Biomass retention on electrodes rather than electrical current enhances stability in anaerobic digestion

Abstract

Anaerobic digestion (AD) is a well-established technology for energy recovery from organic waste streams. Several studies noted that inserting a bioelectrochemical system (BES) inside an anaerobic digester can increase biogas output, however the mechanism behind this was not explored and primary controls were not executed. Here, we evaluated whether a BES could stabilize AD of molasses. Lab-scale digesters were operated in the presence or absence of electrodes, in open (no applied potential) and closed circuit conditions. In the control reactors without electrodes methane production decreased to 50% of the initial rate, while it remained stable in the reactors with electrodes, indicating a stabilizing effect. After 91 days of operation, the now colonized electrodes were introduced in the failing AD reactors to evaluate their remediating capacity. This resulted in an immediate increase in CH 4 production and VFA removal. Although a current was generated in the BES operated in closed circuit, no direct effect of applied potential nor current was observed. A high abundance of Methanosaeta was detected on the electrodes, however irrespective of the applied cell potential. This study demonstrated that, in addition to other studies reporting only an increase in methane production, a BES can also remediate AD systems that exhibited process failure. However, the lack of difference between current driven and open circuit systems indicates that the key impact is through biomass retention, rather than electrochemical interaction with the electrodes.

Mitigation of Volatile Fatty Acid Build-Up by the Use of Soft Carbon Felt Electrodes: Evaluation of Anaerobic Digestion in Acidic Conditions

“…Although acetoclastic/hydrogenotrophic methanogenesis cannot be ruled out, the introduction of the electrodes enhanced the methanogenic processes as a consequence of better biomass proliferation, as observed from the responses obtained from OC experiments versus CC tests. In these experiments, applying an electric charge did not cause higher methane production rates, which was in accordance with results reported by De Vrieze et al [23]. The measurement of the electric current of the closed-circuit system allows for the calculation of the methane produced by electromethanogenesis using the stoichiometry of this reaction.…”

“…In addition, a combined AD and BES (AD-BES) reactor could represent a useful way to prevent inhibition of the AD process in pre-installed digesters, optimizing the efficiency and treatment capacity of pre-existing infrastructure. In fact, this combined approach has been evaluated for the treatment of different substrates, such as fecal wastewater [20], waste activated sludge [21,22], molasses [23], domestic wastewater [24], and toxic contaminants [25]. However, these studies were performed at non-inhibitory organic loads and offered no information on how the combined AD-BES reactor behaves when VFAs accumulate.…”

Lab-Scale Experiment and Model Study on Enhanced Digestion of Wastewater Sludge using Bioelectrochemical Systems

“…A recent study performed concluded that such anode biofilms increase the retention of slowly growing microorganisms (e.g., acetoclastic methanogens) in the EAD and thus enhance the rate of anaerobic digestion (De Vrieze et al, 2014). However, in our study, such thick biofilms were not observed on the graphite brushes in the control digester, indicating that solid retention near the graphite brush did not play an important role in enhancing the rate of anaerobic digestion in this study.…”

Direct Interspecies Electron Transfer between Geobacter metallireducens and Methanosarcina barkeri

“…Previous studies that have reported stimulation in methane production when carbon cloth was introduced into digesters have attributed this response to the cloth retaining methanogens (48)(49)(50). However, the possibility that these conductive materials might promote DIET should also be considered.…”