Process intensification (PI) of CO uptake: Investigation of cellular biocomposites of Clostridium ljungdahlii OTA1 immobilized on paper for the design of bioreactors with reduced power and water requirements

PI in a cellular biocomposite concentrates cells, reduces mass transfer resistance, decreases water and reduces power input. Our model system is C. ljungdahlii OTA1, which takes up CO/H₂, produces ethanol/acetate, and grows to 10⁸ cells/mL in suspension culture. A concentrated biocomposite formulation of cells and growth limiting media (10¹⁰ cells/mL) can be spray coated in argon onto chromatography paper generating a thin coating of 10¹³ cells/m² without affecting viability. The coatings are tested in sealed, horizontal Balch tubes, hydrated with growth limiting media, and the headspace flushed with an H₂/N₂/CO mixture (45%/10%/45%).  The cells’ are hydrated by the media moving through the paper pores while the coating remains completely in the gas phase.  The slowly moving liquid phase aids diffusion of nutrients and C₂ products. Measurement of gas concentration (GC) determines specific CO adsorption. Results show that when tube shaking speed is reduced, the biocomposites outperform the same number of cells in suspension. Using an extrusion coating method, at 100rpm the CO uptake rate of OTA1 is ~10% faster in a biocomposite than in suspension.  However, at 25rpm the biocomposite is ~300% faster. This is the result of the paper maintaining a high interfacial area without vigorous stirring which is required for high substrate uptake in CSTR gas fermentations. In both cases, <1% of the cells escaped the coating into the liquid phase. We also investigate the desiccation tolerance of the OTA1 by the addition of dessication protectants, controlled drying, storage, and rehydration conditions for rapid recovery of coating reactivity.