Optimisation of continuous gas fermentation by immobilisation of acetate-producing Acetobacterium woodii

Hydrogen from water electrolysis is often suggested as a way of storing the excess energy from wind and solar power plants. However, unlike natural gas, hydrogen is difficult to store and distribute. One solution is to convert the hydrogen into other fuels or bulk chemicals. In this study we investigated fermentation in which homoacetogenic clostridia apply the Wood-Ljungdahl pathway to generate acetate from H₂ and CO₂. Acetate can be used as a bulk chemical or further transformed into biofuels. Autotrophic growth with CO₂ as the sole carbon source is slow compared to heterotrophic growth, so the aim of this work was to improve continuous gas fermentation by immobilising the acetate-producing clostridia, thus preventing their wash out from the bioreactor. Two homoacetogenic bacterial strains (Acetobacterium woodii and Moorella thermoacetica) were tested for their acetate production potential, with A. woodii proving to be the better strain with maximum acetate concentration of 29.57 g l^-1. Due to its stability during fermentation and good bacterial immobilisation, linen was chosen as immobilisation material for continuous fermentation. This study demonstrates the successful continuous fermentation of acetate from H₂ and CO₂ using A. woodii immobilised on a low-cost surface at high volumetric productivity of 1.21 ± 0.05 g acetate l^-1 d^-1. This has great industrial potential and future studies should focus on the scale-up of this process.