CH4 Production Rate Research Articles

Carbon dioxide conversion to acetate and methane in a microbial electrosynthesis cell employing an electrically-conductive polymer cathode modified by nickel-based coatings

In this study, CO2 conversion to acetate and CH4 was achieved in a flow-through laboratory-scale microbial electrosynthesis (MES) cell composed of a 3D conductive polylactic acid (cPLA) lattice cathode with electrodeposited metal electrocatalyst coatings. The MES cell with a bare cPLA cathode showed the poorest performance with the lowest H2 and CH4 production rates and low Coulombic efficiency. This was ascribed to a poor electrocatalytic activity of cPLA towards H2 production and high electrode resistivity. When the cPLA electrode was modified with metal coatings, the CH4, acetate and H2 production rate increased significantly, with the following trend: cPLA < Ni < NiFe < NiFeMn. The better performance of the metal-coated cPLA in terms of CH4 production was attributed to the lower electrical resistance, enhanced H2 production and enhanced electron transfer between the cathode and the biofilm. At the cell potential of 2.8 V, the best-performing NiFeMn cPLA cathode showed stable production of CH4 (50 ± 6 mL d−1), acetate (185 ± 27 mg d−1), and H2 (545 ± 175 mL d−1) at close to 100% Coulombic efficiency.

Effect of low digestate recirculation ratio on biofuel and bioenergy recovery in a two-stage anaerobic digestion process

A relatively high (0.2–4.3) digestate recirculation ratio (RR) is typically adopted to raise the pH and provide the dark fermentation reactor (DF) with alkalinity and hydrogen-producing microorganisms in a two-stage anaerobic digestion process. This study examined the production of bio-H2 and bio-CH4 from readily biodegradable organic waste in a large scale recirculated two-stage thermophilic anaerobic system to determine the effect of low RR on biofuel and bioenergy recovery. The performance of the two-stage system was evaluated at 2 hydraulic retention times (HRT) (1.1 and 2.5 d) in DF and 4 RR (0, 0.11, 0.18 and 0.25). The pH in DF was not controlled and ranged from 3.8 to 4.2. Hydrogen yield was negatively affected by digestate recirculation, while CH4 yield, as well as H2 and CH4 production rates, first tended to increase and then decrease with increasing RR. Overall, biofuel and bioenergy were best recovered at an RR of 0.11, namely 1.48 L H2/L/d, 0.88 L CH4/L/d, 106.2 mL H2/g VSinit.,161.3 mL CH4/g VSinit., 7.7 kJ/g VSinit. and 88.2 kJ/L/d were obtained depending on HRT in DF. It has been shown that a low RR can improve the performance of the two-stage anaerobic digestion process.

Co-metabolic Effect of Glucose on Methane Production and Phenanthrene Removal in an Enriched Phenanthrene-Degrading Consortium Under Methanogenesis.

Anaerobic digestion is used to treat diverse waste classes, and polycyclic aromatic hydrocarbons (PAHs) are a class of refractory compounds that common in wastes treated using anaerobic digestion. In this study, a microbial consortium with the ability to degrade phenanthrene under methanogenesis was enriched from paddy soil to investigate the cometabolic effect of glucose on methane (CH4) production and phenanthrene (a representative PAH) degradation under methanogenic conditions. The addition of glucose enhanced the CH4 production rate (from 0.37 to 2.25mg⋅L−1⋅d−1) but had no influence on the degradation rate of phenanthrene. Moreover, glucose addition significantly decreased the microbial α-diversity (from 2.59 to 1.30) of the enriched consortium but showed no significant effect on the microbial community (R2=0.39, p=0.10), archaeal community (R2=0.48, p=0.10), or functional profile (R2=0.48, p=0.10). The relative abundance of genes involved in the degradation of aromatic compounds showed a decreasing tendency with the addition of glucose, whereas that of genes related to CH4 synthesis was not affected. Additionally, the abundance of genes related to the acetate pathway was the highest among the four types of CH4 synthesis pathways detected in the enriched consortium, which averagely accounted for 48.24% of the total CH4 synthesis pathway, indicating that the acetate pathway is dominant in this phenanthrene-degrading system during methanogenesis. Our results reveal that achieving an ideal effect is diffcult via co-metabolism in a single-stage digestion system of PAH under methanogenesis; thus, other anaerobic systems with higher PAH removal efficiency should be combined with methanogenic digestion, assembling a multistage pattern to enhance the PAH removal rate and CH4 production in anaerobic digestion.

Methane Production from Carbondioxide in Polluted Areas Using Graphene Doped Ni/ NiO Nanocomposite via Photocatalysis

Background: Photocatalysis for the production of solar fuels and particularly to perform CO2 reduction with a sufficiently high efficiency depends to the presence of H2 and absence of H2 O using graphene doped Ni/ NiO nanocomposite. Objective: To examine the methane production from carbondioxide in polluted areas using graphene doped Ni/ NiO nanocomposite via photocatalysis with the use of dimethylaniline and xylene as electron donors. Methods: With 2 mg/l graphene doped Ni/ NiO nanocomposite with a Ni content of 19% wt from 786 μmol/h CO2 gas 563 μl CH4 /g Ni.h was obtained at 160oC temperature and the quantum yield was detected as 1.99%. It was found that H2 O has a negative influence on the photocatalytic activity. Under continuous flow operation, water molecules were easier desorbed from the graphene doped Ni/NiO photocatalyst Results: The maximum CH4 production rate was 650 μl/h for 2.4 mg of graphene doped Ni/ NiO nanocomposite after a detectione time of 17 min. Dimethylaniline and xylene were used as electron donors and 1.2 ml/l dimethylaniline and 0.9 ml/l xylene enhanced the CH4 productions by 8% and 12% as quenching factor. Conclusion: Photocatalysis methods was effected for methane production from carbondioxide in polluted areas using graphene doped Ni/ NiO nanocomposite with the use of dimethylaniline and xylene as electron donors.

EPR Investigation on Electron Transfer of 2D/3D g‐C3N4/ZnO S‐Scheme Heterojunction for Enhanced CO2 Photoreduction

AbstractPhotocatalytic conversion of CO2 into useful products is a promising technology from environmental and economic viewpoints. However, the efficiency of current photocatalytic materials is still unsatisfactory. In this work, a robust S‐scheme heterojunction composed of 3D ZnO hollow spheres wrapped by 2D g‐C3N4 layers is fabricated. The electrostatic attraction between both components not only facilitates the exfoliation of g‐C3N4 layers but also strengthens the photocatalyst framework. The prepared g‐C3N4/ZnO photocatalyst afforded an enhanced CH4 production rate, which is ≈40 and 7 times higher than those of pure ZnO and g‐C3N4, respectively. The improved activity is attributed to the extended light absorption and suppressed charge carrier recombination. Moreover, apart from traditional techniques, electron paramagnetic resonance (EPR) is used to probe charge movement in the fabricated S‐scheme heterojunction. An observable shift of the relevant EPR peak is noticed after the construction of g‐C3N4/ZnO heterostructure, indicating the electron transfer process. This study sheds light on S‐scheme heterojunctions as efficient candidates for CO2 photocatalytic conversion.

Improved Membrane Inlet Mass Spectrometer Method for Measuring Dissolved Methane Concentration and Methane Production Rate in a Large Shallow Lake

Natural water bodies, such as lakes, rivers, and oceans, are important sources of atmospheric methane (CH4). Therefore, quantitative and accurate determination of the dissolved CH4 concentration in water is of great significance for studying CH4 emissions and providing an in-depth understanding of the carbon cycle. Headspace gas chromatography (HGC) is the traditional method for measuring CH4 in water. Despite its long success, it has a lot of problems in use, such as complex pretreatment and a long measurement time, and it is not suitable for the CH4 determination of a large number of samples. In view of these shortcomings, a more convenient and efficient method based on membrane inlet mass spectrometry (MIMS) for quantitative measurements of the dissolved CH4 concentration in water was established. In our study, the standard curves showed that the method had high accuracy, both at low and high CH4 concentrations. After a laboratory test, to evaluate the sensitivity of this method, samples were collected from a large shallow lake (Lake Taihu). Both the HGC method and MIMS method were used to determine the dissolved CH4 to compare these two methods. The small difference in CH4 concentration obtained from the MIMS and HGC methods and the significant correlation between the CH4 concentrations derived from the MIMS method with those derived from the HGC method showed that the MIMS method could replace the HGC method in the determination of dissolved CH4 in natural waters. In addition, we also measured the sediment CH4 production rates in three different areas of Lake Taihu using a laboratory incubation experiment. During the experiment, significant CH4 accumulations were observed, indicating that sediment CH4 production was an important source of dissolved CH4 in the water column. Our study concluded that the MIMS method was sufficient and a better alternative than the HGC method owing to its capacity to measure a broad range of values plus the fact that it was relatively easy to use with less manipulation of the samples.

When dewatered swine manure-derived biochar meets swine wastewater in anaerobic digestion: A win-win scenario towards highly efficient energy recovery and antibiotic resistance genes attenuation for swine manure management

This work explored the feasibility of dewatered swine manure-derived biochar (DSMB) as an additive to facilitate anaerobic digestion (AD) of swine wastewater for energy recovery and antibiotic resistance genes (ARG) attenuation enhancements. With 20 g/L DSMB assistance, the methanogenic lag time of swine wastewater was shortened by 17.4-21.1%, and the maximum CH4 production rate increased from 40.8 mL/d to 48.3-50.5 mL/d, among which DSMB prepared under 300 °C exhibited a better performance than that prepared under 500 °C and 700 °C. Integrated analysis of DSMB electrochemical properties, microbial electron transfer system activity, and microbial community succession revealed the potential of DSMB-300 to act as redox-active electron transfer mediators between syntrophic microbes to accelerate syntrophic methanogenesis via potential direct interspecies electron transfer. Meanwhile, DSMB preparation by pyrolysis dramatically reduced ARG abundance by almost 4 logs. Adding DSMB into AD not only strengthened the attenuation efficiency of ARG in the original swine wastewater, but also effectively controlled the potential risk of horizontal gene transfer by mitigating 74.8% of the mobile gene elements abundance. Accordingly, we proposed a win-win scenario for bio-waste management in swine farms, highlighting the more advanced energy recovery and ARG attenuation compared to the current status.

An electrolytic-hydrogen-fed moving bed biofilm reactor for efficient microbial electrosynthesis of methane from CO2

Microbial electrosynthesis (MES) is an emerging technology for CO2 fixation and renewable energy storage. Currently, the low current density hinders the practical application of this technology. In situ coupling of electrochemical hydrogen production, and hydrogen gas fermentation has been proposed as a promising way to enhance the current density of MES. However, due to the low solubility of hydrogen, how to ensure a high coulombic efficiency (i.e. hydrogen utilization efficiency) under high current density is the main challenge for this type of reactor. Here, we report a novel electrolytic-hydrogen-fed moving bed biofilm reactor (electro-MBBR) reactor for methane production. The reactor consists of an electrochemical cell at the bottom and an MBBR column at the top. The MBBR column prolongs the retention time of the electrolytically-produced hydrogen and enhances the mass transfer of hydrogen. Consequently, the methanogens in the reactor could efficiently convert the electrolytically-produced hydrogen and the externally-supplied CO2 into methane. The 4.5 L reactor achieved a maximum methane production rate of 1.42 L·L−1·d−1 (141.5 L·m−2cat·d−1) at 5 A (111.1 A·m−2cat). The CH4 production rate is more than two times higher than the maximum reported value based on biofilm-driven MES (0.54 L·L−1·d−1, 65 L·m−2cat·d−1). Microbial community analysis showed that the Methanobrevibacter was the dominant methane-producing archaea. These results demonstrated under high current density high coulombic efficiency could be achieved if proper hydrogen retention time and good hydrogen mass transfer were ensured in hydrogen-mediated MES reactors. The electro-MBBR appears to be a promising MES setup for scaling up and practical application.

Carbon nano-layer coated TiO2 nanoparticles for efficient photocatalytic CO2 reduction into CH4 and CO

Solar-driven photocatalytic reduction of CO2 into value-added fuel is an exciting concept that can address serious global challenges such as greenhouse effect and energy crisis. However, the low sunlight utilization by the photocatalysts and their high production costs have greatly hindered their mass adoption. Herein, we proposed a facile and efficient approach to prepare the carbon nano-layer coated TiO2 as an efficient photocatalyst for the photocatalytic reduction of CO2 into CH4 and CO. By pyrolyzing linear low-density polyethylene, a carbon nano-layer was formed on the surface of TiO2. The obtained carbon nano-layer coated TiO2 was able to exhibit superior photocatalytic CO2 reduction activity as compared to its unmodified counterpart, whereby, a CH4 production rate of ~2.30 μmol · g−1 h−1 was achieved (prepared at a dwelling temperature of 600 °C). Meanwhile, a CO production rate of ~16.76 μmol · g−1 h−1 was recorded for the materials prepared at a dwelling temperature of 500 °C. The enhanced CH4 and CO production can be attributed to the elevated absorbance in the visible-light region, decreased band gap and effective restriction of photogenerated electron-hole pair recombination. Thus, based on the results, carbon nano-layer coated TiO2 can be a promising alternative for the photocatalytic reduction of CO2 into valuable fuel for large-scale application.

Decoupling of microbial community dynamics and functions in Arctic peat soil exposed to short term warming.

Temperature is an important factor governing microbe-mediated carbon feedback from permafrost soils. The link between taxonomic and functional microbial responses to temperature change remains elusive due to the lack of studies assessing both aspects of microbial ecology. Our previous study reported microbial metabolic and trophic shifts in response to short-term temperature increases in Arctic peat soil, and linked these shifts to higher CH4 and CO2 production rates (Proceedings of the National Academy of Sciences of the United States of America, 112, E2507-E2516). Here, we studied the taxonomic composition and functional potential of samples from the same experiment. We see that along a high-resolution temperature gradient (1-30°C), microbial communities change discretely, but not continuously or stochastically, in response to rising temperatures. The taxonomic variability may thus in part reflect the varied temperature responses of individual taxa and the competition between these taxa for resources. These taxonomic responses contrast the stable functional potential (metagenomic-based) across all temperatures or the previously observed metabolic or trophic shifts at key temperatures. Furthermore, with rising temperatures we observed a progressive decrease in species diversity (Shannon Index) and increased dispersion of greenhouse gas (GHG) production rates. We conclude that the taxonomic variation is decoupled from both the functional potential of the community and the previously observed temperature-dependent changes in microbial function. However, the reduced diversity at higher temperatures might help explain the higher variability in GHG production at higher temperatures.

An empirical model to predict methane production in inland water sediment from particular organic matter supply and reactivity

AbstractThe highest CH4 production rates can be found in anoxic inland water surface sediments however no model quantifies CH4 production following fresh particular organic matter (POM) deposition on anoxic sediments. This limits our capability of modeling CH4 emissions from inland waters to the atmosphere. To generate such a model, we quantified how the POM supply rate and POM reactivity control CH4 production in anoxic surface sediment, by amending sediment at different frequencies with different quantities of aquatic and terrestrial POM. From the modeled CH4 production, we derived parameters related to the kinetics and the extent of CH4 production. We show that the extent of CH4 production can be well predicted by the quality (i.e., C/N ratio) and the quantity of POM supplied to an anoxic sediment. In particular, within the range of sedimentation rates that can be found in aquatic systems, we show that CH4 production increases linearly with the quantity of phytoplankton‐derived and terrestrially derived POM. A high frequency of POM addition, which is a common situation in natural systems, resulted in higher peaks in CH4 production rates. This suggests that relationships derived from earlier incubation experiments that added POM only once, may result in underestimation of sediment CH4 production. Our results quantitatively couple CH4 production in anoxic surface sediment to POM sedimentation flux, and are therefore useful for the further development of mechanistic models of inland water CH4 emission.

Ex-situ biogas upgrading in thermophilic up-flow reactors: The effect of different gas diffusers and gas retention times

Four different types of ceramic gas distributors (Al2O3 of 1.2 μm and SiC of 0.5, 7 and 14 μm) were evaluated to increase biomethane formation during ex-situ biogas upgrading process. Each type of gas diffuser was tested independently at three different gas retention times of 10, 5 and 2.5 h, at thermophilic conditions. CH4 production rate increased by increasing input gas flow rate for all type of distributors, whereas CH4 concentration declined. Reactors equipped with SiC gas distributors effectively improved biomethane content fulfilling natural gas standards. Microbial analysis showed high abundance of hydrogenotrophic methanogens and proliferated syntrophic bacteria, i.e. syntrophic acetate oxidizers and homoacetogens, confirming the effect of H2 to alternate anaerobic digestion microbiome and enhance hydrogenotrophic methanogenesis. A detailed anaerobic bioconversion model was adapted to simulate the operation of the R1-R4 reactors. The model was shown to be effective for the simulation of biogas upgrading process in up-flow reactors.

The enhanced photo-catalytic CO2 reduction performance of g-C3N4 with high selectivity by coupling CoNiSx

In photo-catalysis, noble metals generally serve as co-catalysts, while the rarity limited their practical applications. In this case, transition metal sulfide was proved to potential substitute for noble metals owing to their peculiar photo-electrochemical properties. Here, bimetallic CoNiSx was coupled with g-C3N4 to form composite and played as a role of co-catalyst to improve the photo-catalytic activity. Notably, compared with pristine g-C3N4, the CoNiSx-CN composite exhibits superior photo-catalytic CO2 reduction activity and manifests high selectivity to CO production. The production rate of CH4 and CO in this reaction can reach 0.904 and 11.77 μmol‧ g−1 h−1, correspondingly, where the CO yield is nearly 10-fold comparable to solely g-C3N4. The excellent performance of composite can be attributed to the enhanced charge separation, light absorption ability and photo-thermal effect with the existence of CoNiSx. This study shed light on the enhancement of photo-catalytic CO2 reduction based on bimetallic co-catalyst loaded on g-C3N4.

Simulation study on influencing factors of coal low temperature oxidation based on a one-step chemical reaction equation

ABSTRACT Coal spontaneous combustion is one of the important factors affecting coal mine safety production. However, the numerical simulation of coal spontaneous combustion influence factors is mostly carried out by gas concentration, using chemical equation to simulate has higher accuracy. In this paper, a temperature-programmed experiment investigating low-temperature oxidation based on small-scale coal samples was performed, and an one-step chemical reaction equation for coal low-temperature oxidation was established. Then, SIMTEC was used to conduct a numerical simulation of the equation, the simulation results are compared with the experimental results to verify the accuracy of the simulation. Finally, the effects of different porosity, wind speed and heating rate on low temperature oxidation of coal were simulated. The results show that when the temperature is lower than 398.15 K, the O2 consumption rate as well as CO and CH4 production rates change slowly and then change rapidly when the temperature exceeds 398.15 K. The porosity and inlet flow rate of the coal samples were positively correlated with the O2 consumption rate and gas production rate, O2 consumption rate and production rates of CO and CH4 increase with increasing inlet flow rate and decrease with increasing heating rate.

Investigation of Hydrogen and Methane Production from Flue Gas Released from the Steel Industry

This paper concerns a potential multigeneration system where the flue gas is generated to be utilized for power, hydrogen and methane production, and an additional subsystem where the blending of 20% hydrogen into the methane is accomplished for residential usages. The proposed system mainly involves combined Brayton and organic Rankine cycles, water–gas shift reactor, Sabatier reactor, and combustion chamber. The Aspen Plus simulations are performed for the development and assessment of the presented multigeneration system. When the flue gas temperature is 815 °C, the work rate generated is 41.6 MW. As another beneficial output, the Sabatier reactor’s CH4 production rate is 0.26 kg/s, and a part of it is sent to blend with hydrogen at amolar fraction of 0.20 in the blend for household applications. It is observed that when combustion temperature of the blend increases from 800 °C to 1200 °C, the CO2 emissions decrease from 2.63 kmol/h to 1.42 kmol/h, and the net released heat from the combustion reactor decreases from 1175 kW to 944 kW. Furthermore, the overall energetic and exergetic efficiencies of the system are calculated as 52.1% and 68.9%, respectively.

Prepared Pd/MgO/BiVO4 composite for photoreduction of CO2 to CH4

AbstractIn this study, several Pd/MgO/BiVO4 nanocomposites were prepared using the hydrothermal method and their effective photocatalyst activity for the photoreduction of CO2 to CH4 was investigated. The microstructure and photocatalytic behavior of the prepared photocatalysts were studied through X‐ray diffraction, scanning electron microscopy, tunneling electron microscopy, X‐ray photoelectron spectroscopy, ultraviolet–visible spectrophotometry, Fourier‐transform infrared spectroscopy, Brunauer–Emmett–Teller specific surface area, and photoluminescence spectroscopy. CH4 yields of 5.9 and 9.4 μmol g−1 hr−1 were obtained using BiVO4 and 25% MgO/BiVO4 nanocomposites, respectively. The highest production rate of CH4 was measured as 12.8 μmol g−1 hr−1 on overloading 0.3% Pd on 25% MgO/BiVO4. Pd/MgO/BiVO4 exhibited excellent CO2 photoreduction stability with negligible loss after five consecutive cycles. Finally, a possible mechanism of CO2 photoreduction using Pd/MgO/BiVO4 was proposed for understanding the synergistic effects. The results of the experiment revealed that Pd/MgO/BiVO4 can be a promising photocatalyst in solar energy conversion and storage application and can achieve excellent CO2 photoreduction performance.

Selecting carrier material for efficient biomethanation of industrial biogas-CO2 in a trickle-bed reactor

Biogas upgrading via biomethanation of renewable H2 and the CO2 in biogas poses a potential key technology in the decarbonized energy system. This study uses a methanogenic trickle-bed reactor for upgrading raw biogas from an industrial anaerobic digester to natural gas-grade biomethane through ex situ biomethanation. The carrier material comprises a critical design parameter for trickle-bed reactors by supporting the catalytic methanogenic biofilm and thereby the interfacial area for H2 mass transfer. The present study applied an abiotic mass transfer-based screening of six different carrier types to identify a candidate material that can facilitate efficient H2 mass transfer in a biomethanation trickle-bed reactor. The screening included gas-liquid mass transfer experiments in an air-water system in addition to characterizations of the carriers’ surface area, dynamic liquid hold-up volumes, and external porosities. The abiotic experiments showed that the gas-liquid mass transfer coefficient is highly influenced by material type and cannot be predicted solely from differences in the carriers’ surface area. The abiotic characterizations led to the selection of crushed clay particles, whose application in biomethanation-based upgrading of raw biogas from an industrial anaerobic digester was examined. The clay particles facilitated an apparent volumetric mass transfer coefficient for H2 of 10 min−1, enabling CH4 production rates up to 6.1 L · L−1 · d−1 and more than 99 % H2 conversion over 30 days of continuous operation, where after accumulation of metabolic water in the packed bed impaired H2 mass transfer. Apart from the adverse effect of water accumulation, there was no indication that the maximum H2 mass transfer rate was reached, demonstrating the selected carrier material’s industrial potential for biomethanation-based biogas upgrading.

Methane production and oxidation potentials along a fen-bog gradient from southern boreal to subarctic peatlands in Finland.

Methane (CH4 ) emissions from northern peatlands are projected to increase due to climate change, primarily because of projected increases in soil temperature. Yet, the rates and temperature responses of the two CH4 emission-related microbial processes (CH4 production by methanogens and oxidation by methanotrophs) are poorly known. Further, peatland sites within a fen-bog gradient are known to differ in the variables that regulate these two mechanisms, yet the interaction between peatland type and temperature lacks quantitative understanding. Here, we investigated potential CH4 production and oxidation rates for 14 peatlands in Finland located between c. 60 and 70°N latitude, representing bogs, poor fens, and rich fens. Potentials were measured at three different temperatures (5, 17.5, and 30℃) using the laboratory incubation method. We linked CH4 production and oxidation patterns to their methanogen and methanotroph abundance, peat properties, and plant functional types. We found that the rich fen-bog gradient-related nutrient availability and methanogen abundance increased the temperature response of CH4 production, with rich fens exhibiting the greatest production potentials. Oxidation potential showed a steeper temperature response than production, which was explained by aerenchymous plant cover, peat water holding capacity, peat nitrogen, and sulfate content. The steeper temperature response of oxidation suggests that, at higher temperatures, CH4 oxidation might balance increased CH4 production. Predicting net CH4 fluxes as an outcome of the two mechanisms is complicated due to their different controls and temperature responses. The lack of correlation between field CH4 fluxes and production/oxidation potentials, and the positive correlation with aerenchymous plants points toward the essential role of CH4 transport for emissions. The scenario of drying peatlands under climate change, which is likely to promote Sphagnum establishment over brown mosses in many places, will potentially reduce the predicted warming-related increase in CH4 emissions by shifting rich fens to Sphagnum-dominated systems.

Application of biogas recirculation in anaerobic granular sludge system for multifunctional sewage sludge management with high efficacy energy recovery

This study investigated the possibility of biogas recirculation-driven anaerobic granular sludge system for sewage sludge treatment, aiming to develop an energy sufficient and multifunctional anaerobic digestion (AD) system for sewage sludge with biogas upgrading, sludge stabilization and self-aggregation. Results show that biogas recirculation could enhance the CH4 production rate by 31–44% and shorten the lag-phase duration to 0.08–0.2 day with simultaneous increment of CH4 content (> 83% in this study). The reason is mainly associated with the stronger interspecies electron transfer under the biogas recirculation condition. In addition, 37–40% better dewaterability of the digested sludge was achieved, implying the occurrence of self-aggregation of microbial cells induced by biogas recirculation. Energy balance analysis reflects that this sewage sludge treatment system could enhance the net energy recovery by 78–85%. Moreover, almost no obvious influence was noticed on the seed granules’ compositionand properties. These findings suggest that the biogas recirculation-driven anaerobic granular sludge system could be a promising alternative for sewage sludge treatment, which can improve biogas quality and sludge dewaterability simultaneously towards sludge self-aggregation with no addition of other chemicals.

Enhancing stability and resilience of electromethanogenesis system by acclimating biocathode with intermittent step-up voltage

Electromethanogenesis (EMG) system could efficiently convert CO2 to CH4 by using excess renewable electricity. However, the fluctuation and interruption of renewable electricity will adversely affect the biocathode and therefore the CH4 production of the EMG system. In this work, a novel biocathode acclimation strategy with intermittent step-up voltage (ISUV) was proposed to improve the stability and resilience of the EMG system against the unstable input of renewable power. Compared with the intermittent application of constant voltage (IACV), the ISUV increased the rate of CH4 production by 11.7 times with the improvement of the stability and resilience by 56% and 500%, respectively. Morphology and microflora structure analysis revealed that the biofilm enriched with ISUV exhibited a compact microflora structure with high-density cells and nanowires interconnected. This study provided a novel effective strategy to regulate the biofilm structure and enhance the performance of the EMG system.