The role of electrical voltage application in enhancing anaerobic digestion of long chain fatty acids: Connection Matters!

Although electrical voltage (EV) application is widely used for direct interspecies electron transfer stimulation (DIET-S) in anaerobic digestion (AD), three relevant questions are yet to answer, i.e., i) can EV stimulate AD of recalcitrant/toxic wastes, e.g., long chain fatty acid (LCFA)?, ii) is it beneficial to combine EV with other DIET-S strategy?, and iii) after microbial community enrichment, if EV is disconnected, will the enhanced AD performance be affected?. For answering these questions, we operated four reactors: CR, EVR, MR, and (EV-M)R, referring to control, EV-applied, magnetite-supplemented, and "EV-applied with magnetite-supplemented" reactors, respectively, using oleate as substrate. Maximal treatable organic loading rate in CR and DIET-S reactors was 0.75, and 2.00 g COD/L/d. CH4 production yields from EVR, MR, and (EV-M)R were 0.19, 0.16, and 0.22 L CH4/g CODadded, respectively. DIET-S enriched DIET-implicated species, and thermodynamically enhanced LCFA beta-oxidation. Genes involved in CH4 generation via CO2 reduction in EVR, MR, and (EV-M)R, were 2.4, 2.6, and 4.0 times higher than CR, respectively. Considering its CO2 reduction capability, Methanothrix sp. is claimed to establish DIET network with Syntrophomonas sp. in DIET-S reactors. For answering the third question, we further operated EVR and (EV-M)R. EV disconnection sharply lessened CH4 production, however, EV reconnection caused performance recovery, without significant change in microbial community structure. Relative abundances of pilA, ATPase, and hydrogenotrophic methanogenesis-related genes were increased by EV reconnection, and vice versa. This study provides engineering tool for enhancing LCFAs degradation, and scientific clues for the importance of continuous EV application, as DIET-S warranty.