A method for improving the efficiency of methane production in anaerobic digestion of pig manure by adding a ferric compound

Abstract

The present application belongs to the technical field of organic waste anaerobic digestion, and particularly relates to a method for improving the methanogenic efficiency of pig manure anaerobic digestion by adding ferric compounds, comprising the following steps: mixing pig manure and anaerobic inoculated sludge in a proportion to prepare an initial digestion liquid, and performing first-stage anaerobic digestion, wherein the anaerobic digestion conditions are as follows: TS is 10%, SRT is 15 d, and the temperature is 37 DEG C; then sequentially performing second-stage anaerobic digestion, third-stage anaerobic digestion, fourth-stage anaerobic digestion, fifth-stage anaerobic digestion and sixth-stage anaerobic digestion. The pig manure and anaerobic inoculated sludge are mixed in a proportion to form an initial digestion liquid, and the anaerobic digestion is carried out in stages, and the ferric compounds are added from the second stage, so that the system stability and the methanogenic efficiency are effectively improved through the mechanisms of adjusting VFA metabolism, relieving toxic substance accumulation, promoting microbial synergistic metabolism and the like.

Description

Technical Field

[0001] This invention belongs to the field of anaerobic digestion technology of organic waste, specifically relating to a method for improving the efficiency of methanogenesis from anaerobic digestion of pig manure by adding ferric compounds. Background Technology

[0002] Pig manure, a typical high-concentration organic waste, is rich in complex organic matter, and its improper treatment can cause serious pollution to the ecological environment. Anaerobic digestion technology is widely used in pig manure treatment because it can reduce the volume of organic waste, render it harmless, and recover energy. In particular, the high efficiency of 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 practical engineering applications, anaerobic digestion systems often experience problems such as VFA accumulation, pH decrease, and reduced methane yield due to factors such as fluctuations in organic load, high TS (solids content) conditions, changes in SRT (sludge retention time), and rising temperature, which seriously restrict the efficient resource utilization of pig manure.

[0004] Previous studies have shown that ferric iron (Fe³⁺) has good electron acceptor properties, can buffer pH fluctuations, accelerate VFA metabolism, and promote the growth of methanogenic microorganisms, thereby improving anaerobic digestion efficiency to some extent. However, systematic research on its comprehensive regulatory effects is still lacking, especially in semi-continuous high-load operation modes, where its mechanisms and application modes remain unclear.

[0005] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to provide a method for improving the efficiency of methanogenesis in the anaerobic digestion of pig manure by adding ferric iron compounds, so as to solve the technical problems such as low methane yield and poor system stability in the traditional anaerobic treatment of high-concentration organic matter.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for improving the methanogenesis efficiency of anaerobic digestion of pig manure by adding a ferric compound includes the following steps:

[0009] S1. Pig manure and anaerobic inoculated sludge were mixed in a certain proportion to prepare an initial digestion solution for the first stage of anaerobic digestion. The anaerobic digestion conditions were: TS = 10%, SRT = 15 days, and temperature = 37℃.

[0010] S2. Maintain the anaerobic digestion conditions of the first stage, and add ferric iron compounds to the digestion solution to carry out the second stage of anaerobic digestion;

[0011] S3. Based on the second stage of anaerobic digestion, the SRT is extended to 24 days to carry out the third stage of anaerobic digestion;

[0012] S4. Based on the third stage of anaerobic digestion, the temperature is increased to 55℃ to carry out the fourth stage of anaerobic digestion;

[0013] S5. Based on the fourth stage of anaerobic digestion, increase TS to 15% and proceed to the fifth stage of anaerobic digestion;

[0014] S6. Based on the fifth stage of anaerobic digestion, increase TS to 20% and proceed to the sixth stage of anaerobic digestion.

[0015] Furthermore, in step S2, the concentration of the trivalent iron compound added is 5.99 g / L and 0.81 g / L.

[0016] Furthermore, in step S2, the trivalent iron compound is FeCl3 or Fe2O3.

[0017] Furthermore, in steps S1-S6, methane production and pH levels are monitored daily.

[0018] The present invention also provides an anaerobic digestion system for implementing the method, comprising: a thermostatic reactor for controlling the temperature of the digestate, a stirring system for maintaining uniform mixing of materials in the thermostatic reactor, a biogas collection and metering system, and a data acquisition system for pH value, temperature, and methanogen production.

[0019] Furthermore, the effective volume of the isothermal reactor is 8L, supporting semi-continuous feeding and discharging.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) In this invention, pig manure and anaerobic inoculated sludge are mixed in a certain proportion to form an initial digestate. Anaerobic digestion is carried out in stages, and ferric iron compounds are added from the second stage onwards. At the same time, the sludge retention time is extended, the reaction temperature is increased, and the total solids content of the raw materials is increased in sequence to gradually improve the system load and methane production efficiency. Among them, ferric iron compounds effectively improve the system stability and methane conversion efficiency through mechanisms such as regulating VFA metabolism, alleviating the accumulation of toxic substances, and promoting microbial synergistic metabolism.

[0022] (2) The method of the present invention can significantly increase methane production and enhance the system energy recovery efficiency.

[0023] (3) The method of the present invention improves the system’s tolerance to high TS and high temperature anaerobic digestion.

[0024] (4) The method of the present invention is simple to add, has strong process adaptability, does not rely on high-cost auxiliary substrates or exogenous microorganisms, and has controllable cost, which is conducive to the promotion of distributed or small and medium-sized biogas projects in rural areas.

[0025] (5) The present invention is equipped with a dedicated anaerobic reaction system, which can realize intelligent control of complex anaerobic processes. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the process flow under different operating stages of the present invention;

[0027] Figure 2 This is a schematic diagram of the semi-continuous anaerobic digestion system of the present invention;

[0028] Figure 3 The graph shows the effect of trivalent iron (Fe2O3 and FeCl3) on the daily cumulative methane production under different conditions.

[0029] Figure 4 The curves show the dynamic changes in system pH during anaerobic digestion in each treatment group. Detailed Implementation

[0030] The technical solution of this invention patent will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0031] 1. Materials and Methods

[0032] 1.1 Construction of Experimental Materials and Apparatus

[0033] In this study, pig manure and anaerobic inoculum were collected from a pig farm in Yongning District, Nanning City, and used directly after sieving to remove large impurities. Two typical ferric compounds, FeCl3 and Fe2O3, were selected and configured as experimental groups A and B, respectively. The unadded iron group served as the blank control group (CK).

[0034] Reaction apparatus 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, temperature and pH monitoring probes, a CO2 absorption bottle and a data acquisition terminal, and has the functions of temperature control, dosing control, stirring control and automatic gas metering.

[0035] Three identical CSTR anaerobic reactors with a working volume of 8L were selected at room temperature. A blank control group (CK) and two experimental groups (R1 with Fe2O3 added, R2 with FeCl3 added) were set up. Pig manure and inoculum sludge were mixed at a TS ratio of 3:1. After mixing, the final TS was adjusted to 10%. The reactor was sealed after feeding, and the system temperature was maintained at 37℃. The stirring system was started (80 rpm, intermittent stirring). The initial sludge retention time (SRT) was 15 days, and 1.6L of feed was discharged every 3 days.

[0036] 1.2 Operational Strategy and Phase Division

[0037] a. First Phase (Initiation Period)

[0038] With a TS of 10%, SRT of 15 days, and a temperature of 37°C, the reactor was started up and operated for 48 days 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.

[0039] b. Second Phase

[0040] 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. The operation continued for 45 days, with ferric compounds added simultaneously with each feed. Results showed that the daily methane production of both groups significantly increased, pH fluctuations decreased, and the system's resistance to acidification improved. The total methane production of R2 increased by 8.4%.

[0041] c. Third Stage

[0042] The residence time (SRT) was extended from 15 days to 24 days, and the reactors operated continuously for 78 days. Under this high residence time condition, both R1 and R2 groups maintained high gas production performance, and the daily methane production did not decrease, indicating that ferric iron helps to extend the system load tolerance brought about by the extended SRT. The R1 reactor gradually recovered to enhance methane production, and the total methane production increased by 5.8%; the methane production of R2 increased by 13.1%.

[0043] d. Fourth stage

[0044] The system temperature was increased from 37℃ to 55℃, while other parameters remained unchanged. The reactor operated for 81 days. The results showed that the methane production of groups R1 and R2 was 32.87% and 34.31% higher than that of group CK, respectively, indicating that ferric iron can effectively buffer high-temperature inhibition and improve microbial activity under high temperatures.

[0045] e. Fifth stage

[0046] The system was maintained at a high temperature, with the feed total sulfide (TS) increased to 15%, and the system operated for 96 days. Under high TS conditions, the methanogenesis rate of the control group decreased significantly, while groups R1 and R2 maintained good gas production performance, indicating that ferric iron can mitigate the impact of organic load. The total methanogenesis of R1 and R2 was increased by 193% and 152% respectively compared to the control group.

[0047] f. Sixth stage

[0048] The total sulfide (TS) was further increased to 20% and operated for 99 days. This stage further verified the adaptability of trivalent iron under high-load operating conditions. The results showed that the gas production of group R1 continued to increase, while that of group R2 decreased slightly but was still better than that of CK. The system pH remained stable, and the reactor did not acidify. The total methan production of R1 and R2 was 290% and 206% higher than that of group CK, respectively.

[0049] Daily methane production and pH levels were collected at each stage.

[0050] This invention's technical solution is based on a multi-stage regulation concept, combining four dimensions: ferric iron addition, temperature control, total sulfide (TS) control, and total sulfide (SRT) optimization, to construct a six-stage operating system. Through this multi-stage coupled regulation strategy, ferric compounds not only act as exogenous electron acceptors to promote hydrogen-form methanogenesis, but also enhance organic matter degradation and energy conversion efficiency by buffering pH, inhibiting acidification, and synergistically regulating the metabolic balance between acid-producing and methanogenic bacteria. The supporting system includes an anaerobic reactor equipped with temperature control, water bath heating, quantitative feeding, and stirring devices, and gas collection and metering devices to ensure continuous process, stable parameters, and data traceability. After adopting this method, the daily methane production of the FeCl3 and Fe2O3 groups was significantly higher than that of the control group at each operating stage. Figure 3 Furthermore, the pH fluctuation range is small and the system stability is enhanced. Figure 4 ).

[0051] This invention achieves an innovative combination of ferric iron addition and synergistic control of multiple parameters including SRT, temperature, and TS, providing a theoretical basis and feasible path for the efficient anaerobic digestion and engineering scale-up of complex organic waste. This method is applicable to small and medium-sized anaerobic reactors using high-solids-content pig manure as raw material, and has advantages such as simple operation, precise control, and high energy recovery efficiency. It is particularly suitable for distributed manure treatment and resource utilization scenarios in rural areas.

[0052] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for improving the efficiency of methanogenesis from anaerobic digestion of pig manure by adding a ferric compound, characterized in that, The CSTR anaerobic reactor is used, and the following steps are included: S1. Pig manure and anaerobic inoculated sludge were mixed at a solids content of TS 3:1 to prepare an initial digestion solution with a TS content of 10%. The first stage of anaerobic digestion was carried out under the following conditions: TS 10%, SRT 15d, and temperature 37℃. S2. Maintain the anaerobic digestion conditions of the first stage, and add ferric iron compounds to the digestion solution to carry out the second stage of anaerobic digestion; S3. Based on the second stage of anaerobic digestion, the SRT is extended to 24 days to carry out the third stage of anaerobic digestion; S4. Based on the third stage of anaerobic digestion, the temperature is increased to 55℃ to carry out the fourth stage of anaerobic digestion; S5. Based on the fourth stage of anaerobic digestion, increase TS to 15% and proceed to the fifth stage of anaerobic digestion; S6. Based on the fifth stage of anaerobic digestion, increase TS to 20% and proceed with the sixth stage of anaerobic digestion; In step S2, the trivalent iron compound is FeCl3 or Fe2O3.

2. The method according to claim 1, characterized in that, The amount of Fe2O3 added is 5.99 g / L, and the amount of FeCl3 added is 0.81 g / L.

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

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