. This result indicates that biogas residue recirculation can significantly enrich [the biogas slurry] with [the following slurry]: Methanosarcina Methanogenic archaea, represented by [specific species], enhance the conversion of methanogens to acetic acid, hydrogen, and partially methylated substrates, thereby reducing the accumulation of propionic acid and VFAs and improving methane generation capacity and system stability under high organic load conditions.

[0075] Table 3 shows the test results of various indicators of the biogas residue, biogas slurry, and digestion residue of the methanogenic phase reactor in Example 1.

[0076] Table 3. Test results of various indicators of biogas residue, biogas slurry, and digestion residues in the methanogenic phase reflux.

[0077] As shown in Table 3, biogas slurry contains abundant nutrients, which can promote the growth and metabolism of microorganisms. Simultaneously, the high ammonia nitrogen content in biogas slurry can act as an acid buffer, preventing acid shock to the methanogenic system caused by excessively low pH values. Biogas residue contains even more microorganisms, with a high content of methanogens; its recirculation can increase the microbial population in the system.

[0078] Comparative Example 3 The rest is the same as in Example 1, except that the sludge recirculation ratio is 0.08. The lower sludge recirculation rate results in insufficient retention of anaerobic microorganisms in the methanogenic phase, especially methanogens with longer generation cycles. This weakens the system's ability to utilize volatile fatty acids, particularly propionic acid, leading to propionic acid accumulation and inhibiting the methanogenesis process. Compared to Example 1, the system stability decreases under these conditions, and both organic matter removal efficiency and methanogenesis performance are reduced, indicating that an excessively low sludge recirculation ratio is detrimental to the stable operation of the two-phase anaerobic digestion system under high organic load conditions.

[0079] In the above fermentation method, the pH value of the methanogenic phase rapidly decreases to between 4.5 and 5. At the same time, the biogas production of the system decreases, the biogas composition is mainly carbon dioxide, the proportion of hydrogen increases, the methanogenic system gradually collapses, and the methane production per unit VS is less than 5 mL CH4 / (g·VS) / d.

[0080] Comparative Example 4 The process is the same as in Example 1, except that the volume of biogas slurry returned to the acid-producing phase remains 1 L, while the volume of biogas residue returned to the methanogenic phase is increased to 1.4 L, with a biogas residue return ratio of 0.525. The results show that the total saturation (TS) in the methanogenic phase increases to 32-40 g / L, while the volume of volatile solids (VSS) / TS decreases to 45%-52%. Accumulation of inert solids and humic substances worsens mass transfer. The pH of the methanogenic phase is 6.5-6.8, total volatile fatty acids (VFAs) is 2500-4200 mg / L, propionic acid is 1200-2000 mg / L, methane volume fraction is 35%-45%, and methane yield per unit of VS is 120-180 mL CH4 / (g·VS) / d. In terms of microorganisms, the total relative abundance of methanogenic archaea was only 20%-30%, and the ETS activity was 0.15-0.22 mg INT / (g VSS·h), indicating that excessive recirculation of biogas residue could not further enhance methanogenesis, but instead weakened the system due to solid accumulation.

[0081] Comparative Example 5 The process was the same as in Example 1, except that only 1 L of biogas slurry was recycled to the acidogenic phase, and the biogas residue was not recycled to the methanogenic phase. The results showed that the pH of the acidogenic phase could be maintained at 5.0-5.4, and the ammonia nitrogen was 420-650 mg / L, indicating that the biogas slurry had a buffering effect. However, due to the shortened HRT, biomass was washed out in the methanogenic phase, with VSS decreasing to 12-18 g / L, the pH of the methanogenic phase decreasing to 5.8-6.3, total VFAs at 3500-6000 mg / L, propionic acid at 1600-2600 mg / L, methane volume fraction at 20%-32%, and methane yield per unit VS at 60-120 mL CH4 / (g·VS) / d. Methanosaeta and Methanosarcina The total relative abundance was 12%-22%, and the ETS activity was 0.10-0.18 mg INT / (g VSS·h), indicating that biogas slurry recirculation alone cannot solve the problem of methanogenic bacteria loss under high load and short HRT.

[0082] Comparative Example 6 The process was the same as in Example 1, except that only 0.6 L of biogas residue was recycled to the methanogenic phase, not to the acidogenic phase, and 1 L of distilled water was used instead of the recycled biogas slurry. The results showed that the acidogenic phase lacked alkalinity and ammonia nitrogen buffering in the biogas slurry, maintaining a pH of 4.3-4.8. The total VFAs in the acidogenic phase were 9000-12000 mg / L, and the acetic acid content was too high. Although some bacteria were added to the methanogenic phase, the acid shock from the influent was not mitigated, resulting in a pH of 5.6-6.2, total VFAs of 4200-6800 mg / L, propionic acid of 1800-3000 mg / L, methane volume fraction of 18%-30%, and methane yield per unit of VS of 50-110 mL CH4 / (g·VS) / d. The total relative abundance of methanogenic archaea was 15%-25%, and the ETS activity was 0.10-0.16 mg INT / (g VSS·h). The results showed that the effect of simply refluxing the biogas residue in two-phase anaerobic digestion was not good, because it could only partially replenish the microbial cells and could not provide the necessary buffer for the acid-producing phase or reduce the acid load entering the methanogenic phase.

[0083] Comparative Example 7 The rest is the same as in Example 1, except that the slurry recirculation ratio is 0.10 and the sludge recirculation ratio is 0.525, that is, only 0.25 L of slurry is recirculated to the acidogenic phase and 1.40 L of sludge is recirculated to the methanogenic phase. In this comparative example, both slurry and sludge participate in the recirculation, but the coupled recirculation ratios are also mismatched, resulting in insufficient buffering in the acidogenic phase and excessive solid load in the methanogenic phase. The results showed that the pH of the acid-producing phase was 4.3-4.8, ammonia nitrogen was only 250-380 mg / L, and total VFAs rose to 9500-12500 mg / L, indicating a high acid load entering the methanogenic phase. The total saturation (TS) of the methanogenic phase increased to 32-42 g / L, and the VSS / TS ratio decreased to 45%-52%. Accumulation of inert solids and humic substances led to poor mass transfer. The pH was 5.8-6.4, propionic acid was 1800-3000 mg / L, methane volume fraction was 20%-35%, and methane yield per unit VS was 70-130 mL CH4 / (g·VS) / d. Microbiologically, although the recycled biogas residue introduced some methanogenic bacteria, the combined effects of acid shock and solid accumulation... Methanosaeta and Methanosarcina The total relative abundance was only 18%-30%, and the ETS activity was 0.12-0.19 mg INT / (g VSS·h). These results indicate that excessively high biogas residue recirculation flow cannot compensate for the acidification impact of insufficient biogas slurry recirculation, and excessive biogas residue will reduce effective mass transfer. Therefore, coupled recirculation must be controlled within an appropriate matching range.

[0084] Comparative Example 8 The process is the same as in Example 1, except that after the methanogenic phase is discharged, solid-liquid separation is not performed. Instead, the digestion residue is thoroughly mixed and refluxed to the acidogenic and methanogenic phases at 1 L and 0.6 L respectively. The results show that the acidogenic phase simultaneously receives ammonia nitrogen, salts, and fine inert solids, with a pH fluctuation range of 4.8-6.2, ammonia nitrogen of 650-950 mg / L, and unstable acidification. In the methanogenic phase, the total TS (total volatile organic compounds) increases to 28-38 g / L, total VFAs are 2800-4800 mg / L, propionic acid is 1200-2100 mg / L, methane volume fraction is 32%-45%, and methane yield per unit VS is 130-200 mL CH4 / (g·VS) / d. Microbiologically, the total relative abundance of methanogenic archaea is approximately 22%-35%, and the ETS activity is 0.16-0.24 mg INT / (g VSS·h). This comparative example illustrates that the reflux of ordinary fully digested residues will simultaneously introduce liquid buffer substances and solid inert particles into both phases, significantly reducing the improvement effect on both phases.

[0085] Comparative Example 9 The process is the same as in Example 1, except that the centrifugation speed was increased from 1000 rpm to 1400 rpm and the centrifugation time was extended to 20 min during solid-liquid separation, reducing the moisture content of the refluxed biogas residue to 80%. Then, the same biogas slurry reflux ratio of 0.25 and biogas residue reflux ratio of 0.18 were used for coupled reflux. The results showed that excessive centrifugation compacted the biogas residue, damaging the extracellular polymers and microbial floc structure. After reflux, the dispersibility of the biogas residue in the methanogenic phase decreased. The pH of the acidogenic phase remained at 5.0-5.4, with ammonia nitrogen at 420-650 mg / L. However, the pH of the methanogenic phase was 6.2-6.7, with total VFAs at 2200-3800 mg / L, propionic acid at 1000-1800 mg / L, and methane volume fraction at 35%-48%. The methane yield per unit VS was 140-220 mL CH4 / (g·VS) / d. In terms of microorganisms, the copy number of the 16S rRNA gene of methanogenic archaea in the recycled biogas residue decreased to approximately 1.0 × 10^6 - 5.0 × 10^6 copies / g wet substrate. Methanosaeta and Methanosarcina The total relative abundance was 18%-32%, and the ETS activity was 0.14-0.23 mg INT / (g VSS·h), significantly lower than in Example 1. This result indicates that solid-liquid separation conditions not only affect the moisture content of the biogas residue but also the preservation of active methanogenic bacteria and bioflocs within it. When the centrifugation speed of the biogas residue was reduced from excessively high to around 80%, the returned biogas residue no longer achieved the desired effect of replenishing active bacteria and maintaining biomass as required in Example 1. Comparative Example 10 Other aspects are the same as in Example 1, except that the biogas residue refluxed to the methanogenic phase is taken from the digestion residue solid phase after long-term low-temperature storage or short-term air exposure. Its moisture content, total sulfide (TS), and reflux volume are basically the same as in Example 1. At the gate level, Firmicutes It is 25%. Bacteroidota It is 5%; at the genus level, Fastidiosipila 2%, W5053 Approximately 1%, Christensenellaceae_R-7_group It is 1%. Meanwhile, Methanosarcina , Methanosaeta , Methanobacterium and Methanolinea The abundance of key methanogenic archaea was significantly reduced, among which Methanosarcina Approximately 1%, Methanosaeta Approximately 0.5%, Methanobacterium Approximately 0.2%, [[ID=!15]]MethanolineaThe result was approximately 0.2%. Operational results showed that the acid-producing phase maintained a pH of 5.0-5.5 and ammonia nitrogen of 400-650 mg / L due to the recirculation of biogas slurry. However, the methanogenic phase could not obtain effective microbial replenishment through biogas residue recirculation, and the pH gradually decreased to 5.8-6.4. The total VFAs were 3000-5200 mg / L, propionic acid was 1400-2400 mg / L, methane volume fraction was 18%-32%, methane yield per unit VS was 50-120 mL CH4 / (g·VS) / d, and ETS activity was only 0.08-0.16 mg INT / (g VSS·h). The results indicate that the role of refluxed biogas residue in this invention is not simply to reflux solids or supplement inert carriers, but to retain sufficient methanogenic archaea and symbiotic fatty acid degrading bacteria in the refluxed biogas residue. When the above-mentioned key bacterial groups are missing or have very low abundance, even if the biogas residue reflux ratio is within the range of Example 1, it is difficult to restore the stability of the high-load methanogenic phase.

[0086] Comparative Example 11 The process is the same as in Example 1, except that the solid-liquid separation step does not use centrifugation. Instead, the methanogenic digestion residue is allowed to settle naturally for 12 hours. After the supernatant is discharged, the bottom sediment layer is taken as the reflux sludge, and coupled reflux is performed using the same sludge reflux ratio of 0.25 and sludge reflux ratio of 0.18 as in Example 1. Since natural settling mainly relies on gravity, its ability to remove fine flocs, colloidal particles, and extracellular polymeric water bound in the kitchen waste-sludge digestion residue is insufficient. The resulting reflux sludge has a water content of 96%-98%, a total saturation (TS) of only 2%-4%, and a volumetric sludge concentration (VSS) of 8-15 g / L, which is significantly lower than the effective solid phase concentration of the sludge obtained by centrifugation in Example 1. The results showed that the pH of the acidogenic phase could still be maintained at 5.0-5.5 through biogas slurry recirculation, with ammonia nitrogen at 400-650 mg / L. However, in the methanogenic phase, due to insufficient actual recirculated solid biomass, the VSS was only 16-23 g / L, the pH was 6.0-6.6, the total VFAs were 2400-4200 mg / L, propionic acid was 1100-1900 mg / L, the methane volume fraction was 30%-42%, and the methane yield per unit VS was 100-170 mL CH4 / (g·VS) / d. Regarding microorganisms, in the methanogenic phase... Methanosaeta and Methanosarcina The total relative abundance was 18%-30%, the copy number of the methanogenic archaea 16S rRNA gene was 1.0×10^6-8.0×10^6 copies / g wet substrate, and the ETS activity was 0.13-0.22 mg INT / (g VSS·h), all lower than in Example 1. These results indicate that if natural sedimentation is used without centrifugation, the returned biogas residue has a high moisture content and insufficient effective solid-phase bacteria, failing to fully utilize its role as a carrier and biomass supplement for methanogenic bacteria.

[0087] Figure 12 This diagram illustrates the process flow and material balance of two-phase anaerobic digestion in Example 1. It shows the process flow and material flow of the co-anaerobic digestion of kitchen waste and dewatered sludge in this example. The mixed feed first enters the acidogenic phase reactor for hydrolysis and acidification. The resulting hydrolyzed acidified liquid then enters the methanogenic phase reactor for further anaerobic methanogenesis, where biogas is produced. The residue generated during the reaction is separated into solid and liquid components; a portion is discharged, while the remaining biogas slurry is recycled back to the reaction system. This invention, through two-phase staged reaction and biogas slurry recycling, can improve the organic matter conversion efficiency, promote biogas production, and achieve the stabilization treatment of residues.

[0088] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for enhanced two-phase anaerobic digestion with high organic loading through coupled reflux of biogas residue and biogas slurry, characterized in that, Includes the following steps: (1) After mixing and stirring the kitchen waste and dewatered sludge, anaerobic fermentation is carried out in the acid-producing phase reactor to obtain hydrolyzed acidified liquid, which is used as the substrate of the methanogenic phase reactor. In the methanogenic phase reactor, under the action of methanogenic bacteria, the hydrolyzed acidified liquid is converted into gaseous products containing methane and carbon dioxide. (2) After the system reaches steady state, the digestion residue in the methanogenic reactor is subjected to solid-liquid separation to obtain biogas slurry and biogas residue; (3) The biogas slurry is returned to the acid-producing phase reactor and the biogas residue is returned to the methanogenic phase reactor, wherein the biogas slurry return ratio is 0.15-0.30 and the biogas residue return ratio is 0.15-0.

25.

2. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The mass ratio of VS in the kitchen waste and dewatered sludge is (3-5):

1.

3. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The kitchen waste mentioned is kitchen waste that has undergone oil removal treatment, with a pH of 3-5, TS of 8wt%-10wt%, and VS of 7wt%-9wt%. The dewatered sludge has a moisture content of 80wt%-90wt%, contains polyaluminum chloride, and has a total saturation (TS) of 12wt%-16% and a total saturation (VS) of 7wt%-9wt%. The pH of the mixed substrate of food waste and dewatered sludge is 4-5.

4. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The organic loading of the acid-producing phase reactor is 10-20 g VS / (L·d); the organic loading of the methanogenic phase reactor is 3-10 g VS / (L·d).

5. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The system reaches stability specifically when the daily VFA content and daily biogas production differ by less than 5%.

6. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The temperature in the acid-producing phase reactor and the methanogenic phase reactor is 37±1℃.

7. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The solid-liquid separation in step (2) is centrifugal separation, natural sedimentation, or plate and frame filtration.

8. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The solid-liquid separation in step (2) is centrifugal separation, with a centrifugation speed of 800-1000 rpm and a centrifugation time of 5-10 min.

9. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The system is carried out in a semi-continuous flow two-phase anaerobic digester, with semi-continuous treatment performed once a day at set times, while the biogas slurry and biogas residue are coupled and refluxed.

10. The two-phase anaerobic digestion method as described in claim 1, characterized in that, The moisture content of the biogas residue is 90-95 wt%. The biogas residue contains functional microbial communities related to organic matter hydrolysis, fermentation, intermediate metabolite conversion, and methanogenesis; at the phylum level, the biogas residue contains... Firmicutes Relative abundance of 45% or higher Bacteroidota Relative abundance is 20% or higher; At the genus level, the biogas residue in Fastidiosipila Relative abundance of 10% or higher W5053 Relative abundance of 8% or higher Christensenellaceae_R-7_group Relative abundance of 10% or higher Lactobacillus The relative abundance of methanogenic archaea was less than or equal to 5%, while the total relative abundance of methanogenic archaea was greater than or equal to 20%.

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