Kinetics of hydrogen production with continuous anaerobic cultures utilizing sucrose as the limiting substrate

Improving the Continuous Multiple Tube Reactor: an Innovative Bioreactor Configuration with Great Potential for Dark Fermentation Processes

The continuous multiple tube reactor (CMTR) has been developed as a promising technology to maximize biohydrogen production (BHP) by dark fermentation (DF) by preventing excess biomass accumulation, leading to suboptimum values of specific organic loading rates (SOLR). However, previous experiences failed to achieve stable and continuous BHP in this reactor, as the low biomass retention capacity in the tube region limited controlling the SOLR. This study goes beyond the evaluation of the CMTR for DF by inserting grooves in the inner wall of the tubes to ensure better cell attachment. The CMTR was monitored in 4 assays at 25ºC using sucrose-based synthetic effluent. The hydraulic retention time (HRT) was fixed at 2 h, while the COD varied between 2-8 g L-1 to obtain organic loading rates in the 24 - 96 g COD L-1 d-1. Long-term (90 d) BHP was successfully attained in all conditions due to the improved biomass retention capacity. Optimal values for the SOLR (4.9 g COD g-1 VSS d-1) were observed when applying up to 48 g COD L-1 d-1, in which BHP was maximized. These patterns indicate a favorable balance between biomass retention and washout was naturally achieved. The CMTR looks promising for continuous BHP and is exempt from additional biomass discharge strategies.

Towards industrial biological hydrogen production: a review

Increased production of renewable energy sources is becoming increasingly needed. Amidst other strategies, one promising technology that could help achieve this goal is biological hydrogen production. This technology uses micro-organisms to convert organic matter into hydrogen gas, a clean and versatile fuel that can be used in a wide range of applications. While biohydrogen production is in its early stages, several challenges must be addressed for biological hydrogen production to become a viable commercial solution. From an experimental perspective, the need to improve the efficiency of hydrogen production, the optimization strategy of the microbial consortia, and the reduction in costs associated with the process is still required. From a scale-up perspective, novel strategies (such as modelling and experimental validation) need to be discussed to facilitate this hydrogen production process. Hence, this review considers hydrogen production, not within the framework of a particular production method or technique, but rather outlines the work (bioreactor modes and configurations, modelling, and techno-economic and life cycle assessment) that has been done in the field as a whole. This type of analysis allows for the abstraction of the biohydrogen production technology industrially, giving insights into novel applications, cross-pollination of separate lines of inquiry, and giving a reference point for researchers and industrial developers in the field of biohydrogen production.

Processing of Biomass Prior to Hydrogen Fermentation and Post-Fermentative Broth Management

Using bioconversion and simultaneous value-added product generation requires purification of the gaseous and the liquid streams before, during, and after the bioconversion process. The effect of diversified process parameters on the efficiency of biohydrogen generation via biological processes is a broad object of research. Biomass-based raw materials are often applied in investigations regarding biohydrogen generation using dark fermentation and photo fermentation microorganisms. The literature lacks information regarding model mixtures of lignocellulose and starch-based biomass, while the research is carried out based on a single type of raw material. The utilization of lignocellulosic and starch biomasses as the substrates for bioconversion processes requires the decomposition of lignocellulosic polymers into hexoses and pentoses. Among the components of lignocelluloses, mainly lignin is responsible for biomass recalcitrance. The natural carbohydrate-lignin shields must be disrupted to enable lignin removal before biomass hydrolysis and fermentation. The matrix of chemical compounds resulting from this kind of pretreatment may significantly affect the efficiency of biotransformation processes. Therefore, the actual state of knowledge on the factors affecting the culture of dark fermentation and photo fermentation microorganisms and their adaptation to fermentation of hydrolysates obtained from biomass requires to be monitored and a state of the art regarding this topic shall become a contribution to the field of bioconversion processes and the management of liquid streams after fermentation. The future research direction should be recognized as striving to simplification of the procedure, applying the assumptions of the circular economy and the responsible generation of liquid and gas streams that can be used and purified without large energy expenditure. The optimization of pre-treatment steps is crucial for the latter stages of the procedure.

Enhancing biohydrogen gas production in anaerobic system via comparative chemical pre-treatment on palm oil mill effluent (POME)

Biological hydrogen production using palm oil mill effluent (POME) as a carbon source through dark fermentation process has been suggested to be a promising bioenergy potential and enacts as alternative renewable energy source. Results have indicated that among various 1.5% (w/v) chemical pre-treatments (sodium hydroxide, NaOH; hydrochloric acid, HCl; sulphuric acid, H₂SO₄; phosphoric acid, H₃PO₄ and nitric acid, HNO₃) on POME, using H₃PO₄ would generate maximum biohydrogen production of 0.193 mmol/L/h, which corresponded to a yield of 1.51 mol H₂/mol TC_consumed with an initial total soluble carbohydrate concentration of 23.52 g/L. Among H₃PO₄ concentrations (1%-10%), the soluble carbohydrate content and the biohydrogen produced was highest and increased by 1.70-fold and 2.35-fold respectively at 2.5% (w/v), as compared to untreated POME. The batch fermentation maximum hydrogen production rate and yield of 0.208 mmol/L/h and 1.69 mol H₂/mol TC_consumed were achieved at optimum pre-treatment conditions of pH 5.5 and thermophilic temperature (60 °C). This study suggests that chemical pre-treatment approaches manage to produce and improve the carbohydrate utilisation process further. Continuous fermentation in CSTR at the optimum conditions produce heightened 1.5-fold biohydrogen yield for 2.5% H₃PO₄ at 6 h HRT as compared to batch scale. Bacterial community via next-generation sequencing analysis at optimum HRT (6 h) revealed that Thermoanaerobacterium thermosaccharolyticum registered the highest relative frequency of 20.24%. At the class level, Clostridia, Bacilli, Bacteroidia, Thermoanaerobacteria, and Gammaproteobacteria were identified as the biohydrogen-producing bacteria in the continuous system. Insightful findings from this study suggest the substantial practical utility of dilute chemical pre-treatment in improving biohydrogen production.

Reliable determination of the growth and hydrogen production parameters of the photosynthetic bacterium Rhodobacter capsulatus in fed batch culture using a combination of the Gompertz function and the Luedeking-Piret model

In this study, experimental results of hydrogen producing process based on anaerobic photosynthesis using the purple non-sulfur bacterium Rhodobacter capsulatus are scrutinized. The bacterial culture was carried out in a photo-bioreactor operated in a quasi-continuous mode, using lactate as a carbon source. The method is based on the continuous stirred tank reactors (CSTR) technique to access kinetic parameters. The dynamic evolution of hydrogen production as a function of time was accurately simulated using Luedeking-Piret model and the growth of R. capsulatus was computed using Gompertz model. The combination of both models was successfully applied to determine the relevant parameters (λ, μ_max, α and β) for two R. capsulatus strains studied: the wild-type strain B10 and the H₂ over-producing mutant IR3. The mathematical description indicates that the photofermentation is more promising than dark fermentation for the conversion of organic substrates into biogas.

Biohydrogen production in cassava processing wastewater using microbial consortia: Process optimization and kinetic analysis of the microbial community

Biohydrogen production was evaluated using cassava processing wastewater (CPW) and two microbial consortia (Vir and Gal) from different Brazilian environments. The biohydrogen production was optimized using a Box-Behnken design (T, pH, C/N, and % v/v inoculum). Maximum yields were obtained with hydrolyzed substrate: 4.12 and 3.80 mol H₂ / for Vir and Gal, respectively. Similarly, the kinetic parameters µ, k, and q were higher with hydrolyzed CPW in both consortia. The molecular analysis of the consortia through Illumina high-throughput sequencing showed the presence of bacteria from the families Porphyromonadaceae, Clostridiaceae, Ruminococcaceae, and Enterococcaceae. The relative abundance of microbial families varies as fermentation progresses. In both consortia, Clostridiaceae reached the maximum relative abundance in the media between 16 and 24 h, interval in which approximately 90% of the biohydrogen is generated.

Optimization of Cattle Manure and Food Waste Co-Digestion for Biohydrogen Production in a Mesophilic Semi-Continuous Process

Biohydrogen production from organic solid waste has shown particular advantages over other methods owing to the combination of waste reduction and bioenergy production. In this study, biohydrogen production from the co-digestion of cattle manure and food waste was optimized in a mesophilic semi-continuous process. To maximize hydrogen production, the effects of the mixing ratio (the proportion of food waste in the substrate), substrate concentration, and hydraulic retention time (HRT) on the co-digestion were systematically analyzed using a Box–Behnken design. The results showed that strong interactive effects existed between the three factors, and they had a direct effect on the responses. Hydrogen was primarily produced via the butyrate pathway, which was accompanied by the competing heterolactic fermentation pathway. Propionate and valerate produced from lipids and proteins, respectively, were obtained along with butyrate. The optimal process parameters included a mixing ratio of 47% to 51%, a substrate concentration of 76 to 86 g L−1, and an HRT of 2 d. Under these optimal conditions, the hydrogen production rate and hydrogen yield were higher than 1.00 L L−1 d−1 and 30.00 mL g−1 VS, respectively, and the predicted results were consistent with the experimental data. The results indicate that the co-digestion of cattle manure and food waste is a practical and economically promising approach for biohydrogen production.

Kinetic modeling and microbial community analysis for high-rate biohydrogen production using a dynamic membrane

This study investigated the kinetic parameters of high-rate continuous performance and biofilm layer formation in a H₂-producing dynamic membrane bioreactor, composed of a continuously stirred tank reactor along with an external module containing polyester mesh with a pore size of 100 µm. A maximum H₂ production rate of 48.9 L/L-day and hydrogen yield of 2.8 mol/mol glucose_added were attained at a hydraulic retention time of 3 h. The maximum specific growth rate and Monod constant were estimated as 14.92 d^-1 and 1.02 g COD/L, respectively. During the entire operation without backwashing, the transmembrane pressure remained below 1.7 kPa, while tightly bound extracellular polymeric substances increased as the dynamic membrane was developed. Fluorescent in situ hybridization and quantitative polymerase chain reaction assays revealed that Clostridium butyricum was dominant in all samples; however, the biofilm had a higher proportion of Prevotella spp. than the fermentation liquor.

Mesophilic continuous fermentative hydrogen production from acid pretreated de-oiled jatropha waste hydrolysate using immobilized microorganisms

Mesophilic hydrogen production from acid pretreated hydrolysate (biomass concentration of 100g/L and 2% hydrochloric acid) of de-oiled jatropha waste was carried out in continuous system using immobilized microorganisms at various hydraulic retention times (HRTs) ranging from 48 to 12h. The experimental results of the reusability of immobilized microorganisms showed their stability up to 10 cycles with an average cumulative hydrogen production of 770mL/L. The peak hydrogen production rate and hydrogen yield were 0.9L/L*d and 86mL/g_{reducing sugars added}, respectively at 16h HRT, with butyrate as the predominant volatile fatty acid. The microbial community analysis revealed that majority of the PCR-DGGE bands were assigned to genus Clostridium and were perhaps the key drivers of the higher hydrogen production.

Biohydrogen production from sugarcane bagasse hydrolysate: effects of pH, S/X, Fe2+, and magnetite nanoparticles

Batch dark fermentation experiments were conducted to investigate the effects of initial pH, substrate-to-biomass (S/X) ratio, and concentrations of Fe²⁺ and magnetite nanoparticles on biohydrogen production from sugarcane bagasse (SCB) hydrolysate. By applying the response surface methodology, the optimum condition of steam-acid hydrolysis was 0.64% (v/v) H₂SO₄ for 55.7 min, which obtained a sugar yield of 274 mg g^-1. The maximum hydrogen yield (HY) of 0.874 mol (mol glucose^-1) was detected at the optimum pH of 5.0 and S/X ratio of 0.5 g chemical oxygen demand (COD, g VSS^-1). The addition of Fe²⁺ 200 mg L^-1 and magnetite nanoparticles 200 mg L^-1 to the inoculum enhanced the HY by 62.1% and 69.6%, respectively. The kinetics of hydrogen production was estimated by fitting the experimental data to the modified Gompertz model. The inhibitory effects of adding Fe²⁺ and magnetite nanoparticles to the fermentative hydrogen production were examined by applying Andrew's inhibition model. COD mass balance and full stoichiometric reactions, including soluble metabolic products, cell synthesis, and H₂ production, indicated the reliability of the experimental results. A qPCR-based analysis was conducted to assess the microbial community structure using Enterobacteriaceae, Clostridium spp., and hydrogenase-specific gene activity. Results from the microbial analysis revealed the dominance of hydrogen producers in the inoculum immobilized on magnetite nanoparticles, followed by the inoculum supplemented with Fe²⁺ concentration. Graphical abstract ᅟ.

Effect of hydraulic retention time on suppression of methanogens during a continuous biohydrogen production process using molasses wastewater

This study was undertaken to investigate the reduction of the hydraulic retention time (HRT) to decrease methane generation and recover hydrogen production during the long-term operation of biohydrogen production in a continuous stirred tank reactor (CSTR) using molasses wastewater. Reduction of HRT can be a simple and economic method to immediately control unfavorable methane generated during continuous operation of a hydrogen production system. The steady-state performance of the CSTR showed a hydrogen content of 41.3 ± 3.30% and a hydrogen production rate (HPR) of 63.7 ± 10.01 mmol-H₂L^-1d^-1 under an organic loading rate (OLR) of 29.7 g CODL^-1 at an HRT of 24 h. Increase in the methane level above 40% during long-term operation caused decrease in the hydrogen content and HPR to 5.9 ± 1.6% and 2.1 ± 1.1 mmoL-H₂L^-1d^-1, respectively. When methane increased to a high level over 40%, the CSTR at the HRT of 24 h was operated at the HRT of 12 h. Reduction of the HRT from 24 to 12 h led to decrease in the methane content of 12.1 ± 4.44% and recovery of the HPR value to 48.9 ± 15.37 mmol-H₂L^-1d^-1 over a duration of 13-22 d. When methane is generated in a continuously operated reactor, reduction of the HRT can be an easy way to suppress methanogens and recover hydrogen production without any additives or extra treatments.

Fermentation of residual glycerol by Clostridium acetobutylicum ATCC 824 in pure and mixed cultures

The aim of this research was to estimate the production of hydrogen, organic acids and alcohols by the strain of Clostridium acetobutylicum ATCC 824 using residual glycerol as a carbon source. The experiments were carried out in pure and mixed cultures in batch experiments. Three different sources of inocula for mixed culture were used. Ruminal liquid from goats and sludge collected from two upflow anaerobic sludge blanket reactors treating municipal wastewater and brewery effluent were tested for hydrogen, organic acids and alcohols production with or without C. acetobutylicum ATCC 824. The main detected end-products from the glycerol fermentation were hydrogen, organic acids (acetic, propionic, butyric and caproic) and alcohol (ethanol and 1,3-propanediol - 1,3PD). High hydrogen (0.44 mol H2/mol glycerol consumed) and 1,3PD (0.32 mol 1,3PD/mol glycerol consumed) yields were obtained when the strain C. acetobutylicum ATCC 824 was bioaugmented into the sludge from municipal wastewater using 5 g/L of glycerol. Significant concentrations of n-caproic acid were detected in the ruminal liquid when amended with C. acetobutylicum ATCC 824. The results suggest that glycerol can be used for the generation of H2, 1,3PD and n-caproic acid using C. acetobutylicum ATCC 824 as agent in pure or mixed cultures.

Batch and continuous biogenic hydrogen fermentation of acid pretreated de-oiled jatropha waste (DJW) hydrolysate

In an attempt to tailor the efficient hydrogen fermentation from hydrochloric acid-pretreated hydrolysate of de-oiled jatropha waste (DJW), batch tests were conducted to find the optimal hydrolysate concentration, temperature and pH.

The application of an innovative continuous multiple tube reactor as a strategy to control the specific organic loading rate for biohydrogen production by dark fermentation

Biohydrogen production in fixed-bed reactors often leads to unstable and decreasing patterns because the excessive accumulation of biomass in the bed negatively affects the specific organic loading rate (SOLR) applied to the reactor. In this context, an innovative reactor configuration, i.e., the continuous multiple tube reactor (CMTR), was assessed in an attempt to better control the SOLR for biohydrogen production. The CMTR provides a continuous discharge of biomass, preventing the accumulation of solids in the long-term. Sucrose was used as the carbon source and mesophilic temperature conditions (25°C) were applied in three continuous assays. The reactor showed better performance when support material was placed in the outlet chamber to enhance biomass retention within the reactor. Although the SOLR could not be effectively controlled, reaching values usually higher than 10gsucroseg(-1)VSSd(-1), the volumetric hydrogen production and molar hydrogen production rates peaked, respectively, at 1470mLH2L(-1)d(-1) and 45mmolH2d(-1), indicating that the CMTR was a suitable configuration for biohydrogen production.

Performance of mesophilic biohydrogen-producing cultures at thermophilic conditions

In this study, batch tests were conducted to investigate the performance of mesophilic anaerobic digester sludge (ADS) at thermophilic conditions and estimate kinetic parameters for co-substrate fermentation. Starch and cellulose were used as mono-substrate and in combination as co-substrates (1:1 mass ratio) to conduct a comparative assessment between mesophilic (37 °C) and thermophilic (60 °C) biohydrogen production. Unacclimatized mesophilic ADS responded well to the temperature change. The highest hydrogen yield of 1.13 mol H2/mol hexose was observed in starch-only batches at thermophilic conditions. The thermophilic cellulose-only yield (0.42 mol H2/mol hexose) was three times the mesophilic yield (0.13 mol H2/mol hexose). Interestingly, co-fermentation of starch-cellulose at mesophilic conditions enhanced the hydrogen yield by 26% with respect to estimated mono-substrate yields, while under thermophilic conditions no enhancement in the overall yield was observed. Interestingly, the estimated overall Monod kinetic parameters showed higher rates at mesophilic than thermophilic conditions.

Suppression of methanogenesis in hydrogen fermentation by intermittent feeding

This study investigated whether intermittent feeding by using a concentrated carbon source is an appropriate method for selective enrichment of hydrogenesis by means of methanogen suppression. In a conventional reactor fed continuously for 10 d, methanogens increased from 2.8 × 10(7) to 1.1 × 10(9) gene copy number (GCN)/mg-cell dry weight, and methane concentration in the resulting biogas was 5.8%. However, when a carbon source was intermittently supplied for 10 d to the reactor, the number of methanogens was reduced 98.9% from 2.77 × 10(7) to 1.2 × 10(3) GCN/mg-cell dry weight, and methane was not detected during this period of intermittent feeding. Intermittent feeding shifted the dominants in the reactor from Clostridiaceae (70.5%) and Lactobacillaceae (11.0%) to Acetobacteraceae (62.0%) and Clostridiaceae (38.0%). In the reactor operated in continuous feeding mode after intermittent feeding, methane concentration was below 0.3% and the portion of methanogens in the bacterial community was maintained below 0.2%. These results suggest that the intermittent feeding of a carbon source during hydrogen production processes is a suitable method to suppress the activity of methanogens.

Impact of organic loading rate on biohydrogen production in an up-flow anaerobic packed bed reactor (UAnPBR)

This study assesses the impact of organic loading rate on biohydrogen production from glucose in an up-flow anaerobic packed bed reactor (UAnPBR). Two mesophilic UAPBRs (UAnPBR1 and 2) were tested at organic loading rates (OLRs) ranging from 6.5 to 51.4 g COD L(-1)d(-1). To overcome biomass washout, design modifications were made in the UAnPBR2 to include a settling zone to capture the detached biomass. The design modifications in UAnPBR2 increased the average hydrogen yield from 0.98 to 2.0 mol-H2 mol(-1)-glucose at an OLR of 25.7 g COD L(-1)d(-1). Although, a maximum hydrogen production rate of 23.4 ± 0.9 L H2 L(-1)d(-1) was achieved in the UAnPBR2 at an OLR of 51.4 g COD L(-1)d(-1), the hydrogen yield dropped by 50% to around 1 mol-H2 mol(-1)-glucose. The microbiological analysis (PCR/DGGE) showed that the biohydrogen production was due to the presence of the hydrogen and volatile acid producers such as Clostridium beijerinckii, Clostridium butyricum, Megasphaera elsdenii and Propionispira arboris.

The maximum specific hydrogen-producing activity of anaerobic mixed cultures: definition and determination

Fermentative hydrogen production from wastes has many advantages compared to various chemical methods. Methodology for characterizing the hydrogen-producing activity of anaerobic mixed cultures is essential for monitoring reactor operation in fermentative hydrogen production, however there is lack of such kind of standardized methodologies. In the present study, a new index, i.e., the maximum specific hydrogen-producing activity (SHA_m) of anaerobic mixed cultures, was proposed, and consequently a reliable and simple method, named SHA_m test, was developed to determine it. Furthermore, the influences of various parameters on the SHA_m value determination of anaerobic mixed cultures were evaluated. Additionally, this SHA_m assay was tested for different types of substrates and bacterial inocula. Our results demonstrate that this novel SHA_m assay was a rapid, accurate and simple methodology for determining the hydrogen-producing activity of anaerobic mixed cultures. Thus, application of this approach is beneficial to establishing a stable anaerobic hydrogen-producing system.

Biohydrogen production from anaerobic fermentation

Significant progress has been achieved in China for biohydrogen production from organic wastes, particularly wastewater and agricultural residues, which are abundantly available in China. This progress is reviewed with a focus on hydrogen-producing bacteria, fermentation processes, and bioreactor configurations. Although dark fermentation is more efficient for hydrogen production, by-products generated during the fermentation not only compromise hydrogen production yield but also inhibit the bacteria. Two strategies, combination of dark fermentation and photofermentation and coupling of dark fermentation with a microbial electrolysis cell, are expected to address this issue and improve hydrogen production as well as substrate utilization, which are also discussed. Finally, challenges and perspectives for biohydrogen production are highlighted.

A kinetic study of biohydrogen production from glucose, molasses and cheese whey by suspended and attached cells of Thermotoga neapolitana

Batch tests of H2 production from glucose, molasses and cheese whey by suspended and immobilized cells of Thermotoga neapolitana were conducted to develop a kinetic model of the process. H2 production was inhibited by neither H2 (up to 0.7 mg L(-1)) nor O2 (up to 0.2 mg L(-1)). The H2 specific rates obtained at different substrate concentrations were successfully interpolated with Andrew's inhibition model. With glucose and molasses, biofilms performed better than suspended cells. The suspended-cell process was successfully scaled-up to a 19-L bioreactor. Assays co-fed with molasses and cheese whey led to higher H2 productivities and H2/substrate yields than the single-substrate tests. The simulation of the suspended-cell continuous-flow process indicated the potential attainment of H2 productivities higher than those of the batch tests (up to 3.6 mmol H2 h(-1) L(-1) for molasses and 0.67 mmol H2 h(-1) L(-1) for cheese whey) and allowed the identification of the optimal dilution rate.

The influence of total solids content and initial pH on batch biohydrogen production by solid substrate fermentation of agroindustrial wastes

Hydrogen is a valuable clean energy source, and its production by biological processes is attractive and environmentally sound and friendly. In México 5 million tons/yr of agroindustrial wastes are generated; these residues are rich in fermentable organic matter that can be used for hydrogen production. On the other hand, batch, intermittently vented, solid substrate fermentation of organic waste has attracted interest in the last 10 years. Thus the objective of our work was to determine the effect of initial total solids content and initial pH on H2 production in batch fermentation of a substrate that consisted of a mixture of sugarcane bagasse, pineapple peelings, and waste activated sludge. The experiment was a response surface based on 2(2) factorial with central and axial points with initial TS (15-35%) and initial pH (6.5-7.5) as factors. Fermentation was carried out at 35 °C, with intermittent venting of minireactors and periodic flushing with inert N2 gas. Up to 5 cycles of H2 production were observed; the best treatment in our work showed cumulative H2 productions (ca. 3 mmol H2/gds) with 18% and 6.65 initial TS and pH, respectively. There was a significant effect of TS on production of hydrogen, the latter decreased with initial TS increase from 18% onwards. Cumulative H2 productions achieved in this work were higher than those reported for organic fraction of municipal solid waste (OFMSW) and mixtures of OFMSW and fruit peels waste from fruit juice industry, using the same process. Specific energetic potential due to H2 in our work was attractive and fell in the high side of the range of reported results in the open literature. Batch dark fermentation of agrowastes as practiced in our work could be useful for future biorefineries that generate biohydrogen as a first step and could influence the management of this type of agricultural wastes in México and other countries and regions as well.

Kinetics study of fermentative hydrogen production from liquid swine manure supplemented with glucose under controlled pH

Kinetics of H2 production from liquid swine manure supplemented with glucose by mixed anaerobic cultures was investigated using batch experiments under four different pH conditions (4.4, 5.0, 5.6, and uncontrolled). The temperature for the experiments was controlled at 37 ± 1°C and the length of experiments varied between 50 and 120 hours, depending upon the time needed for completion of each individual experiment. The modified Gompertz model was evaluated for its suitability for describing the H2 production potential, H2 production rate, and substrate consumption rate for all the experiments. The results showed that the Gompertz model could adequately fit the experimental results. The effect of pH was significant on all kinetic parameters for H2 production including yield, production rate and lag time, and the substrate utilization rate. The optimal pH was found to be 5.0, at which a maximum H2 production rate (0.64 L H2/h) was obtained, and deviation from the optimal pH could result in substantial reductions in H2 production rate (0.32 L H2/h for pH 4.0 and 0.43 L H2/h for pH 5.6). The results also showed that if pH was not controlled for the batch fermentation process, the substrate utilization efficiency could steeply decrease from 98.8% to 33.7%.

Hydrodynamic behaviors in fermentative hydrogen bioreactors by pressure fluctuation analysis

Three bioreactor configurations were employed in these investigations, which consisted of working volumes of 10, 1.2 and 1.2L. Power spectrum diagrams of bed pressure fluctuation were used with hydraulic retention times (HRT) and geometric factors to identify the flow regimes in the bioreactors, where HRT varied from 8 to 1h. It was found that the flow regimes in the bioreactors changed from a dispersed regime to coalesced and slugging regimes, when the biogas production rate (BPR) increased, as a result of decreasing the operating HRT. The flow regime was a dispersed bubble regime when the HRT was higher than 4h in the bioreactor, whereas when the HRT was 2h the coalesced bubble phenomena occurred in the bioreactor. A slugging regime was found when the HRT was lower than 1h in thinner bioreactor.

Hydrogen production by the newly isolated Clostridium beijerinckii RZF-1108

A fermentative hydrogen-producing strain, RZF-1108, was isolated from a biohydrogen reactor, and identified as Clostridium beijerinckii on the basis of the 16S rRNA gene analysis and physiobiochemical characteristics. The effects of culture conditions on hydrogen production by C. beijerinckii RZF-1108 were investigated in batch cultures. The hydrogen production and growth of strain RZF-1108 were highly dependent on temperature, initial pH and substrate concentration. Yeast extract was a favorable nitrogen source for hydrogen production and growth of RZF-1108. Hydrogen production corresponded to cell biomass yield in different culture conditions. The maximum hydrogen evolution, yield and production rate of 2209ml H2/l medium, 1.97 mol H2/mol glucose and 104.20 ml H2/g CDWh(-1) were obtained at 9 g/l of glucose, initial pH of 7.0, inoculum volume of 8% and temperature of 35 °C, respectively. These results demonstrate that C. beijerinckii can efficiently produce H2, and is another model microorganism for biohydrogen investigations.

Challenges in developing biohydrogen as a sustainable energy source: implications for a research agenda

The U.S. Department of Energy's Hydrogen Program aims to develop hydrogen as an energy carrier to decrease emissions of greenhouse gases and other air pollutants and reduce the use of fossil fuels. However, current hydrogen production technologies are not sustainable as they rely heavily on fossil fuels, either directly or indirectly through electricity generation. Production of hydrogen by microorganisms, biohydrogen, has potential as a renewable alternative to current technologies. The state-of-the-art for four different biohydrogen production mechanisms is reviewed, including biophotolysis, indirect biophotolysis, photofermentation, and dark fermentation. Future research challenges are outlined for bioreactor design, optimization of bioreactor conditions, and metabolic engineering. Development of biohydrogen technologies is still in the early stages, although some fermentation systems have demonstrated efficiencies reasonable for implementation. To enhance the likelihood of biohydrogen as a feasible system to meet future hydrogen demands sustainably, directed investment in a strategic research agenda will be necessary.

Evaluation of metabolism using stoichiometry in fermentative biohydrogen

We first constructed full stoichiometry, including cell synthesis, for glucose mixed-acid fermentation at different initial substrate concentrations (0.8-6 g-glucose/L) and pH conditions (final pH 4.0-8.6), based on experimentally determined electron-equivalent balances. The fermentative bioH2 reactions had good electron closure (-9.8 to +12.7% for variations in glucose concentration and -3 to +2% for variations in pH), and C, H, and O errors were below 1%. From the stoichiometry, we computed the ATP yield based on known fermentation pathways. Glucose-variation tests (final pH 4.2-5.1) gave a consistent fermentation pattern of acetate + butyrate + large H2, while pH significantly shifted the catabolic pattern: acetate + butyrate + large H2 at final pH 4.0, acetate + ethanol + modest H2 at final pH 6.8, and acetate + lactate + trivial H2 at final pH 8.6. When lactate or propionate was a dominant soluble end product, the H2 yield was very low, which is in agreement with the theory that reduced ferredoxin (Fd(red)) formation is required for proton reduction to H2. Also consistent with this hypothesis is that high H2 production correlated with a high ratio of butyrate to acetate. Biomass was not a dominant sink for electron equivalents in H2 formation, but became significant (12%) for the lowest glucose concentration (i.e., the most oligotrophic condition). The fermenting bacteria conserved energy similarly at approximately 3 mol ATP/mol glucose (except 0.8 g-glucose/L, which had approximately 3.5 mol ATP/mol glucose) over a wide range of H2 production. The observed biomass yield did not correlate with ATP conservation; low observed biomass yields probably were caused by accelerated rates of decay or production of soluble microbial products.

Biohydrogen production in a three-phase fluidized bed bioreactor using sewage sludge immobilized by ethylene-vinyl acetate copolymer

Ethylene-vinyl acetate (EVA) copolymer was used to immobilize H(2)-producing sewage sludge for H(2) production in a three-phase fluidized bed reactor (FBR). The FBR with an immobilized cell packing ratio of 10% (v/v) and a liquid recycle rate of 5l/min (23% bed expansion) was optimal for dark H(2) fermentation. The performance of the FBR reactor fed with sucrose-based synthetic medium was examined under various sucrose concentration (C(so)) and hydraulic retention time (HRT). The best volumetric H(2) production rate of 1.80+/-0.02 H(2) l/h/l occurred at C(so)=40 g COD/l and 2h HRT, while the optimal H(2) yield (4.26+/-0.04 mol H(2)/mol sucrose) was obtained at C(so)=20 g COD/l and 6h HRT. The H(2) content in the biogas was stably maintained at 40% or above. The primary soluble metabolites were butyric acid and acetic acid, as both products together accounted for 74-83% of total soluble microbial products formed during dark H(2) fermentation.