Internal mass transfer considerations in biofilms of succinic acid producing Actinobacillus succinogenes

Abstract

The rumen bacterium Actinobacillus succinogenes is reputable for its high productivity, -yield and -titre fermentative production of succinic acid under biofilm conditions. The paper presents an analysis of internal mass transfer effects in biofilm fermentations of A. succinogenes. Internal mass transfer effects were investigated by batch fermentations using attached- and resuspended biofilms as biocatalysts. In the latter, the biofilms were resuspended after initial development to simulate mass transfer free (free cell) fermentations. Intrinsic kinetics for succinic production obtained from resuspended free cell fermentations predicted faster production rates than for the attached biofilms runs (biofilm thicknesses in the range of 120–200 µm), indicating internal mass transfer restrictions. A developed biofilm reaction diffusion model gave good predictions of attached biofilm batch results by accounting for internal mass transfer in the biofilm. Biofilm effectiveness factors ranged from 75% to 97% for all batches at the inception of batch conditions but increased with progression of batch operation due to increased succinic acid titres which inhibited production rates. Biofilm thickness and succinic acid concentrations were shown to have a significant effect on internal mass transfer. A simplified algorithm was developed to estimate the pseudo-steady state glucose penetration and biofilm effectiveness of A. succinogenes biofilms without the requirement to solve the overall mass transfer model. The results clearly showed that internal mass transfer need to be considered in biofilm fermentations involving A succinogenes as high biomass concentrations may not always equate to increased productivities if mass transfer effects dominate.