Utilizing acid-rich effluents of fermentative hydrogen production process as substrate for harnessing bioelectricity: An integrative approach

Lower substrate degradation is one of the limiting factors associated with fermentative hydrogen production process. To overcome this, an attempt was made to integrate microbial fuel cell (MFC) as a secondary energy generating process with the fermentative hydrogen (H₂) production. The acid-rich effluents generated from the acidogenic sequential batch biofilm reactor (AcSBBR) producing H₂ by fermenting vegetable waste was subsequently used as substrate for bioelectricity generation in single chambered MFC (air cathode; non-catalyzed electrodes). AcSBBR was operated at 70.4 kg COD/m³-day and the outlet was fed to the MFC at three variable organic loading rates. The final outlet from AcSBBR was composed of fermentative soluble acid intermediates along with residual carbon source. Experimental data illustrated the feasibility of utilizing acid-rich effluents by MFC for both additional energy generation and wastewater treatment. Higher power output (111.76 mW/m²) was observed at lower substrate loading condition. MFC also illustrated its function as wastewater treatment unit by removing COD (80%), volatile fatty acids (79%), carbohydrates (78%) and turbidity (65.38%) effectively. Fermented form of vegetable wastewater exhibited higher improvement (94%) in power compared to unfermented wastewater. The performance of MFC was characterized with respect to polarization behavior, cell potentials, cyclic voltammetry and sustainable power. This integration approach enhanced wastewater treatment efficiency (COD removal, 84.6%) along with additional energy generation demonstrating both environmental and economic sustainability of the process.     

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.     

Impact of the microbial inoculum source on pre-treatment efficiency for fermentative H2 production from glycerol

Hydrogen (H₂) production by dark fermentation can be performed from a wide variety of microbial inoculum sources, which are generally pre-treated to eliminate the activity of H₂-consuming species and/or enrich the microbial community with H₂-producing bacteria. This paper aims to study the impact of the microbial inoculum source on pre-treatment behavior, with a special focus on microbial community changes. Two inocula (aerobic and anaerobic sludge) and two pre-treatments (aeration and heat shock) were investigated using glycerol as substrate during a continuous operation. Our results show that the inoculum source significantly affected the pre-treatment efficiency. In aerobic sludge no pre-treatment is necessary, while in anaerobic sludge the heat pre-treatment increased H₂ production but aeration caused unstable H₂ production. In addition, biokinetic control was key in Clostridium selection as dominant species in all microbial communities. Lower and unstable H₂ production were associated with a higher relative abundance of Enterobacteriaceae family members. Our results allow a better understanding of H₂ production in continuous systems and how the microbial community is affected. This provides key information for efficient selection of operating conditions for future applications.     

Influence of butyrate on fermentative hydrogen production and microbial community analysis

The effect of butyrate on hydrogen production and the potential mechanism were investigated by adding butyric acid into dark fermentative hydrogen production system at different concentrations at pH range of 5.5–7.0. The results showed that under all the tested pH from 5.5 to 7.0, the addition of butyric acid can inhibit the hydrogen production, and the inhibitory degree (from 10.5% to 100%) increased with the increase of butyric acid concentration and with the decrease of pH values, which suggested that the inhibition effect is highly associated with the concentration of undissociated acids. Substrate utilization rate and VFAs accumulation also decreased with the addition of butyric acid. The microbial community analysis revealed that butyrate addition can decrease the dominant position of hydrogen-producing microorganisms, such as Clostridium, and increase the proportion of other non-hydrogen-producing bacteria, including Pseudomonas, Klebsiella, Acinetobacter, and Bacillus.     

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.     

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.     

Ultrasonic pretreatment of palm oil mill effluent: Impact on biohydrogen production,bioelectricity generation,and underlying microbial communities

Substrate bioavailabity is one of the critical factors that determine the relative biohydrogen (bioH₂) yield in fermentative hydrogen production and bioelectricity output in a microbial fuel cell (MFC). In the present undertaking, batch bioH₂ production and MFC-based biolectricity generation from ultrasonically pretreated palm oil mill effluent (POME) were investigated using heat-pretreated anaerobic sludge as seed inoculum. Maximum bioH₂ production (0.7 mmol H₂/g COD) and COD removal (65%) was achieved at pH 7, for POME which was ultrasonically pretreated at a dose of 195 J/mL. Maximum value for bioH₂ productivity and COD removal at this sonication dose was higher by 38% and 20%, respectively, than unsonicated treatments. In batch MFC experiments, the same ultrasound dose led to reduced lag-time in bioelectricity generation with concomitant 25% increase in bioelectricity output (18.3 W/m³) and an increase of COD removal from 30% to 54%, as compared to controls. Quantitative polymerase chain reaction (qPCR) tests on sludge samples from batch bioH₂ production reflected an abundance of gene fragments coding for both clostridial and thermoanaerobacterial [FeFe]-hydrogenase. Fluorescence in situ hybridization (FISH) tests on sludge from MFC experiments showed Clostridium spp. and Thermoanaerobacterium spp. as the dominant microflora. Results suggest the potential of ultrasonicated POME as sustainable feedstock for dark fermentation-based bioH₂ production and MFC-based bioelectricity generation.     

A review on single stage integrated dark-photo fermentative biohydrogen production: Insight into salient strategies and scopes

In recent times, systematic integration of dark and photofermentation has attracted a lot of attention due to the advantage of enhanced H₂ yields and better substrate conversion efficiencies. This integration is done either in a sequential two stage or in a single stage manner, between which the single stage integration (SSI) seems more cost effective. This article thoroughly reviews the salient operational strategies, key factors affecting the H₂ yields and overall increment in H₂ yields attained in the SSI biohydrogen processes. Selection of more complementing pair of dark and photofermentative microbes, optimization of composition of common growth medium, better strategies for consistent pH control and facilitation of lignocellulosic feedstocks have been identified as major areas requiring in-depth focus and subsequent improvements. Based on the insightful discussions, the current state-of-the-art of SSI bio-H₂ technology has been evaluated and its potential to become a reliable hydrogen production technology has been factually assessed.     

Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: A review

Oxygen Reduction Reactions (ORR) are one of the main factors of major potential loss in low temperature fuel cells, such as microbial fuel cells and proton exchange membrane fuel cells. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells. This review outlines important classes of abiotic catalysts and biocatalysts as electrochemical oxygen reduction reaction catalysts in microbial fuel cells. It was shown that manganese oxide and metal macrocycle compounds are good candidates for Pt catalyst replacements due to their high catalytic activity. Moreover, nitrogen doped nanocarbon material and electroconductive polymers are proven to have electrocatalytic activity, but further optimization is required if they are to replace Pt catalysts. A more interesting alternative is the use of bacteria as a biocatalyst in biocathodes, where the ORR is facilitated by bacterial metabolism within the biofilm formed on the cathode. More fundamental work is needed to understand the factors affecting the performance of the biocathode in order to improve the performance of the microbial fuel cells.     

Assessment of immobilized cell reactor and microbial fuel cell for simultaneous cheese whey treatment and lactic acid/electricity production

Two biological methods for treatment of cheese whey and concentrated cheese whey were investigated in this research. As the first method, fermentation of cheese whey for production of lactic acid, in an immobilized cell reactor (ICR) was successfully carried out. The immobilisation of Lactobacillus bulgaricus was performed by the enriched cells cultured media harvested at exponential growth phase. Furthermore, the FTIR analysis has been done to prove the production of lactic acid. The COD removal during the continuous process for both whey and concentrated whey was above 70% which showed the capability of reaction for wastewater treatment. The cells were immobilised by sodium alginate as a perfect polymer in this regard. The maximum produced lactic acid from whey was 10.7 g l^?1 at 0.125 h^?1 and 19.5 g l^?1 from concentrated whey at 0.063 h^?1. Finally it can be concluded that the process is efficient for lactic acid production and COD removal simultaneously. As the second studied method, whey and concentrated cheese whey were used as the sources of carbon in a microbial fuel cell. The power densities of 188.8 and 288.12 mW m^?2 were recorded for whey-fed and concentrated whey-fed MFCs while the COD removal were 95% and 86% respectively. Biological wastewater treatment can be a very efficient alternative for traditional wastewater treatment which selecting any and or integrating of them depends on specific applications needed to be achieved.     

Processing of electroplating industry wastewater through dual chambered microbial fuel cells (MFC) for simultaneous treatment of wastewater and green fuel production

Microbial fuel cells (MFC) provide a breakthrough development for wastewater treatment combined with electricity production. Though, MFC applications are restricted in laboratory scale level. Present study an effort has been made to employ the electroplating industrial wastewater as feedstock in dual chambered anaerobic microbial fuel cell for organic content removal as well as energy production. The ultimate goal of this research is to analyze the effect of organic load (OL) on removal of organic matter and power production. The maximum removal efficiency of total, soluble oxygen demands (TCOD, SCOD) and total suspended solids (TSS) of about 87%, 79% and 72% respectively was obtained at the OL of 1.5 gCOD/L. The maximum power and current density of about 260 mW/m² (6.2 W/m³) and 364 mA/m² was also recorded at a same OL of 1.5 gCOD/L. From the above findings proposed that utilization of high strength organic wastewater in MFC could pave the way to handle the problem of electroplating industries as well as minimize a small portion of energy demand.     

Ceramic-based microbial fuel cells (MFCs): A review

Recently, porous ceramic and clayware membranes have been widely used in microbial fuel cells (MFCs) as separators. Chemical, thermal and mechanical stability, low-cost and many other advantages of ceramic membranes make them an appropriate substitute for expensive polymeric ion exchange membranes. Moreover, good power performances in short and long-term periods were observed using ceramic membranes. In this review, we attempted to gather and assort all the experiments which applied ceramic or other earthenware membranes as the separator of MFCs. The effects of physical and chemical properties of ceramic membranes on the power efficiency of MFCs as well as scale-up challenges and future aspects were also studied.     

A review on carbon and non-precious metal based cathode catalysts in microbial fuel cells

Microbial fuel cells, an emerging technology has been paid a great attention in recent years, due to its unique advantages in treating wastewater to portable water, together with the generation of useful electricity, with the help of bio-active anodes and electrochemical cathodes, simultaneously. When applying this technology in a practical scale, the indigenous bacteria present in the wastewater catalyze the breakdown of organic matter in the anode compartment, with generation of electrons and in the cathode compartment an oxidant, usually the oxygen present in the air, take the electron and reduce to water (oxygen reduction reaction, ORR). An ideal ORR catalyst should be highly active, durable, scalable, and most importantly it should be cost effective. Generally, platinum-based catalyst is utilized, however, due to the high cost of Pt based catalysts, many cheap, cost effective catalyst have been identified as efficient ORR catalyst. Carbon based catalysts known to possess good electronic conductivity, desirable surface area, high stability, together when doped with heteroatoms and cheap metals is found to remarkably enhance the ORR activity. Although a lot of research has been done in view of developing carbon based cheap, cost-effective catalysts, still their collective information has not been reviewed. In this article we anticipate reviewing various non-precious metal and metal-free catalysts that are synthesized and investigated for MFCs, factors that affect the ORR activity, catalyst designing strategies, membranes utilized for MFCs, together with the cost comparison of non-precious and metal-free catalysts with respect to Pt based catalysts have been summarized. We anticipate that this review could offer researchers an overview of the catalyst developed so far in the literatures and provides a direction to the young researchers.     

Response of ceramic microbial fuel cells to direct anodic airflow and novel hydrogel cathodes

The presence of air in the anode chamber of microbial fuel cells (MFCs) might be unavoidable in some applications. This study purposely exposed the anodic biofilm to air for sustained cycles using ceramic cylindrical MFCs. A method for improving oxygen uptake at the cathode by utilising hydrogel was also trialled. MFCs only dropped by 2 mV in response to the influx of air. At higher air-flow rates (up to 1.1 L/h) after 43–45 h, power did eventually decrease because chemical oxygen demand (COD) was being consumed (up to 96% reduction), but recovered immediately with fresh feedstock, highlighting no permanent damage to the biofilm. Two months after the application of hydrogel to the cathode chamber, MFC power increased 182%, due to better contact between cathode and ceramic surface. The results suggest a novel way of improving MFC performance using hydrogels, and demonstrates the robustness of the electro-active biofilm both during and following exposure to air.     

Recent advances in constructed wetland‐microbial fuel cells for simultaneous bioelectricity production and wastewater treatment: A review

The coupling of constructed wetlands (CWs) to microbial fuel cells (MFCs) has turned out to be a source of renewable energy for the production of bioelectricity and for the simultaneous wastewater treatment. Both technologies have an aerobic zone in the air‐water interface and an anaerobic zone in the lower part, where the anode and the cathode are strategically placed. This hybridization is a promising bioelectrochemical technology that exerts a symbiosis between plant‐bacteria in the rhizosphere of an aquatic plant, converting solar energy into bioelectricity through the formation of root exudates as an endogenous substrate and a microbial activity. The difference between CW‐MFC and MFC conventional lies in the bioelectricity and substrate production in situ, where exogenous substrates are not required for example wastewater. However, CW‐MFC can take organic content present in wastewater, promoting the removal of some pollutants. Different areas that comprise the study of a CW‐MFC have been explored, including the structures and their operation. This review aims to provide concise information on the state of the art of CW‐MFC systems, where a summary on important aspects of the development of this technology, such as bioelectricity production, configurations, plant species, rhizodeposits, electrode materials, wastewater treatment, and future perspectives, is presented. This system is a promising technology, not only for the production of bioenergy but also to maintain a clean environment, since during its operation, no toxic byproducts were formed.     

Investigating the effect of membrane layers on the cathode potential of air-cathode microbial fuel cells

This study investigates the effect of cation exchange membrane (CEM) diffusion layers on cathode potential behavior in microbial fuel cells based on a cobalt electrodeposited anode that works in actual industrial wastewater. The structural properties of the modified anode materials were evaluated using scanning electron microscopy (SEM), which showed a strong and clear biofilm layer on the anode surface. Additionally, the structural properties of the utilized cathode materials were evaluated using energy dispersive X-ray (EDX) spectrometry and field emission scanning electron microscopy (FE-SEM) techniques, which confirmed the transfer of cobalt ions through the CEM to the cathode surface. Finally, the performance of the modified anode material with various CEMs as diffusion layers was investigated in air-cathode microbial fuel cells. The results indicate that the metal electrodeposition strategy, which utilizes multiple CEM layers, enhanced the power and current generation by 498.2 and 455%, respectively. Moreover, the Columbic efficiency (CE) increased by 77%, 154.5%, and 232% for the MFC with one, two and three CEM layers, respectively.     

Effects of architectural changes and inoculum type on internal resistance of a microbial fuel cell designed for the treatment of leachates from the dark hydrogenogenic fermentation of organic solid wastes

A new design of a single chamber MFC-A based on extended electrode surface (larger σ, specific surface or surface area of electrode to cell volume) and the assemblage or ‘sandwich’ arrangement of the anode-proton exchange membrane-cathode (AMC arrangement) and a standard single chamber MFC-B with separated electrodes were tested with several inocula (sulphate-reducing, SR-In; methanogenic, M-In, and aerobic, Ab-In) in order to determine the effects on the internal resistance R_int and other electrical characteristics of the cells. In general, the R_int of the new design cell MFC-A was consistently lower than that of the standard MFC-B, for all inocula used in this work. Resistances followed the order R_int,SR-In < R_int,M-In ? R_int,Ab-In.These results were consistent with reports on reduction of ohmic resistance of cells by decreasing inter-electrode distance. Also, the volumetric power P_V output was higher for the MFC-A than for MFC-B; this was congruent with doubling the σ in the MFC-A compared to MFC-B. Yet, power density P_An delivered was higher for MFC-A only when operated with SR-In and Ab-In, but not with M-In. The MFC-A loaded with SR-In showed a substantial improvement in P_V (ca. 13-fold, probably due to the combined effects of increased σ and decreased of R_int) and a 6.4-fold jump in P_An compared to MFC-B. The improvement was higher than the expected improvement factors (or algebraic factors; 6.5 improvement expected for P_V due to combined effects of increase of σ and lowering the R_int; 3.25 improvement expected for P_An due to lowering the R_int).Our results point out to continuing work using the two-set, sandwich-electrode MFC and sulphate-reducing inoculum as a departing model for further studies on effects of inoculum enrichment and electrode material substitution on cell performance. Also, the MFC-A model seems to hold promise for future studies of bioelectricity generation and pollution abatement processing leachates produced during biohydrogen generation in dark fermentation processes of organic solid wastes.     

Efficient biohydrogen and bioelectricity production from xylose by microbial fuel cell with newly isolated yeast of Cystobasidium slooffiae

Although xylose is the secondary dominant sugar derived from biomass, the conversion of xylose to energy products is quite challenging. In this work, a new exoelectrogenic yeast strain (Cystobasidium slooffiae strain JSUX1) that can generate electricity in microbial fuel cell (MFC) by using xylose as the substrate was isolated and identified. After adaptation, it produced significant current output with rapid xylose metabolism. More surprisingly, this strain produced hydrogen gas either in anerobic flask incubation or in MFC, which delivered a 67 mW/m² power output and 23 L/m³ hydrogen gas in MFC with xylose as fuel. Further electrochemical analysis indicated that riboflavin was secreted by this strain as the electron mediator for efficient electron transfer between cells and electrode in MFC. This is the first microorganism identified that can simultaneously produce bio-hydrogen and bio-electricity from xylose, which would diversify the toolbox of biomass energy.