Ultrasonication for biohydrogen production from food waste

The applicability of ultrasonication for enhancement of hydrogen production from food wastes was evaluated in three different systems. System A is a conventional continuously-stirred tank reactor (CSTR) fed raw food waste; system B is a conventional continuously-stirred tank reactor fed sonicated food waste, and system C (US patent-pending) is the sonicated biological hydrogen reactor (SBHR) which comprised a CSTR connected with an ultrasonic probe at the bottom of the reactor. In this study, the increase in hydrogen production rate relative to the control (system A) due to sonication of the feed before the digestion was 27%, compared to 90% in the SBHR. Similarly, the CSTR with sonicated feed exhibited a 23% increase in hydrogen yield as mol H₂/mol hexose_consumed compared to a 62% increase in the SBHR relative to the control (system A). The VSS destruction in the SBHR was higher than those in the CSTR and CSTR with sonicated feed by 50% and 60%, respectively.     

Effect of organic loading on a novel hydrogen bioreactor

This study investigated the impact of six organic loading rates (OLR) ranging from 6.5 gCOD/L-d to 206 gCOD/L-d on the performance of a novel integrated biohydrogen reactor clarifier systems (IBRCSs) comprised a continuously stirred reactor (CSTR) for biological hydrogen production, followed by an uncovered gravity settler for decoupling of solids retention time (SRT) from hydraulic retention time (HRT). The system was able to maintain a high molar hydrogen yield of 2.8 mol H₂/mol glucose at OLR ranging from 6.5 to 103 gCOD/L-d, but dropped precipitously to approximately 1.2 and 1.1 mol H₂/mol glucose for the OLRs of 154 and 206 gCOD/L-d, respectively. The optimum OLR at HRT of 8 h for maximizing both hydrogen molar yield and volumetric hydrogen production was 103 gCOD/L-d. A positive statistical correlation was observed between the molar hydrogen production and the molar acetate-to-butyrate ratio. Biomass yield correlated negatively with hydrogen yield, although not linearly. Analyzing the food-to-microorganisms (F/M) data in this study and others revealed that, both molar hydrogen yields and biomass specific hydrogen rates peaked at 2.8 mol H₂/mol glucose and 2.3 L/gVSS-d at F/M ratios ranging from 4.4 to 6.4 gCOD/gVSS-d. Microbial community analysis for OLRs of 6.5 and 25.7 gCOD/L-d showed the predominance of hydrogen producers such as Clostridium acetobutyricum, Klebsiella pneumonia, Clostridium butyricum, Clostridium pasteurianum. While at extremely high OLRs of 154 and 206 gCOD/L-d, a microbial shift was clearly evident due to the coexistence of the non-hydrogen producers such as Lactococcus sp. and Pseudomonas sp.     

Comparative assessment of decoupling of biomass and hydraulic retention times in hydrogen production bioreactors

This study compared biological hydrogen production from glucose in two continuously stirred tank reactors (CSTRs) and two integrated biohydrogen reactor clarifier systems (IBRCSs) comprising CSTRs with gravity settlers to decouple the hydraulic retention time (HRT) from solids retention time (SRT). The four systems were operated at organic loading rates of 6.5–42.8 gCOD/L-d, and HRTs of 8–12 h. The SRT was maintained at 2 days in the two IBRCSs. The decoupling of SRT from HRT not only increased glucose conversion from 29–50% in the CSTR to 99.9% in the IBRCSs, but also the volumetric hydrogen production from 0.55–1.8 in the CSTRs to 2.4–9.6 L/L-d. Biomass yields in the two IBRCSs were 0.09 and 0.13 g VSS/g glucose converted, about 50% lower than the CSTR yields of 0.19 and 0.29 g VSS/g glucose converted. Hydrogen yield increased from 0.5–1.0 mol H₂/mol glucose converted in the CSTR to 2.8 mol H₂/mol glucose converted in the IBRCSs. The inverse relationship between hydrogen yield and biomass yield observed in this study implies that the hydrogen yield is maximized with the minimization of biomass yield, thus necessitating decoupling of SRT from HRT to ensure sufficient reactor biomass. DGGE analysis confirmed the specificity of the microbial culture in the IBRCSs with the high-hydrogen producing Clostridium species, as compared to the more diverse cultures in the CSTR.     

Aspect ratio effect of bioreactor on fermentative hydrogen production with immobilized sludge

The phenomenon of bacterial wash-out frequently occurs in the traditional continuous stirred tank reactor (CSTR) systems at low hydraulic retention time (HRT). In this study, the effect of different aspect ratios, height (H) to diameter (D) of 1:1, 3:1 and 5:1, of a CSTR with immobilized anaerobic sludge on hydrogen (H₂) production were investigated. The pH, volatile suspended solids (VSS) and total solids (TS) concentrations of the seed sludge were 6.8, 33.3 and 65.1 g/L, respectively. Thermally treated sludge was immobilized by silicone gel entrapment approach. The entrapped-sludge system operated stably at a low HRT without suffering from cell wash-out. Hence, the hydrogen production rate (HPR) was enhanced by increasing organic loading rates. The immobilized sludge CSTRs were operated at 40 °C with sucrose (10, 20, 30 and 40 g COD/L) and Endo nutrient medium at different HRTs (4, 2, 1 and 0.5 h). It was found that the granule formation enhanced HPR. The maximum HPR and the H₂ yield were found to be 15.36H₂ L/h/L and 3.16 mol H₂/mol sucrose, respectively, with the H₂ content in the biogas above 44% for all tests runs.     

An integrated system for hydrogen and methane production during landfill leachate treatment

The patent-pending integrated waste-to-energy system comprises both a novel biohydrogen reactor with a gravity settler (Biohydrogenator), followed by a second stage conventional anaerobic digester for the production of methane gas. This chemical-free process has been tested with a synthetic wastewater/leachate solution, and was operated at 37 °C for 45 d. The biohydrogenator (system (A), stage 1) steadily produced hydrogen with no methane during the experimental period. The maximum hydrogen yield was 400 mL H₂/g glucose with an average of 345 mL H₂/g glucose, as compared to 141 and 118 mL H₂/g glucose for two consecutive runs done in parallel using a conventional continuously stirred tank reactor (CSTR, System (B)). Decoupling of the solids retention time (SRT) from the hydraulic retention time (HRT) using the gravity settler showed a marked improvement in performance, with the maximum and average hydrogen production rates in system (A) of 22 and 19 L H₂/d, as compared with 2–7 L H₂/d in the CSTR resulting in a maximum yield of 2.8 mol H₂/mol glucose much higher than the 1.1–1.3 mol H₂/mol glucose observed in the CSTR. Furthermore, while the CSTR collapsed in 10–15 d due to biomass washout, the biohydrogenator continued stable operation for the 45 d reported here and beyond. The methane yield for the second stage in system (A) approached a maximum value of 426 mL CH₄/gCOD removed, while an overall chemical oxygen demand (COD) removal efficiency of 94% was achieved in system (A).     

Comparison of thermophilic and hyperthermophilic dark fermentation with subsequent mesophilic methanogenesis in expanded granular sludge bed reactors

Herein, dark fermentation (DF, V = 5.5 L) and subsequent mesophilic methanogenesis (V = 43.5 L) are run as expanded granular sludge bed reactors (EGSB) at thermophilic (υDF = 60 °C) and hyperthermophilic (υDF = 80 °C) temperatures. A synthetic glucose wastewater is run with a 22.5 g/L chemical oxygen demand (COD) and 48–9 h hydraulic retention times (HRTs), giving organic loading rates (OLRs) of 11–60 g COD/L/d for DF. The maximum hydrogen production rate (HPR) is HPR = 3.0 m³/m³/d for HRT = 9 h with a 50 L/kg COD hydrogen yield (HY) and 40 vol% H₂. Methane production rate (MPR) reaches MPR = 2.6 m³/m³/d with 70 vol% CH₄ at HRT = 2.8 d. The highest H₂ yields are HY = 180 L/kg COD with 53 vol% H₂ (thermophilic, HRT = 48 h). Hyperthermophilic temperatures led to lower HPRs (0.7 m³/m³/d) and MPRs (1.6 m³/m³/d). 53% of Thermoanaerobacterium thermosaccharolyticum as an H₂ producer are found. Discoloration of granular sludge from black to white and granule stability was observed in DF.     

Biological hydrogen production in continuous stirred tank reactor systems with suspended and attached microbial growth

Fermentative H₂ production in continuous stirred tank reactor (CSTR) system with bacteria attached onto granular activated carbon (GAC) was designed to produce H₂ continuously. The H₂ production performances of CSTR with suspended and attached-sludge from molasses were examined and compared at various organic loading rates (8–40 g COD/L/d) at hydraulic retention time of 6 h under mesophilic conditions (35 °C). Both reactor systems achieved ethanol-type fermentation in the pH ranges 4.5–4.8 and 3.8–4.4, respectively, while ORP ranges from −450 to −470 mV and from −330 to −350 mV, respectively. The hydrogen production rate in the attached system was higher compared to that of the suspended system (9.72 and 6.65 L/d/L, respectively) while specific hydrogen production rate of 5.13 L/g VSS/d was higher in the suspended system. The attached-sludge CSTR is more stable than the suspended-sludge CSTR with regard to hydrogen production, pH, substrate utilization efficiency and metabolic products (e.g., volatile fatty acids and ethanol) during the whole test.     

Dark fermentative hydrogen production from xylose by a hot spring enrichment culture

Dark fermentative hydrogen production by a hot spring culture was studied from different sugars in batch assays and from xylose in continuous stirred tank reactor (CSTR) with on-line pH control. Batch assays yielded hydrogen in following order: xylose > arabinose > ribose > glucose. The highest hydrogen yield in batch assays was 0.71 mol H₂/mol xylose. In CSTR the highest H₂ yield and production rate at 45 °C were 1.97 mol H₂/mol xylose and 7.3 mmol H₂/h/L, respectively, and at 37 °C, 1.18 mol H₂/mol xylose and 1.7 mmol H₂/h/L, respectively. At 45 °C, microbial community consisted of only two bacterial strains affiliated to Clostridium acetobutulyticum and Citrobacter freundii, whereas at 37 °C six Clostridial species were detected. In summary hydrogen yield by hot spring culture was higher with pentoses than hexoses. The highest H₂ production rate and yield and thus, the most efficient hydrogen producing bacteria were obtained at suboptimal temperature of 45 °C for both mesophiles and thermophiles.     

Effect of organic loading rate on fermentative hydrogen production from continuous stirred tank and membrane bioreactors

The influence of organic loading rates (OLRs) on the performance of fermentative hydrogen-producing bioreactors operating in continuous stirred tank reactor (CSTR) and membrane bioreactor (MBR) modes was examined. Five OLRs were examined, ranging from 4.0 to 30 g COD L^?1 d^?1, with influent glucose concentrations ranging from 1.3 to 10 g COD L^?1. At OLRs up to 13 g COD L^?1 d^?1, all influent glucose was utilized and the H₂ yield was not significantly influenced by OLR, although the yield in the CSTR mode was significantly higher than that in the MBR mode, 1.25 versus 0.97 mol H₂ (mol Gluc. Conv.)^?1, respectively. At an OLR of 30 g COD L^?1 d^?1, both reactor modes were overloaded with respect to glucose utilization and also had significantly higher H₂ yields of 1.77 and 1.49 mol H₂ (mol Gluc. Conv.)^?1 for the CSTR and MBR modes, respectively, versus the underloaded operation. At the intermediate OLR of 22 g COD L^?1 d^?1, the H₂ yield was maximized at 1.78 mol H₂ (mol Gluc. Conv.)^?1 for both the CSTR and MBR operation. Overall H₂ production was 50% higher in the MBR mode, 0.78 versus 0.51 moles d^?1, because the CSTR mode was overloaded with respect to glucose utilization at this OLR. These results suggest that an optimum OLR that maximizes H₂ yield and H₂ production may be near the OLR that causes overload with respect to substrate utilization. Additionally, while the CSTR mode is easier to operate and provides higher H₂ yields at underloaded and overloaded OLRs, the MBR mode may be preferable when operating near the optimum OLR.     

Feasibility of biohydrogen production from Gelidium amansii

The feasibility of hydrogen production from red algae was investigated. Galactose, the main sugar monomer of red algae, was readily converted to hydrogen by dark fermentation. The maximum hydrogen production rate and yield of galactose were 2.46 L H₂/g VSS/d and 2.03 mol H₂/mol galactose_added, respectively, which were higher than those for glucose (0.914 L H₂/g VSS/d and 1.48 mol H₂/mol galactose_added). The distribution of soluble byproducts showed that H₂ production was the main pathway of galactose uptake. 5-HMF, the main byproduct of acid hydrolysis of red algae causes noncompetitive inhibition of H₂ fermentation. 1.37 g/L of 5-HMF decreased hydrogen production rate by 50% compared to the control. When red algae was hydrolyzed at 150 °C for 15 min and detoxified by activated carbon, 53.5 mL of H₂ was produced from 1 g of dry algae with a hydrogen production rate of 0.518 L H₂/g VSS/d. Red algae, cultivable on vast tracts of sea by sunlight without any nitrogen-based fertilizer, could be a suitable substrate for biohydrogen production.     

Evaluation of hidden H2-consuming pathways using metabolic flux-based analysis for a fermentative side-stream dynamic membrane bioreactor using untreated seed sludge

This study investigates the performance and hidden hydrogen consuming metabolic pathways of a fermentative side stream dynamic membrane (DM) bioreactor using flux balance analysis (FBA). The bioreactor was inoculated with untreated methanogenic seed sludge. It was found that fouling rate aggravated with increasing COD concentration (10–30 g/L) and was positively correlated to it rather than to the applied solid flux on the DM module. Due to increased fouling rate the hydraulic retention time (HRT) could not be reduced less than 0.82 ± 0.02 d. An increase in the organic loading rate (OLR) led to an increase in H₂ yield from 0.01 to 0.76 mol H₂/mol of sucrose. FBA revealed that homoacetogenesis was the main H₂-consuming pathway at lower OLRs (corresponding to 10 and 15 g COD/L), while for the OLR corresponding to 30 g COD/L, homoacetogens were suppressed. More importantly, caproic acid production pathway was identified for the first time as another H₂-consuming pathway at high OLR which was not significant at lower OLRs during fermentative dynamic membrane bioreactor operations.     

Direct fermentation of Laminaria japonica for biohydrogen production by anaerobic mixed cultures

A few studies have been made on fermentative hydrogen production from marine algae, despite of their advantages compared with other biomass substrates. In this study, fermentative hydrogen production from Laminaria japonica (one brown algae species) was investigated under mesophilic condition (35 ± 1 °C) without any pretreatment method. A feasibility test was first conducted through a series of batch cultivations, and 0.92 mol H₂/mol hexose_added, or 71.4 ml H₂/g TS of hydrogen yield was achieved at a substrate concentration of 20 g COD/L (based on carbohydrate), initial pH of 7.5, and cultivation pH of 5.5. Continuous operation for a period of 80 days was then carried out using anaerobic sequencing batch reactor (ASBR) with a hydraulic retention time (HRT) of 6 days. After operation for approximately 30 days, a stable hydrogen yield of 0.79 ± 0.03 mol H₂/mol hexose_added was obtained. To optimize bioenergy recovery from L. japonica, an up-flow anaerobic sludge blanket reactor (UASBr) was applied to treat hydrogen fermentation effluent (HFE) for methane production. A maximum methane yield of 309 ± 12 ml CH₄/g COD was achieved during the 90 days operation period, where the organic loading rate (OLR) was 3.5 g COD/L/d.     

Application of immobilized upflow anaerobic sludge blanket reactor using Clostridium LS2 for enhanced biohydrogen production and treatment efficiency of palm oil mill effluent

Polyethylene glycol (PEG) gel was used to immobilize hydrogen producing Clostridium LS2 bacteria for hydrogen production in an upflow anaerobic sludge blanket (UASB) reactor. The UASB reactor with a PEG-immobilized cell packing ratio of 10% weight to volume ratio (w/v) was optimal for dark hydrogen production. The performance of the UASB reactor fed with palm oil mill effluent (POME) as a carbon source was examined under various hydraulic retention time (HRT) and POME concentration. The best volumetric hydrogen production rate of 365 mL H₂/L/h (or 16.2 mmol/L/h) with a hydrogen yield of 0.38 L H₂/g COD_added was obtained at POME concentration of 30 g COD/L and HRT of 16 h. The average hydrogen content of biogas and COD reduction were 68% and 65%, respectively. The primary soluble metabolites were butyric acid and acetic acid with smaller quantities of other volatile fatty acid and alcohols formed during hydrogen fermentation. More importantly, the feasibility of PEG-immobilized cell UASB reactor for the enhancement of the dark-hydrogen production and treatment of wastewater is demonstrated.     

Continuous biohydrogen production using cheese whey: Improving the hydrogen production rate

Due to the renewed interest in finding sustainable fuels or energy carriers, biohydrogen (Bio-H₂) from biomass is a promising alternative. Fermentative Bio-H₂ production was studied in a continuous stirred tank reactor (CSTR) operated during 65.6 d with cheese whey (CW) as substrate. Three hydraulic retention times (HRTs) were tested (10, 6 and 4 h) and the highest volumetric hydrogen production rate (VHPR) was attained with HRT of 6 h. Therefore, four organic loading rates (OLRs) at a fixed HRT of 6 h were tested thereafter, being: 92.4, 115.5, 138.6 and 184.4 g lactose/L/d. The highest VHPR (46.61 mmol H₂/L/h) and hydrogen molar yield (HMY) of 2.8 mol H₂/mol lactose were found at an OLR of 138.6 g lactose/L/d; a sharp fall in VHPR occurred at an OLR of 184.4 g lactose/L/d. Butyric, propionic and acetic acids were the main soluble metabolites found, with butyric-to-acetic ratios ranging from 1.0 to 2.4. Bacterial community was identified by partial sequence analysis of the 16S rRNA and polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE). The results showed that at HRT of 10 h and 6 h were dominated by the Clostridium genus. The VHPR attained in this study is the highest reported value for a CSTR system using CW as substrate with anaerobic sludge as inoculum and represents a 33-fold increase compared to a previous study. Thus, it was demonstrated that continuous fermentative Bio-H₂ production from CW can be significantly enhanced by an appropriate selection of parameters such as HRT and OLR. Enhancements in VHPR are significant because it is a critical parameter to determine the full-scale practical application of fermentation technologies that will be used for sustainable and clean energy generation.     

Biological fermentative hydrogen and ethanol production using continuous stirred tank reactor

Hydrogen and ethanol are promising biofuels and have great potential to become alternatives to fossil fuels. The influence of organic loading rates (OLRs) on the production of fermentative hydrogen and ethanol were investigated in a continuous stirred tank reactor (CSTR) from fermentation using molasses as substrate. Four OLRs were examined, ranging from 8 to 32 kg/m³·d. The H₂ and ethanol production rate in CSTR initially increased with increasing OLR (from 8 to 24 kg/m³ d). The highest H₂ production rate (12.4 mmol/h l) and ethanol production rate (20.27 mmol/h l) were obtained in CSTR both operated at OLR = 24 kg/m³ d. However, the H₂ and ethanol production rate tended to decrease with an increase of OLR to 32 kg/m³ d. The liquid fermentation products were dominated by ethanol, accounting for 31-59% of total soluble metabolities. Linear regression results show that ethanol production rate (y) and H₂ production rate (x) were proportionately correlated which can be expressed as y = 0.5431x + 1.6816 (r² = 0.7617). The total energy conversion rate based on the heat values of H₂ and ethanol was calculated to assess the overall efficiency of energy conversion rate. The best energy conversion rate was 31.23 kJ/h l, occurred at OLR = 24 kg/m³ d.     

Technological advances in hydrogen production by Enterobacter bacteria upon substrate,luminosity and anaerobic conditions

The enhancement of hydrogen production by Enterobacter aerogenes and Enterobacter cloacae from fermentation of carbon sources such as glucose and lactose (from cheese whey permeate) was investigated. Also, the influence of the luminosity (2200 lux) and anaerobic condition (nitrogen and argon gases) were evaluated. The assays were carried out in 50 mL reactors during 108 h. To E. aerogenes/nitrogen/luminosity condition and using glucose as substrate, H₂ production (73.8 mmol/L.d) was higher than using lactose (15.5 mmol/L.d). In the dark fermentation, hydrogen yields were 1.60 mol H₂/mol glucose and 1.36 molH₂/mol lactose. When using E. cloacae, the light fermentation using nitrogen gas resulted in 77 mmol H₂/L.d and 1.62 mol H₂/mol glucose. In addition, for E. cloacae, hydrogen yields using argon gas and luminosity provided 2.39 mol/mol glucose and 2.53 mol/mol lactose. In general, butyric and acetic acid fermentation were observed and favored the target-product (H₂).     

Two-stage UASB reactor converting coffee drink manufacturing wastewater to hydrogen and methane

In this study, the feasibility of a continuous two-stage up-flow anaerobic sludge blanket (UASB) reactor system, consisted of thermophilic (55 °C) dark fermentative H₂ production and mesophilic (35 °C) CH₄ production from coffee drink manufacturing wastewater (CDMW), was tested. A recently proposed operational strategy was used to overcome a major drawback of the long start-up period of the UASB reactor. Firstly, a completely stirred tank reactor (CSTR) was operated for 8 days to prepare seeding. The seed was then directly transferred to the UASB reactor. Microbial aggregation took place in the initial period, and the floc size was gradually increased over time. In UASB reactor, the maximum H₂ yield of 2.57 mol H₂/mol hexose_added and a stable H₂ production rate of 4.24 L H₂/L/h were observed at a hydraulic retention time (HRT) of 6 h and substrate concentration of 20 g Carbo. COD/L. In this novel method using CDMW, thermophilic H₂-producing granules with an average particle size of 1.3 mm was successfully developed after 100 days. The more bioenergy recovery was attempted in a post-treatment process using a mesophilic UASB reactor for CH₄ production from the H₂ fermented effluent. The maximum CH₄ yield of 325 mL of CH₄/g COD was achieved with removing 93% of the COD at an organic loading rate of 3.5 g COD/L/d. The developed two-stage UASB reactor system achieved biogas conversion by 88.2% (H₂ 15.2% and CH₄ 73%) and COD removal by 98%.     

Converting waste molasses liquor into biohydrogen via dark fermentation using a continuous bioreactor

This study investigated the effects of substrate concentration, HRT (hydraulic retention time), and pre-treatment of the substrate molasses on biohydrogen production from waste molasses (condensed molasses fermentation solubles, CMS) with a CSTR (continuously-stirred tank reactor). First, the hydrogen production was performed with various CMS concentrations (40–90 g COD/L, total sugar 8.7–22.6 g/L) with 6 h HRT. The results show that the maximal hydrogen production rate (HPR) occurred at 80 g COD/L substrate (19.8 g ToSu/L, ToSu: Total Sugar), obtaining an HPR of 0.417 mol/L/d. However, maximum hydrogen yield (HY) of 1.44 mol H₂/mol hexose and overall hydrogen production efficiency (HPE) of 25.6% were achieved with a CMS concentration of 70 g COD/L (17.3 g ToSu/L). The substrate inhibition occurred when CMS concentration was increased to 90 g COD/L (22.6 g ToSu/L). Furthermore, it was observed that the optimal HPR, HY, and HPE all occurred at HRT 6 h. Operating at a lower HRT of 4 h decreased the hydrogen production performance because of lower substrate utilization efficiency. The employment of pre-heating treatment (60 °C for 1 h) of the substrate could markedly enhance the fermentation performance. With 6 h HRT and substrate pre-heating treatment, the HPE raised to 29.9%, which is 18% higher than that obtained without thermal pretreatment.     

Enhanced bio-hydrogen production from sugarcane juice by immobilized Clostridium butyricum on sugarcane bagasse

Immobilized Clostridium butyricum TISTR 1032 on sugarcane bagasse improved hydrogen production rate (HPR) approximately 1.2 times in comparison to free cells. The optimum conditions for hydrogen production by immobilized C. butyricum were initial pH 6.5 and initial sucrose concentration of 25 g COD/L. The maximum HPR and hydrogen yield (HY) of 3.11 L H₂/L substrate·d and 1.34 mol H₂/mol hexose consumed, respectively, were obtained. Results from repeated batch fermentation indicated that the highest HPR of 3.5 L H₂/L substrate·d and the highest HY of 1.52 mol H₂/mol hexose consumed were obtained at the medium replacement ratio of 75% and 50% respectively. The major soluble metabolites in both batch and repeated batch fermentation were butyric and acetic acids.     

Enhancing the performance of dark fermentative hydrogen production using a reduced pressure fermentation strategy

The partial pressure of hydrogen is an extremely important factor for hydrogen generation. This study investigated the effect of reduced pressure (via vacuum) on hydrogen production in a CSTR reactor. The results show that the reduced pressure condition is more effective in enhancing H₂ production at lower HRT (e.g., 8–4 h) than at higher HRT (e.g., 12 h). The optimal hydrogen yield and overall hydrogen production efficiency occurred at a HRT of 6 h with a value of 4.50 mol H₂/mol sucrose and 56.2%, respectively. Meanwhile, at HRT 6 h the hydrogen production rate was 0.937 mol/L/d. In addition, the HPR could be further improved to 1.196 mol/L/d when the HRT was shortened to 4 h, obtaining a 37–271% increase in HPR when compared with that described in the relevant reports. For all experiments, butyrate and acetate were the two primary soluble metabolites, accounting for 85–99% of total soluble microbial products. Predominant production of acetate and butyrate demonstrates the efficient H₂ fermentation with reduced pressure processes.