High efficient biohydrogen production from palm oil mill effluent by two-stage dark fermentation and microbial electrolysis under thermophilic condition

Biohydrogen production from palm oil mill effluent by two-stage dark fermentation and microbial electrolysis was investigated under thermophilic condition. The optimum chemical oxygen demand (COD) concentration and pH for dark fermentation were 66 g·L⁻¹ and 6.5 with a hydrogen yield of 73 mL-H₂·gCOD⁻¹. The dark fermentation effluent consisted of mainly acetate and butyrate. The optimum voltage for microbial electrolysis was 0.7 V with a hydrogen yield of 163 mL-H₂·gCOD⁻¹. The hydrogen yield of continuous two-stage dark fermentation and microbial electrolysis was 236 mL-H₂·gCOD⁻¹ with a hydrogen production rate of 7.81 L·L⁻¹·d⁻¹. The hydrogen yield was 3 times increased when compared with dark fermentation alone. Thermoanaerobacterium sp. was dominated in the dark fermentation stage while Geobacter sp. and Desulfovibrio sp. dominated in the microbial electrolysis cell stage. Two-stage dark fermentation and microbial electrolysis under thermophilic condition is a highly promising option to maximize the conversion of palm oil mill effluent into biohydrogen.     

Maximizing biohydrogen production from water hyacinth by coupling dark fermentation and electrohydrogenesis

Biohydrogen production via dark fermentation has shown immense potential for simultaneous energy generation and waste remediation. However, the low substrate conversion rates limit its practical feasibility. Therefore, the present work attempts to develop a single chamber microbial electrolysis cell (MEC) as an additional means for biohydrogen production. Different organic substrates including simple sugars and volatile fatty acids were demonstrated as potential substrates for H₂ production in MEC. The use of water hyacinth as sole substrate for H₂ production was examined. Furthermore, the feasibility of using MEC for second stage energy recovery after dark fermentation was explored. The two-stage process exhibited improved performance as compared to single stage MEC process with overall hydrogen yield of 67.69 L H₂/kg COD_consumed, COD removal of 70.33% and energy recovery of 46%. These results suggest that coupled dark fermentation-MEC process can be a promising means for obtaining high yield biohydrogen from water hyacinth.     

A monetary comparison of energy recovered from microbial fuel cells and microbial electrolysis cells fed winery or domestic wastewaters

Microbial fuel (MFCs) and electrolysis cells (MECs) can be used to recover energy directly as electricity or hydrogen from organic matter. Organic removal efficiencies and values of the different energy products were compared for MFCs and MECs fed winery or domestic wastewater. TCOD removal (%) and energy recoveries (kWh/kg-COD) were higher for MFCs than MECs with both wastewaters. At a cost of $4.51/kg-H₂ for winery wastewater and $3.01/kg-H₂ for domestic wastewater, the hydrogen produced using MECs cost less than the estimated merchant value of hydrogen ($6/kg-H₂). 16S rRNA clone libraries indicated the predominance of Geobacter species in anodic microbial communities in MECs for both wastewaters, suggesting low current densities were the result of substrate limitations. The results of this study show that energy recovery and organic removal from wastewater are more effective with MFCs than MECs, but that hydrogen production from wastewater fed MECs can be cost effective.     

Hydrogen generation in microbial electrolysis cell feeding with fermentation liquid of waste activated sludge

The short-chain fatty acids (SCFAs) accumulated in waste activated sludge (WAS) fermentation was adopted as an alternative extra carbon source for biohydrogen production in microbial electrolysis cells (MECs). WAS was pretreated by bi-frequency ultrasonic and the highest SCFAs were accumulated at 3rd day. Three groups of tests were conducted in single chamber MECs for H₂ production under different SCOD concentrations. SCOD removals were up to 60% at diluted influent, but reduced to 50% at original concentration. Highest H₂ yield was 1.2 mL H₂/mg COD at 2-fold dilution with 155% energy efficiency. Results showed that >90% of acetate and ∼90% of propionate were effectively converted to hydrogen, and next were n-butyrate and n-valerate (at dilutions), but <20% of iso-butyrate and iso-valerate were converted. The overall biohydrogen recovery in this study was 120 ml H₂/g VSS/d. This work shows a possibility of cascade utilization of WAS fermentation liquid and H₂ generation in MEC.     

Manipulating the hydrogen production from acetate in a microbial electrolysis cell-microbial fuel cell-coupled system

A biological hydrogen-producing system is configured through coupling an electricity-assisting microbial fuel cell (MFC) with a hydrogen-producing microbial electrolysis cell (MEC). The advantage of this biocatalyzed system is the in-situ utilization of the electric energy generated by an MFC for hydrogen production in an MEC without external power supply. In this study, it is demonstrated that the hydrogen production in such an MEC-MFC-coupled system can be manipulated through adjusting the power input on the MEC. The power input of the MEC is regulated by applying different loading resistors connected into the circuit in series. When the loading resistance changes from 10 Ω to 10 kΩ, the circuit current and volumetric hydrogen production rate varies in a range of 78 ± 12 to 9 ± 0 mA m⁻² and 2.9 ± 0.2 to 0.2 ± 0.0 mL L⁻¹ d⁻¹, respectively. The hydrogen recovery (RH₂), Coulombic efficiency (CE), and hydrogen yield (YH₂) decrease with the increase in loading resistance. Thereafter, in order to add power supply for hydrogen production in the MEC, additional one or two MFCs are introduced into this coupled system. When the MFCs are connected in series, the hydrogen production is significantly enhanced. In comparison, the parallel connection slightly reduces the hydrogen production. Connecting several MFCs in series is able to effectively increase power supply for hydrogen production, and has a potential to be used as a strategy to enhance hydrogen production in the MEC-MFC-coupled system from wastes.     

Key factors affecting microbial anode potential in a microbial electrolysis cell for H2 production

In order to optimize operations of microbial electrolysis cell (MEC) for hydrogen production, microbial anode potential (MAP) was analyzed as a function of factors in biofilm anode system, including pH, substrate and applied voltage. The results in “H” shape reactor showed that MAP reflected the information when any factor became limiting for hydrogen production. Commonly, hydrogen generation started around anode potential of −250 mV to −300 mV. While, higher current density and higher hydrogen rate were obtained when MAP went down to −400 mV or even lower in this study. Biofilm anode could work normally between pH 6.5 and 7.0, while the lowest anode potential appeared around 6.8–7.0. However, when pH was lower 6.0 or substrate concentration was less than 50 mg L⁻¹ in anode chamber, MAP went up to −300 mV or above, leading to hydrogen reduction. Applied voltage did not affect MAP much during the process of hydrogen production. Anode potential analysis also showed that planktonic bacteria in suspended solution presented positive effects on biofilm anode system and they contributed to enhance electron transfer by reducing internal resistance and lowering minimum voltage needed for hydrogen production to some extent.     

Maximizing performance of microbial electrolysis cell fed with dark fermentation effluent from water hyacinth

The performance of microbial electrolysis cell (MEC) fed with dark fermentation effluent (DEF) from water hyacinth (WH) was enhanced in this study. First, the single effects of the auxiliary processes, including centrifugation, dilution, buffering, and external power input, were investigated. Then, the interaction of these processes was further evaluated using response surface methodology (RSM) and a combination of artificial neural network (ANN) and particle swarm optimization (PSO). Statistical analysis results revealed that ANN-PSO outperformed RSM in predictability. Consequently, the ANN-PSO approach determined that a 2.2-fold dilution of centrifuged-DFE (～1.64 g of soluble metabolite products per L), buffer concentration of 75 mM, and an applied voltage of 0.7 V were the optimal conditions for simultaneously maximizing H₂ production yield and energy efficiency of DFE@WH-fed MEC. Under co-optimized conditions, H₂ yield (560.8 ± 10.8 mL/g-VS) and electrical energy recovery (162.2 ± 4.7%) significantly improved compared to unoptimized conditions.     

Mathematical model of biohydrogen production in microbial electrolysis cell: A review

Microbial electrolysis cell (MEC) is a promising reactor. However, currently, the reactor cannot be adapted for industrial-scale biohydrogen production. Nevertheless, this drawback can be overcome by modeling studies based on mathematical equations. The limitation of analytical instrumentation to record the non-linearity of the dynamic behavior for biohydrogen processes in an MEC has led to the introduction of computational approach that has the potential to reduce time constraints and optimize experimental costs. Reviews of comparative studies on bioelectrochemical models are widely reported, but there is less emphasis on the MEC model. Therefore, in this paper, a comprehensive review of the MEC mathematical model will be further discussed. The classification of the model with respect to the assumptions, model improvement, and extensive studies based on the model application will be critically analyzed to establish a methodology algorithm flow chart as a guideline for future implementation.     

Promoting dark fermentation for biohydrogen production: Potential roles of iron-based additives

Dark fermentation (DF) is a promising technology for biohydrogen production. Low efficiency of biohydrogen production is a bottleneck of the scale-up prospects for DF. Additives have been extensively studied to improve the biohydrogen production efficiency. Among of them, iron-based additives present a promising application potential due to their demonstrated significant enhancement of DF efficiency and among the low-cost bioactive agents. However, current reviews mainly examined the effects of nano-materials on DF and an in-depth analysis of enhancing mechanisms with addition of iron-based additives in DF is still lacking. To this end, this article comprehensively reviewed and evaluated the effects of iron-based additives on DF. Further, the potential mechanisms, including altering metabolic pathways, improving activities of microbes and enzymes, promoting electron delivery, and enriching hydrogen-producing bacteria, were discussed. Lastly, prospects and challenges of iron-based additives for subsequent research and large-scale application for DF were summarized.     

Optimization of NiMo catalyst for hydrogen production in microbial electrolysis cells

Non-platinum based cathodes were recently developed by electrodepositing NiMo on carbon cloth, which demonstrated good electrocatalytic activity for hydrogen evolution in microbial electrolysis cells (MECs). To further optimize the electrodeposition condition, the effects of electrolyte bath composition, applied current density, and duration of electrodeposition were systematically investigated in this study. The developed NiMo catalysts were characterized with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) and evaluated using chronopotentiometry and in MECs. The optimal condition for electrodeposition of NiMo on carbon cloth was determined as: a Mo/Ni mass ratio of 0.65 in electrolyte bath, an applied current density of 50 mA/cm² and electrodeposition duration of 10 min. Under this condition, the NiMo catalyst has a formula of Ni₆MoO₃ with a nodular morphology. The NiMo loading on the carbon cloth was reduced to 1.7 mg/cm² and the performance of MEC with the developed NiMo cathode was comparable to that with Pt cathode with a similar loading. This result indicates that a much lower cathode fabrication cost can be achieved compared to that using Pt catalyst, and thereby significantly enhancing the economic feasibility of the MEC technology.     

Advances towards the understanding of microbial communities in dark fermentation of enzymatic hydrolysates: Diversity,structure and hydrogen production performance

Microbial communities involved in hydrogen (H₂) production from enzymatic hydrolysates of agave bagasse were analyzed through 16S rRNA sequencing. Two types of reactor configurations and four different enzymatic hydrolysates were evaluated. Trickling bed reactors led to highly-diverse microbial communities, but low volumetric H₂ production rates (VHPR, maximum: 5.8 L H₂/L-d). On the contrary, well-controlled environments of continuous stirred-tank reactors favored the establishment of low diverse microbial communities composed by Clostridium-Sporolactobacillus leading to high-performance H₂-production (VHPR maximum: 13 L H₂/L-d). Cellulase-Viscozyme and Celluclast-Viscozyme hydrolysates led to the co-dominance of Clostridium and Sporolactobacillus, possibly due to the presence of xylose and hemicellulose-derived carbohydrates. Cellulase hydrolysates were linked to communities dominated by Clostridium, while maintaining low abundance of Sporolactobacillus. Stonezyme hydrolysates favored microbial communities co-dominated by Clostridium-Lachnoclostridium-Leuconostoc. Moreover, contrary to the prevailing theory, it was demonstrated that H₂ production performance was inversely related to microbial diversity.     

Optimization of high-solid waste activated sludge concentration for hydrogen production in microbial electrolysis cells and microbial community diversity analysis

To enhance hydrogen recovery from high-solid waste activated sludge (WAS), microbial electrolysis cells (MECs) were used as an efficient device. The effects of WAS concentrations were firstly investigated. Optimal concentration for hydrogen production was 7.6 g VSS/L. Maximum hydrogen yields reached to 4.66 ± 1.90 mg-H₂/g VSS and 11.42 ± 2.43 mg-H₂/g VSS for MECs fed with raw WAS (R-WAS) and alkaline-pretreated WAS (A-WAS) respectively, which was much higher than that obtained traditional anaerobic digestion. Moreover, no propionic acid accumulation was achieved at the optimal concentration. Effective sludge reduction was also achieved in MECs feeding with A-WAS. 52.9 ± 1.3% TCOD were removed in A-WAS MECs, meanwhile, protein degradation were 50.4 ± 0.8%. The 454 pyrosequencing analysis of 16S rRNA gene revealed the syntrophic interactions were existed between exoelectrogen Geobacter and fermentative bacteria Petrimonas, which apparently drove the efficient performance of MECs fed with WAS.     

Effects of pretreatment method of natural bacteria source on microbial community and bio-hydrogen production by dark fermentation

The effects of pretreatment method of cow dung compost, which was employed as natural hydrogen bacteria source, on the microbial community, population distribution of microbes and hydrogen production potential were investigated in the batch tests. The maximum hydrogen yield of 290.8 mL/L-culture appeared in the pretreated method A (infrared drying) by dark fermentation. The pretreated method of compost significantly affected microbial succession, population distribution of microbes. Both Clostridium sp. and Enterobacter sp. were found to be two species of preponderant hydrogen-producing bacteria, the next best was Bacteroides sp. and Veillonella sp., the last was Lactobacillus sp. and Streptococcus sp., which were also essential. The results showed that the mutualism and symbiosis relations of the mixed bacteria played a critical role in hydrogen fermentation process.     

Domestic wastewater treatment in parallel with methane production in a microbial electrolysis cell

Microbial electrolysis cells (MECs) have great potential as a technology for wastewater treatment in parallel to energy production. In this study we explore the feasibility of using a low-cost, membraneless MEC for domestic wastewater treatment and methane production in both batch and continuous modes. Low-strength wastewater can be successfully treated by means of an MEC, obtaining significant amounts of methane. The results also suggest that hydrogenotrophic methanogenesis reduce the incidence of homoacetogenic activity, thus improving the overall MEC performance. However, gas production rates are low and important aspects such as methane solubility in water still remain a challenge. Overall, MECs can offer competitive advantages not only for low-strength wastewater treatment but also as an aid to anaerobic methane production by improving the chemical oxygen demand (COD) removal and methane production rates.     

Lactate wastewater dark fermentation: The effect of temperature and initial pH on biohydrogen production and microbial community

Biohydrogen production using dark fermentation (hydrolysis and acidogenesis) is one of the ways to recover energy from lactate wastewater from the food-processing industry, which has high organic matter. Dark fermentation can be affected by the temperature, pH and the microbial community structure. This study investigated the effects of temperature and initial pH on the biohydrogen production and the microbial community from a lactate wastewater using dark fermentation. Biohydrogen production was successful only at lower temperature levels (35 and 45 °C) and initial pH 6.5, 7.5 and 8.5. The highest hydrogen yield (0.85 mol H₂/mol lactate consumed) was achieved at 45 °C and initial pH 8.5. The COD reduction achieved by fermenting the lactate wastewater at 35 °C ranged between 21 and 30% with the maximum COD reduction at pH 8.5, and at 45 °C, the COD reduction ranged between 12 and 21%, with the maximum at pH 7.5. At 35 °C, the lactate degradation ranged between 54 and 95%, while at 45 °C, it ranged between 77 and 99.8%. 16S rRNA sequencing revealed that at 35 °C, bacteria from the Clostridium genera were the most abundant at the end of the fermentation in the reactors that produced hydrogen, while at 45 °C Sporanaerobacter, Clostridium and Pseudomonas were the most abundant.     

A new cathodic electrode deposit with palladium nanoparticles for cost-effective hydrogen production in a microbial electrolysis cell

Microbial electrolysis cell (MEC) provides a sustainable way for hydrogen production from organic matters, but it still suffers from the lack of efficient and cost-effective cathode catalyst. In this work carbon paper coated with Pd nanoparticles was prepared using electrochemical deposition method and used as the cathodic catalyst in an MEC to facilitate hydrogen production. The electrode coated with Pd nanoparticles showed a lower overpotential than the carbon paper cathode coated with Pt black. The coulombic efficiency, cathodic and hydrogen recoveries of the MEC with the Pd nanoparticles as catalyst were slightly higher than those with a Pt cathode, while the Pd loading was one order of magnitude less than Pt. Thus, the catalytic efficiency normalized by mass of the Pd nanoparticles was about fifty times higher than that of the Pt black catalyst. These results demonstrate that utilization of the cathode with Pd nanoparticles could greatly reduce the costs of the cathodic catalysts when maintaining the MEC system performance.     

Hydrogen production from macroalgae by simultaneous dark fermentation and microbial electrolysis cell with surface-modified stainless steel mesh cathode

An affordable cathode material for microbial electrolysis cells (MECs) was synthesized via surface-modification of stainless steel mesh (SSM) by anodization. The anodization parameters, such as wire mesh size, temperature, applied voltage, operating time, were optimized. The surface-modified SSM (smSSM) exhibited porous surface and higher specific surface area. The as synthesized smSSMs were utilized as freestanding cathodes in a conventional microbial electrolysis cell (MEC) and a simultaneous dark fermentation and MEC process (sDFMEC). The H₂ production in MEC and sDFMEC with smSSM as cathode was approximately 150% higher than that with SSM. The performance of smSSM was 67–75% of that of Pt/C. The sDFMEC with smSSM as cathode was stable for 12 cycles of fed-batch operation in 60 days. Overall, energy conversion from S. japonica by sDFMEC was as high as 23.4%.     

Enhanced hydrogen production in a UASB reactor by retaining microbial consortium onto carbon nanotubes (CNTs)

Biohydrogen and subsequent biomethane generation from biomass is a promising strategy for renewable energy supply, because this combination can lead to higher energy recovery efficiency and faster fermentation than single methane fermentation. Microbial consortium control by retaining hydrogen-producers through the addition of microbial carriers is an alternative to constructing hydrogen-producing reactors. Here we report the use of carbon nanotubes (CNTs) as microbial carriers to enhance microbial retention and the production of biohydrogen. Laboratory-scale upflow anaerobic sludge blanket (UASB) reactors with CNTs at 100 mg/L achieved a maximal hydrogen production rate of 5.55 L/L/d and a maximal hydrogen yield of 2.45 mol/mol glucose. Compared to frequently used activated carbon (AC) particles, CNTs resulted in quicker startup and better performance of hydrogen fermentation in UASB reactors. Scanning electron microscopy (SEM) and pyrosequencing results revealed that the reactor with CNTs led to a high proportion of hydrogen-producing bacteria among the microbial consortium, which endowed the microbes with strong flocculation capacity and hydrogen productivity.     

Enhancing biohydrogen production from sugar industry wastewater using metal oxide/graphene nanocomposite catalysts in microbial electrolysis cell

Biohydrogen production through Microbial Electrolysis Cell (MEC) has drifted towards the development of suitable cost-effective cathode catalysts. In this study, two graphene hybrid metal oxide nanocomposites were used as catalysts to investigate hydrogen production in the MEC operated with sugar industry wastewater as substrate against phosphate buffer catholyte. Electrochemical characterizations exposed the better performance of NiO.rGO coated cathode which showed lesser overpotential at 600 mV and overall lowest resistance in the Nyquist plots than Ni-foam and Co₃O₄.rGO cathodes. The experimental results showed that at an applied voltage 1.0 V, NiO.rGO nanocomposite had exhibited maximum hydrogen production rate of 4.38 ± 0.11 mmol/L/D, Coloumbic efficiency of 65.6% and Cathodic hydrogen recovery of 20.8% respectively. The MEC performance in terms of biohydrogen production was 1.19 and 2.68 times higher than Co₃O₄.rGO and uncoated Ni-Foam. Hence, economical hybrid nanocomposite catalysts were demonstrated in MEC using industrial effluent for energy and environment sustainability.     

Evaluation of sludge liquors from acidogenic fermentation and thermal hydrolysis process as feedstock for microbial electrolysis cells

In this study, mesophilic acidogenic fermentation, thermophilic acidogenic fermentation, and thermal hydrolysis process (THP) were compared to generate sludge liquors for bioenergy recovery with microbial electrolysis cells (MECs). The results showed that THP at 170 °C was the most effective for hydrolysis of particulate organics in sewage sludge, while fermentation under thermophilic temperature led to the highest accumulation of volatile fatty acids (VFAs) in sludge liquor. However, THP yielded the highest percentage of acetate in VFAs, which resulted in superior MEC performance compared to fermented sludge liquors in terms of current density (2.7 vs. ~1.3 A/m²), coulombic efficiency (50% vs. 31–34%), bio-H₂ potential (1114 vs. 839–881 mL), and H₂ production rate (50.3 mL/d vs. 28–32 mL/d). The utilization sequence of the VFAs was found to be acetate > butyrate > propionate. Overall, our results show that generating sludge liquors through THP could provide a feasible solution to produce bio-H₂ from sewage sludge; however, coulombic efficiencies should be further improved before practical application.