Maximum Methane Production Rate Research Articles

Using amino acid waste liquid as functional supplement to change microbial community in up-flow anaerobic sludge blanket treatment of methanolic wastewater.

In this study, amino acid waste liquid was employed as a functional supplement (designated as amino acid-rich FS) in the up-flow anaerobic sludge blanket (UASB) treatment of methanolic wastewater. The effect of amino acid-rich FS was evaluated through repeated batch tests, showing that a 0.5% and 1% dosage increased the maximum methane production rate by 93.60% and 123.04%, respectively, by promoting faster methanol degradation. Additionally, long-term operation of the UASB reactor was conducted with increased dosages of amino acid-rich FS, resulting in improved performance. Microbial community analysis demonstrated that the addition of amino acid-rich FS enhanced microbial diversity, with the abundance of Sporomusa increasing by 47.5 times. Beyond the original cooperative relationships, an additional synergy between Sporomusa and Methanosarcina was observed. These findings could address the key challenge of limited microbial diversity in the anaerobic treatment of methanolic wastewater.

Solid-state anaerobic digestion of sweet corn waste: The effect of mixing and recirculation interval

The mixing and sludge recirculation interval is one of the key successes of the solid-state anaerobic digestion process, a promising technology for converting high-solid agro-industrial wastes to renewable energy. This study employed a pilot-scale completely stirred tank reactor to examine biogas production from sweet corn waste, including corn cobs, husks, and seeds. The reactor was operated as solid-state anaerobic digestion at an ambient temperature. The mixing and recirculating intervals were set to non-mixing and mixing for 10 minutes every 3, 6, and 12 hours. The initial total solid of the feedstock was 25%, while the hydraulic retention time was 30 days. The results showed that during the mixing and recirculation every three hours, the highest chemical oxygen demand, total solid, and volatile solid (VS) removal efficiencies were 85.42%, 62.92%, and 64.59%, respectively. The ratio between volatile fatty acid and alkalinity ranged between 0.20 and 0.30 without any sign of system failure. The highest specific methane yield of 766 L/kg VSadded was obtained in the experiment with mixing and recirculating intervals every 3 hours. It was found that the modified Gompertz model could effectively fit the methane yields with an R2 of 0.9667. The modeled methane production potential and the maximum methane production rate were 867.40 NL/kg VSadded and 132.01 NL/kg VSadded-d, respectively. Additionally, the levelized cost of the biogas produced from the solid-state anaerobic digestion of the sweet corn waste was calculated to be 0.61 USD/kg. The findings of this study can serve as a guide for the design and operation of the SS-AD system, which aims to transform various types of lignocellulosic waste into environmentally friendly energy.

Magnetite-mediating direct interspecies electron transfer is not pivotal role in directing electrons toward methane production from sulfate containing wastewater

Sulfate reduction can compete for electrons with methane production, thereby reducing methane production. Several factors, such as magnetite addition, carbon sources, and the chemical oxygen demand (COD)/SO42− ratio, can influence the direction of electron flow toward methanogenesis. However, their contributions to methane production remain unclear. In this study, the addition of magnetite significantly enhanced the maximum methane production rate (Rmax). The enrichment of Methanothrix and Methanobacterium in the conductive material groups indicated the establishment of a methanogenesis process through direct interspecies electron transfer (DIET). Although the DIET pathway enhanced Rmax in the conductive material groups, this increase did not translate into a higher methane yield in this study. The contribution of DIET pathway to methane yield was negligible compared with the effect of COD/SO42− ratio and carbon sources. The relative contribution of COD/SO42− ratio exceeded 40 % in directing electron flow.

Use of aqueous liquor from digested sludge pyrolysis for biogas production: characterization, toxicity assessment, and rate-limiting step determination

This study assessed the characteristics and toxicity of aqueous pyrolytic liquid (APL) derived from digested sewage sludge on anaerobic digestion (AD) and determined its rate-limiting step. Digested sewage sludge was pyrolyzed at multiple temperatures (350–650 °C) and moisture levels (0–40.4 %), resulting in APLs with varying AD toxicities. APL 350 °C-0 % showed the least toxicity, whereas APL 650 °C-40.4 % exhibited the greatest toxicity. Glucose (GL) and sodium acetate (SA) were introduced to elucidate the rate-limiting steps. SA, but not GL, enhanced APL conversion to CH4. And volatile fatty acid lack was observed in treatments without SA addition. This suggested that acidification was the primary rate-limiting step. This finding was confirmed using the modified Gompertz model: SA considerably improved the maximum methane production rate, whereas GL did not. Insights gained from this research clarified the feasibility and potential of AD for APL utilization and conversion.

Determination of the biochemical methane potential of swine hydrolyzate

Swine farming generates remains of births and animals that die during the process. Implementing a plan to eliminate these materials by hydrolysis process results in an animal by-product called swine hydrolyzate. The treatment of this by-product through its application in the anaerobic digestion process can also represent its conversion into a resource for energy production. Thus, the present study focuses on this by-product as a potential substrate for anaerobic digestion, evaluating the methane production potential. The results show biogas production with an average methane content of about 70 %. Theoretical biochemical methane potential was 967.95 ± 0.11 mL g VS−1, which represented a bioconversion efficiency of 59.97 ± 3.54 % and 62.26 ± 0.43 %, considering the experimental results of 580.47 ± 34.29 mL g VS−1 and 602.67 ± 4.16 mL g VS−1, respectively. By comparing the experimental biochemical methane potential with the predicted value using the modified Gompertz model, it was possible to conclude that the maximum methane production rate was 73.46 ± 0.36 mL g VS−1 day−1, with a digestion time of 18 days to obtain 90 % of the methane production potential.

Assessment of the Biochemical Methane Potential of Wastewater and Sludge Cake from a Poultry Slaughterhouse

Objectives : This study evaluates the potential for resource recovery through anaerobic digestion of organic waste generated in a poultry slaughterhouse, specifically focusing on poultry slaughterhouse wastewater and sludge cake obtained from in-house wastewater treatment.Methods : Basic characteristics (total solids, alkalinity, etc.), total nitrogen/ammonia nitrogen, and elemental analysis (macro and trace elements) were performed to determine the properties of the samples and calculate theoretical methane potential. Experimental methane potential was determined through BMP tests, with parameters such as lag period (λ) and maximum methane production rate (CH 4 mL/g VS/d) obtained using the modified Gompertz equation.Results and Discussion : The wastewater exhibited low organic matter concentration (1.79 g VS/kg; 3.74 g COD/L), while the sludge cake showed high total solids content (TS 170.8 g/kg) and organic matter concentration (131.5 g VS/kg; 220 g COD/L). Elemental analysis revealed that the C/N ratio was 9.17 for the sludge cake and 8.24 for the wastewater, indicating high nitrogen content. Methane production modeling using the modified Gompertz equation revealed a lag period of 7.5 days and T80 (time to produce 80% of total methane) of 21 days for wastewater, and a lag period of 2.4 days and T80 of 14 days for sludge cake.Conclusion : Wastewater and sludge cake from poultry slaughterhouses show significant potential as substrates for anaerobic digestion. However, the wastewater has low organic matter content, and the sludge cake, while high in organic matter, has low moisture and high ammonia levels. Therefore, it may be appropriate to co-digest these two substrates, or to co-digest the sludge cake with other substrates that have low nitrogen content and high water content. These findings provide fundamental data for the anaerobic digestion of slaughterhouse waste and highlight the need for further research on co-digestion and continuous processes.

Biostimulation of styrene-containing wastewater in carrier-supported anaerobic bioreactor towards enhancing biogas production and styrene degradation

Biological treatment of styrene-containing petrochemical wastewater usually suffers from styrene volatility and its toxicity to microorganisms. The main aim of the present study was to enhance the treatment performance of synthetic petrochemical wastewater using carrier-supported anaerobic bioreactor, in terms of biogas production and styrene degradation. For this purpose, three types of carrier, including magnetic granular activated carbon (MGAC), cuttlefish bone (CFB) and pumice, were used. A control reactor and the three carrier-supported reactors were operated under three styrene concentrations of 20 (cycle 1), 50 (cycle 2), and 100 (cycle 3) mg/L. Ethanol was used as a co-solvent and the total chemical oxygen demand (COD) was 500 mg/L in each cycle. According to the results, under all examined styrene concentrations, all the carrier-supported reactors exhibited higher styrene removal efficiency and gas production than the control reactor. Among carrier-containing rectors, the reactor supplemented with MGAC showed the best performance with styrene removal of 85.4 %–91.4 % and gas production of 368–890 mL/g COD added. The corresponding values for the control reactor were 72.3–79.2 % and 174–358 mL/g COD added. In addition, Carrier-containing reactors showed 31.2–67.2 % less styrene volatilization than the control reactor. The process performance was also affected by the styrene concentration so that the highest and the lowest gas productions were achieved for styrene contents of 50 and 100 mg/L, orderly. Using modified Gompertz model, it was found that the carriers shortened the lag phase and increased the maximum methane production rate, proving the establishment of stable conditions in carrier-supported reactors for the treatment of styrene-containing wastewater.

Effects of biochars derived from different feedstocks and pyrolysis temperatures on the anaerobic digestion of kitchen waste

The aim of this work was to investigate the effects of biochars derived from different feedstocks and pyrolysis temperatures on the anaerobic digestion (AD) of kitchen waste (KW). Nine biomass feedstocks (corn straw (CS), Dicranopteris dichotoma (DD), bamboo (B), KW, tea residues (TR), mushroom cultivation waste (MW), cassava lees (CL), Chlorella (C), and sargassum (S)) were pyrolyzed at different temperatures (300 °C, 500 °C, and 800 °C). Biochar varied in physicochemical properties (e.g., specific surface area, total pore volume, and organic functional group) depending on both feedstock type and pyrolysis temperature. This further impacted the enrichment of functional microbial consortia and development of methanogenic pathways, resulting in a varied AD performance. The addition of biochars generated respectively from CS, MW, and S at 800 °C, 300 °C, and 500 °C significantly improved the maximum methane production rate (Rm) and methane yield, while other biochars enhanced either Rm or methane yield. Therefore, the efficacy of biochar on methanogenesis associated with both the feedstock type and pyrolysis temperature. The findings offer a beneficial reference for the selection and application of biochar to improve the AD performance.

Enhancement of anaerobic digestion of dairy wastewater by addition of conductive materials with or without the combination of external voltage application

AbstractBACKGROUNDThe addition of conductive materials or the application of external voltage is a promising novel methodology to enhance methane production in anaerobic digesters. However, there is limited knowledge about their individual or combined effects on the anaerobic digestion of dairy wastewater. The aim of this study was to examine the effects of granular activated carbon (GAC) or magnetite addition and external voltage application, both individually and in combination, for the anaerobic digestion of real, undiluted dairy wastewater.RESULTSThe results showed that the estimated maximum methane production rate increased significantly (P < 0.05) from 143 ± 8 mL gVSadded−1 day−1 in the control reactors to 163 ± 11 mL gVSadded−1 day−1 and 166 ± 11 mL gVSadded−1 day−1 in the reactors containing GAC, with or without the combination of external voltage, respectively. The addition of GAC, in both cases, resulted in higher consumption rate of acetate, indicating a promotion of the methanogenesis step. By contrast, the application of external voltage or the addition of magnetite had no significant effect on either methane production rate nor methane yield. Moreover, the analysis of the microbial community showed that the addition of GAC resulted in the enrichment of Desulfobacterota, Methanosarcina, Candidatus Methanofastidiosum and Methanolinea.CONCLUSIONOverall, GAC addition seems a promising strategy to increase methane production in anaerobic digesters treating dairy wastewater. © 2024 The Author(s). Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Interspecies electron transfer regulation in a stage-separated anaerobic digestion system: The role of inoculum strategies, activated carbon integration, and organic loading rate adjustment

Understanding and regulation of interspecies electron transfer, including hydrogen-mediated interspecies electron transfer (MIET) and direct interspecies electron transfer (DIET), is crucial for enhancing anaerobic digestion (AD). In this study, MIET and DIET were regulated in a multistage anaerobic hythane reactor (MAHR) via inoculum management, activated carbon (AC) addition, and organic loading rate (OLR) adjustment. The operational stability, organic removal, and methane production can be enhanced, with AC addition in the methane production zone (Mm) (RpCMm and RnpCMm). The reactors of RpCMm and RnpCMm achieved start-up within 20 days and showed a 20 % and 104 % increase in maximum methane production rates, respectively. Microbial community characteristics confirmed that AC in Mm could facilitate DIET establishment, but its addition in Mh could not. This assertion was supported by the observed tighter and more positive correlation between biofilm conductivity and DIET-related microbes in RpCMm and RnpCMm, as opposed to RpCMh and RnpCMh. Further, the dual drive of DIET and MIET was revealed through the comprehensive analysis of operational performances and microbial community with the structure equation model. The electron fluxes analysis highlighted a dynamic shift of DIET and MIET in the methanogenic stage with AC addition and OLR variation. The DIET was proved more efficient than MIET in Mm with AC addition and OLR below 40 g COD/L/d, while high OLR (80 g COD/L/d) favored MIET with lower methane production efficiency. The results of this study shed light on the nuanced characteristics of interspecies electron transfer, providing valuable insights for the further design and operation of stage-separated AD.

Unlocking potential: Exploring the methane production potential in anaerobic co-digestion of cassava wastewater and glycerol

This study aimed at evaluating the feasibility of CH4 production in a mesophilic (30 °C) single-stage anaerobic co-digestion of cassava wastewater (CW) and glycerol in an anaerobic fluidized bed reactor (AFBR) with mixing ratio of 50 % / 50 % CW/Gly on a (chemical oxygen demand)COD basis. Thus, the following strategy was used: increasing the organic loading rate (OLR) (1–20 g COD.L−1.d−1) by increasing the substrate concentration (1–20 g COD.L−1) and applying a hydraulic detention time (HRT) of 24 h. The AFBR presented a maximum methane (CH4) content of 88.38±1.94 % and a CH4 yield (MY) of 293.48±18.37 mL of CH4.mL−1CODrem in the OLR of 1 g COD.L−1.d−1 and the maximum methane production rate (MPR) of 62.09±2.75 mL of CH4.L−1.h−1 in the OLR of 20 g COD.L−1.d−1. The conversion of glycerol and carbohydrates remained above 91±0.54 % and 82±2.79 %, respectively, and the COD conversion was above 80±0.65 %. The main metabolites were HAc, HPr, HBu, EtOH and HVa. The most abundant genera identified in the AFBR were Izemoplasmatales and Enterobacteriaceae for the bacteria domain, and Methanosaeta, Methanobacterium for the archaea domain. The single-stage system is robust and can operate with higher OLR without reducing the efficiency of wastewater treatment and CH4 production.

Optimizing the Mixing Ratios of Source-Separated Organic Waste and Thickened Waste Activated Sludge in Anaerobic Co-Digestion: A New Approach

Anaerobic co-digestion (AnCoD) presents several advantages over conventional mono-digestion. Various factors can impact the efficiency of the co-digestion process, including the mixing ratio of the feedstocks. This study primarily investigates the effects of different mixing ratios on methane production during the co-digestion of source-separated municipal organic waste (SSO) with thickened waste activated sludge (TWAS). While the C/N or COD/N ratio has generally been used for optimizing the mixing ratios of co-digested feedstocks, a new approach is introduced in this study to evaluate the effects of the lipid, protein, and carbohydrate (L:P:C) ratios on the efficiency of AnCoD with respect to methane production, kinetics, and synergism at mixing ratios of TWAS:SSO of 10:90, 30:70, 50:50, 70:30, and 10:90. AnCoD improved methane production and kinetics relative to TWAS at all mixing ratios, the highest of which was at the 10:90 ratio, corresponding to a methane yield, maximum methane production rate, and an L:P:C ratio of 353 mL CH4/g COD, 25 mL CH4/g COD/d, and 8:1:18, respectively. Improvements in methane yields and kinetics due to synergy were evident at all mixing ratios, with improvements in methane yields ranging from 11 to 23% and improvements in kinetics ranging from 18 to 58%. Improvements in methane yields and kinetics were insensitive to the feedstock composition beyond the 50:50 mixing ratio.

Enhancement of Anaerobic Digestion from Food Waste via Ultrafine Wet Milling Pretreatment: Simulation, Performance, and Mechanisms

Particle size reduction is a commonly used pretreatment technique to promote methane production from anaerobic digestion (AD) of food waste (FW). However, limited research has focused on the effect of micron-sized particles on AD of FW. This research presented an ultrafine wet milling (UFWM) pretreatment method to reduce the particle size of FW particles. After four hours of milling, D90 was reduced to 73 μm and cumulative methane production boosted from 307.98 mL/g vs. to 406.75 mL/g vs. without ammonia inhibition. We evaluated the performance of the AD systems and explored their facilitation mechanisms. Kinetic analysis showed that the modified Gompertz model predicted experimental values most accurately. UFWM pretreatment increased the maximum methane production rate by 44.4% and reduced the lag time by 0.65 days. The mechanical stress and collisions of milling resulted in a scaly surface of the particles, which greatly increased the voids and surface area. A rise in the XPS peak area of the C–N and C=O bonds proved the promotion of the liberation of carbohydrates and fats. Further microbial community analysis revealed that the relative abundance of Bacteroidota and Methanosarcina were enriched by UFWM. Meanwhile, methane metabolism pathway analysis confirmed that module M00567, module M00357, and related enzymes were stimulated. This study provided a theoretical basis for UFWM pretreatment applications and improvements in AD of FW.

A cathodic electro-fermentation system for enhanced methane production from high-concentration potato starch industrial wastewater

The high chemical oxygen demand (COD) concentration of potato starch industrial wastewater makes it very challenging for traditional anaerobic digestion (AD) to treat. Here we proposed to use the cathodic electro-fermentation system to enhance the methane production from potato starch industrial wastewater (7546 mg·L−1 COD). Compared with the traditional AD, the electro-fermentation system enhanced the maximum methane production rate more than three times and increased the cumulative methane production by 30 %. What's more, the lag phase of the AD was dramatically reduced, and the COD degradation rate was significantly improved in the electro-fermentation system. The better performance could be attributed to the fact the electro-fermentation system could avoid the accumulation of the intermediate metabolites (e.g. volatile fatty acids (VFAs)) and maintain a suitable pH for methanogens. Additionally, the applied potential at the cathode enhanced the acidification and methanation process and enabled a more stable microbial community. This study demonstrated that electro-fermentation is an efficient way to treat high-concentration potato starch industrial wastewater for methane production.

Biochar and modified magnetic biochar enhanced anaerobic digestion of swine wastewater under ammonia stress: Performance and microbial dynamics

This study aimed to investigate the effectiveness and mechanism of biochar and modified magnetic biochar addition in improving the anaerobic digestion (AD) efficiency of swine wastewater under ammonia stress. Two batches of serial experiments were carried out with ammonia levels of 3000, 4500, and 6000 mg/L. The results indicated that a one-time addition of 15 g/L of both types of carbon materials could sustainably optimize the AD performance of swine wastewater at ammonia nitrogen levels of 3000 and 4500 mg/L. In the second batch of experiments, at 4500 mg/L ammonia, groups with C and FeC additions showed 6.5%, 16.2% higher cumulative methane yield and 45.5%, 60.0% greater maximum gas production rates than controls. However, under the ammonia stress condition of 6000 mg/L, the reactor exhibited a mismatch between the carbon material addition and the degree of stress. The maximum methane production rate was intensified only in the first batch, which was 71.8% and 105.1% higher in the C and FeC groups, respectively, than in the control group. The microbiological analysis revealed that biochar optimized the AD performance by enriching the abundance of fermentative bacteria and enhancing the activity of Methanosaeta. In contrast, modified magnetic biochar promoted the enrichment of acetogens and shifted the dominant methanogens from sensitive Methanosaeta to the highly ammonia-tolerant Methanosarcina, thus causing the optimization of AD performance. The findings of this study provided new insights into the mitigation of inhibition and enhancement of efficacy in the anaerobic digestion of swine wastewater.

Comparing the mechanisms of syntrophic volatile fatty acids oxidation and methanogenesis recovery from ammonia stress in regular and biochar-assisted anaerobic digestion: Different roads lead to the same goal

Biochar has been recognized as a promising additive to mitigate ammonia inhibition during syntrophic methanogenesis, while the key function of biochar in this process is still in debates. This study clarified the distinct mechanisms of syntrophic volatile fatty acids -oxidizing and methanogenesis recovery from ammonia inhibition in regular and biochar-assisted anaerobic digestion. Under 5 g/L ammonia stress, adding biochar shortened the methanogenic lag time by 10.9% and dramatically accelerated the maximum methane production rate from 60.3 to 94.7 mLCH4/gVSsludge/d. A photometric analysis with a nano-WO3 probe revealed that biochar enhanced the extracellular electron transfer (EET) capacity of suspended microbes (Pearson's r = −0.98), confirming that biochar facilitated methanogenesis by boosting EET between syntrophic butyrate oxidizer and methanogens. Same linear relationship between EET capacity and methanogenic rate was not observed in the control group. Microbial community integrating functional genes prediction analysis uncovered that biochar re-shaped syntrophic partners by enriching Constridium_sensu_stricto/Syntrophomonas and Methanosarcina. The functional genes encoding Co-enzyme F420 hydrogenase and formylmethanofuran dehydrogenase were upregulated by 1.4–2.3 times, consequently enhanced the CO2-reduction methanogenesis pathway. Meanwhile, the abundances of gene encoding methylene-tetrahydrofolate transformation, a series of intermediate processes involved in acetate oxidation, in the biochar-assisted group were 28.2–63.7% higher than these in control group. Comparatively, Methanosaeta played a pivotal role driving aceticlastic methanogenesis in the control group because the abundance of gene encoding acetyl-CoA decarbonylase/synthase complex increased by 1.9 times, suggesting an aceticlastic combining H2-based syntrophic methanogenesis pathway was established in control group to resist ammonia stress. A 2nd period experiment elucidated that although depending on distinct mechanisms, the volatile fatty acid oxidizers and methanogens in both groups developed sustained and stable strategies to resist ammonia stress. These findings provided new insights to understand the distinct methanogenic recovery strategy to resist toxic stress under varied environmental conditions.

Boosting anaerobic digestion of long chain fatty acid with microbial electrolysis cell combining metal organic framework as cathode: Biofilm construction and metabolic pathways

The role of metal organic framework (MOF) modified cathode in promoting long chain fatty acid (LCFA) methanation was identified in microbial electrolysis cell coupled anaerobic digestion (MEC-AD) system. The maximum methane production rate of MEC-AD-MOF achieved 49.8 ± 3.4 mL/d, which increased by 41 % compared to MEC-AD-C. The analysis of bio-cathode biofilm revealed that microbial activity, distribution, population, and protein secretion prompted by MOF cathode, which in turn led to an acceleration of electron transfer between the cathode and microbes. Specifically, the relative abundance of acetate-oxidizing bacterium (Mesotoga) in MEC-AD-MOF was 1.5–3.6 times higher than that in MEC-AD-C, with a co-metabolized enrichment of Methanobacterium. Moreover, MOF cathode reinforced LCFA methanation by raising the relative abundance of genes coded key enzymes involved in CO2-reducing pathway, and elevating the tolerance of microbes to LCFA inhibition. These results indicate that MOF can enhance biofilm construction in MEC-AD, thereby improving the treatment performance of lipid wastewater.

Residence Time Reduction in Anaerobic Reactors: Investigating the Economic Benefits of Magnetite-Induced Direct Interspecies Electron Transfer Mechanism

Existing research on direct interspecies electron transfer (DIET) has predominantly focused on the types and concentrations of conductive materials across diverse anaerobic digestion. However, insufficient understanding of the impact of residence time, a critical economic factor, prompted this investigation. Magnetite, a conductive material, was introduced into the anaerobic digestion of food wastewater, leading to a significant increase in ultimate methane production (Bu) with 25 mM-Fe3O4 (p < 0.05). Despite a subsequent decline in methane production efficiency from 388.9% to 7.1% over the 15- to 65-day anaerobic digestion period, the initial impact of increased methane production due to magnetite addition was evident. Control’s maximum methane production rate (Rm) was 27.5 mL/day, reaching its highest point at 37.4 mL/day with 15 mM-Fe3O4, accompanied by a noteworthy 56.6% reduction in the attainment day of Rm (Rm-day), shortened to 8.2 days. Even with 100 mM-Fe3O4, while Bu showed no significant difference, Rm-day exhibited a substantial reduction of 22.8. Despite the lower overall anaerobic digestion efficiency under some magnetite input conditions, this study confirmed a substantial shortening of Rm-day, suggesting that the DIET mechanism induced by conductive materials such as magnetite could reduce the residence time in continuous-type anaerobic reactors, contributing to improved economic feasibility.

Evaluation of batch mesophilic anaerobic digestion of waste Euro banknotes for methane Production: Preliminary studies and kinetic approach

The European Central Bank is striving to find environmentally friendly alternative methods of waste disposal. In 2020, it decided to end the disposal of Waste Euro Banknotes (W€B) in landfills and planned to use them for recycling and/or energy recovery. Despite being recognized as an effective tool in a circular economy model, there are no reported studies on the use of W€B as a substrate in anaerobic digestion (AD). Thus, the aim of this research was to assess the viability of W€B to be converted into high-value-added energy products (mainly methane) through AD. W€B (€10 and €20 denominations) provided by the Bank of Spain were used. Biochemical methane potential (BMP) tests of pre-treated (grinded) and untreated W€B were assessed at mesophilic temperature. The ultimate methane yield was considerably higher for pre-treated W€B (334 ± 23 NmL CH4 g VS-1added) than for untreated W€B (297 ± 14 NmL CH4 g VS-1added). The Logistic or Sigmoidal kinetic model adequately fit the experimental data and allowed for obtaining the kinetic parameters of each case studied. In this sense, an increase of 22.4 % in the maximum methane production rate (Rmax) was observed for the pre-treated W€B (52.5 ± 0.9 mL CH4 (g VS·d)-1 compared to the untreated W€B (16.2 ± 1.8 mL CH4 (g VS·d)-1). According to the obtained results, AD may be a good alternative for the energetic valorization and recycling of W€B.

Mechanistic insights into synergistic facilitation of copper/zinc ions and dewatered swine manure-derived biochar on anaerobic digestion of swine wastewater

Biochar-assisted anaerobic digestion (AD) has been proposed an advanced system for swine wastewater (SW) management. However, the effects of metallic nutrients in SW, such as copper/zinc ions (Cu2+/Zn2+), on the biochar-assisted AD of SW are not well understood. This study investigated the influences of individual Cu2+/Zn2+ or dewatered swine manure-derived biochar, as well as their combined additions, on the AD of SW. The results showed that exposure to 50 mg/L Cu2+/Zn2+ temporary inhibited methane production, but the addition of 20 g/L biochar alleviated this inhibition by shortening the methanogenic lag time and increasing methane yield. Following a period of acclimation, both Cu2+/Zn2+ and biochar promoted methane production, although metagenomic analysis revealed distinct mechanisms underlying their promotion. Cu2+/Zn2+ enhanced ATP processing, including electron exchange between NADH/NAD+ and succinate/fumarate transformation, by 26.0–35.8%. Additionally, the gene encoding Coenzyme M methylation was upregulated by 36.2% along with enrichments of Methanocullus and Methanosarcina, contributing to accelerated hydrolysis and methanogenesis rates by 54.7% and 44.8%, respectively. On the other hand, biochar mainly stimulated bacterial F-type ATPase activities by 28.4%, likely facilitating direct interspecies electron transfer between Geobacter and Methanosarcina for syntrophic methanogenesis. The combined addition of Cu2+/Zn2+ and biochar resulted in “win-win” benefits, significantly increasing the maximum methane production rate from 40.3 mL CH4/d to 53.7 mL CH4/d. Moreover, the introduction of biochar into AD of SW facilitated the transformation of more Cu2+/Zn2+ from a reducible Fe–Mn oxides form to a residual form, which potentially reduced the metallic toxicity of the digestate for soil amendment. The findings of this study provide novel insights into understanding the synergistic impacts of heavy metals and biochar in regulating SW during AD, as well as the management of associated digestate.