Microbial electrolysis cells using complex substrates achieve high performance via continuous feeding-based control of reactor concentrations and community structure

Abstract

Microbial electrolysis cells have potential to be integrated into biorefineries for waste valorization in the form of renewable hydrogen, a reagent needed for bio-oil upgrading. However, reaching high productivities and sustaining them during long-term operation requires a better understanding of the impact of process conditions on the biocatalyst and overall performance. The impact of feeding regime and different concentration levels in continuous vs. fed-batch delivery of a switchgrass-derived substrate were comparatively investigated. At an organic loading of 20 g/L-d, continuous operation resulted in a stable average H2 productivity of 7.9 ± 0.4 L/L-d over a 3-week period, the highest reported value for continuous hydrogen production from complex streams via microbial electrolysis. Corresponding fed-batch operation yielded 46% lower productivity on average, which dropped further to <1 L/L-d after two-weeks of operation. Principle component analysis (PCA) of the data linked changes in exoelectrogen population to the decrease in performance at high concentrations during fed-batch operation. Accumulation of propionic acid was linked to electron diversion to undefined sinks. The differences in working concentration between the two modes of feeding resulted in different selective pressures, leading to an increase in fermenting microbes such as Firmicutes, and emergence of a more tolerant Geobacter sp. at high substrate concentrations. These insights increase the understanding of how different microbial families change with process conditions, and their impact on performance. This study highlights how process design can improve microbial community management and sustain long-term performance, which is key for commercial applications of microbial electrolysis systems.