Regulation of biohydrogen production by heat-shock pretreatment facilitates selective enrichment of Clostridium sp.

The effect of heat-shock treatment to selectively enrich acidogenic, H₂ producing consortia was investigated for inoculum preparation and to control the process operation. Long term operation (520 days) in suspended-batch mode bioreactors illustrated relative efficiency and feasibility of heat-shock treated consortia (15.78 mol/kg COD_R) in enhancing H₂ production (3.31 mol/kg COD_R) when compared to parent (control) consortia. On the contrary, substrate degradation was higher in the control operation (ξ_COD, 62.86%; substrate degradation rate (SDR), 1.34 kg COD_R/m³-day) compared to heat-shock operation (ξ_COD, 52.6%; SDR, 1.10 kg COD_R/m³-day). Heat-shock pretreatment has resulted in a marked fermentation pathway shift towards acetic-butyric acid type production. The microbial diversity illustrated dominance in the Clostridia class after applying heat-shock pretreatment. The redox catalytic currents and Tafel analysis strongly support the conclusion of an improved biocatalyst performance after pretreatment with regards to H₂ production.     

Behavior of acidogenesis during biohydrogen production with formate and glucose as carbon source: Substrate associated dehydrogenase expression

Experiments were designed to enumerate variation in biohydrogen (H₂) production pattern with formate and glucose as carbon source under acidogenic mixed microenvironment. High H₂ production was observed with glucose (180 ml) when compared to formate (152 ml). The process was validated with modified Gompertz model (R² = 0.98). Substrate degradation also showed higher removal of glucose (ξ_COD, 82%) compared to formate (ξ_COD, 53%). Nevertheless, specific H₂ yield of formate (6.6 mol H₂/kg COD_R) obtained was comparatively higher than glucose (5 mol H₂/kg COD_R). Variation in H₂ production was manifested by change in fatty acid composition and substrate degradation pattern. Acetate was obtained as a major metabolic intermediate from the degradation of glucose and a shift in the biochemical pathway towards formation of butyrate occurred after maximum substrate degradation. The role of substrate-dependent dehydrogenase activity was deciphered during H₂ production and was evidenced with bio-electrochemical analysis.     

Harnessing of biohydrogen by acidogenic fermentation of Citrus limetta peelings: Effect of extraction procedure and pretreatment of biocatalyst

The extracts of Citrus limetta (sweet lime) peelings were evaluated as a fermentable substrate for hydrogen (H₂) production by dark-fermentation (acidogenic) using both anaerobic mixed consortia and selectively enriched acidogenic mixed consortia. Extraction was carried by pretreating sweet lime peelings at 121 °C (1 bar pressure) at variable pH (6 and 7) and digestion time (20 and 40 min). Maximum organic matter extraction was observed at pH 7.0 (40 min). Fermentation was performed at different organic loading conditions [OL1, 1.17 kg COD/m³; OL2, 2.35 kg COD/m³; OL3, 4.69 kg COD/m³] under acidophilic microenvironment. H₂ production was found to depend on the concentration of the substrate and composition. Increase in organic load showed consistent improvement in H₂ production. Operation at OL3 employing selectively enriched inoculum documented higher cumulative H₂ production (10.07 mmol) and H₂ production rate (0.345 mmol/h) (pH 7; 40 min). Substrate degradation was also found to increase with increase in organic loading. Maximum substrate degradation (SD) was registered at pH 6 (40 min) with anaerobic culture (2.80 kg COD_R/m³; ξ_COD 31.82%) and at pH 7 (40 min) with selectively enriched acidogenic culture (3.20 kg COD_R/m³; ξ_COD 36.36%). Concentration of volatile fatty acids (VFAs) also improved with increase in organic load. Maximum VFA concentration (1098 mg/l) was observed with OL3 (pH 7; 40 min) by using selectively enriched culture.     

Acetate and butyrate as substrates for hydrogen production through photo-fermentation: Process optimization and combined performance evaluation

Organic acids viz., acetate and butyrate were evaluated as primary substrates for the production of biohydrogen (H₂) through photo-fermentation process using mixed culture at mesophilic temperature (34 °C). Experiments were performed by varying parameters like operating pH, presence/absence of initiator substrate (glucose) and vitamin solution, type of nitrogen source (mono sodium salt of glutamic acid and amino glutamic acid) and gas (nitrogen/argon) used to create anaerobic microenvironment. Experimental data showed the feasibility of H₂ production along with substrate degradation utilizing organic acids as metabolic substrate but was found to be dependent on the process parameters evaluated. Maximum specific H₂ production and substrate degradation were observed with acetic acid [3.51 mol/Kg COD_R-day; 1.22 Kg COD_R/m³-day (92.96%)] compared to butyric acid [3.33 mol/Kg COD_R-day; 1.19 Kg COD_R/m³-day (88%)]. Higher H₂ yield was observed under acidophilic microenvironment in the presence of glucose (co-substrate), mono sodium salt of glutamic acid (nitrogen source) and vitamins. Argon induced microenvironment was observed to be effective compared to nitrogen induced microenvironment. Combined process efficiency viz., H₂ production and substrate degradation was evaluated employing data enveloping analysis (DEA) methodology based on the relative efficiency. Integration of dark fermentation with photo-fermentation appears to be an economically viable route for sustainable biohydrogen production if wastewater is used as substrate.     

Metabolic shift and electron discharge pattern of anaerobic consortia as a function of pretreatment method applied during fermentative hydrogen production

We have made an attempt to evaluate the variation in the electron discharge (ED) pattern of anaerobic consortia as a function of pretreatment viz., chemical, heat-shock, acid and oxygen-shock in comparison with untreated mixed consortia during fermentative hydrogen (H₂) production. Experiments were performed with dairy wastewater as substrate using anaerobic mixed consortia as biocatalyst (pretreated individually and in combination). Cyclic voltammetry (CV) elucidated significant variation in the ED pattern of mixed consortia along with H₂ production and substrate degradation (SD) as a function of pretreatment method applied. Higher ED was observed with all pretreated consortia which can be attributed to the stable proton (H⁺) shuttling due to the suppression of methanogenic activity. Oxygen-shock method and untreated consortia showed lower H₂ production and higher SD among the variations studied, while, combined pretreated consortia resulted higher H₂ production and lower SD. Lower ED observed with untreated consortia suggests the H⁺ reduction during methanogenesis rather than the inter-conversion of metabolites, which is presumed to be necessary for H₂ production. ED observed with combined pretreated consortia corroborated well with the observed H₂ production. Redox pairs were visualized on the voltammograms with almost all the experimental variations studied except untreated consortia. The potentials (E₀) of redox pairs observed were corresponding to intracellular electron carriers viz., NAD⁺/NADH (E₀ −0.32 V) and FAD⁺/FADH₂ (E₀ −0.24 V).     

Behavior of single chambered mediatorless microbial fuel cell (MFC) at acidophilic, neutral and alkaline microenvironments during chemical wastewater treatment

Single chamber mediatorless microbial fuel cell (MFC; non-catalyzed graphite electrodes; open air cathode) behaviour was evaluated under different pH microenvironments [acidophilic (pH 6), neutral (pH 7) and alkaline (pH 8)] during chemical wastewater treatment employing anaerobic mixed consortia as anodic biocatalyst at room temperature (29 ± 2 °C). The performance was found to depend on the feed pH used. Higher current density was observed at acidophilic conditions [pH 6; 186.34 mA/m²; 100 Ω] compared to neutral [pH 7; 146.00 mA/m²; 100 Ω] and alkaline [pH 8; 135.23 mA/m²; 100 Ω]. On the contrary, substrate degradation was found to be effective at neutral pH conditions (ξ_COD – 58.98%; SDR – 0.67 kg COD/m³-day) followed by alkaline (ξ_COD – 55.76%; SDR – 0.62 kg COD/m³-day) and acidophilic (ξ_COD of 47.80%; SDR 0.58 kg COD/m³-day) conditions studied. However, relatively higher specific power yield was observed at acidophilic microenvironment (46 mW/kg COD_R) compared to neutral (35 mW/kg COD_R) and alkaline (34 mW/kg COD_R) conditions. The behaviour of the MFC was also evaluated employing electron discharge, cyclic voltammetry, cell potentials, Coulombic efficiency and sustainable power analysis. Acidophilic operation showed higher Coulombic efficiency and effective electron discharge at relatively higher resistance compared to neutral and alkaline conditions studied.     

Effluents with soluble metabolites generated from acidogenic and methanogenic processes as substrate for additional hydrogen production through photo-biological process

The feasibility of utilizing effluents generated from acidogenic [producing biohydrogen (H₂)] and methanogenic [producing methane] processes was studied for additional H₂ production by terminally integrating with photo-biological process employing enriched mixed culture. Experimental data has depicted enhanced process efficiency with respect to additional H₂ production and substrate degradation through photo-biological process. However, the efficiency was found to depend on the process used in the first stage along with nature and composition of the substrate. Acidogenic process in the first stage had more positive influence on photo-biological H₂ production [synthetic wastewater – 14.40 mol/Kg COD_R and 15.16 mol/Kg COD_R (with vitamins); dairy wastewater – 13.29 mol/Kg COD_R and 13.70 mol/Kg COD_R (with vitamins)] over the corresponding methanogenic process. Effluent generated from acidogenic treatment of dairy wastewater yielded high substrate degradation rate (SDR) [1.20 Kg COD/m³ day and 1.34 Kg COD/m³ day (vitamins)] followed by synthetic wastewater [0.92 Kg COD/m³ day and 1.05 Kg COD/m³ day (vitamins)]. Among the studied experimental variations chemical wastewater evidenced poor H₂ production and SDR. Vitamin solution showed positive influence on both H₂ production and wastewater treatment irrespective of the experimental variations studied.     

Sugarcane vinasse as substrate for fermentative hydrogen production: The effects of temperature and substrate concentration

The present study aimed to evaluate the hydrogen production of a microbial consortium using different concentrations of sugarcane vinasse (2–12 g COD L⁻¹) at 37 °C and 55 °C. In mesophilic tests, the increase in vinasse concentration did not significantly impact the hydrogen yield (HY) (from 1.72 to 2.23 mmol H₂ g⁻¹ COD_influent) but had a positive effect on the hydrogen production potential (P) and hydrogen production rate (R_m). On the other hand, the increase in the substrate concentration caused a drop in HY from 2.31 to 0.44 mmol H₂ g⁻¹ COD_influent in the tests performed at 55 °C with vinasse concentrations from 2 to 12 g COD L⁻¹. The mesophilic community was composed of different species within the Clostridium genus, and the thermophilic community was dominated by organisms affiliated with the Thermoanaerobacter genus. Not all isolates affiliated with the Clostridium genus contributed to a high HY, as the homoacetogenic pathway can occur.     

Adaptation of biohydrogen producing reactor to higher substrate load: Redox controlled process integration strategy to overcome limitations

Adaptation of acidogenic sequencing batch biofilm reactor (AcSBBR) to higher loading conditions of vegetable waste extract was studied during biohydrogen production at pH 6.0 under ambient conditions. H₂ production rate (HPR) and cumulative H₂ production (CHP) were found to improve with increase in organic load from 4.50 to 26.44 kg COD/m³ and later at 35.25 kg COD/m³ stabilization was observed. Acid metabolic intermediates production tends to lower the system pH which limits the substrate degradation and H₂ production at higher loading conditions. To overcome these limitations, redox controlled strategy (pH 7.0) was applied by integrating another AcSBBR. Upon redox controlled integration, CHP and substrate degradation were found to improve by 42.81% and 36.82% respectively. This approach helped to maintain the favorable redox microenvironment for fermentation at higher VFA concentrations. This process integration methodology will help to overcome some persistent limitation observed during biohydrogen production and make the process sustainable especially with high strength waste/wastewaters.     

Bio-electrochemical evaluation of fermentative hydrogen production process with the function of feeding pH

Fermentative hydrogen (H₂) production process in concurrence with feeding pH [aciodophilic (pH 6.0) and neutral (pH 7.0)] and reactor operation mode (continuous and fed-batch) was evaluated in a biofilm configured reactor [upflow mode; retention time, 24 h; operating temperature, 28 ± 2 °C; organic loading rate, 3.4 kg COD/m³ day] using anaerobic mixed consortia. Acidophilic pH showed relatively effective performance with respect to H₂ production compared to neutral operation. Neutral pH illustrated effective substrate removal efficiency over the corresponding acidophilic operation. Fed-batch mode of operation with acidophilic pH showed highest H₂ production among the studied experimental variations. The pattern of soluble metabolites distribution showed the persistence of acid-forming metabolic flow associated with acidogenesis which may be considered as optimum microenvironment for effective H₂ production. Bio-electrochemical behavior of mixed anaerobic consortia (whole cell) during H₂ production process was evaluated employing cyclic voltammetry (CV) in electrochemical cell [platinum as working electrode; Ag/AgCl as reference electrode; graphite rod as counter electrode; wastewater as electrolyte] to gain insight into the possible mechanism based on intracellular electron transfer involved in the fermentative metabolic process. Voltammogram profiles visualized well defined redox pairs in forward and reverse scans at both pH conditions and the signals corresponded to intracellular electron carrier, NADH/NAD⁺ (E^0′, −0.32 V). Relatively higher energy output was observed in acidophilic operation which might be attributed to the possibility of efficient proton (H⁺) transfer between metabolic intermediates.     

Microbial hydrogen production from sewage sludge bioaugmented with a constructed microbial consortium

A constructed microbial consortium was formulated from three facultative H₂-producing anaerobic bacteria, Enterobacter cloacae IIT-BT 08, Citrobacter freundii IIT-BT L139 and Bacillus coagulans IIT-BT S1. This consortium was tested as the seed culture for H₂ production. In the initial studies with defined medium (MYG), E. cloacae produced more H₂ than the other two strains and it also was found to be the dominant member when consortium was used. On the other hand, B. coagulans as a pure culture gave better H₂ yield (37.16 ml H₂/g COD_consumed) than the other two strains using sewage sludge as substrate. The pretreatment of sludge included sterilization (15% v/v), dilution and supplementation with 0.5% w/v glucose, which was found to be essential to screen out the H₂ consuming bacteria and ameliorate the H₂ production. Considering (1:1:1) defined consortium as inoculum, COD reduction was higher and yield of H₂ was recorded to be 41.23 ml H₂/g COD_reduced. Microbial profiling of the spent sludge showed that B. coagulans was the dominant member in the constructed consortium contributing towards H₂ production. Increase in H₂ yield indicated that in consortium, the substrate utilization was significantly higher. The H₂ yield from pretreated sludge (35.54 ml H₂/g sludge) was comparatively higher than that reported in literature (8.1–16.9 ml H₂/g sludge). Employing formulated microbial consortium for biohydrogen production is a successful attempt to augment the H₂ yield from sewage sludge.     

Biohydrogen production from Enterobacter cloacae IIT-BT 08 using distillery effluent

Distillery effluent poses severe environmental pollution problem mainly due to its high organic content. During alcohol fermentation, most of the essential macro- and micro-nutrients get utilized. Therefore, supplementation of these nutrients becomes imperative for the improvement of biohydrogen production. In the present study, starch based distillery effluent was used for dark fermentative hydrogen production using Enterobacter cloacae IIT-BT 08. Hence, this study was undertaken to evaluate the effect of supplementation of yeast extract, malt extract, Fe⁺⁺, Cu⁺⁺ and Mg⁺⁺ on biohydrogen production. The interaction among supplements and their mutual effect on the hydrogen production was studied using five factor–five level central composite design). Optimum hydrogen yield of 7.4 mol H₂/kg COD_reduced was predicted by the model, which showed an excellent correlation with experimental hydrogen yield of 7.38 ± 0.24 mol H₂/kg COD_reduced. An average hydrogen production rate of 80 mL/L h was achieved after supplementation, having 2.2 times higher hydrogen yield as compared to non-supplemented distillery effluent.     

Enhancing biohydrogen production from chemical wastewater treatment in anaerobic sequencing batch biofilm reactor (AnSBBR) by bioaugmenting with selectively enriched kanamycin resistant anaerobic mixed consortia

The basic aim of this study was to investigate the feasibility of bioaugmentation strategy in the process of enhancing biohydrogen (H₂) production from chemical wastewater treatment (organic loading rate (OLR)—6.3 kg COD/m³-day) in anaerobic sequencing batch biofilm reactor (AnSBBR) operated at room temperature (28±2$28\pm 2$ °C) under acidophilic microenvironment (pH 6) with a total cycle period of 24 h. Parent augmented inoculum (kanamycin resistant) was acquired from an operating upflow anaerobic sludge blanket (UASB) reactor treating chemical wastewater and subjected to selective enrichment by applying repetitive/cyclic pre-treatment methods [altering between heat-shock treatment (100 °C; 2 h) and acid treatment (pH 3; 24 h)] to eliminate non-spore forming bacteria and to inhibit the growth of methanogenic bacteria (MB). Experimental data revealed the positive influence of bioaugmentation strategy on the overall H₂ production. Specific H₂ production almost doubled after augmentation from 0.297 to 0.483 mol H₂/kg COD_R-day. Chemical wastewater acted as primary carbon source in the metabolic reactions involving molecular H₂ generation leading to substrate degradation. The augmented culture persisted in the system till the termination of the experiments. The survival and retention of the augmented inoculum and its positive effect on process enhancement may be attributed to the adopted reactor configuration and operating conditions. Scanning electron microscope (SEM) images documented the selective enrichment of morphologically similar group of bacteria capable of producing H₂ under acidophilic conditions in anaerobic microenvironment. This depicted work corroborated successful application of bioaugmentation strategy to improve H₂ production rate from anaerobic chemical wastewater treatment.     

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.     

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.     

Influence of reactor configuration on fermentative hydrogen production during wastewater treatment

Influence of reactor configuration [biofilm/suspended growth] on fermentative hydrogen (H₂) production and substrate degradation was evaluated employing anaerobic mixed consortia. Reactors were operated at acidophilic (pH 6.0) condition employing designed synthetic wastewater as substrate at an organic loading rate of 3.4 Kg COD/m³-day with a retention time of 24 h at 28 ± 2 °C. Experimental data enumerated the influence of reactor configuration on both H₂ production and wastewater treatment. Biofilm reactor (28.98 mmol H₂/day; 1.25 Kg COD/m³-day) showed relatively efficient performance over the corresponding suspended growth configuration (20.93 mmol H₂/day; 1.08 Kg COD/m³-day). Specific H₂ yields of 6.96 mmol H₂/g-COD_L-day (19.32 mmol H₂/g-COD_R-day) and 5.03 mmol H₂/g-COD_L-day (16.10 mmol H₂/g-COD_R-day) were observed during stabilized phase of operation of biofilm and suspended growth reactors respectively. Higher concentration of VFA generation was observed in the biofilm reactor. Both the configurations recorded higher acetate concentration over other soluble metabolites indicating the dominance of acid-forming metabolic pathway during the H₂ production process.     

Deoiled algal cake as feedstock for dark fermentative biohydrogen production: An integrated biorefinery approach

An integrated biorefinery approach utilizing deoiled algal cake (after lipid extraction) as potential feed-stock for biohydrogen (H₂) production using selectively enriched acidogenic consortia as biocatalyst was evaluated. Algae pretreated extract (AP-E) documented maximum H₂ production rate (HPR), cumulative H₂ production (CHP) and specific H₂ yield (SHY) with higher substrate degradation (65%) in terms of COD removal efficiency than other conditions, which is a good sign for waste remediation. Along with the biohydrogen production and substrate removal the consortia also produced good amount of volatile fatty acids (VFA). VFA production in fermentation media resulted in reactor pH drop. The study depicted the feasible use of deoiled algal biomass as feed-stock for H₂ production in the framework of biorefinery.     

Thermophilic biohydrogen recovery from palm oil mill effluent

Enhancement of biological H₂ production efficiency with pre-ozonation process of palm oil mill effluent (POME) prior to thermophilic dark fermentation (55 °C) was investigated. H₂ fermentation experiments were conducted using varying concentrations of raw and ozonated POME. Results revealed that H₂ can be produced from both raw and ozonated POME under thermophilic fermentation. Maximum H₂ production yield of 77 mL.g⁻¹COD_removed was obtained from ozonated POME, which was higher than that of 51 mL·g⁻¹ COD_removed obtained from raw POME at the highest concentration of 35,000 mg COD.L⁻¹. Meanwhile, the specific H₂ production rate (R'_max) of 1.9 and 1.5 mL·h⁻¹·g⁻¹ TVS were observed in raw and ozonated POME at the concentration of 25,000 mg COD.L⁻¹, respectively. The main metabolic products during POME fermentation were acetic and butyric acids and trace amount of valeric acid. Propionic acid and ethanol have contributed, which could be reduced H₂ production in all batch experiments for both POME. The highest efficiency of total and soluble COD removal of 24 and 25% was obtained from the raw POME, and those of 19 and 25% was obtained from the ozonated POME. The present study demonstrates that the POME loading was greatly influenced on the H₂ production yields and rates. The comparative results showed that the ozonated POME gave higher H₂ yields than the raw POME. Thus, demonstrating that the ozonation process significantly improved the POME biodegradability, which is able to enhance H₂ production yields. However, the ozone pre-treatment was not improved in the specific H₂ production rates.     

Electrode performance and analysis of reversible solid oxide fuel cells with proton conducting electrolyte of BaCe0.5Zr0.3Y0.2O3−δ

Reversible solid oxide fuel cells (R-SOFCs) are regarded as a promising solution to the discontinuity in electric energy, since they can generate electric powder as solid oxide fuel cells (SOFCs) at the time of electricity shortage, and store the electrical power as solid oxide electrolysis cells (SOECs) at the time of electricity over-plus. In this work, R-SOFCs with thin proton conducting electrolyte films of BaCe_0.5Zr_0.3Y_0.2O_3−δ were fabricated and their electro-performance was characterized with various reacting atmospheres. At 700 °C, the charging current (in SOFC mode) is 251 mA cm⁻² at 0.7 V, and the electrolysis current densities (in SOEC mode) reaches −830 mA cm⁻² at 1.5 V with 50% H₂O-air and H₂ as reacting gases, respectively. Their electrode performance was investigated by impedance spectra in discharging mode (SOFC mode), electrolysis mode (SOEC mode) and open circuit mode (OCV mode). The results show that impedance spectra have different shapes in all the three modes, implying different rate-limiting steps. In SOFC mode, the high frequency resistance (R_H) is 0.07 Ωcm² and low frequency resistances (R_L) are 0.37 Ωcm². While in SOEC mode, R_H is 0.15 Ωcm², twice of that in SOFC mode, and R_L is only 0.07 Ωcm², about 19% of that in SOFC mode. Moreover, the spectra under OCV conditions seems like a combination of those in SOEC mode and SOFC mode, since that R_H in OCV mode is about 0.13 Ωcm², close to R_H in SOEC mode, while R_L in OCV mode is 0.39 Ωcm², close to R_L in SOFC mode. The elementary steps for SOEC with proton conducting electrolyte were proposed to account for this phenomenon.     

Thermophilic anaerobic digestion of cheese whey: Coupling H2 and CH4 production

The thermophilic anaerobic digestion of cheese whey was evaluated using a single and two stage configuration (H₂–CH₄) in a sequencing batch reactor (SBR). The single stage process presented stable performance with a specific methane production (SMP) of 314.5 ± 6.6 L CH₄ kg⁻¹ COD_feed (Chemical oxygen demand) at a hydraulic retention time (HRT) of 8.3 days. On the contrary, the two stage process presented instabilities at an HRT of 12.5 days with acid accumulation being observed in the methanogenesis phase at an earlier stage. This behaviour was indicative of process inhibition by high concentrations of sodium and potassium ions as a consequence of pH control during the H₂ producing stage. In spite of this phenomenon, this condition attained the highest SMP value (340.4 ± 40 L CH₄ kg⁻¹ COD_feed). The performance of the anaerobic digestion process was also analysed by means of Fourier transform infrared (FTIR) and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy. The two stage process showed higher content in triacylglycerol groups probably associated with changes in archaeal lipid complexes as a microbial response to a higher salinity environment.