Biohydrogen production from sago starch in wastewater using an enriched thermophilic mixed culture from hot spring

We investigated the feasibility of producing biohydrogen from sago starch in wastewater using a thermophilic mixed culture enriched from a hot spring in southern Thailand (PGK). The effects of repeated batch cultivations, the dependence of the initial pH values ranging from 5.0 to 8.0 and the initial sago starch concentrations ranging from 2.5 to 60 g/l as well as the utilization of 4 different starch types (gelatinized dry starch, non-gelatinized dry starch, gelatinized wet starch, and non-gelatinized wet starch) were examined in batch experiments at 60 °C. Microbial community structures were also investigated. The methane-free biogas evolved contained up to 55% hydrogen, with the remainder comprising carbon dioxide. Repeated batch cultivation under identical conditions improved the hydrogen yield. Gelatinized dry starch at an initial pH of 6.5 and an initial starch concentration of 2.5 g/l gave the maximum hydrogen yield of 422 ml-H₂/g-starchadded (80% of the theoretical limit). PCR-DGGE profiles of 16S rRNA gene fragments from the cultures showed that the predominant hydrogen producers were closely related to thermophilic anaerobic bacteria, including Thermoanaerobacterium saccharolyticum, Thermoanaerobacterium thermosulfurigenes, and uncultured Thermoanaerobacterium sp., all of which probably produced hydrogen with simultaneous generation of ethanol. The thermophilic bacilli capable of utilizing starch, including Bacillus sp., Anoxybacillus sp., Bacillus lentus, and Geobacillus sp., were also found in the starch cultures, but mainly in the non-gelatinized wet starch culture.     

Biohydrogen production from cheese processing wastewater by anaerobic fermentation using mixed microbial communities

Hydrogen (H₂) production from simulated cheese processing wastewater via anaerobic fermentation was conducted using mixed microbial communities under mesophilic conditions. In batch H₂ fermentation experiments H₂ yields of 8 and 10 mM/g COD fed were achieved at food-to-microorganism (F/M)$(F/M)$ ratios of 1.0 and 1.5, respectively. Butyric, acetic, propionic, and valeric acids were the major volatile fatty acids (VFA) produced in the fermentation process. Continuous H₂ fermentation experiments were also performed using a completely mixed reactor (CSTR). The pH of the bioreactor was controlled in a range of 4.0–5.0 by addition of carbonate in the feed material. Maximum H₂ yields were between 1.8 and 2.3 mM/g COD fed for the loading rates (LRs) tested with a hydraulic retention time (HRT) of 24 h. Occasionally CH₄ was produced in the biogas with concurrent reductions in H₂ production; however, continuous H₂ production was achieved for over 3 weeks at each LR. The 16S rDNA analysis of DNA extracted from the bioreactors during periods of high H₂ production revealed that more than 50% of the bacteria present were members of the genus Lactobacillus and about 5% were Clostridia. When H₂ production in the bioreactors decreased concurrent reductions in the genus Lactobacillus were also observed. Therefore, the microbial populations in the bioreactors were closely related to the conditions and performance of the bioreactors.     

Biohydrogen production from mixed xylose/arabinose at thermophilic temperature by anaerobic mixed cultures in elephant dung

Biohydrogen production by batch fermentation of mixed xylose/arabinose at thermophilic temperature using anaerobic mixed cultures in elephant dung as the seed inoculums was investigated. Elephant dung was heat-treated in boiling water for 2 h before used as the seed inoculum in order to inhibit methanogenic activity. Biohydrogen was successfully produced from mixed xylose/arabinose. The optimum conditions for hydrogen production were the initial concentration of mixed xylose/arabinose 5 g/L each, initial cultivation pH 5.5 and temperature 55 °C. Under the optimum conditions, a maximum hydrogen yield of 2.49 mol-H₂/mol-sugar consumed was obtained. The optimum conditions were then used to produce hydrogen from sugar derived from acid-hydrolysed sugarcane bagasse (SCB) at a reducing sugar concentration of 10 g/L in which a lower hydrogen yield of 1.48 mol-H₂/mol-sugar consumed was achieved. Main soluble product was acetate suggesting the hydrogen fermentation from mixed xylose/arabinose is the acetate type. The dominant hydrogen producers found in both fermentation broth were Thermoanaerobacterium thermosaccharolyticum and Clostridium sp. Lower hydrogen yield in the SCB hydrolysate fermentation broth may be due to the present of Clostridium ragsdalei and microorganisms in the class Bacilli viz. Lactococus lactis subsp., Lactobacillus delbrueckii, and Sporolactobacillus sp. as well as the inhibitors (acetic acid and furfural) contained in the SCB hydrolysate.     

Microbial community analysis of thermophilic mixed culture sludge for biohydrogen production from palm oil mill effluent

The microbial community structure of thermophilic mixed culture sludge used for biohydrogen production from palm oil mill effluent was analyzed by fluorescence in situ hybridization (FISH) and 16S rRNA gene clone library techniques. The hydrogen-producing bacteria were isolated and their ability to produce hydrogen was confirmed. The microbial community was dominated by Thermoanaerobacterium species (∼66%). The remaining microorganisms belonged to Clostridium and Desulfotomaculum spp. (∼28% and ∼6%, respectively). Three hydrogen-producing strains, namely HPB-1, HPB-2, and HPB-3, were isolated. 16S rRNA gene sequence analysis of HPB-1 and HPB-2 revealed a high similarity to Thermoanaerobacterium thermosaccharolyticum (98.6% and 99.0%, respectively). The Thermoanaerobacterium HPB-2 strain was a promising candidate for thermophilic fermentative hydrogen production with a hydrogen yield of 2.53 mol H₂ mol⁻¹hexose from organic waste and wastewater containing a mixture of hexose and pentose sugars. Thermoanaerobacterium species play a major role in thermophilic hydrogen production as confirmed both by molecular and cultivation-based analyses.     

The effect of hydrolysis and sterilization in biohydrogen production from cassava processing wastewater medium using anaerobic bacterial consortia

We evaluated the production of bioH₂ from Cassava Processing Wastewater (CPW) using three microbial consortia (Vac, Esg, and Lod) from different Brazilian environments. These consortia consisted of bacteria of the genera Clostridium, Sporanaerobacter, Coprococcus, Enterococcus, and others. The CPW was supplemented with nitrogen and used raw or hydrolyzed and sterilized or not. Four independent variables were optimized (Box-Behnken design): pH, temperature, C/N ratio, and inoculum ratio. Three quadratic models were obtained and explain production of bioH₂ (R² of 0.93, 0.87 and 0.82 for the consortia Vac, Esg and Lod, respectively). The quadratic effects were the most significant in comparison to linear effects and interactions. The optimal conditions were: pH: 5.5–7.0; temperature 37-39 °C; inoculum ratio 15%, and C/N ratio 5-3,5. After 48 h, the maximum yields of hydrogen obtained with hydrolyzed and sterilized CPW were 1.82, 1.7 and 1.68 mols of H₂/mol of maltose for Lod, Esg and Vac, respectively. While, for the only sterilized substrate the yields are in the range 1.33–1.54 mol H₂/mol maltose.     

Biohydrogen production from starch wastewater and application in fuel cell

Biohydrogen was produced from starch in wastewater by anaerobic fermentation. The effects of parameters, such as pH, starch concentration were investigated and optimum operating conditions were determined. The optimal pH and starch concentration for hydrogen production at 37 °C were 6.5 and 5 g/L, respectively with a maximum hydrogen yield of 186 ml/g-starch. The produced biogas contains 99% of hydrogen after passing through KOH solution to remove CO₂. The anaerobic fermentation installation was integrated with a proton-exchange-membrane fuel cell (PEMFC) system for on-line electricity generation. This combination system of biohydrogen and fuel cell achieved a power output of 0.428 W at 0.65 V per cell.     

Hydrogen production from xylose by moderate thermophilic mixed cultures using granules and biofilm up-flow anaerobic reactors

Continuous H₂ production from xylose by granules and biofilm up-flow anaerobic reactor using moderate thermophilic mixed cultures was investigated. The maximum H₂ yield of 251 mL H₂/g-xylose with H₂production rate of 15.1 L H₂/L⋅d was obtained from granules reactor operating at the organic loading rate (OLR) of 60 g-xylose/L⋅d and hydraulic retention time (HRT) of 4 h. Meanwhile the highest H₂ production rate of 13.3 L H₂/L⋅d with an H₂ yield of 221 mL H₂/g–xylose was achieved from the biofilm reactor. Both reactors were dominated by Thermoanaerobacterium species with acetate and butyrate as main fermentation products. The microbial community of the biofilm reactor was composed of Thermoanaerobacterium species, while granules reactor was composed of Clostridium sp., Thermoanaerobacterium sp. and Caloramator sp. The granular reactor was more microbial diversity and more balance between economic efficiency in term of the hydrogen production rate and technical efficiency in term of hydrogen yield.     

Comparative evaluation of the biohydrogen production potential of thermophilic microorganisms isolated from hot springs located in Izmir

Biohydrogen production with the utilization of thermophilic microorganisms is a green process contributing to the mitigation of fossil fuel dependency. This study reports an improved water-gas shift reaction (WGSR) methodology with isolation of thermophilic and anaerobic microorganisms from 5 different closely-located hot springs, employing enrichment and characterization techniques to evaluate their biohydrogen production capacities with 100% CO in the headspace. Enriched mixed and pure cultures were screened for biohydrogen production. An isolate from Doğanbey hot spring reached the highest H₂ yield of 0.43 mmol H₂/mmol CO with a 43% CO conversion. Using growth profiling and morphologically with SEM imaging, this isolate was characterized biochemically, and was identified as an isolated bacilli shaped bacteria capable of producing hydrogen through WGSR. This is the first study to report potential thermophilic hydrogen producers in selected hot springs in the Izmir region.     

Improving hydrogen production from cassava starch by combination of dark and photo fermentation

The combination of dark and photo fermentation was studied with cassava starch as the substrate to increase the hydrogen yield and alleviate the environmental pollution. The different raw cassava starch concentrations of 10–25 g/l give different hydrogen yields in the dark fermentation inoculated with the mixed hydrogen-producing bacteria derived from the preheated activated sludge. The maximum hydrogen yield (HY) of 240.4 ml H₂/g starch is obtained at the starch concentration of 10 g/l and the maximum hydrogen production rate (HPR) of 84.4 ml H₂/l/h is obtained at the starch concentration of 25 g/l. When the cassava starch, which is gelatinized by heating or hydrolyzed with α-amylase and glucoamylase, is used as the substrate to produce hydrogen, the maximum HY respectively increases to 258.5 and 276.1 ml H₂/g starch, and the maximum HPR respectively increases to 172 and 262.4 ml H₂/l/h. Meanwhile, the lag time (λ) for hydrogen production decreases from 11 h to 8 h and 5 h respectively, and the fermentation duration decreases from 75–110 h to 44–68 h. The metabolite byproducts in the dark fermentation, which are mainly acetate and butyrate, are reused as the substrates in the photo fermentation inoculated with the Rhodopseudomonas palustris bacteria. The maximum HY and HPR are respectively 131.9 ml H₂/g starch and 16.4 ml H₂/l/h in the photo fermentation, and the highest utilization ratios of acetate and butyrate are respectively 89.3% and 98.5%. The maximum HY dramatically increases from 240.4 ml H₂/g starch only in the dark fermentation to 402.3 ml H₂/g starch in the combined dark and photo fermentation, while the energy conversion efficiency increases from 17.5–18.6% to 26.4–27.1% if only the heat value of cassava starch is considered as the input energy. When the input light energy in the photo fermentation is also taken into account, the whole energy conversion efficiency is 4.46–6.04%.     

Comparison of bio-hydrogen production from hydrolyzed wheat starch by mesophilic and thermophilic dark fermentation

Hydrogen gas production potentials of acid-hydrolyzed and boiled ground wheat were compared in batch dark fermentations under mesophilic (37 °C) and thermophilic (55 °C) conditions. Heat-treated anaerobic sludge was used as the inoculum and the hydrolyzed ground wheat was supplemented by other nutrients. The highest cumulative hydrogen gas production (752 ml) was obtained from the acid-hydrolyzed ground wheat starch at 55 °C and the lowest (112 ml) was with the boiled wheat starch within 10 days. The highest rate of hydrogen gas formation (7.42 ml H₂ h⁻¹) was obtained with the acid-hydrolyzed and the lowest (1.12 ml H₂ h⁻¹) with the boiled wheat at 55 °C. The highest hydrogen gas yield (333 ml H₂ g⁻¹ total sugar or 2.40 mol H₂ mol⁻¹ glucose) and final total volatile fatty acid (TVFA) concentration (10.08 g L⁻¹) were also obtained with the acid-hydrolyzed wheat under thermophilic conditions (55 °C). Dark fermentation of acid-hydrolyzed ground wheat under thermophilic conditions (55 °C) was proven to be more beneficial as compared to mesophilic or thermophilic fermentation of boiled (partially hydrolyzed) wheat starch.     

Biohydrogen production from acid hydrolyzed wastewater treatment sludge by dark fermentation

Waste generation, waste management, sustainable energy production, and global warming are interrelated environmental issues to be considered together. Wastewater treatment sludge is an organic substance rich waste which causes significant environmental problems. However, these wastes can be used as raw material in biofuel generation. This study was designed to investigate the possible utilization of waste sludge in biohydrogen production by taking these facts into consideration. For this purpose, the sludge was first pre-treated with acid and then, the solid (sludge) and liquid (filtrate) phases of acid pre-treated sludge were used as the substrates for biohydrogen generation dark fermentation. Two-factor factorial experimental design method was used in acid hydrolysis of sludge to determine the effect of pH (pH = 2–6) and reaction period (time, min) elution of chemical oxygen demand (COD), total organic carbon (TOC) and total sugar (TS), NH₄N and PO₄P. Statistical evaluation of the results indicated that pH significantly affects the elution of organic carbon and nutrient content of sludge while the reaction time is significant for only organic carbon content. The optimum pretreatment conditions for maximum organic and nutrient elution were determined as pH = 2 and t = 1440 min. The pretreated products, named as filtrate sludge and sludge, conducted to dark fermentation under mesophilic conditions for biohydrogen generation showed that pretreatment of waste sludge at pH = 6 is the best condition giving the maximum yields (Y_H2) as Y_H2 = 24 mmol g⁻¹ Total Sugar _consumed and Y_H2 = 41 mmol g⁻¹ Total sugar consumed, for filtrate and sludge, respectively.     

Impact of furfural on biological hydrogen production kinetics from synthetic lignocellulosic hydrolysate using mesophilic and thermophilic mixed cultures

The impact of furfural on hydrogen production and microbial growth kinetics was assessed using mixed anaerobic cultures at mesophilic and thermophilic conditions. Mesophilic experiments showed a hydrogen yield of 1.6 mol H₂/mol initial sugars at 1 g/L furfural which is a 45% enhancement from the control (0 g/L furfural) at a substrate-to-biomass ratio (S°/X°) of 4 gCOD/gVSS. On the other hand, thermophilic experiments showed no enhancement at 1 g/L furfural but rather a 53% decrease in hydrogen yield from its control. Furfural inhibition threshold limit was observed to be greater than 1 g/L for mesophilic experiments and less than 1 g/L for thermophilic experiments. In both cases, 4 g/L was the most recalcitrant furfural concentration, with propionate and lactate the most predominant soluble metabolites in the mesophilic and thermophilic experiments respectively. It was also noted that in the presence of furfural, hydrogen-producers in both mesophilic and thermophilic mixed cultures were inactivated as no hydrogen was produced until furfural was completely degraded irrespective of sugars degradation. This study also presents the kinetics of microbial growth and substrate degradation obtained using the Monod model on MATLAB^®, ignoring an inhibition term. IC₅₀ of the mesophilic and thermophilic experiments were 1.03 g/L and 0.5 g/L respectively indicating that the thermophilic hydrogen producers were more strongly affected by furfural than the mesophilic cultures.     

Developing a thermophilic hydrogen-producing microbial consortia from geothermal spring for efficient utilization of xylose and glucose mixed substrates and oil palm trunk hydrolysate

Xylose and glucose are the major sugar components of lignocellulosic hydrolysate. This study aims to develop thermophilic hydrogen-producing consortia from eight sediments-rich samples of geothermal springs in Southern Thailand by repeated batch cultivation at 60 °C with glucose, xylose and xylose-glucose mixed substrates. Significant hydrogen production potentials were obtained from thermophilic enriched cultures encoded as PGR and YLT with the maximum hydrogen yields of 241.4 and 231.6 mL H₂/g sugar_consumed, respectively. After repeated batch cultivation the hydrogen yield from xylose-glucose mixed substrate of PGR increased to 375 mL H₂/g sugar_consumed which was 30% higher than that of YLT (287 mL H₂/g sugar_consumed). Soluble metabolites from xylose-glucose mixed substrates were composed mostly of butyric acid (20.6-21.8 mM), acetic acid (7.2-13.5 mM), lactic acid (8.2-11.7 mM) and butanol (4.4-13.0 mM). Denaturing gradient gel electrophoresis (DGGE) profiles illustrated small difference in microbial patterns of PGR enriched with glucose, xylose-glucose mixed substrate and xylose. The dominant populations were affiliated with low G + C content Gram-positive bacteria, Thermoanaerobacterium sp., Thermoanaerobacter sp., Caloramater sp. and Anoxybacillus sp. based on the 16S rRNA gene. Cultivation of the enriched culture PGR in oil palm trunk hydrolysate, the maximum hydrogen yield of 301 mL H₂/g sugar_consumed was achieved at hydrolysate concentration of 40% (v/v). At higher concentration to 80% (v/v), the hydrogen fermentation process was inhibited. Therefore, the efficient thermophilic hydrogen-producing consortia PGR has successfully developed and has great potential for production of biohydrogen from mixed sugars hydrolysate.     

High-solid dark fermentation of cassava pulp and cassava processing wastewater for hydrogen production

Dark fermentation (DF) is a promising biological process for hydrogen production from biomass. However, low hydrogen yield (HY) is a major hurdle impeding its use at large-scale operation. A potential way to mitigate this problem is to increase the concentration of substrate in the process to increase hydrogen production. The present study investigated the possibility of using high-solid DF to produce hydrogen from cassava processing wastes, i.e., cassava pulp (CP) and cassava processing wastewater (CPW). CP was suspended in CPW and hydrolyzed enzymatically under optimum conditions of 150 g-CP/L, 29 U/g of α-amylase, 47 U/g of glucoamylase, and 60 FPU/g of cellulase. The hydrolysis performed at 50 °C for 24 h yielded a reducing sugar concentration of 117.7 ± 1.8 g/L, equivalent to 0.78 g-reducing-sugar/g-CP. Subsequent DF of CP-CPW enzymatic slurry, which contained 12.1% water insoluble solids, resulted in a cumulative production of 13.72 ± 0.22 L-H₂, equivalent to 225.2 ± 3.7 mL-H₂/g-VS. This was 83.1% of a maximum stoichiometric HY, based on carbohydrate content of CP and soluble metabolites production. The present study shows clearly the applicability of high-solid DF in the production of hydrogen from cassava processing wastes.     

Biohydrogen production from poplar leaves pretreated by different methods using anaerobic mixed bacteria

Leaves are one of the main by-products of forestry. In this study, batch experiments were carried out to convert poplar leaves pretreated by different methods into hydrogen using anaerobic mixed bacteria at 35 °C. The effects of acid (HCl), alkaline (NaOH) and enzymatic (Viscozyme L, a mixture of arabanase, cellulase, β-glucanase, hemicellulase and xylanase) pretreatments on the saccharification of poplar leaves were studied. Furthermore, the effects of acid and enzymatic pretreatment on hydrogen production, together with their corresponding degradation efficiencies for the total reducing sugar (TRS) and metabolites were compared. A maximum cumulative hydrogen yield of 44.92 mL/g-dry poplar leaves was achieved from substrate pretreated with 2% Vicozyme L, which was approximately 3-fold greater than that in raw substrate and 1.34-fold greater than that from substrate pretreated with 4% HCl. The results show that enzymatic pretreatment is an effective method for enhancing the hydrogen yield from poplar leaves.     

Dark fermentation of starch factory wastewater with acid- and base-treated mixed microorganisms for biohydrogen production

Biohydrogen (Bio-H₂) can be produced from starch factory wastewater and mixed microorganisms using dark fermentation. Acidic and basic chemicals were used to treat the microorganisms to select the hydrogen (H₂)-producing culture. The experiment used a 120 mL bioreactor at 35 °C and the operation commenced with the initial pH level of wastewater in the pH range 4–7 in batch mode. The bacteria:chemical oxygen demand (COD) ratio was 0.2. The initial pH level of the wastewater in the fermentation process affected the H₂ yield and the specific hydrogen production rate (SHPR). For acid-treated bacteria, the maximum H₂ yield and SHPR were produced at an initial pH of 6.5. The maximum H₂ yield and SHPR were 138 mL/g COD degraded and 7.42 mL/g cells?h, respectively. For the base-treated bacteria, the maximum H₂ yield and SHPR were produced at initial pH of 6.5 and pH 7, respectively. The maximum H₂ yield and SHPR were 182 mL/g COD degraded and 25.60 mL/g cells?h, respectively. The COD degradation efficiency levels were 16 and 20% for acid- and base-treated bacteria, respectively. The digested wastewater remained acidic at pH 4.79–4.83. Throughout the study, no methane gas was observed in the gas mixture produced.     

Biohydrogen production from waste glycerol and sludge by anaerobic mixed cultures

Key factors affecting biohydrogen production from waste glycerol and sludge by anaerobic mixed cultures were optimized using response surface methodology (RSM) with central composite design (CCD). Investigated parameters were waste glycerol concentration, sludge concentration, and the amount of Endo–nutrient addition. Concentrations of waste glycerol and sludge had a significant individual effect on hydrogen production rate (HPR) (p ≤ 0.05). The interactive effect on HPR (p ≤ 0.05) was found between waste glycerol concentration and sludge concentration. The optimal conditions for the maximum HPR were: waste glycerol concentration 22.19 g/L, sludge concentration 7.16 g-total solid (TS/L), and the amount of Endo–nutrient addition 2.89 mL/L in which the maximum HPR of 1.37 mmol H₂/L h was achieved. Using the optimal conditions, HPR from a co-digestion of waste glycerol and sludge (1.37 mmol H₂/L h) was two times greater than the control (waste glycerol without addition of sludge) (0.76 mmol H₂/L h), indicating a significant enhancement of HPR by sludge. Major metabolites of the fermentation process were ethanol, 1,3-propanediol (1,3-PD), lactate, and formate.     

Improvement of hydrogen production with thermophilic mixed culture from rice spent wash of distillery industry

Biohydrogen production processes were investigated using thermophilic bacterial consortia enriched from sludge of the anaerobic digester. A multiple parameter optimization viz. temperature, pH and substrate concentration was performed for maximization of hydrogen production. Heat shock pre-treatment followed by BES (2-bromo ethane sulfonate) treatment was done for the enrichment of hydrogen producing bacteria. Box–Behnken design and response surface methodology were adopted to investigate the mutual interaction among the process parameters. Experimental optimization of process parameters (60 °C, pH 6.5 and 10 g/L) gave the maximum hydrogen production and yield of 3985 mL/L and 2.7 mol/mol glucose respectively in the batch system which is higher than the reported value on UASB. These experimental parameters found concurrent with the values obtained from the theoretical model i.e. 58.4 °C, pH 6.6, 10.8 g/L and yield of 2.71 mol/mol glucose. At optimized conditions, maximum hydrogen production rate (R_m) of 850 mL/h, gas production potential (P) of 4551 mL/L and lag time (λ) of 1.98 h were determined using modified Gompertz equation. Using the optimum conditions, hydrogen production from rice spent wash was conducted in which hydrogen yield of 464 mL/g carbohydrate and hydrogen production rate of 168 mL/L h were obtained. PCR-DGGE profile showed that the thermophilic mixed culture was predominated with species closely affiliated to Thermoanaerobacterium sp.     

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

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

Exploring optimal conditions for thermophilic fermentative hydrogen production from cassava stillage

This study investigated the effects of seed sludges, alkalinity and HRT on the thermophilic fermentative hydrogen production from cassava stillage. Five different kinds of sludges were used as inocula without any pretreatment. Though batch experiments showed that mesophilic anaerobic sludge was the best inoculum, the hydrogen yields with different seed sludges were quite similar in continuous experiments in the range of 82.9–92.3 ml H₂/gVS without significant differences which could be attributed to the establishment of Uncultured Thermoanaerobacteriaceae bacterium-dominant microbial communities in all reactors. It is indicated that results obtained from batch experiments are not consistent with those from continuous experiments and all the tested seed sludges are good sources for continuous thermophilic hydrogen production from cassava stillage. The influent alkalinity of 6 g NaHCO₃/L and HRT 24 h were optimal for hydrogen production with hydrogen yield of 76 ml H₂/gVS and hydrogen production rate of 3215 ml H₂/L/d. Butyrate was the predominant metabolite in all experiments. With the increase in alkalinity of more than 6 g/L, the concentration of VFA/ethanol increased while hydrogen yield decreased due to the higher concentration of acetate and propionate. The decrease in HRT resulted in the higher hydrogen production rate but lower hydrogen yield. Variation of hydrogen yields were quite correlated with butyrate/acetate (B/A) ratio with different influent alkalinities, however, butyrate was important parameter to justify the hydrogen yields with various HRTs.