An investigation of bacterial composition and biofilm structure in mixed-community bioanodes

Abstract

Microbial fuel cells (MFC) are devices that convert chemical energy in soluble organic matter into electrical energy. They can be used for wastewater treatment coupled with energy production as well as for sensing, hydrogen production, electrosynthesis and metal recovery. Implementing these technologies is hindered by low current production. Currently, little is known about anodic communities regarding growth, electrode coverage, bacterial composition, biofilm structure, metabolism and how are they affected by operational factors. Such knowledge is needed to engineer MFCs that can overcome current limitations. The subject of the present study is the mixed-community bioanode. The effects of light, anode-tocathode surface ratio (A/C), substrate composition and anode potential on bioanodes were investigated. Two types of substrates were used: the first was based on sodium acetate and the second was a synthetic wastewater which simulated the chemical composition of real wastewater. First bioanodes were studied in presence and absence of light. A different set of bioanodes were grown at 9 different A/C ratios in single-chamber MFCs. Another set of bioanodes were grown in half-cells at 3 different anode potentials (-400 mV, -200 mV and 0 mV vs Ag/AgCl). The development of anodic biofilms and their long-term dynamics were investigated using a multi-anode reactor which allowed for better replication of running conditions. Geobacter was identified in all bioanodes but its abundance was highly variable and dependent on running conditions. Over time the bacterial composition of bioanodes under constant conditions continuously changed during the first 33 days but stabilised by the 67th day. Bioanodes fed on acetate had higher cell counts, Geobacter percentage, and current output than bioanodes fed on synthetic wastewater. Light exposure decreased coulombic efficiency by almost 14 times and favored growth of Rhodopseudomonas species in the detriment of Geobacter. Abundance of Geobacter increased with anode potential when fed on acetate (from 609.98 106 cells/gram at -400 mV to 5212.38 106 cells/gram at 0 mV) but decreased when fed on synthetic wastewater (from 200.6 106 cells/gram at -400 mV to 49 106 cells/gram at 0 mV). Current density and Geobacter density decreased by an order of magnitude when A/C ratio was varied from 1:12 to 1:1 but remained relatively constant when A/C was increased further to 8:1. Uneven biomass coverage on bioanodes and a decrease of biofilm volume with depth inside bioanodes were observed suggesting that anodes were only partially used by electrigenic bacteria. Results reported here have important implications for future reactor designs, on the use of three-dimensional bioanodes and on long-term applications of Microbial Fuel Cells.