The demand for renewable energy is increasing due to increasing energy demand and global warming associated with increasing use of fossil fuels. Renewable energy can be derived from biological production of energy carriers from cellulosic biomass. These biochemical processes include biomass fermentation to hydrogen, methane and alcohols, and bioelectricity production in microbial fuel cells (MFCs). The objective of this study was to investigate the production of different energy carriers (hydrogen, methane, ethanol, butanol, bioelectricity) through biochemical processes. Hydrogen production potential of a hot spring enrichment culture from different sugars was determined, and hydrogen was produced continuously from xylose. Cellulolytic and hydrogenic cultures were enriched on cellulose, cellulosic pulp materials, and on silage at different process conditions. The enrichment cultures were further characterized. The effect of acid pretreatment on hydrogen production from pulp materials was studied and compared to direct pulp fermentation to hydrogen. Electricity and alcohol(s) were simultaneously produced from xylose in MFCs and the exoelectrogenic and alcohologenic enrichment cultures were characterized. In the end, the energy yields obtained from different biochemical processes were determined and compared.
Hydrogen production potential from various hexose and pentose sugars was investigated with a hot spring enrichment culture. Lignocellulosic and cellulosic materials contain hexose and pentose sugars and thus, their efficient utilization for hydrogen production is important. The culture favored pentoses over hexoses for hydrogen fermentation with the highest yield of 0.71 mol H₂/mol xylose. Hydrogen was further produced continuously from xylose in a completely stirred tank reactor at 37°C and 45°C. Highest hydrogen yield and production rate at 45°C were 1.97 mol H₂/mol xylose and 7.3 mmol H₂/L/h, respectively, and were considerably higher than at 37°C. Clostridium acetobutylicum and Citrobacter freundii were the only bacteria detected at 45°C.
Cellulolytic and hydrogenic cultures were enriched on cellulose from compost and rumen fluid materials at elevated temperatures. Elevated temperatures are associated with increased chemical and enzymatic reaction rates and hydrogen yields. Furthermore, elevated temperatures may inhibit hydrogen consuming bacteria and enhance biomass hydrolysis. The need and effects of heat treatments on hydrogen production potentials were determined. Hydrogen consumers remained absent even in cultures that were not heat-treated, while heat treatment enhanced hydrogen production at certain conditions.
The highest hydrogen and ethanol yields of 0.4 mol H₂/mol hexose (1.9 mol H₂/mol hexosedegraded) and 0.2 mol EtOH/mol hexose (1.0 mol EtOH/mol hexosedegraded), respectively, were obtained with rumen fluid culture without heat treatment at 60°C and associated with 21 % cellulose hydrolysis. The rumen fluid enrichment culture contained mainly Clostridial species, from which a cellulolytic hydrogen-producer Clostridium stercorarium dominated. With compost enrichment culture, the highest hydrogen yields were obtained after heat treatment at 80°C for 20 min, although hydrogen was also produced without heat treating the culture. At 52°C, 1.4 mol H₂/mol hexose (2.4 mol H₂/mol hexosedegraded) and 0.4 mol EtOH/mol hexose (0.8 mol EtOH/mol hexosedegraded) were produced with 57 % cellulose degradation, while hydrogen production was negligible at temperatures above 52°C. Compost enrichment culture consisted of bacteria belonging to genera Thermoanaerobacterium and Clostridium, from which Clostridium cellulosi and C. stercorarium dominated. With both enrichment cultures, hydrogen yields were controlled by cellulose degradation efficiencies.
Hydrogen and methane were produced from dry and wet pulp materials at different pH values. Compost enrichment culture did not produce methane at pH 9, whilst at pH 6 methane was produced from all tested substrates but dry conifer pulp. These pH values could be successfully used to enrich cellulolytic hydrogen-producing cultures. Fermentation of dry pulps at pH 6 resulted in 160 mL H₂/g TS. The highest hydrogen and methane yields were 560 mL H₂/g TS from wet birch pulp at pH 6 and 4800 mL CH₄/g TS from wet conifer pulp at pH 7, respectively. Inhibition of methanogens with BESA (2-Bromoethanesulfonic acid) resulted in decreased hydrogen yields, which may have resulted from the inhibitory effects of BESA on some Clostridial species. Cellulolytic and hydrogenic cultures enriched on pulp materials belonged mainly to phyla Bacteroidetes, Firmicutes and Proteobacteria.
Direct pulp fermentation to hydrogen was compared to hydrogen fermentation from acid hydrolyzed pulps. Wet and dry pulps were hydrolyzed with concentrated sulfuric acid at 37°C. The optimal times for hydrolysis and the following sugars yields were 33-37 % after 90 min with wet pulps and 70-84 % after 180 min with dry pulps, respectively. Fermentation of dry conifer pulp hydrolysate resulted in 63 mL H₂/g TS. In conclusion, higher hydrogen yields were obtained from direct pulp fermentation to hydrogen (120 mL H₂/g TS). However, hydrogen production from acid hydrolyzed pulp took 10 days, while direct fermentation was completed in 28 days.
Indigenous grass silage bacteria were enriched for hydrogen production at different silage concentrations. Lowest silage concentration of 25 g/L resulted in the highest hydrogen yield of 163 mL H₂/g TS, while increasing silage concentrations up to 200 g/L decreased the hydrogen yields but increased the cumulative hydrogen production. Silage fermentation to hydrogen was associated with bacteria related to Ruminobacillus xylanolyticum, Acetanaerobacterium elongatum and Clostridium populeti.
Compost and anaerobic digester samples were enriched on xylose in MFCs resulting in simultaneous production of electricity and ethanol/butanol. Alcohol production was dependent on xylose concentration. Low xylose concentration of 1.0 g/L resulted in electron recoveries of 13-24 % and 40-65 % as electricity and ethanol, respectively. With higher xylose concentration of 4.0 g/L, electrons were directed mainly to butanol (33 %) and 4 % of the electrons were recovered as electricity. Ruminobacillus xylanolyticum was mainly responsible for xylose degradation in MFCs, while denitrifying bacteria, Comamonas denitrificans and Paracoccus pantotrophus, produced electricity from soluble metabolites.
In this study, hydrogen, methane, alcohols and electricity were produced at laboratory scale in batch systems. The highest overall energy yields of 167 kJ/g TS and 113-130 kJ/g TS were obtained from direct pulp fermentation to both hydrogen and methane and from simultaneous production of electricity and butanol in MFCs, respectively. Cellulose fermentation resulted in the simultaneous production of hydrogen and ethanol with the highest overall energy yield of 4.9 kJ/g TS with compost enrichment culture. The highest energy yield as hydrogen, 5.3-6.0 kJ/g TS, was obtained from wet pulps.
In summary, bacterial cultures producing different energy carrier(s) can be enriched from the same environmental sample by controlling the enrichment conditions. For example, compost sample was enriched for the production of (i) hydrogen and ethanol from cellulose at elevated temperatures by heat-treating the sample, (ii) hydrogen and/or methane from pulp materials at 37°C by changing the pH values, and (iii) electricity and alcohol(s) at 37°C in MFCs by changing xylose concentrations. It was shown that different operational conditions enrich for different microbial communities that are responsible for changes in fermentation patterns. In this study, cultures carrying out simultaneous cellulose hydrolysis and hydrogen fermentation were enriched from different sources at different operational conditions. These cultures were successfully utilized for cellulose to hydrogen fermentation in batch systems. Based on these results further research should be conducted on continuous hydrogen production from cellulosic materials.