Method for relieving inhibition of polyvinyl chloride microplastics on anaerobic digestion process by sludge and coal gangue based biochar

Abstract

The invention belongs to the technical field of sewage treatment, and provides a method for relieving inhibition of polyvinyl chloride micro-plastics on an anaerobic digestion process by using sludge and coal gangue based biochar, which comprises the following steps: adding pretreated sludge and simulated wastewater into an SBR (Sequencing Batch Reactor); soaking PVC-MPs in alcohol for 48 hours, washing the PVC-MPs with distilled water for 3-5 times to remove impurities in the PVC-MPs, and drying the PVC-MPs at 105 DEG C for 24 hours for later use; pVC-MPs is added into a reaction device according to the proportion of 0.1 g. (g. VS)-1 and placed in a constant-temperature shaking table at the temperature of 35.0 + / -0.1 DEG C in a dark place; sCBC is added into the reaction device according to the adding amount of 1.0 g.L <-1 >, other conditions are kept the same, and sampling and analysis are conducted at 0, 1, 2, 3, 5, 7, 9 and 12 h respectively. The invention proves that the SCBC can effectively relieve the inhibition of PVC-MPs on anaerobic digestion, promote the enrichment of dominant methanogens, and provide a feasible technical path for the resource utilization of coal gangue and sludge at the same time.

Description

Technical Field

[0001] The invention belongs to the technical field of sewage treatment, and specifically relates to a method for alleviating the inhibition of polyvinyl chloride microplastics on anaerobic digestion process by using sludge and coal gangue-based biochar. Background Art

[0002] Microplastics (MPs) refer to plastic fragments or particles smaller than 5 mm in size. Due to their inherent toxicity, environmental persistence, and bioaccumulation, they pose a major threat to the ecological environment and human health. Therefore, MPs have become the focus of research in the field of emerging pollutants in recent years [1]. MPs discharged from urban sewage treatment plants are one of the key sources of MPs pollution in the water environment. Existing studies have confirmed that MPs in sewage treatment plants can interfere with the anaerobic digestion (AD) process, reduce the decomposition efficiency of organic matter, inhibit the production of methane, and weaken the stability of the system [2].

[0003] At present, the main methods for removing MPs from water environments include physical methods (such as filtration, adsorption, density separation), chemical methods (such as electrocoagulation, advanced oxidation) and biological methods (such as biodegradation, membrane bioreactor, etc.). However, these technologies usually have the limitations of high cost and difficulty in large-scale application [3]. Therefore, it is of great significance to develop new and cost-effective technologies to alleviate the inhibitory effect of MPs on AD.

[0004] In recent years, biochar has attracted attention for its mitigating effect on MPs in AD systems. Singh et al. [4] showed that biochar can adsorb MPs of different particle sizes; Yan et al. [5] found that the adsorption capacity of biochar on MPs affects the structure of microbial communities; Palansooriya et al. [6] further pointed out that the coexistence of biochar and MPs can increase the relative abundance of functional genes, promote microbial growth and optimize the stability of the AD system. Summary of the invention

[0005] In order to solve the problem of the inhibitory effect of microplastics on methane production during anaerobic digestion, the present invention provides a method for sludge-coal waste biochar to alleviate the inhibition of polyvinyl chloride microplastics on the anaerobic digestion process. Sludge-coal waste biochar (SCBC) is applied to the anaerobic digestion system containing polyvinyl chloride microplastics (PVC-MPs), and the effect of SCBC on AD performance, methane production and microbial community structure is systematically explored, aiming to verify that SCBC can alleviate the inhibitory effect of PVC-MPs on anaerobic digestion and promote the enrichment of dominant methanogens. In addition, the present invention also provides a new technical path for the resource utilization of coal gangue and sludge.

[0006] The present invention is achieved by the following technical scheme: a method for alleviating the inhibition of polyvinyl chloride microplastics on anaerobic digestion process by using sludge and coal gangue-based biochar, wherein 40 ml of pretreated sludge and 40 ml of configured simulated wastewater are added to an SBR reactor; The properties of the pretreated sludge were as follows: reaction period of 12 h, pH of 7.0 ± 0.2, and ρ[total solids (TS)] of (12.0 ± 0.1) g·L -1 , ρ[volatile solids (VS)] is (8.0±0.1) g·L -1 ; The substrate is COD 300 mg·L -1 The glucose matrix simulated wastewater contains macroelements and trace elements, as shown in Table 1: Table 1: ; After immersing PVC-MPs in alcohol for 48 hours, wash them with distilled water for 3-5 times to remove impurities in the material, and dry them at 105°C for 24 hours before use; The concentration of PVC-MPs is calculated based on VS, 0.1 g·(g·VS) -1 Add to the reaction device and place in a constant temperature shaker at 35.0±0.1℃ away from light; Sludge gangue-based biochar SCBC was 1.0 g·L -1 The dosage was added into the reaction device, and other conditions remained the same. Samples were taken and analyzed at 0, 1, 2, 3, 5, 7, 9, and 12 h.

[0007] The specific steps are as follows: (1) Each group of reactors was added with 40 ml of pretreated sludge and 40 ml of simulated wastewater, and three groups were set up in parallel; the concentration of polyvinyl chloride microplastics (PVC-MPs) was calculated as VS, at 0.1 g·(g·VS) -1Add to the reaction device, named Z1, and the control group K0 without PVC-MPs; place in a constant temperature shaker at 35.0±0.1℃ away from light; (2) Sludge gangue-based biochar SCBC was calculated at 1.0 g·L -1 The dosage of was added to Z1, and other conditions remained the same, named Z1C1; the above three groups of reactors were sampled and analyzed at 0, 1, 2, 3, 5, 7, 9, and 12 hours respectively; (3) Performance analysis: The water sample was filtered with a 0.45 μm water filter, and the chemical oxygen demand (COD) was determined by potassium dichromate titration. The cumulative gas production was analyzed by direct reading with a 25 mL syringe. The gas components H2, CH4, and CO2 were analyzed by gas chromatography, and the temperatures of the injection port and the chromatographic column were 250°C and 60°C respectively. The pH value was determined by a pH meter. (4) High-throughput sequencing analysis: The extracted DNA of the three groups of sludge samples was subjected to high-throughput sequencing analysis on the Illumina platform. DNA was extracted using a DNA kit, and the V3-V4 region of the microbial 16S rRNA gene was amplified and purified to simultaneously obtain information on bacteria and archaea. The bacterial primers used were as shown in SEQ ID NO.1 and SEQ ID NO.2 in the sequence listing, specifically as follows: 341F (5'-CCTACGGGNGGCWGCAG-3') and 805R (5'-GACTACHVGGGTATCTAATCC-3'); The archaeal primers used in the first round are shown in SEQ ID NO.3 and SEQ ID NO.4, specifically as follows: 340F (5'-CCCTAYGGGGYGCASCAG-3') and 1000R (5'-GGCCATGCACYWCYTCTC-3'); the second round are shown in SEQ ID NO.5 and SEQ ID NO.6, specifically as follows: 349F (5'-GYGCASCAGKCGMGAAW-3'); 806R (5'-GGACTACVSGGGTATCTAAT-3').

[0008] The particle size of PVC microplastics is 1.0±0.1mm, the melting point is 200.0±20.0℃, and the bulk density is 1.0±0.2g·cm -3 After soaking in alcohol for 48 hours, wash with distilled water 3-5 times to remove impurities in the material, dry at 105℃ for 24 hours and set aside.

[0009] SCBC is made by the following steps: 1) Mix the coal gangue and sludge powder in a mass ratio of 3:2 and mix them evenly; 2) According to the material-liquid ratio of m activator: m carbonized material of 1:2, 4 mol·L -1The ZnCl2 was used as an activator and immersed for 24 hours, and then filtered; 3) Place in a blast drying oven and dry at 105±2℃; 4) Activate at 550°C for 60 min in a tube furnace under N2 atmosphere and cool to room temperature; 5) Use 1.2 mol·L -1 Soak in hot hydrochloric acid for 60 minutes, then rinse with deionized water until neutral; 6) Dry in a forced air drying oven at 105±2℃ to constant weight.

[0010] By analyzing the COD removal rate, pH value changes, cumulative gas production, gas component proportions and changes in microbial community structure, the present invention proves that SCBC can effectively alleviate the inhibition of PVC-MPs on anaerobic digestion, promote the enrichment of dominant methanogens, and provide a feasible technical path for the resource utilization of coal gangue and sludge.

[0011] The results showed that the concentration of PVC-MPs was 0.1 g·(g·VS) -1 When the pH value was 1.0 g·L, the anaerobic digestion was inhibited, and the COD, gas production, and methane content decreased by 17.5%, 11.5%, and 26.7%, respectively. SCBC can most effectively reduce the inhibitory effect of PVC-MPs. The addition amount of SCBC was 1.0 g·L -1 When the pH value was adjusted to 7.0±0.2, the inhibition of PVC-MPs on anaerobic digestion was effectively alleviated, and COD, gas production, and CH4 content increased by 16.7%, 51.4%, and 101.2%, respectively. At the same time, SCBC enhanced the biomass of functional bacteria and the aggregation and colonization ability of methanogens. In the reaction system, the relative abundances of Chloroflexi, Proteobacteria, and Bacteroidetes increased to 46.9%, 10.9%, and 10.8%, respectively; the relative abundances of Methanobacteria, Methanobacterium, and Methanosarcina increased to 28.8%, 14.3%, and 28.8%, respectively. PVC microplastics are hydrophobic and non-degradable, and they remain in the reactor after the reaction. The particle size is (1.0±0.1) mm, the melting point is (200.0±20.0)℃, and the bulk density is (1.0±0.2) g·cm -3 After soaking in alcohol for 48 hours, wash with distilled water 3-5 times to remove impurities in the material, and dry in an oven at 105℃ for 24 hours before use. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is the COD removal rate at different PVC-MPs concentrations; Figure 2 Middle: (a) is the cumulative gas production under different PVC-MPs concentrations; (b) is the change of pH under different PVC-MPs concentrations; Figure 3 is the change of gas component content under different PVC-MPs concentrations; Figure 4 The effect of SCBC on COD removal efficiency; Figure 5 is the effect of SCBC on cumulative gas production and pH; Figure 6 is the change of SCBC content of gas components; Figure 7 Middle: Relative abundance of (a) bacterial community at the phylum level; (b) archaeal community at the genus level. DETAILED DESCRIPTION

[0013] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0014] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs, and the disclosure and materials cited therein are hereby incorporated by reference.

[0015] Technical equivalents to the specific embodiments described that are apparent to those skilled in the art using no more than routine experimentation are intended to be encompassed by this application.

[0016] The experimental methods in the following examples are all conventional methods unless otherwise specified. The instruments and equipment used in the following examples are all conventional laboratory instruments and equipment unless otherwise specified; the experimental materials used in the following examples are all purchased from conventional biochemical reagent stores unless otherwise specified.

[0017] 1. Experimental materials: The inoculated sludge was taken from the reflow tank of a sewage treatment plant. After settling for 48 hours, the supernatant was discarded for later use. The substrate was COD of 300 mg·L -1 The specific composition of the glucose-based wastewater can be found in the literature [7] and is shown in Table 1. The properties of the sludge after pretreatment are as follows: the reaction period is 12 h, the pH is 7.0 ± 0.2, and the ρ [total solids (TS)] is (12.0 ± 0.1) g·L -1 ,ρ[volatile solid (VS)]: (8.0±0.1) g·L -1 .

[0018] Table 1: Specific components of wastewater PVC-MPs were purchased from Fengtai Polymer Materials. They are hydrophobic and non-degradable and remain in the reactor after the reaction. The particle size is (1.0±0.1) mm, the melting point is (200.0±20.0)℃, and the bulk density is (1.0±0.2) g·cm -3 , After soaking in alcohol for 48 hours, wash with distilled water 3 to 5 times to remove impurities in the material, and dry in an oven at 105℃ for 24 hours before use.

[0019] Biochar (SCBC) was prepared from sewage sludge and coal gangue according to the literature [8].

[0020] SCBC was prepared by the following steps: 1) mixing coal gangue and sludge powder in a mass ratio of 3:2; 2) mixing 4 mol·L -1 1) Activate with ZnCl2 as activator for 24h and filter; 2) Place in a forced air drying oven at (105±2)°C for drying; 3) Activate in a tube furnace at 550°C for 60min under N2 atmosphere and cool to room temperature; 4) Use 1.2mol·L -1 Soak in hot hydrochloric acid for 60 minutes, then rinse with deionized water until neutral; 6) Dry in a forced air drying oven at (105±2)°C to constant weight.

[0021] 2. Experimental method: The water sample was filtered with a 0.45 μm water filter, and the chemical oxygen demand (COD) was determined by potassium dichromate titration. The cumulative gas production was analyzed by direct reading with a 25 mL syringe. The gas components (H2, CH4, CO2) were analyzed by gas chromatography (Agilent 7890B, Agilent, USA), and the temperatures of the injection port and the chromatographic column were 250 °C and 60 °C respectively. The pH value was determined by a pH meter (pHS-3C, Leici, China).

[0022] Experimental design: The experiment was carried out in 100 mL serum bottles. The first batch was set up with 4 groups, and 3 groups were set up in parallel. 40 ml of pretreated sludge and 40 ml of simulated wastewater were added to each reactor. The concentration of PVC-MPs (in terms of VS) was 0.01 g·(g·VS) -1 、0.1 g·(g·VS) -1 ,1.0 g·(g·VS) -1 They were added into the reaction device and named as D1, Z1, G1 respectively. The control group K0 did not add PVC-MPs. They were placed in a constant temperature shaker at (35.0±0.1)℃ away from light.

[0023] The second batch was set up with 6 groups, and 3 groups were set up in parallel. Other conditions remained the same. SCBC was set at 0.5 g·L -1 , 1.0 g·L -1 , 2.0 g·L -1The dosages were added to D1 and Z1 respectively and named as D1C0.5, D1C1, D1C2, Z1C0.5, Z1C1 and Z1C2 respectively.

[0024] High-throughput sequencing analysis: K0, Z1 and Z1C1 sludge samples were selected for high-throughput sequencing analysis of the extracted DNA on the Illumina platform (Illumina, USA). DNA was extracted using a DNA kit (Omega, USA), and the V3-V4 region of the microbial 16SrRNA gene was amplified and purified to simultaneously obtain the information of bacteria and archaea. The bacterial primers used were as shown in SEQ ID NO.1 and SEQ ID NO.2 in the sequence table, which are as follows: 341F (5'-CCTACGGGNGGCWGCAG-3') and 805R (5'-GACTACHVGGGTATCTAATCC-3'); The archaeal primers used in the first round are shown in SEQ ID NO.3 and SEQ ID NO.4, specifically as follows: 340F (5'-CCCTAYGGGGYGCASCAG-3') and 1000R (5'-GGCCATGCACYWCYTCTC-3'); the second round are shown in SEQ ID NO.5 and SEQ ID NO.6, specifically as follows: 349F (5'-GYGCASCAGKCGMGAAW-3'); 806R (5'-GGACTACVSGGGTATCTAAT-3').

[0025] 3. Results and Discussion 1. Effect of PVC-MPs concentration on AD performance: Changes in COD removal rate: Effects of different PVC-MPs concentrations on COD removal rate Figure 1 As shown in the figure, COD of K0 and Z1 were 91.7% and 83.1% respectively; compared with blank K0, D1 (PVC-MPs concentration was 0.01 g·(g·VS) -1 ) and Z1 (PVC-MPs concentration was 0.1 g·(g·VS) -1 ) decreased by 5.2% and 17.5% respectively. PVC-MPs in the range of 0.01~0.1 g·(g·VS) -1 The concentration range has an inhibitory effect on the AD process, resulting in a decrease in COD removal efficiency. This inhibitory effect may be due to the fact that the exposure of PVC-MPs inhibits the secretion of extracellular polymers (EPS), causing the granular sludge and internal microorganisms to lose the protection of EPS, resulting in the rupture of granular sludge and the decrease of cell viability [9].

[0026] On the contrary, G1 (PVC-MPs concentration is 1 g·(g·VS)-1 ), the COD removal rate increased by 3.5%, and the effect of PVC-MPs on the AD process changed from inhibition to promotion, which may be due to the combined effect of the particle size and concentration of PVC-MPs. The large specific surface area of ​​high-concentration PVC-MPs may promote biofilm formation. The inherent characteristics (plastic additives and plastic oligomers) can be released from PVC-MPs into the water and may be used by certain microbial groups to change the structure of the microbial community, thereby promoting the substrate conversion rate

[10] . In summary, the concentration effect of PVC-MPs on the AD system is bidirectional, that is, at a specific concentration, it may have both inhibitory and promoting effects, which requires detailed analysis based on specific circumstances.

[0027] Cumulative gas production and pH changes: Effects of different PVC-MPs concentrations on cumulative gas production Figure 2 (a) shows that in one cycle, the cumulative gas production showed a trend of first increasing and then decreasing, and reached a peak at the 9th hour. Compared with the blank K0, the cumulative gas production of D1 and Z1 decreased by 10.9% and 11.5%, respectively. This shows that PVC-MPs in the range of 0.01~0.1 g·(g·VS) -1 The concentration range has an inhibitory effect on the biogas production in the AD process, indicating that the presence of PVC-MPs will reduce the methane production efficiency and gas production level by affecting the activity of key enzymes (α-glucosidase, protease) in the AD process. K0, D1, Z1, and G1 decreased from 10.2 mL, 9.1 mL, 9.0 mL, and 11.3 mL to 6.2 mL, 5.4 mL, 4.1 mL, and 7.4 mL at the 12th hour. The decrease in gas production may be due to the reaction of hydrogen produced by hydrogenotrophic methanogens with carbon dioxide to produce methane, resulting in a decrease in gas production

[11] . The concentration of PVC-MPs is 1 g·(g·VS) -1 When the temperature was 0.5 °C, the cumulative gas production increased by 10.7%, which was consistent with the above-mentioned COD changes. This may be because under medium temperature conditions, PVC-MPs were enriched with hydrolytic and acidifying bacteria, and the increased activity of these microorganisms may have improved the gas production capacity

[10] .

[0028] Effects of different PVC-MPs concentrations on reactor pH Figure 2 (b) As the reaction proceeds, the pH value first decreases, then gradually increases, and finally stabilizes. The pH value gradually decreases within the first 5 hours, and the lowest values ​​of K0, D1, Z1, and G1 are 6.8, 6.7, 6.4, and 6.9, respectively; from 5 to 12 hours, the pH value of the reactor gradually increases and tends to stabilize, among which the decrease in Z1 is the most significant, and it fails to return to the appropriate pH level (7.0±0.2) during the entire period. Therefore, when the PVC-MPs concentration is 0.01~0.1g·(g·VS) -1The effect of pH on the AD process changed significantly when PVC-MPs were in contact with cells, which may be because PVC-MPs caused severe physical damage to the cells, significantly reducing the thickness of the formed biofilm and destroying its structural stability. This physical damage may change the integrity and charge distribution of the cell membrane

[12] .

[0029] 2. Changes in gas component content: The changes in gas component content due to different PVC-MPs concentrations are as follows: Figure 3 As shown in the figure, with the increase of PVC-MPs concentration, the H2 content showed a significant downward trend, the CO2 content showed a significant upward trend, the CH4 content gradually decreased in the D1 and Z1 reactors, and slightly increased in the G1 reactor. Compared with the blank K0, the CH4 content in D1, Z1, and G1 decreased by 16.6%, 26.7%, and increased by 3.9%, respectively; the CO2 content increased by 23.7%, 32.8%, and 16.2%, respectively; and the H2 content decreased by 35.9%, 41.8%, and 56.2%, respectively. It can be seen that PVC-MPs in wastewater inhibited the production of CH4 and H2 in the AD process, while promoting the production of CO2. This may be because the attachment of nanoplastics to hydrogen-producing bacteria resulted in a large number of nanopores in the cell wall, which would change the permeability of the cytoplasmic membrane and further interfere with the synthesis of cell DNA and protein

[13] . It can be inferred that PVC-MPs can cause the destruction of microbial cells, release nutrients in the cells, and produce toxic effects on microorganisms, thereby inhibiting the ability to produce methane in the AD process.

[0030] The CH4 content in G1 increases. When the PVC particle size is 1 mm, the concentration is 10particles·(g·VS) -1 PVC can increase the cumulative production of methane by 5.9±0.1%. This may be because PVC-MPs have a large specific surface area and have a significant adsorption capacity for pollutants. This adsorption may change the composition and function of the microbial community, thereby affecting the microbial activity and interaction during the AD process

[14] .

[0031] 3. Different SCBC addition amounts alleviate AD inhibition Changes in COD removal efficiency: Effects of different SCBC addition amounts on COD removal efficiency in D1 and Z1 reactors Figure 4 Compared with D1, D1C0.5 (PVC-MPs concentration is 0.01 g·(g·VS) -1 ,SCBC addition amount was 0.5 g·L -1 )、D1C1(PVC-MPs concentration is 0.01 g·(g·VS) -1 ,SCBC addition amount was 1.0 g·L -1)、D1C2(PVC-MPs concentration is 0.01 g·(g·VS) -1 ,SCBC addition amount was 2.0 g·L -1 ) increased by 2.4%, 8.2% and 8.1% respectively; compared with Z1, Z1C0.5 (PVC-MPs concentration is 0.1 g·(g·VS) -1 ,SCBC addition amount was 0.5 g·L -1 )、Z1C1(PVC-MPs concentration is 0.1 g·(g·VS) -1 ,SCBC addition amount was 1.0 g·L -1 ) with a COD of 97.0% and a Z1C2 (PVC-MPs concentration of 0.1 g·(g·VS) -1 ,SCBC addition amount was 2.0 g·L -1 ) increased by 3.1%, 16.7%, and 16.3%, respectively. Obviously, SCBC helps alleviate the negative impact of PVC-MPs on COD removal during the AD process. This may be because the pore structure of biochar after chemical activation is more complete, thereby increasing its specific surface area and active sites, thereby improving its adsorption capacity

[15] . Specifically, SCBC can significantly increase the specific surface area of ​​biochar after chemical activation treatment, forming more active sites, which provide more adsorption sites for microorganisms and help adsorb and remove PVC-MPs.

[0032] It is worth noting that when the amount of SCBC added was 2.0 g·L -1 and 1.0 g·L -1 When SCBC was added in excess, the difference in COD removal rate was not significant, and even inhibition was observed at higher addition amounts. Since the pH value of biochar is relatively high (6.8-11.3)

[16] , when SCBC is added in excess, it may cause the AD environment to become alkaline, which may interfere with the optimal pH level for microbial growth (7.0±0.2), thereby reducing the ability of microorganisms to decompose substrates. Therefore, it can be concluded that the optimal addition amount of SCBC is 1.0 g·L -1 , in order to avoid adverse effects on microbial activity.

[0033] Cumulative gas production and pH changes: The changes in cumulative gas production within a cycle with different SCBC additions are shown in Figure 2. Figure 5As shown in (a) and (b), with the increase of SCBC addition, the cumulative gas production gradually increased and reached the highest point at the 9th hour. Compared with D1, the cumulative gas production of D1C0.5, D1C1, and D1C2 increased by 6.3%, 38.2%, and 10.9% on average, and compared with Z1, the cumulative gas production of Z1C0.5, Z1C1, and Z1C2 increased by 0.8%, 34.1%, and 0.3% on average. This shows that adding an appropriate amount of SCBC can enhance the ability of PVC-MPs to reduce gas production during AD, and it can be inferred that the promoting effect of SCBC is greater than the inhibitory effect of PVC-MPs.

[0034] The changes of pH with different SCBC addition amounts are as follows Figure 5 (c) and (d). As shown in the figure, SCBC can effectively reduce the acidic inhibition effect caused by PVC-MPs in the AD process and help maintain the pH value at an appropriate level. D1C0.5, D1C1, and D1C2 are stabilized at 6.7, 7.0, and 6.8, respectively, and Z1C0.5, Z1C1, and Z1C2 are stabilized at 6.8, 6.9, and 6.8, respectively. Some studies have shown that biochar, as a carbon-based conductive material, can aggregate conductive microorganisms and promote electron transfer between them, which helps to solve the problem of insufficient electron concentration under adverse conditions, thereby facilitating the smooth progress of the AD process

[17] . Therefore, this experiment shows that the appropriate addition of SCBC helps prevent the pH value from dropping too low and is conducive to restoring an environment suitable for microbial growth.

[0035] Changes in gas components: The changes in the content of gas components due to different SCBC additions are as follows: Figure 6 Compared with D1 and Z1, the contents of CH4 and H2 increased, while the content of CO2 decreased after adding SCBC, especially when the addition amount of SCBC was 1 g·L -1 When , the CH4 content reached the highest level. Compared with D1, the CH4 content of D1C0.5, D1C1, and D1C2 increased by 25.1%, 118.8%, and 94.6%, respectively; the H2 content increased by 6.6%, 75.3%, and 43.7%, respectively; the CO2 content decreased by 13.1%, 69.1%, and 52.2%, respectively. Compared with Z1, the CH4 content of Z1C0.5, Z1C1, and Z1C2 increased by 10.7%, 101.1%, and 46.5%, respectively; the H2 content increased by 33.0%, 41.3%, and 73.2%, respectively; the CO2 content decreased by 9.0%, 45.1%, and 27.8%, respectively. This result shows that SCBC, as an effective additive, can promote the improvement of gas production efficiency under the inhibition of PVC-MPs in the AD process, thereby enhancing the production of CH4 and H2.

[0036] Studies have shown that biochar can enhance hydrogenotrophic methanogenesis by promoting direct electron transfer (DIET) between microorganisms and promote the syntrophic metabolism of intermediate products in the AD system, which helps to improve the efficiency of methane production

[18] . Therefore, SCBC can provide a suitable growth space for syntrophic microorganisms, enhance the electron activity of methanogens, and thus more effectively utilize CO2 to convert into H2 and CH4, promoting the progress of the methanogenesis stage. This process may involve the DIET mechanism, in which SCBC, as a conductive material, may promote the direct transfer of electrons between syntrophic microorganisms and improve the AD efficiency.

[0037] 4. Microbial community analysis Alpha diversity of microbial communities: The alpha diversity index of bacterial and archaeal communities in the three groups of reactors K0, Z1, and Z1C1 is shown in Table 1.

[0038] Table 1: Alpha diversity index of bacterial and archaeal communities The Chao and ACE indices are both proportional to the richness of the microbial community. The Chao and ACE indices of the bacterial and archaeal communities in the Z1 reactor are lower than those in the blank K0. Studies have shown that PVC-MPs can significantly reduce the richness of bacterial and archaeal communities, change the permeability of cell membranes, and allow nutrients to flow out due to their inherent toxicity or the release of harmful chemicals (such as bisphenol A)

[19] . The Chao and ACE indices of the bacterial and archaeal communities in the Z1C1 group were higher than those in the Z1 group. This indicates that the addition of SCBC increases the richness of the microbial community. It can be inferred that SCBC can increase the richness of the microbial community during the AD process, and its promoting effect is greater than the inhibitory effect of PVC-MPs, which is conducive to the positive and stable operation of the AD process.

[0039] The smaller the Simpson index and the larger the Shannon index, the higher the diversity of the microbial community. Among the three groups of reactors, the Simpson index of the bacterial and archaeal communities in Z1C1 was the smallest and the Shannon index was the largest, which shows that the addition of SCBC can significantly increase the diversity of the microbial community, and the diversity of the microbial community has a positive effect on the performance of AD. Highly diverse microbial communities can support more ecosystem functions, including methane production, thereby improving the efficiency and stability of AD

[20] . Therefore, SCBC can alleviate the inhibitory effect of PVC-MPs and promote the development of microbial communities towards a richer diversity.

[0040] The Shannon index can also reflect the proportion of dominant bacteria in the total microorganisms. The CH4 and H2 contents of the Z1C1 group were higher than those of the Z1 group, and the CO2 content was lower, which indicates that the addition of SCBC can promote the enrichment of dominant bacteria such as methanogens, enhance their dominant position in the microbial community, and thus increase the cumulative gas production and the proportion of methane content. The Coverage index of the three groups of samples exceeded 0.99, which indicates that the sequencing results are representative and can accurately reflect the true microbial diversity.

[0041] Changes in the composition of the microbial community: Changes in the relative abundance of bacterial communities at the phylum level such as Figure 7 (a) shows. Chloroflexi ( Chloroflexi ), Proteobacteria ( Proteobacteria ), Bacteroidetes ( Bacteroidetes ) are the dominant bacterial phyla. These three bacteria are all involved in the process of acetic acid production. They can provide sufficient substrate for subsequent methane production and play a vital role in the hydrolysis process of the substrate, which has an important impact on the acidification and hydrolysis stages of the AD process

[21] . Compared with the blank K0, the relative abundance of these three bacteria in Z1 decreased by 45.3%, 26.9%, and 19.8%, respectively; compared with Z1, the relative abundance of these three bacteria in Z1C1 increased by 76.1%, 21.9%, and 10.8%, respectively. This shows that the presence of PVC-MPs reduces the acidification and hydrolysis capacity in the AD process, and the content of intermediate products such as acetic acid is reduced, which in turn affects the substrate level in the subsequent methane production stage, and ultimately reduces the methane production rate. Studies have confirmed that the microbial community structure and composition in the biofilm on the surface of PVC-MPs are significantly different from those in the surrounding environment, and may enrich potential pathogens and drug-resistant bacteria, which seriously affects the acid production capacity of microorganisms

[22] . On the other hand, it shows that the addition of SCBC can reduce the inhibitory effect of PVC-MPs on the AD process and enhance the degradation ability of AD on organic matter, which helps to provide more substrates such as acetic acid, creates more favorable conditions for the methanogenesis stage, and further improves the efficiency of methanogenesis.

[0042] Firmicutes Firmicutes ) is different from the above three types of bacteria and plays an important role in decomposing carbohydrates, cellulose, proteins, etc.

[23] . Firmicutes ( Firmicutes ) increased by 7.9 times compared with K0, while in Z1C1, Firmicutes ( Firmicutes ) was 8.05 times lower than that of Z1. Under the inhibition of PVC-MPs, Firmicutes ( Firmicutes) increased. The reason for this phenomenon may be that microplastics, as a potential carbon source, induce stronger microbial activity and biofilm formation. This increased microbial activity may affect the spread of antibiotic resistance genes (ARGs), thereby increasing the number of Firmicutes ( Firmicutes ) abundance

[24] . After adding SCBC, the Firmicutes ( Firmicutes ) decreased until it returned to its original level, indicating that the addition of SCBC reduced the abundance of Firmicutes ( Firmicutes ) and promoted the growth of other bacterial phyla. You et al.

[25] found that magnetic biochar significantly changed the microbial composition during composting, with the dominant bacterial community shifting from Firmicutes ( Firmicutes ) into Bacteroidetes ( Bacteroidota ), Actinobacteria ( Actinobacteria ) and Proteobacteria ( Proteobacteria ).

[0043] The relative abundance changes of archaeal communities at the genus level are as follows Figure 7 (b) The dominant archaeal genus in the three reactors was Methanobasidium ( Methanosaeta ), Methanobacterium ( Methanobacterium ) and Methanosarcina ( Methanosarcina ) Methanosaeta ) is an obligate acetic acid-producing methanogen, which mainly produces CH4 through the acetate metabolism pathway. Methanosarcina ) can use a variety of substrates such as acetic acid and methanol to produce CH4. These two genera can enhance the mutualistic relationship between microorganisms through DIET mediated by conductive pili, thus promoting the production of CH4 during AD

[26] . Methanobacterium ) is an important hydrogenotrophic methanogen in anaerobic systems and can produce CH4 using H2 and CO2

[27] . Compared with the blank K0, the relative abundance of the three archaea in the Z1 group decreased by 20.2%, 1.3%, and 20.5%, respectively, indicating that the presence of PVC-MPs reduced the relative abundance of acetic acid and hydrogenotrophic methanogens. Studies have confirmed that the toxicity mechanism of PVC-MPs itself, such as toxic leachate and excessive oxidative stress, may affect the activity and abundance of methanogens, thereby affecting the methanogenesis rate

[28] . It may also be because the addition of PVC-MPs changed the Chloroflexi ( Chloroflexi ), Proteobacteria ( Proteobacteria ), Bacteroidetes ( Bacteroidetes ) indirectly affects the relative proportion of methanogens, including Methanobacterium ( Methanobacterium ) and Methanobristle ( Methanosaeta ) abundance.

[0044] Compared with Z1, the relative abundance of the three archaea in the Z1C1 group increased by 19.0%, 29.3%, and 18.9%, respectively. Wang et al.

[29] also pointed out that biochar provides a stable attachment carrier for microorganisms, and the surface of biochar can effectively enrich a large number of methanogenic microorganisms, such as Methanobacterium ( Methanobacterium ) and Methanococcus ( Methanococcus ), increasing the conversion rate of propionic acid to acetic acid, helping methanogenic microorganisms to withstand higher volatile fatty acids (VFAs), and further enhancing the activity of microorganisms in the methanogenic stage.

[0045] In summary, sludge gangue-based biochar can enhance the hydrogenotrophic methanogenesis process by promoting direct electron transfer (DIET) between microorganisms and promote the syntrophic metabolism of intermediates in the anaerobic digestion system, which helps to improve the efficiency of methane production. Therefore, SCBC can provide a suitable growth space for syntrophic microorganisms, enhance the electron activity of methanogens, and more effectively utilize CO2 to convert into H2 and CH4, thereby promoting the progress of the methanogenesis stage. This process may involve the DIET mechanism, in which SCBC, as a conductive material, may promote the direct transfer of electrons between syntrophic microorganisms and improve the efficiency of anaerobic digestion.

[0046] 5. Technical and economic analysis: According to the literature [7], the production of 1 ton of SCBC requires 1.2 tons of coal gangue, 0.8 tons of sludge, 0.04 tons of ZnCl2 and 0.3 tons of HCl. The costs involved include equipment assets, transportation, water, gas and electricity, chemical materials and labor costs. The life cycle cost (LCC) covers all costs in the production process. T ) including variable costs (C V ) and fixed costs (C F )

[30] . For this technology, variable costs include transportation costs (C O ), water, gas and electricity (C P ), chemical material cost (C A ), fixed costs include labor costs (C H ) and equipment asset depreciation (C E ). The main costs are as follows, see Table 2 for details.

[0047] Table 2: Production cost of SCBC Transportation cost (C O ): Assuming the total transportation distance is 100km, a small diesel car consumes about 7 liters of diesel per 100km, and the market price of diesel is 8 yuan / liter, the transportation cost is 56 yuan. Electricity and water cost (C P): The total electricity consumption is 100 kWh. Based on the industrial electricity price of 0.5 yuan / kWh provided by the China Power Grid, the electricity cost is 50 yuan. The cost of N2 equipment is 2 yuan / liter, so the N2 cost is 40 yuan. Assuming that the amount of water used for washing is 15 times that of raw materials during the process water washing process

[31] , it can be concluded that the water consumption is 15 tons, the unit price of industrial water is 4 yuan / ton, and the water price is 60 yuan. Chemical material cost (C A ): The market price of ZnCl2 is 6,000 yuan / ton, so 240 yuan is needed. The market price of hydrochloric acid is 500 yuan / ton, so 150 yuan is needed. Equipment asset depreciation (C E ): Mainly refers to foreign exchange transaction costs. Referring to the biochar plant in Selangor, Malaysia

[32] , the total investment in foreign exchange assets is US$170.2 per year (US$1 = RMB 7.29, exchange rate on December 18, 2024); if this cost is evenly distributed to each month, the cost of the equipment is about RMB 103.4 per month.

[0048] In summary, the total cost of preparing 1 ton of SCBC is about 1,700 yuan, that is, the cost of SCBC is 1.7 yuan / kg. SCBC technology is economically superior to traditional methods. Biochar has become an effective solution for removing microplastics and nanoplastics due to its renewability, high specific surface area and cost-effectiveness

[33] . This study successfully developed a cost-effective innovative technical strategy that effectively alleviated the inhibitory effect of MPs in the AD process. This technology is expected to be widely used in the future.

[0049] The effect of PVC-MPs on AD varies with concentration, and careful analysis is needed to accurately evaluate its effect. -1 Increase to 0.1 g·(g·VS) -1 When the concentration increased to 1.0 g·(g·VS), the pH value decreased, inhibiting AD, and causing COD, gas production, and methane content to decrease by 17.5%, 11.5%, and 26.7%, respectively. -1 When , the pH value decreased slowly, AD changed from inhibition to promotion, COD, gas production and methane content increased by 3.5%, 10.7% and 3.9% respectively.

[0050] SCBC has an optimal dosage, which can most effectively reduce the inhibitory effect of PVC-MPs and has the highest cost-effectiveness. -1 To 1.0g·L -1 When the pH value was adjusted to 7.0±0.2, the inhibition of PVC-MPs on AD was effectively alleviated. -1 The best effect was achieved when COD, gas production and CH4 content increased by 16.7%, 51.4% and 101.2% respectively.-1 After that, the promotion effect weakened, and COD, gas production, and CH4 content decreased by 0.3%, 25.2%, and 27.2%, respectively.

[0051] SCBC enhanced the biomass of functional bacteria and the aggregation and colonization ability of methanogens. In the reaction system, the relative abundances of Chloroflexi, Proteobacteria, and Bacteroidetes increased to 46.9%, 10.9%, and 10.8%, respectively; the relative abundances of Methanobacterium, Methanobacterium, and Methanosarcina increased to 28.8%, 14.3%, and 28.8%, respectively.

[0052] The results of technical and economic analysis showed that the cost of SCBC was 1.7 yuan / kg, which was cost-effective compared with physical and chemical methods, and showed the dual advantages of economic and environmental benefits in mitigating PVC-MPs in AD systems.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

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Claims

1. A method for alleviating the inhibition of polyvinyl chloride microplastics on anaerobic digestion process by using sludge and coal gangue-based biochar, characterized in that: 40 ml of pretreated sludge and 40 ml of simulated wastewater were added to the SBR reactor; The properties of the pretreated sludge were as follows: reaction period of 12 h, pH of 7.0 ± 0.2, and ρ[total solids (TS)] of (12.0 ± 0.1) g·L -1 , ρ[volatile solids (VS)] is (8.0±0.1) g·L -1 ; The substrate is COD 300 mg·L -1 The glucose matrix simulated wastewater contains macroelements and trace elements, as shown in Table 1: Table 1: ； After PVC-MPs are soaked in alcohol for 48 hours, they are washed with distilled water for 3-5 times to remove impurities in the material, and dried at 105°C for 24 hours before use; The concentration of PVC-MPs is calculated based on VS, 0.1 g·(g·VS) -1 Add to the reaction device and place in a constant temperature shaker at 35.0±0.1℃ away from light; Sludge gangue-based biochar SCBC was 1.0 g·L -1 The dosage was added into the reaction device, and other conditions remained the same. Samples were taken and analyzed at 0, 1, 2, 3, 5, 7, 9, and 12 h.

2. The method according to claim 1, characterized in that: The specific steps are as follows: (1) Each group of reactors was added with 40 ml of pretreated sludge and 40 ml of simulated wastewater, and three groups were set up in parallel; the concentration of polyvinyl chloride microplastics (PVC-MPs) was calculated as VS, at 0.1 g·(g·VS) -1 Add to the reaction device, named Z1, and the control group K0 without PVC-MPs; place in a constant temperature shaker at 35.0±0.1℃ away from light; (2) Sludge gangue-based biochar SCBC was calculated at 1.0 g·L -1 The dosage of was added to Z1, and other conditions remained the same, named Z1C1; the above three groups of reactors were sampled and analyzed at 0, 1, 2, 3, 5, 7, 9, and 12 hours respectively; (3) Performance analysis: The water sample was filtered with a 0.45 μm water filter, and the chemical oxygen demand (COD) was determined by potassium dichromate titration. The cumulative gas production was analyzed by direct reading with a 25 mL syringe. The gas components H2, CH4, and CO2 were analyzed by gas chromatography, and the temperatures of the injection port and the chromatographic column were 250°C and 60°C respectively. The pH value was determined by a pH meter. (4) High-throughput sequencing analysis: The extracted DNA of the three groups of sludge samples was subjected to high-throughput sequencing analysis on the Illumina platform. DNA was extracted using a DNA kit, and the V3-V4 region of the microbial 16S rRNA gene was amplified and purified to simultaneously obtain information on bacteria and archaea. The bacterial primers used were as shown in SEQ ID NO.1 and SEQ ID NO.2 in the sequence listing, specifically as follows: 341F (5'-CCTACGGGNGGCWGCAG-3') and 805R (5'-GACTACHVGGGTATCTAATCC-3'); The archaeal primers used in the first round are shown in SEQ ID NO.3 and SEQ ID NO.4, specifically as follows: 340F (5'-CCCTAYGGGGYGCASCAG-3') and 1000R (5'-GGCCATGCACYWCYTCTC-3'); the second round are shown in SEQ ID NO.5 and SEQ ID NO.6, specifically as follows: 349F (5'-GYGCASCAGKCGMGAAW-3'); 806R (5'-GGACTACVSGGGTATCTAAT-3').

3. The method according to claim 2, characterized in that: The particle size of PVC microplastics is 1.0±0.1mm, the melting point is 200.0±20.0℃, and the bulk density is 1.0±0.2g·cm -3 After soaking in alcohol for 48 hours, wash with distilled water 3-5 times to remove impurities in the material, dry at 105℃ for 24 hours and set aside.

4. The method according to claim 2, characterized in that: SCBC is made by the following steps: 1) Mix the coal gangue and sludge powder in a mass ratio of 3:2 and mix them evenly; 2) According to the material-liquid ratio of m activator: m carbonized material of 1:2, 4 mol·L -1 The ZnCl2 was used as an activator and immersed for 24 hours, and then filtered; 3) Place in a blast drying oven and dry at 105±2℃; 4) Activate at 550°C in a tube furnace for 60 min under N2 atmosphere and cool to room temperature; 5) Use 1.2 mol·L -1 Soak in hot hydrochloric acid for 60 minutes, then rinse with deionized water until neutral; 6) Dry in a forced air drying oven at 105±2℃ to constant weight.

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