Understanding the fundamentals and application of electrically conductive biofilm in dry digestion

Abstract

The application of microbial electrolysis cell assisted anaerobic digestion (MEC-AD) system for dry digestion to recover methane from food waste (FW) is promising. Understanding the extracellular electron transfer (EET) based syntrophy between ARB and other microorganisms is important to improve the MEC-AD system performance during FW dry digestion. Therefore, the objective of this thesis was (1) to demonstrate an engineered MEC-AD system, bioelectrochemical leach bed reactor (BLBR), for FW dry digestion with enhanced performance, (2) to establish a new standard method for quantification of biofilm conductivity with improved accuracy and reproducibility and to apply the developed method to the anode biofilms in BLBR together with other tools to systematically demonstrate and assess direct interspecies electron transfer (DIET) and conduction-based syntrophy during dry AD process of FW in BLBR. A traditional leach bed reactor (LBR) was modified for FW dry digestion and methane production. Inoculum should be acclimated with potential inhibitors such as salinity and ammonia which may be encountered with elevated levels in LBR systems before starting the LBR operation, in order to accumulate desired microorganisms adapted to those inhibitors and to achieve the methane production. The cumulative methane yield with the inoculum to substrate ratio (ISR) of 60% was 3.35 times more than that with ISR of 10%, while VS reduction with ISR of 60% decreased by 20% to that with ISR of 10%. Increasing in leachate recirculation also promoted the methane yield. The cumulative methane yield increased by 78% when the recirculation rate increased from 0.3 L/hr to 7.5 L/hr while ISR was kept unchanged. These results displayed the feasibility of LBR application for methane production from FW. The voltage application in BLBR has proved to improve the acidogenesis and methanogenesis during FW AD process. Optimal applied voltage for methane production was 0.9 V, at which methane yield was 293 mLCH4/gVS and was 46.7% higher than the control. Further increase the voltage to 1.2 V led to decrease on methane yield. VFA profiles also showed enhanced acidogenesis and acetogenesis with voltage application, with the observed propionate accumulation occurred in the BLBR with 0.3 and 1.2 V but without any significant inhibition on methane yield. The contribution of direct electron transfer through closed circuit on methane yield enhancement was less than 10% and the coulombic efficiency (CE) was less than 6.1%, even with a high conductive anode biofilm in BLBR. The microbial community structure in cathode biofilms indicated the enhanced methane production could be attributed to the enrichment of hydrogenotrophic methanogens through syntrophic methanogenesis because of the enhanced acidogenesis and acetogenesis by applied voltage. Acetoclastic methanogens enriched at the anode may have also improved the methane production via acetate dismutation by directly accepting electrons from ARB in the anode biofilm, although this hypothesis requires future experimental evidence. The results from the present experiments proved the feasibility of BLBR on FW dry digestion and enhanced methane production. Biofilm conductance is a key parameter to estimate that how feasible the electron could be transferred within it. The biofilm conductance in the MECs showed a sigmoidal profile with anode potential. In comparison, biofilm conductance with a fixed anode potential of -0.4V showed little difference from that without a fixed anode potential, which indicated that anode potential control was not critical for measuring biofilm conductance. The Ohmic-response range was identified at a voltage range between 0 and 100 mV where the current-voltage profile of biofilm followed Ohm's law. Increased monitoring time at each voltage step showed up to 69% decrease in measured biofilm conductance and further deviation from Ohm’s law on the current-voltage profile. The relationship between biofilm conductance and operational parameters also suggested that biofilm conductance should be quantified and displayed with these parameters together for the purpose of comparison. The methods developed in this chapter will be applied to investigate the conductance of biofilms developed in BLBR systems. Biofilm from LBR and BLBR in previous experiments were developed on the split gold electrodes in the MEC for biofilm conductivity measurement. Although both LBR and BLBR biofilms showed smaller conductivity than Geobacter-enriched biofilms, the BLBR biofilm was also reported highly conductive (145.6 ~ 158.9 μS/cm) under the anode potential observed during BLBR operation, indicating that the anode biofilm would be conductive for EET in BLBR. The electron transfer in those methanogenic types of biofilms would be dominated by metallic conduction, because the biofilm conductivity increased against the anode potential. The abundance of Geobacter sp. in biofilm community was positively correlated with the biofilm conductivity. Microbial community structure analysis identified similar bacterial and archaea genera in the biofilm communities grown on split gold anodes of MECs and on carbon fiber anodes of BLBR. Therefore, MEC equipped with split gold anodes could be a useful tool for in situ study on the bioelectrochemical characteristics including biofilm conductivity of those biofilms grown on electrodes of BLBR and other MEC-AD systems.