(163d) Advanced Membrane Pre-Concentration in Continuous Downstream Bioprocessing

The conversion of biogenic feedstocks to microbial processes holds substantial promise to displace fossil carbon-derived fuels and chemicals. For the production of many bio-based chemicals, downstream separations are often the most expensive, energy-intensive unit operations, accounting for up to 90% of the total production cost because bioprocesses often yield dilute streams with multiple impurities, making bioproduct recovery a major technical hurdle. Towards improved separations in bioprocessing, we integrated a membrane pre-concentration step into a continuous in situ product recovery (ISPR) process. The membrane-assisted ISPR process consists of a cell retention device, a membrane for product concentration, a continuous solvent extraction system, a flash tank, and two distillation columns to continuously extract and purify bioproducts from fermentation broth. By removing the product on-the-fly, this system can mitigate product inhibition, improve biological productivity and apparent titer, and enable continuous fermentation.

In a case study, we investigated the use of both reverse osmosis (RO) and high-pressure reverse osmosis (HPRO) membranes for concentrating an exemplary fermentation product, butyric acid—an important precursor for sustainable aviation fuel—prior to downstream extraction. The experimental measurements indicated a maximum achievable concentration factor (CF) of butyric acid to 4.0 with an HPRO membrane, representing a notable improvement over the 2.6-3.2 CF range typical of conventional reverse osmosis (RO) membranes. The membrane pre-concentration step further resulted in a more than five-fold increase in the final butyric acid concentration in the organic phase, and a substantial 76% reduction in organic solvent consumption, both of which contributed to a 53% decrease in the overall cost of butyric acid production and a 46% reduction in greenhouse gas (GHG) emissions.

Notably, there is a tradeoff between product CF and product recovery during membrane-based product concentration. As we concentrated butyric acid, increased product loss was observed in the membrane permeate as the driving force for product permeation through the membrane (i.e., transmembrane concentration gradient) increased. Consequently, considering the typical tradeoff between CF and product recovery, we made further efforts to optimize process energy consumption and provide guidance for general downstream processing regarding benefits and tradeoffs for use of a membrane product concentration unit.

Overall, the integration of membrane preconcentration has the potential, when optimized, to offer a more economically viable and environmentally sustainable approach to the recovery of bioproducts from dilute aqueous streams, providing clear benefits in both process efficiency and sustainability.