Enhancing anaerobic digestion of agricultural residues by microaerobic conditions

Biogas plants treating agricultural residues are associated with limited bioenergy output due to the high content of hardly degradable lignocellulosic fibers in the feedstock. Hence, effective treatment techniques are needed to enhance holocellulose deconstruction, and thus, increase the energy budget of anaerobic digestion (AD) process. In the present research, microaerobic conditions were applied in reactors that were previously operated under strict anaerobic conditions as a tool to increase the biodegradability of the lignocellulosic material and thereby improve the methane production in lignocellulose-based AD. Initially, two levels of oxygen loads (i.e., 5 and 15 mLO₂/gVS) were examined during the AD of lignocellulosic biomass at batch mode. Low and high oxygen loads were connected with positive (+ 10%) and negative (− 4%) impact in methanation process, respectively. Subsequently, the experimental results were validated by the amended BioModel. Furthermore, continuous mode experiments were conducted to more closely mimic real-life applications. Monitoring of a continuous digester fed with agricultural residues showed that the injection of 7.3 mLO₂/gVS/day—which value was defined from mathematical optimization—was capable of improving the methane yield by ∼ 7%. In addition, oxygen injection did not create any risk of inhibition incidents. Concerning microbial community structure, the bacterial population was relatively robust and was not markedly affected by oxygen addition. In contrast, some archaeal representatives were found to have increased relative abundance on oxygen exposure. More specifically, the aero-tolerant Methanosarcina and Methanobacterium spp. were the most dominant methanogens at microaerobic conditions.