Ecological interactions in microbial electrochemical systems (MXCs) that enhance electron recovery

Microbial electrochemical systems (MXCs) hold promise for renewable energy capture from organic waste streams. Coulombic efficiency (CE) is crucial for high energy conversion efficiency (ECE) in microbial electrochemical systems (MXCs). Low coulombic efficiencies are a major impediment during utilization of fermentable substrates in the MXC anode. Complete electron balance was established for consumption of ethanol in an MEC (microbial electrolysis cell) anode. In experiments where methanogenesis was allowed, 60% electrons were recovered as electric current, while 26% of the total electrons ended up as methane. When methanogenesis was selectively inhibited, coulombic efficiency increased to 84%. Ethanol had to be first fermented to acetate and hydrogen gas (H2) before producing electric current. Detection of hydrogenotrophic methanogens alone in the control reactor confirmed the loss of H2 electrons to methane. Under methanogenic inhibition, homo-acetogenic bacteria channeled the H2 electrons to produce acetate, which was further used by the anode respiring bacteria (ARB) to produce current. H2 scavengers formed an essential link in a three-way syntrophy along with fermenters and ARB in the anode, and the microbial community structure of the anode correlated well with the electron flow from ethanol.;Homo-acetogenic bacteria also formed a robust partnership with ARB in a H2-fed biofilm anode to produce high current density (∼ 10A/m 2). The partnership was sustained at very low hydraulic retention time (HRT) of 1 hour, providing homo-acetogens a competitive advantage over methanogens, which are severely limited under these conditions.;The positive role of homo-acetogens to increase CE was re-established when high ammonium-N was present in the anode, as occurs with animal wastes. Beneficial ecological management is crucial for efficient MXC operation through sound engineering and microbiological practices.