Continuous autotrophic production of alcohols with Clostridium autoethanogenum LAbrini in a submerged membrane bioreactor.

Due to the generally low growth rates of acetogens, conventional batch and continuously operated syngas fermentation processes in suspension cultures, e.g., stirred tank bioreactors, typically result in low cell concentrations and, thus, limited volumetric productivities (space-time yields). To increase cell concentrations in syngas fermentations, a continuously operated submerged microfiltration membrane bioreactor (MBR) was applied on a L-scale with C. autoethanogenum LAbrini for the production of alcohols. Cell retention was achieved by inserting a sintered metal microfiltration rod into a standard stirred-tank bioreactor. A batch phase followed by continuous operation with full cell retention until 95% CO conversion was used to start the MBR operation with partial cell retention. Three continuous syngas fermentation processes were operated until a quasi-steady state was achieved with total process times of up to 10 days. Higher cell (4.4 g L-1), ethanol (4.8 g L-1), and D-2,3-butandiol (0.9 g L-1) concentrations, and very much improved volumetric productivities were achieved at a medium dilution rate (1.4 d-1) below the maximum growth rate of C. autoethanogenum LAbrini, compared to an autotrophic batch process used as reference. However, at a medium dilution rate (2.8 d-1) above the maximum growth rate, solely the volumetric productivities were improved, e.g., 7.73 g L-1 d-1 ethanol compared to 0.44 g L-1 d-1 in the reference batch process. The start-up time of the MBR operation with partial cell retention was successfully reduced by applying an initial mixotrophic batch phase with fructose as a heterotrophic carbon source. The submerged microfiltration membrane rod made of sintered metal, applied here for the first time in a stirred-tank bioreactor for cell retention, demonstrated long-term operational stability in continuous gas fermentations with cell concentrations up to 4.4 g L-1. Due to its low specific membrane area of ~ 70 cm2 L- 1, the scalability of a submerged MBR is facilitated, especially when operated at the relatively low permeate flow rates of < 1.65 mL cm-2 h-1, as demonstrated here.