Cumulative Methane Production Research Articles

Microbial mechanism underlying the effect of biochar supported nano zero-valent iron on the anaerobic digestion of food waste

Anaerobic digestion (AD) is one of the effective methods to treat the food waste (FW). In this study, biochar supported nano zero-valent iron (BC-nZVI) was prepared by carbon thermal reduction process. The effects of concentrations (1, 2 and 3 g/L) of BC-nZVI on methane production, microorganisms and metabolic mechanism during the AD of food waste were investigated. The results showed that 1 g/L of BC-nZVI was the best concentration for methane production. The cumulative methane production was enhanced by 29.54% at BC-nZVI 1 g/L. Microbial community analysis showed that BC-nZVI mainly increased the abundances of hydrolytic bacteria (including Bacteroidetes_vadinHA17, Proteiniphilum, Syntrophobacter) and acetoclast (Methanosaeta). Further exploration of the mechanism using PICRUSt2 version based on 16 S rRNA gene sequence analysis indicated that the addition of BC-nZVI promoted the abundances of enzymes related to methane metabolism such as the phosphate acetyltransferase and the methyl-coenzyme M reductase alpha subunit, which made the AD system stable.

Comprehensive study of the combined effects of biochar and iron-based conductive materials on alleviating long chain fatty acids inhibition in anaerobic digestion

This study investigated the feasibility of alleviating the negative influence of long-chain fatty acids (LCFAs) on anaerobic digestion by biochar, micron zero-valent iron, micron-magnetite (mFe3O4) and their combination. The results demonstrate that co-addition of biochar and 6 g/L mFe3O4 (BC+6 g/L mFe3O4) increased cumulative methane production by 50% as suffered from LCFAs inhibition exerted by 2 g/L glycerol trioleate. The BC+6 g/L mFe3O4 did best in accelerating total organic carbon degradation and volatile fatty acids conversion, through successively enriching Bacteroides, Corynebacterium, and DMER64 to dominant the bacterial community. The proportion of acetotrophic Methanothrix that could alternatively reduce CO2 to methane by accepting electrons via direct interspecies electron transfer (DIET) was 0.09% with BC+6 g/L mFe3O4, nine times more than the proportion in control. Prediction of functional genes revealed the enrichment of the bacterial secretion system, indicating that BC+6 g/L mFe3O4 promoted DIET by stimulating the secretion of extracellular polymeric substances. This study provided novel insights into combining biochar and iron-based conductive materials to enhance AD performance under LCFAs inhibition.

Effects of biochar, granular activated carbon, and magnetite on the electron transfer of microbials during the anaerobic digestion process: Insights into nitrogen heterocyclic compounds degradation

Anaerobic digestion (AD) of the hydrothermal liquefaction aqueous phase is often inhibited by high levels of nitrogen heterocyclics, which hinder its valorization. Despite the potential benefits of conductive materials in promoting the degradation of toxic compounds through the establishment of direct interspecies electron transfer (DIET), there is still a gap in the understanding of their role and related mechanisms in nitrogen heterocyclic degradation. In this study, the AD process was improved by the addition of conductive materials. Subsequently, the cumulative methane production increased by 7.2% (with biochar), 15.4% (with granular activated carbon), and 24.3% (with magnetite). Besides, The addition of conductive materials significantly improved the complexity and stability of the microbial communities, especially the DIET potential microorganisms—Methanosaetaceae and Syntrophobacteraceae, Anaerolineaceae, Clostridiaceae, Geobacteraceae, Desulfovibrionaceae. Thermodynamic analysis and pyridine degradation pathways further supported enhanced pyridine degradation via the addition of conductive materials.

Investigation into the treatment and resource recovery of rifamycin mycelial dreg with thermal alkaline pretreatment-anaerobic digestion

Rifamycin, a highly potent drug for the treatment of tuberculosis should raise environmental concerns due to its massive production of mycelial dreg (RMD). The RMD treatment by anaerobic digestion (AD) is a highly promising endeavor, but has still far from explicit. Herein, the thermal alkaline pretreatment (TAP)-AD process with RMD as substrate was performed. The optimal temperature, pH, and reaction time for TAP were 140 °C, 12, and 3 h, respectively. The optimal RMD concentration (1%) and inoculation ratio (2:1, sludge: RMD, mass ratio) were subsequently determined, leading to an increase of 86.4% in cumulative methane production. Correspondingly, a functional core taxa including fermentative bacteria and methanogens was enriched. Molecular ecological network (pMEN) revealed an ecological niche-based functional redundancy, which facilitated the efficiency and stability of the AD system. Notably, representative draft genome recovered from raw RMD possessed the potential fermentation capability, and the phosphotransferase component for carbohydrate transport may not be affected by rifamycin-B according to the results of molecular docking. Taking these factors, an innovative insight into the natural suitability of RMD as native fermentation inoculum was carefully presented. This study clarifies the feasibility of TAP-AD in the treatment of RMD. Moreover, it contributes to a deeper understanding of the ecological properties and microbial mechanisms in AD systems.

Effects of thermal pretreatment and biotin supplementation on the anaerobic digestion of pineapple (Ananas comosus) wastes, kinetics of methane production, and statistical analysis

The anaerobic digestion (AD) of pineapple wastes (PW) was employed to understand the effects of design parameters, namely thermal pretreatment (TP) temperature (X1) and reaction time (X2), and biotin concentration (X3), to methane production. Kinetic parameters were evaluated by utilizing sigmoidal growth functions, whereas modified Gompertz (MGM) and Logistic function (LF) best described the system among all batch assays. Principal components analysis (PCA) and Pearson correlation matrix were used to understand the correlation and relationship between the design parameters, evaluated kinetic parameters, methane yield (Y), and process performance parameters, wherein it was found that the significantly statistical correlations were Rm-A, λ-X1, A-Y, A-%VS reduction, A-%pH change, Rm-Y, Y-%VS reduction, Y-%pH change, and %VS reduction-%pH change. Furthermore, the cumulative methane production profile in all reactors showed an increase of 19.2–169.2 % in Y compared to the control, suggesting that TP and biotin supplementation were suitable for the optimization of Y from the AD of PW. The Y among all reactors followed the order of A0 <A1 <A2 <A4 <A7 <A3 <A5 <A8 <A6 wherein the lowest Y from pretreated PW was observed at A1 (77.6478 mL/gVS), while the highest was recorded at A6 (175.3053 mL/gVS). The ANOVA results demonstrated that the contribution of main and interaction parameters to Y (X3 >X1X3 >X2X3 >X1 >X2 >X1X2) were statistically significant (p < 0.05), and therefore relevant in the microbial syntrophies during the AD of PW. From the regression analysis, a surrogate mathematical model was constructed with a goodness of fit R2adj= 0.998. Herein, the optimum design conditions in the methane production from AD of PW were found to be X1 = 80ºC, X2 = 120 min, and X3 = 0.25 mg/L.

Hydrothermal carbonization of food waste: Process parameters optimization and biomethane potential evaluation of process water

Food waste (FW) is one of the major biomasses produced in large quantities in urban areas, which contributes to more than one-third of global greenhouse gas emissions. FW must be properly managed to minimize its environmental consequences. Hydrothermal carbonization (HTC) of FW is a promising technology compared to conventional methods. The objective of the present study is to maximize the mass yield (MY), higher heating value (HHV) and energy yield (EY) of FW by optimizing the operational variables of HTC process. Additionally, process water generated during HTC of FW under optimal conditions was evaluated for methane yield using anaerobic digestion. To optimize the HTC process, three operational variables, including solid-to-liquid (S/L) ratio, temperature, and reaction time, were manipulated using response surface methodology (RSM). According to RSM studies, the optimum operating conditions are 198.5 °C for 150 min with a 0.2 S/L ratio, resulting in MY, HHV and EY as 62.5%, 21.24 MJ/kg and 81.71%, respectively. Proximate and elemental analysis for the hydrochars synthesized at various operating conditions reveals that the temperature and reaction time have a significant impact on fixed carbon and carbon percentage. The anaerobic digestion results showed that the combination of process water and hydrochar, yielded a maximum cumulative methane production of 298.5 ± 16.34 mL/g COD. To mimic methane production, the modified Gompertz model was utilized. Thus, this finding contributes towards the commercialization of the HTC process to produce solid fuel (hydrochar) and provides a way to find an alternative energy source that enhances the HTC process and tackles the problem of process water disposal.

Investigating the performance of different applied voltages on lignite biomethanation in microbial electrolytic cell coupled anaerobic digestion

The performance of different applied voltages on the biomethanation of lignite is investigated in microbial electrolytic cells coupled with anaerobic digestion (MEC-AD). The cumulative methane production is only 35.1 mL/g lignite in the control reactor. However, the cumulative methane production is increased to 44.3 mL/g lignite in the MEC-AD reactor at the applied voltage of 1 V, and followed by 62.6 mL/g lignite (2 V), 102.8 mL/g lignite (4 V), and 123.8 mL/g lignite (8 V). The 3D fluorescence spectra further clarified that the applied voltage favored the hydrolysis of macromolecular organic matter such as humic acid in lignite. The microbial community abundance in the bulk solution shows that some specific microorganisms such as electroactive bacteria (Chloroflexi) and hydrogenotrophic methanogenic archaea (Methanobacterium) were well enriched in the MEC-AD reactors. These results indicate that MEC-AD facilitates direct interspecies electron transfer (DIET) between electroactive bacteria and archaea, and significantly increases methane production. However, the economic analysis concludes that the methane production at the applied voltage of 8 V could not compensate for the cost incurred by electricity. The MEC-AD reactor with an applied voltage of 4 V showed excellent biomethanation characteristics of lignite, which provides a basis for the resource utilization of lignite.

In vitro CHEMICAL AND FERMENTATIVE CHARACTERISTICS OF TROPICAL PASTURES AT DIFFERENT AGES OF REGROWTH

The low nutritive value of pastures in tropical regions represents a limitation in the productivity of grazing ruminants. The objective was to determine the in vitro chemical and fermentative characteristics of nine tropical grasses (Aruana [Panicum maximun Jacq. cv. Aruana], Bermuda [Cynodon dactylon L.], Estrella de África [Cynodon Cynodon nlemfuensis Vanderyst], Insurgente [Brachiaria brizantha Hochst. Stapf.], Llanero [Andropogon gayanus Kunth], Mombaza [Panicum maximun Jacq. cv. Mombaza], Pará [Brachiaria mutica Stapf], Pangola [Digitaria eriantha Steud.], and Tanzania [Panicum maximun Jacq. cv. Tanzania]) at three regrowth ages (30, 45, and 60 days). The chemical analysis was used to determine the concentrations of dry matter (DM), crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), and ash (Ce). The in vitro trial measured partial (24, 48, and 72 h) and cumulative biogas and methane production, fermentation kinetics estimators (A, b, and k), dry matter degradation (DMD) and detergent neutral fiber degradation (NDFD), pH, and ammonia nitrogen (N-NH3). The experimental design was completely randomized at each regrowth age. In conclusion, based on chemical analysis and fermentative characteristics, Pangola and Estrella pastures performed better at each regrowth age.

Granular activated carbon enhances the anaerobic digestion of solid and liquid fractions of swine effluent at different mesophilic temperatures

ObjectivesThis study evaluated the effect of particle size and dosage of granular activated carbon (GAC) on methane production from the anaerobic digestion of raw effluent (RE) of swine wastewater, and the solid (SF) and liquid (LF) fractions. The effect of temperature using the selected size and dosage of GAC was also evaluated. Methods60 mL of swine wastewater were inoculated with anaerobic granular sludge and GAC at different dosages and particle size. The cultures were incubated at different temperatures at 130 rpm. The kinetic parameters from experimental data were obtained using the Gompertz model. ResultsThe cultures with the LF and GAC (75–150 μm, 15 g/L) increased 1.87-fold the methane production compared to the control without GAC. The GAC at 75–150 μm showed lower lag phases and higher Rmax than the cultures with GAC at 590–600 μm. The cumulative methane production at 45 °C with the RE + GAC was 7.4-fold higher than the control. Moreover, methane production at 45 °C significantly increased with the cultures LF + GAC (6.0-fold) and SF + GAC (2.0-fold). The highest production of volatile fatty acids and ammonium was obtained at 45 °C regardless of the substrate and the addition of GAC contributed to a higher extent than the cultures lacking GAC. In most cases, the kinetic parameters at 30 °C and 37 °C were also higher with GAC. ConclusionsGAC contributed to improving the fermentative and methanogenesis stages during the anaerobic digestion of fractions, evidenced by an improvement in the kinetic parameters.

Improving methane production from palm leaflets waste with thermal and ultrasound pre-treatment

The use of anaerobic digestion (AD) technology for energy recovery from date palm leaflet waste (PLW), is attracting growing interest due to its multiple economic, social and environmental benefits. However, challenges persist in optimizing anaerobic digestion to harness its full potential for efficient methane production. This research aims to address these challenges by identifying a potent pre-treatment approach for enhancing methane generation through anaerobic digestion of palm leaflet waste (PLW) by comparing the results of thermal and ultrasound techniques. Moreover, the experimental methane production obtained from the AD of PLW was adjusted using Logistic and modified Gompertz models. This study showed that the cumulative methane productions were 2341, 1789, and 1461 Nml, respectively, in thermal, ultrasound pre-treatment, and untreated (witness). The results further conclude that by applying high ultrasonic and thermal energies, the biodegradation rate could be enhanced, leading to increased methane production. The simulated maximum of cumulative methane production for both the modified Logistic and modified Gomperz for all sorts of substrate was 1692,18 NmL and 1723,40 NmL, respectively, untreated, ultrasound, and thermal pre-treated. The findings of this study highlight the potential of innovative pre-treatment strategies to elevate anaerobic digestion's efficiency, thereby offering a sustainable solution for waste management and energy production.

Experimental evaluation on the effect of biochar addition with anaerobic digestion of the tannery wastewater to improve biogas production

Abstract The use of biochar as an additive material in various compositions to the anaerobic digestion of the tannery wastewater was investigated. Pyrolytic biochar made from Khat waste with different compositions was added to anaerobic batch digesters in laboratory-scale experiments. Biogas digesters of 500 ml volume were used to explore the effects of biochar on anaerobic digestion in the ratios of 5, 10, 15, and 20% (w/w) at constant mesophilic temperature (38 °C). The results show that for five experimental tests in digesters (D0, D1, D2, D3, and D4), which contains 0, 5, 10, 15, and 20% of biochar, the cumulative methane and biogas yields were, respectively, 17.04, 41.2, 43.8, 51.6, 48.6% and 150, 520, 680, 760, 690ml. Compared to the cumulative methane production efficiency of the digester without biochar (D0), the addition of 5, 10, 15, and 20% biochar had more favorable effects and increased the efficiency by 58.6, 61.1, 66.98, and 64.94%, respectively. From these results, adding 15% of biochar to anaerobic digestion causes a three-fold increase in methane compared to the control and more positive effects than all other biochar compositions. According to this result, the right dose of biochar must be added to maximize biogas production.

Impact of Varying Mass Concentrations of Ammonia Nitrogen on Biogas Production and System Stability of Anaerobic Fermentation

High ammonium release from chicken manure poses a significant limitation to aerobic digestion, impeding microbial processes and inhibiting biogas production. In this study, we conducted anaerobic digestion of a mixture consisting of chicken manure and corn straw as the fermented raw material. The inoculum used was obtained from the residue of previously fermented chicken manure. To assess the inhibitory effect, we varied the ammonia levels within the range of 750–4250 mg/L by introducing ammonium chloride. The efficiency of aerobic digestion was monitored through the measurement of volatile fatty acids (VFA), chemical oxygen demand (COD), total inorganic carbon (TOC), and methane yield. Our results indicated that elevated levels of ammonia nitrogen had a suppressive impact on methane release, and this decrease followed a linear relationship with the increasing ammonia nitrogen load. Moreover, the addition of ammonia led to a slower release, with the maximum daily ammonia concentration observed at 15 days compared to the 6th day at lower ammonia levels. Furthermore, on the 40th day of aerobic digestion, the cumulative methane production at 4250 mg/L was inhibited by 41% compared to the 750 mg/L condition. The patterns of VFA, inorganic carbon, and COD reduction were consistent across all ammonia levels, with VFA and TOC levels being highest at the highest ammonia concentration and lowest at the lowest ammonia concentration. The accumulation of VFA resulted in a decrease in pH and a decline in methanogenic activity. Additionally, high ammonia levels altered the relative abundance of methanogens. Acetoclastic methanogens (Methanosaeta) exhibited a decrease in abundance, while hydrogenotrophic methanogens (Methanosaeta, Methanoculleus) and methylotrophic methanogens (Candidatus Methanoplasma) demonstrated an increase in abundance. Overall, our findings highlight the inhibitory effects of high ammonia concentrations on biogas production, providing insights into the changes in microbial composition and activity during anaerobic fermentation.

Constructing carbon-based materials loaded with MOFs to realize efficient anaerobic digestion of rural organic waste

The application of anaerobic digestion (AD) technology could convert rural organic waste (ROW) into renewable energy such as methane, which can help to mitigate the scarcity of fossil fuels and positively impact the global environment. However, the inhibition of ammonia nitrogen remains a significant obstacle to the methane production process with high concentrations of AD. Hence, in this study, three adsorption materials for ammonia nitrogen, namely FeMn-MOF, FeMn/MOF-CFs, and FeMn/MOF-CFC, were synthesized through distinct protocols. Their ability to mitigate the effect of ammonia nitrogen inhibition was investigated in a Continuous Stirred-Tank Reactor (CSTR) with total solid (TS) concentration of 10% under a semi-continuous operation of ROW digestion system. The results show that the addition of Metal-Organic Frameworks (MOFs) material substantially mitigated the inhibition of ammonia nitrogen and enhanced methane production. Compared with the control group, FeMn/MOF-CFC exhibited the best performance, with a 40.21% decrease in ammonia nitrogen concentration and 66.96 L/L-reactor cumulative methane production. Furthermore, the potential mechanisms underlying microbial community characteristics were explored, indicating that the addition of FeMn/MOF-CFC to AD provides the optimal enhancement of methane production. The addition of FeMn/MOF-CFC can enrich Methanosarcina, enhance the acetoclastic pathway for methane production, and increase the relative activity of coenzyme F420, achieving a 193.68% increase.

Densification of Synechococcus subsalsus biomass by chitosan coagulation for biogas production

ABSTRACT This study evaluated the coagulation/flocculation process using chitosan as a natural coagulant to concentrate suspensions of the cyanobacterium Synechococcus subsalsus and enable biogas production from concentrated biomass. The chitosan performance was tested and compared with the inorganic ferric chloride (FeCl3) coagulant. Using the liquid fraction of the coagulation/flocculation process in subsequent biomass cultivations proved viable, with similar growths in culture media with up to 80% of the liquid fraction. At pH 6 and 400 mg/L FeCl3, the biomass concentrated almost seven times, increasing the total suspended solids (TSS) of the suspension from 0.4–0.6 g/L to 2.6–4.0 g/L. With 80 mg/L chitosan and pH 7, the TSS concentration attained values in the range of 7.0–9.7 g/L, an increase of more than 30 times, clearly showing that chitosan has a much higher capacity for biomass concentration at a lower concentration. A ratio of 0.3 g chitosan/g dry mass of the biomass was established to reach the maximum densification. The production of methane from chitosan-densified biomass proved to be feasible. Chitosan-densified biomass showed a two-phase cumulative methane production when digested, with slower methane production and 23% lower methane yield after 30 days of digestion (207 NmL CH4/g CODi) compared to the biomass from cultivation (non-densified, 270 NmL CH4/g CODi). However, optimizing the digestion conditions of the densified biomass should increase the methane yield and reduce process time.

Cold isotherm pressing pretreatment for effective improvement of methane production from Hybrid Pennisetum

Ultra-high pressing pretreatment has been employed to effectively improve methane production from Hybrid Pennisetum. In this study, Hybrid Pennisetum was pretreated by cold isostatic pressing (CIP) to improve methane production. The pretreatment enhanced the accessibility between the substrate and inoculum. The decomposition of the compact biomass structure was analyzed through Fourier Transform Infrared Spectrometer (FT-IR) and X-ray Diffractometer (XRD). Moreover, the electron transfer capability in the system was evaluated by electrochemical analysis. The systematic experiment yielded the highest methane production at a pressure of 400Mpa, reaching 227.67 mL/g VS. The pretreatment changed the reducing sugar content (RS), pH, and volatile fatty acids (VFAs) in the AD system, which in turn increased the cumulative methane production. This study provides a promising platform for the development of an efficient methanogenic pretreatment.

Critical evaluation of biochar effects on methane production and process stability in anaerobic digestion

Biochar is an emerging biomaterial for managing residual biomass while simultaneously sequestering carbon. To extend the biochar value chain, applying biochar to enhance anaerobic digestion (AD) processes is gaining attention in the context of a circular economy and cascading use of biomass. However, the comparative effects of various biochar dosages under normal and severe AD conditions are still unclear. To further our understanding of its potential application, this work investigated the impact of adding various biochar dosages on AD processes under normal and high substrate loadings. Three inoculum-to-substrate ratios (ISRs): one representing normal substrate loading (ISR 2) and two representing substrate overloading (ISR 1 and 0.5) were investigated. Each substrate loading rate was tested with a biochar dosage of 0% (control), 10%, and 25% based on substrate volatile solids. The results revealed that under the severe condition of high substrate overload (ISR 0.5), a high biochar dosage of 25% significantly increased cumulative methane production by 5.6% (p = 0.06) when compared to the control. Under the same condition (ISR 0.5, 25%), the time required to achieve a particular extent of ultimate methane potential was significantly reduced (p = 0.04), indicating that the methane production rate was increased. At ISR 0.5, the increase of process stability was also significant with 25% biochar addition, while the control (0%) and 10% biochar addition exhibited high variance among replicates. However, biochar did not affect AD processes under normal substrate loading (ISR 2) and mild substrate overload (ISR 1). Thus, a positive effect of biochar on the AD process was only observed under severe conditions with the highest biochar dosage. Future works should consider optimising substrate loadings and biochar dosages under real conditions when testing the practical application of biochar addition in AD processes.

Effects of cold isostatic press pretreatment of rice straw on microstructure and efficiency of anaerobic digestion for methane production

In this study, rice straw was pretreated using by cold isostatic pressure to disrupt its microstructure and improve the performance of anaerobic digestion, and the optimal process parameters were optimized using the response surface methodology. The results showed that cold isostatic pressure pretreatment under optimal conditions (pressure of 400 MPa and holding time of 9 min) was effective in disrupting the structure of rice straw and improving its biodegradability. The cumulative methane production of the cold isostatic pressure pretreatment group increased by 76% compared to the untreated group. In addition, microbial community analysis showed that the relative abundance of Firmicutes, Halobacterota, DMER64 and Methanosaeta was higher in groups pretreated with cold isostatic pressure than in untreated groups. This study demonstrated the potential of pretreatment of rice straw with cold isostatic pressure to increase methane production during anaerobic digestion.

Green and chemical-free pretreatment of corn straw using cold isostatic pressure for methane production

In this study, the effect of cold isostatic pressure (CIP) pretreatment on the physicochemical properties and subsequent anaerobic digestion (AD) performance of corn straw (CS) was explored. The CS was subjected to CIP pretreatment by pressures of 200, 400 and 600 MPa, respectively, while AD was carried out at medium temperature (35 ± 2 °C). The results showed that CIP pretreatment disrupted the dense structure of the CS and altered the crystallinity index and surface hydrophobicity of the CS, thereby affecting the AD process. The presence of CIP pretreatment increased the initial reducing sugar concentration by 0.11–0.27 g/L and increased the maximum volatile fatty acids content by 112.82–436.64 mg/L, which facilitated the process of acidification and hydrolysis of the AD. It was also observed that the CIP pretreatment maintained the pH in the range of 6.37–7.30, maintaining the stability of the overall system. Moreover, the cumulative methane production in the CIP pretreatment group increased by 27.17 %–64.90 % compared to the control group. Analysis of the microbial results showed that CIP pretreatment increased the abundance of cellulose degrading bacteria Ruminofilibacter from 21.50 % to 27.53 % and acetoclastic methanogen Methanosaeta from 45.48 % to 56.92 %, thus facilitating the hydrolysis and methanogenic stages. The energy conversion analysis showed that CIP is a green and non-polluting pretreatment strategy for the efficient AD of CS to methane.

Co-production of biohydrogen and biomethane utilizing halophytic biomass Atriplexcrassifolia by two-stage anaerobic fermentation process.

The excessive use of fossil has resulted in the drastic exhaustion of natural energy sources, leading to environmental challenges and energy crises. Owing to rising energy demand there is a dire need to shift towards renewable energies from lignocellulosic biomass. The present study assessed the co-production of biohydrogen (H2) and biomethane (CH4) by utilizing a less explored halophyte Atriplexcrassifolia. Various reaction parameters were evaluated for their effect on biohydrogen and biomethane production in batch experiments. One parameter at a time experimental strategy was chosen for production optimization. Hydrogen and methane yields along with their production rates were assessed at different incubation times, temperatures, pH, substrate concentrations, and inoculum sizes in acidogenesis and methanogenesis stages, respectively. In the first stage, maximum cumulative hydrogen production of 66 ± 0.02mL, with hydrogen yield of 13.2 ± 0.03mL/g, and hydrogen production rate (HPR) of 1.37 ± 0.05mL/h was attained when the reaction mixture (5g Atriplexcrassifolia and 10mL pretreated sewage sludge) was processed at 37°C and pH 5.5 after 48h of incubation. While in the second stage, maximum cumulative methane production, i.e., 343 ± 0.12mL, methane yield (MY) of 8.5 ± 0.07mL/mL, and methane production rate (MPR) of 0.8 ± 0.05mL/h was achieved after 18days of incubation of reaction mixture (40mL of hydrogenic slurry with 80mL inoculum) at 45°C and pH 8. Furthermore, a 51% and 24% rise in biohydrogen and biomethane production respectively were recorded when the gases were produced at these optimized reaction conditions. The results ensure halophyte Atriplexcrassifolia as an imperative renewable energy resource and proposed that effective optimization of the process further increased the coproduction of biohydrogen and biomethane.

Heat-enhanced sulfite pretreatment improves the release of soluble substances and the stimulation of methanogenic pathways for anaerobic digestion of waste activated sludge

Waste activated sludge pretreatment is effective in improving sludge hydrolysis rate and methane production. Our previous study revealed that sulfite could be a viable method for sludge pretreatment. To further improve methane production, this study employed an integrated strategy by combining sulfite and heat pretreatment. WAS from a local full-scale plant was pretreated with heat-enhanced sulfite pretreatment (HESP) at 25, 35, 55 °C. Compared with the control (no sulfite, 25 °C), soluble substances (e.g., chemical oxygen demand, protein, polysaccharides, nitrogen and phosphorus) could be significantly enhanced by the absence of sulfite and the increasing temperature. A variation of − 8.32–19.90% in cumulative methane production was observed. Moreover, methane production could be significantly enhanced by 14.15% at sulfite of 300 mg S/L and temperature of 35 °C, while kinetic analysis showed that the highest methane production potential (1.43 times of the control) was obtained at this level. Besides, the microbial structure and pathways reveal that HESP promotes the enrichment of methanogenic bacteria and facilitates methanogenic metabolism. The findings of this study suggest that the HESP could be promising in enhancing methane production, particularly considering the reuse of sulfite-laden industrial wastes and the heat as the by-products of the anaerobic digestion plant with biogas power generation, but certain experimental conditions should be verified case by case.