Abstract
The use of straw for biofuel production is encouraged by the European Union. A previous study showed the feasibility of producing biomethane in upflow anaerobic sludge blanket (UASB) reactors using hydrolyzed, steam-pretreated wheat straw, before and after dark fermentation with Caldicellulosiruptor saccharolyticus, and lucerne. This study provides information on overall microbial community development in those UASB processes and changes related to acidification. The bacterial and archaeal community in granular samples was analyzed using high-throughput amplicon sequencing. Anaerobic digestion model no. 1 (ADM1) was used to predict the abundance of microbial functional groups. The sequencing results showed decreased richness and diversity in the microbial community, and decreased relative abundance of bacteria in relation to archaea, after process acidification. Canonical correspondence analysis showed significant negative correlations between the concentration of organic acids and three phyla, and positive correlations with seven phyla. Organic loading rate and total COD fed also showed significant correlations with microbial community structure, which changed over time. ADM1 predicted a decrease in acetate degraders after a decrease to pH ≤ 6.5. Acidification had a sustained effect on the microbial community and process performance.
Highlights
Wheat straw is one of the most abundant agricultural residues in the world [1] and the most abundant in the European Union (EU) [2]
It has been demonstrated that some sugars are still needed for good granulation in upflow anaerobic sludge blanket (UASB) reactors, when the feed is composed of only volatile fatty acids (VFAs) [28]. These findings suggest that partial pre-acidification, rather than no pre-acidification or complete pre-acidification for anaerobic digestion (AD), could have beneficial effects on the process in an UASB reactor
chemical oxygen demand (COD) was determined with LCK114 test kits (HACH, Loveland, CO, USA), methane production with AMPTS (Bioprocess Control AB, Lund, Sweden), pH with a pH meter, total and partial alkalinity with titration, and the concentrations of acids and other metabolites by HPLC (Jasco Co., Tokyo, Japan) using an Aminex HPX-87H column (Bio-Rad, Hercules, CA, USA) and refractive index detector (Shimadzu Co., Kyoto, Japan)
Summary
Wheat straw is one of the most abundant agricultural residues in the world [1] and the most abundant in the European Union (EU) [2]. Use of wheat straw for biofuel production does not compete with land use for food production, and is encouraged by the EU Renewable Energy Directive 2009/28/EC [3] and its amendment EU 2015/1513 [4]. Wheat straw has been evaluated in different processes for microbial conversion to various energy carriers such as ethanol, hydrogen, methane, and combinations of these (e.g., [5,6,7,8,9,10,11,12,13,14,15,16]) Among these energy carriers, methane is highly interesting, as it can be used as a vehicle fuel (after upgrading the biogas) and for the production of electricity and other fuels and chemicals [17,18]. The environmental impact of the entire methane production and utilization chain is relatively low [13,17]
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