Impact of cathodic pH and bioaugmentation on acetate and CH4 production in a microbial electrosynthesis cell

Abstract

This study compares carbon dioxide conversion in carbonate-fed microbial electrosynthesis (MES) cells operated at low (5.3), neutral (7) and high (8) pH levels and inoculated either with wild-type or bioaugmented mixed microbial populations. Two 100 mL (cathode volume) MES cells inoculated with anaerobic digester sludge were operated with a continuous supply of carbonate solution (5 g L⁻¹ as CO₃²⁻). Acetate production was highest at low pH, however CH₄ production still persisted, possibly due to pH gradients within the cathodic biofilm, resulting in acetate and CH₄ volumetric (per cathode compartment volume) production rates of 1.0 ± 0.1 g (L_c d)⁻¹ and 0.84 ± 0.05 L (L_c d)⁻¹, respectively. To enhance production of carboxylic acids, four strains of acetogenic bacteria (Clostridium carboxidivorans, Clostridium ljungdahlii, Clostridium autoethanogenum, and Eubacterium limosum) were added to both MES cells. In the bioaugmented MES cells, acetate production increased to 2.0 g (L_c d)⁻¹. However, production of other carboxylic acids such as butyrate and caproate was insignificant. Furthermore, 16S rRNA gene sequencing of cathodic biofilm and suspended biomass suggested a low density of introduced acetogenic bacteria implying that selective pressure rather than bioaugmentation led to improved acetate production.