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.     

Biohydrogen production from soluble condensed molasses fermentation using anaerobic fermentation

Using anaerobic micro-organisms to convert organic waste to produce hydrogen gas gives the benefits of energy recovery and environmental protection. The objective of this study was to develop a biohydrogen production technology from food wastewater focusing on hydrogen production efficiency and micro-flora community at different hydraulic retention times. Soluble condensed molasses fermentation (CMS) was used as the substrate because it is sacchariferous and ideal for hydrogen production. CMS contains nutrient components that are necessary for bacterial growth: microbial protein, amino acids, organic acids, vitamins and coenzymes. The seed sludge was obtained from the waste activated sludge from a municipal sewage treatment plant in Central Taiwan. This seed sludge was rich in Clostridium sp.A CSTR (continuously stirred tank reactor) lab-scale hydrogen fermentor (working volume, 4.0 L) was operated at a hydraulic retention time (HRT) of 3–24 h with an influent CMS concentration of 40 g COD/L. The results showed that the peak hydrogen production rate of 390 mmol H₂/L-d occurred at an organic loading rate (OLR) of 320 g COD/L-d at a HRT of 3 h. The peak hydrogen yield was obtained at an OLR of 80 g COD/L-d at a HRT of 12 h. At HRT 8 h, all hydrogenase mRNA detected were from Clostridium acetobutylicum-like and Clostridium pasteurianum-like hydrogen-producing bacteria by RT-PCR analysis. RNA based hydrogenase gene and 16S rRNA gene analysis suggests that Clostridium exists in the fermentative hydrogen-producing system and might be the dominant hydrogen-producing bacteria at tested HRTs (except 3 h). The hydrogen production feedstock from CMS is lower than that of sucrose and starch because CMS is a waste and has zero cost, requiring no added nutrients. Therefore, producing hydrogen from food wastewater is a more commercially feasible bioprocess.     

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.     

Steady-state and dynamic modeling of biohydrogen production in an integrated biohydrogen reactor clarifier system

Steady-state operational data from the integrated biohydrogen reactor clarifier system (IBRCS) during anaerobic treatment of glucose-based synthetic wastewater at HRT of 8 h and SRT ranging from 26 to 50 h and organic loading rates of 6.5–206 gCOD/L-d were used to calibrate and verify a process model of the system developed using BioWin. The model accurately predicted biomass concentrations in both the bioreactor and the clarifier supernatant with average percentage errors (APEs) of 4.6% and 10%, respectively. Hydrogen production rates and hydrogen yields predicted by the model were in close agreement with the observed experimental results as reflected by an APE of less than 4%, while the hydrogen content was well correlated with an APE of 10%. The successful modeling culminated in the accurate prediction of soluble metabolites, i.e. volatile fatty acids in the reactor with an APE of 14%. The calibrated model confirmed the advantages of decoupling of the solids retention time (SRT) from the hydraulic retention time (HRT) in biohydrogen production, with the average hydrogen yield decreasing from 3.0 mol H₂/mol glucose to 0.8 mol H₂/mol glucose upon elimination of the clarifier. Dynamic modeling showed that the system responds favorably to short-term hydraulic and organic surges, recovering back to the original condition. Furthermore, the dynamic simulation revealed that with a prolonged startup periods of 10 and 30 days, the IBRCS can be operated at an HRT of 4 h and OLR as high as 206 gCOD/L-d without inhibition and/or marked performance deterioration.     

Comparison of suspended and granular cell anaerobic bioreactors for hydrogen production from acid agave bagasse hydrolyzates

Acid agave bagasse hydrolyzates have been used as a substrate for hydrogen production, however, bioreactors are unstable and with poor performance. Granular biomass could be more successful in producing hydrogen from acid agave bagasse hydrolyzates in comparison with suspended biomass. Thus, this study aimed to evaluate the effect of increasing concentrations of acid agave hydrolyzates on hydrogen production, to compare the hydrogen productivity and stability of granular biomass in an expanded granular sludge bed (EGSB) reactor and suspended biomass in an anaerobic sequencing batch reactor (AnSBR) fed with acid hydrolyzates, and finally to determine the variation of microbial communities established in both bioreactor configurations. In batch tests, the heat-treated inoculum produced hydrogen from acid agave hydrolyzates without observing inhibition at 6.3 g/L of carbohydrates (_CHO). This hydrolyzate concentration was used to start up the AnBSR, which reached a productivity of 226 ± 53 mL H₂/L⋅d at organic loading rates (OLR) from 3.2 to 4.5 g_CHO/L⋅d. The hydrogen production stability index decreased from 0.8 to 0.6 at increasing OLR, and the AnSBR failed at the highest OLR of 5.7 g/L⋅d. The EGSB reactor reached the highest productivity of 361 ± 130 mL H₂/L⋅d at an OLR of 7.4 g_CHO/L⋅d, but with a low stability index of 0.6. Independently of the bioreactor configuration, microbial communities associated with the production of acetate/lactate were successfully established in both configurations with the prevalence of Lactobacillus spp. A low abundance of typical H₂ producers like Clostridium was always observed over the whole period of operation (<10% of the total abundance). In sum, the hydrogen productivity from acid agave hydrolyzates was higher for the EGSB reactor than for the AnSBR, but with much lower stability. The evidence provided by this study suggests the establishment of metabolic pathways for hydrogen production from organic acids.     

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.     

Humic substances improve the co-production of hydrogen and carboxylic acids by anaerobic mixed cultures

The addition of external redox mediators (e.g., humic substances) as a strategy to enhance the production of H₂ and carboxylic acids through mixed-culture dark fermentation was investigated. Pahokee Peat and Leonardite humic substances along the anthraquinone-2,6-disulfonate (AQDS) humic model compound were evaluated. The use of Pahokee peat into dark fermentation assays reached an H₂ potential of 618.2 ± 28.9 mL H₂/L, equivalent to an increase of 8% compared with the control experiment. In contrast, the addition of Leonardite (in its basal redox state) as a redox mediator increased the H₂ potential by up to 14.5% and achieved 655.8 mL H₂/L-d, which was higher than the control's H₂ production of 572.6 mL H₂/L-d. Moreover, the use of chemically reduced Leonardite led to the highest H₂ potential (1158.6 mL H₂/L-d) which was about 1.8-fold the H₂ potential found in the control. Overall, experimental and production-modeling results suggest that external redox mediators not only served as an additional source of electrons for dark-fermentative pathways but also as electron acceptors. Electrons from redox mediators were likely transported through membrane proteins and could be used to reduce NAD⁺ and Fd_ox. Results indicated that electrons were preferably channeled through the butyrate pathway when reduced substances were applied.     

Different start-up strategies to enhance biohydrogen production from cheese whey in UASB reactors

The effect of different operational strategies and inoculum structure (granules and disaggregated granules) during the start-up of four up-flow anaerobic sludge blanket hydrogenogenic reactors was investigated. The more stable volumetric hydrogen production rates obtained were 0.38 and 0.36 L H₂/L-d, in reactors operated with a constant organic loading rate (OLR) with both inoculum structures, whereas in reactors operated with an increasing OLR methane started to be produced earliest in time. Specific hydrogenogenic activity results proved that the disaggregated inoculum produced a more active biomass than the granular one, but not granule formation was evident. The methane hydrogenotrophic activity was the main limitation of the systems evaluated. In the reactors inoculated with disaggregated sludge the start-up strategy did not influence the bacterial DGGE fingerprint, in contrast to the reactors started-up with granular sludge; members of the Clostridium genus were always present. The results demonstrated that operational conditions during the start-up period are crucial for the production of hydrogenogenic biomass.     

Biological stoichiometric analysis of nutrition and ammonia toxicity in thermophilic anaerobic co-digestion of organic substrates under different organic loading rates

This study used chemostats under different Organic Loading Rates (OLRs) to investigate the co-digestion of kitchen waste, swine wastewater sludge, and fruit and vegetable waste. In kitchen waste, mean Total Chemical Oxygen Demand (TCOD), Oil and Grease (O&G), and moisture content (MC) was 101.5 g/L, 33.0 g/L, and 82.7%, respectively. The TCOD/Total Kjeldahl Nitrogen (TKN) and TCOD/Total Phosphorus (TP) of kitchen waste were 319.5 and 230, respectively. In swine wastewater sludge, TCOD/TKN was 4.56–43.9 and TCOD/TP was 2.02–31.8. Biodegradability tests of fruit and vegetable waste showed that COD removal exceeded 56%, and methane recovery exceeded 80%. Co-digestion of these three organic wastes in chemostats suggests that the system functioned stably up to an OLR of 9.52 g COD/L-d at a Hydraulic Retention Time (HRT) of 5 days. When the OLR increased to 12.54 g COD/L-d, average COD and Volatile Suspended Solids (VSS) removal efficiencies decreased from 90% to 76.5% and from 93% to 76.5%, respectively. The analyzed NH₃–N concentration is 28% less than the stoichiometry-predicted concentration. The discrepancy may be due to differences in substrate biodegradabilities, TKN sampling and analysis procedures, and parameters used for stoichiometry calculations.     

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.     

Effects of substrate concentration and hydraulic retention time on hydrogen production from common reed by enriched mixed culture in continuous anaerobic bioreactor

This study aims to investigate the effect of substrate concentration and hydraulic retention time (HRT) on hydrogen production in a continuous anaerobic bioreactor from unhydrolyzed common reed (Phragmites australis) an invasive wetland and perennial grass. The bioreactor has capacity of 1 L and working volume of 600 mL. It was operated at pH 5.5, temperature at 37 °C, hydraulic retention time (HRT) 12 h, and variation of substrate concentration from 40, 50, and 60 g COD/L, respectively. Afterward, the HRT was then varied from 12, 8, to 4 h for checking the optimal biohydrogen production. Each condition was run until reach steady state on hydrogen production rate (HPR) which based on hydrogen percentage and daily volume. The results were obtained the peak of substrate concentration was at the 50 g COD/L with HRT 12 h, average HPR and H₂ concentration were 28.71 mL/L/h and 36.29%, respectively. The hydrogen yield was achieved at 106.23 mL H₂/g CODre. The substrate concentration was controlled at 50 g COD/L for the optimal HRT experiments. It was found that the maximum of average HPR and H₂ concentration were 43.28 mL/L/h and 36.96%, respectively peak at HRT 8 h with the corresponding hydrogen yield of 144.35 mL H₂/g CODre. Finally, this study successful produce hydrogen from unhydrolyzed common reed by enriched mixed culture in continuous anaerobic bioreactor.     

Optimizing energy harvest in wastewater treatment by combining anaerobic hydrogen producing biofermentor (HPB) and microbial fuel cell (MFC)

This study focused on the optimization of energy harvest from wastewater treatment by integrating two novel biotechnologies: anaerobic hydrogen production and microbial fuel cell (MFC). The simultaneous production of hydrogen and electricity from wastewater was examined at continuous flow at different organic loading rates (OLR) by changing chemical oxygen demand (COD) and hydraulic retention time (HRT). The experimental results showed that the specific hydrogen yield (SHY, mole H₂/mole glucose) increased with the decrease in OLR, and reached at the maximum value of 2.72 mol H₂/mole glucose at the lowest OLR of 4 g/L.d. The effluent from hydrogen producing biofermentor (HPB) was fed to a single chamber MFC (SCMFC), obtaining the highest power density and coulombic efficiency (CE) of 4200 mW/m³ and 5.3%, respectively. The energy conversion efficiency (ECE) increased with OLR and reached the peak value of 4.24% at the OLR of 2.35 g/L.d, but decreased with higher OLR. It was demonstrated that the combination of HPB and MFC improved the ECE and COD removal with the maximum total ECE of 29% and COD removal of 71%. The kinetic analysis was conducted for the HPB-MFC hybrid system. The maximum hydrogen production was projected to be 2.85 mol H₂/mole glucose. The maximum energy recovery and COD removal efficiency from MFC were projected to be 559 J/L and 97%, respectively.     

Effect of hydraulic retention time on the hydrogen production in a horizontal and vertical continuous stirred-tank reactor

Hydraulic retention time (HRT) is the main process parameter for biohydrogen production by anaerobic fermentation. This paper investigated the effect of the different HRT on the hydrogen production of the ethanol-type fermentation process in two kinds of CSTR reactors (horizontal continuous stirred-tank reactor and vertical continuous stirred-tank reactor) with molasses as a substrate. Two kinds of CSTR reactors operated with the organic loading rates (OLR) of 12kgCOD/m3•d under the initial HRT of the 8 h condition, and then OLR was adjusted as 6kgCOD/m3•d when the pH drops rapidly. The VCSTR and HCSTR have reached the stable ethanol-type fermentation process within 21 days and 24 days respectively. Among the five HRTs settled in the range of 2–8 h, the maximum hydrogen production rate of 3.7LH₂/Ld and 5.1LH₂/Ld were investigated respectively in the VCSTR and HCSTR. At that time the COD concentration and HRT were 8000 mg/L and 5 h for VCSTR, while 10000 mg/L and 4 h for HCSTR respectively.Through the analysis on the composition of the liquid fermentation product and biomass under the different HRT condition in the two kinds of CSTR, it can found that the ethanol-type fermentation process in the HCSTR is more stable than VCSTR due to enhancing biomass retention of HCSTR at the short HTR.     

Biohydrogen production from crude glycerol by immobilized Klebsiella sp. TR17 in a UASB reactor and bacterial quantification under non-sterile conditions

Biohydrogen production from crude glycerol by immobilized Klebsiella sp. TR17 was investigated in an up-flow anaerobic sludge blanket (UASB) reactor. The reactor was operated under non-sterile conditions at 40^○C and initial pH 8.0 at different hydraulic retention times (HRTs) (2–12 h) and glycerol concentrations (10–30 g/L). Decreasing the HRT led to an increase in hydrogen production rate (HPR) and hydrogen yield (HY). The highest HPR of 242.15 mmol H₂/L/d and HY of 44.27 mmol H₂/g glycerol consumed were achieved at 4 h HRT and glycerol concentrations of 30 and 10 g/L, respectively. The main soluble metabolite was 1,3-propanediol, which implies that Klebsiella sp. was dominant among other microorganisms. Fluorescence in situ hybridization (FISH) revealed that the microbial community was dominated by Klebsiella sp. with 56.96, 59.45, and 63.47% of total DAPI binding cells, at glycerol concentrations of 10, 20, and 30 g/L, respectively.     

Bioconversion of de-oiled Jatropha Waste (DJW) to hydrogen and methane gas by anaerobic fermentation: Influence of substrate concentration,temperature and pH

Cellulosic materials-based de-oiled Jatropha Waste (DJW) was fermented to H₂ and CH₄ using sewage sludge inoculum. Batch assays were performed at various substrate concentrations (40–240 g/L), temperatures (25–65 °C) and pHs (5.5–7.5). The peak hydrogen production rate (HPR) and hydrogen yield (HY) of 744.0 ± 11.3 mL H₂/L-d and 10.6 ± 0.2 mL H₂/g VS obtained when the optimal substrate concentration, pH, temperature were 200 g/L, 6.5, 55 °C, respectively. The peak methane production rate (MPR) of 178.4 ± 5.6 mL CH₄/L-d obtained while DJW concentration, pH, temperature were 200 g/L, 7.0, 45 °C, however, peak methane yield (MY) of 23.3 ± 0.1 mL CH₄/g VS obtained at 40 g/L, 7.0 and 55 °C, respectively. Effect of substrate concentration on HPR and MPR was elucidated using Monod model. Butyrate and acetate were the main soluble metabolic products. Maximal carbohydrate removal and COD reduction were achieved as 51.7 ± 0.7% and 68.3 ± 1.6%, respectively.     

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.     

Pilot-scale of biohythane production from palm oil mill effluent by two-stage thermophilic anaerobic fermentation

The pilot-scale of two-stage thermophilic (55 °C) for biohythane production from palm oil mill effluent (POME) was operated at hydraulic retention time (HRT) of 2 days and organic loading rate (OLR) of 27.5 gCOD/L⋅d) for first stage and HRT of 10 days and OLR of 5.5 gCOD/L⋅d for second stage. Biohythane production rate was 1.93 L-gas/L⋅d with biogas containing 11% H₂, 37% CO₂, and 52% CH₄. Recirculation of methane effluent mixed with POME at a ratio of 1:1 can control pH in the first stage at an optimal range of 5.0–6.5. Microbial community in hydrogen stage dominated by Thermoanaerobacterium sp., while methane stage dominated by Methanosarcina sp. The H₂/CH₄ ratio of biohythane was 0.13–0.18 which suitable for vehicle fuel. Biohythane production from POME could be promising cleaner biofuel with flexible and controllable H₂/CH₄ ratio.     

Decreasing methane production in hydrogenogenic UASB reactors fed with cheese whey

One of the problems in fermentative hydrogen producing reactors, inoculated with pre-treated anaerobic granular sludge, is the eventual methane production by hydrogen-consuming methanogens. In this study, strategies such as reduction of pH and HRT, organic shock loads and repeated biomass heat treatment were applied to hydrogenogenic UASB reactors fed with cheese whey, that showed methane production after certain time of continuous operation (between 10 and 60 days). The reduction of pH to 4.5 not only decreased methane production but also hydrogen production. Organic shock load (from 20 to 30 g COD/L-d) was the more effective strategy to decrease the methane production rate (75%) and to increase the hydrogen production rate (172%), without stopping reactor operation. Repeated heat treatment of the granular sludge was the only strategy that inhibited completely methane production, leading to high volumetric hydrogen production rates (1.67 L H₂/L-d), however this strategy required stopping reactor operation; in addition homoacetogenesis, another hydrogen-consuming pathway, was not completely inhibited. This work demonstrated that it was possible to control the methane activity in hydrogen producing reactors using operational strategies.     

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.     

Continuous fermentative hydrogen and methane production from Laminaria japonica using a two-stage fermentation system with recycling of methane fermented effluent

In this study, a two-stage fermentation system to produce H₂ and CH₄ from Laminaria japonica was developed. In the first stage (dark fermentative H₂ production, DFHP), response surface methodology (RSM) with a Box-Behnken design (BBD) was applied for optimization of operational parameters, including cycle-frequency, HRT, and substrate concentration, using an intermittent-continuously stirred tank reactor (i-CSTR). Overall performance revealed that the degree of importance of the three variables in terms of H₂ yield is as follows: cycle-frequency > substrate concentration > HRT. In the confirmation test, H₂ yield of 113.1 mL H₂/g dry cell weight (dcw) was recorded, corresponding with 96.3% of the predicted response value under desirable operational conditions (cycle-frequency of 17 hr, HRT of 2.7 days, and substrate concentration of 31.1 g COD/L). In the second stage, an anaerobic sequencing batch reactor (ASBR) and an up-flow anaerobic sludge blanket reactor (UASBr) were employed for CH₄ production from H₂ fermented solid state (HFSS) and H₂ fermented liquid state (HFLS), respectively. The CH₄ producing ASBR and UASBr showed a stable CH₄ yield and COD removal until a HRT of 12 days and OLR of 3.5 g COD/L/d, respectively. Subsequently, for recycling of CH₄ fermented effluent from the UASBr (MFE_UASBr) as diluting water in DFHP, the tap water and MFE_UASBr mixing ratio (T/M ratio) was optimized (a T/M ratio of 5:5) in a batch test using heat pretreated MFE_UASBr at 90 °C for 20 min, resulting in the best performance. Although slight decreases of H₂ yield (7.6%) and H₂ production rate (3.5%) were recorded, 100% reduction of alkali addition was possible, indicating potential to maximize economic benefits. However, a drastic decrease of H₂ productivity and a change of liquid-state metabolites were observed with the use of non-heat pretreated MFE_UASBr. These results coincided with those of the microbial analysis, where non-H₂ producing bacteria, such as Selenomonas sp., were detected. The results indicate that pretreatment of MFE_UASBr may be required in order to recycle it in DFHP.