Oxygen tolerance capacity of upflow anaerobic solid-state (UASS) with anaerobic filter (AF) system
Graphical abstract
Introduction
Anaerobic digestion (AD) is a promising and competent technology for treating various types of organic wastes and simultaneously producing biogas as a renewable energy carrier (Li et al., 2011). Unavoidable oxygen would be taken into anaerobic digesters unintentionally as the reactors are operated within an aerobic open environment, especially through interactions with the surroundings such as by feeding and mixing (Kato et al., 1997). Some enzyme synthesizing of strict anaerobes can be inhibited and rapid cell lysis of obligatory anaerobic species can occur in the presence of oxygen so that oxygen is thought to be an inhibitor to anaerobic process (Botheju and Bakke, 2011). Methanogens will be inhibited by oxygen in anaerobic digesters (Ren and Wang, 2004). On the other hand, the rate-limiting step, hydrolysis of particulate matter in AD can be enhanced (Ramos and Fdz-Polanco, 2013) because oxygen can promote facultative microorganisms excrete a higher amount of enzymes hydrolysis (Johansen and Bakke, 2006, Sheets et al., 2015) and limited aeration could increase synthesis and activity of cellular hydrolytic enzymes (Zhu et al., 2009). Lim and Wang (2013) reported that microaerobic treatment could reduce the formation of toxic metabolites (e.g., lactic acid and ethanol) as well as promote the synthesis of certain lipids required for the stability of anaerobe cell membrane. Previous studies about microaeration pretreatment were conducted under batch conditions (Charles et al., 2009, Mshandete et al., 2005). However, the effect of oxygen on anaerobic digestion under semi-continuous conditions was still unclear.
A UASS with anaerobic filter (AF) system was first described by Mumme et al. (2010), and worked well with maize silage (Mumme et al., 2010), wheat straw (Pohl et al., 2012, Pohl et al., 2013), horse manure (Böske et al., 2014, Böske et al., 2015), and maize straw (Meng et al., 2016a, Meng et al., 2016b). Different from other anaerobic reactors, this system included an upflow anaerobic solid-state (UASS) reactor in which solid feedstock was digested in a plug-flow mode and an AF reactor in which most methanogens existed as biofilm. The effect of oxygen on anaerobic digestion in UASS and AF system cannot be predicted from performance data of other reactors.
Therefore, the overall aim of this research is to investigate the oxygen tolerance capacity of the UASS with AF system. Further aims are to investigate the effect of microaeration on maize straw anaerobic digestion under batch conditions; to investigate the effect of oxygen on maize straw anaerobic digestion in two-stage semi-continuous reactors.
Section snippets
Substrates and inoculum properties
Maize straw was collected from a farm in Cadenberge, Germany. After harvest, the straw was chopped to a final average cutting length of 2–5 cm. Afterwards, it was air-dried to achieve a moisture content of less than 10% and stored at room temperature in a woven bag prior to the experiment.
The inoculum was obtained from previous biogas experiments, which were incubated under thermophilic (55°C) conditions at the Leibniz Institute for Agricultural Engineering. The inoculum was stored at room
Impact of microaeration under batch digestions
The inoculum used was found to be in good condition, as the cellulose reference yielded 600 ± 23 mL/gvs under thermophilic condition was in accordance with the lower limit stated in the VDI guideline 4630 (VDI, 2006). Biogas yields were calculated from the first day of AD until daily biogas yield was less than 1% of the cumulative biogas yield as stated in the VDI guideline 4630 (VDI, 2006). As shown in Fig. 2a, the biogas yields increased sharply in the first ten days. The start of biogas
Conclusions
Aeration pretreatment with an equivalent aeration intensity of 431 mL O2/gvs improved methane yield by 82.2%. Aeration pretreatment can enhance methane production of maize straw under batch conditions. Two stage-system showed relative stability as aeration intensity increased. Although methane yield of the UASS and AF reactor were affected by aeration, the total methane yield of the whole system was relatively stable. The UASS with AF system showed high oxygen tolerance capacity.
Acknowledgments
This collaborative research received funding from the German Federal Ministry of Education and Research (No. 03SF0381A), the National Natural Science Foundation of China (No. 21206084), and the National Key Technology Support Program of China (No. 2014BAC27B01). Additionally, the authors would like to thank Ulf Lüder and Laureen Herklotz for their technical and analytical support.
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