Method for improving anaerobic digestion methane production efficiency of pig manure by adding ferric iron compound under different operation conditions

Abstract

The invention belongs to the technical field of anaerobic digestion of organic wastes, and particularly relates to a method for improving the methane production efficiency of anaerobic digestion of pig manure by adding a ferric iron compound under different operation conditions, which comprises the following steps: mixing pig manure and anaerobic inoculation sludge in proportion to prepare an initial digestion solution, and carrying out first-stage anaerobic digestion to obtain a second-stage anaerobic digestion solution; the anaerobic digestion conditions are as follows: TS is 10%, SRT is 15 days, and the temperature is 37 DEG C; and then sequentially carrying out second-stage anaerobic digestion, third-stage anaerobic digestion, fourth-stage anaerobic digestion, fifth-stage anaerobic digestion and sixth-stage anaerobic digestion. Pig manure and anaerobic inoculation sludge are mixed in proportion to form an initial digestion solution, anaerobic digestion is carried out in stages under different operation conditions, a ferric iron compound is added in the second stage, and the ferric iron compound can be used for improving the VFA metabolism through the mechanisms of regulating VFA metabolism, relieving accumulation of toxic substances, promoting synergistic metabolism of microorganisms and the like. The system stability and the methane conversion efficiency are effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of anaerobic digestion of organic waste, and particularly relates to a method for improving the methane production efficiency of anaerobic digestion of pig manure by adding a trivalent iron compound under different operating conditions. Background Art

[0002] Pig manure, a typical high-concentration organic waste, is rich in complex organic matter. Improper treatment can cause serious ecological pollution. Anaerobic digestion technology is widely used in pig manure treatment because it can reduce and decontaminate organic waste and recover energy. Efficient methane production not only improves energy recovery efficiency but is also a key indicator for evaluating the operational stability of anaerobic systems.

[0003] However, in actual engineering applications, anaerobic digestion systems often experience problems such as VFA accumulation, pH decrease, and reduced methane yield due to factors such as organic load fluctuations, high TS (solid content) conditions, SRT (sludge retention time) changes, and increased temperature, which seriously restricts the efficient resource utilization of pig manure.

[0004] Previous studies have shown that ferric iron (Fe³⁺) has excellent electron acceptor properties, capable of buffering pH fluctuations, accelerating VFA metabolism, and promoting the growth of methanogenic microorganisms, thereby improving anaerobic digestion efficiency to a certain extent. However, there is currently a lack of systematic research on its comprehensive regulatory effects under different operating conditions, especially under semi-continuous high-load operation, where its mechanism and application model remain unclear.

[0005] The information disclosed in this background technology section is only intended to enhance understanding of the overall background of the invention and should not be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for improving the methane production efficiency of anaerobic digestion of pig manure by adding trivalent iron compounds under different operating conditions, so as to solve the technical problems of low methane yield and poor system stability in the traditional anaerobic treatment process of high-concentration organic matter.

[0007] To achieve the above object, the present invention provides the following technical solutions: A method for improving the methane production efficiency of anaerobic digestion of pig manure by adding a ferric iron compound under different operating conditions comprises the following steps: S1. Pig manure and anaerobic inoculum sludge were mixed in appropriate proportions to produce an initial digestate for the first stage of anaerobic digestion. Anaerobic digestion conditions were: TS 10%, SRT 15 days, and temperature 37°C. S2. maintaining the first stage anaerobic digestion conditions and adding ferric iron to the digestate to conduct the second stage anaerobic digestion; S3. Based on the second stage of anaerobic digestion, the SRT is extended to 24 days and the third stage of anaerobic digestion is carried out; S4. Based on the third stage of anaerobic digestion, the temperature is raised to 55°C and the fourth stage of anaerobic digestion is carried out; S5. Based on the fourth stage of anaerobic digestion, increase the TS to 15% and proceed to the fifth stage of anaerobic digestion; S6. Based on the fifth stage of anaerobic digestion, increase the TS to 20% and proceed to the sixth stage of anaerobic digestion.

[0008] Furthermore, in step S2, the ferric iron compound is added at a concentration of 5.99 g / L and 0.81 g / L.

[0009] Furthermore, in step S2, the trivalent iron compound is FeCl3 or Fe2O3.

[0010] Furthermore, in steps S1-S6, methane production and pH value indicators are monitored daily.

[0011] The present invention also provides an anaerobic digestion system for implementing the method, comprising: a constant temperature reactor for controlling the temperature of the digestate, a stirring system for maintaining uniform mixing of materials in the constant temperature reactor, a biogas collection and metering system, and a data acquisition system for pH value, temperature and methane production.

[0012] Furthermore, the effective volume of the constant temperature reactor is 8 L, supporting semi-continuous feeding and discharging.

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention mixes pig manure and anaerobic inoculum sludge in proportion to form an initial digestate, conducts anaerobic digestion in stages under different operating conditions, and adds trivalent iron compounds from the second stage onwards. Simultaneously, the sludge retention time is extended, the reaction temperature is increased, and the total solids content of the raw materials is increased, thereby gradually increasing the system load and methane production efficiency. The trivalent iron compounds effectively improve system stability and methane conversion efficiency by regulating VFA metabolism, alleviating the accumulation of toxic substances, and promoting microbial co-metabolism.

[0014] (2) The method of the present invention can significantly increase methane production and enhance the energy recovery efficiency of the system.

[0015] (3) The method of the present invention improves the system's tolerance to high TS and high-temperature anaerobic digestion.

[0016] (4) The method of the present invention has a simple dosing method, strong process adaptability, does not rely on high-cost auxiliary matrix or exogenous microorganisms, and has controllable costs, which is conducive to the promotion of rural distributed or small and medium-sized biogas projects.

[0017] (5) The present invention is equipped with a dedicated anaerobic reaction system, which can realize intelligent control of complex anaerobic processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the process flow of the present invention at different operation stages; Figure 2 Schematic diagram of the structure of the semi-continuous anaerobic digestion system of the present invention; Figure 3 The graph shows the effect of ferric iron (Fe2O3 and FeCl3) on daily cumulative methane production under different conditions; Figure 4 The dynamic change curve of system pH during anaerobic digestion in each treatment group. DETAILED DESCRIPTION

[0019] The following is a clear and complete description of the technical solution of the present invention. Obviously, the embodiments described are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts are within the scope of protection of the present invention.

[0020] 1. Materials and Methods 1.1 Experimental materials and device construction For this study, pig manure and anaerobic inoculum sludge were collected from a pig farm in Yongning District, Nanning City, and used directly after screening to remove bulky impurities. Two representative ferric iron compounds, FeCl₃ and Fe₂O₃, were used as experimental groups A and B, respectively. A blank control group (CK) was prepared without iron supplementation.

[0021] Reaction device such as Figure 2 As shown, it consists of a main reaction cylinder (effective volume 8L), a constant temperature water bath, a constant speed stirring device, a biogas collection and metering system, a temperature and pH monitoring probe, a CO2 absorption bottle and a data acquisition terminal. It has the functions of temperature control, dosing control, stirring control and automatic gas metering.

[0022] Three identical CSTR anaerobic reactors with an 8-L working volume were selected at room temperature. A blank control (CK) and two experimental groups (R1 supplemented with Fe₂O₃ and R2 supplemented with FeCl₃) were set up. Pig manure was mixed with inoculum sludge at a TS ratio of 3:1. After mixing, the final TS was adjusted to 10%. After addition of feed, the reactors were sealed, the system temperature was maintained at 37°C, and the agitation system was activated (80 rpm, intermittent stirring). The initial sludge retention time (SRT) was 15 days, with 1.6 L of feed and discharge every three days.

[0023] 1.2 Operation strategy and phase division a. Phase 1 (Start-up Period) The reactor was started and operated for 48 days at a TS of 10%, a SRT of 15 days, and a temperature of 37°C to establish a stable system foundation. Daily methane production, pH, temperature, and other indicators were monitored to ensure stable system operation. This phase served as the baseline data establishment period.

[0024] b. Phase II Maintaining the conditions of the first stage, 5.99 g / L Fe₂O₃ and 0.81 g / L FeCl₃ were added to R1 and R2, respectively. Operation continued for 45 days, with the addition of ferric iron compound simultaneously with each feed. The results showed a significant increase in daily methane production in both groups, reduced pH fluctuations, and enhanced the system's resistance to acidification. Total methane production in R2 increased by 8.4%.

[0025] c. Phase 3 The SRT was extended from 15 days to 24 days, and the reactors operated continuously for 78 days. Under this high residence time, both R1 and R2 maintained high gas production performance, with no decrease in daily methane production, indicating that the ferric iron helped extend the system load tolerance brought on by the SRT. The R1 reactor gradually resumed enhanced methane production, with total methane production increasing by 5.8%, while R2 production increased by 13.1%.

[0026] d. Stage 4 The system temperature was increased from 37°C to 55°C, while other parameters remained unchanged. The reactor operated for 81 days. Results showed that methane production in the R1 and R2 groups was higher than that in the CK group by 32.87% and 34.31%, respectively, indicating that ferric iron can effectively buffer high-temperature inhibition and enhance microbial activity at high temperatures.

[0027] e. Stage 5 The system maintained a high temperature and increased the feed TS to 15% for 96 days. Under high TS conditions, the blank group experienced a significant decrease in methane production, while the R1 and R2 groups maintained good gas production, demonstrating that ferric iron can mitigate organic loading shock. Total methane production in R1 and R2 increased by 193% and 152%, respectively, compared to the CK group.

[0028] f. Stage 6 The TS was further increased to 20% for 99 days. This phase further verified the adaptability of ferric iron under high-load conditions. Results showed that gas production in the R1 group continued to increase, while that in the R2 group decreased slightly but still outperformed the CK group. The system pH remained stable, and the reactor did not acidify. Total methane production in the R1 and R2 groups increased by 290% and 206%, respectively, compared to the CK group.

[0029] Methane production and pH indicators were collected daily during each stage.

[0030] The technical solution of the present invention is based on the concept of multi-stage control, combining the four dimensions of ferric iron addition, temperature control, TS control and SRT optimization to construct a six-stage operation system. Through this multi-stage coupled control strategy, ferric iron compounds not only promote the hydrogen-type methanogenesis process as an exogenous electron acceptor, but also enhance the degradation of organic matter and energy conversion efficiency by buffering pH, inhibiting acidification, and synergistically regulating the metabolic balance of acid-producing bacteria and methanogens. The supporting system includes an anaerobic reactor with temperature control, water bath heating, quantitative feeding and stirring devices, and is equipped with gas collection and metering devices to ensure process continuity, parameter stability and data traceability. After adopting this method, the daily methane production of the FeCl3 and Fe2O3 groups in each operation stage was significantly higher than that of the control group ( Figure 3 ), and the pH fluctuation is small and the system stability is enhanced ( Figure 4 ).

[0031] This method innovatively combines the addition of ferric iron with the coordinated regulation of multiple parameters, including SRT, temperature, and TS, providing a theoretical basis and a feasible path for efficient anaerobic digestion and scale-up of complex organic waste. This method is suitable for small and medium-sized anaerobic reactors using high-solids pig manure as feedstock. It offers advantages such as ease of operation, precise control, and high energy recovery efficiency, making it particularly well-suited for distributed manure treatment and resource utilization in rural areas.

[0032] The foregoing descriptions of specific exemplary embodiments of the present invention are for purposes of illustration and description. These descriptions are not intended to limit the invention to the precise forms disclosed, and it is apparent that many variations and modifications are possible in light of the foregoing teachings. The exemplary embodiments have been selected and described for the purpose of explaining the specific principles of the invention and their practical application, thereby enabling those skilled in the art to realize and utilize a variety of exemplary embodiments of the invention and various options and modifications. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for improving the methane production efficiency of anaerobic digestion of pig manure by adding a trivalent iron compound under different operating conditions, characterized in that: The following steps are involved: S1. Pig manure and anaerobic inoculum sludge were mixed at a solids-to-solids ratio of 3:1 to create a 10% TS initial digestate. Anaerobic digestion conditions were: 10% TS, 15-day SRT, and 37°C. S2. maintaining the first stage anaerobic digestion conditions and adding ferric iron to the digestate to conduct the second stage anaerobic digestion; S3. Based on the second stage of anaerobic digestion, the SRT is extended to 24 days and the third stage of anaerobic digestion is carried out; S4. Based on the third stage of anaerobic digestion, the temperature is raised to 55°C and the fourth stage of anaerobic digestion is carried out; S5. Based on the fourth stage of anaerobic digestion, increase the TS to 15% (the TS ratio of pig manure to inoculum sludge is 3:1) and proceed to the fifth stage of anaerobic digestion; S6. Based on the fifth stage of anaerobic digestion, increase the TS to 20% (the TS ratio of pig manure to inoculated sludge is 3:1) and proceed to the sixth stage of anaerobic digestion.

2. The method according to claim 1, characterized in that In step S2, the concentrations of the ferric iron compound added are 5.99 g / L and 0.81 g / L respectively.

3. The method according to claim 1, characterized in that In step S2, the trivalent iron compound is FeCl3 or Fe2O3.

4. The method according to claim 1, characterized in that In steps S1-S6, methane production and pH value indicators are monitored daily.

5. An anaerobic digestion system for implementing the method according to any one of claims 1 to 3, characterized in that: include: A constant temperature reactor for controlling the temperature of the digestate, a stirring system for maintaining uniform mixing of materials in the constant temperature reactor, a biogas collection and metering system, and a data acquisition system for pH value, temperature and methane production.

6. The anaerobic digestion system according to claim 5, characterized in that: The effective volume of the constant temperature reactor is 8 L, supporting semi-continuous feeding and discharging.

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