A method for improving the methane production of cadmium-containing sludge anaerobic digestion

Abstract

The application discloses a method for improving the methane production of cadmium-containing sludge anaerobic digestion, which comprises the following steps: adjusting the pH value of the cadmium-containing sludge to alkaline, and performing alkali pretreatment on the cadmium-containing sludge; and performing anaerobic digestion on the alkali pretreated cadmium-containing sludge under light conditions. The method for improving the methane production of cadmium-containing sludge anaerobic digestion can perform anaerobic digestion on the cadmium-containing sludge under the assistance of light, can construct a microorganism-semiconductor biological hybrid system, and can promote electron transfer, improve the methane generation efficiency, and improve the anaerobic digestion efficiency and the methane production through the formation and light activation of the microorganism-semiconductor biological hybrid system, so that the method is more green and environmentally friendly, can be widely used for treating the cadmium-containing sludge, is convenient for realizing the harmless treatment and resource utilization of the cadmium-containing sludge, has high use value, and has good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of sludge treatment and bioenergy technology, and relates to a method for increasing methane production from anaerobic digestion of cadmium-containing sludge. Background Technology

[0002] Anaerobic digestion (AD) is a technology that converts organic matter into biogas (mainly methane) through microbial action, and it is widely used in sludge treatment and resource recovery. However, traditional anaerobic digestion technology faces the problem of low efficiency when treating sludge containing heavy metals, especially when the sludge contains heavy metals such as cadmium (Cd), which significantly inhibits methane formation.

[0003] Currently, the common method for increasing methane production from anaerobic digestion of cadmium-containing sludge is to add chemical agents (such as biochar, nano-zero-valent iron, or polyacrylamide) to the anaerobic fermentation system. This reduces the content of harmful components or slows down their toxic effects on microorganisms, thereby increasing methane production from anaerobic digestion. However, the above methods still have the following drawbacks: (1) The chemical agents themselves are toxic to the microorganisms in the sludge, making it difficult to effectively improve the activity of the microorganisms, resulting in low anaerobic digestion efficiency and methane production; (2) The chemical agents used are prone to remain in the sludge, causing secondary pollution and making it difficult to achieve harmless treatment and resource utilization of the sludge. Therefore, obtaining an anaerobic digestion method for cadmium-containing sludge that is highly efficient, produces high levels of methane, and is environmentally friendly is of great significance for promoting the resource utilization of sludge. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for increasing methane production from anaerobic digestion of cadmium-containing sludge. This method can not only improve anaerobic digestion efficiency and methane production, but is also more environmentally friendly.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A method for increasing methane production from anaerobic digestion of cadmium-containing sludge includes the following steps:

[0007] S1. Adjust the pH value of the cadmium-containing sludge to alkaline and perform alkaline pretreatment on the cadmium-containing sludge.

[0008] S2. Anaerobic digestion of cadmium-containing sludge pretreated with alkali in step S1 under light conditions.

[0009] In a further improvement to the above method, in step S1, an alkaline agent is used to adjust the pH value of the cadmium-containing sludge; the alkaline agent is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide, and sodium bicarbonate.

[0010] In a further improvement to the above method, in step S1, the pH value of the cadmium-containing sludge is adjusted to 9-11.

[0011] In a further improvement to the above method, in step S1, the alkali pretreatment is carried out at a temperature ≤10℃; and the alkali pretreatment time is 2h.

[0012] In a further improvement to the above method, in step S1, the cadmium-containing sludge contains methanogenic bacteria and sulfate-reducing bacteria; the initial concentration of cadmium in the cadmium-containing sludge is 0.1±0.02 mg / L, and the initial concentration of sulfate is 150 mg / L~200 mg / L; the cadmium-containing sludge also includes the following treatment before use: filtering the cadmium-containing sludge with a sieve with a pore size of 0.45 mm to remove large particles, condensing it at 4℃ for 24 hours, removing the supernatant, and collecting the remaining sludge.

[0013] In a further improvement to the above method, in step S1, cadmium ions are added to the cadmium-containing sludge to make the concentration of cadmium ions in the cadmium-containing sludge 2.26 mg / g TSS to 19.72 mg / g TSS.

[0014] In a further improvement to the above method, in step S2, the light intensity used during the anaerobic digestion process is 150 W / m². 2 ~200W / m 2 .

[0015] In a further improvement to the above method, in step S2, the anaerobic digestion is carried out at a temperature of 35℃~40℃; and the anaerobic digestion time is 29 days.

[0016] In a further improvement to the above method, in step S2, the pH value of the cadmium-containing sludge is adjusted to neutral during the anaerobic digestion process.

[0017] In a further improvement to the above method, step S2 further includes adding cadmium sulfide to the cadmium-containing sludge during the anaerobic digestion process, so that the concentration of cadmium sulfide in the cadmium-containing sludge is 4 mg / g TSS to 16 mg / g TSS.

[0018] In this invention, the microbial-semiconductor biohybrid system is an emerging treatment technology that combines the characteristics of microorganisms and semiconductor materials. By anaerobic fermentation of sludge under light conditions, the treatment efficiency and effect of sludge can be significantly improved through the combined action of microorganisms and semiconductors.

[0019] Compared with the prior art, the advantages of the present invention are as follows:

[0020] To address the shortcomings of existing anaerobic digestion methods for cadmium-containing sludge, such as low anaerobic digestion efficiency, low methane yield, and the risk of secondary pollution, this invention creatively proposes a method to improve methane yield from the anaerobic digestion of cadmium-containing sludge. First, the pH of the cadmium-containing sludge is adjusted to alkaline for alkaline pretreatment, which disrupts the extracellular polymeric substances (EPS) of the sludge. This effectively eliminates the physical barrier caused by EPS (especially T-EPS), thereby promoting effective contact between microorganisms and electron acceptors, facilitating electron transfer. This forms the basis for constructing a microbial-semiconductor biohybrid system. Based on this, the pretreated cadmium-containing sludge is anaerobically digested under light conditions. During anaerobic digestion, sulfate-reducing bacteria reduce sulfate to sulfides, which then react with Cd. 2 The reaction generates CdS particles, which can then be used to form a microbial-semiconductor biohybrid system with methanogens. In this system, the CdS particles, acting as a semiconductor material, can generate a large number of photogenerated electrons under light conditions, promoting the rapid transfer of electrons from CdS to methanogens. On one hand, the rapid electron transfer within the microbial-semiconductor biohybrid system facilitates the rapid acquisition of electrons by methanogens and accelerates methane production, thus improving anaerobic digestion efficiency. On the other hand, the continuous generation and consumption of large amounts of electrons by methanogens, leading to biochemical methanogenesis, also results in a continuous accumulation of methane and a significant increase in yield. More importantly, the microbial-semiconductor biohybrid system constructed in this invention optimizes the microbial community structure, increasing the abundance of acid-producing and methanogens while reducing the abundance of reactive oxygen species (ROS)-producing microorganisms (such as Planctomycetota), thereby significantly mitigating Cd toxicity. Furthermore, anaerobic digestion under light conditions eliminates the need for additional chemical reagents, making it more environmentally friendly. This invention relates to a method for increasing methane production from the anaerobic digestion of cadmium-containing sludge. By performing anaerobic digestion of cadmium-containing sludge under light assistance, a microbial-semiconductor biohybrid system can be constructed. Through the formation of this biohybrid system and photoactivation, electron transfer is promoted, thereby increasing methane production efficiency. This method not only improves anaerobic digestion efficiency and methane production but is also more environmentally friendly. It can be widely used to treat cadmium-containing sludge, facilitating the harmless treatment and resource utilization of cadmium-containing sludge. It has high practical value and promising application prospects. Attached Figure Description

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0022] Figure 1 This is a comparison chart showing the anaerobic digestion effect of cadmium-containing sludge under different conditions in Examples 1 and 2 of the present invention.

[0023] Figure 2 This is a graph showing the changes in electron capacity and rate constant when treating cadmium-containing sludge under different anaerobic digestion conditions in Example 2 of the present invention.

[0024] Figure 3 This is a graph showing the changes in protein and polysaccharide content in sludge EPS before and after alkali pretreatment in Example 1 of the present invention.

[0025] Figure 4 The images show the XRD patterns of the sludge after anaerobic digestion in Examples 1 and 2 of this invention.

[0026] Figure 5 These are SEM images of the sludge after anaerobic digestion in Examples 1 and 2 of this invention.

[0027] Figure 6 This is a graph showing the changes in sulfate and sulfide concentrations in the sludge during the anaerobic digestion process in Example 2 of the present invention.

[0028] Figure 7 This is a graph showing the effect of different cadmium sulfide concentrations on methane yield from anaerobic digestion in Example 3 of the present invention.

[0029] Figure 8 This is a graph showing the effect of different treatment conditions on the yield of methane from anaerobic digestion in Example 4 of the present invention. Detailed Implementation

[0030] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1

[0032] A method for increasing methane production from anaerobic digestion of cadmium-containing sludge includes the following steps:

[0033] S1. Adjust the pH value of the cadmium-containing sludge to alkaline and perform alkaline pretreatment on the cadmium-containing sludge, specifically as follows:

[0034] S1-1. Take cadmium-containing sludge from the secondary sedimentation tank of a sewage treatment plant in Changsha City. The test results show that the cadmium-containing sludge contains methanogenic bacteria and sulfate-reducing bacteria. The initial concentration of cadmium is 0.1±0.02 mg / L and the initial concentration of sulfate is 168.6±12.4 mg / L. First, filter the cadmium-containing sludge through a stainless steel screen with a pore size of 0.45 mm to remove large particles. Then, condense it at 4℃ for 24 hours, remove the supernatant, and collect the remaining sludge at the bottom to obtain the original cadmium-containing sludge.

[0035] S1-2. Add sodium hydroxide to the original cadmium-containing sludge obtained in step S1-1 to adjust the pH value of the cadmium-containing sludge to 10 (alkaline). Perform alkaline pretreatment on the cadmium-containing sludge to be treated at a temperature of 4℃ for 2 hours to obtain alkaline pretreated cadmium-containing sludge.

[0036] S2. Anaerobic digestion of the cadmium-containing sludge pretreated with alkali in step S1 under light conditions, specifically:

[0037] S2-1. Add hydrochloric acid solution to the cadmium-containing sludge pretreated in step S1-2 and adjust the pH of the cadmium-containing sludge pretreated in step S1-2 to 7 (neutral) to ensure that methane can be produced through anaerobic digestion.

[0038] S2-2, under a light intensity of 200W / m 2 Anaerobic digestion of cadmium-containing sludge pretreated with alkali at pH 7 was carried out under light conditions. The anaerobic digestion was conducted in an anaerobic environment with the temperature controlled at 35℃ for 29 days, completing the anaerobic digestion treatment of the cadmium-containing sludge, which is denoted as PL-WAS.

[0039] Control group 1 (WAS): Anaerobic digestion of raw cadmium-containing sludge was carried out under dark conditions, with other conditions being the same.

[0040] Control group 2 (L-WAS): The original cadmium-containing sludge was anaerobically digested under light, with other conditions being the same.

[0041] Control group 3 (P-WAS): Anaerobic digestion of cadmium-containing sludge pretreated with alkali was carried out in the dark, with other conditions being the same.

[0042] Example 2

[0043] A method for increasing methane production from anaerobic digestion of cadmium-containing sludge includes the following steps:

[0044] S1. Adjust the pH value of the cadmium-containing sludge to alkaline and perform alkaline pretreatment on the cadmium-containing sludge, specifically as follows:

[0045] S1-1. Take cadmium-containing sludge from the secondary sedimentation tank of a sewage treatment plant in Changsha City. The test results show that the cadmium-containing sludge contains methanogenic bacteria and sulfate-reducing bacteria. The initial concentration of cadmium is 0.1±0.02 mg / L and the initial concentration of sulfate is 168.6±12.4 mg / L. First, filter the cadmium-containing sludge through a stainless steel screen with a pore size of 0.45 mm to remove large particles. Then, condense it at 4℃ for 24 hours, remove the supernatant, and collect the remaining sludge at the bottom to obtain the original cadmium-containing sludge.

[0046] S1-2. Add a cadmium ion solution with a concentration of 6g Cd / L to the original cadmium-containing sludge obtained in step S1-1 to adjust the concentration of cadmium ions in the cadmium-containing sludge to 5mg / g TSS, thereby obtaining modified cadmium-containing sludge (i.e., cadmium-containing sludge with added cadmium ions).

[0047] S1-3. Add sodium hydroxide to the modified cadmium-containing sludge obtained in step S1-2, adjust the pH value of the modified cadmium-containing sludge to 10 (alkaline), and perform alkaline pretreatment on the cadmium-containing sludge to be treated at a temperature of 4℃ for 2 hours to obtain alkaline pretreated modified cadmium-containing sludge.

[0048] S2. Anaerobic digestion of the cadmium-containing sludge pretreated with alkali in step S1 under light conditions, specifically:

[0049] S2-1. Add hydrochloric acid solution to the alkali pretreated modified cadmium-containing sludge obtained in step S1-3 to adjust the pH of the alkali pretreated modified cadmium-containing sludge to 7 (neutral) to ensure that methane can be produced through anaerobic digestion.

[0050] S2-2, under a light intensity of 200W / m 2 Anaerobic digestion of modified cadmium-containing sludge with alkaline pretreatment at pH 7 was carried out under light conditions. The anaerobic digestion was conducted in an anaerobic environment with the temperature controlled at 35℃ for 29 days, and the anaerobic digestion treatment of the cadmium-containing sludge was completed. This was denoted as PL-CdW.

[0051] Control group 1 (CdW): Modified cadmium-containing sludge was anaerobically digested under dark conditions, with other conditions being the same.

[0052] Control group 2 (L-CdW): Modified cadmium-containing sludge was anaerobic digested under light conditions, with other conditions being the same.

[0053] Control group 3 (P-CdW): Anaerobic digestion of alkali-pretreated modified cadmium-containing sludge was carried out under dark conditions, with other conditions being the same.

[0054] In this invention, TSS represents the total suspended solids content in sludge, and VSS represents the volatile suspended solids content in sludge.

[0055] In Examples 1 and 2, after anaerobic digestion, the methane production from anaerobic digestion of cadmium-containing sludge under different conditions was measured, and the results are as follows: Figure 1 As shown.

[0056] Figure 1 This is a comparison chart showing the anaerobic digestion effect of cadmium-containing sludge under different conditions in Examples 1 and 2 of the present invention. Figure 1 In the figure, a is a comparison chart of methane production under different anaerobic digestion conditions, and b is a comparison chart of the incremental methane production under different anaerobic digestion conditions.

[0057] Depend on Figure 1 It was found that Cd significantly inhibited anaerobic digestion (AD), causing the cumulative methane yield to decrease from 33.9 ± 2.2 (WAS) to 27.1 ± 1.6 (CdW) L CH4 / kg VSS. Furthermore, alkali pretreatment of sludge before anaerobic digestion was beneficial in increasing methane yield. Simultaneously, introducing light during anaerobic digestion also increased methane yield, from 27.1 (CdW) L CH4 / kg VSS to 45.0 (L-CdW) L CH4 / kg VSS. In particular, introducing light during the anaerobic digestion of alkali-pretreated sludge was even more beneficial in increasing methane yield, from 46.4 (P-CdW) L CH4 / kg VSS to 111 (PL-CdW) L CH4 / kg VSS. Under light conditions, the relative methane yield of PL-CdW compared to P-CdW increased by 139.3%. Figure 1 (b) This value far exceeds the methane enhancement (64.7%–66.0%) caused solely by light exposure, a phenomenon not observed in L-CdW (relative to CdW) without alkali pretreatment. These results suggest that, in addition to light directly promoting anaerobic digestion, other factors may also contribute to the increased methane production observed in PL-CdW. A plausible explanation is that electron transfer may occur between photoactivated cadmium sulfide particles and methanogens, and the disruption of extracellular polymers caused by alkali pretreatment may promote this electron transfer, further amplifying the methane generation process under light.

[0058] In Example 2, after anaerobic digestion, the electron transfer capacity (ETC), electron acceptor capacity (EAC), electron donor capacity (EDC), apparent electron transfer rate constant (kapp), and reactive oxygen species (ROS) of samples collected under different conditions (P-CdW, PL-CdW) were tested. The results are as follows: Figure 2 As shown.

[0059] Figure 2 This is a graph showing the changes in electron capacity and rate constant when treating cadmium-containing sludge under different anaerobic digestion conditions in Example 2 of the present invention. Figure 2 In the figure, a is a comparison chart of electron capacity under different anaerobic digestion conditions for treating cadmium-containing sludge, and b is a comparison chart of rate constants under different anaerobic digestion conditions for treating cadmium-containing sludge.

[0060] Depend on Figure 2 It can be seen that the ETC, EAC, and EDC of PL-CdW are all higher than those of P-CdW. For example, the ETC value of PL-CdW is 4.1 ± 0.9 μmol e - / g VS, higher than 2.6±0.8μmol e of P-CdW - / g VS indicates that electron transfer is more intense in this light-assisted AD system. The increased EDC and EAC values ​​also suggest that cadmium sulfide formed in PL-CdW generates a large number of electrons, and that the methanogens' electron-accepting ability is enhanced. The Kapp value is generally correlated with the interfacial electron transfer rate. The Kapp value for PL-CdW is 2.9s. -1 It is 2.6 times higher than P-CdW, which means that under light conditions, cadmium sulfide activates a faster rate of extracellular electron transfer.

[0061] In Example 1, the protein and polysaccharide contents of EPS in sludge before and after alkali pretreatment were detected, and the results are as follows: Figure 3 As shown.

[0062] Figure 3 This is a graph showing the changes in protein and polysaccharide content in sludge EPS before and after alkali pretreatment in Example 1 of the present invention. Figure 3 In the diagram, a represents the change in protein content, and b represents the change in phosphorus content of polysaccharides. Figure 3 In this study, S-EPS (super-explosive polysaccharide) is an extracellular polymeric substance (EPS) that exists outside the sludge flocs, unbound to cells in solution. It acts as a buffer and protectant in the sludge and can be utilized by microorganisms as a carbon and energy source. L-EPS (lean super-explosive polysaccharide) is an extracellular polymeric substance attached to the surface of the sludge flocs, loosely bound to cells. It significantly influences the flocculation, settling, and dewatering properties of sludge, exhibiting a loose structure and some fluidity. T-EPS (trans-explosive polysaccharide) is an extracellular polymeric substance tightly bound to the cell wall, typically located inside the sludge flocs. It plays a crucial role in the structural stability of the sludge flocs and significantly affects their settling and dewatering performance. These three types of EPS collectively constitute the external protective layer of the sludge flocs, influencing the physicochemical properties and treatment performance of the sludge.

[0063] Depend on Figure 3 It can be seen that after alkali pretreatment, the protein and polysaccharide contents in L-EPS and T-EPS decreased, while the contents in S-EPS increased. This indicates that alkali pretreatment effectively destroyed the EPS structure in the sludge, allowing proteins and polysaccharides to be released into the liquid phase, while also enhancing the direct interaction between methanogens and cadmium sulfide.

[0064] In Example 2, after anaerobic digestion, samples were extracted from the two reactors corresponding to P-CdW and PL-CdW for metatranscriptome analysis. The results are as follows:

[0065] The commodity coverage index in both reactors was close to 1, indicating that the sequencing depth was reasonable and included all species in the digested samples, thus ensuring the reliability of the results, as detailed below:

[0066] (1) Microbial community structure at the phylum level in the two reactors. In P-CdW, Actinobacteria, Bacteroidetes, Firmicutes, Archaea, and Chlorconoids were the main phyla, accounting for 96.4%; while in PL-CdW, these phyla accounted for 91.5%. Bacteroidetes, Firmicutes, and Chlorconoids mainly participate in the degradation of macromolecular organic matter, such as carbohydrates and proteins, and belong to the phylum Bacteria. Their total abundance in PL-CdW (43.3%) was higher than that in P-CdW (29.4%), indicating that the organic matter decomposition in PL-CdW was more complete, thus providing sufficient soluble substrates for methane production. This phenomenon is consistent with the photo-assisted methanogenesis effect of Cd-containing sludge.

[0067] (2) Significant differences in acetic acid and methanogenesis at the genus level. In P-CdW, the dominant genera were *Tesciella*, *Methanobacterium*, and *Methanococcus*; while in PL-CdW, the dominant genera were *Methanobacterium*, *Clostridium*, and *Methanococcus*. These genera were identified as key microbial components in our study of anaerobic digestion systems. Overall, PL-CdW exhibited a higher abundance of acid-producing and methanogenic bacteria than P-CdW. This implies more active acetic acid and methanogenesis, consistent with the cumulative methane production observed during photo-assisted anaerobic digestion of cadmium-containing sludge. Figure 1(b) Specifically, *Tissierella* and *Proteiniborus* were recorded as promoting sludge hydrolysis and acidification, with their abundance increasing from 5.41% to 5.45% and from 3.10% to 6.02%, respectively. Similarly, the abundance of *Methanobacterium* and *Methanosarcina*, associated with methanogenesis, also increased from 4.70% to 6.16% and from 3.67% to 5.80%, respectively. These microbial changes were further confirmed by assays of key enzyme activities associated with hydrolysis (protease), acidification (AK), and methanogenesis (F420). It can be seen that the relative activities of protease, AK, and F420 in PL-CdW increased significantly, from 48.5±4.1% to 69.9±2.5%, 56.0±1.9%, and 70.2±4.0%, and from 35.1±2.8% to 56.8±3.4%, compared to P-CdW. The enhanced enzyme activity in PL-CdW is partly attributed to the formation of cadmium sulfide, which reduces free Cd. 2+ The binding to the enzyme protein reduces Cd-induced enzyme inhibition, which helps to facilitate more efficient hydrolysis, acidification, and methanogenesis processes.

[0068] (3) Key genes encoding cytochrome C, pili, and membrane proteins were screened in P-CdW and PL-CdW using metatranscriptomics. The results showed that genes related to electron transport were actively expressed in PL-CdW. For example, the relative abundances of petD (K03888) and sdhc (K00241), which are related to cytochrome C, increased from 2.00 × 10⁻⁶ to 2.00 × 10⁻⁶. -6 Increased to 1.87 × 10 -4 and from 4.00×10 -6 Increased to 6.00×10 -5 Similarly, the relative abundances of the pilA (K02650, K02651) and pilB (K02652) genes involved in conductive fimbriae synthesis in PL-CdW increased by 239.3% and 343.8%, respectively, compared to P-CdW. Furthermore, the relative abundances of the functional genes encoding transmembrane electron transport proteins mtrA (K00577) and mtrC (K00579, K03585) increased by 152.00% and 247.37%, respectively. The relative abundance of etfB (K03521), a gene involved in encoding the β subunit of electron transport flavoprotein to transport electrons to the mitochondrial respiratory chain, increased from 3.80 × 10⁻⁶ in PL-CdW. -5 3.01 × 10⁻⁶ added to P-CdW -4 .

[0069] In Examples 1 and 2, after anaerobic digestion, samples were extracted from the two reactors corresponding to PL-WAS and PL-CdW, respectively. These samples were then analyzed using X-ray diffraction (XRD) and a scanning electron microscope equipped with energy-dispersive spectroscopy (SEM-EDS). The results are as follows: Figure 4 and Figure 5 As shown.

[0070] Figure 4 The images show the XRD patterns of the sludge after anaerobic digestion in Examples 1 and 2 of this invention. Figure 4 In the diagram, a and b are the characteristic diffraction peaks corresponding to CdS. Figure 4 It can be seen that after anaerobic digestion, cadmium sulfide was formed in the sludge of both PL-WAS and PL-CdW, and the sludge of PL-CdW showed a stronger CdS characteristic diffraction peak.

[0071] Figure 5 These are SEM images of the sludge after anaerobic digestion in Examples 1 and 2 of this invention. Figure 5 In the image, a is a SEM image of the sludge after anaerobic digestion in PL-WAS, and c is a SEM image of the sludge after anaerobic digestion in PL-CdW. From... Figure 5 As shown in a and 5c, the sludge in both PL-WAS and PL-CdW exhibits a loose, flocculent structure, a typical characteristic of sludge. Furthermore, PL-CdW is slightly darker than PL-WAS, indirectly indicating that PL-CdW has a higher degree of humification and a more complete anaerobic digestion process. In addition, EDS results show that the sludge in PL-CdW contains large amounts of sulfur and cadmium, while in PL-WAS, due to the lower cadmium sulfide content, only sulfur was detectable. Therefore, the presence of cadmium and sulfur ions in the sludge is an important prerequisite for the formation of cadmium sulfide semiconductors.

[0072] In Example 2, during the first 16 days of anaerobic digestion, samples were taken every two days from the reactor corresponding to PL-CdW, and the concentrations of sulfate and sulfide in the samples were measured. The results are as follows: Figure 6 As shown.

[0073] Figure 6 This is a graph showing the changes in sulfate and sulfide concentrations in the sludge during the anaerobic digestion process in Example 2 of the present invention. Figure 6 The results showed that during anaerobic digestion, sulfate levels decreased while sulfide concentrations gradually increased. This phenomenon is mainly attributed to the activity of sulfate-reducing bacteria, which promote sulfate reduction in the anaerobic system. Subsequently, sulfides and Cd in the sludge... 2+ The interaction between them leads to the spontaneous formation of cadmium sulfide in PL-CdW. Therefore, S originating from sulfate reduction... 2- With soluble Cd2+ Cadmium sulfide is formed through this process. Under light conditions, the photoelectrons generated by cadmium sulfide are rapidly consumed by methanogens to drive methanogenesis. This continuous electron generation establishes a synergistic cycle between cadmium sulfide and methanogens, promoting efficient electron transfer and ultimately increasing methane production.

[0074] The results show that in the Cd-containing reactor pretreated with alkali, methane production increased by 74.6% under light conditions compared to under dark conditions. SEM-EDS and XRD analyses confirmed the formation of cadmium sulfide. Electrochemical analysis indicated successful assembly of the CdS-methanogenic biohybrid, highlighting its active role in biomethane production: light-induced electrons were rapidly captured by the methanogens for methane production; under continuous light, a sustained electron transfer cycle was established between cadmium sulfide and the methanogens; furthermore, the reaction between sulfides produced by sulfate reduction and Cd in the sludge was observed. 2+ The reaction forms cadmium sulfide. Metatranscriptomic analysis showed an increase in the abundance of key microbial communities involved in the anaerobic digestion process, as well as enhanced gene expression related to electron transport under light. These findings highlight the enhanced methane production achieved by photo-assisted anaerobic digestion of Cd-containing wastewater.

[0075] Example 3

[0076] A method for increasing methane production from anaerobic digestion of cadmium-containing sludge includes the following steps:

[0077] S1. Adjust the pH of the cadmium-containing sludge to alkaline and perform alkaline pretreatment on the cadmium-containing sludge, specifically as follows:

[0078] S1-1. Take cadmium-containing sludge from the secondary sedimentation tank of a sewage treatment plant in Changsha City. The test results show that the cadmium-containing sludge contains methanogenic bacteria and sulfate-reducing bacteria. The initial concentration of cadmium is 0.1±0.02 mg / L and the initial concentration of sulfate is 168.6±12.4 mg / L. First, filter the cadmium-containing sludge through a stainless steel screen with a pore size of 0.45 mm to remove large particles. Then, condense it at 4℃ for 24 hours, remove the supernatant, and collect the remaining sludge at the bottom to obtain the original cadmium-containing sludge.

[0079] S1-2. Add sodium hydroxide to the original cadmium-containing sludge obtained in step S1-1 to adjust the pH value of the cadmium-containing sludge to 10 (alkaline). Perform alkaline pretreatment on the cadmium-containing sludge to be treated at a temperature of 4℃ for 2 hours to obtain alkaline pretreated cadmium-containing sludge.

[0080] S2. Anaerobic digestion of the cadmium-containing sludge pretreated with alkali in step S1 under light conditions, specifically:

[0081] S2-1. Add hydrochloric acid solution to the cadmium-containing sludge pretreated in step S1-2 and adjust the pH of the cadmium-containing sludge pretreated in step S1-2 to 7 (neutral) to ensure that methane can be produced through anaerobic digestion.

[0082] S2-2. Add cadmium sulfide to the cadmium-containing sludge pretreated with alkali at pH 7 to make the concentration of cadmium sulfide in the sludge 4 mg / g TSS, 8 mg / g TSS, and 16 mg / g TSS.

[0083] S2-3, under a light intensity of 200W / m 2 Anaerobic digestion of cadmium-containing sludge pretreated with cadmium sulfide was carried out under light conditions. The anaerobic digestion was conducted in an anaerobic environment with the temperature controlled at 35℃ for 29 days to complete the anaerobic digestion treatment of the cadmium-containing sludge. The cadmium sulfide concentrations in the cadmium-containing sludge were 4 mg / g TSS, 8 mg / g TSS, and 16 mg / g TSS, respectively, and were designated as PL-CdS 4, PL-CdS 8, and PL-CdS16.

[0084] Control group: Anaerobic digestion of cadmium-containing sludge pretreated with alkali at concentrations of 4 mg / g TSS, 8 mg / g TSS, and 16 mg / g TSS was carried out under dark conditions, with other conditions being the same. The cadmium sulfide concentrations in the cadmium-containing sludge at 4 mg / g TSS, 8 mg / g TSS, and 16 mg / g TSS were respectively designated as P-CdS 4, P-CdS 8, and P-CdS16.

[0085] After anaerobic digestion, the effects of different cadmium sulfide concentrations on methane yield were investigated, and the results are as follows: Figure 7 As shown.

[0086] Figure 7 This is a graph showing the effect of different cadmium sulfide concentrations on methane yield from anaerobic digestion in Example 3 of the present invention.

[0087] Depend on Figure 7It can be seen that under light conditions, as the concentration of cadmium sulfide increases, the cumulative methane production also gradually increases (from 4 mg / g total suspended solids to 8 mg / g total suspended solids and then to 16 mg / g total suspended solids). Furthermore, the cumulative methane production under light conditions is higher than that in the corresponding dark group, with relative methane production increases of 130.0% (PL-CdS4 vs. P-CdS4), 130.9% (PL-CdS8 vs. P-CdS8), and 131.8% (PL-CdS16 vs. P-CdS16), respectively. These increases significantly exceed the 64.7% increase caused by light alone. This phenomenon is consistent with the methanogenic effect of cadmium-containing sludge under light assistance mentioned above, providing strong evidence to confirm the role of cadmium sulfide generated in cadmium-containing sludge under light assistance.

[0088] Example 4

[0089] To further verify the synergistic effect of CdS-methanogenic biohybrids in enhancing methane production during anaerobic digestion, the experiment used sterilized raw sludge and was conducted under light conditions, including the following steps:

[0090] (1) Take the cadmium-containing sludge pretreated with alkali with a pH of 7 from Example 3, treat it at 100°C for 60 min to kill methanogenic bacteria, and divide it into three groups of three bottles each.

[0091] (2) Take one group of sterilized alkaline pretreated cadmium-containing sludge, add cadmium sulfide to make the concentration of cadmium sulfide in the alkaline pretreated cadmium-containing sludge 4 mg / g TSS, and inoculate with methanogenic bacteria inoculum (original cadmium-containing sludge) to obtain sludge A. Take the second group of sterilized alkaline pretreated cadmium-containing sludge, add cadmium sulfide to make the concentration of cadmium sulfide in the alkaline pretreated cadmium-containing sludge 4 mg / g TSS, and add an equal volume of deionized water to the methanogenic bacteria inoculum to keep the system volume constant to obtain sludge B. The third group of sterilized alkaline pretreated cadmium-containing sludge, without adding cadmium sulfide, but inoculated with methanogenic bacteria inoculum (original cadmium-containing sludge), is recorded as sludge C.

[0092] (3) When the light intensity is 200W / m 2 Anaerobic digestion was carried out on sludge A, sludge B and sludge C under light conditions. The anaerobic digestion was carried out in an anaerobic environment with the temperature controlled at 35℃ for 29 days to complete the anaerobic digestion treatment of cadmium-containing sludge.

[0093] After anaerobic digestion, the effects of different treatments on methane yield were investigated, and the results are as follows: Figure 8 As shown.

[0094] Figure 8 This is a graph showing the effect of different treatment conditions on methane yield from anaerobic digestion in Example 4 of the present invention. Figure 8 It was observed that in the CdS-only system, the cumulative methane production remained essentially constant throughout the anaerobic digestion process, with almost no methane produced, primarily due to the lack of methanogens. Furthermore, it was found that the cumulative methane production of CdS-containing sludge under light irradiation with inoculum (i.e., methanogens) was significantly higher than that of CdS-only sludge. These results indicate that under light-assisted anaerobic digestion conditions, maximum cumulative methane production can only be achieved through the synergistic effect between cadmium sulfide and methanogens. In the method of this invention, the formation of biohybrids and electron transfer between cadmium sulfide and methanogens promote the methanogenesis process of the methanogens.

[0095] The results above show that, compared with conventional methods, the method of the present invention for improving methane production from anaerobic digestion of cadmium-containing sludge, by performing anaerobic digestion of cadmium-containing sludge under light assistance, can construct a microbial-semiconductor biohybrid system. Furthermore, through the formation of the microbial-semiconductor biohybrid system and photoactivation, electron transfer is promoted, thereby increasing methane production efficiency. This method not only improves anaerobic digestion efficiency and methane production but is also more environmentally friendly. It can be widely used to treat cadmium-containing sludge, facilitating the harmless treatment and resource utilization of cadmium-containing sludge, demonstrating high practical value and promising application prospects.

[0096] The above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for increasing methane yield from anaerobic digestion of cadmium-containing sludge, characterized in that, Includes the following steps: S1. Adjust the pH value of the cadmium-containing sludge to alkaline and perform alkaline pretreatment on the cadmium-containing sludge. S2. Anaerobic digestion of cadmium-containing sludge pretreated with alkali in step S1 under light conditions.

2. The method according to claim 1, characterized in that, In step S1, an alkaline agent is used to adjust the pH value of the cadmium-containing sludge; the alkaline agent is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide and sodium bicarbonate.

3. The method according to claim 2, characterized in that, In step S1, the pH value of the cadmium-containing sludge is adjusted to 9-11.

4. The method according to claim 3, characterized in that, In step S1, the alkali pretreatment is carried out at a temperature ≤10℃; the alkali pretreatment time is 2h.

5. The method according to claim 4, characterized in that, In step S1, the cadmium-containing sludge contains methanogenic bacteria and sulfate-reducing bacteria; The initial concentration of cadmium in the cadmium-containing sludge is 0.1±0.02 mg / L, and the initial concentration of sulfate is 150 mg / L~200 mg / L. Before use, the cadmium-containing sludge also includes the following treatment: filtering the cadmium-containing sludge with a sieve with a pore size of 0.45 mm to remove large particles, condensing it at 4℃ for 24 hours, removing the supernatant, and collecting the remaining sludge.

6. The method according to claim 5, characterized in that, In step S1, cadmium ions are also added to the cadmium-containing sludge, so that the concentration of cadmium ions in the cadmium-containing sludge is 2.26 mg / g TSS to 19.72 mg / g TSS.

7. The method according to any one of claims 1 to 6, characterized in that, In step S2, the light intensity used in the anaerobic digestion process is 150 W / m². 2 ~200W / m 2 .

8. The method according to any one of claims 1 to 6, characterized in that, In step S2, the anaerobic digestion is carried out at a temperature of 35℃~40℃; the anaerobic digestion time is 29 days.

9. The method according to any one of claims 1 to 6, characterized in that, In step S2, the pH value of the cadmium-containing sludge is adjusted to neutral during the anaerobic digestion process.

10. The method according to any one of claims 1 to 5, characterized in that, In step S2, the anaerobic digestion process also includes adding cadmium sulfide to the cadmium-containing sludge, so that the concentration of cadmium sulfide in the cadmium-containing sludge is 4 mg / g TSS to 16 mg / g TSS.

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