Fermentation strategies for PHB production in a novel membrane bioreactor: investigating batch and fed-batch operations

Gas-transfer membranes have been successfully deployed as efficient aeration devices in wastewater treatment. There is an increasing interest in using such membrane technology in industrial biotechnology. This study proposes membrane bioreactors as a novel bioreactor setup for polyhydroxybutyrate (PHB) production using Cupriavidus necator, whereby gas-transfer membranes are used for aeration. A proof-of-concept membrane bioreactor was built by combining a 50 ml centrifuge tube with hollow fiber membrane bundles. Different numbers and length of polydimethylsiloxane (PDMS) hollow fiber membranes were used to create membrane bundles with varying specific surface areas for oxygen transfer. In batch mode, a maximum biomass concentration of 10.3 g/L, which corresponds to a yield of 0.67 g biomass/g substrate, was achieved with 250 m²/m³ as the specific surface area of the membranes and 40 rpm as the liquid recirculation rate. Two different fed-batch modes were investigated to induce PHB production by applying nitrogen source limitation via fill-and-draw and two-step feeding strategies. A PHB level of 22% was obtained with fill-and-draw feeding by supplying 0.25 g/L NH₄Cl after initial cultivation. Results indicate that membrane bioreactors are promising for C. necator cultivation, but further research is needed to enhance the PHB productivity.