Influence of applied voltage and conductive material in DIET promotion for methane generation

The utilization of biological-, electrode- and conductive material-mediated direct interspecies electron transfer (DIET) between exoelectrogenic bacteria and methanogenic archaea for enhancing methane productivity is widely reported in the literature. However, two cardinal questions are still controversial, i.e., which applied voltage value would be more recommended to enhance methane generation? and how the DIET over IIET has the upper hand in enhancing methane productivity? Herein, the influence of different applied voltages to promote biological-, conductive- and electrode-mediated DIET was investigated in MEC-AD reactors with conductive material. Polarized bioelectrodes induced electrode-mediated DIET (eDIET) and biological DIET (bDIET), in addition to cDIET (conductive material-mediated DIET), improved the methane yield to 315.40 mL/g COD_r with an applied voltage of 0.9 V. Whereas further increase of applied voltage 1.2 V, lessened methane production efficiency due to high-voltage inhibition and adverse effect on DIET promotion. The anaerobic digestion coupled microbial electrolysis cells with optimal electric potential selectively promotes the DIET through polarized electrodes were confirmed through microbial analysis. As the contribution of DIET increased to 80%, the methane yield increased, and the substrate residue decreased, resulting in a significant improvement in methane production.     

Effect of organic fraction of municipal solid waste addition to high rate activated sludge system for hydrogen production from carbon rich waste sludge

Municipal solid waste has been used for bio-methane production for many years. However, both methane and carbon dioxide that is produced during bio-methanization increases the greenhouse gas emissions; therefore, hydrogen production can be one of the alternatives for energy production from waste. Hydrogen production from the organic substance was studied in this study with the waste activated sludge from the municipal wastewater treatment. High rated activated sludge (HRAS) process was applied for the treatment to reduce energy consumption and enhance the organic composition of WAS. The highest COD removal (76%) occurred with the 12 g/L organic fraction of municipal solid waste (OFMSW) addition at a retention time of 120 min. The maximum hydrogen and methane yields for the WAS was 18.9 mL/g VS and 410 mL/g VS respectively. Total carbon emission per g VS of the substrate (OFMSW + waste activated sludge) was found as 0.087 mmol CO₂ and 28.16 mmol CO₂ for dark fermentation and bio-methanization respectively. These kinds of treatment technologies required for the wastewater treatment plantcompensate it some of the energy needs in a renewable source. In this way, the HRAS process decreases the energy requirement of wastewater treatment plant, and carbon-rich waste sludge enables green energy production via lower carbon emissions.     

System optimization for effective hydrogen production via anaerobic digestion and biogas steam reforming

To construct a system for the effective hydrogen production from food waste, the conditions of anaerobic digestion and biogas reforming have been investigated and optimized. The type of agitator and reactor shape affect the performance of anaerobic digestion reactors. Reactors with a cubical shape and hydrofoil agitator exhibit high performance due to the enhanced axial flow and turbulence as confirmed by simulation of computational fluid dynamics. The stability of an optimized anaerobic digestion reactor has been tested for 60 days. As a result, 84 L of biogas is produced from 1 kg of food waste. Reaction conditions, such as reaction temperature and steam/methane ratio, affect the biogas steam reforming reaction. The reactant conversions, product yields, and hydrogen production are influenced by reaction conditions. The optimized reaction conditions include a reaction temperature of 700 °C and H₂O/CH₄ ratio of 1.0. Under these conditions, hydrogen can be produced via steam reforming of biogas generated from a two-stage anaerobic digestion reactor for 25 h without significant deactivation and fluctuation.     

Effect of ammonia concentration on hythane (H2 and CH4) production in two-phase anaerobic digestion

Co-production of hydrogen and methane by two-phase anaerobic digestion (AD) may offer a sustainable solution for the centralized treatment of food waste (FW), while ammonia accumulation is potentially encountered. A mesophilic two-phase AD was investigated for hydrogen and methane production from FW at varying ammonia concentrations. The process achieved a hydrogen yield of 47.7 mL/g VS and a methane yield of 335 mL/g VS by optimizing the organic loading rate (OLR) and recirculation ratio. Total ammonia nitrogen (TAN) concentration of 4044 mg/L corresponded to a threshold in the hydrogen reactor, above which ammonia would initiate inhibition of hydrogenogenesis and acidogenesis. Methane yield was recovered in the methane reactor after acute inhibiting effects of TAN below 5800 mg/L, while TAN above 6200 mg/L caused chronic inhibition of methanogens. Adjusting hydraulic retention time (HRT) and recirculation ratio in hydrogen and methane reactors reduced TAN to 960 and 2105 mg/L respectively, resulting in successful recovery was achieved in the hydrogen reactor but not in the methane reactor. The two-phase AD for methane and hydrogen production can be a promising solution for ammonia accumulation in AD from FW.     

Photocatalytic pretreatment for the redox conversion of waste activated sludge to enhance biohydrogen production

Photocatalytic pretreatment of waste activated sludge (WAS) using a flat photocatalytic reactor was undertaken. Photocatalytic pretreatment enhanced the release of soluble substances from WAS, in which the soluble protein and soluble carbohydrate concentration increased by about 50% and 80%, respectively. Significant removal of heavy metal ions from the liquid phase of WAS was also achieved after photocatalytic pretreatment. In addition, the highest hydrogen yield and the highest concentration of volatile fatty acids (VFAs) were achieved from the photocatalysis pretreated WAS by batch anaerobic digestion (55 °C). The cumulative hydrogen yield from photocatalysis pretreated WAS was 211.0 ml/l-sludge, much higher than those from UV pretreated WAS (111.0 ml/l-sludge) and from raw WAS (93.0 ml/l-sludge). The results indicate that photocatalysis is a promising WAS pretreatment method for the enhancement of biohydrogen production, probably due to the photo-oxidation of organics and simultaneous photo-reduction of heavy metal ions in WAS.     

Co-generation of hydrogen and electricity from biodiesel process effluents

In this study, we apply a short-term voltage (0.2–0.8 V) to both crude glycerol (CG) and an anaerobic digestion (AD) effluent in a single-chamber microbial fuel cell (MFC) for power production. This improves the bioelectrogenesis in both CG (in MFC-1) and the AD effluent (in MFC-2), but higher power generation is attained in MFC-2. The use of domestic and synthetic wastewaters in the AD process leads to the generation of 195 and 350 mL H₂/L-medium, respectively. MFC-2 performs better than MFC-1 in terms of both voltage generation and chemical oxygen demand (COD) reduction. The application of 0.8 V yields a power density of 311 mW/m² (1.94 times higher than that of the control (160 mW/m²)). In addition, MFC-2 exhibits a 70% COD removal at 0.8 V, which decreases to 56% at 0.2 V. Thus, the application of a short-term voltage in MFC can stimulate both bioelectrogenesis and COD removal.     

A new side-looking at the dark fermentation of sugarcane vinasse: Improving the carboxylates production in mesophilic EGSB by selection of the hydraulic retention time and substrate concentration

In recent years, a lot of scientific effort has been put into reusing the energy potential of sugarcane vinasse by dark fermentation. However, the findings so far indicate that new pathways need to be followed. In this context, this study assessed the effect of hydraulic retention time (HRT, from 24 to 1 h) on vinasse fermentation (10, 20, and 30 g COD L^?1) in three mesophilic expanded granular sludge bed reactors (EGSB). The carbohydrate conversion remained above 60% at all organic loading rates applied. The maximum hydrogen production rate (8.77 L day^?1 L^?1) was obtained for 720 kg COD m^?3 day^?1 and associated to the lactate-acetate pathway. The highest productivities of propionic, acetic, and butyric acids were 3.11, 1.68, and 2.45 g L^?1 h^?1, respectively, at a HRT of 1 h. At this HRT, the degrees of acidification remained between 54% and 76% in all EGSB reactors. This research provides insights for carboxylate production from sugarcane vinasse and suggests applying the EGSB setup in the acidogenic stage of two-stage processes.     

Degradation of volatile fatty acids in highly efficient anaerobic digestion

To improve the efficiency of anaerobic digestion, we examined the effects of C₂–C₆ volatile fatty acids (VFAs) on methane fermentation, as well as the behavior of VFAs in anaerobic digestion. The VFA concentrations and methane production in anaerobic digestion were increased by pretreatment of waste activated sludge (WAS), such as ultrasonic disintegration, thermal and freezing treatments. The major intermediate products of anaerobic digestion for untreated and pretreated WAS, such as acetate, propionate, isobutyrate, butyrate, isovalerate, valerate, isocaproate and caproate, were used as substrates and the anaerobic degradation of these was carried out under the same conditions. It was found that decomposition rates of the VFAs (C₂–C₆) with a straight chain (normal form) were greater than those of their respective isomers with a branched chain (iso form). It was shown that the decomposition rates of the iso and normal forms of butyrate were greater than those of valerate and caproate. This was caused by the isomerization between butyrate and isobutyrate which occurred during the digestion process. Anaerobic bacteria in digested sludge converted butyrate to isobutyrate and vice versa by migration of the carboxyl group to the adjacent carbon atom. In addition, inhibition of degradation of the VFAs by acetate in a digester was also examined.     

Supercritical water gasification mechanism of polymer-containing oily sludge

In the offshore petroleum industry, polymer-containing oily sludge (PCOS) hinders oil extraction and causes tremendous hazards to the marine ecological environment. In this paper, an effective pretreatment method is proposed to break the adhesive structure of PCOS, and the experiments of supercritical water gasification are carried out under the influencing factors including residence time (5–30 min) and temperature (400–750 °C) in batch reactors. The increase of time and temperature all show great promoting effects on gas production. Polycyclic aromatic hydrocarbons, including naphthalene and phenanthrene, are considered as the main obstacles for a complete gasification. Carbon gasification efficiency (CE) reaches maximum of 95.82% at 750 °C, 23 MPa for 30 min, while naphthalene makes up 70% of the organic compounds in residual liquid products. The highest hydrogen yield of 19.79 (mol H₂/kg of PCOS) is observed in 750 °C for 25 min. A simplified reaction pathway is presented to describe the gaseous products (H₂, CO, CO₂, CH₄). Two intermediates are defined for describing the reaction process bases on the exhaustive study on organic matters in residual liquid products. The results show that the calculated data and the experimental data have a high degree of fit and tar formation reaction is finished within 10 min.     

Bioelectrochemical enhancement of hydrogen and methane production from the anaerobic digestion of sewage sludge in single-chamber membrane-free microbial electrolysis cells

Batch tests were carried out to investigate the bioelectrochemical enhancement of hydrogen and methane production from the anaerobic digestion of sewage sludge in single-chamber membrane-free microbial electrolysis cells (MEC) and non-MECs. Hydrogen and methane were produced from the anaerobic digestion of sewage sludge in all reactors. Compared with controls, hydrogen production was enhanced 1.7–5.2-fold, and methane production 11.4–13.6-fold with Ti/Ru electrodes at applied voltages of 1.4 and 1.8 V, respectively. Most of hydrogen was produced in the first 5 days of digestion and most of methane was generated after 5 days. No oxygen was detected in the biogas and no hydrogen production was detected in the control test with water. The applied voltages can enhance the removal of suspended and volatile suspended solids, increase the transformation of soluble chemical oxygen demand, accelerate the conversion of volatile fatty acids and maintain an optimal pH range for methanogen growth.     

Investigation of anaerobic digestion of Chlorella sp. and Micractinium sp. grown in high-nitrogen wastewater and their co-digestion with waste activated sludge

This study investigated two wildtype green algae, Micractinium sp. and Chlorella sp., for their growth in high nitrogen wastewater (mixture of sludge centrate and primary effluent wastewater) and subsequent anaerobic digestion under mesophilic conditions. Extraction and analysis of extracellular polymeric substances (EPS) in both algal species during cultivation showed that Micractinium generated larger quantity of EPS-proteins than Chlorella. Anaerobic digestion of harvested algae showed the opposite trend that Chlorella allowed a higher CH₄ yield on the volatile solids fed the digester (VS_fed) of 230 dm³ kg⁻¹ than Micractinium (209 dm³ kg⁻¹). These results suggested that different growth patterns of two types of algae, with different quantity of EPS expressed, affected anaerobic digestibility and biogas yield. Co-digestion of algae with waste activated sludge (WAS) improved the volatile solids reduction, hydrolysis efficiency as well as the biogas yields of algae.     

Dark fermentative biohydrogen production using pretreated Scenedesmus obliquus biomass under an integrated paradigm of biorefinery

In recent times, biohydrogen production from microalgal feedstock has garnered considerable research interests to sustainably replace the fossil fuels. The present work adapted an integrated approach of utilizing deoiled Scenedesmus obliquus biomass as feedstock for biohydrogen production and valorization of dark fermentation (DF) effluent via biomethanation. The microalgae was cultivated under different CO₂ concentration. CO₂-air sparging of 5% v/v supported maximum microalgal growth and carbohydrate production with CO₂ fixation ability of 727.7 mg L^?1 d^?1. Thereafter, lipid present in microalgae was extracted for biodiesel production and the deoiled microalgal biomass (DMB) was subjected to different pretreatment techniques to maximize the carbohydrate recovery and biohydrogen yield. Steam heating (121 °C) in coherence with H₂SO₄ (0.5 N) documented highest carbohydrate recovery of 87.5%. DF of acid-thermal pretreated DMB resulted in maximum H₂ yield of 97.6 mL g^?1 VS which was almost 10 times higher as compared to untreated DMB (9.8 mL g^?1 VS). Subsequent utilization of DF effluent in biomethanation process resulted in cumulative methane production of 1060 mL L^?1. The total substrate energy recovered from integrated biofuel production system was 30%. The present study envisages a microalgal biorefinery to produce biohydrogen via DF coupled with concomitant CO₂ sequestration.     

Impact of operational conditions on development of the hydrogen-producing microbial consortium in an AnSBBR from cassava wastewater rich in lactic acid

Biohydrogen production from cassava starch wastewater was evaluated in anaerobic sequencing batch biofilm reactor (AnSBBR) using different inoculum (mixed cultures from naturally fermented wastewater and anaerobic sludge thermally treated) and feeding strategies (batch and fed-batch). The highest hydrogen productivity (2.4 LH₂ L⁻¹ d⁻¹) and yield (11.7 molH₂ kg⁻¹Carbohydrates) were verified in low and intermediate organic load rates (12 and 14 g L⁻¹ d⁻¹) and longer cycle time (4 h), respectively. The productivity was favored by fed-batch strategy, and yield by batch. The hydrogen production was verified in both inoculum sources. However, in the assays inoculated from naturally fermented wastewater, with higher organic load rate (18 g L⁻¹ d⁻¹) and intermediate cycle time (3 h) no hydrogen was observed, regardless the feeding strategy, indicating that the inhibitory effects of the indigenous microorganisms present in cassava starch wastewater were more expressive in these conditions. The operational conditions applied to hydrogen production in AnSBBR from cassava starch wastewater may influence the microflora development in the reactor. In this study three possible scenarios were verified: hydrogen-producing bacteria (HPB) growth; hydrogen-producing bacteria inhibition or coexistence between ones and lactic acid bacteria (LAB), which are autochthones of this wastewater.     

HRT control as a strategy to enhance continuous hydrogen production from sugarcane juice under mesophilic and thermophilic conditions in AFBRs

The objective of this study was to evaluate the effects of hydraulic retention time (HRT) (8–1 h) on H₂ production from sugarcane juice (5000 mg COD L⁻¹) in mesophilic (30 °C, AFBR-30) and thermophilic (55 °C, AFBR-55) anaerobic fluidized bed reactors (AFBRs). At HRTs of 8 and 1 h in AFBR-30, the H₂ production rates were 60 and 116 mL H₂ h⁻¹ L⁻¹, the hydrogen yields were 0.60 and 0.10 mol H₂ mol⁻¹ hexose, and the highest bacterial diversities were 2.47 and 2.34, respectively. In AFBR-55, the decrease in the HRT from 8 to 1 h increased the hydrogen production rate to 501 mL H₂ h⁻¹ L⁻¹ at the HRT of 1 h. The maximum hydrogen yield of 1.52 mol H₂ mol⁻¹ hexose was observed at the HRT of 2 h and was associated with the lowest bacterial diversity (0.92) and highest bacterial dominance (0.52).     

Enhanced anaerobic digestion of sewage sludge by thermal or alkaline-thermal pretreatments: Influence of hydraulic retention time reduction

The influence of hydraulic retention times (HRTs) reduction from 25 days to 15 days on the enhancement effects of two pretreatments (thermal pretreatment and alkaline-thermal pretreatment) on the continuous anaerobic digestion (AD) of sewage sludge was studied in a long-term experiment (196 days). The operation of the semi-continuous AD fed with raw sludge or pretreated sludge was stable at the three HRTs. The methane production increased from 70.6 to 165.8 ml/L·d to 75.2–172.6 ml/L·d and the methane yield decreased from 98.9 to 234.9 ml/g added volatile solid to 65.6–144.9 ml/g added volatile solid when the HRT reduced from 25 days to 15 days. The two pretreatments reduced the HRT of raw sludge AD by over 40%, and the effects of the alkaline-thermal pretreatment were greater than those of the thermal pretreatment. The reduction of HRT from 25 days to 15 days increased the enhancement effects of the two pretreatments on the removal of organic matter (4.7–15.9% for volatile solid), average hydrolysis ratio (36.9–116.4%), and specific hydrolysis rate (44.1–155.6%) but decreased the enhancement effects of the pretreatments on the methane production (0.9–4.6%) and yield (4.0–15.8%), average reaction ratios (0.4–8.2%), and specific rates of the last three AD steps (0.1–13.9%). The influence of HRT reduction on the enhancement effects of the alkaline-thermal pretreatment for sludge AD was slightly greater than on the enhancement effects of the thermal pretreatment.     

Enhanced methane fermentation of municipal sewage sludge by microbial electrochemical systems integrated with anaerobic digestion

Sewage sludge from a municipal wastewater treatment plant was fed into a microbial electrochemical system, combined with an anaerobic digester (MES-AD), for enhanced methane production and sludge stabilization. The effect of thermally pretreating the sewage sludge on MES-AD performance was investigated. These results were compared to those obtained from control operations, in which the sludge was not pretreated or MES integration was absent. The soluble chemical oxygen demand (SCOD) in the raw sewage sludge after pretreatment was 31% higher than the SCOD in untreated sludge (5804.85 mg/L vs. 4441.46 mg/mL). The methane yield and proportion of methane in biogas generated by the MES-AD were higher than those of the control systems, regardless of the pretreatment process. The maximum methane yield (0.28 L CH₄/g COD) and methane production (1139 mL) were obtained with the MES inoculated with pretreated sewage sludge. Methane yield and production with this system using pretreated sewage were 47% and 56% higher, respectively, than those of the control (0.19 L CH₄/g COD, 730 mL). Additionally, the maximum SCOD removal (89%) and current generation were obtained with the MES inoculated with a pretreated substrate. These results suggested that sewage sludge could be efficiently stabilized with enhanced methane production by synergistic combination of MES-AD system with pretreatment process.     

Optimizing external voltage for enhanced energy recovery from sludge fermentation liquid in microbial electrolysis cell

Waste activated sludge (WAS), which is rich in organic substances, provides an energy resource. To recover hydrogen from the organic wastes, microbial electrolysis cell may be used as an efficient device. Since different extra applied voltages have significant effects on the efficiency of microbial electrolysis cell, this paper explores different extra applied voltages (0.6 V–1.2 V) affecting the utilization of sludge fermentation liquid (SFL) that is treated with synchronous double-frequency (28 + 40 kHz) and alkali coupling 72-bacth mesothermal anaerobic fermentation (35 °C). It is found that 0.8 V was the optimum extra applied voltage. With this voltage, the highest energy recovery efficiency will be 169 ± 1% and the peak of soluble chemical oxygen demand (SCOD) removal efficiency can be found at 51.4 ± 0.6%; Coulombic efficiency is 98.9 ± 1.0%. The order of complex matter consumption is found to be HAc > HPr > nHBu > nHVa > total carbohydrates > protein. The processing methods of synchronous double-frequency, alkaline, coupling with anaerobic fermentation are feasible for microbial electrolysis cell to transform large amount of waste activated sludge into energy.     

Corrosion-resistant non-noble metal electrodes for PEM-type water electrolyzer

Developing cheap and highly durable non-noble metal catalysts for water electrolysis to sustainably produce hydrogen as alternatives to platinum-based catalysts is essential. Herein, we report graphene-encapsulated NiMo alloys as acid-stable non-noble metal catalyst electrodes. The graphene-encapsulated NiMo cathode showed a highly stable performance in the potential cycling test (10,000 cycles) from 0 to 5.0 A cm⁻² and 100 h of durability at a 2.2 V constant cell voltage. A balance between catalytic activity and corrosion in acidic environments was achieved by tuning the number of N-doped graphene layers. Through their application in a full-cell PEM-type water electrolyzer, we verify that noble metal catalysts can be replaced by non-noble metal catalysts. Such cheap acid-stable non-noble metal electrodes have promising practical applications in PEM-type water electrolysis.     

Enhancement of bio-hydrogen generation by spirulina via an electrochemical photo-bioreactor (EPBR)

The biological production of hydrogen by microalgae is considered as an advantageous process. However, its yields are sometimes limited. To go beyond this limit, the improvement of the H₂ generation rate by Spirulina was studied via an electrochemical photo-bioreactor (EPBR). This EPBR led to hydrogen evolution rates of up to 27.49 and 13.37 mol of H₂.d⁻¹.m⁻³ for the anode and cathode chambers, respectively, under 0.3 V voltage and ~2.5 mA current. These results represent about a 4-fold increase compared to the H₂ production rate recorded without the application of a voltage. This increase in bio-hydrogen production is correlated with a drop in the concentration of NADPH. The Electrochemical Sequential Batch Reactor (ESRB) provided a more interesting total production rate which was 2.65 m³ m⁻³ d⁻¹, compared to the batch mode, which gave 1.2 m³ m⁻³.d⁻¹. The results show, for the first time, the boosting effect of the voltage on the metabolism of H₂ production by the Spirulina strain.     

Biohydrogen and biomethane production from cassava wastewater in a two-stage anaerobic sequencing batch biofilm reactor

The objective of the present study was to determine the energetic potential from cassava starch wastewater in a two-stage system (BioH₂ + BioCH₄) composed by anaerobic sequencing batch biofilm reactors (AnSBBR). Included in this general objective, the behavior of the methanogenic AnSBBR regarding organic matter removal and biomethane production will be investigated. The acidogenic AnSBBR was operated with organic loading rate (OLR) of 14 gCarb.L⁻¹.d⁻¹, influent concentration of 5 gCarb.L⁻¹ and cycle time of 4 h. The methanogenic AnSBBR was submitted to OLR increase (3.7–12 gCOD.L⁻¹.d⁻¹), provided by arrangements between influent concentration (2.8; 4.0 and 6.0 gCOD.L⁻¹) and cycle time (6; 8 and 12 h). For the evaluated condition, the acidogenic reactor presented productivity of 0.7 LH₂.L⁻¹.d⁻¹ and yield of 1.1 molH₂.kg⁻¹Carb. The methanogenic reactor presented stable methane production (%CH₄ > 78) during the 260-days operating period. The maximum methane productivity (2.71 LCH₄.L⁻¹.d⁻¹) and yield (0.263 LCH₄.g⁻¹COD) were obtained at OLR of 12 gCOD.L⁻¹.d⁻¹ and cycle time of 6 h. The estimated energy production rate in the two-stage system (BioH₂ + BioCH₄) was 105.2 kJ.L⁻¹.d⁻¹.