Achieving superior carbon transfer efficiency and pH control using membrane carbonation with a wide range of CO2 contents for the coccolithophore Emiliania huxleyi

The economic viability of microalgal-derived products relies on rapid CO2 transfer in a cost-effective manner. Many industrial gas streams contain concentrated CO2 that, if converted to useful products, would lower greenhouse gas emissions and valorize the wasted CO2. Membrane carbonation (MC) uses non-porous hollow-fiber gas-transfer membranes to deliver CO2 without bubble formation, which makes it possible to achieve a high carbon-transfer efficiency (CTE). However, inert gasses in the industrial streams (e.g., N-2, O-2, and H2O) can significantly lower the CO2-delivery rate. The means to overcome the buildup of inert gases in the membrane lumen is to manage the distal end of the membranes to sweep out inert gases while not wasting significant CO2. A MC-venting strategy was evaluated for CO2 inputs from 5% to 100%. Abiotic tests using a restricted exit flow could achieve >95% CTEabiotic for industrial CO2 streams. When integrated with semi-continuous cultivation of a marine coccolithophore, Emiliania huxleyi, CO2 delivery and venting were on-demand based on a pH set points and pH-actuated feed and venting valves. MC using the venting strategy achieved 100% CTEbiotic when delivering 100% and 50% CO2, which was better than 50% CTEbiotic obtained from pH-controlled sparging of 100% CO2-sparging. E. huxleyi consistently fixed similar to 80% of the delivered CO2 into biomass, and the remaining similar to 20% to calcite coccoliths. The compact size of MC modules, stable pH control, and no shear forces from bubble agitation during the CO2 delivery made MC an ideal match for cultivation of coccolithophores, which are sensitive to shear forces and pH fluctuations.