Application of desirability function based on neural network for optimizing biohydrogen production process

A fractional factorial design was carried out to investigate the effects of temperature, initial pH and glucose concentration on fermentative hydrogen production by mixed cultures in batch tests and then the experimental data of substrate degradation efficiency, hydrogen yield and average hydrogen production rate were described by a neural network, based on which the simultaneous optimization of the three responses was performed by the method of desirability function. The analysis showed that the neural network could successfully describe the effects of temperature, initial pH and glucose concentration on the substrate degradation efficiency, hydrogen yield and average hydrogen production rate of this study. The maximum substrate degradation efficiency of 95.3%, hydrogen yield of 305.3 mL/g glucose and average hydrogen production rate of 23.9 mL/h were all obtained at the optimal temperature of 39.0 °C, initial pH of 7.0 and glucose concentration of 24.6 g/L identified by the method of desirability function based on a neural network. In sum, the method of desirability function based on a neural network was a useful tool to optimize several responses for fermentative hydrogen production processes simultaneously.     

Effect of temperature on fermentative hydrogen production by mixed cultures

Effect of temperatures ranging from 20 °C to 55 °C on fermentative hydrogen production by mixed cultures was investigated in batch tests. The experimental results showed that, at initial pH 7.0, during the fermentative hydrogen production using glucose as substrate, the substrate degradation efficiency and hydrogen production potential increased with increasing temperatures from 20 °C to 40 °C. The maximal substrate degradation efficiency was 98.1%, the maximal hydrogen production potential was 269.9 mL, the maximal hydrogen yield was 275.1 mL/g glucose and the shortest lag time was 7.0 h. The temperature for fermentative hydrogen production by mixed cultures was optimized to be 40 °C. The expanded Ratkowsky models could be used to describe the effect of temperatures on the hydrogen production potential, maximum hydrogen production rate and the lag time during fermentative hydrogen production.     

Inhibitory effect of ethanol,acetic acid,propionic acid and butyric acid on fermentative hydrogen production

The inhibitory effect of added ethanol, acetic acid, propionic acid and butyric acid on fermentative hydrogen production by mixed cultures was investigated in batch tests using glucose as substrate. The experimental results showed that, at 35 °C and initial pH 7.0, during the fermentative hydrogen production, the substrate degradation efficiency, hydrogen production potential, hydrogen yield and hydrogen production rate all trended to decrease with increasing added ethanol, acetic acid, propionic acid and butyric acid concentration from 0 to 300 mmol/L. The inhibitory effect of added ethanol on fermentative hydrogen production was smaller than those of added acetic acid, propionic acid and butyric acid. The modified Han–Levenspiel model could describe the inhibitory effects of added ethanol, acetic acid, propionic acid and butyric acid on fermentative hydrogen production rate in this study successfully. The modified Logistic model could describe the progress of cumulative hydrogen production.     

Dark fermentative biohydrogen production by mesophilic bacterial consortia isolated from riverbed sediments

Dark fermentative bacterial strains were isolated from riverbed sediments and investigated for hydrogen production. A series of batch experiments were conducted to study the effect of pH, substrate concentration and temperature on hydrogen production from a selected bacterial consortium, TERI BH05. Batch experiments for fermentative conversion of sucrose, starch, glucose, fructose, and xylose indicated that TERI BH05 effectively utilized all the five sugars to produce fermentative hydrogen. Glucose was the most preferred carbon source indicating highest hydrogen yields of 22.3 mmol/L. Acetic and butyric acid were the major soluble metabolites detected. Investigation on optimization of pH, temperature, and substrate concentration revealed that TERI BH05 produced maximum hydrogen at 37 °C, pH 6 with 8 g/L of glucose supplementation and maximum yield of hydrogen production observed was 2.0–2.3 mol H₂/mol glucose. Characterization of TERI BH05 revealed the presence of two different bacterial strains showing maximum homology to Clostridium butyricum and Clostridium bifermentans.     

Combined effects of temperature and pH on biohydrogen production by anaerobic digested sludge

A full factorial design was conducted to investigate the combined effects of temperatures and initial pH on fermentative hydrogen production by mixed cultures in batch tests. The experimental results showed that the modified Logistic model can be used to describe the progress of cumulative hydrogen production in the batch tests of this study. The modified Ratkowsky model can be used to describe the combined effects of the temperatures and initial pH on the substrate degradation efficiency, hydrogen yield and average hydrogen production rate. The temperatures and initial pH had interactive impact on fermentative hydrogen production. The maximum substrate degradation efficiency, the maximum hydrogen yield and the maximum average hydrogen production rate was predicted at the temperature of 37.8 °C and the initial pH of 7.1, 37.4 °C and 6.9, and 38.2 °C and 7.2, respectively. In general, the optimal temperature for the fermentative hydrogen production was around 37.8 °C and the optimal initial pH for the fermentative hydrogen production was around 7.1.     

Fermentative hydrogen production from starch using natural mixed cultures

Batch and continuous tests were conducted to evaluate fermentative hydrogen production from starch (at a concentration of chemical oxygen demand (COD) 20 g/L) at 35 °C by a natural mixed culture of paper mill wastewater treatment sludge. The optimal initial cultivation pH (tested range 5–7) and substrate concentration (tested range 5–60-gCOD/L) were evaluated by batch reactors while the effects of hydraulic retention time (HRT) on hydrogen production, as expressed by hydrogen yield (HY) and hydrogen production rate (HPR), were evaluated by continuous tests. The experimental results indicate that the initial cultivation pH markedly affected HY, maximum HPR, liquid fermentation product concentration and distribution, butyrate/acetate concentration ratio and metabolic pathway. The optimal initial cultivation pH was 5.5 with peak values of HY 1.1 mol-H₂/mol-hexose maximum HPR 10.4 mmol-H₂/L/h and butyrate concentration 7700 mg-COD/L. In continuous hydrogen fermentation, the optimal HRT was 4 h with peak HY of 1.5 mol-H₂/mol-hexose, peak HPR of 450 mmol-H₂/L/d and lowest butyrate concentration of 3000 mg-COD/L. The HPR obtained was 280% higher than reported values. A shift in dominant hydrogen-producing microbial population along with HRT variation was observed with Clostridium butyricum, C. pasteurianum, Klebshilla pneumoniae, Streptococcus sp., and Pseudomonas sp. being present at efficient hydrogen production at the HRTs of 4–6 h. Strategies based on the experimental results for optimal hydrogen production from starch are proposed.     

Characteristics of fermentative hydrogen production with Klebsiella pneumoniae ECU-15 isolated from anaerobic sewage sludge

Klebsiella pneumoniae ECU-15 (EU360791), which was isolated from anaerobic sewage sludge, was investigated in this paper for its characteristics of fermentative hydrogen production. It was found that the anaerobic condition favored hydrogen production than that of the micro-aerobic condition. Culture temperature and pH of 37 °C and 6.0 were the most favorable for the hydrogen production. The strain could grow in several kinds of monosaccharide and disaccharide, as well as the complicated corn stalk hydrolysate, with the best results exhibited in glucose. The maximum hydrogen production rate and yield of 482 ml/l/h and 2.07 mol/mol glucose were obtained at initial glucose concentration of 30 g/L and 5 g/L, respectively. Fermentation results in the diluent corn stalk hydrolysate showed that cell growth was not inhibited. However, the hydrogen production of 0.65 V/V was relatively lower than that of the glucose (1.11 V/V), which was mainly due to the interaction between xylose and glucose.     

Hydrogen production using Clostridium saccharoperbutylacetonicum N1-4 (ATCC 13564)

Fermentative hydrogen production was carried out using Clostridium saccharoperbutylacetonicum N1-4 (ATCC 13564). This work investigates the effects of initial substrate concentration, initial medium pH, and temperature. The hydrogen yield was about 3.1 mol (mol glucose)⁻¹ when starting with an initial glucose concentration of 10 gl⁻¹ and initial a pH of 6.0 ± 0.2 at a temperature of 37 °C. The volume of hydrogen produced decreased when higher initial glucose concentrations were applied. The most suitable conditions for hydrogen production in a batch reactor were observed at initial pH 6.0 ± 0.2 and 37 °C.     

High yield simultaneous hydrogen and ethanol production under extreme-thermophilic (70 °C) mixed culture environment

The effect of pH and medium composition on extreme-thermophilic (70 °C) dark fermentative simultaneous hydrogen and ethanol production (process performance and microbial ecology) was investigated. Hydrogen and ethanol yields were optimized with respect to glucose, peptone, FeSO₄, NaHCO₃, yeast extract, trace mineral salts, vitamins, and phosphate buffer concentrations as well as initial pH as independent variables. A combination of low levels of both glucose (≤2 g/L) and vitamin solutions (≤1 mL/L) and high levels of initial pH (≥7), mineral salts solution (≥5 mL/L) and FeSO₄ (≥100 mg/L) stimulated the hydrogen production, while high level of glucose (≥5 g/L) and low levels of both initial pH (≤5.5) and mineral salts solution (≤1 mL/L) enhanced the ethanol production. High yield of simultaneous hydrogen and ethanol production (1.58 mol H₂/mol glucose combined with an ethanol yield of 0.90 mol ethanol/mol glucose) was achieved under extreme-thermophilic mixed culture environment. Results obtained showed that the shift of the metabolic pathways favouring either hydrogen or ethanol production was affected by the change in cultivation conditions (pH and medium composition). The mixed culture in this study demonstrated flexible ability for simultaneous hydrogen and ethanol production, depending on pH and nutrients formulation. The microorganisms involved could be regarded as simultaneous hydrogen/ethanol producers, as hydrogen and ethanol fermentation under all conditions was carried out by a group of extreme-thermophilic bacterial species related to Thermoanaerobacter, Thermoanaerobacterium and Caldanaerobacter.     

Enhancement of biohydrogen producing using ultrasonication

Ultrasonication was evaluated as a pretreatment for biological hydrogen production from glucose in batch studies, in comparison with heat-shock pretreatment, acid pretreatment, and base pretreatment. The optimized sonication energy for hydrogen production using anaerobic digester sludge was 79 kJ/gTS. Sonication with temperature control (less than 30 °C) increased volumetric hydrogen production by 120% over the untreated sludge, and by 40% over the heat-shock and acid pretreated sludge, with a marginal (∼10%) increase in hydrogen production rate. Upon comparing the molar hydrogen yield in sonicated sludge with and without temperature control, the deleterious effect of heat on some hydrogen producers as reflected by a 30% decrease in yield to 1.03 mol H₂/mol glucose is evident. Sonication with temperature control affected a 45% increase in molar hydrogen yield to 1.55 mol H₂/mol glucose over heat-shock pretreatment at 70 °C for 30 min and acidification to pH 3.0 for 24 h at 4 °C. Sonication with temperature control produced a biomass yield of 0.13 g VSS/g COD, as compared to 0.24 g VSS/g COD for the untreated sludge. The hydrogen yield increased linearly with the molar acetate to butyrate ratio and decreased linearly with the biomass yield.     

Fermentative hydrogen production by a new mesophilic bacterium Clostridium sp. 6A-5 isolated from the sludge of a sugar mill

The fermentative hydrogen production capability of the newly isolated Clostridium sp. 6A-5 bacterium was studied in a batch cultivation experiment. Various culture conditions (temperature, initial pH, and glucose concentration) were evaluated for their effects on cell growth and hydrogen production (including the yield and rate) of Clostridium sp. 6A-5. Optimal cell growth was observed at 40 °C, initial pH 7.5–8, and glucose concentration 16–26 g/L. The optimal hydrogen yield was obtained at 43 °C, initial pH 8, and glucose concentration 10–16 g/L. Hydrogen began to evolve when cell growth entered the mid-exponential phase and reached the maximum production rate at the late exponential and stationary phases. The maximum hydrogen yield, and rate were 2727 mL/L, and 269.3 mL H₂/L h, respectively. These results indicate that Clostridium sp. 6A-5 is a good candidate for mesophilic fermentative hydrogen production.     

Optimization of hydrogen production by hyperthermophilic eubacteria, Thermotoga maritima and Thermotoga neapolitana in batch fermentation

The batch fermentations of two hyperthermophilic eubacteria Thermotoga maritima strain DSM 3109 and Thermotoga neapolitana strain DSM 4359 were carried out to optimize the hydrogen production. The simple and economical culture medium using cheap salts with strong buffering capacity was designed based on T. maritima basal medium (TMB). Both strains cultivated under strictly anaerobic conditions showed the best growth at temperature of 75–80 °C and pH of 6.5–7.0. The maximum cell growth of 3.14 g DCW/L and hydrogen production of 342 mL H₂ gas/L were obtained, respectively, in the modified TB medium containing 7.5 g/L of glucose and 4 g/L of yeast extract. Hydrogen accumulation in the headspace was more than 30% of the gaseous phase. Cells were also cultivated in cellulose-containing medium to test the feasibility of hydrogen production.     

Fermentative production of hydrogen and soluble metabolites from crude glycerol of biodiesel plant by the newly isolated thermotolerant Klebsiella pneumoniae TR17

A thermotolerant fermentative hydrogen-producing strain was isolated from crude glycerol contaminated soil and identified as Klebsiella pneumoniae on the basis of the 16S rRNA gene analysis as well as physiological and biochemical characteristics. The selected strain, designated as K. pneumoniae TR17, gave good hydrogen production from crude glycerol. Culture conditions influencing the hydrogen production were investigated. The strain produced hydrogen within a wide range of temperature (30–50 °C), initial pH (4.0–9.0) and crude glycerol concentration (20–100 g/L) with yeast extract as a favorable nitrogen source. In batch cultivation, the optimal conditions for hydrogen production were: cultivation temperature at 40 °C, initial pH at 8.0, 20 g/L crude glycerol and 2 g/L yeast extract. This resulted in the maximum cumulative hydrogen production of 27.7 mmol H₂/L and hydrogen yield of 0.25 mol H₂/mol glycerol. In addition, the main soluble metabolites were 1,3-propanediol, 2,3-butanediol and ethanol corresponding to the production of 3.52, 2.06 and 3.95 g/L, respectively.     

Effects of pH value and substrate concentration on hydrogen production from the anaerobic fermentation of glucose

A series of batch experiments were conducted to investigate the effects of pH and glucose concentrations on biological hydrogen production by using the natural sludge obtained from the bed of a local river as inoculant. Batch experiments numbered series I and II were designed at an initial and constant pH of 5.0–7.0 with 1.0 increment and four different glucose concentrations (5.0, 7.5, 10 and 20 g glucose/L). The results showed that the optimal condition for anaerobic fermentative hydrogen production is 7.5 g glucose/L and constant pH 6.0 with a maximum H₂ production rate of 0.22 mol H₂ mol⁻¹ glucose h⁻¹, a cumulative H₂ yield of 1.83 mol H₂ mol⁻¹ glucose and a H₂ percentage of 63 in biogas.     

Kinetics of cotton cellulose hydrolysis using concentrated acid and fermentative hydrogen production from hydrolysate

The kinetics of cotton cellulose hydrolysis using concentrated sulfuric acid and the performance of fermentative hydrogen production from the hydrolysate in the batch system was carried out in this study. Effects of sulfuric acid concentrations, cotton cellulose concentrations and operating temperatures on the cotton cellulose hydrolysis were investigated. It was found that cotton cellulose can dissolve completely in sulfuric acid concentration above 55% (by volume) at room temperature. The reduced sugar yields were varied from 64.3 to 73.9% (g R-sugar/g cotton cellulose) with the initial cotton cellulose concentrations of 30-70 g/L at a temperature of 40 °C.The reduced sugar concentrations and the initial pH of biohydrogen production were investigated at 37 °C. It was found that the optimal values of the hydrogen yield and substrate utilization were 0.95 mol H₂/mol R-sugar and 98% with an initial pH of 8.2, when substrate concentration was fixed at 20 g R-sugar/L. The maximum hydrogen yield was 0.99 mol H₂/mol R-sugar at a substrate concentration of 15 g R-sugar/L. Using the Gompertz Equation Model simulation, the maximum hydrogen production rate was 253 mL H₂/h/L at a substrate of 30 g/L and initial pH of 8.4.     

Biological hydrogen production in an anaerobic sequencing batch reactor: pH and cyclic duration effects

An anaerobic sequencing batch reactor (ASBR) was used to evaluate biological hydrogen production from carbohydrate-rich organic wastes. The goal of the proposed project was to investigate the effects of pH (4.9, 5.5, 6.1, and 6.7), and cyclic duration (4, 6, and 8 h) on hydrogen production. With the ASBR operated at 16-h HRT, 25 g COD/L, and 4-h cyclic duration, the results showed that the maximum hydrogen yield of 2.53 mol H₂/mol sucrose_consumed appeared at pH 4.9. The carbohydrate removal efficiency declined to 56% at pH 4.9, which indirectly resulted in the reduction of total volatile fatty acid production. Acetate fermentation was the dominant metabolic pathway at pH 4.9. The concentration of mixed liquor volatile suspended solid (MLVSS) also showed a decrease from nearly 15,000 mg/L between pHs 6.1 and 6.7 to 6000 mg/L at pH 4.9. Investigation of the effect of cyclic duration found that hydrogen yield reached the maximum of 1.86 mol H₂/mol sucrose_consumed at 4-h cyclic duration while ASBR was operating at 16-h HRT, 15 g COD/L, and pH 4.9. The experimental results showed that MLVSS concentration increased from 6200 mg/L at 4-h cyclic duration to 8500 mg/L at 8-h cyclic duration. However, there was no significant change in effluent volatile suspended solid concentration. The results of butyrate to acetate ratio showed that using this ratio to correlate the performance of hydrogen production is not appropriate due to the growth of homoacetogens. In ASBR, the operation is subject to four different phases of each cycle, and only the complete mix condition can be achieved at react phase. The pH and cyclic duration under the unique operations profoundly impact fermentative hydrogen production.     

Fermentative hydrogen production by the newly isolated Enterobacter asburiae SNU-1

A new fermentative hydrogen-producing bacterium was isolated from a domestic landfill and identified as Enterobacter asburiae using 16S rRNA gene sequencing and DNA–DNA hybridization methods. The isolated bacterium, designated as Enterobacter asburiae SNU-1, is a new species that has never been examined as a potential hydrogen-producing bacterium. This study examined the hydrogen-producing ability of Enterobacter asburiae SNU-1. During fermentation, the hydrogen was mainly produced in the stationary phase. The hydrogen yield based on the formate consumption was 0.43 mol hydrogen/mol formate. This strain was able to produce hydrogen over a wide range of pH (4–7.5), with the optimum pH being pH 7. The level of hydrogen production was also affected by the initial glucose concentration, and the optimum value was found to be 25 g glucose/l. The maximum and overall hydrogen productivities were 398 and 174 ml/l/hr, respectively, at pH 7 with an initial glucose concentration of 25 g/l. This strain could produce hydrogen from glucose and many other carbon sources such as fructose, sucrose, and sorbitol.     

Continuous biohydrogen production from liquid swine manure supplemented with glucose using an anaerobic sequencing batch reactor

Liquid swine manure supplemented with glucose (10 g/L) was used as substrate for hydrogen production using an anaerobic sequencing batch reactor at 37 ± 1 °C and pH 5.0 under different hydraulic retention times (HRTs). Decreasing HRT from 24 to 8 h caused an increasing hydrogen production rate from 0.05 to 0.15 L/h/L. Production rates of both total biogas and hydrogen were linearly correlated to HRT with R² being 0.993 and 0.997, respectively. The hydrogen yield ranged between 1.18 and 1.63 mol-H₂/mol glucose and the 12 h HRT was preferred for high production rate and efficient yield. For all the five HRTs examined, the glucose utilization efficiency was over 98%. The biogas mainly consisted of carbon dioxide and hydrogen (up to 43%) with no methane detected throughout the experiment. Ethanol and organic acids were the major aqueous metabolites produced during fermentation, with acetic acid accounting for 56–58%. The hydrogen yield was found to be related to the acetate/butyrate ratio.     

Optimization of fermentative hydrogen production process using genetic algorithm based on neural network and response surface methodology

A central composite design was carried out to investigate the effect of temperature, initial pH and glucose concentration on fermentative hydrogen production by mixed cultures in batch test. The modeling abilities of the response surface methodology model and neural network model, as well as the optimizing abilities of response surface methodology and the genetic algorithm based on a neural network model were compared. The results showed that the root mean square error and the standard error of prediction for the neural network model were much smaller than those for the response surface methodology model, indicting that the neural network model had a much higher modeling ability than the response surface methodology model. The maximum hydrogen yield of 289.8 mL/g glucose identified by response surface methodology was a little lower than that of 360.5 mL/g glucose identified by the genetic algorithm based on a neural network model, indicating that the genetic algorithm based on a neural network model had a much higher optimizing ability than the response surface methodology. Thus, the genetic algorithm based on a neural network model is a better optimization method than response surface methodology and is recommended to be used during the optimization of fermentative hydrogen production process.     

Fermentative hydrogen production by the newly isolated Clostridium beijerinckii Fanp3

A hydrogen producing strain newly isolated from anaerobic sludge in an anaerobic bioreactor, was identified as Clostridium beijerinckii Fanp3 by 16S rDNA gene sequence analysis and detection by BioMerieux Vitek. The strain could utilize various carbon and nitrogen sources to produce hydrogen, which indicates that it has the potential of converting renewable wastes into hydrogen. In batch cultivations, the optimal initial pH of the culture medium was between 6.47 and 6.98. Using 0.15 M phosphate as buffer could alleviate the medium acidification and improve the overall performance of C. beijerinckii Fanp3 in hydrogen production. Culture temperature of 35 °C was established to be the most favorable for maximum rate of hydrogen production. The distribution of soluble metabolic products (SMP) was also greatly affected by temperature. Considering glucose as a substrate, the activation energy (E_a) for hydrogen production was calculated as 81.01 kcal/mol and 21.4% of substrate energy was recovered in the form of hydrogen. The maximal hydrogen yield and the hydrogen production rate were obtained as 2.52 mol/mol-glucose and 39.0 ml/g-glucose h⁻¹, respectively. These results indicate that C. beijerinckii Fanp3 is an ideal candidate for the fermentative hydrogen production.