Influence of pH on fermentative hydrogen production from sweet sorghum extract

The present study focused on the influence of pH on the fermentative hydrogen production from the sugars of sweet sorghum extract, in a continuous stirred tank bioreactor. The reactor was operated at a Hydraulic Retention Time of 12 h and a pH range of 3.5–6.5. The maximum hydrogen production rate and yield were obtained at pH 5.3 and were 1752 ± 54 mL H₂/d or 3.50 ± 0.07 L H₂/L reactor/d and 0.93 ± 0.03 mol H₂/mol glucose consumed or 10.51 L H₂/kg sweet sorghum, respectively. The main metabolic product at this pH value was butyric acid. The hydrogen productivity and yield were still at high levels for the pH range of 5.3–4.7, suggesting a pH value of 4.7 as optimum for hydrogen production from an economical point of view, since the energy demand for chemicals is lower at this pH. At this pH range, the dominant fermentation product was butyric acid but when the pH culture sharply decreased to 3.5, hydrogen evolution ceased and the dominant metabolic products were lactic acid and ethanol.     

Effect of substrate concentration on fermentative hydrogen production from sweet sorghum extract

The aim of the present study was to assess the influence of substrate concentration on the fermentative hydrogen production from sweet sorghum extract, in a continuous stirred tank bioreactor. The reactor was operated at a Hydraulic Retention Time (HRT) of 12 h and carbohydrate concentrations ranging from 9.89 to 20.99 g/L, in glucose equivalents. The maximum hydrogen production rate and yield were obtained at the concentration of 17.50 g carbohydrates/L and were 2.93 ± 0.09 L H₂/L reactor/d and 0.74 ± 0.02 mol H₂/mol glucose consumed, corresponding to 8.81 ± 0.02 L H₂/kg sweet sorghum, respectively. The main metabolic product at all steady states was butyric acid, while ethanol production was high at high substrate concentrations. The experiments showed that hydrogen productivity depends significantly on the initial carbohydrate concentration, which also influences the distribution of the metabolic products.     

Application of a modified Anaerobic Digestion Model 1 version for fermentative hydrogen production from sweet sorghum extract by Ruminococcus albus

The aim of the present study was to evaluate the effectiveness of a developed, ADM1-based kinetic model for the hydrogen production process in batch and continuous cultures of the bacterium Ruminococcus albus grown on sweet sorghum extract as the sole carbon source. Although sorghum extract is known to contain at least two different sugars, i.e. sucrose and glucose, no biphasic growth was observed in batch cultures as such growth is reported to occur in cultures of R. albus with mixed substrates. Thus, taking into account that the main sugar of sweet sorghum extract is sucrose, batch experiments with different initial concentrations of sucrose were performed in order to estimate the growth kinetics of the bacterium on this substrate. The kinetic parameters used, concerning the endogenous metabolism of the bacterium as well as those concerning the effect of pH and hydrogen partial pressure (P_H2), were the same as those estimated in a previous study with glucose as carbon source. Subsequently, the experimental data of batch and continuous experiments with sweet sorghum extract were simulated based on the already developed, modified ADM1 model accounting for the use of sugar-based substrate. It was shown that the model which was developed on synthetic substrates was successful in adequately describing the behavior of the microorganism on a real substrate such as sweet sorghum extract and predicting the experimental results quite well with a deviation of the model predictions from the experimental results being between 5-18% for the hydrogen yield.     

Modeling fermentative hydrogen production from sucrose supplemented with dairy manure

Our previous studies had shown that fermentative hydrogen production from sucrose could be improved with dairy manure as a supplement. In addition to contributing to nearly 10% more hydrogen yield at ambient temperature, dairy manure was shown to be capable of providing the required nutritional needs, buffering capacity, and hydrogen-producing organisms, improving the practical viability of fermentative hydrogen production. In this report, we present a kinetic model for fermentative hydrogen production from sucrose supplemented with dairy manure. This model includes hydrogen production from sucrose as well as from the soluble products hydrolyzed from particulate manure. The integrated model was calibrated using experimental data from one batch reactor and validated with dissolved COD, hydrogen, and volatile fatty acid data from four other reactors. Predictions by this model agreed well with the temporal trends in the experimental data, with r² averaging 0.85 for dissolved COD; 0.94 for total COD; 0.84 for hydrogen; 0.84 for acetic acid; and 0.89 for butyric acid; quality of fit in the case of propionic acid was lower with r² averaging 0.57.     

Pretreatment of sweet sorghum bagasse for hydrogen production by Caldicellulosiruptor saccharolyticus

Pretreatment of sweet sorghum bagasse, an energy crop residue, with NaOH for the production of fermentable substrates, was investigated. Optimal conditions for the alkaline pretreatment of sweet sorghum bagasse were realized at 10% NaOH (w/w dry matter). A delignification of 46% was then observed, and improved significantly the efficiency of enzymatic hydrolysis. Under hydrolysis conditions without pH control, up to 50% and 41% of the cellulose and hemicellulose contained in NaOH-pretreated sweet sorghum bagasse were converted by 24 h enzymatic hydrolysis to soluble monomeric sugars. The extreme thermophilic bacterium Caldicellulosiruptor saccharolyticus showed normal growth on hydrolysates of NaOH-pretreated biomass up to a sugar concentration of 20 g/L. Besides hydrogen, the main metabolic products detected in the fermentations were acetic and lactic acid. The maximal hydrogen yield observed in batch experiments under controlled conditions was 2.6 mol/mol C6 sugar. The maximal volumetric hydrogen production rate ranged from 10.2 to 10.6 mmol/(L h). At higher substrate concentrations the production of lactic acid increased at the expense of hydrogen production.     

Coproduction of hydrogen and volatile fatty acid via thermophilic fermentation of sweet sorghum stalk from co-culture of Clostridium thermocellum and Clostridium thermosaccharolyticum

This study was focused on investigating the potential of hydrogen and volatile fatty acid (VFA) coproduction. Sweet sorghum stalks (SS) were used as substrate along with Clostridium thermocellum and Clostridium thermosaccharolyticum as production microbes. Inoculation ratio of C. thermosaccharolyticum to C. thermocellum (0:1–1.5:1 and 1:0 v/v), substrate concentrations (2.5–15.0 g/L) and inoculation time intervals of C. thermosaccharolyticum followed by C. thermocellum (0–48 h) were investigated. Experimental data showed that higher yields of hydrogen and VFA were obtained in the co-culture than their individual cultures. The optimum conditions for the highest yield of products found as 1:1 inoculation ratio of both strains, 24 h of time gap between C. thermosaccharolyticum followed by C. thermocellum after the first inoculation and 5 g/L of substrate concentration. The maximum yield of products was observed as hydrogen (5.1 mmol/g-substrate), acetic acid (1.27 g/L) and butyric acid (1.05 g/L) at optimum conditions. The results suggest that SS can be used for simultaneous production of hydrogen and VFA employing co-culture of C. thermocellum and C. thermosaccharolyticum strains. This approach can contribute to the sustainability of biorefinery.     

Optimization of biohydrogen production from sweet sorghum syrup using statistical methods

This study employed statistically based experimental designs to optimize fermentation conditions for hydrogen production from sweet sorghum syrup by anaerobic mixed cultures. Initial screening of important factors influencing hydrogen production, i.e., total sugar, initial pH, nutrient solution, iron (II) sulphate (FeSO₄), peptone and sodium bicarbonate was conducted by the Plackett–Burman method. Results indicated that only FeSO₄ had statistically significant (P ≤ 0.005) influences on specific hydrogen production (P_s) while total sugar and initial pH had an interdependent effect on P_s. Optimal conditions for the maximal P_s were 25 g/L total sugar, 4.75 initial pH and 1.45 g/L FeSO₄ in which P_s of 6897 mL H₂/L was estimated. Estimated optimum conditions revealed only 0.04% difference from the actual P_s of 6864 mL H₂/L which suggested that the optimal conditions obtained can be practically applied to produce hydrogen from sweet sorghum syrup with the least error.     

Continuous dark fermentative hydrogen production by mesophilic microflora: Principles and progress

Continuous, dark fermentative hydrogen production technology using mixed microflora at mesophilic temperatures may be suitable for commercial development. Clostridial-based cultures from natural sources have been widely used, but more information on the need for heat treatment of inocula and conditions leading to germination and sporulation are required. The amount of nutrients given in the literature vary widely. Hydrogen production is reported to proceed without methane production in the reactor in the pH range 4.5–6.7, with hydraulic retention times optimally between a few hours and 3 days depending on substrate. Higher substrate concentrations should be more energy-efficient but there are product inhibition limitations, for example from unionised butyric acid. Inhibition by H₂ can be reduced by stirring, sparging or extraction through membranes. Of the reactor types investigated, while granules have the best performance with soluble substrate, for particulate feedstock biofilm reactors or continuous stirred tank reactors may be most successful. A second stage is required to utilise the fermentation end products which, when cost-effective reactors are developed, may be photofermentation or microbial fuel cell technologies. Anaerobic digestion is a currently-available technology and the two-stage process is reported to give greater conversion efficiency than anaerobic digestion alone.     

Biological hydrogen production from sweet sorghum syrup by mixed cultures using an anaerobic sequencing batch reactor (ASBR)

Continuous biological hydrogen production from sweet sorghum syrup by mixed cultures was investigated by using anaerobic sequencing batch reactor (ASBR). The ASBR was conducted based on the optimum condition obtained from batch experiment i.e. 25 g/L of total sugar concentration, 1.45 g/L of FeSO₄ and pH of 5.0. Feasibility of continuous hydrogen fermentation in ASBR operation at room temperature (30 ± 3 °C) with different hydraulic retention time (HRT) of 96, 48, 24 and 12 hr and cycle periods consisting of filling (20 min), settling (20 min), and decanting (20 min) phases was analyzed. Results showed that hydrogen content decreased with a reduction in HRT i.e. from 42.93% (96 hr HRT) to 21.06% (12 hr HRT). Decrease in HRT resulted in a decrease of solvents produced which was from 10.77 to 2.67 mg/L for acetone and 78.25 mg/L to zero for butanol at HRT of 96 hr-12 hr, respectively. HRT of 24 hr was the optimum condition for ASBR operation indicated by the maximum hydrogen yield of 0.68 mol H₂/mol hexose. The microbial determination in DGGE analysis indicated that the well-known hydrogen producers Clostridia species were dominant in the reacting step. The presence of Sporolactobacillus sp. which could excrete the bacteriocins causing the adverse effect on hydrogen-producing bacteria might responsible for the low hydrogen content obtained.     

Effect of temperature on anaerobic fermentative hydrogen gas production from feedlot cattle manure using mixed microflora

Hydrogen production from the anaerobic fermentation of feedlot cattle manure was examined in batch cultures over a temperature range from 36 to 60 °C at a pH of 5.2. The amount of hydrogen produced increased with temperature to a maximum of 65 L H₂ kg TS⁻¹ at 52 °C. At temperatures > 52 °C, acetate was the main volatile fatty acid (VFA) accumulated, while at <52 °C butyrate accumulated the most. Formate was detected in the 56 and 60 °C treatments but was absent in all others. Thermophilic conditions resulted in the highest hydrogen production rates, with maximum hydrogen production occurring 52 °C. Changing incubation temperature by small (4 °C) increments up or down from 52 °C resulted in changes in the metabolic flux (conversion of substrate to VFA and gaseous products) of the anaerobic digestion system. These findings indicate that the hydrogen production potential of anaerobic systems utilizing heat treated cattle manure as inoculum is affected greatly by incubation temperature.     

Modeling dark fermentation for biohydrogen production: ADM1-based model vs. Gompertz model

Biohydrogen production by dark fermentation in batch reactors was modeled using the Gompertz equation and a model based on Anaerobic Digestion Model (ADM1). The ADM1 framework, which has been well accepted for modeling methane production by anaerobic digestion, was modified in this study for modeling hydrogen production. Experimental hydrogen production data from eight reactor configurations varying in pressure conditions, temperature, type and concentration of substrate, inocula source, and stirring conditions were used to evaluate the predictive abilities of the two modeling approaches. Although the quality of fit between the measured and fitted hydrogen evolution by the Gompertz equation was high in all the eight reactor configurations with r² ∼0.98, each configuration required a different set of model parameters, negating its utility as a general approach to predict hydrogen evolution. On the other hand, the ADM1-based model (ADM1BM) with predefined parameters was able to predict COD, cumulative hydrogen production, as well as volatile fatty acids production, albeit at a slightly lower quality of fit. Agreement between the experimental temporal hydrogen evolution data and the ADM1BM predictions was statistically significant with r² > 0.91 and p-value <1E-04. Sensitivity analysis of the validated model revealed that hydrogen production was sensitive to only six parameters in the ADM1BM.     

Enhanced hydrogen and volatile fatty acid production from sweet sorghum stalks by two-steps dark fermentation with dilute acid treatment in between

This study investigated the potential of hydrogen and volatile fatty acid coproduction from two steps dark fermentation with dilute acid treatments of the residual slurry after 1st step fermentation. Sweet sorghum stalks (SS) was used as substrate along with Clostridium thermosaccharolyticum as production microbe. Residual lignocelluloses after 1st step fermentation were treated for 1 h by sulfuric acid concentration of 0.25, 0.5, 1.0, 1.5, 2.0 and 2.5% (w/v) with different reaction temperature of 120, 90 and 60 °C were studied. The optimum severity conditions for the highest yield of products found from the treatment acid concentration of 1.5% (w/v) at 120 °C for 10 g/L of substrate concentration. Experimental data showed that two-step fermentation increased 76% hydrogen, 84% acetic acid and 113% of butyric acid production from single step. Maximum yields of hydrogen, acetic acid and butyric acid were 5.77 mmol/g-substrate, 2.17 g/L and 2.07 g/L respectively. This two-step fermentation for hydrogen and VFA production using the whole slurry would be a promising approach to SS biorefinery.     

Value analysis for commercialization of fermentative hydrogen production from biomass

In addition to producing hydrogen gas, biohydrogen production is also used to process wastewater. Therefore, this study specifically conducted value analyses of two different scenarios of fermentative hydrogen production from a biomass system: to increase the value of a wastewater treatment system and to specifically carry out hydrogen production. The analytical results showed that fermentative hydrogen production from a biomass system would increase the value of a wastewater treatment system and make its commercialization more feasible. In contrast, fermentative hydrogen production from a biomass system designed specifically for producing hydrogen gas would have a lower system value, which indicated that it is not yet ready for commercialization. The main obstacle to be overcome in promoting biohydrogen production technology and system application is the lack of sales channels for the system's products such as hydrogen gas and electricity. Thus, in order to realize its commercialization, this paper suggests that governments provide investment subsidies for the use of biohydrogen production technology and establish a buy-back tariff system for fuel cells.     

Recent advance in inhibition of dark fermentative hydrogen production

Dark fermentative hydrogen production is an effective and feasible technology for biological hydrogen production. However, this technology has not been commercially applied yet. One of the major reasons is that several inhibitory factors limit hydrogen production and the commercial potential. In this review paper, the various inhibitory factors which influence the dark fermentation hydrogen production were systematically analyzed and summarized, including inorganic inhibitors (heavy metal ions, light metal ions, ammonia, sulfate and hydrogen gas), organic inhibitors (volatile fatty acids, furan derivative and phenolic components), and bio-inhibitors (bacteriocins and thiosulfinate). The inhibitory concentration and mechanism were discussed in-depth and comprehensively. The strategies for mitigating these inhibitory factors were also introduced and discussed. Suggestion for future study in this aspect was proposed to promote the scale-up and commercial application of dark fermentative hydrogen production.     

Lipid production from sweet sorghum bagasse through yeast fermentation

Cryptococcus curvatus has great potential in fermenting unconditioned hydrolysates of sweet sorghum bagasse. With hydrolysates obtained by enzymatic hydrolysis of the solid pretreated by microwave with lime, the maximal yeast cell dry weight and lipid content were 10.83 g/l and 73.26%, respectively. For hydrolysates obtained in the same way but without lime, these two parameters were 15.50 g/l and 63.98%, respectively. During yeast fermentation, glucose and xylose were consumed simultaneously while cellobiose was released from the residual bagasse. The presence of lime, on one hand, made cellulose more accessible to enzymes as evidenced by higher total reducing sugar release compared to that without during enzymatic hydrolysis step; on the other hand, it caused the degradation of sugars to non-sugar chemicals during pretreatment step. As a result, higher lipid yield of 0.11 g/g bagasse or 0.65 ton/hectare of land was achieved from the pathway of microwave pretreatment and enzymatic hydrolysis while 0.09 g/g bagasse or 0.51 ton/hectare of land was attained from the process of lime-assisted microwave pretreatment followed by the same enzymatic saccharification.     

Bioethanol production from sweet sorghum: Evaluation of post-harvest treatments on sugar extraction and fermentation

Three experimental sweet sorghum varieties (M81, Topper and Theis) and three post-harvest conditions were evaluated for ethanol production: juices extracted by milling were obtained from the whole plant, plant without panicle, and stalk (plant without panicle and leaves), respectively. A linear relationship was found between the total fermentable sugar concentrations and Brix degrees of the juices, which can predict the potential ethanol yield by field analytical tests. The juice extractability presented different behavior among the sweet sorghum varieties with respect to the treatments studied. However such treatments did not affect the level of sugar concentration of the juices obtained and the fermentation efficiency. Topper and Theis showed the best performance in terms of ethanol concentration, fermentation efficiency and ethanol yield. The variety used and its post-harvest treatment should be appropriately selected in order to improve the ethanol production from sweet sorghum.     

Refining bioethanol from stalk juice of sweet sorghum by immobilized yeast fermentation

The relationship between total soluble sugar content and Brix in stalk juice of sweet sorghum was determined through one-dimensional linear regression. Meanwhile, bioethanol fermentation experiments were conducted in shaking flasks and 10 l fluidized bed bioreactor with stalk juice of Yuantian No. 1 sweet sorghum cultivar when immobilized yeast was applied. The experimental results in the shaking flasks showed that the order of influence on improving ethanol yield was (NH₄)₂SO₄>MgSO₄>K₂HPO₄, and the optimum inorganic salts supplement dose was determined as follows: K₂HPO₄ 0%, (NH₄)₂SO₄ 0.2%, MgSO₄ 0.05%. When the optimum inorganic salts supplement dose was used in fermentation in 10 l fluidized bed reactor, the fermentation time and ethanol content were 5 h and 6.2% (v/v), respectively, and ethanol yield was 91.61%, which was increased by 9.73% than blank. In addition, the results showed that the fermentation time was about 6–8 times shorter in fluidized bed bioreactor with immobilized yeast than that of conventional fermentation technology. As a result, it can be concluded that the determined optimum inorganic salts supplement dose could be used as a guide for commercial ethanol production. The fluidized bed bioreactor with immobilized yeast technology has a great potential for ethanol fermentation of stalk juice of sweet sorghum.