Application of calcium peroxide pretreatment coupled with two-phase anaerobic digestion in promoting methanogenesis

Abstract

The present invention discloses the application of calcium peroxide pretreatment coupled with two-phase anaerobic digestion in promoting methane production. The invention utilizes calcium peroxide pretreatment technology to give play to the advantages of the separation of the acid-producing phase and the methane-producing phase in the two-phase anaerobic digestion process, effectively promotes the enrichment of methanogenic functional bacteria in sludge anaerobic digestion, greatly improves the methane production rate, shortens the methane production hysteresis period, and realizes efficient energy conversion of sludge.

Description

Technical Field

[0001] The present invention relates to the field of resource utilization of organic solid waste, and in particular to application of calcium peroxide pretreatment coupled with two-phase anaerobic digestion in promoting methane production. Background Art

[0002] As a core component of urban environmental infrastructure, sewage collection, treatment, and resource utilization facilities play a vital role in winning the battle against pollution and accelerating the development of an ecological civilization. 80,000 kilometers of new and renovated sewage collection pipelines will be added, 20 million cubic meters of sewage treatment capacity will be added daily, and at least 15 million cubic meters of recycled water production capacity will be upgraded, renovated, or expanded daily. Excess activated sludge from sewage treatment plants is an extremely complex, heterogeneous mixture composed of organic debris, bacterial cells, inorganic particles, and colloids. According to statistics, my country's sludge production exceeded 60 million tons (at an 80% moisture content) in 2019. With urbanization, this figure is expected to exceed 90 million tons by 2025. Over 80% of this sludge is not properly treated. If sludge production continues to rise without proper treatment and disposal, not only will resources such as organic carbon and chemical energy not be effectively utilized, but the resulting secondary pollution will also pose a threat to the ecological environment and human health.

[0003] Anaerobic digestion is an excellent sludge treatment method, utilizing anaerobic sludge bacteria to degrade organic matter while generating renewable energy biogas. This energy is recovered for use in heating and power generation, further stabilizing sludge and reducing its volume. However, the bioconversion rate of anaerobic digestion in my country is only 20-50%, far lower than that of developed countries in Europe and the United States (50-70%), hindering the promotion and application of this technology. Hydrolysis is the rate-limiting step in anaerobic digestion, and the use of pretreatment technology can overcome the barriers of the hydrolysis stage and improve the efficiency of anaerobic digestion to a certain extent.

[0004] CaO2 is a white or pale yellow inorganic peroxide containing high-energy peroxide covalent bonds. It has the advantages of thermal stability, environmental friendliness, and a wide range of applications. When it comes into contact with aqueous media, it generates H2O2 and Ca(OH)2. Existing research has shown that the oxidative power of H2O2 and its stable free radicals, combined with the alkaline environment provided by Ca(OH)2, can enhance the degradation of carbohydrates and proteins, as well as compounds that are difficult to degrade under standard anaerobic conditions, significantly improving sludge hydrolysis. However, this can inhibit the activity of methanogens and reduce methane production during anaerobic digestion. Previous studies have shown that calcium peroxide can improve the acid quality and acetic acid ratio of anaerobic fermentation of excess activated sludge (CN201210556900.5), and that a combined calcium peroxide-microwave method can enhance methane production in single-phase anaerobic digestion (CN201510712997.8). However, this combined technology significantly increases operating energy consumption and costs.

[0005] The two-phase anaerobic digestion process has independent and separate acid-producing phases and methane-producing phases, which can solve the problems of differences in physiology, nutritional requirements, growth dynamics and sensitivity to environmental conditions between acid-producing and methane-producing microorganisms in conventional applications. Improving methane production efficiency through process improvements will help popularize anaerobic digestion technology. Summary of the Invention

[0006] Purpose of the invention: The present invention aims to overcome the defects of the prior art and provide an application of calcium peroxide pretreatment coupled with two-phase anaerobic digestion in promoting methanogenesis, thereby promoting anaerobic fermentation of sludge to produce methane.

[0007] Technical solution: Application of calcium peroxide pretreatment coupled with two-phase anaerobic digestion in promoting methanogenesis.

[0008] Furthermore, the calcium peroxide pretreatment is to add CaO2 powder during the anaerobic acid production stage of the sludge to perform anaerobic digestion to produce acid.

[0009] Furthermore, the addition amount of the CaO2 powder is 0.05g / g VSS.

[0010] Furthermore, the coupled two-phase anaerobic digestion is to perform anaerobic digestion on the acidogenic phase mud-water mixture obtained after anaerobic digestion to produce acid in the sludge anaerobic methanogenesis stage to produce methane.

[0011] Furthermore, the mass ratio of the seed sludge in the anaerobic methanogenesis stage of the sludge to the acidogenic phase sludge-water mixture is 1.5:1 (calculated in terms of VSS).

[0012] Furthermore, the seed mud is taken from an anaerobic digestion tank of a sewage treatment plant. The present invention has no particular limitation on the seed mud, as long as the seed mud contains archaea, and is usually taken from an anaerobic digestion tank of a conventional sewage treatment plant.

[0013] Furthermore, the pH of the sludge anaerobic methanogenesis stage is controlled to be 7.0±0.1.

[0014] Furthermore, the pH is controlled with 1 M HCl and / or 1 M NaOH.

[0015] Furthermore, the temperature of the sludge anaerobic acid production stage is controlled at 35° C. and the time is 5 days.

[0016] Furthermore, the temperature of the sludge anaerobic methanogenesis stage is controlled at 35°C for 40 days.

[0017] Beneficial effects:

[0018] 1) The present invention greatly enriches the types of methanogens by coupling CaO2 pretreatment with a two-phase anaerobic digestion process, significantly improving the metabolic pathways of acetic acid methanogenesis, hydrogen methanogenesis, methyl methanogenesis, and methanol methanogenesis. Among them, acetic acid methanogenesis and hydrogen methanogenesis increased by 2.6 times and 4.2 times respectively compared with traditional anaerobic digestion processes;

[0019] 2) The present invention significantly improves the methane production rate and amount of sludge anaerobic digestion by coupling CaO2 pretreatment with a two-phase anaerobic digestion process, and shortens the hysteresis period of methane production. Compared with the traditional single-phase anaerobic digestion process, the hysteresis period disappears, and the maximum methane production rate and methane production potential are increased by 2.0 times and 2.1 times, respectively;

[0020] 3) The method of the present invention adds a small amount of CaO2, only 0.05g / gVSS, which has the advantages of good economic benefits and high energy efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a comparison chart of the cumulative methane production from anaerobic digestion of sludge in Example 1 and Comparative Examples 1-2;

[0022] Figure 2 This is a comparison chart of the abundance of Methanogens in anaerobic digestion of sludge in Example 1 and Comparative Examples 1-2;

[0023] Figure 3 This is a comparison chart of the main metabolic pathways of anaerobic digestion of sludge in Example 1 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0024] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] It should be noted here that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions of the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0026] In the following examples and comparative examples, unless otherwise specified, raw materials or processing techniques are conventional commercially available raw materials or conventional processing techniques in the art.

[0027] The sludge used in the following examples and comparative examples was residual sludge from the secondary sedimentation tank of a municipal sewage treatment plant. The sludge was filtered through a 1 mm x 1 mm sieve to remove larger particles and suspended solids. The sludge was then allowed to stand at 4°C for 24 hours to obtain concentrated sludge, which was then refrigerated for later use. Initial properties of the concentrated sludge were tested, revealing a total sedimentation stress (TSS) of 18.3 g / L and a volumetric sedimentation stress (VSS) of 12.8 g / L.

[0028] Example 1:

[0029] A reaction bottle with a volume of 250 mL was used for the sludge anaerobic acid production phase test. 200 mL of concentrated sludge was added to the reaction bottle. The air above the reaction bottle was removed by nitrogen blow-off, and the nitrogen blow-off time was 2 minutes. CaO2 pretreatment was then carried out, and 0.128 g of CaO2 powder (0.05 g / g VSS) was quickly added to the reaction bottle and the bottle mouth was immediately plugged with a rubber stopper. The reaction bottle was placed in a medium temperature (35°C) air bath constant temperature shaker with a shaker speed of 200 rpm. The acid production phase operation time was 5 days, and three parallel samples were set. After the acid production phase operation was completed, an acid production phase mud-water mixture was obtained.

[0030] The anaerobic methanogenesis phase of sludge was also tested using a 250 mL reaction bottle. 60 mL of seed sludge and 82 mL of the acidogenic phase sludge-water mixture were added to the methanogenic phase reaction bottle. The seed sludge was taken from the anaerobic digester of the sewage treatment plant. The VSS was 26.4 g / L, the proportion of archaea in the microbial community composition (the relative abundance of archaea in all microorganisms) was 3.6%, and the mass ratio of seed sludge to the acidogenic phase sludge-water mixture was 1.5:1 (in terms of VSS). 1 M HCl and 1 M NaOH were then used to control the initial pH to 7.0 ± 0.1. The reaction bottle was placed in a medium temperature (35°C) air bath constant temperature shaker with a shaker speed of 200 rpm. The methanogenic phase ran for 40 days, and three parallel samples were set.

[0031] Comparative Example 1

[0032] A CaO2 pretreatment coupled with single-phase anaerobic digestion test was conducted. 200 mL of concentrated sludge was added to a reaction bottle with a volume of 250 mL. The air above the reaction bottle was removed by nitrogen blow-off, and the nitrogen blow-off time was 2 minutes. CaO2 pretreatment was then carried out, and 0.128 g of CaO2 powder (0.05 g / gVSS) was quickly added to the reaction bottle and the bottle mouth was immediately plugged with a rubber stopper. The reaction bottle was placed in an air bath constant temperature shaker at medium temperature (35°C) with a shaker speed of 200 rpm. The single-phase anaerobic digestion operation time was 40 days, and three parallel samples were set.

[0033] Comparative Example 2

[0034] Conduct a single-phase anaerobic digestion test. Add 200 mL of concentrated sludge to a 250 mL reaction flask. Remove the air above the flask using nitrogen purge for 2 minutes. Immediately seal the flask with a rubber stopper. Place the flask in a medium-temperature (35°C) air bath shaker at 200 rpm. Run the single-phase anaerobic digestion for 40 days, with three replicates.

[0035] For each of the above examples and comparative examples, the gas production was measured using a polyvinyl chloride tube and a two-way valve connected to a glass syringe according to the set sampling nodes, and 5 to 10 mL of gas was collected for methane content determination. In order to ensure the anaerobic conditions of the system, a low-flow nitrogen gas purge was used after each sampling and the cap was then placed. The measurement results are shown in the figure. Figure 1 .

[0036] like Figure 1 As shown, Figure 1 The cumulative methane production from anaerobic digestion of sludge from Example 1 and Comparative Examples 1-2 is shown. It can be seen that the final cumulative methane production from CaO2 pretreatment coupled with two-phase anaerobic digestion was 84.31 mL / g VSS, a 2.1-fold increase compared to single-phase anaerobic digestion with CaO2 pretreatment (39.94 mL / g VSS). The cumulative methane production from the conventional anaerobic digestion process (single-phase anaerobic digestion without CaO2 pretreatment) was 38.86 mL / g VSS. This indicates that applying CaO2 pretreatment to single-phase anaerobic digestion does not effectively increase its methane production, while properly coupling CaO2 pretreatment with two-phase anaerobic digestion significantly increases methane production.

[0037] On the 22nd day, 5 mL of the sludge-water mixture was collected from the reaction glass bottles of each group of the above examples and comparative examples. After centrifugation using a refrigerated centrifuge, the sludge samples were subjected to metagenomic analysis to reveal the methanogenic functional bacteria and the methanogenic metabolic pathway. The results are as follows: Figure 2-3 .

[0038] The steps of metagenomic sequencing are: DNA extraction and library construction, data quality control and assembly, gene prediction, and functional annotation. Microbial and gene abundance analysis is performed based on the latest Non-Redundant Protein Sequence Database (NR) and Kyoto Encyclopedia of Genes and Genomes (KEGG).

[0039] like Figure 2 As shown, Figure 2The abundance of Methanogens in the anaerobic digestion of sludge from Example 1 and Comparative Examples 1-2 is shown. CaO2 pretreatment coupled with two-phase anaerobic digestion significantly altered the species and abundance of methanogens, effectively enriching the acetic acid methanogen Methanothrix, increasing its relative abundance from 38.1% to 82.8% compared to CaO2 pretreatment coupled with single-phase anaerobic digestion. In addition, Methanobacterium, unclassified_f__Methanobacteriaceae, and Methanobrevibacter were enriched, which are hydrogen-type methanogens; methyl-type methanogens such as Candidatus_Methanofastidiosum, unclassified_c__Candidatus_Methanofastidiosa, Methanomassiliicoccus, and unclassified_o__Methanomassiliicoccales were also further enriched, thus CaO2 pretreatment coupled with two-phase anaerobic digestion effectively broadened the various methane production pathways.

[0040] like Figure 3 As shown, Figure 3 The main metabolic pathways of anaerobic digestion of sludge in Example 1 and Comparative Examples 1-2 are shown. The methane metabolic pathway (ko00680) contains four methane production pathways, which are divided into four modules in the KEGG database, namely M00357 (acetic acid type methanogenesis), hydrogen type methanogenesis (M00567), methyl type methanogenesis (M00563) and methanol type methanogenesis (M00356). Figure 3 It can be seen that CaO2 pretreatment coupled with two-phase anaerobic digestion effectively increased the relative abundance of the four methanogenic modules, among which acetic acid methanogenesis and hydrogen methanogenesis increased by 2.6 times and 4.2 times respectively compared with the traditional anaerobic digestion process.

[0041] After the anaerobic digestion was completed, the improved Gompertz model was used to calculate the methane production rate and hysteresis period of the above embodiments and comparative examples. The results are shown in Table 1.

[0042] Table 1

[0043]

[0044] As can be seen from Table 1, in Comparative Example 1, CaO2 pretreatment is coupled with single-phase anaerobic digestion, and its hysteresis period is 3.08 days. When CaO2 pretreatment is coupled with two-phase anaerobic digestion, the hysteresis period disappears, methane can be produced more efficiently, the maximum methane production rate is increased by 77.5%, and the methane production potential is increased by more than 2 times. Compared with Comparative Example 2, the effect is more obvious. Compared with the traditional anaerobic digestion process, the maximum methane production rate and methane production potential of CaO2 pretreatment coupled with two-phase anaerobic digestion are increased by 2.0 times and 2.1 times, respectively.

[0045] The above embodiments of the present invention have the following beneficial effects compared with the prior art:

[0046] 1) The present invention greatly enriches the types of methanogens by coupling CaO2 pretreatment with a two-phase anaerobic digestion process, significantly improving the metabolic pathways of acetic acid methanogenesis, hydrogen methanogenesis, methyl methanogenesis, and methanol methanogenesis. Among them, acetic acid methanogenesis and hydrogen methanogenesis increased by 2.6 times and 4.2 times respectively compared with traditional anaerobic digestion processes;

[0047] 2) The present invention significantly improves the methane production rate and amount of sludge anaerobic digestion by coupling CaO2 pretreatment with a two-phase anaerobic digestion process, and shortens the hysteresis period of methane production. Compared with the traditional single-phase anaerobic digestion process, the hysteresis period disappears, and the maximum methane production rate and methane production potential are increased by 2.0 times and 2.1 times, respectively;

[0048] 3) The method of the present invention adds a small amount of CaO2, only 0.05g / gVSS, which has the advantages of good economic benefits and high energy efficiency.

[0049] Although the present invention has been illustrated and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made to the present invention without departing from the scope of the present invention as defined by the claims.

Claims

1. Application of calcium peroxide pretreatment coupled with two-phase anaerobic digestion in promoting methanogenesis, characterized in that: The calcium peroxide pretreatment is to add CaO2 powder to the sludge anaerobic acid production stage for anaerobic digestion and acid production, add concentrated sludge to the reaction bottle during the sludge anaerobic acid production stage, and remove the air above the reaction bottle by nitrogen stripping; The coupled two-phase anaerobic digestion is to perform anaerobic digestion of the acid-producing phase mud-water mixture obtained after anaerobic digestion to produce acid in the sludge anaerobic methanogenesis stage to produce methane; The mass ratio of the seed sludge in the anaerobic methanogenesis stage to the acidogenic phase sludge-water mixture is 1.5:1 in terms of VSS; The pH of the sludge anaerobic methanogenesis stage is controlled at 7.0±0.1; the temperature of the sludge anaerobic acidogenesis stage is controlled at 35°C for 5 days; and the temperature of the sludge anaerobic methanogenesis stage is controlled at 35°C for 40 days.

2. The use according to claim 1, characterized in that The added amount of the CaO2 powder is 0.05g / gVSS.

3. The use according to claim 1, characterized in that The seed mud is taken from the anaerobic digestion tank of a sewage treatment plant.

4. The use according to claim 1, characterized in that The pH was controlled with 1 M HCl and / or 1 M NaOH.

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