Abstract
BackgroundCurrent EU directives demand increased use of renewable fuels in the transportation sector but restrict governmental support for production of biofuels produced from crops. The use of intercropped lucerne and wheat may comply with the directives. In the current study, the combination of ensiled lucerne (Medicago sativa L.) and wheat straw as substrate for hydrogen and methane production was investigated. Steam-pretreated and enzymatically hydrolysed wheat straw [WSH, 76% of total chemical oxygen demand (COD)] and ensiled lucerne (LH, 24% of total COD) were used for sequential hydrogen production through dark fermentation and methane production through anaerobic digestion and directly for anaerobic digestion. Synthetic co-cultures of extreme thermophilic Caldicellulosiruptor species adapted to elevated osmolalities were used for dark fermentation.ResultsBased on 6 tested steam pretreatment conditions, 5 min at 200 °C was chosen for the ensiled lucerne. The same conditions as applied for wheat straw (10 min at 200 °C with 1% acetic acid) would give similar sugar yields. Volumetric hydrogen productivities of 6.7 and 4.3 mmol/L/h and hydrogen yields of 1.9 and 1.8 mol/mol hexose were observed using WSH and the combination of WSH and LH, respectively, which were relatively low compared to those of the wild-type strains. The combinations of WSH plus LH and the effluent from dark fermentation of WSH plus LH were efficiently converted to methane in anaerobic digestion with COD removal of 85–89% at organic loading rates of COD 5.4 and 8.5 g/L/day, respectively, in UASB reactors. The nutrients in the combined hydrolysates could support this conversion.ConclusionsThis study demonstrates the possibility of reducing the water addition to WSH by 26% and the phosphorus addition by 80% in dark fermentation with Caldicellulosiruptor species, compared to previous reports. WSH and combined WSH and LH were well tolerated by osmotolerant co-cultures. The yield was not significantly different when using defined media or hydrolysates with the same concentrations of sugars. However, the sugar concentration was negatively correlated with the hydrogen yield when comparing the results to previous reports. Hydrolysates and effluents from dark fermentation can be efficiently converted to methane. Lucerne can serve as macronutrient provider in anaerobic digestion. Intercropping with wheat is promising.
Highlights
Current EU directives demand increased use of renewable fuels in the transportation sector but restrict governmental support for production of biofuels produced from crops
enzymatic hydrolysis (EH) of steam-pretreated wheat straw at 10% water-insoluble solids (WIS) in the present study resulted in a hydrolysate containing 57 ± 3 g/L glucose and 31 ± 3.5 g/L xylose, which correspond to a sugar yield of approximately 0.5 g sugar per g Dry matter (DM) raw wheat straw, which is above 90% of the theoretical yield according to data found in Bondesson and Galbe [45]
The rather high residual xylan content in the solid fractions after pretreatment together with a relatively low concentration of xylose in the liquid fractions (Table 2) indicated that the severity of the pretreatment was low. This was supported by the observation that after steam pretreatment the concentrations of HMF and furfural were below the detection limit
Summary
Current EU directives demand increased use of renewable fuels in the transportation sector but restrict governmental support for production of biofuels produced from crops. Research in the production of these energy carriers is further motivated by their use in electricity generation and chemical synthesis, e.g. ammonia [7] or oil and fat hydrogenation [8]. Another advantage of anaerobic digestion is that the residues, i.e. the biodigestates, are widely applied as fertilizers in agriculture [9, 10]
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